Effect of mutations in the 2b protein of tomato aspermy virus on RNA silencing suppressor activity, virulence, and virus-induced gene silencing.

Tomato aspermy virus (TAV) and cucumber mosaic virus (CMV) belong to the genus Cucumovirus in the family Bromoviridae. The cucumovirus 2b protein is one of the first identified viral suppressors of RNA silencing (VSR). The cucumovirus 2b protein contains a conserved amino acid motif consisting of several highly conserved amino acid residues. Here, we demonstrate that the TAV 2b protein N-terminal region, Arg46, Ser40, and Ser42 as well as the CMV 2b protein are essential for VSR activity, virulence, and viral RNA accumulation. Furthermore, we developed the first TAV-induced gene silencing (VIGS) vector. In contrast to other cucumovirus vectors, such as CMV and peanut stunt virus, the TAV vector did not induce a silencing phenotype in Nicotiana benthamiana when 2b protein VSR activity was retained. These findings suggest that the cucumovirus 2b proteins share amino acid residues for VSR activity but may have different roles in VIGS induction.