HY5 enhances Arabidopsis tolerance to combined high light and heat stress by coordinating photoprotection and hormone signaling.

High light (HL) and heat stress (HS) are two major abiotic factors that commonly co-occur in nature and severely impair photosynthetic performance when combined. The bZIP transcription factor HY5 is a well-known integrator of light and temperature cues, but its role under combined HL + HS stress remains largely unexplored. Here, we investigated the role of HY5 in Arabidopsis tolerance to HL + HS using wild-type (Col-0), HY5-deficient (hy5-215), and HY5-overexpressing (HY5OX) lines. Physiological and biochemical analyses revealed that HY5OX plants maintained higher photosynthetic efficiency, lower membrane damage, and improved leaf health under HL + HS, while hy5-215 mutants were hypersensitive. Proteomic profiling showed that HL + HS induced distinct HY5-dependent changes in the accumulation of photosynthesis-related proteins, particularly Photosystem II core subunits D1 and D2. NPQ4/PsbS, a key component of non-photochemical quenching (NPQ), was related to the presence of HY5, with impaired NPQ activation in hy5-215 correlating with lower Fv/Fm and higher increased oxidative damage. Hormonal profiling further revealed that HY5 is required for ABA and JA signaling under HL + HS. Our findings highlight HY5 as a central regulator of tolerance to combined HL + HS stress, acting through the transcriptional coordination of photoprotective proteins and hormonal signaling networks.