Brems1 Mutation Induced Tapetum Deficiency Leading to Male Sterility in Chinese Cabbage (Brassica Rapa L. Ssp. Pekinensis)

The mutation in Brems1 resulting in male sterility in Chinese cabbage were validated through two allelic mutations. Male sterile lines are ideal for hybrid seed production in Chinese cabbage. Herein, the complete male sterile mutants M5026 and M5073 were obtained through ethyl methanesulfonate (EMS) mutagenesis in the Chinese cabbage double haploid line 'FT'. Cytological observations revealed that M5026 exhibited an absence of the tapetum, an overabundance of microsporocytes, and abnormal exine formation in pollen. The male sterility phenotype of M5026 was controlled by a single recessive nuclear gene. Using mutmap sequencing and kompetitive allele-specific PCR (KASP) identification and gene cloning, two distinct SNPs in BraA10g029920.3.5C, encoding EMS1 (excess microsporocytes 1), were identified to be associated with the male sterility of M5026 and M5073. The gene was named as Brems1. M5026 and M5073 were determined to be allelic variants. Both BrEMS1 and Brems1 were subcellularly localized at the cell membrane. Brems1 exhibited the highest expression level in buds, while no expression was detected in roots. Transcriptomic analysis revealed that Brems1 mutations reduced the expression levels of genes associated with the tapetum, pollen tube, and LRR-RLK family. These results suggested that Brems1 played a critical role in pollen development and contributes to elucidating the molecular mechanisms underlying tapetum development and male sterility in Chinese cabbage.