Mitotic stability of infection-induced resistance to plum pox potyvirus associated with transgene silencing and DNA methylation

Plum pox potyvirus (PPV) infection of transgenic Nicotiana benthamiana plan ts that expressed the PPV NIb RNA replicase carrying a Gly to Val mutation at the GDD motif (NIbV lines) induced a phenotype of virus resistance and t ransgene silencing, which was not transmissible to the progeny after self:f ertilization (H. S. Guo and J. A. Garcia, Mol. Plant-Microbe Interact. 10:1 60-170, 1997), Here, we demonstrate that the induced resistance of NIbV pla nts is mitotically stable after plant propagation by grafting and by in vit ro regeneration. Virus replication or residual virus RNA seem not to be req uired to maintain transgene silencing and virus resistance. Analysis by PCR (polymerase chain reaction) amplification after treatment with methylation -sensitive restriction nucleases indicates that DNA methylation is associat ed with establishment and maintenance of transgene silencing and virus resi stance. Restoration of transgene activity and susceptibility to PPV in sexu al progeny correlated with resetting of transgene DNA methylation. On the b asis of these and other published results, we present a general model for p ost-transcriptional gene silencing in which RNA signals, generated either b y a silenced nuclear gene or by virus replication, both activate a specific cytoplasmic RNA degradation pathway and induce changes (in particular, DNA methylation) in homologous nuclear genes that switch them from an active t o a silenced status.