Brassinosteroid regulates thermotolerance through affecting the antioxidant capacity and the auxin pathway under heat stress in Brassica rapa ssp. pekinensis.

Heat stress threatens the yield and quality of plants. Brassinosteroid (BR) is widely used in agricultural production and plays important roles in regulating plant growth and stress response, but it is rarely reported on its function in thermotolerance in Chinese cabbage. In this study, we found that BR improves thermotolerance by alleviating the inhibition of leaf growth, and the BR biosynthesis inhibitor brassinazole (BRZ) treatment significantly reduced thermotolerance and plant growth in Chinese cabbage. Under heat stress, the accumulation of malondialdehyde and H2O2 was inhibited, and antioxidant-related enzyme activities and the contents of soluble protein and free proline were increased under BR treatment. Multiple transcriptome assays showed that BR may affect the expression levels of some BrSAUR genes in the auxin pathway, and ABA biosynthesis and metabolism genes to improve heat tolerance. Dual-luciferase reporter assay and VIGS assay showed that BrBZR1 can activate BrSAUR9 expression and the BrSAUR9-silenced plants inhibit plant growth. Furthermore, BR alleviated photosynthetic system damage and increased the fresh weight under heat stress. Our results provide new insights about the function of BR in conferring thermotolerance and biomass increase under heat stress in Chinese cabbage.