RNAi-mediated viral immunity requires amplification of virus-derived siRNAs in Arabidopsis thaliana

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 107; no. 1; pp. 484 - 489
• Main Authors: Wang, Xian-Bing, Wu, Qingfa, Ito, Takao, Cillo, Fabrizio, Li, Wan-Xiang, Chen, Xuemei, Yu, Jia-Lin, Ding, Shou-Wei
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 05-01-2010; National Acad Sciences
• Subjects: Animals; Antivirals; Arabidopsis - genetics; Arabidopsis - immunology; Arabidopsis - virology; Arabidopsis thaliana; Biological Sciences; Cucumber mosaic virus; Cucumovirus - genetics; Cucumovirus - metabolism; disease resistance; Double stranded RNA; Eukaryotes; Flowers & plants; gene amplification; Gene Expression Profiling; Gene Expression Regulation, Viral; Gene Silencing; genome; Genomics; Immunity; Immunity - genetics; Infections; Isoenzymes - genetics; Isoenzymes - metabolism; Mutation; Plant Diseases - genetics; Plant Diseases - virology; Plant Proteins - genetics; Plant Proteins - metabolism; Ribonuclease III - genetics; Ribonuclease III - metabolism; Ribonucleic acid; RNA; RNA Interference; RNA Replicase - genetics; RNA Replicase - metabolism; RNA, Small Interfering - genetics; RNA, Small Interfering - metabolism; RNA, Viral - genetics; RNA, Viral - metabolism; RNA-directed RNA polymerase; Small interfering RNA; Untranslated regions; Virus Diseases - genetics; Virus Diseases - virology; Virus Replication - genetics; Viruses
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0904086107

In diverse eukaryotic organisms, Dicer-processed, virus-derived small interfering RNAs direct antiviral immunity by RNA silencing or RNA interference. Here we show that in addition to core dicing and slicing components of RNAi, the RNAi-mediated viral immunity in Arabidopsis thaliana requires host RNA-directed RNA polymerase (RDR) 1 or RDR6 to produce viral secondary siRNAs following viral RNA replication-triggered biogenesis of primary siRNAs. We found that the two antiviral RDRs exhibited specificity in targeting the tripartite positive-strand RNA genome of cucumber mosaic virus (CMV). RDR1 preferentially amplified the 5'-terminal siRNAs of each of the three viral genomic RNAs, whereas an increased production of siRNAs targeting the 3' half of RNA3 detected in rdr1 mutant plants appeared to be RDR6-dependent. However, siRNAs derived from a single-stranded 336-nucleotide satellite RNA of CMV were not amplified by either antiviral RDR, suggesting avoidance of the potent RDR-dependent silencing as a strategy for the molecular parasite of CMV to achieve preferential replication. Our work thus identifies a distinct mechanism for the amplification of immunity effectors, which together with the requirement for the biogenesis of endogenous siRNAs, may play a role in the emergence and expansion of eukaryotic RDRs.