Illuminating UVR8: Signal Transduction Pathways of the Plant UV-B Photoreceptor

Ultraviolet-B (UV-B) radiation is an intrinsic component of the solar spectrum that acts as an environmental cue, exerting a strong influence on plant physiology, morphology, and environmental acclimation. UV RESISTANCE LOCUS 8 (UVR8) is recognized as the sole specific UV-B photoreceptor, mediating perception and initiating a sophisticated signaling cascade that facilitates developmental and protective responses. UV-B irradiation triggers the dissociation of the cytosolic UVR8 homodimer into biologically active monomers. This structural transition enables rapid, regulated nuclear translocation, where the UVR8 monomer interacts with the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1). This interaction, involving a critical two-interface binding mechanism, inhibits COP1 activity toward key transcription factors, notably ELONGATED HYPOCOTYL 5 (HY5), thereby stabilizing them and orchestrating the UV-B acclimation transcriptome. Furthermore, UVR8 functions as a crucial hub for signaling integration, directly modulating multiple phytohormone pathways, coordinating spectral responses with other photoreceptors, and regulating novel non-canonical modules in both the nucleus and the cytoplasm. This comprehensive review examines the molecular architecture, photocycle dynamics, integrated signaling mechanisms, and critical physiological roles of UVR8. Finally, we provide perspectives on unresolved questions concerning its full array of post-translational modifications and the potential to apply this knowledge to enhance crop resilience.