Expression of genes for resistance to potato virus Y in potato plants and protoplasts

SUMMARY Plants of several potato clones with major gene resistance to potato virus Y (PVY) developed necrotic local lesions and systemic necrosis after manual inoculation with common (PVY o ) or veinal necrosis (PVY N ) strains of the virus. The clones reacted similarly, although their resistance genes are thought to be derived from four different wild species of Solarium. Mesophyll protoplasts from each clone became infected when inoculated with RNA of PVY o by the polyethylene glycol method. The proportion of protoplasts infected, assessed by staining with fluorescent antibody to virus particles, was similar to that of protoplasts of susceptible potato cultivars. In contrast, plants of potato cultivars Corine and Pirola, which possess gene Ry from S. stoloniferum , developed few or no symptoms when manually inoculated or grafted with PVY o . Moreover, only very few protoplasts of these cultivars produced virus particle antigen after inoculation with PVY o RNA. The extreme resistance to PVY of cvs Corine and Pirola was therefore expressed by inoculated protoplasts whereas the resistance of the necrotic‐reacting potato clones was not.