A single amino acid change in viral genome-associated protein of potato virus Y correlates with resistance breaking in 'Virgin A Mutant' tobacco

Tobacco cultivar Virgin A Mutant (VAM) is reported to have the recessive po tyvirus resistance gene va. Varied levels of resistance were observed in VA M plants inoculated with Japanese potato virus Y (PVY) isolates. VAM was hi ghly resistant to most of the PVY isolates tested and tolerant to three nec rotic strain isolates of PVY-T. Based on data obtained from tissue printing and press blotting, the resistance appeared to be mainly at the level of c ell-to-cell movement. PW replicated in VAM protoplasts, but the replication was 30% lower than in susceptible tobacco, suggesting that impairment of r eplication also contributes to resistance. To identify the viral gene produ ct or products involved in VAM resistance, we isolated spontaneous resistan ce-breaking mutants by passing vein-banding (O strain) isolates several tim es through VAM plants. By comparing the amino acid sequences of the mutants with their original isolates, we identified a single amino acid substituti on in the viral genome-associated protein (VPg) domain that is correlated w ith VAM resistance breaking. Together, these results suggest that, in addit ion to its role in replication, VPg plays an important role in the cell-to- cell movement of PVY.