Screening of clubroot-resistant varieties and transfer of clubroot resistance genes to Brassica napus using distant hybridization.

Clubroot is an economically important disease affecting plants in the family Cruciferae worldwide. In this study, a collection of 50 Cruciferae accessions was screened using Plasmodiophora brassicae pathotype 4 in China. Eight of these demonstrated resistance, including three Chinese cabbages, two cabbages, one radish, one kale, and one Brassica juncea. The three clubroot-resistant Chinese cabbages (1003, 1007 and 1008) were then used to transfer the clubroot resistance genes to B. napus by distant hybridization combined with embryo rescue. Three methods including morphological identification, cytology identification, and molecular marker-assisted selection were used to determine hybrid authenticity, and 0, 2, and 4 false hybrids were identified by these three methods, respectively. In total, 297 true hybrids were identified. Clubroot resistance markers and artificial inoculation were utilized to determine the source of clubroot resistance in the true hybrids. As a result, two simple sequence repeat (SSR) and two intron polymorphic (IP) markers linked to clubroot resistance genes were identified, the clubroot resistance genes of 1007 and 1008 were mapped to A03. At last, 159 clubroot-resistant hybrids were obtained by clubroot resistance markers and artificial inoculation. These intermediate varieties will be used as the 'bridge material' of clubroot resistance for further B. napus breeding.