Rapid introgression of the clubroot resistance gene CRa into cabbage skeleton inbred lines through marker assisted selection.

Clubroot, caused by Plasmodiophora brassicae, is a globally pervasive soil-borne disease that poses a significant challenge primarily in cruciferous crops. However, the scarcity of resistant materials and the intricate genetic mechanisms within cabbage present major obstacles to clubroot resistance (CR) breeding. In our previous research, we developed an Ogura CMS cabbage variety, "17CR3", which harbors the CRa gene, crucial for CR. The fertility of this variety can be restored through crossing with an Ogura cytoplasmic male sterile (CMS) restore line. In the current investigation, offspring from fertile hybrids were utilized as donor parents in backcrossing with five cabbage inbred lines, with the goal of introducing the CRa gene into elite cabbage cultivars possessing superior agronomic traits. Following five years of continuous field selection combined with molecular marker-assisted selection (MAS), we successfully developed BC₄ individuals exhibiting excellent agronomic traits and diverse genetic backgrounds. Whole-genome resequencing revealed a mere 54,213 SNP differences between the genetic makeup of BC₄ individuals and their recurrent parents. The results of inoculation identification demonstrated a high degree of co-segregation between the CRa-specific marker KBrH129J18 and resistance to Plasmodiophora brassicae in both inoculated resistant seedlings and cabbage plants harboring CRa across three distinct regions of China. Additionally, results from Semi-Quantitative RT-PCR experiments revealed minimal to no expression of CRa in the majority of susceptible individuals, underscoring the pivotal role of CRa in conferring CR. Moreover, BC₃ individuals resulting from the cross between "SK308" and "18CR3", which carried CRa, exhibited resistance to clubroot under the natural conditions of disease-prone fields in Wulong, China. In summary, through a combination of traditional breeding methods and MAS, we successfully bred five cabbage inbred lines carrying the CRa gene from diverse genetic backgrounds, thereby establishing a robust foundation for their integration into breeding programs.

Supplementary information: The online version contains supplementary material available at 10.1007/s11032-024-01532-2.