Short Defective Interfering RNAs of Tombusviruses Are Not Targeted but Trigger Post-Transcriptional Gene Silencing against Their Helper Virus

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Abstract
Post-transcriptional gene silencing (PTGS) is a sequence-specific degradation mechanism that operates in almost all eukaryotic cells. In plants, double-stranded RNA triggers PTGS, generating 21- to 25-nucleotide guide RNAs responsible for specific degradation of cognate mRNA. The double stranded RNA intermediates of replicating plant viruses often induce PTGS, leading to symptom attenuation. Here we demonstrate the role of PTGS in defective interfering (DI) RNA–mediated symptom attenuation in plants infected with Cymbidium ringspot tombusvirus (CymRSV). Analysis of 21- to 25-nucleotide RNAs in Nicotiana benthamiana infected with CymRSV indicated that PTGS was not spread homogeneously along the viral genome. The 21- to 25-nucleotide RNAs derived mainly from plus-stranded RNA and likely arose from local basepaired structures. In contrast to helper viral RNA, short DI RNAs were not accessible to helper virus–induced RNA degradation guided by the 21- to 25-nucleotide RNAs. Our results suggest a model in which PTGS plays an important role in the selective accumulation and symptom attenuation mediated by DI RNAs. Because PTGS operates in a wide variety of different organisms, this model is applicable to DI RNA generation and accumulation in both plant and animal cells.