Plant Vectors, Compositions and Uses Relating Thereto
The present disclosure relates to a single stranded RNA vector suitable for introducing a therapeutic agent, such as a peptide, a protein or a small RNA, into a host plant. The vector does not encode for any movement protein or coat protein, but is capable of capable of systemic and phloem-limited movement and replication within the host plant.
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This application is based on US Provisional Application No. 63/023,712, filed May 12, 2020, which application is incorporated herein by reference in its entirety and to which priority, and the benefit of, is claimed.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTThis invention was made with government support under Grant No. AP18PPQS&T00C223 awarded by the United States Department of Agriculture (USDA), and under Grant No. 10S1912025 and MCB1818229 awarded by the National Science Foundation (NSF). The United States government has certain rights in this invention.
REFERENCE TO SEQUENCE LISTINGThis application includes one or more Sequence Listings pursuant to 37 C.F.R. 1.821 et seq., which are disclosed in computer-readable media (file name: 2105_0077P_ST25, created on May 11, 2021, and having a size of 37,396 bytes), which file is herein incorporated by reference in its entirety.
FIELD OF THE INVENTIONThe present disclosure relates to a single stranded RNA vector suitable for introducing a therapeutic agent, such as a peptide, a protein or a small RNA, into a host plant, wherein movement thereof is substantially limited to the phloem and targeted to control or manage a plant disease or condition.
BACKGROUND OF THE INVENTIONBoth general and highly targeted anti-microbial agents have been developed for animals (e.g., humans) whose circulatory systems provide a delivery system for widespread application throughout the animal. In contrast, much less research has been conducted to develop general or targeted therapeutic agents for non-genetically modified plants since lack of a simplified circulatory system complicates delivery throughout the host plant. This is especially problematic in large, long-lived trees (e.g., citrus), where injection of anti-microbial agents may be rapidly diluted. As a result, few solutions exist for treating systemic plant infections or conditions beyond external application of pesticides, e.g., to control the pathogen's vector during the growing season, foliar applications to strengthen a plant's health in general, or expensive, short-duration injection of agents targeting the pathogen or vector.
Particularly concerning are diseases and conditions affecting the citrus industry. Huanglongbing (HLB), also known as Citrus Greening, is the most serious citrus disease globally. HLB is associated with three species of the bacterium Candidatus Liberibacter spp. (asiaticus, africanus, and americanus) and is transmitted by two psyllid species, Asian citrus psyllid (ACP) (Diaphorina citri, Kuwayama) and African citrus psyllid (Trioza erytreae, Del Guercio). HLB is graft-transmissible and spreads naturally when a bacteria-containing psyllid feeds on a citrus tree and deposits the pathogenic bacteria into the phloem where the bacteria reproduce. Once a tree is infected, there is no cure. While the diseased fruit pose no health threat to humans, HLB has devastated millions of acres of citrus groves throughout the world. In the United States alone, ACP and CL asiaticus (CLas) have decimated the Florida citrus industry, causing billions of dollars of crop losses within a very short time span. Moreover, HLB has spread into every citrus producing region in the United States. Most infected trees die within a few years after infection, and fruit develops misshapen and off flavored and thus is unsuitable for consumption. According to the United States Department of Agriculture (USDA), the entire citrus industry is at substantial risk.
Consideration of plant physiology aids in the development and implementation of strategies for managing plant diseases and conditions. The vascular system of plants is the key conduit for sugars and amino acids, as well as signaling molecules such as small ribonucleic acids (RNAs), proteins, peptides and hormones, which are required for a large number of developmental processes and responses to biotic and abiotic stress (
Confusion in the mRNA movement literature is pervasive. Some studies have indicated that the major determinant of RNA mobility is their abundance in companion cells (Kim, G. et al. (2014), Genomic-scale exchange of mRNA between a parasitic plant and its hosts, Science 345:808-811; Thieme, C. J. et al. (2015), Endogenous Arabidopsis messenger RNAs transported to distant tissues, Nature Plants 1(4):15025; Yang, Y. et al. (2015), Messenger RNA exchange between scions and rootstocks in grafted grapevines, BMC Plant Biol 15, 251). Mathematical modeling has been used to propose a non-selective, Brownian diffusion model for mRNA movement based mainly on their abundance, with half-life and transcript length also playing roles (Calderwood, A. et al. (2016), Transcript Abundance Explains mRNA Mobility Data in Arabidopsis thaliana, Plant Cell 28:610-615). However, other studies reached opposing conclusions, finding that mRNA abundance in companion cells does not correlate with movement (Xia, C. et al. (2018), Elucidation of the Mechanisms of Long-Distance mRNA Movement in a Nicotiana benthamiana/Tomato Heterograft System, Plant Physiol 177:745-758). In addition, while it is generally assumed that the phloem does not contain RNases that target the transiting RNAs (Morris, R. J. (2018), On the selectivity, specificity and signaling potential of the long-distance movement of messenger RNA, Curr Opin Plant Biol 43:1-7), Xia et al. also found that most mobile mRNAs are degraded and never reach the root or upper stem. Other studies found that the presence of a predicted tRNA-like structure is associated with over 11% of mobile mRNAs (Zhang, W. N. et al. (2016), tRNA-Related Sequences Trigger Systemic mRNA Transport in Plants, Plant Cell 28:1237-1249), suggesting that mobile mRNAs might harbor specific “zip-codes”. However, other abundant mRNAs containing similar tRNA-like motifs were not mobile (Xia, C. et al. (2018), Elucidation of the Mechanisms of Long-Distance mRNA Movement in a Nicotiana benthamiana/Tomato Heterograft System, Plant Physiol 177:745-758). Thus, prior studies have failed to identify and develop a model system consisting of a highly abundant, mobile RNA whose movement is traceable in living tissue under different cellular conditions.
Plant viruses, many of which move through the plant as a ribonucleoprotein complex (vRNP), have evolved to use the same pathway as used by mobile endogenous RNAs. Plant viruses can accumulate in substantial amounts, and most initiate infection in epidermal or mesophyll cells and then move cell-to-cell through highly selective intercellular connectors called plasmodesmata, which allow for continuity between the cytoplasm of neighboring cells (
For viruses that transit through the phloem as vRNPs, movement is similar to that of host mRNAs. All plant viruses encode at least one movement protein necessary for movement, which bind to viral RNA and also dilate plasmodesmata. Thus, host mRNA movement also likely requires similar host-encoded movement proteins. Viral movement proteins are non-specific RNA binding proteins. However, questions remain with regard to how vRNPs load into the phloem and unload in distal tissues, although reprograming companion cell gene expression may be required (Collum, T. D. et al. (2016), Tobacco mosaic virus-directed reprogramming of auxin/indole acetic acid protein transcriptional responses enhances virus phloem loading, Proc Natl Acad Sci USA 113:E2740-E2749). If mRNA trafficking is so widespread and non-specific, it has remained unclear why RNA viruses require their own encoded movement proteins. Some researchers have suggested that RNA viruses require movement proteins if they move as preformed replication complexes that include a large RNA-dependent RNA polymerase (Heinlein, M. (2015), Plant virus replication and movement, Virology 479:657-671), which is beyond the size-exclusion limit (˜70 kDa) of companion cell plasmodesmata. It has also remained unclear why and how some viruses are phloem-limited. For example, phloem-limited Closteroviruses have at least 5 movement proteins, and phloem-limitation can be relieved by over-expressing the silencing suppressor and downregulating host defenses (Folimonova, S. Y. and Tilsner, J. (2018), Hitchhikers, highway tolls and roadworks: the interactions of plant viruses with the phloem, Curr Opin Plant Biol 43:82-88), suggesting that phloem-limitation is a complex process for some viruses. Phloem-limitation can also be an active process (as opposed to lack of a cell-to-cell movement protein). For example, altering a domain of the Potato leaf role virus movement protein conferred the ability to exit the phloem (Bendix, C., and Lewis, J. D. (2018), The enemy within: phloem-limited pathogens, Mo Plant Path 19:238-254).
A direct connection between host movement of mRNAs and vRNP movement was established when the origin of plant virus movement proteins was solved. A pumpkin protein (RPB50) related to the Cucumber mosaic virus movement protein was discovered that was capable of transporting its own mRNA, as well as other mRNAs, into the phloem (Xoconostle-Cazares, B. et al. (1999), Plant paralog to viral movement protein that potentiates transport of mRNA into the phloem, Science (New York, N.Y.) 283:94-98; Ham, B. K. et al. (2009), A polypyrimidine tract binding protein, pumpkin RBP50, forms the basis of a phloem-mobile ribonucleoprotein complex, Plant Cell 21:197-215). A complex population of these endogenous movement proteins, known as non-cell-autonomous proteins (NCAPs), have been proposed as being responsible for the long-distance phloem trafficking of mRNAs (Gaupels, F. et al. (2008), Nitric oxide generation in Vicia faba phloem cells reveals them to be sensitive detectors as well as possible systemic transducers of stress signals, New Phytol 178:634-646; Gomez, G. et al. (2005), Identification of translocatable RNA-binding phloem proteins from melon, potential components of the long-distance RNA transport system, Plant J 41:107-116; Kim, M. et al. (2001), Developmental changes due to long-distance movement of a homeobox fusion transcript in tomato, Science (New York, N.Y.) 293:287-289; Pallas, V. and Gomez, G. (2013), Phloem RNA-binding proteins as potential components of the long-distance RNA transport system, Front Plant Sci 4:130; Yoo, B.C. et al. (2004), A systemic small RNA signaling system in plants, Plant Cell 16:1979-2000).
Since their discovery (Deom, C. M. et al. (1987), The 30-kilodalton gene product of tobacco mosaic virus potentiates virus movement, Science (New York, N.Y.) 237:389-394), a number of viral movement proteins have been identified that are responsible for intracellular trafficking of vRNPs to the plasmodesmata, as well as for cell-to-cell and long-distance movement (Tilsner, J. (2014), Techniques for RNA in vivo imaging in plants, J Microscopy 258(1):1-5). For some viruses (e.g., umbraviruses), cell-to-cell and long-distance movement are associated with multiple movement proteins (Ryabov, E. V. et al. (2001), Umbravirus-encoded proteins both stabilize heterologous viral RNA and mediate its systemic movement in some plant species, Virology 288:391-400). For example, closteroviruses such as Citrus tristeza virus contain three movement proteins. However, for most viruses, all movement activities are thought to be associated with a single movement protein.
