Clubroot disease 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 disease. For protection from this disease, genes for resistance to clubroot were introduced from the European turnip. CRa is a gene that confers specific resistance to the clubroot pathogen Plasmodiophora brassicae isolate M85. Fine mapping of the CRa locus using synteny to the Arabidopsis thaliana genome and partial genome sequences of B. rapa revealed a candidate gene encoding a TIR-NBS-LRR protein. There were several structural differences in this candidate gene between susceptible and resistant lines, and CRa 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 these mutant lines carried independent mutations in the candidate TIR-NBS-LRR gene. This genetic and molecular evidence strongly suggests that the revealed gene is CRa. This is the first 2 report, on the identification of a clubroot Resistance gene in Brassicaceae, and on the identification of the disease resistance gene in B. rapa.

Many clubroot resistant (CR) cultivars of Chinese cabbage (Brassica rapa ssp. pekinensis) have been bred so far, but their usage is limited because the capacity for resistance breaks down with time. This degradation is caused by a pathogenic variation in the causal fungus Plasmodiophora brassicae. We attempted to accumulate 3 CR genes, CRa, CRk, and CRc, through marker-assisted selection. Five doubled haploid CR lines with an individual CR locus were used as breeding materials. The CR lines were crossed with each other. A subsequent selection for resistance was performed using sequence characterized amplified region markers in segregating generations. As a result, 4 homozygous lines for 3 resistance genes and the F 1 hybrids between them were developed. CR pyramiding lines were inoculated with 6 field isolates of P. brassicae. The homozygous lines for 3 CR genes, whether selfed or crossed, exhibited exceedingly high resistance against all of the isolates. Morphological characters of F 1 hybrids were comparable to those of a control cultivar, but the degree of heterosis was less than expected, which is probably because of genetic similarity of the parents. The results of this study prove that clubroot resistance can be reinforced through the accumulation of varied resistance genes in B. rapa.

A restriction fragment length polymorphism (RFLP) marker, HC352b, has identified as closely linked to the CRa locus responsible for clubroot resistance (CR) in a study dissecting CR loci in Chinese cabbage (Brassica rapa L. ssp pekinensis). Unfortunately, the RFLP pattern, including HC352b detected by the cDNA clone HC352, was complicated and confused in its interpretation because it represented multiple-copy loci. To provide a practical HC352b marker for CR breeding programs, a sequence characterized amplified region (SCAR) marker was constructed along the analysis of HC352 genes, followed by its evaluation for CR selection. Schematic characterization of signals was achieved by identification of HC352 homologous genes with their restriction sites, employing CRa-positive and -negative doubled haploid (DH) lines. Genomic sequence information from a set of HC352 homologous genes was analyzed to identify the CRa linked paralog HC352b, followed by the successful designation of a SCAR marker. This correctly predicted the CR phenotypes of all tested individuals of F 2 and back-crossed progenies.

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