Ultraviolet-B (UV-B) is a key environmental signal that is specifically perceived by plants and promotes UV acclimation and survival in sunlight. We discovered recently that the Arabidopsis UVR8 protein is absolutely required for UV-B acclimation and functions as the major UV-B photoreceptor in plants: UVR8 dimer perception of UV-B photons leads to monomerization and direct interaction with the E3 ubiquitin ligase COP1, the central regulator of light signalling. We proposed that the initial UV-B perception step by the UVR8 photoreceptor involves the aromatic amino acid tryptophan as a chromophore. The UVR8 protein contains 14 tryptophans, localized mostly at the top of the predicted UVR8 beta-propeller structure. This hydrophobic surface is probably the surface through which UVR8 dimerizes, generating a tryptophan-rich “interphase” between the two monomers. Although it is clear from our combined genetic, physiological, molecular and biochemical data that UVR8 functions as the long-sought-after UV-B photoreceptor in plants, the mechanism of UV-B absorption and the immediate impact on the UVR8 protein and its interactors remains elusive. We would like to address this current major gap in photobiology in frame of the ERC project “UV-B Perception”. The proposed project will provide mechanistic insights into this newly discovered perception mechanism and photoreceptor pathway, which is of utmost importance for the survival of plants.