Brassinosteroid‐Mediated <scp> <i>BraSERKs</i> </scp> Regulation and Antioxidant Defense Enhancement Confer Cd/Zn Stress Tolerance in Chinese Cabbage

Somatic embryogenesis receptor kinases (SERKs) are pivotal regulators of plant development and stress adaptation, known to integrate multiple signaling pathways, including brassinosteroid (BR)-mediated responses to coordinately modulate stress-related gene expression. While SERKs participate in biotic/abiotic stress regulation, their roles in heavy metal (HM) stress responses and BR-mediated transcriptional control under HM exposure remain unexplored. Therefore, we systematically identified 20 BraSERK genes in Chinese cabbage (Brassica rapa subsp. Pekinensis), revealing conserved domain architectures and stress-responsive expression patterns. Three BraSERK members (BraSERK9, 12, and 17) exhibited significant induction under Cd/Zn stress. The potential regulatory roles of BR on Chinese cabbage exposed to Cd/Zn stress were investigated in this study by exogenous spraying of 24-epibrassinolide (EBR). The results showed that EBR could significantly increase total chlorophyll content, reduce Zn/Cd translocation, increase antioxidant capacity, and thus attenuate Cd/Zn-induced toxicity. Moreover, EBR dynamically regulated BraSERKs expression in a tissue- and metal-specific manner, potentiating BraSERK12/17 induction in Zn-stressed shoots while suppressing BraSERK9. Our findings suggest that BraSERKs may contribute to BR-mediated HM adaptation, while EBR appears to play a dual protective role through both direct phytoprotection and potential modulation of SERK pathway activity under Cd/Zn co-stress conditions. This study advances our understanding of BR-mediated metal tolerance in crops, offering potential strategies to mitigate HM toxicity through targeted SERK pathway modulation.