An Insertion Mutation in Encoding a Histone Methyltransferase Is Responsible for Early Bolting in Chinese Cabbage ( L. ssp. ).

Bolting is an important agronomic character of the Chinese cabbage, but premature bolting can greatly reduce its commercial value, yield, and quality. Here, early-bolting mutant 1 () was obtained from a Chinese cabbage doubled haploid (DH) line "FT," by using an isolated microspore culture and ethyl methanesulfonate (EMS) mutagenesis. The was found to bolt extremely earlier than the wild type "FT." Genetic analysis indicated that the phenotype of the was controlled by a single recessive nuclear gene. Using a mapping population of 1,502 recessive homozygous F individuals with the phenotype, the gene was mapped to between the markers SSRhl-53 and SSRhl-61 on chromosome A04 by using SSR markers, and its physical distance was 73.4 kb. Seven genes were predicted in the target region and then cloned and sequenced; the only difference in the sequences of the and "FT" genes was with . Unlike that in "FT," the in had a novel 53 bp insertion that caused the termination of amino acid coding. The mutation was not consistent with EMS mutagenesis, and thus, may have been caused by spontaneous mutations during the microspore culture. Based on the gene annotation information, was found to encode the histone methyltransferase () in . regulates the expression of flowering-related genes. Further genotyping revealed that the early-bolting phenotype was fully co-segregated with the insertion mutation, suggesting that was the most likely candidate gene for . No significant differences were noted in the expression levels between the and "FT." However, the expression levels of the flowering-related genes , , , and were significantly higher in the than in the "FT." Thus, the mutation of is responsible for the early-bolting trait in Chinese cabbage. These results provide foundation information to help understand the molecular mechanisms of bolting in the Chinese cabbage.