The CRY1-COP1-HY5 axis mediates blue-light regulation of Arabidopsis thermotolerance.

High-temperature stress, also referred to as heat stress, often has detrimental effects on plant growth and development. Phytochromes have been implicated in regulating plant heat stress responses, but the role of blue-light receptors, such as cryptochromes, in plant blue light-dependent heat stress response has remained unclear. We found that the blue light receptor cryptochrome 1 (CRY1) negatively regulates heat stress tolerance (thermotolerance) in Arabidopsis. Heat stress represses CRY1 phosphorylation. Unphosphorylated CRY1 exhibits decreased activity suppressing CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and ELONGATED HYPOCOTYL 5 (HY5) interaction, leading to the excessive degradation of HY5 under heat stress in blue light. This reduction in HY5 protein levels subsequently relieves its repression on the transcription of HY5 target genes, especially the heat shock transcription factors (HSFs). Our study unveils a novel mechanism by which CRY1-mediated blue-light signaling suppresses plant thermotolerance and highlights the dual function of CRY1-COP1-HY5 module in both light and heat stress signaling, providing insights on how heat stress and light signals integrate to optimize plant survival under heat stress.