Homology-dependent virus resistance in transgenic plants with the coat protein gene of sweet potato feathery mottle potyvirus: Target specificity and transgene methylation

Nicotiana benthamiana plants were transformed with the coat protein (CP) coding sequence and the 3' nontranslated region (NTR) of the severe strain of sweet potato feathery mottle potyvirus (SPFMV-S). Regenerated lines were screened for virus resistance using recombinant potato virus X (PVX) engineered to contain the sequence homologous to the transgene. Out of 19 transgenic lines, 7 showed virus resistance after inoculation by the recombinant PVX. In most of the resistant lines, relatively low steady-state accumulation of the CP gene mRNA and little or no protein products were observed, suggesting that the resistance was manifested by a posttranscriptional gene-silencing mechanism. The resistant lines could be divided into two groups according to the target specificity of the silencing mechanism; one group recognizing the 3' part of the transgene mRNA and the other not only the 3' part, but also the 5' and the central part of the transgene mRNA. Particular regions of the transgene corresponding to the RNA target in the resistant lines were differentially methylated compared with the transgene sequence in a susceptible line.