In Chinese cabbage (
Brassica rapa), the clubroot resistance (CR) gene
CRb is effective against
Plasmodiophora brassicae isolate No. 14, which is classified as pathotype group 3. Although markers linked to
CRb have been reported, an accurate position in the genome and the gene structure are unknown. To determine the genomic location and estimate the structure of
CRb, we developed 28 markers (average distance, 20.4 kb) around
CRb and constructed a high-density partial map. The precise position of
CRb was determined by using a population of 2,032 F
2 plants generated by selfing
B. rapa ‘CR Shinki.’ We determined that
CRb is located in the 140-kb genomic region between markers KB59N07 and B1005 and found candidate resistance genes. Among other CR genes on chromosome R3, a genotype of
CRa closest marker clearly matched those of
CRb and
Crr3 did not confer resistance to isolate No. 14. Based on the genotypes of 11 markers developed near
CRb and resistance to isolate No. 14, 82 of 108 cultivars showed a strong correlation between genotypes and phenotypes. The results of this study will be useful for isolating
CRb and breeding cultivars with resistance to pathotype group 3 by introducing
CRb into susceptible cultivars through marker-assisted selection.
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