Transgenic resistance to PVYO associated with post-transcriptional silencing of P1 transgene is overcome by PVYN strains that carry highly homologous P1 sequences and recover transgene expression at infection

Resistance to Potato virus Y (PVY) has been obtained in our previous studies through expression of the PVY P1 gene in sense or antisense orientation in potato cv. Pito, In the present study, the mechanism and strain specificity of the resistance were analyzed. Several features including low steady-state P1 mRNA expression in the resistant P1 plants indicated that resistance was based on posttranscriptional gene silencing (PTGS). Resistance was specific to PVYO isolates, the PVY strain group from which the P1 transgene was derived. However, according to group analyses, there was no distinguishing characteristic between the PVYO and PVYN strains P1 gene sequences. Therefore, the ability of the PVYN strains to overcome resistance could not be explained solely based on their P1 gene sequences. Infection with PVYN of the PVYO-resistant transgenic lines led to a recovery of expression of the P1 transgene, These data suggested that factors other than sequence homology are required in determination of the resistance specificity.