Abstract
The rapid spread of clubroot disease caused by Plasmodiophora brassicae threatens radish (Raphanus sativus) production in China because some cultivation types lack clubroot-resistant (CR) genes. However, few molecular markers for clubroot resistance have been developed and used in hybrid breeding programs. In this study, 27 immune and 6 highly resistant accessions were identified among 95 radish inbred lines. The genes Rsa10003637 and Rsa10025569/Rsa10025571 were respectively identified from an XYB36-2 reference genome as the homologs of Crr1 and CRa from Brassica rapa by means of homology and synteny analysis. The association between the degree of clubroot resistance and the genotype of these CR genes suggested that Rsa10025569-H3 can be used as a clubroot-resistant haplotype. The sequence identity of Rsa10025569 in clubroot-resistant lines (CR-60 and CR-88) and clubroot-susceptible lines (CR-10 and CR-35) was 92.47%, and there was a 699 bp insertion at the end of the fourth exon in the clubroot-susceptible line. Association analysis of a BC1F1 population derived from the cross CR-88 (resistance) × CR-10 (susceptible) revealed an apparent correlation between polymorphisms at the Rsa10025569 locus and degree of clubroot resistance. On the basis of the results, molecular marker-assisted selection was used to transfer disease resistance genes to susceptible varieties and a new CR germplasm of Xinlimei was obtained.
Highlights
Clubroot disease caused by Plasmodiophora brassicae is a serious threat to cruciferous crop production worldwide [1]
A total of 95 radish inbred lines were evaluated for resistance to P. brassicae race 4
The majority of the tested lines were susceptible to P. brassicae: 21 lines were slightly susceptible (1 < DI ≤ 2), 21 lines showed moderate susceptibility (2 < DI ≤ 3), and 20 lines were highly susceptible (3 < DI ≤ 4) (Table 1)
Summary
Clubroot disease caused by Plasmodiophora brassicae is a serious threat to cruciferous crop production worldwide [1]. Multiple CR genes have been identified or cloned in Brassica crops, such as CRa [12,13], CRb [14], Crr, Crr, and Crr4 [15–17], Crr3 [18,19], CRc and CRk [20], PbBa3.1 and PbBa3.3 [21], QS_B3.1 [22], CRd [23], and PbBrA08 [24]. Gan et al identified five QTLs (RsCrl, RsCr2, RsCr3, RsCr4, and RsCr5) associated with radish clubroot resistance, among which RsCr4 showed synteny with the Crr region in B. rapa [31]. The crucial resistance genes were identified using a homolog-based cloning method, and molecular markers were developed to assist in the introduction of the resistance genes into clubroot-susceptible radish cultivars by backcrossing. The results will be useful for marker-assisted breeding of clubroot-resistant cultivars to reduce disease-related yield loss in radish
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