RNA-dependent RNA polymerase-mediated antiviral defense and gene silencing

The purpose of the present study was to investigate the molecular mechanisms of RNA-dependent RNA polymerase (RDR)-mediated antiviral defense and gene silencing. Arabidopsis plants expressing a tobacco NtRDR1 transgene showed enhanced resistance to RNA viruses. Early induction of NtRDR1 transgene expression before virus challenge inhibited viral transcript accumulation both locally and systemically. 3' truncated transcripts from the NtRDR1 transgene and a transgene with three direct repeats of GUS reporter gene were detected in rdr6 mutant plants but not in a wild-type genetic background, indicating an RNA degradation mechanism mediated by RDR6. A correlation between aberrant RNA production and RDR6-dependent RNA silencing was uncovered by directly comparing the GUS accumulation levels of various constructs. Aberrant RNA probably serves as a direct target of nuclear-localized RDR6 for dsRNA generation. In addition, long Arabidopsis cDNAs were efficiently suppressed in Arabidopsis plants in an RDR6-independent manner. Methylation inhibitor treatment enhanced RNA accumulation from the transgenes. No significant differences were observed in methylation status of the promoter that was used to drive expression of Arabidopsis cDNAs of various sizes. It is possible that methylation and other epigenetic modifications affect transcription elongation and therefore, suppress the expression of large transgenes.