Integrated Physio-Biochemistry and Transcriptome Analysis Reveals the Mechanism of 24-Epibrassinolide in Alleviating...

Cadmium (Cd) contamination is widely recognized as a major risk factor affecting the security and quality of crop production. Watermelon (Citrullus lanatus) is a globally cultivated fruit that is susceptible to Cd stress. 24-Epibrassinolide (EBR), an active brassinosteroid, is essential for plant growth and abiotic stress responses. However, its protective role in watermelon under Cd stress remains unclear. This study elucidates the physiological and molecular processes underlying EBR-mediated alleviation of Cd toxicity in watermelon seedlings. The results showed that exogenous EBR application effectively mitigated Cd-induced growth inhibition through decreased Cd deposition, reduced the accumulation of reactive oxygen species (ROS), lowered membrane lipid peroxidation, and increased antioxidant capacity in watermelon leaves under Cd treatment. Transcriptome (RNA-Seq) analysis revealed that EBR triggered substantial reprogramming of gene expression patterns, identifying 530 differentially expressed genes (DEGs) in Cd + EBR co-treatment compared with Cd treatment alone, including 204 down-regulated genes and 326 up-regulated genes. These DEGs are vital for controlling several physiological processes, including phenylpropane metabolism, phenylpropanoid biosynthesis, endoplasmic reticulum's protein production, cell wall organization, and others. Further physiological assays confirmed that EBR increased the activities of PAL and 4CL, the core enzymes driving phenylpropanoid biosynthesis, leading to a significant accumulation of total phenols and flavonoids. Together, the above results give concrete proof of the powerful functions of 24-EBR, acting as an enhancer of plant performance under Cd stress by enhancing the antioxidant system and by activating the phenylpropanoid pathway and its derived metabolic networks.