Mapping of a novel clubroot resistance QTL using ddRAD-seq in Chinese cabbage (Brassica rapa L.)

Plasmodiophora brassicae is a soil-borne plant pathogen that causes clubroot disease, which results in crop yield loss in cultivated Brassica species. Here, we investigated whether a quantitative trait locus (QTL) in B. rapa might confer resistance to a Korean P. brassicae pathotype isolate, Seosan. We crossed resistant and susceptible parental lines and analyzed the segregation pattern in a F₂ population of 348 lines. We identified and mapped a novel clubroot resistance QTL using the same mapping population that included susceptible Chinese cabbage and resistant turnip lines. Forty-five resistant and 45 susceptible F₂ lines along with their parental lines were used for double digest restriction site-associated DNA sequencing (ddRAD-seq). High resolution melting (HRM)-based validation of SNP positions was conducted to confirm the novel locus.

A 3:1 ratio was observed for resistant: susceptible genotypes, which is in accordance with Mendelian segregation. ddRAD-seq identified a new locus, CRs, on chromosome A08 that was different from the clubroot resistance (CR) locus, Crr1. HRM analysis validated SNP positions and constricted CRs region. Four out of seventeen single nucleotide polymorphisms (SNPs) positions were within a 0.8-Mb region that included three NBS-LRR candidate genes but not Crr1.

The newly identified CRs locus is a novel clubroot resistance locus, as the cultivar Akimeki bears the previously known Crr1 locus but remains susceptible to the Seosan isolate. These results could be exploited to develop molecular markers to detect Seosan-resistant genotypes and develop resistant Chinese cabbage cultivars.