Two distinct mechanisms of transgenic resistance mediated by groundnut rosette virus satellite RNA sequences

Transformation of Nicotiana benthamiana with full-length sequences of a mild variant of the groundnut rosette virus (GRV) satellite RNA (sat-RNA) yielded plants that did not produce symptoms when inoculated with GRV and a virulent sat-RNA. Two different resistance mechanisms operated in different transformed lines. In the first, plants contained high levels of transcript RNA, and replication of both sat-RNA and GRV genomic RNA was inhibited. This mechanism is analogous to the down-regulation of GRV genomic and sat-RNA replication in infections containing the mild sat-RNA, and indeed infection of sat-RNA transgenic plants with GRV was shown to lead to liberation of unit-length sat-RNA from transgene transcripts. In the second resistance mechanism, plants contained low transcript RNA levels, and replication of sat-RNA but not of GRV genomic RNA was inhibited. These plants were also resistant to infection by potato virus X derivatives containing GRV sat-RNA sequences. This mechanism is an example of homology-dependent gene silencing or cosuppression. Resistant plants were also produced by transformation with sequences representing only the 5' terminal one-third of the mild sat-RNA; the mechanism of resistance in these plants was of the cosuppression type.