Structural insights into the arms race between host and virus along RNA silencing pathways in Arabidopsis thaliana

RNA silencing refers to a conserved sequence-specific gene-regulation mechanism mediated by small RNA molecules. In plants, microRNA (miRNA) and small interfering RNA (siRNA) represent two major types of small RNA molecules which play pivotal roles in plant developmental control and antiviral defences. To escape these plant defences, plant viruses have encoded a vast array of viral suppressors of RNA silencing (VSRs) to attack the host antiviral silencing pathway by interfering with small RNA processing, RNA-induced silencing complex (RISC) assembly, viral mRNA cleavage etc. Transgenic plants expressing distinct VSRs often show developmental aberrations that resemble the phenotype of miRNA-deficient mutants, implying a potential intrinsic link between VSRs and the miRNA pathway (at least in Arabidopsis thaliana) even though their pathogenic mechanisms remain largely unknown. In this review, we summarise our current structural understandings of the arms race between the host and virus along the RNA silencing pathway in A. thaliana by focusing on several important ribonucleoprotein (RNP) structures involved in RNA silencing and unique structural features adopted by VSRs.