Transformation of Recalcitrant Melon (Cucumis melo L.) Cultivars is Facilitated by Wounding with Carborundum

Transformation of the recalcitrant melon (Cucumis melo L.) cultivars Kirkagac 637 and Noi Yarok was accomplished by wounding cotyledon explants by vortexing with carborundum prior to inoculation with Agrobacterium tumefaciens. The addition of silver nitrate to the regeneration-selection medium reduced the transformation efficiency, as the percentage of the explants forming putative transgenic calli and bud-like protuberances was decreased and no transgenic shoots were produced. Chimeric transgenic plants were obtained after the regeneration of putatively transformed callus, bud-like protuberances, buds and shoots on selective medium with kanamycin. The treatments producing the most buds or shoots from explants after 30-40 days of cultivation were the most successful for the production of transgenic plants. Only treatments where explants were vortexed with carborundum produced transgenic melon shoots of either cultivar. Subculture every 18-20 days on fresh regeneration-selection medium containing 50 mg/L kanamycin after either a relatively high (100 mg/L) or low level (50 mg/ L) of kanamycin in the first regeneration-selection medium was necessary for the successful transformation of cultivar Kirkagac 637. These techniques are now being used in breeding programs for the production of melon lines bearing resistances to zucchini yellow mosaic virus and cucumber mosaic virus, important viruses limiting agricultural production.