Coordinated physiological and molecular reprogramming by brassinosteroids improves soybean tolerance to combined salt and drought stress

This study investigates how brassinosteroids (BRs) enhance stress tolerance in soybean under combined salt and drought stress by examining growth, chlorophyll content, photosynthesis, and reactive oxygen species (ROS) homeostasis. Salt and drought stress significantly reduced soybean growth and photosynthetic performance, as reflected by lower SPAD chlorophyll values and decreased photosystem II (PSII) efficiency. In contrast, BR (24-epibrassinolide, EBL) significantly improved growth parameters and spectral indices, indicating a healthier pigment status and improved canopy function. EBL-treated plants also exhibited enhanced PSII performance, as indicated by increased Fv/Fm and a higher performance index (PI). Furthermore, BRs modulated ROS levels and promoted cellular homeostasis by elevating the activities of antioxidant enzymes such as APX, CAT, and POX, thereby mitigating oxidative damage. Consistently, expression of key stress-responsive genes (<i>GmCAT, GmSOD</i>, and <i>GmP5CS</i>) was strongly induced under combined salt, drought, and EBL treatment, highlighting the synergistic role of EBL in transcriptional activation under combined stress. EBL treatment increased the proline content and the activities of ProDH and P5CS, supporting proline-mediated osmoprotection, while BR-treated plants exhibited reduced malondialdehyde (MDA) accumulation and electrolyte leakage (EL), indicating lower lipid peroxidation and better membrane integrity under stress. Overall, this study demonstrates that EBL enhances soybean resilience to combined salt and drought stress by improving growth, photosynthetic efficiency, antioxidant defense, osmotic adjustment, and membrane stability.