RNA-Targeted Antiviral Immunity: More Than Just RNA Silencing

RNA silencing, RNA decay, and RNA quality control are three major RNA degradation pathways that target redundant/damaged/detrimental cellular and exogenous RNAs in plants. Virus infection may activate RNA degradation pathways. RNA silencing is the most important line of defense that restricts virus infection. Viruses have evolved diverse mechanisms to suppress RNA silencing. The best effective mechanism is to express VSRs. In response, plants may activate a countersuppression mechanism. RNA decay may function as an antiviral defense, possibly through concerted actions of RNA silencing. Viruses can repress RNA decay to promote viral proliferation. RNA quality control may recognize specific features of certain plant RNA viruses to restrict virus infection. Many viruses have also evolved strategies to escape or avoid this degradation pathway. RNA silencing is a fundamental, evolutionarily conserved mechanism that regulates gene expression in eukaryotes. It also functions as a primary immune defense in microbes, such as viruses in plants. In addition to RNA silencing, RNA decay and RNA quality-control pathways are also two ancestral forms of intrinsic antiviral immunity, and the three RNA-targeted pathways may operate cooperatively for their antiviral function. Plant viruses encode viral suppressors of RNA silencing (VSRs) to suppress RNA silencing and facilitate virus infection. In response, plants may activate a counter-counter-defense mechanism to cope with VSR-mediated RNA silencing suppression. In this review, we summarize current knowledge of RNA silencing, RNA decay, and RNA quality control in antiviral defense, and highlight the mechanisms by which viruses compromise RNA-targeted immunity for their infection and survival in plants. RNA silencing is a fundamental, evolutionarily conserved mechanism that regulates gene expression in eukaryotes. It also functions as a primary immune defense in microbes, such as viruses in plants. In addition to RNA silencing, RNA decay and RNA quality-control pathways are also two ancestral forms of intrinsic antiviral immunity, and the three RNA-targeted pathways may operate cooperatively for their antiviral function. Plant viruses encode viral suppressors of RNA silencing (VSRs) to suppress RNA silencing and facilitate virus infection. In response, plants may activate a counter-counter-defense mechanism to cope with VSR-mediated RNA silencing suppression. In this review, we summarize current knowledge of RNA silencing, RNA decay, and RNA quality control in antiviral defense, and highlight the mechanisms by which viruses compromise RNA-targeted immunity for their infection and survival in plants. virus infection induces the generation of three types of dsRNA, including replicative intermediates, intramolecular pairing of genomic RNA, and de novo products catalyzed by cellular RNA-dependent RNA polymerases (RdRs). viruses of the family Geminiviridae; they have a small, single-stranded DNA (ssDNA) genome and they replicate in the nucleus of host plant cells by double-stranded DNA (dsDNA) intermediates. NGD targets mRNAs with elongation-inhibiting features such as secondary structures or modified nucleotides and then mediates their degradation. NMD recognizes premature termination codons containing mRNAs in order to mediate their exonucleatic degradation. recognizes mRNAs lacking an in-frame stop codon to mediate their exonucleatic degradation. is a common RNA silencing pathway triggered by dsRNA to downregulate a gene at the RNA level. As a result, RNA translation is repressed and RNA is degraded. the biosynthesis of siRNAs is dependent on endogenous RDRs to amplify dsRNA. siRNAs are short dsRNAs of 20–24 nt in length with 2 nt 3′ overhangs that have 5′-monophosphate and 3′-hydroxyl termini. the transcription of genomic DNA is repressed or inactive due to DNA methylation or any modifications leading to the structural reorganization of chromatin (also known as chromatin remodeling). are siRNAs that are activated by viruses. siRNAs derived from viral RNA. are also referred to as to RNA silencing suppressor (RSS). VSR or RSS suppresses the RNA silencing pathway.