Abstract
Two cabbage (Brassica oleracea) cultivars ‘Tekila’ and ‘Kilaherb’ were identified as resistant to several pathotypes of Plasmodiophora brassicae. In this study, we identified a clubroot resistance gene (Rcr7) in ‘Tekila’ for resistance to pathotype 3 of P. brassicae from a segregating population derived from ‘Tekila’ crossed with the susceptible line T010000DH3. Genetic mapping was performed by identifying the percentage of polymorphic variants (PPV), a new method proposed in this study, through bulked segregant RNA sequencing. Chromosome C7 carried the highest PPV (42%) compared to the 30–34% in the remaining chromosomes. A peak with PPV (56–73%) was found within the physical interval 41–44 Mb, which indicated that Rcr7 might be located in this region. Kompetitive Allele-Specific PCR was used to confirm the association of Rcr7 with SNPs in the region. Rcr7 was flanked by two SNP markers and co-segregated with three SNP markers in the segregating population of 465 plants. Seven genes encoding TIR-NBS-LRR disease resistance proteins were identified in the target region, but only two genes, Bo7g108760 and Bo7g109000, were expressed. Resistance to pathotype 5X was also mapped to the same region as Rcr7. B. oleracea lines including ‘Kilaherb’ were tested with five SNP markers for Rcr7 and for resistance to pathotype 3; 11 of 25 lines were resistant, but ‘Kilaherb’ was the only line that carried the SNP alleles associated with Rcr7. The presence of Rcr7 in ‘Kilaherb’ for resistance to both pathotypes 3 and 5X was confirmed through linkage analysis.
Highlights
Clubroot, caused by the obligate pathogen Plasmodiophora brassicae Woronin, attacks several economically important members of the family Brassicaceae, including canola/oilseed rape (Brassica napus L.), broccoli and cabbage (B. oleracea L.)[1]
The objectives of the current study were: (i) to identify resistance gene in the cabbage cultivars; (ii) to test if identification of polymorphic variants (PPV) could be used for gene mapping; (iii) to characterize transcriptome-wide variation and map the resistance gene; (iv) to develop single nucleotide polymorphisms (SNPs) markers tightly linked to the resistance gene; and (v) to examine DNA variation in the target region and identify the most probable candidates for the gene
A previous study identified a high proportion of PPV on chromosome A03 of B. rapa adjacent to the clubroot resistance gene Rcr[128]
Summary
Clubroot, caused by the obligate pathogen Plasmodiophora brassicae Woronin, attacks several economically important members of the family Brassicaceae, including canola/oilseed rape (Brassica napus L.), broccoli and cabbage (B. oleracea L.)[1]. BSR-Seq of genome-wide DNA variants in a B. rapa population were used to map clubroot resistance gene Rcr[1] onto chromosome A03. We hypothesized that a gene could be genetically mapped by identifying the PPV in a genome through BSR-Seq. A previous study identified two cabbage (B. oleracea) cultivars, ‘Tekila’ and ‘Kilaherb’, with strong resistance to the major strains of P. brassicae in western Canada[37]. The objectives of the current study were: (i) to identify resistance gene in the cabbage cultivars; (ii) to test if identification of PPV could be used for gene mapping; (iii) to characterize transcriptome-wide variation and map the resistance gene; (iv) to develop SNP markers tightly linked to the resistance gene; and (v) to examine DNA variation in the target region and identify the most probable candidates for the gene
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