Arabidopsis SGS2 and SGS3 genes are required for posttranscriptional gene silencing and natural virus resistance

Posttranscriptional gene silencing (PTGS) in plants results from the degrad ation of mRNAs and shows phenomenological similarities with quelling in fun gi and RNAi in animals. Here, we report the isolation of sgs2 and sgs3 Arab idopsis mutants impaired in PTGS. We establish a mechanistic link between P TGS, quelling, and RNAi since the Arabidopsis SGS2 protein is similar to an RNA-dependent RNA polymerase like N. crassa QDE-1, controlling quelling, a nd C. elegans EGO-1, controlling RNAi. In contrast, SGS3 shows no significa nt similarity with any known or putative protein, thus defining a specific step of PTGS in plants. Both sgs2 and sgs3 mutants show enhanced susceptibi lity to virus, definitively proving that PTGS is an antiviral defense mecha nism that can also target transgene RNA for degradation.