Phosphorylation of the plant photoreceptor UVR8 regulates its dimer/monomer status and enhances UV-B signaling across species.

Ultraviolet-B (UV-B) is an integral component of sunlight that profoundly shapes plant growth, development, and survival. Plant photoreceptor UV RESISTANCE LOCUS 8 (UVR8) homodimers perceive UV-B and dissociate into active monomers to initiate signaling, yet how UVR8 dimer stability and monomer activity are post-translationally regulated remains unclear. We identify UV-B-enhanced phosphorylation at threonine 356 of UVR8, which promotes UV-B-induced UVR8 dimer dissociation and delays redimerization, shifting dimer/monomer equilibrium towards active monomer in Arabidopsis. Importantly, the same regulatory mechanism is conserved in tomato, demonstrating that phosphorylation at this site represents a broadly applicable molecular switch across species. Moreover, this modification enhances the binding of UVR8 to CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), prolongs UVR8 nuclear residence, and amplifies UV-B signaling outputs. Together, these findings uncover phosphorylation at T356 as an evolutionarily conserved molecular switch that fine-tunes photoreceptor conformational change and signaling, highlighting phosphorylation as a critical posttranslational control of light perception in plants.