De novo methylation and co‐suppression induced by a cytoplasmically replicating plant RNA virus

The relationship between co‐suppression and DNA methylation was explored in transgenic plants showing inducible co‐suppression following infection with a cytoplasmically replicating RNA virus. Induction resulted in a loss of transgene mRNA and resistance to further infection, factors typical of post‐transcriptional gene silencing. In infected plants, de novo methylation of the transgene appeared to precede the onset of resistance and only occurred in plants where the outcome was co‐suppression. The methylation was limited to sequences homologous to the viral RNA and occurred at both symmetric and non‐symmetric sites on the DNA. Although methylation is predicted to occur in mitotic cells, the virus was found not to access the meristem. A diffusible sequence‐specific signal may account for the epigenetic changes in those tissues.