Delivering engineered therapeutic agents into plants for combating diseases, insects or other adverse conditions (e.g., HLB and/or the carrier insects) using virus vectors is an established means of introducing traits such as resistance to pathogens or other desired properties into plants for research purposes. Various methods of providing vectors to plants are known in the art. This is often achieved by delivery of the virus vector into a plant cell's nucleus by Agrobacteria tumefactions-mediated “agroinfiltration,” which may result in a modification of that cell's genome, or by delivering the virus vector directly into a cell's cytoplasm, which results in infection without a requirement for genomic modification. In the case of agroinfiltration of RNA viruses, the cDNA of the viral genome is incorporated into the T-DNA, which Agrobacteria delivers into the plants. Such T-DNA includes further regulatory DNA components (e.g., promoter for RNA polymerase), which allow for transcription of the viral genome within plant cells. The incorporated virus, containing therapeutic DNA inserts, is transcribed into RNA within the plant cells, after which the virus behaves like a normal RNA virus (amplification and movement). Thus, to act as an effective vector, a virus should be engineered to accept inserts without disabling its functionality and to ensure that the engineered virus is able to accumulate systemically in the host to a level sufficient to deliver and in some cases express the insert(s). These inserts, whether open reading frames (ORFs) that will be translated into proteins or RNAs that will be used for a beneficial function, should be delivered into the targeted tissue in a manner that is effective and sufficiently non-toxic to the host or to any downstream consumption of the host or the environment. However, only a limited number of viral vectors exist that meet the above criteria and are available for only certain plants (e.g., Tobacco rattle virus for tobacco). Unfortunately, there is either no known suitable viral vector, or only suboptimal viral vectors, for most plants, particularly for long lived trees and vines.
Thus, the ability to implement RNA or DNA therapies on a broad basis is substantially limited with existing technologies. Over 1,000 plant viruses have been identified with many plants subject to infection by multiple viruses. For example, citrus trees are subject to Citrus leaf blotch virus, Citrus leaf rugose virus, Citrus leprosis virus C, Citrus psorosis virus, Citrus sudden death-associated virus, Citrus tristeza virus (CTV), Citrus variegation virus, Citrus vein enation virus and Citrus yellow mosaic virus, among others. However, CTV, the causal agent of catastrophic citrus diseases such as quick decline and stem pitting, is currently the only virus that has been developed as a vector for delivering agents into citrus phloem.
CTV is a member of the genus Closterovirus. It has a flexuous rod-shaped virion composed of two capsid proteins with dimensions of 2000 nm long and 12 nm in diameter. With a genome of over 19 kb, CTV (and other Closteroviruses) are the largest known RNA viruses that infect plants. It is a virulent pathogen that is responsible for killing or rendering useless millions of citrus trees worldwide, although the engineered vector form is derived from a less virulent strain, at least for Florida citrus trees (still highly virulent in California trees). Prior studies have purportedly demonstrated that CTV-based vectors can express engineered inserts in plant cells (U.S. Pat. No. 8,389,804; US 20100017911 A1). However, it has not been commercialized due to its inconsistent ability to accumulate in plants and achieve its targeted beneficial outcome. It is thought that CTV's inability to replicate to sufficiently high levels and heat sensitivity limits its ability to generate a sufficient quantity of RNA for treatment.
Thus, CTV-based vectors have a very limited ability to deliver an effective beneficial payload where needed. Moreover, CTV is difficult to work with due to its large size. CTV is also subject to superinfection exclusion, wherein a CTV-based vector is unable to infect a tree already infected with CTV. CTV is also highly transmissible from plant to plant via several aphid species, a property disliked by regulators concerned with uncontrolled escape into the environment where it might mutate or interact with other hosts in undesirable ways. In addition, strains suitable for one region (e.g., Florida) are unsuitable for varieties of trees in another region (e.g., California). Despite such problems, CTV is the only viral vector platform available for citrus trees.
Accordingly, there is a need for an infectious agent that solves some or all of the above-noted problems, and which is capable of introducing a desirable property and/or delivering a therapeutic agent(s) into a plant, particularly a long-lived plant such as a tree or vine.
SUMMARYThe present disclosure relates to a novel infectious agent(s) capable of delivering an exogenous insert(s) into a plant, compositions comprising a plant infected by the disclosed agent(s), and methods and uses relating thereto. The disclosed agents are sometimes referred to herein as “independently mobile RNAs” or “iRNAs.” Despite being infectious single-stranded RNAs, iRNAs are not viruses given they do not code for any movement protein(s) or RNA silencing suppressors, which are key characteristics of plant viruses. In addition, unlike virtually all plant RNA viruses, with the exception of umbraviruses, iRNAs also do not encode a coat protein for encapsidating the RNA into virions, which is a requirement for vectored movement of viruses from plant to plant. Despite the lack of movement protein expression, iRNAs are able to move systemically within the phloem in a host plant. As compared to viruses, iRNAs have additional advantageous properties, such as: the ability to accumulate to levels exceeding those of most known plant viruses; relatively small size, e.g., being only about two-thirds the size of the smallest plant RNA virus and thus much easier to work with compared to such conventional plant RNA viruses; and the inability to spread on their own to other plants (given their inability to encode for any coat protein).
In accordance with disclosed embodiments, an infectious agent comprises an iRNA-based vector containing an engineered insert, which triggers in a plant expression of a targeted peptide, protein(s) and/or produces targeted small interfering RNAs that are cleaved from the vector for beneficial application. Aspects of the present disclosure include: an iRNA-based vector for delivery of targeted anti-pathogenic agents; an anti-bacterial enzybiotic targeted at bacteria infecting a plant or bacteria required by the insect vector; an enzybiotic that is generated from the TEV IRES; incorporation of siRNAs into the iRNA genome; incorporation of inserts into a lock and dock structure to stabilize the base of a scaffold that supports the inserts; incorporation of siRNAs from a iRNA genome that has been modified to enhance the stability of the local region to precisely counter the destabilizing effects of the inserts; incorporation of an siRNA that disrupts or kills a targeted insect vector; incorporation of an siRNA that mitigates the negative impacts of a tree's callose production; incorporation of an siRNA that mitigates the plant's recognition of the pathogen; incorporation of an iRNA that targets bacterial or fungal pathogens; and incorporation of an insert that triggers a particular plant trait (e.g., dwarfism). Thus, the infectious agents and compositions disclosed herein possess superior and advantageous properties as compared to conventional technologies.
The iRNA-based vectors of the present disclosure are suitable for use as a general platform for expression of various proteins and/or delivery of small RNAs into the phloem of citrus and other host plants. In some implementations, a CYVaV-based vector is provided, which accumulates to massive levels in companion cells and phloem parenchyma cells. The vectors of the present disclosure may be utilized to examine the effects of silencing specific gene expression in the phloem of trees. In addition, CYVaV may be developed into a model system for examining long-distance movement of mRNAs through sieve elements. Since CYVaV is capable of infecting virtually all varieties of citrus, with few if any symptoms generated in the infected plants, movement of RNAs within woody plants may be readily examined.
In accordance with disclosed embodiments, the present disclosure is directed to a plus-sense single stranded ribonucleic acid (RNA) vector comprising a replication element(s) and a heterologous segment(s), wherein the RNA vector lacks a functional coat protein(s) open reading frames (ORFs) and a functional movement protein ORFs. The RNA vector is capable of movement in a host plant, for example systemic movement, movement through the phloem, long-distance movement and/or movement from one leaf to another leaf. In some implementations, the RNA vector also lacks any silencing suppressor ORF(s). In some implementations, the RNA vector comprises a 3′ Cap Independent Translation Enhancer (3′ CITE) comprising the nucleic acid sequence(s) of SEQ ID NO: 4 and/or SEQ ID NO: 5. In some embodiments, the 3′ CITE comprises the nucleic acid sequence of SEQ ID NO: 3.
In some embodiments, the replication element(s) of the RNA vector comprises one or more conserved polynucleotide sequence(s) having the nucleic acid sequence of: SEQ ID NO: 10, SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, and/or SEQ ID NO: 14. In some implementations, the replication element(s) additionally or alternatively comprises one of more conserved polynucleotide sequence(s) having the nucleic acid sequence of: SEQ ID NO: 15 and/or SEQ ID NO: 16.
In some embodiments, the RNA vector is derived from citrus yellow associated vein virus (SEQ ID NO:1) or an iRNA relative thereof. The RNA vectors of the present disclosure are capable of systemic and phloem-limited movement and replication within a host plant. The RNA vectors of the present disclosure are functionally stable for replication, movement and/or translation within the host plant for at least one month after infection thereof
In some embodiments, the heterologous segment(s) of the RNA vector of the present disclosure comprises a polynucleotide that encodes at least one polypeptide selected from the group consisting of a reporter molecule, a peptide, and a protein or is an interfering RNA. In some implementations, the polypeptide is an insecticide or an insect control agent, an antibacterial, an antiviral, or an antifungal. In some implementations, the antibacterial is an enzybiotic. In some implementations, the antibacterial targets a bacterium Candidatus Liberibacter species, e.g. Candidatus Liberibacter asiaticus (CLas).
In some embodiments, the heterologous segment(s) of the RNA vector of the present disclosure comprises a small non-coding RNA molecule and/or an RNA interfering molecule. In some implementations, the small non-coding RNA molecule and/or the RNA interfering molecule targets an insect, a bacterium, a virus, or a fungus. In some implementations, the small non-coding RNA molecule and/or the RNA interfering molecule targets a nucleic acid of the insect, the bacterium, the virus, or said fungus. In some implementations, the small non-coding RNA molecule and/or the RNA interfering molecule targets a virus, for example a virus selected from the group consisting of Citrus vein enation virus (CVEV) and Citrus tristeza virus (CTV). In some implementations, a targeted bacteria is Candidus Liberibacter asiaticus (CLas). In some implementations, the iRNA comprises an siRNA hairpin that targets and renders the targeted bacteria non-pathogenic.
It should be understood that the RNA vector may include multiple heterologous segments, each providing for the same or different functionality. In some embodiments, the heterologous segment(s) is a first heterologous segment, wherein the RNA vector further comprising a second heterologous segment(s), wherein the replication element(s) is intermediate the first and second heterologous segments.
In some embodiments, the heterologous segment(s) of the RNA vector of the present disclosure comprises a polynucleotide that encodes for a protein or peptide that alters a phenotypic trait. In some implementations, the phenotypic trait is selected from the group consisting of pesticide tolerance, herbicide tolerance, insect resistance, reduced callose production, increased growth rate, and dwarfism.
The present disclosure is also directed to a host plant comprising the RNA vector of the present disclosure. The host plant may be a whole plant, a plant organ, a plant tissue, or a plant cell. In some implementations, the host plant is in a genus selected from the group consisting of citrus, vitis, ficus and olea. In some implementations, the host plant is a citrus tree or a citrus tree graft.
The present disclosure also relates to a composition comprising a plant, a plant organ, a plant tissue, or a plant cell infected with the RNA vector of the present disclosure. In some implementations, the plant is in a genus selected from the group consisting of citrus, vitis, ficus and olea. In some implementations, the plant is a citrus tree or a citrus tree graft.
