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

Abstract

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 (BrCRT2R) and tipburn-susceptible (BrCRT2S) lines and identified a 51-bp deletion in BrCRT2S. Overexpression of BrCRT2R 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.