24-Epibrassinolide modulates antioxidant regulation and redox homeostasis in soybean exposed to cadmium stress

Abstract Cadmium (Cd) is a toxic heavy metal that severely impairs plant growth and productivity by disrupting cellular homeostasis. This study investigated the effects of foliar-applied 24-Epibrassinolide (EBL) on antioxidant capacity and redox coordination in Cd-stressed Glycine max (L.) through integrated physiological, biochemical, and transcriptional analyses. Seedlings were grown hydroponically and exposed to 20 or 40 mg L − 1 Cd for 10 days, while 1 µM EBL was applied as a foliar treatment. Increasing Cd concentrations reduced plant growth and photosynthetic pigment contents, whereas Cd accumulation in leaves increased markedly. The combined application of Cd and EBL significantly improved shoot growth and pigment contents compared to plants treated with Cd alone. Cadmium stress increased hydrogen peroxide and malondialdehyde levels, while co-application of EBL significantly decreased these oxidative markers. Exposure to Cd stimulated the activities of key antioxidant enzymes, including superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase, with EBL application further enhancing these activities, particularly at 20 mg L − 1 Cd. In addition, EBL promoted the accumulation of non-enzymatic antioxidants, including proline, phenolics, and flavonoids. Transcriptional analysis revealed that EBL coordinately regulates antioxidant-related gene expression, supporting a gene-specific response under Cd stress. Overall, foliar application of EBL strengthened antioxidant defences and maintained redox homeostasis, thereby improving soybean tolerance, particularly at the lower Cd concentration.