The present disclosure also relates to a method for introducing a heterologous segment(s) into a host plant comprising introducing into the host plant the RNA vector of the present disclosure. In some embodiments, the step of introducing the heterologous segment(s) into the host plant comprises grafting a plant organ or plant tissue of a plant that comprises the RNA vector of the present disclosure to a plant organ or plant tissue of another plant that does not comprise the RNA vector prior to said introduction. The RNA vectors of the present disclosure are capable of systemically infecting the host plant.
The present disclosure is also directed to a process of producing in a plant, a plant organ, a plant tissue, or a plant cell a heterologous segment(s), comprising introducing into said plant, said plant organ, said plant tissue or said plant cell the RNA vector of the present disclosure. In some embodiments, the plant is in a genus selected from the group consisting of citrus, vitis, ficus and olea.
The present disclosure also relates to a kit comprising the RNA vector of the present disclosure.
The present disclosure is also directed to use of the RNA vector(s) of the present disclosure for introducing the heterologous segment(s) into a plant, a plant organ, a plant tissue, or a plant cell. The present disclosure is also directed to use of the host plant(s) of the present disclosure, or use of the composition(s) of the present disclosure, for introducing the RNA vector(s) into a plant organ or plant tissue that does not, prior to said introducing, comprise the RNA vector. In some implementations, the step of introducing the RNA vector comprises grafting a plant organ or plant tissue of a plant that comprises the RNA vector to a plant organ or plant tissue of another plant that does not comprise the RNA vector.
The present disclosure is also directed to a method of making a vector for use with a plant comprising the steps of inserting one or more heterologous segment(s) into an RNA, wherein the RNA is selected from the group consisting of: CYVaV; a relative of CYVaV; other RNA vectors having least 50% or at least 70% RdRp identity with CYVaV; and another iRNA. The present disclosure also relates to a vector produced by the disclosed method(s).
The present disclosure also relates to the use of an RNA molecule as a vector, wherein the RNA is selected from the group consisting of: CYVaV; a relative of CYVaV; other RNA vectors having at least 50% or at least 70% RdRp identity with CYVaV; and, another iRNA. In some implementations, the RNA is used in the treatment of a plant, for example the treatment of a viral or bacterial infection of a plant, for example the treatment of CTV infection or Citrus Greening in a Citrus plant, or in the control of insects that are vectors and/or feed on the plant. The RNA is modified with one or more inserted heterologous segment(s), for example an enzybiotic.
The present disclosure is also directed to the use of an RNA molecule characterized by being in the manufacture of a medicament to treat a disease or condition of a plant, wherein the RNA is selected from the group consisting of: CYVaV; a relative of CYVaV; other RNA vectors having at least 50% or at least 70% RdRp identity with CYVaV; and, another iRNA. In some implementations, the disease or condition is a viral or bacterial infection of a plant, for example CTV or Citrus Greening in a Citrus plant.
The present disclosure is also directed to an RNA molecule for use as a medicament or in the treatment of a disease or condition of a plant, wherein the RNA is selected from the group consisting of: CYVaV; a relative of CYVaV; other RNA vectors having at least 50% or at least 70% RdRp identity with CYVaV; and, another iRNA.
The present disclosure is also related to a ribonucleic acid (RNA) vector, for example a plus-sense single stranded ribonucleic acid (RNA) vector, comprising one or more heterologous segment(s), wherein said heterologous element(s) is attached to the main structure of the RNA vector through a lock and dock structure, optionally a branched structure comprising an insert site for the heterologous element and a relatively stable and/or locking structure that does not participate in folding of the heterologous element or the main structure of the RNA vector. In some implementations, the RNA vector is an iRNA-based vector or a virus-based vector. In some implementations, a lock portion of the lock and dock structure comprises a crystallography scaffold. In some implementations, the lock and dock structure comprises a branched element, wherein a stem and a branch of the branched element are located within a relatively stable structure forming the lock, such as a tetraloop-tetraloop dock, and another branch of the branched element comprises an insert site for the heterologous element.
The present disclosure is also related to an iRNA-based vector having one or more heterologous segment(s) having an siRNA effective against a plant pathogenic bacteria. In some implementations, the siRNA targets a Candidatus Liberibacter species such as Candidatus Liberibacter asiaticus (CLas).
The present disclosure is also related to an iRNA-based vector having a heterologous element comprising a hairpin having a sequence on one side complementary to a sequence within Citrus tristeza virus (CTV). In some implementations, the sequence within CTV is conserved in multiple CTV strains. In some implementations, the sequence one on side of the hairpin is complementary with a sequence in multiple CTV strains, or all known CTV strains, despite differences in the CTV sequences.
The present disclosure is also related to a method for introducing a heterologous segment(s) into a host plant comprising introducing into said host plant an iRNA-based vector after a) encapsidating the iRNA vector in a capsid protein other than the capsid protein of CVEV, or b) by agroinfiltration after inoculating an agroinfiltration site with Xanthomonas citri subsp. citri (Xcc).
The present disclosure is also related to an iRNA-based vector comprising one or more inserts at one or more of positions 2250, 2301, 2319, 2330, 2331, 2336 and 2083 of a CYVaV based RNA. In some implementations, the iRNA-based vector is stabilized, for example by converting G:U pairs to G:C pairs in the 3′UTR structure.
The present disclosure is also related to a method of making a plus-sense single stranded ribonucleic acid (RNA) vector comprising stabilizing the 3′ UTR structure of a parental construct and inserting one or more destabilizing heterologous segment(s) into the stabilized parental construct.
The present disclosure describes many CYVaV-based vectors, but in some implementations analogous vectors are produced using another iRNA or a virus as the starting material or sequence. In these implementations, descriptions relating to CYVaV may be modified accordingly. For example, positions described for CYVaV may be substituted with a corresponding position in another type of iRNA.
In some implementations, an iRNA-based vector or a virus-based vector is constructed using starting material (i.e. an iRNA or virus) obtained from the wild, or multiplied cloned or otherwise reproduced from starting material obtained from the wild. The starting material is modified, for example to change, delete and/or replace, one or more elements of the wild-type structure and/or to add one or more inserts. In other implementations an iRNA-based vector or virus based vector is synthetic. For example, an iRNA-based vector or virus based vector may be made by creating a synthetic replica of the wild type RNA and then modifying the synthetic replica, or directly creating a synthetic replica of a modified RNA.
The present disclosure is also related to compositions and methods comprised of combinations or sub-combinations of one or more other compositions or methods described herein, to compositions produced by methods described herein, to methods of making compositions described herein, and to methods of treating plants using compositions described herein.
The present disclosure relates to novel infectious agents for use as vectors for plants, compositions comprising a plant infected by the disclosed agent(s), and uses and methods relating thereto. The infectious agents of the present disclosure are sometimes referred to herein as “independently mobile RNAs” or “iRNAs” and exhibit superior characteristics as compared to conventional viral vectors. In accordance with disclosed embodiments, the iRNAs are RNA molecules capable of infecting plants and encoding for an RNA polymerase to sustain their own replication, but lacking the ability to encode for any movement protein or coat protein. In addition, iRNAs do not code for any RNA silencing suppressors.
As used herein, a “host” refers to a cell, tissue or organism capable of being infected by and capable of replicating a nucleic acid. A host may include a whole plant, a plant organ, plant tissue, a plant protoplast, and a plant cell. A plant organ refers to a distinct and visibly differentiated part of a plant, such as root, stem, leaf, seed, graft or scion. Plant tissue refers to any tissue of a plant in whole or in part. Protoplast refers to an isolated cell without cell walls, having the potency for regeneration into cell culture, tissue or whole plant. Plant cell refers to the structural and physiological unit of plants, consisting of a protoplast and the cell wall.
As used herein, “nucleic acid sequence,” “polynucleotide,” “nucleotide” and “oligonucleotide” are used interchangeably and refer to a polymeric form of nucleotides of any length. Polynucleotides may have any three-dimensional structure, and may perform any function. A “gene” refers to a polynucleotide containing at least one open reading frame that is capable of encoding a particular polypeptide sequence. “Expression” refers to the process by which a polynucleotide is transcribed into mRNA and/or the process by which the transcribed mRNA is translated into peptides, polypeptides, or proteins.
A vector “derived from” a particular molecule means that the vector contains genetic elements or sequence portions from such molecule. In some embodiments, the vector comprises a replicase open reading frame (ORF) from such molecule (e.g., iRNA). One or more heterologous segment(s) may be added as an additional sequence to the vectors of the present disclosure. In some implementations, said heterologous segment(s) is added such that high level expression (e.g., of a particular protein or small RNA) is achieved. The resulting vector is capable of replicating in plant cells by forming further RNA vector molecules by RNA-dependent RNA polymerization using the RNA vector as a template. An iRNA vector may be constructed from the RNA molecule from which it is derived (e.g., CYVaV).
As used herein, an “infection” or “capable of infecting” includes the ability of a vector to transfer or introduce its nucleic acid into a host, such that the nucleic acid or portion(s) thereof is replicated and/or proteins or other agents are synthesized or delivered in the host. Infection also includes the ability of a selected nucleic acid sequence to integrate into a genome of a target host.
As used herein, a “phenotypic trait” refers to an observable, measurable or detectable characteristic or property resulting from the expression or suppression of a gene or genes. Phenotype includes observable traits as well as biochemical processes.
As used herein, “endogenous” refers to a polypeptide, nucleic acid or gene that is expressed by a host. “Heterologous” refers to a polypeptide, nucleic acid or gene that is not naturally expressed by a host. A “functional heterologous ORF” refers to an open reading frame (ORF) that is not present in the respective unmodified or native molecule and which can be expressed to yield a particular agent such as a peptide, protein or small RNA. For being expressible from the vector in a plant, plant tissue or plant cell, the vector comprising a functional heterologous ORF comprises one or more subgenomic promoters or other sequence(s) required for expression.
Various assays are known in the art for determining expression of a particular product, including but not limited to: hybridization assays (e.g. Northern blot analysis), amplification procedures (e.g. RT-PCR), and array-based technologies. Expression may also be determined using techniques known in the art for examining the protein product, including but not limited to: radioimmunoassay, ELISA (enzyme linked immunoradiometric assays), sandwich immunoassays, immunoradiometric assays, in situ immunoassays, western blot analysis, immunoprecipitation assays, immunofluorescent assays, GC-Mass Spec, and SDS-PAGE.
An “exogenous RNA segment” refers to a segment of RNA inserted into a native molecule, whereby the source of the exogenous RNA segment is different from the native molecule. The source may be another virus, a living organism such as a plant, animal, bacteria, virus or fungus, a chemically synthesized material, or a combination thereof. The exogenous RNA segment may provide any function appropriate for a particular application, including but not limited to: a non-coding function RNA, a coding function in which the RNA acts as a messenger RNA encoding a sequence which, translated by the host cell, results in synthesis of a peptide (e.g., a molecule comprising between about 2 and 50 amino acids) or a protein (e.g. a molecule comprising 50 or more amino acid) having useful or desired properties.
As used herein, “movement protein” refers to a protein(s) required for cell-to-cell and/or long distance movement. “Coat protein” refers to protein(s) comprising or building the virus coat.
