Integrated Phylogenomics and Expression Profiling of the MED Gene Family in Brassica napus Uncover Their Roles in Plant Development and Stress Tolerance

Rapeseed (Brassica napus L.) is a globally vital oilseed crop for edible oil, biofuel and animal feed production, yet its yield and quality are severely threatened by a narrow genetic foundation, scarce germplasm and recurrent abiotic stresses including drought and salinity. Mediator subunits have been proven to be critical for abiotic stress tolerance in model plants, but systematic research on the Brassica napus Mediator (BnaMED) gene family and its stress regulatory mechanism in Brassica napus remains largely lacking. In this study, we identified 189 BnaMED members phylogenetically clustered into five subfamilies, with extensive family expansion mainly driven by whole-genome duplications, accompanied by functional differentiation and redundancy to bolster polyploid adaptability. Chromosomal mapping revealed 176 BnaMED genes unevenly distributed across A/C subgenomes. Through gene collinearity analysis among Brassica napus, Arabidopsis thaliana, Glycine max, Oryza sativa, and Zea mays, we found that Brassica napus shares high collinearity with dicotyledonous species, such as Glycine max and Arabidopsis thaliana, indicating evolutionary conservation specific to dicots. Protein physicochemical properties and promoter cis-element analysis uncovered diverse structural features and extensive involvement in light, hormone and stress signaling pathways. Tissue expression profiling demonstrated distinct subfamily specificity and functional divergence. The quantitative real-time PCR (qRT-PCR) analysis revealed that under salt stress, endogenous transcript levels of BnaMED family members BnaA09g34000D, BnaA09g23620D, BnaC03g31860D and BnaA06g10800D were up-regulated, whereas BnaA08g23170D and BnaA08g22420D were transcriptionally repressed. Following drought-stress treatment, all six genes (BnaA09g34000D, BnaA09g23620D, BnaA09g50540D, BnaA06g10800D, BnaA08g23170D and BnaA08g22420D) displayed markedly reduced expression. These observations reflect stress-specific regulatory strategies underlying abiotic-stress adaptation. In addition, several BnaMED genes showed dynamic expression during floral transition in the semi-winter Zhong Shuang 11 (ZS11) cultivar. This study provides a foundational insight into functional roles of BnaMED genes in rapeseed abiotic stress responses.