Natural variation in a calreticulin gene causes reduced resistance to Ca2+ deficiency‐induced tipburn in Chinese cabbage (Brassica rapa ssp. pekinensis)

Tipburn is an irreversible physiological disorder of Chinese cabbage that decreases crop value. Because of a strong environmental component, tipburn‐resistant cultivars are the only solution, although tipburn resistance genes are unknown in Chinese cabbage. We studied three populations of Chinese cabbage over four growing seasons under field conditions: (a) 194 diverse inbred lines, (b) a doubled haploid (DH100) population, and (c) an F₂ population. The 194 lines were genotyped using single nucleotide polymorphism markers, and genome‐wide‐association mapping showed that 24 gQTLs were significantly associated with tipburn disease index. Analysis of the DH100 and F₂ populations identified a shared tipburn‐associated locus, gqbTRA06, that was found to cover the region defined by one of the 24 gQTLs. Of 35 genes predicted in the 0.14‐Mb quantitative trait locus region, Bra018575 (calreticulin family protein, BrCRT2) showed higher expression levels during disease development. We cloned the two BrCRT2 alleles from tipburn‐resistant (BrCRT2^R) and tipburn‐susceptible (BrCRT2^S) lines and identified a 51‐bp deletion in BrCRT2^S. Overexpression of BrCRT2^R increased Ca²⁺ storage in the Arabidopsis crt2 mutant and also reduced cell death in leaf tips and margins under Ca²⁺‐depleted conditions. Our results suggest that BrCRT2 is a possible candidate gene for controlling tipburn in Chinese cabbage.