Similar to umbraviruses, iRNAs do not possess a functional coat protein(s) ORF and/or otherwise encode for any coat protein. In addition, the RNA polymerase of iRNAs is similar to that of umbraviruses. However, unlike umbraviruses, iRNAs do not possess a functional movement protein(s) ORF and/or otherwise encode for any cell-to-cell movement protein(s) or any long-distance movement protein(s) that serves as a stabilization protein for countering nonsense mediated decay.
Conventional viruses lacking coat proteins are generally less stable inside a plant cell given their genomes are vulnerable to the host RNA silencing defense system. However, iRNAs are surprisingly stable in the intracellular environment, which is an important characteristic for an effective vector. iRNAs are also restricted to the inoculated host plant in the absence of a specific helper virus, since without associated virions they are not transmissible by an insect vector. It is believed that iRNAs are encapsidated into virions only when in the presence of a specific helper virus, e.g., such as an enamovirus, including Citrus vein enation virus (CVEV), which is a rarely seen virus in the United States.
In disclosed embodiments, a recombinant plus-sense single stranded RNA vector is provided that comprises a replication element (s) (e.g., a portion(s) of the vector molecule responsible for replication) and a heterologous segment(s). The RNA vectors of the present disclosure are capable of accumulating to high levels in phloem, and are capable of delivering a therapeutic agent(s) such as a protein, a peptide, an antibacterial and/or an insecticide (e.g., siRNAs) directly into the plant tissue. In certain implementations, the RNA vector is derived from an iRNA molecule, which lacks the ability to encode for any coat protein(s) or movement protein(s). For example, the vector is derived from and/or includes structural elements of the iRNA molecule known as Citrus yellow vein associated virus (CYVaV), an unclassified molecule associated with yellow-vein disease of citrus.
Thus, disclosed embodiments provide for an iRNA-based vector built on or derived from a plus-sense single-stranded RNA molecule using genetic components from an iRNA molecule, e.g., CYVaV. In addition, the present disclosure is directed to kits and/or mixtures comprising an iRNA-based (e.g. a CYVaV-based) vector(s). Such mixtures may be in a solid form, such as a dried or freeze-dried solid, or in a liquid, e.g. as aqueous solution, suspension or dispersion, or as gels. Such mixtures can be used to infect a plant, plant tissue or plant cell. Such kits and mixtures may be used for successfully infecting a plant(s) or plant cell(s) with the iRNA-based vectors of the present disclosure and/or for expression of heterologous proteins or delivery of other therapeutic agents to such plant or plant cell(s).
The present disclosure also relates to a plant, plant tissue, or plant cell comprising said iRNA-based vector as disclosed herein, and/or a plant, plant tissue, or plant cell comprising a therapeutic agent or heterologous polypeptide encoded or delivered by said vector. The present disclosure also provides for methods of isolating such heterologous polypeptide from the plant, plant tissue, or plant cell. Methods for isolating proteins from a plant, plant tissue or plant cell are well known to those of ordinary skill in the art.
CYVaV was found in four limequat trees in the 1950s independent of any helper virus (Weathers, L. (1957), A vein yellowing disease of citrus caused by a graft-transmissible virus, Plant Disease Reporter 41:741-742; Weathers, L. G. (1960), Yellow-vein disease of citrus and studies of interactions between yellow-vein and other viruses of citrus, Virology 11:753-764; Weathers, L. G. (1963), Use of synergy in identification of strain of Citrus yellow vein virus, Nature 200:812-813). Further analysis and sequencing of CYVaV was conducted years later by Georgios Vidalakis (University of California, Davis, Calif.; GenBank: JX101610). Dr. Vidalakis's lab conducted analysis on samples collected from previously established tree sources (Weathers, L. G. (1963), Use of synergy in identification of strain of Citrus yellow vein virus, Nature 200:812-813) and maintained in the disease bank of the Citrus Clonal Protection Program (CCPP). Studies by the Vidalakis lab to characterize CYVaV were inconclusive. However, many of the infected samples containing CYVaV also contained the enamovirus citrus vein enation virus (CVEV); it was relatively common in the 1950s through 1980s for CCPP personnel to mix infect plants with yellow-vein and vein enation for symptom enhancement.
CYVaV is a small (−2.7 kb) iRNA molecule composed of a single, positive sense strand of RNA. It replicates to extremely high levels, is very stable, is limited to the phloem, and has no known mechanism of natural spread. As such, CYVaV is ideal as a vector platform for introducing an agent(s) into a plant host, e.g., such as a small RNA (e.g., non-coding RNA molecule of about 50 to about 250 nt in length) and/or proteins for disease and/or pest management. The production of proteins that bolster (or silence) defenses, antimicrobial peptides that target bacterium, and/or small RNAs that target plant gene expression or the insect vectors of disease agents provide an effective management strategy. To be efficacious, the proteins and small RNAs should be produced in sufficient quantities and accumulate to sufficient levels in the phloem, particularly small RNAs designed to be taken up by targeted insects or fungal pathogens.
CYVaV is only transmissible in nature with a helper virus but may be moved from tree to tree by grafting, and has been shown to infect nearly all varieties of citrus with the exception of hearty orange, including but not limited to infecting citron, rough lemon, calamondin, sweet orange, sour orange, grapefruit, Rangpur and West Indian lime, lemon, varieties of mandarin, varieties of tangelo, and kumquat. It produces a yellowing of leaf veins in the indicator citron tree and has no or very mild yellow vein symptoms in sweet orange and other citrus with no reported impact on fruit quality, or otherwise causing harm to trees.
The polynucleotide sequence (bases 1 to 2692) of CYVaV is presented below (SEQ ID NO: 1):
Relatedness of CYVaV with other viruses including Tombusviridae viruses is shown in
The polynucleotide sequence of the 3′ end of CYVaV (bases 2468 to 2692) is presented below (SEQ ID NO: 2):
The polynucleotide sequence of the 3′ Cap Independent Translation Enhancer (3′ CITE) of CYVaV (bases 2468 to 2551) is presented below (SEQ ID NO: 3):
The 3′ end (and 3′ CITE) of CYVaV comprises the following conserved polynucleotide sequence(s) (bolded and underlined above):
The polynucleotide sequence of CYVaV that encodes for protein p21 (bases 9 to 578) is presented below (SEQ ID NO: 6):
The amino acid sequence of protein p21 is presented below (SEQ ID NO:7):
The polynucleotide sequence of CYVaV that encodes for protein p81 (bases 752 to 2158) is presented below (SEQ ID NO: 8):
The amino acid sequence of protein p81 is presented below (SEQ ID NO:9):
The replication element of CYVaV (e.g., that encodes for protein p81) comprises the following conserved polynucleotide sequence(s) (highlighted and underlined above):
In addition, CYVaV may additionally comprise the following conserved polynucleotide sequence(s) (highlighted and underlined above):
The polynucleotide sequences of recoding frameshift sites of CYVaV (see also
Highly similar iRNAs have also been found in Opuntia (GenBank: MH579715), fig trees, and Ethiopian corn (
The polynucleotide sequence of a similar iRNA identified in a fig tree (sometimes referred to herein as “iRNA relative 1” or “iRNA r1”) is presented below (SEQ ID NO: 20):
The polynucleotide sequence of an iRNA identified in another fig tree (sometimes referred to herein as “iRNA relative 2” or “iRNA r2”) is presented below (SEQ ID NO: 21):
The polynucleotide sequence of an iRNA identified in maize (sometimes referred to herein as “iRNA relative 3” or “iRNA r3”) is presented below (SEQ ID NO: 22):
Note that iRNA relatives (e.g., iRNA r1, iRNA r2, and iRNA r3) may comprise conserved polynucleotide sequence(s) (bolded and underlined above): auagcacug (SEQ ID NO: 4); and/or gauuuguga (SEQ ID NO: 5). For example, the iRNA molecule comprises both of conserved polynucleotide sequence(s): auagcacug (SEQ ID NO: 4); and gauuuguga (SEQ ID NO: 5).
In addition, iRNA relatives (e.g., iRNA r1, iRNA r2, and iRNA r3) may comprise conserved polynucleotide sequence(s) (bolded and underlined above): cguuc (SEQ ID NO: 10); gaacg (SEQ ID NO: 11); gguuca (SEQ ID NO: 12); ggag (SEQ ID NO: 13); and/or aaauggga (SEQ ID NO: 14). For example, the iRNA molecule comprises all of conserved polynucleotide sequence(s): cguuc (SEQ ID NO: 10); gaacg (SEQ ID NO: 11); gguuca (SEQ ID NO: 12); ggag (SEQ ID NO: 13); and aaauggga (SEQ ID NO: 14).
Further, iRNA relatives (e.g., iRNA r1, iRNA r2, and iRNA r3) may comprise conserved polynucleotide sequence(s) (bolded and underlined above): ucgacg (SEQ ID NO: 15); and/or cuccga (SEQ ID NO: 16). The iRNA molecule may comprise both conserved polynucleotide sequence(s): ucgacg (SEQ ID NO: 15); and cuccga (SEQ ID NO: 16). In some embodiments, the iRNA molecule are highly related to CYVaV (or to iRNA r1, iRNA r2, or iRNA r3), and comprise a polynucleotide sequence having 50%, 60%, 70% or more identity for the recoding site for synthesis of RdRp thereof, e.g., 75% or 85% or 90% or 95% or 98% identify of the RdRp of CYVaV (or of iRNA r1, iRNA r2, or iRNA r3).
Thus, in accordance with disclosed embodiments, an RNA vector (e.g., derived from an iRNA molecule) comprises a frameshift ribosome recoding site for synthesis of the RNA-dependent RNA polymerase (RdRp). In addition, the RNA vector may include a 3′ end comprising a polynucleotide sequence that terminates with three cytidylates ( . . . CCC). The penultimate 3′ end hairpin may also contain three guanylates in the terminal loop ( . . . GGG . . . ). Further, the 3′ CITE includes an extended hairpin or portion thereof that binds to Eukaryotic translation initiation factor 4 G (eIF4G) and/or Eukaryotic initiation factor 4F (eIF4F),
In certain embodiments, an RNA vector comprises a 3′CITE comprising conserved sequences auagcacug (SEQ ID NO: 4) and gauuuguga (SEQ ID NO: 5). The RNA vector may also comprise one or more of the following polynucleotide sequences (conserved sequences of identified iRNA molecules): cguuc (SEQ ID NO: 10) and gaacg (SEQ ID NO: 11); and/or gguuca (SEQ ID NO: 12) and ggag (SEQ ID NO: 13); and/or aaauggga (SEQ ID NO: 14). Alternatively, or in addition, the RNA vector may comprise one or both of the following polynucleotide sequences (conserved sequences of identified iRNA molecules): ucgacg (SEQ ID NO: 15) and cuccga (SEQ ID NO: 16).
