Molecular dissection of the TMV-encoded p126 protein in suppression of virus-induced gene silencing

Post-transcriptional gene silencing (PTGS) is a conservative phenomenon across kingdoms. In plants, it has been demonstrated that PTGS functions as an intrinsic defense mechanism during viral infection. On the other hand, viruses have evolved counter-defense strategies by encoded suppressors to circumvent PTGS. In previous studies, Tobacco mosaic virus (TMV) encoded p126 protein has been exhibited as a suppressor of PTGS although its mechanism remains unknown. In this current study, we found that there are multiple suppressor domains/motifs within the p126 protein, including methyltransferase, helicase and non-conserved region II, implicating that these domains/motifs could function synergistically against PTGS. Nevertheless, these suppressor domains might exhibit differential consequences on local and/or systemic gene silencing. Moreover, the enzymatic activities of methyltransferase and helicase are dispensable in the suppression pathways. Interestingly, several amino acids within the non-conserved region II pivotal for pathological consequence seem to intimately correlate with extend of the ability of gene silencing suppression. Together, these results suggest that a role of gene silencing suppression of TMV determinates its susceptibility on host plants. Furthermore, the MAPK signal cascade(s) was involved in the functioning of the p126-mediated gene silencing in tobacco. Our findings will therefore help elucidate the function and evolution of different virus suppressor and lead to new crop protection strategies.