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

Key message Mapping and resequencing of two allelic early bolting mutantsebm5-1andebm5-2revealed that theBrSDG8gene is related to bolting in Chinese cabbage (Brassica rapassp.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,ebm5-1andebm5-2, 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 linesebm5-1andebm5-2were located to the same region in chromosome A07. Using the 1741F(2)individuals with theebm5-1phenotype 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 theBraA07g040740.3C(BrSDG8) region by whole-genome resequencing ofebm5-1mutant and 'FT.'BrSDG8is a homolog ofArabidopsis thaliana SDG8encoding a histone methyltransferase affecting H3K4 trimethylation inFLOWERING LOCUS Cchromatin. Comparative sequencing established that the SNP occurred onBrSDG817th exon inebm5-1. Genotype analysis showed full co-segregation of the early bolting phenotype with this SNP. Cloning of allelic mutantebm5-2indicated that it harbors a deletion mutation on the 12th exon ofBrSDG8. Quantitative real-time PCR analysis indicated thatBrSDG8expression level was observably lower in mutantebm5-1than in 'FT.' Overall, the present results provide strong evidence thatBrSDG8mutation leads to early bolting in Chinese cabbage, thereby providing a basis to understand the molecular mechanisms underlying this phenotype.