DE-NOVO METHYLATION AND CO-SUPPRESSION INDUCED BY A CYTOPLASMICALLY REPLICATING PLANT RNA VIRUS

The relationship between co-suppression and DNA methylation was explor ed in transgenic plants showing inducible co-suppression following inf ection with a cytoplasmically replicating RNA virus. Induction resulte d in a loss of transgene mRNA and resistance to further infection, fac tors typical of post-transcriptional gene silencing. In infected plant s, de novo methylation of the transgene appeared to precede the onset of resistance and only occurred in plants where the outcome was co-sup pression. The methylation was limited to sequences homologous to the v iral 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-spe cific signal may account for the epigenetic changes in those tissues.