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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 potyvirus resistance gene va. Varied levels of resistance were observed in VAM plants inoculated with Japanese potato virus Y (PVY) isolates. VAM was highly resistant to most of the PVY isolates tested and tolerant to three necrotic 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 cell-to-cell movement. PVY replicated in VAM proto-plasts, but the replication was 30% lower than in susceptible tobacco, suggesting that impairment of replication also contributes to resistance. To identify the viral gene product or products involved in VAM resistance, we isolated spontaneous resistance-breaking mutants by passing vein-banding (O strain) isolates several times through VAM plants. By comparing the amino acid sequences of the mutants with their original isolates, we identified a single amino acid substitution in the viral genome-associated protein (VPg) domain that is correlated with VAM resistance breaking. Together, these results suggest that, in addition to its role in replication, VPg plays an important role in the cell-to-cell movement of PVY.