RNA interference (RNAi) is a post-transcriptional generegulatory mechanism that operates in many eukaryotes. RNAi is induced by double-stranded RNA (dsRNA) and is mainly involved in defence against transposons and viruses. To counteract RNAi, viruses have RNAi suppressors. Here we show a novel mechanism of RNAi suppression by a plant virus Red clover necrotic mosaic virus (RCNMV). To suppress RNAi, RCNMV needs multiple viral components, which include viral RNAs and putative RNA replicase proteins. A close relationship between the RNA elements required for negative-strand RNA synthesis and RNAi suppression suggests a strong link between the viral RNA replication machinery and the RNAi machinery. In a transient assay, RCNMV interferes with the accumulation of small-interfering RNA (siRNAs) in RNAi induced by a hairpin dsRNA and it also interferes with microRNA (miRNA) biogenesis. An Arabidopsis dcl1 mutant showed reduced susceptibility to RCNMV infection. Based on these results, we propose a model in which, to replicate, RCNMV deprives the RNAi machinery of Dicer-like enzymes that are involved in both siRNA and miRNA biogenesis.

The genome of Red clover necrotic mosaic virus (RCNMV) consists of RNA1 and RNA2. RNA1 encodes N-terminally overlapping replication proteins, p27 and p88, which are translated in a cap-independent manner. The 3' untranslated region of RNA1 contains RNA elements essential for cap-independent translation and negative-strand RNA synthesis. In this study, we investigated how p27 and p88 were engaged in the replication of RCNMV genomic RNAs by using DNA vectors or in vitro transcribed RNAs in protoplasts and in a cell-free extract of evacuolated BY-2 protoplasts. Our results show a cis-preferential requirement of p88, but not of p27, for the replication of RNA1. This mechanism might help to facilitate a switch in the role of RNA1 from mRNA to a replication template.