Identified iRNA relatives all have inserts in the 3′UTR and other nucleotide changes that result in the generation of an ORF that encodes a protein (p21.2) of unknown function. One differentiating characteristic of iRNAs such as CYVaV from any plant virus (
In contrast, PEMV2, as with all umbraviruses, encodes for two movement proteins: p26 (long-distance movement) and p27 (cell-to-cell movement) (
The polynucleotide sequence of PEMV2 is presented below (SEQ ID NO:23):
The polynucleotide sequence of the intergenic plus region of PEMV2 (bolded and underlined above) is presented below (SEQ ID NO:24):
The polynucleotide sequences of recoding frameshift sites of PEMV2 (bases 881 to 1019; see also
CYVaV unexpectedly replicates very efficiently in Arabidopsis thaliana protoplasts despite not encoding p26 (or any other movement protein), which is required for accumulation of PEMV2 because of its ability to also counter NMD (see, e.g., May et al. (2020) “The Multifunctional Long-Distance Movement Protein of Pea Enation Mosaic Virus 2 Protects Viral and Host Transcripts from Nonsense Mediated Decay,” mBio 11:300204-20; https://doi.org/10.1128/mBio.00204-20). Indeed, CYVaV was unusually stable, much more stable than most traditional viruses. CYVaV also produced an astonishingly high level of p81 in wheat germ extracts, at least 50-fold more than the p94 orthologue from PEMV2 (
CYVaV had no synergistic effect with any other combination of citrus virus tested. Additional studies showed that CVEV may be utilized as a helper virus for CYVaV in order to allow for transmission from tree to tree. CVEV was likely responsible for the presence of CYVaV in the original limequat trees; however, CVEV is known to be very heat sensitive and thus was likely lost from the limequat trees during a hot summer.
CYVaV moved sporadically into upper, uninoculated leaves and accumulated at extremely high levels, sometimes visible by ethidium staining on gels. Symptoms that began in the ninth leaf of the major bolt comprised stunting, leaf curling, and deformation of floral tissue. Leaves in axillary stems also began showing similar symptoms around the same time. This astonishing result demonstrated that CYVaV moves systemically in the absence of any encoded movement protein(s), which is not possible by traditional plant viruses. Experiments showed that CYVaV moves systemically in N. benthamiana and is strictly confined to the phloem, replicating only in companion cells and phloem parenchyma cells. In citrus, CYVaV is 100% graft-transmissible, but difficult to transmit in other forms.
Fluorescence in situ hybridization (FISH) of symptomatic leaf tissue and roots confirmed that CYVaV is confined to phloem parenchyma cells, companion cells and sieve elements (
Phloem-limited movement of CYVaV explains why it is readily graft-transmissible, but not easily transmissible by any means. CYVaV lacks any encoded movement protein(s) as noted above. Instead, CYVaV utilizes host plant endogenous movement protein phloem protein 2 (PP2), and the pathway for transiting between companion cells, phloem parenchyma cells, and sieve elements. In addition, since host range is believed to involve compatible interactions between viral movement proteins and host plasmodesmata-associated proteins, it is believed that CYVaV is capable of transiting through the phloem of numerous other woody and non-woody host plants using PP2 as it is a very conserved host endogenous movement protein(s). As such, CYVaV provides an exceptional model system for examining RNA movement (e.g., in N. benthamiana and/or citrus) and for use as a vector for numerous applications. Experiments confirmed that CYVaV moves systemically in a host plant and is limited to the phloem, and is readily graft-transmissible but not readily transmissible between plants in other forms.
Systemic infection by CYVaV was also observed in tomato, cucumber and melon. Referring to
Citrus trees have a complex reproductive biology due to apomixis and sexual incompatibility between varieties. Coupled with a long juvenile period that can exceed six years, genetic improvement by traditional breeding methods is complex and time consuming. The present disclosure overcomes such problems by providing an iRNA-based (e.g., CYVaV-based) vector engineered to include therapeutic siRNA inserts. iRNAs such as CYVaV are unique among infectious agents given they encode a polymerase yet move like a viroid (small circular non-coding RNA that also uses PP2 as a movement protein), and thus are capable of transiting through plants other than citrus. Thus, in addition to citrus, the iRNA-based vectors of the present disclosure may be developed for other woody plants (e.g., trees and legumes), and in particular olive trees and grapevines.
In accordance with disclosed embodiments, CYVaV is utilized in the development of a vector for delivery of small RNAs and proteins into citrus seedlings and N. benthamiana. The procedure utilized for CYVaV vector development was similar to that utilized by the present inventors for engineering betacarmovirus TCV to produce small RNAs (see Aguado, L. C. et al. (2017), RNase III nucleases from diverse kingdoms serve as antiviral effectors, Nature 547:114-117). Exemplary and advantageous sites for adding one, two, three, or more small RNA inserts designed to be excised by RNase III-type exonucleases were identified. Exemplary sites include positions 2250, 2301, 2319, 2330 and 2336. A small hairpin was expressed directly from the genome that targets GFP expressed in N. benthamiana plant 16C, which silenced GFP.
In accordance with disclosed embodiments, iRNA vectors disclosed herein may contain small RNA inserts with various functionality including: small RNAs that target an essential fungal mRNA; small RNAs that target an insect for death, sterility, or other incapacitating function; small RNAs that target gene expression in the host plant; small RNAs that target plant pathogenic bacteria; small RNAs that target CTV; and small RNAs that target CVEV (as this virus together with CYVaV causes enhanced yellow-vein symptoms). In addition, the disclosed vectors may include other small RNAs and/or therapeutic agents known in the art. Thus, a phloem-restricted iRNA-based vector may be engineered to produce small RNAs that have anti-bacterial and/or anti-fungal and/or anti-insect and/or anti-viral properties, which provides for a superior treatment and management strategy compared to current methodologies.
CYVaV vectors may be applied manually to infected or uninfected trees by cutting into the phloem and depositing the vector either as RNA, or by agroinfiltration, or after encapsidation in the coat protein of CVEV or another virus, following citrus inoculation procedures well known to those of skill in the art, e.g. such as procedures developed and used routinely under the Citrus Clonal Protection Program (CCPP). Such procedures are routine for inoculation of CTV and other graft-transmissible pathogens of citrus. Since CYVaV does not encode a capsid protein, no virions are made and thus no natural tree-to-tree transmission of CYVaV is possible. When CYVaV is encapsidated in CVEV or other viral coat protein, no other component of CVEV or other virus is present.
A plant may be infected with an iRNA-based vector by way of agroinfiltration without cutting onto the phloem, for example by agroinfiltration into the leaves of the plant. An iRNA-based vector is not a mere replicon that, once injected into a plant cell, is not expected to leave the plant cell. The goal of agroinfiltration of an iRNA-based vector into, for example, the leaf of a plant is not to install the iRNA-based vector in plants cells near the agroinfiltration site, but rather to have at least some of the iRNA-based vector reach the plant's vasculature and thereafter move systemically through the plant. Typically when agroinfiltrated into the leaf of a plant only a portion of the agroinfiltrated iRNA-based vector will reach the plant vasculature and be effective for infecting the plant. In the case of plants recalcitrant to agroinfiltration, the agroinfiltration may be performed first in a related species more susceptible to agroinfiltration followed by grafting from the more susceptible species to the target species. For example, Citrus limon may be more susceptible to agroinfiltration than various species of orange trees. Alternatively or additionally, a species recalcitrant to agroinfiltration may be pretreated to make them more susceptible to agrofiltration. For example, agroinfiltration into Citrus plants may be facilitated by first inoculating the intended agroinfiltration site with an actively growing culture of Xanthomonas citri subsp. citri (Xcc) suspended in water, as described for example in Jia and Wang (2014). Xcc-facilitated agroinfiltration of citrus leaves: a tool for rapid functional analysis of transgenes in citrus leaves. Plant Cell Rep. 33:1993-2001.
When infecting the vasculature of a plant directly, for example by way of contact with a cut in the phloem, the iRNA-based vector may be stabilized with a capsid protein of another type of virus. In some examples, the iRNA-based vector is encapsidated with the coat protein of CVEV, which is believed to be a helper virus able to encapsidate CYVaV in nature. In some examples, one or more iRNA-based vector molecules are encapsidated in a self-assembling capsid protein not naturally associated with CYVaV. For example, methods of assembling capsid protein from cowpea chlorotic mottle virus with RNA molecules of various sizes are described in Cadena-Nava et al. 2012. Self-assembly of viral capsid protein and RNA molecules of different sizes: requirement for a specific high protein/RNA mass ratio. J. Virol. 86:3318-3326.
Once a first plant has been infected with an iRNA-based vector, another plant may be infected by grafting a part of the first plant to the other plant, or by injecting sap from the first plant into the other plant. Grafting in particular allows for transferring the iRNA-based vector over long distances and with long periods of time (e.g., one day or more) between cutting the graft from the first plant and adding the graft to the second plant. In some examples, an iRNA-based vector is transferred between strains or species by way of sap taken from a plant of one strain or species and injected into the vasculature of another plant of a different strain or species. In some examples, an iRNA-based vector is transferred between strains or species by way of a graft taken from a plant of one strain or species and grafted to another plant of a different strain or species.
A first plant (optionally called in some cases a mother tree) infected with an engineered iRNA-based vector can be used to produce grafts for transmitting the iRNA-based vector to other plants either as a preventative or to treat an infection already present in the other plant. The first plant can also be used to produce seedlings (for example by grafting from the first tree to seedlings of the first plant or another plant) which are used to propogate plants having the iRNA-based vector. Once in a seedling, the iRNA-based vector replicates and moves through the plant as it grows.
As noted above, CYVaV has only two ORFs: a 5′ proximal ORF that encodes replication-required protein p21; and a frame-shifting extension of p21, whereby a ribosome recoding element allows ribosomes to continue translation, extending p21 to produce p81, the RNA-dependent RNA polymerase. The organization of these two ORFs is similar to the organization of similar ORFs in viruses in the Tombusviridae and Luteoviridae. However, all viruses in these families, and indeed in all known plant RNA viruses, encode movement proteins or are associated with a secondary virus that encodes a movement protein(s). The ability to encode movement proteins, or associate with a second virus that encodes a movement protein(s), had long been considered a requirement for movement from cell-to-cell and also for transiting through the phloem to establish a systemic infection. As such, the use of iRNAs as vectors had not been proposed, and indeed iRNA molecules were previously considered unsuitable for use as an independent vector due to the lack of any encoded movement protein and belief that they were not independently mobile.
