Resistance to squash mosaic comovirus in transgenic squash plants expressing its coat protein genes

The approach of pathogen-derived resistance was investigated as a means to develop squash mosaic comovirus (SqMV)-resistant cucurbits. Transgenic squa sh lines with both coat protein (CP) genes of the melon strain of SqMV were produced and crossed with nontransgenic squash. Further greenhouse, screen house and field tests were done with R-1 plants from three independent line s that showed susceptible, recovery, or resistant phenotypes after inoculat ions with SqMV. Nearly all inoculated plants of the resistant line (SqMV-12 7) were resistant under greenhouse and field conditions and less so under s creenhouse conditions. Plants of the recovery phenotype line (SqMV-3) were susceptible when inoculated at the cotyledon stage but leaves that develope d later did not show symptoms. The susceptible line (SqMV-22) developed sym ptoms that persisted and spread throughout the plant. Plants were also anal yzed for transcription rates of the CP transgenes and steady state transgen e RNA levels. Results showed that the resistant line SqMV-127 displayed pos t-transcriptional silencing of the CP transgene as evidenced by high transc ription rates but concomitant low accumulation of transgene transcripts. Th is is the first report on the development of transgenic squash that are res istant to SqMV.