Host Recovery and Reduced Virus Level in the Upper Leaves after Potato virus Y Infection Occur in Tobacco and Tomato but not in Potato Plants

In this study, the recovery phenomenon following infection with Potato virus Y (PVY) was investigated in tobacco (Nicotiana tobaccum), tomato (Solanum lycopersicum) and potato (Solanum tuberosum) plants. In tobacco plants, infection of severe strains of PVY (PVY(N) or PVY(N:O)) induced conspicuous vein clearing and leaf deformation in the first three leaves above the inoculated leaves, but much milder symptoms in the upper leaves. The recovery phenotype was not obvious in tobacco plants infected with PVY strain that induce mild symptoms (PVY(O)). However, regardless of the virus strains, reduction in PVY RNA levels was similarly observed in the upper leaves of these plants. Removal of the first three leaves above the inoculated leaves interfered with the occurrence of recovery, suggesting that the signal(s) mediating the recovery is likely generated in these leaves. In PVY(N) or PVY(N:O) but not in PVY(O)-infected tobacco plants, the expression of PR-1a transcripts were correlated with the accumulation level of PVY RNA. Reduced level of PVY RNA in the upper leaves was also observed in infected tomato plants, whereas such phenomenon was not observed in potato plants. PVY-derived small RNAs were detected in both tobacco and potato plants and their accumulation levels were correlated with PVY RNA levels. Our results demonstrate that the recovery phenotype following PVY infection is host-specific and not necessarily associated with the expression of PR-1a and generation of PVY small RNAs.