Specificity of resistance to pea seed-borne mosaic potyvirus in transgenicpeas expressing the viral replicase (NIb) gene

Transgenic pea lines carrying the replicase (Nlb) gene of pea seed-borne mo saic potyvirus (PSbMV) were generated and used in experiments to determine the effectiveness of induced resistance upon heterologous isolates. Three p ea lines showed inducible resistance in which an initial infection by the h omologous isolate (PSbMV-DPD1) was followed by a highly resistant state. Re sistance was observed in plants in either the homozygous or hemizygous cond ition and resulted in no overall yield loss despite the initial infection. Resistance was associated with a loss of both viral and transgene RNA, whic h is indicative of a mechanism based upon post-transcriptional gene silenci ng. There was no correlation between the steady-state levels of transgene R NA and ability of the plants to show resistance. To test the specificity of the resistance, plants were also inoculated with the most distantly relate d sequenced PSbMV isolate, NY. PSbMV-NY varied between experiments in its a bility to induce resistance, suggesting that the sequence identity in the N ib gene is borderline for the specificity required for triggering gene sile ncing. Upon challenge inoculation of virus-free recovered leaves, the speci ficity of the induced resistance varied between the two isolates and indica ted that the virus and transgene additively determined the resistant state. These results suggest that the sequence requirements for triggering gene s ilencing may differ from those involved in the degradation process.