Grapevine fanleaf virus RNA1-encoded proteins 1A and 1B Hel suppress RNA silencing.

Grapevine fanleaf virus (GFLV, genus , family ) causes fanleaf degeneration, one of the most damaging viral diseases of grapevines. Despite substantial advances at deciphering GFLV-host interactions, how this virus overcomes the host antiviral pathways of RNA silencing is poorly understood. In this study, we identified viral suppressors of RNA silencing (VSRs) encoded by GFLV using fluorescence assays and tested their capacity at modifying host gene expression in transgenic expressing (). Results revealed that GFLV RNA1-encoded protein 1A, for which a function had yet to be assigned, and protein 1B, a putative helicase, reverse systemic RNA silencing either individually or as a fused form (1AB) predicted as an intermediary product of RNA1 polyprotein proteolytic processing. The GFLV VSRs differentially altered the expression of plant host genes involved in RNA silencing, as shown by RT-qPCR. In a coinfiltration assay with an hairpin construct, protein 1A upregulated / and , and proteins 1B and 1A+1B upregulated /, / and , while protein 1AB upregulated and . In a reversal of systemic silencing assay, protein 1A upregulated and , and protein 1AB upregulated / and . This is the first report of VSRs encoded by a nepovirus RNA1 and of two VSRs that act either individually or as a predicted fused form to counteract the systemic antiviral host defense, suggesting that GFLV might devise a unique counter-defense strategy to interfere with various steps of the plant antiviral RNA silencing pathways during infection.