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 squash lines with both coat protein (CP) genes of the melon strain of SqMV were produced and crossed with nontransgenic squash. Further greenhouse, screenhouse and field tests were done with R-1 plants from three independent lines that showed susceptible, recovery, or resistant phenotypes after inoculations with SqMV. Nearly all inoculated plants of the resistant line (SqMV-127) were resistant under greenhouse and field conditions and less so under screenhouse conditions. Plants of the recovery phenotype line (SqMV-3) were susceptible when inoculated at the cotyledon stage but leaves that developed later did not show symptoms. The susceptible line (SqMV-22) developed symptoms that persisted and spread throughout the plant. Plants were also analyzed for transcription rates of the CP transgenes and steady state transgene RNA levels. Results showed that the resistant line SqMV-127 displayed post-transcriptional silencing of the CP transgene as evidenced by high transcription rates but concomitant low accumulation of transgene transcripts. This is the first report on the development of transgenic squash that are resistant to SqMV.