Transgenic melon and squash expressing coat protein genes of aphid-borne viruses do not assist the spread of an aphid non-transmissible strain of cucumber mosaic virus in the field

Transgenic melon and squash containing the coat protein (CP) gene of the ap hid transmissible strain WL of cucumber mosaic cucumovirus (CMV) were grown under field conditions to determine if they would assist the spread of the aphid non-transmissible strain C of CMV, possibly through heterologous enc apsidation and recombination. Transgenic melon were susceptible to CMV stra in C whereas transgenic squash were resistant although the latter occasiona lly developed chlorotic blotches on lower leaves. Transgenic squash line ZW -20, one of the parents of commercialized cultivar Freedom II, which expres ses the CP genes of the aphid transmissible strains FL of zucchini yellow m osaic (ZYMV) and watermelon mosaic virus 2 (WMV 2) potyviruses was also tes ted. Line ZW-20 is resistant to ZYMV and WMV 2 but is susceptible to CMV. F ield experiments conducted over two consecutive years showed that aphid-vec tored spread of CMV strain C did not occur from any of the CMV strain C-cha llenge inoculated transgenic plants to any of the uninoculated CMV-suscepti ble non-transgenic plants. Although CMV was detected in 3% (22/764) of the uninoculated plants, several assays including ELISA, RT-PCR-RFLP, identific ation of CP amino acid at position 168, and aphid transmission tests demons trated that these CMV isolates were distinct from strain C. Instead, they w ere non-targeted CMV isolates that came from outside the field plots. This is the first report on field experiments designed to determine the potentia l of transgenic plants expressing CP genes for triggering changes in virus- vector specificity. Our results indicate that transgenic plants expressing CP genes of aphid transmissible strains of CMV, ZYMV, and WMV 2 are unlikel y to mediate the spread of aphid non-transmissible strains of CMV. This fin ding is of practical relevance because transgenic crops expressing the thre e CP genes are targeted for commercial release, and because CMV is economic ally important, has a wide host range, and is widespread worldwide.