As such, the capacity for independent systemic movement of iRNAs throughout a plant's phloem despite not coding for or depending on any exogenous movement protein(s) is quite surprising. The CYVaV-based vectors of the present disclosure unambiguously and repeatedly demonstrated (via fluorescence in situ hybridization and other techniques) systemic movement without the aid of any helper virus. Young, un-infiltrated (systemic) tissue displayed highly visible symptoms on N. benthamiana, including leaf galls and root galls. The disclosed vectors utilize endogenous host movement protein(s) for mobility. In this regard, host phloem protein(s) (25 kDa phloem protein 2 (PP2) and/or 26 kDa Cucumis sativus phloem protein 2-like) known to traffic host RNAs into sieve elements (see Balachandran, S. et al. (1997), Phloem sap proteins from Cucurbita maxima and Ricinus communis have the capacity to traffic cell to cell through plasmodesmata, PNAS 94(25):14150-14155; Gómez, G. and Pallas, V. (2004), A long-distance translocatable phloem protein from cucumber forms a ribonucleoprotein complex in vivo with Hop stunt viroid RNA, J Virol 78(18):10104-10110) were likely shown to interact with CYVaV using Northwestern blots in vitro and RNA pull-downs from infected phloem sap in vivo. Thus, since known plant viruses encode (or are dependent on) a movement protein, iRNAs are quite different structurally and functionally from traditional plant viruses.
In addition to CYVaV, other RNAs of similar size and that encode a polymerase may be utilized in the develop of similarly structured iRNA-based vectors (see, e.g., Chin, L. S. et al. (1993). The beet western yellows virus ST9-associated RNA shares structural and nucleotide sequence homology with Tombusviruses. Virology 192(2):473-482; Passmore, B.K. et al. (1993). Beet western yellows virus-associated RNA: an independently replicating RNA that stimulates virus accumulation. PNAS 90(31):10168-10172). As noted above, other iRNA relatives (e.g., iRNA r1, iRNA r2, and iRNA r3, identified in Opuntia, Fig trees, and Ethiopian corn, respectively) and that encode proteins p21 and p81 (
Although CYVaV is present in the GenBank database (GenBank: JX101610), iRNAs do not belong to any known classification of virus given they lack cistrons that encode movement proteins. Nor are iRNAs dependent on a helper virus for systemic movement within a host. Moreover, iRNAs lack cistrons that encode coat proteins. iRNAs are also dissimilar to viroids, although both are capable of systemic movement in the absence of encoded movement proteins. Viroids are circular single stranded RNAs that have no coding capacity and replicate in the nucleus or chloroplast using a host DNA-dependent RNA polymerase. The vast majority of the tiny viroid genome, typically including about 300 to 400 nucleotides (nt), is needed for the viroid's unusual existence. In addition, viroids do not code for any proteins, which makes them unsuitable for use as vectors. In contrast, iRNAs code for their own RNA-dependent RNA polymerase (RdRp).
iRNAs may be categorized in two classes: a first class is characterized by a frameshift requirement to generate the RdRp and RNA structures proximal to the 3′ end that resemble those of umbraviruses. A second class is characterized by a readthrough requirement to generate the RdRp and 3′ RNA structures that resemble those of Tombusviruses. CYVaV is a member of the first class with properties similar to umbraviruses including a frameshifting recoding site and similar structures at the 3′ end, and similar sequences at the 5′ end. iRNA members of the second class have always been discovered in association with a helper virus.
iRNAs provide a number of benefits as compared to conventional viral vectors. For example, iRNAs are relatively small, making them easier to structurally and functionally map and genetically manipulate. In contrast, viruses such as CTV are 8-fold larger, making them more cumbersome to use as a vector. iRNAs can replicate and accumulate to unexpectedly high levels (e.g., visible by ethidium staining on gels and 4% of reads by RNAseq), which is critical for the vector's ability to deliver a sufficient amount of therapeutic agent(s) into the target plant. In addition, iRNAs are much more stable than many viruses despite not encoding a coat protein or silencing suppressor (
iRNAs are also limited to the host's phloem, which is especially useful for targeting pathogens that either reside in, or whose carriers feed from, or whose symptoms accumulate in, the phloem since the payload will be targeted to where it is most needed. By moving independent of movement proteins (whose interactions with specific host proteins is the primary factor for determining host range), iRNAs are able to transit within a broader range of hosts, thereby increasing the applicability of a single vector platform. Given the lack of coat protein expression and the dispensability of a helper virus for systemic plant infection, iRNAs cannot be vectored from plant-to-plant and instead must be introduced directly into the phloem via grafting. The lack of a coat protein prevents formation of infectious particles and thus unintended reversion to wild type infectious agents into the environment. This is particularly beneficial for streamlining regulatory approval as regulators are often concerned with the possible uncontrolled transmission of introduced biological agents.
iRNAs are also virtually benign in citrus, unlike viruses like CTV whose isolates can be highly pathogenic. Using a common virus as a vector, such as CTV, runs the risk of superinfection exclusion, where trees previously infected and/or exposed to that virus are not able to be additionally infected by the same virus acting as the vector (e.g., most citrus trees in the USA are infected with CTV). Thus, avoiding superinfection exclusion, at a minimum, requires additional steps to the process that makes it more expensive and cumbersome.
The present disclosure also provides for novel therapeutic, prophylactic, or trait enhancing inserts that are engineered into the iRNA vector. A variety of inserts are provided, including inserts that target a particular pathogen, an insect, or a manifestation of the disease(s). Alternatively, or in addition, inserts are provided that strengthen or improve plant health and/or enhance desired characteristics of the plant.
The disclosed infectious agents are capable of accumulation and systemic movement throughout the host plant, and can thus deliver therapies throughout a host over a substantial time period. Characteristics of the disclosed agents are therefore highly beneficial for treating numerous specific diseases. Using an infectious agent composed of either RNA or DNA has an additional advantage of being able to code for therapeutic proteins or peptides that would be expressed within infected cells and/or by engineering the infectious agent to contain a specific sequence or cleavable portion of its genetic material to serve as an RNA-based therapeutic agent.
Products with antimicrobial properties against plant pathogens can take a number of formats and are produced through ribosomal (defensins and small bacteriocins) or non-ribosomal synthesis (peptaibols, cyclopeptides and pseudopeptides). The best known are over 900 cationic antimicrobial peptides (CAPs), such as lactoferrin or defensin, which are generally less than 50 amino acids and whose antimicrobial properties are well known in the art. CAPs are non-specific agents that target cell walls generally, with reported effects against bacteria and fungi. CTV engineered with an insert designed to express defensin has received approval for release by the USDA in Florida, but its widespread efficacy is unknown. Moreover, the isolate of CTV used for the vector makes it unsuitable for trees growing in some regions (e.g., California).
RNA therapies that target viral pathogens are also in widespread development in plants. These therapies use non-coding small interfering RNAs (siRNAs), which are generated from the genome of the plant, and thus include genetic modification of the host. In addition to negative viewpoints of some growers and consumers to genetic modification of citrus trees, the length of time to generate genetically modified trees is measured in decades and may ultimately not have the same attributes (texture/color/taste) as varieties developed over decades, and thus is not a solution to current, time sensitive agricultural diseases, in addition to being very expensive to develop and potentially impacting the quality of the fruit.
siRNAs can be used to target bacteria in plants, for example the Candidus Liberibacter asiaticus (CLas) bacteria. Plant pathogenic bacteria can be targeted using siRNAs that are produced in plants, taken up by the bacteria, and directly reprogram gene expression in the bacteria as described for example by Singla-Rastogi et al. (2019) Plant small RNA species direct gene silencing in pathogenic bacteria as well as disease protection, a preprint posted to bioRxiv Dec. 3, 2019 at https://www.biorxiv.org/content/10.1101/863902v1. In some implementations, CYVaV or another iRNA based vector is provided that contains siRNA hairpins that target a bacteria such as Candidus liberibacter asiaticus and render the bacteria non-pathogenic. For example, an siRNA hairpin provided to a plant by an iRNA based vector may be taken up the CLas or another bacteria in the plant and control gene expression in the bacteria, thereby killing the bacteria and/or inhibiting an increase of the bacterial population. Compared to an enzybiotic which might have, for example, about 500 bases, an siRNA in the form of a hairpin is considerably smaller (<60 bases) and is more likely to be stable in an iRNA based vector.
Recently, highly targeted anti-bacterial enzymes have been developed for use in animals and humans as a replacement for current antibiotics. These enzymes are engineered from bacteriophage lysis proteins and are known as enzybiotics. As with the parental bacteriophage proteins, enzybiotics can lyse bacterial cell walls on contact, but are designed to be used external to both gram positive and gram negative bacteria. Enzybiotics are engineered to lyse only targeted bacterium, leaving other members of the microbiome unaffected. In some implementations, an iRNA vector is provided that includes a non-coding RNA insert that can be translated into an anti-bacterial protein like an enzybiotic.
In some implementations, an iRNA vector is provided that includes an RNA insert that interferes with the functionality of the insect vector at issue. Insects have an RNA silencing system similar to plants; small RNAs ingested by insects are taken up into cells and target critical mRNAs for degradation or blockage of translation within the insect. In some embodiments, a targeted insert is provided that is capable of silencing a critical reproductive function of the insect vector, resulting in sterilization of the insect. Of particular relevance are phloem-feeding insects that transmit phloem-limited pathogens, where a non-coding RNA insert into a phloem-limited vector is readily taken up by feeding insects.
In some implementations, an iRNA vector is provided that includes a non-coding RNA insert that targets a plant response to a pathogen. In some cases, bacteria deposited into a tree by an insect vector does not directly damage the tree. However, the host tree produces excessive callose in their phloem in order to isolate the bacteria, which can ultimately restrict the flow of photoassimilates and kill the tree. Thus, the RNA insert silences and/or depresses such callose production.
In some implementations, an iRNA vector is provided that includes a non-coding RNA insert that targets a virus, for example CTV. In some implementations, an iRNA vector is provided that includes a non-coding RNA insert that is taken up by a pathogenic bacteria or fungus making the non-coding RNA available to silence a critical function within the pathogen that can kill or reduce the virulence of that pathogen to its host.
In some implementations, an iRNA-based vector, e.g., an iRNA vector that includes a non-coding RNA insert, is grafted into rootstocks or seedlings in order to provide protection against a pathogen or in order to make that rootstock or seedling more robust. For example, planting citrus trees on sour orange root stock can be advantageous since trees grown on sour orange rootstock are, among other things, less affected by HLB than trees grown on many other rootstocks. The sour orange rootstock is also tolerant of a wide range of growing conditions. However, sour orange rootstock is also highly susceptible to CTV and many citrus growers abandoned sour orange rootstock after CTV outbreaks. Introducing an iRNA based vector adapted to target CTV into sour orange rootstock thereby produces rootstock that is tolerant to both CTV and HLB. The iRNA-based vector can be introduced into the sour orange rootstock, for example, by grafting a scion containing the iRNA based vector to the rootstock or by grafting a part of plant containing the iRNA-based vector to the rootstock or to a scion grafted to the rootstock. In some examples, seedlings are produce having sour orange rootstock, a scion of sour orange or another citrus species, and the iRNA-based vector containing a heterologous element that targets CTV. In some implementations, the heterologous element is a hairpin including a sequence complimentary to (though not necessarily exactly the same as) a sequence conserved within one or more strains of CTV.
