Transfer of resistance to Xanthomonas campestris pv campestris into Brassica oleracea L. by protoplast fusion.

Black rot caused by the bacterium Xanthomonas campestris pv campestris is one of the most serious diseases of Brassica oleracea. Since sources of resistance to the disease within B. oleracea are insufficient and control means are limited, the development of resistant breeding lines is extremely desirable. Certain lines of B. napus contain very high resistance controlled by a dominant gene, but crossing the two species sexually is very difficult. Therefore, somatic hybrids were produced by protoplast fusion between rapid cycling B. oleracea and a B. napus line highly resistant to X. campestris pv campestris. Hybrid identity was confirmed by morphological studies, flow cytometric estimation of nuclear DNA content, and analysis of random amplified polymorphic DNA (RAPD). Inoculations with the pathogen identified four somatic hybrids with high resistance. The resistant hybrid plants were fertile and set seed when selfed or crossed reciprocally to the bridge line '15' (Quazi 1988). Direct crosses to B. oleracea were unsuccessful, but embryo rescue facilitated the production of a first-backcross generation. The BC1 plants were resistant to the pathogen. Progeny from the crosses to 'line 15' were all susceptible. Embryo rescue techniques were not obligatory for the development of a second-backcross generation, and several resistant BC2 plants were obtained.