Potato Virus Y Strain N-Wi Offers Cross-Protection in Potato Against Strain NTN-NW by Superior Competition

Potato virus Y (PVY) is one of the most economically important pathogens of potato. PVY exhibits different phenotypes in dissimilar potato cultivars. Previously, we observed that two recombinant isolates, PVY^N-Wi-HLJ-BDH-2 (BDH) and PVY^NTN-NW(SYR-II)-INM-W-369-12 (369), exhibited different virulence levels in potato cultivar Kexin 13 despite high genome sequence identity. Indeed, 369 induced severe necrosis and plant death in severe cases in Kexin 13 and severe mosaic in cultivar Yanshu 8, whereas BDH caused mainly mosaic symptoms on the plants of both cultivars. We hypothesized that preinfection of plants with BDH could cross-protect them from 369 infection, and not vice versa. Challenge inoculation, either by mechanical wounding or through grafting, with 369 on plants that were preinfected with BDH did not augment the symptom expression in both cultivars. Reverse transcription quantitative PCR analysis showed that, after challenge inoculation with 369, the titer of the isolate on BDH-preinfected plants remained at a low level (about 3 × 10⁴ copy/µl) during the tested time course (0 h to 30 days). In contrast, in plants that were preinoculated with buffer (mock) and challenge inoculated with 369, the titer of 369 increased continuously until reaching its highest level of about 2 × 10⁷ (Yanshu 8) and about 4 × 10⁸ (Kexin 13) during the time course. Surprisingly, in plants that were preinfected with 369 and challenge inoculated with BDH, the accumulation of BDH reached nearly the same level as that in plants that were preinoculated with buffer and challenge inoculated with BDH. Taken together, these results suggest that PVY^N-Wi mediated cross-protection against PVY^NTN-NW(SYR-II) by superior competition and better fitness.