Additional characteristics and features of the present disclosure will be further understood through reference to the following additional examples and discussion, which are provided by way of further illustration and are not intended to be limiting of the present disclosure.
CYVaV Structure. Full length structure of CYVaV was determined by SHAPE structure probing and phylogenetic comparisons with the CYVaV relatives in Opuntia, Fig and Corn (
The genome organization of CYVaV exhibits some similarities to other RNA molecules, particular PEMV2 (
CYVaV is encapsidated in virions of CVEV. CYVaV or CVEV or CYVaV+CVEV were agroinfiltrated into leaves of N. benthamiana. CYVaV was encapsidated in virions of CVEV, and virions were isolated one week later and the encapsidated RNAs subjected to PCR analysis (see
CYVaV is phloem-limited. Fluorescence in situ hybridization (FISH) imaging clearly detected plus strands of CYVaV, which was completely restricted to the sieve elements, companion cells and phloem parenchyma cells (
CYVaV does not encode a silencing suppressor. N. benthamiana 16C plants were agroinfiltrated with a construct expressing GFP (which is silenced in these plants) and either constructs expressing CYVaV p21 or p81, or constructs expressing known silencing suppressors p19 (from TBSV) or p38 (from TCV) (
Replication of CYVaV in Arabidopsis protoplasts. An infectious clone of CYVaV was generated. Wild-type RNA transcripts (CYVaV) or transcripts containing a mutation in the recoding slippery site that eliminates the synthesis of the RdRp (CYVaV-fsm), and thus does not replicate, were inoculated onto Arabidopsis protoplasts. RNA was extracted and a Northern blot performed 30 hours later. Note that inoculated transcripts of CYVaV-fsm were still present in the protoplasts at 30 hours (whereas in a traditional virus they would be undetectable after 4 hours).
Replication of CYVaV in N. benthamiana. Level of CYVaV accumulating in the infiltrated leaves of N. benthamiana was determined by Northern blot (
Symptoms of N. benthamiana systemically infected with CYVaV. Leaves 4 and 5 were agroinfiltrated with CYVaV. The first sign of a systemically infected plant is a “cupped” leaf (
CYVaV demonstrates an exceptional host range. Sap from a systemically-infected N. benthamiana plant was injected into the petiole of tomato (
CYVaV binds to a highly abundant protein extracted from the phloem of cucumber. Labelled full-length CYVaV binds to a prominent protein as demonstrated in the Northwestern blot (
Referring to
PP2 is believed to be involved with the movement or viroids but has not been reported to be involved in the coating or movement of any virus. Similarly, in the results described above, PP2 did not bind to PEMV2 in the sap of the plant. Without intending to be limited by theory, we believe that PP2 bound to CYVaV in the sap of a plant may also be responsible for the movement of CYVaV. While the early reports of CYVaV suggest that CYVaV does not move within a plant without a helper virus (CVEV) providing a movement protein, we have demonstrated that CYVaV moves systemically within a plant without a helper virus. However, a helper virus may still be required in nature for encapsidation to allow CYVaV to leave the phloem of a host plant and travel to another plant. In other experiments similar to the description above, CYVaV appears to bind to PP2 in the sap of tomato and melon plants. PP2 is found in essentially all plants and may allow iRNA-based vectors to move in, and systemically infect, a wide range of host plants.
CYVaV can express an extra protein from its 3′UTR using a TEV IRES. Location of three separate inserts of nanoluciferase downstream of the Tobacco etch virus (TEV) internal ribosome entry site (IRES) were identified (
Exemplary locations for stable hairpin inserts at positions 2250, 2301 and 2319 were evaluated. The location for each of the inserts falls within an exemplary region noted above (see
The sequences of the insertion regions (underlined below and as shown in
In one embodiment, an iRNA-based vector is provided for treating disease in the citrus industry caused by CLas bacteria (HLB). An isolate of CYVaV is utilized as a vector to target both the bacteria and the psyllid insects that deliver the bacteria into the trees. As discussed above, CYVaV is limited to the phloem where it replicates and accumulates to extremely high levels comparable to the best plant viruses. In addition, its relatively small size makes it exceptionally easy to genetically engineer. Thus, consideration of the structure and biology of CYVaV aided in the development of this novel infectious agent as a vector and model system for phloem transit.
The structure of the 3′UTR of CYVaV was determined based on SHAPE RNA structure mapping (
Certain sites have been identified for potential inserts in the 3′ UTR and the RdRp ORF that can accommodate RNA hairpins, e.g., for generation of siRNAs that target feeding insects, sites that accommodate reporter ORFs and still allow for replication of an engineered CYVaV in agro-infiltrated N. benthamiana, and sites that trigger high level translation of reporter proteins in vitro. An engineered CYVaV incorporating the added ORF and siRNAs is introduced into a storage host tree, and then pieces thereof are usable for straight-forward introduction into field trees by grafting. Given the rarity of CYVaV (to date, it has only been identified in the four limequat trees by Weathers in the 1950s), there is little risk of superinfection exclusion.
Various insert locations were identified wherein replication or translation properties of the vector were not significantly reduced or eliminated. Insert locations adversely affecting such properties (likely due to disrupting the RNA structure or other important aspect of the CYVaV vector) were not pursued further. Four exemplary insert locations on the CYVaV-based vector were identified at positions 2250, 2301, 2319 and 2331. Alternatively or additionally, inserts may be located at positions 2330 and 2336. 50 nt hairpin inserts were successfully deployed in these locations with no disruption to translation in vitro or replication in protoplasts and CYVaV was able to move systemically in N. benthamiana.
Although CYVaV has no additional ORFs, both genomic (g)RNA and a subgenomic (sg)RNA of about 500 nt are detectable using probes to plus- and minus-strands. Investigation of the region that should contain an sgRNA promoter revealed an element with significant similarity to the highly conserved sgRNA promoter of umbraviruses and to a minimal but highly functional sgRNA promoter of carmovirus TCV. In addition, similar RNAs that also only express the RdRp and are related to Tombusviruses all generate a similar sized subgenomic RNA, and may simplify expression of peptides and proteins.
In order to determine where inserts are tolerated downstream of the sgRNA promoter in CYVaV, an evaluation of where critical elements exist in the 3′ UTR of CYVaV was conducted, so that such elements are avoided when inserting heterologous sequences. As described about, the 3′ CITE for CYVaV was identified, as well as several additional 3′ proximal hairpins that are highly conserved in umbraviruses and known to be critical for replication and translation. Using deletions/point mutations, the sequence downstream of the putative sgRNA promoter and upstream of the CAS (˜120 nt) was investigated for regions that do not impact either accumulation in protoplasts or systemic movement in N. benthamiana. A similar strategy was previously utilized by the present inventors to identify regions in the 3′ UTR of TCV that can accommodate hairpins targeted by RNase III-type enzymes (Aguado, L. C. et al. (2017). RNase III nucleases from diverse kingdoms serve as antiviral effectors. Nature 547:114-117).
After identifying suitable regions for accommodating deletions/mutations (e.g., regions not involved in critical functions), heterologous sequences of different lengths were inserted therein to evaluate CYVaV functionality with an extended 3′ UTR. Such investigation aids in determining maximal insert length to ensure that such insert will be tolerated by the CYVaV-based vector while still accumulating to robust levels and engaging in systemic movement. It is believed that the CYVaV-based vector may be able to accommodate an insert having a size of up to 2 kb. In this regard, the nearest related viruses (papaya umbra-like viruses, which like CYVaV, only encode a replicase-associated protein and the RdRp) are 1 to 2 kb larger, with all of the additional sequence length expanding their 3′ UTRs (Quito-Avila, D.F. et al. (2015). Detection and partial genome sequence of a new umbra-like virus of papaya discovered in Ecuador. Eur J Plant Pathol 143:199-204). Various size sequence fragments were evaluated, beginning at 50 nt (the size of an inserted hairpin for small RNA production), up to about 600 nt (the size of an enzybiotic ORF). Initial small RNA fragments include a reporter for knock down of phytoene desaturase, which turns tissue white. The longer size fragments include nano luciferase and GFP ORFS, which may also be used as reporters for examining expression level. Inserts are made in constructs containing the wild-type (WT) sgRNA promoter and the enhanced sgRNA promoter.
Lock and Dock Sequence for stabilizing the base of inserts. Referring to
The use of a scaffold comprising a docked tetraloop as a crystallography scaffold is provided (
A lock and dock structure in accordance with disclosed embodiments is shown in
Lock and dock elements can be inserted into iRNA to stabilize the resulting vector despite the presence of hairpins or other inserts.
The term “lock and dock” is used to indicate that the structure has a highly stable locked or lockable portion and a docking portion suitable for the addition of one or more inserts. In the examples shown, the highly stable portion is provided by way of a tetraloop GNRA (e.g., GAAA) sequence and a tetraloop dock sequence (alternatively called a tetraloop lock sequence). In use, the structure folds with the tetraloop GNRA becoming associated (though not bonded in the sense of forming Watson-Crick pairs) with the tetraloop dock sequence to generate an extremely stable structure, called the “lock”. The “dock”, represented in the Figure by the fragment insert side or a portion of the lock and dock including the fragment insert site, is separated from the iRNA backbone by the lock. One or more inserts added to the dock are inhibited from interfering with folding of the iRNA backbone by the lock. Inserts (hairpins or non-hairpin sequences) may be added to the fragment insert site. In other examples, the two-way stem shown is replaced with a three-way stem to provide a lock and dock structure having a lock and two docks. The examples shown include a dividing (e.g. two-way or three-way) stem, the base and one arm of which are within a tetraloop or other locking structure, and another arm of the dividing stem having an insert site.
In one example, a 30 base non-hairpin sequence was inserted into L&D1, which was in turn inserted into position 2301 in CYVaV to make a CYVaV based vector. The CYVaV vector was agroinfiltrated into an N. benthamiana plant and achieved systemic movement in the plant. [Check for results of stability with the 30 base insert into a lock and dock]
Stabilizing the local 3′UTR structure is detrimental; however insertion of a destabilizing insert nearby restores viability. Referring to
An anti-biotic insert for delivery by the disclosed vector is provided, which comprises either an enzybiotic or small peptide engineered to destroy the CLas bacterium. Enzybiotics prefer sugar rich, room temperature environments such as found in the plant phloem. The enzybiotic is translated in companion cells during the engineered CYVaV infection cycle. Proteins produced in the cytoplasm of the phloem are naturally able to exit into the sieve element (the default pathway for translated proteins), where CLas and other plant pathogenic bacteria take up residence. In the sieve element, the enzyme molecules move with the photo-assimilate up and down the trunk and lyse any bacteria upon contact. Since enzybiotics are targeted towards a specific class of bacteria, they preferably do not disturb the microbiome of the host tree. Various agents that target CLas have been developed (e.g., Hailing Jin, University of California, Riverside, CA). Thus, numerous inserts that target CLas bacterium are known in the art and may be utilized with the CYVaV vectors of the present disclosure.
