Genetic mapping of quantitative trait loci conferring resistance to race 4 of Xanthomonas campestris pv. campestris in cabbage (Brassica oleracea var. capitata)

Abstract

Black rot is one of the most serious bacterial diseases affecting cabbage, yet the genetic architecture underlying its resistance remains poorly characterized. To address this knowledge gap, we explored the quantitative trait loci controlling black rot resistance during the seedling stage in a mapping population generated from well-established resistant and susceptible cabbage lines. We applied a mixed major gene plus polygene inheritance model, utilizing a joint analysis method across multiple generations, which revealed that the resistance to black rot in cabbage is controlled by two major genes with additive-dominant-epistatic effects, as well as multiple additive-dominant genes. Subsequently, we employed bulked-segregant analysis (BSA-seq) and QTL-mapping methods to identify the loci associated with black rot resistance using an F₂ population. In total, we identified eleven candidate loci responsible for black rot resistance on several chromosomes, including C03, C04, C07, C08, and C09. Furthermore, we performed a multi-layered investigation of the top candidate genes within these loci, analyzing their dynamic expression levels in response to black rot disease and identifying multiple promising candidate genes. Our findings provide significant insights into the molecular mechanisms of black rot resistance in cabbage, establishing a foundation for marker-assisted breeding strategies and functional genomic studies targeting this economically important trait.