Dual-Mechanism Induced by 24-Epibrassinolide Synergistically Improves Anatomical Structures and the Antioxidant Defense System in Soybean Plants Under Cadmium Excess

Cadmium (Cd) accumulation in plant tissues causes several damages, including disturbances in anatomical structures, negative impacts on photochemical reactions, and reducing the efficiency of the photosynthetic apparatus. 24-Epibrassinolide (EBR) is a plant steroid that regulates multiple physiological and biochemical processes to counteract the harmful effects of metal stress. The aim of this research was to investigate whether exogenous EBR application affects leaf and root anatomical structures, including stomatal responses, redox-metabolism-related biochemical responses intrinsically related to photosynthetic apparatus, and nutritional status in soybean plants under Cd excess. The experiment was randomized with four treatments: two cadmium concentrations (0 and 500 µM Cd, described as −Cd and +Cd, respectively) and two EBR levels (0 and 100 nM EBR, described as −EBR and +EBR, respectively). Results demonstrated that EBR positively regulated root and leaf structures and stomatal performance, with significant increases in epidermis and cortex (root) and benefits for spongy parenchyma and stomatal density (leaf), clearly protecting the photosynthetic apparatus against Cd excess. Simultaneously, this steroid mitigated Cd-induced oxidative stress by stimulating the activities of superoxide dismutase (25%), catalase (28%), ascorbate peroxidase (30%) and peroxidase (48%), while simultaneously reducing the content of oxidative compounds, including superoxide (16%), hydrogen peroxide (8%), malondialdehyde (12%) and electrolyte leakage (14%). The dual mechanism modulated by EBR protected anatomical structures and stimulated antioxidant defense. Therefore, the results prove that exogenous EBR application effectively attenuates the adverse effects of Cd excess in soybean plants.