Plant viruses versus RNAi: simple pathogens reveal complex insights on plant antimicrobial defense.

RNA interference (RNAi) and related processes serve as a nucleic-acid-mediated surveillance system conserved in almost all eukaryotic organisms. This surveillance system detects various forms of double-stranded RNA (dsRNA) in cells and initiates a cascade of events that degrades dsRNAs into small interfering RNAs (siRNAs) or microRNAs (miRNAs). These small RNAs in turn serve as sequence-specific guides to interfere with the function of other nucleic acids through degradation or translational repression of homologous RNAs, or modification of homologous genome segments. One of the major roles of RNAi in plants and invertebrates is antiviral defense. Conversely, viruses have also evolved to encode suppressors of RNAi (VSRs), which disrupt RNAi at various steps. Research activities focusing on the relationship between plant viruses and RNAi have been essential to our current understanding of RNAi mechanisms.