Fine mapping and candidate gene analysis of QS_B3.1 conferring clubroot resistance in Brassica rapa

Abstract Chinese cabbage is an important leafy vegetable rich in many nutrients widely grown in China. Clubroot disease caused by an obligate biotrophic pathogen Plasmodiophora brassicae was rapidly spread and challenged to Chinese cabbage production. A clubroot resistance (CR) gene, QS_B3.1, was mapped on chromosome A03 of B. rapa. A Chinese cabbage line ‘CR510’, which harbor homozygous resistance locus QS_B3.1 was selected from a BC4F3 family. ‘CR510’ was crossed with a clubroot susceptible Chinese cabbage inbred line ‘59-1’. Total 51 recombinant plants were identified from an F2 population including 3000 individuals. These recombinants were selfed and the clubroot resistance of F2/3 families were evaluated. Finally, QS_B3.1 was fine mapped to an interval of approximately 386 kb between marker syau-InDel3024 and syau-InDel3008. According to the reference genome, total 54 genes including five encoding the TIR-NBS-LRR proteins was annotated in the fine mapped region. Further nine candidate’s gene expression in parental lines at 7, 14 and 21 days after inoculation of P.brassicae were evaluated. Bra019376, Bra019401, Bra019403 and Bra019410 are highly expressed in ‘CR510’ than ‘59-1’. Gene sequence of Bra019410 from ‘CR510’ was cloned and identified different from CRa. Therefore, Bra019376, Bra019401, Bra019403 and Bra019410 are the most likely candidates of QS_B3.1. Our research provides a valuable germplasm resource against P. brassicae and QS_B3.1 closely linked marker for marker-assisted selection in CR cultivars breeding.