A viral movement protein prevents spread of the gene silencing signal in Nicotiana benthamiana

In plants, viruses induce an RNA-mediated defense that is similar to posttr anscriptional gene silencing (PTGS) of transgenes. Here we demonstrate with potato virus X (PVX) that PTGS operates as a systemic, sequence-specific d efense system. However, in grafting experiments or with movement defective forms of PVX, we could not detect systemic silencing unless the 25 kDa vira l movement protein (p25) was made nonfunctional. Investigation of p25 revea led two branches to the PTGS pathway that converge in the production of 25 nucleotide RNAs corresponding to the target RNA. One of these branches is u nique to virus-induced PTGS and is not affected by p25. The second branch i s common to both virus- and transgene-induced PTGS, is blocked by p25, and is likely to generate the systemic silencing signal.