Abstract
Increasing clubroot resistance (CR) of Brassica oleracea by ascertaining the molecular mechanisms has been the key focus in modern B. oleracea breeding. In order to identify the quantitative trait loci (QTLs) associated with CR in B. oleracea, 94 F2 vegetative lines which were developed by tissue culture of selfed seeds from the F1 generation between a clubroot-resistant B. oleracea inbred line and a susceptible line, were identified for disease incidence and six CR-associated traits under a lab inoculation by Plasmodiophora brassicae and were genotyped with the 60K Brassica SNP array. Significant correlations were detected for numbers of fibrous roots and P. brassicae content in roots with disease incidence. Nine linkage groups were constructed from 565 bins which covered around 3,000 SNPs, spanning 1,028 cM of the B. oleracea genome with an average distance of 1.82 cM between adjacent bins. A total of 23 QTLs were identified for disease incidence and the other two correlated traits, individually explaining 6.1–17.8% of the phenotypic variation. Several overlaps were detected among traits, including one three-traits-overlapped locus on linkage group C08 and two important overlapped regions between the two CR-associated traits on C06. The QTLs were compared with known CR loci/genes and the novelty of our QTLs was discussed.
Highlights
Clubroot, caused by the soil-borne obligate Plasmodiophora brassicae, is a devastating disease in Brassica crops including cabbage (Brassica oleracea L. var. capitata) which is one of the most important vegetable crops in the world (Hirai, 2006; Dixon, 2009)
Phenotypic data were collected at 6 weeks post-inoculation, including disease incidence (DIC, percentage of diseased plants in total plants), fresh weight per plant (FW), fibrous root weight/root weight (FR/R), length of root (LR), number of fibrous roots (NFRs), ratio of root surface covered with fibrous roots (RFR), and P. brassicae content in roots (PCR)
Wide variations were detected among the F2 lines for all the seven traits, of which FW, LR, and RFR showed normal distributions, while DIC, FR/R, NFR, and PCR exhibited skew distributions (Table 1)
Summary
Clubroot, caused by the soil-borne obligate Plasmodiophora brassicae, is a devastating disease in Brassica crops including cabbage (Brassica oleracea L. var. capitata) which is one of the most important vegetable crops in the world (Hirai, 2006; Dixon, 2009). Complete resistant accessions against specific pathogen isolates were found in European fodder turnips (B. rapa) and at least eight resistant loci were identified in B. rapa including Crr, Crr, Crr, Crr, CRa, CRb, CRc, and CRk (Matsumoto et al, 1998; Piao et al, 2002, 2004; Hirai et al, 2004; Saito et al, 2006; Suwabe et al, 2006; Sakamoto et al, 2008; Hatakeyama et al, 2013; Kato et al, 2013; Huang et al, 2017; Yu et al, 2017) These loci were reported to control CR in a qualitative plus quantitative manner (Piao et al, 2002; Suwabe et al, 2006; Sakamoto et al, 2008). Identify more CR loci from different B. oleracea resources with wide genetic basis will be benefit to the breeding of clubroot-resistant cabbages
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