Characterising the suppressors of silencing activity of the p19.7 kDa proteins encoded by New Zealand variants of Grapevine leafroll-associated virus 3

Abstract

Grapevine leafroll-associated virus 3 (GLRaV-3) infects grapes and is present in all grapevine growing regions worldwide resulting in an uneven ripeness of grape berries and low quality wine. This virus threatens the largest horticultural export earner for New Zealand, the grape and wine industry. One potential method to prevent the impact of GLRaV-3 is through cross protection, where a virus strain that produces only mild symptoms protects against infection from a more severe strain of the same virus. However, a mild strain of GLRaV-3 has not been identified. Viruses encode suppressors of silencing (VSRs) to bypass the host plant’s RNA interference defence system. VSRs can act either locally or systemically and some VSRs possess both local and systemic VSR activity. Prior to this research, the only described VSR encoded by GLRaV-3 was the p19.7 kDa protein which was shown to act locally. Since the sequence of the p19.7 kDa protein varies greatly among the GLRaV-3 isolates from the six phylogenetic groups, particularly within two novel New Zealand isolates that diverge 20% from other GLRaV-3 sequences, this project used both a novel and traditional methods to investigate the local and systemic VSR activity of the p19.7 kDa protein from the seven described GLRaV-3, Groups I-VI and NZ2. This research made significant findings including 1) local VSR activity of the p19.7 kDa proteins encoded by Group VI and NZ2 isolates of GLRaV-3, 2) the first demonstration of systemic VSR activity from a GLRaV-3 encoded protein (p19.7 encoded by Group II, III, III, V, and VI isolates of GLRaV-3), and 3) the validation of the novel Dual Luciferase Renilla assay for rapid identification of local VSR activity. This project now sets the scene for future research on the VSR activity encoded by GLRaV-3 and use of this knowledge in developing mild strains and infectious clones of GLRaV-3.