Transgenic resistance to PVYO associated with post-transcriptional silencing of P1 transgene is overcome by PVYN strains that carry highly homologousP1 sequences and recover transgene expression at infection

Resistance to Potato virus Y (PVY) has been obtained in our previous studie s through expression of the PVY P1 gene in sense or antisense orientation i n potato cv. Pito, In the present study, the mechanism and strain specifici ty of the resistance were analyzed. Several features including low steady-s tate P1 mRNA expression in the resistant P1 plants indicated that resistanc e was based on posttranscriptional gene silencing (PTGS). Resistance was sp ecific to PVYO isolates, the PVY strain group from which the P1 transgene w as derived. However, according to group analyses, there was no distinguishi ng characteristic between the PVYO and PVYN strains P1 gene sequences. Ther efore, the ability of the PVYN strains to overcome resistance could not be explained solely based on their P1 gene sequences. Infection with PVYN of t he PVYO-resistant transgenic lines led to a recovery of expression of the P 1 transgene, These data suggested that factors other than sequence homology are required in determination of the resistance specificity.