Coat protein gene-mediated resistance to potato virus Y in tobacco: examination of the resistance mechanisms--is the transgenic coat protein required for protection?

We transformed SR1 tobacco with a cDNA fragment of the potato virus Y strain N (PVYN) 605 genome which contained the 3' end of the NIb polymerase gene, the coat protein (CP) coding sequence, and most of the viral 3' untranslated region. Complete resistance to several PVY strains was obtained in plants of the R2 generation. The resistance to PVYN and PVYO strains was maintained when the plants were inoculated dually with the potato potyviruses V or A (PVV or PVA). Only partial resistance to PVYN 605 was observed in plants containing the CP coding sequence in an antisense orientation. Both the "sense" and "antisense" plants accumulated CP transcripts and possessed multiple copies of the transgene. Systemic spread of the virus was studied by grafting experiments. The transgenic PVYN 605 CP was undetectable in healthy plants. This protein accumulated in plants of PVY-susceptible CP-transgenic line 4B5-31 when infected by PVYO, PVA, or PVV, but not in plants of line 4B5-S4, that were resistant to PVY and tolerant to PVA or PVV. To assess the role of the transgenic PVYN 605 CP in protection, we transformed plants with a construct bearing a frame-shift mutation at the beginning of the CP coding sequence, and observed partial protection from PVYN 605. Several hypotheses to explain these resistance mechanisms are discussed.