Abstract: The invention presents DNA constructs comprising a promoter operably linked to DNA which can be transcribed in a plant cell to an RNA transcript, wherein the RNA transcript comprises plant virus sequence from an RNA virus which confers on the RNA transcript the ability to replicate in the cytoplasm of the plant cell, wherein the transcript lacks all or part of the viral genome not required for replication in the cytoplasm, and further comprises at least one targeting sequence which is foreign to the plant virus sequence and causes post-transcriptional gene silencing of one or more target genes. The invention also presents methods to use the DNA construct to cause post-transcriptional gene silencing of a target gene in a plant.