Silencing Suppressor Protein p26 of Areca Palm Velarivirus 1 (APV1) Interacts With SGS3 and Promotes Its Degradation Via the Ubiquitination Pathway.

RNA silencing is a conserved defence mechanism against viral infections in eukaryotes, whereas many plant viruses evolved viral suppressors of RNA silencing (VSRs) to counteract the antiviral silencing in hosts. Areca palm velarivirus 1 (APV1), a member of genus Velarivirus, family Closteroviridae, is a causal agent of yellow leaf disease affecting Areca catechu in Hainan, China. In this work, we identified the APV1-encoded p26 protein as a multifunctional VSR that suppresses local silencing induced by single-stranded RNA (ssRNA), systemic silencing induced by ssRNA and double-stranded RNA (dsRNA), and the systemic spread of silencing signals. p26 was shown to be a viral pathogenicity factor that enhances the accumulation and pathogenicity of recombinant potato virus X (PVX). The suppressor of gene silencing 3 protein of A. catechu (AcSGS3), a key component of the plant antiviral silencing pathway, was proved to be a host factor interacting with p26. Both APV1 infection and p26 overexpression significantly induced NbSGS3 expression in Nicotiana benthamiana. The accumulation level of AcSGS3 protein decreased in the presence of p26, and such degradation was sensitive to the ubiquitination inhibitor MG132. Overexpression of p26 disturbed the interaction between SGS3 and the RNA-dependent RNA polymerase 6 protein (RDR6), another key component coordinating with SGS3 in antiviral silencing. Our study reveals the molecular mechanism of p26-mediated suppression of host antiviral defence by targeting SGS3 for accelerated degradation via the ubiquitination pathway.