Viral pathogenicity determinants are suppressors of transgene silencing inNicotiana benthamiana

Post-transcriptional gene silencing (PTGS) of a green fluorescent protein ( GFP) transgene is suppressed in Nicotiana benthamiana plants infected with potato virus Y (PVY) or with cucumber mosaic virus (CMV), but not in plants infected with potato virus X (PVX). By expressing PVY and CMV-encoded prot eins in a PVX vector we have shown that the viral suppressors of gene silen cing are the HCPro of PVY and the 2b protein of CMV. The HCPro acts by bloc king the maintenance of PTGS in tissues where silencing had already been se t, whereas the 2b protein prevents initiation of gene silencing at the grow ing points of the plants. Combined with previous findings that viruses are both activators and targets of PTGS, these data provide compelling evidence that PTGS represents a natural mechanism for plant protection against viru ses.