BSA-Seq and High-Density Linkage Mapping Identify qBoCR2 as a Major Locus and Implicate a PR5-Family Gene in Resistance to Plasmodiophora brassicae Pathotype 4 in Cabbage

Clubroot, a devastating disease affecting cabbage worldwide, is caused by the soilborne protist Plasmodiophora brassicae. From a cross between the resistant line ‘20CR01’ and the susceptible line ‘27-3-1’, we developed an F2 population to investigate the genetic basis of resistance to P. brassicae pathotype 4. Population-level disease responses were broadly comparable across three consecutive field evaluations. Using whole-genome resequencing of 150 F2 individuals, we constructed a high-density genetic map comprising 2266 bin markers spanning 932.25 cM and detected two QTLs: a major QTL, qBoCR2, on chromosome C02, and a putative minor-effect QTL, qBoCR8, on chromosome C08, whose reproducibility requires further confirmation. qBoCR2 explained 24.0% of the phenotypic variance and, together with BSA-seq evidence, was refined to a 0.42 Mb candidate interval (62.16–62.58 Mb) containing 60 annotated genes. Among five non-redundant candidate genes prioritized within this interval, BolC02g058940.2J, annotated as encoding an endogenous PR5-family thaumatin-like protein, was distinguished by the combination of parental coding polymorphisms and resistant-parent-biased induction following P. brassicae inoculation. Overexpression of BolC02g058940.2J in cabbage hairy roots increased transcript abundance and was associated with reduced P. brassicae accumulation and root gall weight, without a significant effect on hairy-root fresh weight. Overall, BolC02g058940.2J emerges as a prioritized candidate gene potentially contributing to resistance against P. brassicae pathotype 4 in cabbage. These polymorphisms identified here may be useful for marker development, but they require validation in segregating populations and diverse breeding materials.