Plant Virus Silencing Suppressors and RNA Silencing in Plants

Abstract

Plants use ribonucleic acid (RNA) of virus origins to produce small interfering RNAs (siRNAs) that guide sequence‐specific destruction of viral RNAs through a process termed RNA silencing. In addition to siRNAs, antiviral RNA silencing requires the participation of at least four families of plant proteins, namely the dicer‐like (DCL) nucleases that process virus‐specific double‐stranded (ds) RNA to siRNAs, Argonaute proteins (AGO) that use siRNAs as the sequence‐specific guide to cleave (slice) complementary single‐stranded viral RNA, as well as dsRNA‐binding proteins (DRBs) that assist DCL processing and/or DCL‐AGO communication, and RNA‐dependent RNA polymerases (RDRs) that produce a subset of dsRNA substrates of DCLs by converting viral ssRNAs into dsRNAs. To survive the RNA silencing‐based plant defence, plant viruses have evolved to encode strong suppressors of RNA silencing (VSR). Known targets of VSRs include all major steps/components of the RNA silencing pathway(s).

Key Concepts:

  • RNA silencing is an RNA‐mediated, sequence‐specific surveillance system in plants and other eukaryotes.

  • The sequence specificity of RNA silencing is conferred by siRNAs generated through the processing of dsRNAs originated from viruses and other molecular parasites.

  • RNA silencing is analogous to RNA interference (RNAi) in animals. It is a highly conserved mechanism of genome defence in almost all eukaryotic organisms.

  • Evolutionary successful viruses encode VSRs to engage diverse components of the RNA silencing pathway and subvert this robust defence mechanism.

Keywords: RNA silencing; viral suppressor of RNA silencing (VSR); plant; Arabidopsis; plant virus

Figure 1.

The four best characterised endogenous RNA silencing pathways in Arabidopsis plants. The miRNA pathway depicted on the top is initiated with the processing of primary miRNA (pri‐miRNA) in two sequential steps to generate mature miRNA. This process is catalysed by DCL1 with the help of HYL1, a DRB family protein. The miRNAs are then recruited by AGO1 to guide the cleavage of cognate RNAs. The tasiRNA pathway (middle, right) initiates with a TAS transcript that is cleaved at one or both ends by miRNA‐guided slicing (shown as dotted arrow). This transcript then serves as template for RDR6 to produce dsRNA, which in turn is processed into mature tasiRNA by DCL4 with the help of DRB4. tasiRNAs can be recruited by AGO1 or AGO7 to direct target cleavage. The cis‐siRNA pathway (bottom) regulates the gene expression through the modification of the homologous chromosomal DNA. In this pathway, NRPD, a DNA‐dependent RNA polymerase unique to plants, is likely responsible for making the initial transcript from the repetitive DNA to be modified. This NRPD transcript is then copied by RDR2 into dsRNA, the later being processed by DCL3 into 24 nt cis‐siRNAs. The nat‐siRNA pathway (middle, left) is initiated only when both of the genes that are positioned tail‐to‐tail are actively transcribed, creating dsRNA at their 3′ ends. After the initial processing by DCL2, the subsequent steps are similar to that of tasiRNA pathway.

Figure 2.

VSRs from diverse viruses target different steps of the RNA silencing pathway. See main text for details.

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Zhang, Xiuchun, and Qu, Feng(Aug 2014) Plant Virus Silencing Suppressors and RNA Silencing in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020710.pub2]