Abstract: The invention relates to two plant transgene expression systems. The first is comprised of two chromosomally-integrated components that are individually heritable. One component is an inactive replicon, which contains cis-acting viral sequences required for replication and is unable to replicate episomally. The other component is a chimeric transactivating gene comprising a regulated promoter operably-linked to the coding region for a protein that can transactivate replicon replication. Regulated expression of the transactivation protein in plant cells containing the inactive replicon triggers release of free replicon from the integrated inactive replicon and allows episomal replication.