Plants are able to sense UV-B through the UV-B photoreceptor UVR8. UV-B photon absorption by a UVR8 homodimer leads to UVR8 monomerization and interaction with the downstream signaling factor COP1. This then initiates changes in gene expression, which lead to several metabolic and morphological alterations. A major response is the activation of mechanisms associated with UV-B acclimation and UV-B tolerance, including biosynthesis of sunscreen metabolites, antioxidants and DNA repair enzymes. To balance the response, UVR8 is inactivated by regulated re-dimerization. Apart from their importance for plants, UVR8 and its interacting protein COP1 have already proved useful for the optogenetic toolkit used to engineer synthetic light-dependent responses.