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

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Abstract 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. Copyright © 2010 John Wiley & Sons, Ltd. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > RNAi: Mechanisms of Action RNA in Disease and Development > RNA in Disease

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Steps in antiviral RNAi pathways known to be targeted by VSRs. (a) For single‐stranded (ss) RNA viruses, partially double‐stranded hpRNA is frequently detected, which often plays important regulative roles in genome replication, transcription, translation, or assembly. Fully complementary dsRNA can be formed through genome replication or host RDR activity. Both hpRNA and dsRNA can serve as substrates of DCL(s) (green crescent) to produce viral siRNAs (bottom of A). VSRs ('prohibited' sign) disrupt the DCL‐processing step by either binding to unprocessed dsRNA or hairpin RNA (TCV P38), or interact with DCLs/DRBs (light blue ball) (CaMV P6). In the next step, siRNAs are directed to homologous ssRNA, programming the cleavage or translational repression of these RNA through AGOs (purple wedged ball). VSRs are known to thwart siRNA function by either binding to siRNAs (TBSV P19), or AGOs (CMV 2b). (b) DNA viruses as exemplified by geminiviruses produce dsRNA through transcription of inversely overlapping genes, or through the action of host RDRs. Viral siRNAs produced can target viral RNA for degradation or viral DNA for methylation (brown flag). Hence, VSRs encoded by these viruses were shown to suppress both RNA degradation and DNA methylation (AC2 of begomoviruses).

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Regulatory RNAs/RNAi/Riboswitches > RNAi: Mechanisms of Action
RNA in Disease and Development > RNA in Disease

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