Development of clubroot resistant cabbage line through introgressing six CR loci from Chinese cabbage via interspecific hybridization and embryo rescue

Multiple R-gene is required to achieve strong resistance against clubroot, a disease caused by Plasmodiophora brassicae in Cabbage (Brassica oleracea, 2n=18, CC). Most of the R-genes were identified in Chinese cabbage (B. rapa, 2n=20, AA), which could be transferred to cabbage via inter interspecific hybridization. We present a breeding scheme to transfer six major CR-loci namely, Crr1, Crr2, Crr3, CRa, CRb and CRc from Chinese cabbage line, LCR36 to cabbage line, Plimio. Approximately, 48% of the interspecific hybrid embryos were rescued, of which chromosomes of 48.15% colchiploid plants were successfully doubled. The hybridity of the colchiploid interspecific hybrids were confirmed by diagnostic sub-genome specific Conserved Ortholog Set (COS) markers, ploidy level, pollen fertility and meiotic chromosome number analyses. Backcross generations are characterized by very low seed set (0.57-1.33% success rate) in backcrossed generations (using Plimio as the recurrent parent). Marker assisted selection using the six CR-loci specific markers identified the resistant plants in F1 and backcross generations. MAS and bioassay test of BC2F2 plants revealed that plants having all six R-loci are resistant to clubroot. Development of subsequent backcross generations are in progress. More number of back-crosses and/or embryo rescue will be performed to ensure sufficient seed set. The success so far is the first report of successful introgression of multiple R-genes via interspecific hybridization in the initial backcrossed generations, indicating that an elite cabbage line having strong resistance to clubroot can be produced upon the completion of the breeding scheme.