Abstract
RNA silencing or RNA interference (RNAi) is an endogenous mechanism of gene regulation and adaptive defence against invading microbes of higher organisms, triggered by double-stranded RNA. Viruses circumvent this host defence by encoding specific proteins, RNA silencing suppressors (RSSs). Evidence has been mounting that, in common with RNA viruses, the movement of geminiviruses in plants is countered by an RNAi-based resistance. To investigate this idea, the impact of the transient expression of suppressor proteins encoded by three RNA viruses on the infectivity and movement of tomato leaf curl New Delhi virus (ToLCNDV; a bipartite begomovirus family Geminiviridae) was investigated. Both genomic components (designated as DNA A [TA] and DNA B [TB]) are essential for symptomatic infection of plants, although TA alone may inefficiently spread to plants without inducing symptoms. Each of the three heterologous RSSs was shown to enhance the pathogenicity of ToLCNDV when expressed transiently at the site of inoculation. The effects ranged from an increase in the numbers of plants ultimately containing a virus or DNA A component (for inoculations with only TA or only with TA harbouring a mutation of the coat protein gene [TA(Delta CP)]), through to the induction of symptoms (for TA and TA(Delta CP) inoculations of tomato), to a reduction in latent period (for all inoculations involving TB). These results show that heterologous RSSs may enhance ToLCNDV pathogenicity and are consistent with the idea that geminivirus movement in plants is constrained by an RNAi-based resistance.