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

Plants are frequently exposed to multiple simultaneous environmental stresses, yet the mechanisms that underlie their tolerance to such combinations remain poorly understood. 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 HLHS stress remains largely unexplored. Here, we investigated the role of HY5 in Arabidopsis tolerance to HLHS 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 HLHS, while hy5-215 mutants were hypersensitive. Proteomic profiling showed that HLHS 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 identified as a direct downstream target of HY5, with impaired NPQ activation in hy5-215 and npq4-1 mutants correlating with higher Y(NO), lower Fv/Fm, and increased oxidative damage. Hormonal profiling further revealed that HY5 is required for proper ABA and JA signaling under HLHS: hy5-215 mutants failed to induce ABA accumulation and showed disrupted JA signaling despite strong hormone accumulation. Our findings highlight HY5 as a central regulator of tolerance to combined HLHS stress, acting through the transcriptional coordination of photoprotective proteins and hormonal signaling networks. This work provides new insights into how plants integrate environmental and hormonal cues to protect the photosynthetic machinery under multifactorial stress.