Quantitative Trait Locus Mapping of Clubroot Resistance and Plasmodiophora brassicae Pathotype Banglim-Specific Marker Development in Brassica rapa

Clubroot resistance is an economically important trait in Brassicaceae crops. Although many quantitative trait loci (QTLs) for clubroot resistance have been identified in Brassica, disease-related damage continues to occur owing to differences in host variety and constant pathogen variation. Here, we investigated the inheritance of clubroot resistance in a double haploid population developed by crossing clubroot resistant and susceptible lines "09CR500" and "09CR501", respectively. The resistance of "09CR500" to Plasmodiophora brassicae pathotype "Banglim" was controlled as a single dominant gene, with the segregation of resistance and susceptibility being nearly 1:1. PbBrA08 was identified as having a logarithm of odds value of 7.9-74.8, and a phenotypic variance of 26.0-97.1% with flanking marker "09CR.11390652" in A08. After aligning QTL regions to the B. rapa reference genome, 11 genes were selected as candidates. PbBrA08 was located near Crr1, CRs, and Rcr9 loci, but differences were validated by marker analysis, gene structural variations, and gene expression levels, as well as phenotypic responses to the pathotype. Genotyping using the "09CR.11390652" marker accurately distinguished the Banglim-resistance phenotypes in the double haploid population. Thus, the developed marker will be useful in Brassica breeding programs, marker-assisted selection, and gene pyramiding to identify and develop resistant cultivars.