Brems1 mutation induced tapetum deficiency leading to male sterility in Chinese cabbage

Male sterile lines are ideal materials for hybrid seed production in Chinese cabbage. Herein, the male sterile mutant M5026, harboring the loss of tapetum and an excess of microsporocytes, was obtained through ethyl methanesulfonate (EMS) mutagenesis in the cabbage double haploid line FT. Transmission Electron Microscopy (TEM) revealed abnormal exine formation in M5026 microspores. Genetic analysis revealed that the male sterile phenotype of M5026 was caused by a single recessive nuclear gene. By Mutmap sequencing and Kompetitive Allele Specific PCR (KASP) identification, BraA10g029920.3.5C, encoding EMS1, a leucine-rich repeat receptor-like protein kinase, was identified as the candidate gene and named Brems1. Both BrEMS1 and Brems1 were subcellularly localized at the cell membrane. qRT-PCR analysis indicated that Brems1 exhibited the highest expression level in flower buds, while no expression was detected in roots. Transcriptomic analysis revealed that the mutation in Brems1 reduced the expression of genes associated with tapetum, pollen tube, LRR-RLK family and metabolism. These results suggested that Brems1 played a critical role in pollen development and contributed to elucidating the molecular mechanisms underlying tapetum development and male sterility in Chinese cabbage.