RDR 6-dependent production of small interfering RNAs from endogenous mRNAs

Cytoplasmic degradation of endogenous RNAs is an integral part of RNA quality control (RQC) and often relies on the removal of the 5' cap structure and their subsequent 5’ to 3’ degradation in cytoplasmic processing (P-)bodies. In parallel, many eukaryotes degrade exogenous and selected endogenous RNAs through post-transcriptional gene silencing (PTGS). In plants, PTGS depends on small interfering (si)RNAs produced after the conversion of single-stranded RNAs to doublestranded RNAs by the cellular RNA DEPENDENT RNA POLYMERASE 6 (RDR6) in cytoplasmic siRNA-bodies. PTGS and RQC compete for transgene-derived RNAs, but it is unknown whether this competition also occurs for endogenous transcripts. Here, we show that the lethality of decapping mutants is suppressed by impairing RDR6 activity. We also show that upon decapping impairment hundreds of endogenous mRNAs give rise to a new class of siRNAs, a subset of which depends on RDR6 for their production. We observe that, in vivo, Pand siRNA-bodies often are juxtaposed and remain associated while moving in an actin-dependent manner, potentially allowing for cross-talk of the two machineries. Our results suggest that the decapping of aberrant endogenous RNA in P-bodies limits their entry into the PTGS pathway and prevents the subsequent deleterious consequences arising from this entry. We anticipate that the siRNAs identified in decapping mutants represent a subset of a larger ensemble of endogenous siRNAs. We call them rqc-siRNAs due to their over-accumulation when RQC processes are impaired. Running title: Decapping in P-bodies prevents rqc-siRNA biogenesis