Viral pathogenicity determinants are suppressors of transgene silencing in Nicotiana 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 proteins in a PVX vector we have shown that the viral suppressors of gene silencing are the HCPro of PVY and the 2b protein of CMV. The HCPro acts by blocking the maintenance of PTGS in tissues where silencing had already been set, whereas the 2b protein prevents initiation of gene silencing at the growing 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 viruses.