Transcriptomic insights into brassinolide-mediated enhancement of heat tolerance through photosynthetic regulation in tomato

Heat stress severely inhibits photosynthesis, leading to reduced crop yield and quality; however, the molecular mechanisms by which brassinosteroids (BRs) regulate photosynthetic processes in tomato remain elusive. This study demonstrated that exogenous application of 2,4-epibrassinolide (EBR) significantly alleviated heat-induced photoinhibition by coordinating light energy conversion and carbon assimilation. Physiological analyses revealed that EBR treatment preserved chloroplast ultrastructure integrity, reduced malondialdehyde (MDA) and superoxide anion (O 2 . − ) accumulation, and enhanced the maximum photochemical efficiency (Fv/Fm) and photosystem II efficiency (Y(II)), respectively, compared to heat-stressed controls. EBR promoted stomatal opening and transpiration, thereby lowering leaf temperature, while also upregulating the activity and gene expression of key Calvin cycle enzymes, including Rubisco, FBPase, and NADP-GAPDH, ensuring efficient carbon fixation. Transcriptome analysis identified 718 shared differentially expressed genes (DEGs) responsive to both heat and EBR, which were significantly enriched in photosynthesis-antenna proteins, photosynthesis, and carbon fixation pathways. Weighted gene co-expression network analysis (WGCNA) further revealed that the MYB (Solyc02g036370.3) and WRKY (Solyc10g084380.1) transcription factors may be potential key mediators linking BR signaling to the transcriptional regulation of photosynthesis-related genes, including CAB , PsbO , PsaC , and Rbc S . These findings elucidate that EBR enhances tomato heat tolerance by maintaining the photosynthetic electron transport chain and promoting Calvin cycle efficiency through specific transcriptional networks. Under heat stress conditions, BR signaling regulates the expression of downstream photosynthesis-related genes (such as PsbO, PsaD, RbuS, FBPase) by activating key transcription factors (MYB and WRKY), thereby enhancing the light energy capture and electron transfer efficiency of the photosystem, maintaining the stability of Calvin cycle and gas exchange, and ultimately significantly improving the adaptability of tomato to heat stress. A schematic diagram of the mechanism of high temperature and BR on tomato; the arrows in the figure represent promotion and absorption. MYB represents the transcription factor Solyc02g036370.3; WRKY represents transcription factor Solyc10g084380.1; the name of the slant represents the photosynthesis-related differential genes; the circle composed of dotted lines represents the nucleus; the dotted arrow represents an indirect connection between the two, and the solid arrow represents a direct connection between the two.