Upon the irradiation of ultraviolet-B(UV-B)light,UVR8 photoreceptor can undergo dissociation of the protein homodimer and regulate gene expression in plants.We have carried out theoretical calculations and molecular dynamics simulations to study spectra of key tryptophan residues in UVR8 homodimer and to clarify the key role of important charged residues as well as the feasible dissociation mechanism.The present results suggest that W285 is the major chromophore of UVR8,while W233 can also sense the UV-B light and may be responsible for exciton coupling.Geometrical effects as well as electrostatic and polarization interactions with the protein matrix were found to influence optical properties of these tryptophan residues in UVR8.In addition,the UV-B induced dissociation mechanism of the UVR8 homodimer has been proposed.The electrostatic repulsion between the partially-negatively charged benzene ring of W285 in the 1La excited state and the negatively-charged D44/D107,along with electron and/or proton transfers among W285,R286(or R338),W233 and D129,was suggested to result in the breakage of the key salt bridges,and destabilization as well as dissociation of the UVR8 dimer.The proposed mechanism also accounts for the fluorescence quenching in UVR8,and the stability as well as the enhanced red-shifted fluorescence in the W285F mutant.