Improving clubroot resistance of cabbage cultivar ‘Xiyuan 4’ by foreground marker assisted selection and whole-genome re-sequencing background assisted selection

Cabbage is severely threatened by clubroot disease worldwide. To improve the clubroot resistance of cabbage, clubroot resistance from ‘9LQ18-5’ was introgressed into the susceptible parent ‘F416’ of commercial cabbage variety ‘Xiyuan 4’. During the backcrossing program, clubroot resistant individuals were selected by CCR1 and CCR2 markers, whereas the background of backcrossing progenies was detected by whole genome re-sequencing in BC₃F₁ individuals. Among them, 21 − 9 with the highest recurrent parent genomic recovery (83.93%) was identified, and a 47 kb interval (42,986,635 − 43,033,590 bp) on chromosome C07 was defined linking with clubroot resistance. Based on the whole genome re-sequencing, 49 indel markers were developed for background selection of the individuals carrying CCR1 and CCR2 markers in BC₄F₁. Progenies derived from line 21 − 9 were screened for developing BC₄F₂, which exhibited ~ 90% background recovery rate, high morphological similarity to the recurrent parent ‘F416’, and high resistance to clubroot upon artificial inoculation. Among eight hybrids between BC₄F₂ individuals and ‘E2’ (the other parent of ‘Xiyuan 4’), ‘CR64’ displayed comparable agronomic traits to ‘Xiyuan 4’ alongside significantly high clubroot resistance in the field. These results demonstrated that new ‘F416’ line with clubroot resistance was developed, and it could be used for breeding ‘Xiyuan 4R’ variety with enhanced clubroot resistance in cabbage.