High level of resistance to potato virus Y by expressing P1 sequence in antisense orientation in transgenic potato

Potato cultivar Pito was transformed with the P1 sequence of potato virus Y (PVYO) in antisense orientation. Five transgenic lines (ASR) showed a high level of resistance to PVYO on mechanical and side-graft inoculation, expr essed as no symptoms, and no detectable amounts of PVY in the inoculated an d the upper uninoculated leaves, as determined by ELISA 21 and 35 days afte r inoculation. The possibility of accumulation of low levels of PVYO not de tected by ELISA was tested by grafting a scion of untransformed Pito (WT) o n each of the PVYO-inoculated ASR plants. With this method, no PVY was dete cted in the WT plants grafted on the PVYO-inoculated ASR plants. Two of the six plants of the ASR line AI 0623-2 showed PVY accumulation when grafted on the PVYO-inoculated WT plants. Resistance was specific to PVYO, as the A SR lines were fully susceptible to PVYN, PVA and PVX. The ASR lines and PVY -susceptible lines contained 1-5, and 1-3 P1 gene inserts, respectively, as determined by Southern analysis. All lines expressed similarly low levels of the antisense P1 transcript. These results show that effective, virus st rain-specific resistance to PVY(O)can be achieved by expressing the P1 sequ ence in antisense orientation in potato. The low transgene transcript level s suggest that resistance in the ASR plants may be based on gene silencing.