Introgression of downy mildew resistance into grape cv. Thompson Seedless through hybrid embryo rescue in vitro

Using conventional breeding methods, it is not possible to obtain hybrid progeny from controlled crosses where seedless (stenospermocarpic) grapes are used as the female parent, owing to seedlessness of the maternal parent. In vitro embryo culture techniques are being increasingly applied to artificial crosses involving seedless cultivars to rescue hybrid progeny. ‘Thompson Seedless’, a choice seedless variety of grape (Vitis vinifera L), is highly susceptible to the downy mildew disease caused by the fungus Plasmopara viticola resulting in total crop loss in environments favourable to this pathogen. Therefore, a 2-step in vitro method was employed to rescue hybrid embryos from crosses where ‘Thompson Seedless’ served as the female parent and 14 different lines/relatives of grape, variously resistant to downy mildew, served as pollen parents. The technique involved in ovulo embryo culture (i.e., ovule culture) followed eight weeks later by ex ovulo embryo culture (i.e., excised embryo culture) on different synthetic media. Thus, immature zygotic embryos which would have otherwise aborted, were rescued and grown into plantlets in vitro. Hybrid progeny was recovered at an average frequency of 26.19% (364 fully developed plantlets from 1390 ovules cultured). When transplanted into soil, 353 vitro plants survived the final hardening-off. These hybrids await evaluation for downy mildew resistance upon field-planting.