Expression of Potato virus A coat protein gene in a resistant potato cultivar disrupts the virus induced hypersensitive responses

Graft inoculation of potato (Solanum tuberosum) 'Shepody' plants with Potato virus A (PVA) results in hypersensitive resistance responses (HR) in tubers with characteristic necrosis. Increasing the number of PVA-containing scions or increasing the duration of scions on the 'Shepody' rootstock plants resulted in an increased number of tubers showing necrosis. Transgenic 'Shepody' plants with a sense or an antisense copy of PVA coat protein (CP) gene under the direction of the CaMV 35S promoter were developed. The plants were subjected to graft inoculation with PVA in the greenhouse and were monitored by detecting PVA using enzyme-linked immunosorbent assays and reverse transcriptase polymerase chain reaction during the growing season. No PVA was detected in the wild type or antisense-CP transgenic lines. However, PVA was detected from one of the two sense-CP transgenic lines. Potato tubers from graft-inoculated plants were harvested 3 months after inoculation and were examined for necrosis. In the experiments carried out in the greenhouse, approximately 76% of tubers from wild-type plants showed necrosis, as did 43%-84% of tubers from the antisense PVA-CP lines. However, except for one, all tubers (130 in total) from the sense PVA-CP lines were necrosis free, indicating that the PVA-CP transgene suppresses the PVA-mediated HR.