Physiological and Molecular Mechanisms of Brassinolide Alleviating Tomato Blossom-End Rot by Regulating Calcium Uptake and Antioxidant System

Blossom-end rot (BER) in tomatoes is a physiological disorder primarily caused by disrupted calcium absorption and transport. This study aims to investigate the physiological and molecular mechanisms by which exogenous brassinolide (BR) reduces the incidence of tomato BER under calcium-deficient conditions. The results showed that under calcium deficiency, foliar spraying of BR significantly reduced the BER incidence (from 26.67% to 6.67%) and effectively increased calcium ion content in the leaves, stems, roots, and other parts of the plant. At the physiological level, BR treatment significantly enhanced the activities of CAT, POD, and SOD in the leaves (by 105.70%, 117.12%, and 82.77%, respectively), while reducing the contents of H₂O₂ (by 36.90%) and MDA (by 16.38%). This indicates that BR alleviates membrane lipid peroxidation damage by enhancing the antioxidant defense system. Transcriptome analysis further revealed that BR treatment identified 4807, 2807, and 2554 differentially expressed genes (DEGs) in the leaves, stems, and roots, respectively, with the most significant response observed in the leaves. GO functional enrichment analysis showed that these genes are mainly involved in biological processes such as secondary metabolic processes, response to oxygen-containing compounds, and cell wall organization. KEGG pathway analysis further indicated significant enrichment in pathways such as phenylpropanoid biosynthesis, plant hormone signal transduction, and plant-pathogen interaction. Additionally, several key genes, such as the cytochrome C oxidase gene (Solyc03g013460.1), exhibited a gradient up-regulation pattern of "root > stem > leaf" in the oxidative phosphorylation pathway, while photosynthesis-related genes (e.g., PsbA, PsaA, and ND family genes) were also generally up-regulated. In summary, BR likely enhances tomato tolerance to calcium deficiency stress and effectively reduces the occurrence of BER through multiple pathways, including regulating calcium absorption and distribution, activating the antioxidant system, modulating hormone signaling pathways, and enhancing energy metabolism. This provides a theoretical basis for the application of BR in agricultural production.