HY5: a transcription factor integrating environmental and endogenous signals in plant development and acclimation.

As a master bZIP transcription factor, ELONGATED HYPOCOTYL 5 (HY5) coordinates plant growth with environmental acclimation across developmental stages. Beyond its role in photomorphogenesis, HY5 integrates diverse environmental and endogenous signals, linking environmental perception with developmental and metabolic programs to ensure coordinated physiological adaptation. This review synthesizes current understanding of the multilayered regulatory networks controlling HY5, ranging from constitutive photomorphogenic 1/suppressor of phytochrome A (COP1/SPA)-mediated ubiquitin-dependent degradation, transcriptional regulation by upstream factors, and extensive post-translational modifications, to chromatin-level mechanisms that shape HY5-dependent transcriptional outputs. We further summarized how HY5 translates these integrated signals into physiological responses, with particular emphasis on its roles in hormonal crosstalk involving abscisic acid, brassinosteroids, gibberellins, ethylene, and jasmonates, as well as its functions in shaping root system architecture through systemic shoot-to-root communication and local light perception. In addition, HY5 contributes to plant tolerance against multiple abiotic stresses, including UV-B irradiation, extreme temperatures, salinity, drought, and oxidative stress, by coordinating transcriptional reprogramming and metabolic adjustment. Finally, we highlight key unresolved questions regarding the spatiotemporal regulation of HY5, and discuss the potential of targeting HY5-centered regulatory networks for engineering climate-resilient crops with improved growth, stress tolerance, and productivity under dynamic environmental conditions.