A HYPERSENSITIVE RESPONSE-LIKE MECHANISM IS INVOLVED IN RESISTANCE OFPOTATO PLANTS BEARING THE RY(STO) GENE TO THE POTYVIRUSES POTATO-VIRUS-Y AND TOBACCO ETCH VIRUS

Potato plants carrying the Ry(sto) gene from Solanum stoloniferum are extremely resistant to a number of potyviruses, but it is not known at what stage of infection the resistance is expressed, The resistance m ay be due to Ry(sto) or to a closely linked gene. In this investigatio n, we used potato virus Y (PVY) and a tobacco etch virus construct tha t encodes beta-glucuronidase (TEV-GUS) to monitor virus infections of potato plants. Systemic spread of either virus in resistant potato pla nts was not detectable by serology, RT-PCR, GUS assay or bioassay alth ough each replicated in the initially infected cells of leaves from re sistant potato cultivars and was transported into neighbouring cells. However, 3 days post-inoculation (p.i.) a necrotic reaction set in tha t stopped movement and accumulation of both viruses by 7 days p.i. The resistance reaction (probably a hypersensitive reaction) became visib le as necrotic streaks on veins on the lower leaflet surfaces of some potato cultivars carrying the Ry(sto) gene and may be elicited by a co mmon potyviral gene product.