Similar analyses of Arabidopsis single or combinational ago mutants with different wild-type and/or VSR-defective viruses confirmed AGO1 and AGO2 as two major antiviral AGOs against RNA viruses in Arabidopsis. Later, the suspicion that functional VSRs could mask the effects of antiviral silencing functions prompted the use of suppressor-defective viruses to analyze ago activity, and AGO1 was identified as the major antiviral AGO against suppressor-defective Turnip crinkle virus (TCV) in Arabidopsis. Early genetic analyses describing Arabidopsis ago1 hypersusceptibility to Cucumber mosaic virus (CMV) revealed the first antiviral role for a plant AGO. The Arabidopsis thaliana (Arabidopsis) genome encodes 10 AGO genes. Multiple AGOs function in plant antiviral silencing We also discuss how viruses affect AGOs to regulate host gene expression and promote viral infection, and how they suppress AGO antiviral functions in this molecular arms race between plants and viruses. In this article, we review recent progress towards understanding which AGOs have antiviral roles in plants, and how they operate through association with vsiRNAs or through other mechanisms. VSRs can interfere with virtually any step of the antiviral RNA silencing pathway, and attenuate or completely suppress the defense response. As a counter-defense strategy, most if not all plant viruses have evolved specialized proteins known as viral suppressors of RNA silencing (VSRs). Antiviral AGOs associate with vsiRNAs and i) target complementary viral RNA for degradation through endonucleolytic cleavage (slicing) and/or translational arrest, ii) transcriptionally repress complementary viral DNA through hypermethylation, or iii) regulate host gene expression to promote defense. Endogenous RNA-dependent RNA polymerases (RDRs) use single-stranded (ss) viral RNAs to synthesize dsRNA serving as substrate for DCLs to produce secondary vsiRNAs and amplify the antiviral response. ![]() Antiviral RNA silencing is triggered by highly-structured or double-stranded (ds) viral RNAs that are recognized and processed by host Dicer-Like (DCL) proteins into primary 21 to 24 nt virus-derived small interfering RNAs (vsiRNAs). RNA silencing functions as a primary antiviral immune system in plants. Small RNA-programmed AGOs target and silence complementary RNA or DNA through posttranscriptional gene silencing (PTGS) or transcriptional gene silencing (TGS), respectively. Plant AGOs associate with small RNA based on the identity of the 5′ terminal nucleotide of the small RNA and/or other sequence and structural properties of the small RNA duplex. ARGONAUTES (AGOs) are the effector proteins functioning in eukaryotic RNA silencing pathways.
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