Tobacco Rattle virus 16K encodes a suppressor of RNA silencing that allows transient viral entry in meristems

The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK

^* To whom correspondence should be addressed. Email: dcb40{at}cam.ac.uk.

	Abstract

RNA silencing is a host defence mechanism that limits the accumulation and spread of viruses in infected plants. Correspondingly, plant viruses encode suppressors of silencing. In the positive strand RNA virus – Tobacco rattle virus (TRV) – the suppressor of silencing is a 16K protein encoded on RNA1. The suppressor action of the 16K protein is transient and weaker than the P19 suppressor encoded by tomato bushy stunt virus. Mutant TRV that does not produce its suppressor, unlike other suppressor-defective viruses, is competent to accumulate and spread systemically in the infected plant. However this mutant virus does not exhibit the transient invasion of the meristem that is characteristic of the wild type virus. Based on this analysis we propose that the 16K suppressor of silencing allows TRV to transiently invade the meristem. Our data are consistent with a mechanism of long term meristem virus exclusion that is dependent on a transient invasion of the meristem early in the infection cycle. This novel mechanism of meristem exclusion may be associated with the phenomenon of recovery in virus infected plants in which upper leaves have little or no virus and are immune to secondary infection by the same virus.