Rapid identification of the purple stem (Ps) gene of Chinese kale (Brassica oleracea var. alboglabra) in a segregation distortion population by bulked segregant analysis and RNA sequencing

Abundant anthocyanins accumulate in many flowers, leaves, fruits, as well as stems. In this study, an F-2 segregating population derived from a cross between Chinese kale with a purple stem and kale with a green stem was used to decipher the genetic basis for the stem color. Bulked segregant analysis in combination with RNA sequencing (BSR-seq) was used to genetically map the causal gene for the purple stem (Ps), and the allele frequency difference between the two pools showed a single peak on chromosome C09. Ps was fine-mapped to the 0.32 cM interval with 406 kb sequences. A gene-encoding dihydroflavonol 4-reductase (DFR), an enzyme in the anthocyanin biosynthesis pathway, was differentially expressed between the two pools, and was identified as the candidate gene for the purple stem in Chinese kale. Sequence analysis revealed that 1 bp was inserted in the coding sequence of exon 2 of the BoDRF gene in green kale, causing frameshift and loss of function. The expression level of the BoDFR gene was significantly lower than the Chinese kale cultivar with a purple stem. The genetic results of the Ps gene will be useful for future development of purple vegetables.