Molecular Characterization of the CRa Gene Conferring Clubroot Resistance in Brassica rapa

Clubroot disease caused by Plasmodiophora brassicae, is one of the major diseases affecting Brassicaceae crops. For example, Chinese cabbage (Brassica rapa L. ssp. pekinensis) is known to be highly susceptible to clubroot. For protection from this disease, clubroot resistance (CR) genes were introduced into Chinese cabbage from highly resistant European turnips. But so far, developed CR Chinese cabbage cultivars probably carried only the limited CR genes, so that these cultivars showed resistance to the limited variety of clubroot pathogens. From linkage analysis using five doubled-haploid Chinese cabbage lines, we have identified three independent clubroot resistant loci (CRa, CRc and CRk) previously.

  In the present study, CRa, conferring resistance locus to P. brassicae isolate M85, was focused. Fine mapping of the CRa locus revealed that its candidate gene encoded a TIR-NBS-LRR protein. There were several structural differences in this gene between susceptible and resistant lines, and its expression was observed only in the resistant line. Additionally, four mutant lines lacking clubroot resistance were obtained by the UV irradiation of pollen from a resistant line. All of them carried independent mutations in the corresponding gene. These genetic and molecular evidences strongly suggested that the identified TIR-NBS-LRR gene was causal gene of CRa. According to survey the presence of CRa in the commercial Chinese cabbage cultivars, it was distributed in 80% of Japanese CR cultivars. Furthermore, we demonstrated that, in addition to CRa, introduction of CRc and CRk made it possible to develop a strong resistant line.