As a further embodiment, it can be beneficial to target multiple pathways for destroying the disease and the disease psyllid vector. As a result, in certain embodiments the disclosed vectors include the enzybiotic and/or peptides described above, as well as inserts that trigger the production of siRNAs that interfere with either gene expression of the tree or the disease-carrying psyllid. In the case of the ACP, the RNA could kill the vector or render it wingless and thus harmless.
iRNA-based Vector Targeting Host Gene ExpressionAn iRNA-based virus-induced gene-silencing (VIGS) vector (the acronym VIGS being used herein for convenience although the iRNA is not necessarily a virus) is provided that effectively targets host gene expression. An CYVaV-based vector was constructed that included a hairpin that targets green fluorescent protein (GFP) mRNA expressed in N. benthamiana 16C plants. The hairpin sequence (
In a normal, non-infected leaf without an gene for GFP (
Leaves expressing GFP were infected with the constructed iRNA-based VIGS vector including the GFP-suppressing hairpin at position 2301 (CYVaV-GFPhp2301). The infected leaves demonstrated effective gene silencing (
Thus, gene silencing effectively spread throughout much of the entire host plant over time (see
A vector comprising an RNA insert is provided that triggers the reduction of callose production and build-up in a host tree. A sufficiently large amount of the gene that produces callose in the phloem in response to bacteria is silenced via insertion of an siRNA sequence that is excised by the plant.
CYVaV-based vector may be utilized as a virus-induced gene-silencing (VIGS) vector to down-regulate expression of callose synthase in the phloem. VIGS has been widely used to down-regulate gene expression in mature plants to examine plant functional genomics (Senthil-Kumar et al. (2008). Virus-induced gene silencing and its application in characterizing genes involved in water-deficit-stress tolerance. J Plant Physiol 165(13):1404-1421). A complementary sequence is inserted into CYVaV at a suitable location as identified above (either anti-sense or a RNase III-cleavable hairpin). A citrus version of the gene is known (Enrique et al. (2011). Novel demonstration of RNAi in citrus reveals importance of citrus callose synthase in defense against Xanthomonas citri subsp. citri. Plant Biotech J 9:394-407).
Callose is a β 1,3-glucan that is synthesized in various tissues during development and biotic and abiotic stress (Chen, X. Y. and Kim, J. Y. (2009). Callose synthesis in higher plants. Plant Sig Behav 4(6):489-492). Deposition of callose in the sieve plates of sieve elements inhibits photoassimilate flow in the phloem, leading to over accumulation of starch in source (young) leaves, which contributes to the death of trees during bacterial infections such as HLB. All plants contain 12-14 callose synthase genes; one member of this gene family, CalS7 (Arabidopsis nomenclature), is mostly responsible for rapid callose deposition in sieve pores of the phloem in response to wounding and various pathogens (Xie et al. (2011). CalS7 encodes a callose synthase responsible for callose deposition in the phloem. Plant J 65(1):1-14). Complete inhibition of GSL7 impacted both normal phloem transport and inflorescence development in Arabidopsis (Barratt et al. (2011). Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis. Plant Physiol 155(1):328-341). A CYVaV-based vector is utilized to down-regulate the N. benthamiana and orange tree orthologues of CalS7 in mature plants in order to investigate the consequences of reduced (but not eliminated) sieve plate callose deposition. Alternatively, or in addition, the vector provides for an insert that expresses a callose-degrading enzyme.
iRNA-Based Vector Targeting CTVAn iRNA-based VIGS vector was constructed that targets CTV. As demonstrated by the data, disclosed constructs may be utilized for immunization as well as reduction of virus levels in host plants with mature infections. N. benthamiana infected with CTV-GFP (CTV expressing GFP) was used as root stock grafted to wild-type CYVaV (CYVaVwt) and CYVaV-GFPhp2301 scions (
When WT CYVaV was present in the root stock, new leaves from the CTV-GFP scion still fluoresced green under UV light, thus showing that widespread CTV infection was continuing unabated (
As noted above, CTV is composed of two capsid proteins and with a genome of more than 19 kb. 76 CTV isolates have been characterized, which all contain regions of conserved nucleotides. Two sequence portions (18 and 6) of a CTV isolate are identified in Table 1 below, showing fully conserved polynucleotides (underlined below) as well as less-conserved nucleotides (in bold) with other nucleotides present in some isolates (listed as identified and bolded nucleotides in each sequence from left to right). For example, in the sequence portion for CTV18 shown in Table 1, the 3 non-conserved nucleotides include, from left to right: guanine (G) which position instead includes adenine (A) in 10 CTV isolates; cytosine (C) which position instead includes uracil (U) in about half of the CTV isolates; and G which position instead includes A in 6 CTV isolates. In the sequence portion for CTV6, the 6 non-conserved nucleotides include, from left to right: G which position instead includes A in 1 CTV isolate; G which position instead includes A in 3 CTV isolates; U which position instead includes C in 3 CTV isolates; A which position instead includes G in 9 CTV isolates; U which position instead includes C in 1 CTV isolate; and A which position instead includes G in 1 CTV isolate.
Fully CTV-infected N benthamiana were agroinfiltrated with CYVaV-based vector carrying a hairpin at position 2301 that targeted a conserved sequence in the CTV genome (
Leaves co-infiltrated with CTV-GFP and CYVaV wild-type or CYVaV-CTV6 containing another CTV genome-targeting hairpin (
In some embodiments, an insert is provided that targets one or more virus and/or fungal or bacterial pathogens. In some embodiments, a hairpin insert is provided that generates an siRNA that directly targets CVEV, since CVEV is known to slightly intensify the yellowing impacts of CYVaV and to enable transport of CYVaV between trees. In some embodiments, a hairpin insert is provided that targets CTV, since CTV is a highly destructive viral pathogen of citrus (second only to CLas). In other embodiments, an insert is provided that targets another citrus (or other) virus. In some embodiments, an insert is provided that targets a fungal pathogen(s), given that such pathogen(s) are able to take up siRNAs from the phloem. In some embodiments, an insert is provided that targets a bacterial pathogen, given that such pathogen(s) are able to take up siRNAs from the phloem.
In some embodiments, the CYVaV-based (or other iRNA) vector includes an insert(s) engineered to modify a phenotypic property of a plant that emanates from gene expression in companion cells. In one implantation, an insert is provided that triggers dwarfism, so that the fruit is easier to harvest and growth space requirements are reduced. Additional and/or other traits may also be targeted as desired. The iRNA vectors of the present disclosure comprising 1, 2, 3 or more inserts demonstrate stability and functionality.
International Application No. PCT/US2019/060945, Plant Vectors, Compositions and Uses Related Thereto, filed on Nov. 12, 2019, is incorporated herein in its entirety including sequence listings filed with it (file name: 2105 0071PCT ST25, created on Nov. 12, 2019, and having a size of 37,755 bytes) by this reference to it.
All identified publications and references mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference in its entirety. While the invention has been described in connection with exemplary embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the features hereinbefore set forth.
Claims
1. A plus-sense single stranded ribonucleic acid (RNA) vector comprising a heterologous segment(s), wherein said heterologous element(s) is attached to the RNA vector through a lock and dock structure.
2. The RNA vector of claim 1, which comprises a 3′ Cap Independent Translation Enhancer (3′ CITE) comprising the nucleic acid sequence(s) of SEQ ID NO: 4 and/or SEQ ID NO: 5.
3. The RNA vector of claim 2, wherein said 3′ CITE comprises the nucleic acid sequence of SEQ ID NO: 3.
4. The RNA vector of claim 1, having a replication element(s) comprising one or more conserved polynucleotide sequence(s) having the nucleic acid sequence of: SEQ ID NO: 10, SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, and/or SEQ ID NO: 14.
5. The RNA vector of claim 4, wherein said replication element(s) further comprises one of more conserved polynucleotide sequence(s) having the nucleic acid sequence of: SEQ ID NO: 15 and/or SEQ ID NO: 16.
6. The RNA vector of claim 1, which is derived from citrus yellow vein virus (SEQ ID NO:1) or an iRNA relative thereof.
7. The RNA vector of claim 1, which is capable of systemic and phloem-limited movement and replication within a host plant.
8. The RNA vector of claim 1, wherein the lock and dock structure comprises a branched element, wherein a stem and a branch of the branched element are located within a stable structure such as a tetraloop-tetraloop dock, and another branch of the branched element comprises an insert site for the heterologous element.
9. An iRNA-based vector having a heterologous segment(s) comprises an siRNA effective against a plant pathogenic bacteria.
10. The iRNA-based vector of claim 9, wherein said siRNA targets a Candidatus Liberibacter species.
11. The iRNA-based vector of claim 10, wherein said Candidatus Liberibacter species is Candidatus Liberibacter asiaticus (CLas).
12. An iRNA-based vector comprising a heterologous element comprising a hairpin having a sequence on one side complementary to a sequence within Citrus tristeza virus (CTV).
13. The iRNA-based vector of claim 12, wherein the sequence within CTV is conserved in multiple CTV strains.
14. A plant comprising a sour orange rootstock and an iRNA-based vector comprising a heterologous element that targets a nucleic acid of Citrus tristeza virus (CTV).
15. The plant of claim 14, wherein the heterologous element comprises a hairpin having a sequence on one side complementary to a sequence within Citrus tristeza virus (CTV), optionally a sequence conserved in multiple CTV strains.
16. A method for introducing a heterologous segment(s) into a host plant comprising introducing into said host plant an iRNA-based vector after a) encapsidating the iRNA vector in a capsid protein other than the capsid protein of CVEV, or b) by agroinfiltration after inoculating an agroinfiltration site with Xanthomonas citri subsp. citri (Xcc).
17. An iRNA-based vector comprising one or more inserts at one or more of positions 2250, 2301, 2319, 2330, 2331, 2336 and 2083 of a CYVaV based RNA.
18. The iRNA-based vector of claim 17 comprising one or more inserts at one or more of positions 2250, 2301, 2319, 2330 and 2336 of a CYVaV based RNA.
19. The iRNA-based vector of claim 17, wherein the CYVaV based RNA comprises stabilizing changes.
20. The iRNA-based vector of claim 19, wherein the CYVaV based RNA comprises changes converting G:U pairs to G:C pairs in the 3′UTR structure.
21. A method of making a plus-sense single stranded ribonucleic acid (RNA) vector comprising stabilizing the 3′ UTR structure of a parental construct and inserting one or more destabilizing heterologous segment(s) into the stabilized parental construct.
Type: Application
Filed: May 11, 2021
Publication Date: Nov 18, 2021
Applicant: University of Maryland, College Park (College Park, MD)
Inventor: Anne Elizabeth Simon (Bowie, MD)
Application Number: 17/317,596
