Résumé
Short-interfering RNAs (siRNAs) and microRNAs (miRNAs) play an important role in regulation of host gene expression and defense against invasive nucleic acids such as transposons, transgenes, and viruses. In plants, siRNA-directed silencing is a major defense mechanism that restricts replication and spread of RNA and DNA viruses as well as viroids and viral satellites. During viral infection, host Dicer-like (DCL) enzymes catalyze production of 21-, 22-, and 24-nucleotide viral siRNA duplexes from longer double-stranded RNA (dsRNA) precursors generated by viral and/or host RNA polymerases. These duplexes are sorted by several distinct Argonaute (AGO) proteins to form RNA-induced silencing complexes (RISCs) that can potentially target cognate viral RNA for posttranscriptional gene silencing (PTGS) and, in the case of DNA viruses, also viral DNA for cytosine methylation and transcriptional gene silencing (TGS). To establish successful infection, plant viruses have evolved various mechanisms of silencing evasion as well as silencing suppression through effector proteins that interfere with the biogenesis and/or action of viral siRNAs. Furthermore, viruses appear to manipulate host siRNAs and miRNAs which may contribute to antiviral defense indirectly, through regulation of the host genes mediating silencing and innate immunity.