Homology-dependent resistance to tomato ringspot nepovirus in plants transformed with the VPg-protease coding region

The coding region for the VPg (genome-linked viral protein) and protease (P ro) of a raspberry isolate (Rasp-2) of tomato ringspot virus (ToRSV) was cl oned into a plant expression vector and transformed into Nicotiana benthami ana using Agrobacterium tumefaciens. Transgenic lines were also produced th at expressed two mutant derivatives of the VPg-Pro coding region (i.e., con taining point mutations in the putative catalytic triad and substrate-bindi ng pocket of the protease). A large proportion of the R1 transgenic lines d isplayed extreme resistance to ToRSV (raspberry isolate Rasp-1, 99% homolog y to the transgene at the nucleotide level), especially when transformed wi th the wild-type VPg-Pro coding region. Several transgenic lines displaying extreme resistance to ToRSV were further tested at the R2 generation. Thes e R2 lines were resistant to Rasp-1 isolate but were susceptible to GYV iso late (79% homology to the transgene). These transgenic lines were also resi stant to a potato virus X (PVX) hybrid containing the VPg-Pro coding region of ToRSV (Rasp-2 isolate) but were susceptible to the wild-type PVX. These results imply a role for homology-dependent silencing as a mechanism for r esistance in these lines.