HC-Pro suppression of transgene silencing eliminates the small RNAs but not transgene methylation or the mobile signal

Post-transcriptional gene silencing (PTGS) is a sequence-specific RNA degra dation mechanism that is widespread in eukaryotic organisms. It is often as sociated with methylation of the transcribed region of the silenced gene an d with accumulation of small RNAs (21 to 25 nucleotides) homologous to the silenced gene. In plants, PTGS can be triggered locally and then spread thr oughout the organism via a mobile signal that can cross a graft junction. P reviously, we showed that the helper component-proteinase (HC-Pro) of plant potyviruses suppresses PTGS. Here, we report that plants in which PTGS has been suppressed by HC-Pro fail to accumulate the small RNAs associated wit h silencing. However, the transgene locus of these plants remains methylate d. Grafting experiments indicate that HC-Pro prevents the plant from respon ding to the mobile silencing signal but does not eliminate its ability to p roduce or send the signal. These results demonstrate that HC-Pro functions downstream of transgene methylation and the mobile signal at a step precedi ng accumulation of the small RNAs.