Molecular Mechanisms of the Phytohormone–Heat Shock Protein Pathway in Regulating Plant Thermotolerance

Heat stress caused by global climate change poses a significant threat to agricultural production. Phytohormones, as critical signaling molecules, play pivotal roles in modulating plant responses to heat stress. This review systematically summarizes the molecular mechanisms by which eight phytohormones (auxin, gibberellin, cytokinin, ethylene, abscisic acid, brassinosteroid, salicylic acid, and strigolactone) enhance plant thermotolerance through the regulation of heat shock protein (HSP) expression and function. Specifically, auxin enhances thermotolerance by inducing auxin signaling repressor (Aux/IAA) degradation to upregulate HSP transcription, facilitating the formation of the auxin receptor (TIR1)-HSP90 complex to stabilize TIR1, and forming the auxin exporter (PIN)-HSP22 complex to promote auxin transport. Cytokinin enhances thermotolerance by upregulating HSP transcription, with stronger effects in leaves than roots. Gibberellin, salicylic acid, and ethylene enhance thermotolerance primarily by activating heat shock factor (HSF) to induce HSP transcription. Abscisic acid and brassinosteroid improve thermotolerance by inducing HSP transcription and HSP phosphorylation, while strigolactone acts via D14-mediated upregulation of HSP transcription. These phytohormones collaboratively regulate HSPs, forming an intricate network to enhance plant thermotolerance. Deciphering these mechanisms provides a theoretical framework for developing heat-resistant crops and optimizing cultivation techniques.