Role of BrSDG8 on bolting in Chinese cabbage (Brassica rapa)

Mapping and resequencing of two allelic early bolting mutants ebm5-1 and ebm5-2 revealed that the BrSDG8 gene is related to bolting in Chinese cabbage (Brassica rapa ssp. pekinensis). Bolting influences the leafy head formation and seed yield of Chinese cabbage therefore being an important agronomic trait. Herein, two allelic early bolting mutants, ebm5-1 and ebm5-2, stably inherited in Chinese cabbage were obtained from wild-type 'FT' seeds by ethyl methane sulfonate mutagenesis. Both mutants flowered significantly earlier than 'FT,' and genetic analysis revealed that the early bolting of the two mutants was controlled by one recessive nuclear gene. With BSR-seq, the mutations originating lines ebm5-1 and ebm5-2 were located to the same region in chromosome A07. Using the 1741 F₂ individuals with the ebm5-1 phenotype as the mapping population, this region was narrowed to 56.24 kb between markers InDel18 and InDel45. A single-nucleotide polymorphism (SNP) was aligned to the BraA07g040740.3C (BrSDG8) region by whole-genome resequencing of ebm5-1 mutant and 'FT.' BrSDG8 is a homolog of Arabidopsis thaliana SDG8 encoding a histone methyltransferase affecting H3K4 trimethylation in FLOWERING LOCUS C chromatin. Comparative sequencing established that the SNP occurred on BrSDG8 17th exon in ebm5-1. Genotype analysis showed full co-segregation of the early bolting phenotype with this SNP. Cloning of allelic mutant ebm5-2 indicated that it harbors a deletion mutation on the 12th exon of BrSDG8. Quantitative real-time PCR analysis indicated that BrSDG8 expression level was observably lower in mutant ebm5-1 than in 'FT.' Overall, the present results provide strong evidence that BrSDG8 mutation leads to early bolting in Chinese cabbage, thereby providing a basis to understand the molecular mechanisms underlying this phenotype.