UV-B radiations are a major threat for plants. However, they are also a source of information about the environment that influences plant growth and development. UV-B is perceived in plants by the UVR8 photoreceptor. Upon UV-B perception by specific tryptophan residues, the UVR8 photoreceptor converts from an inactive homodimer to a monomer. The UVR8 monomer can bind to the COP1 E3 ubiquitin ligase and thereby block the degradation of COP1 targets. Gene expression changes follow, mainly mediated by the transcription factor HY5. This involves the induction of genes such as CHS, HY5, HYH, RUP1 and RUP2. UVR8 signaling is then shut down via RUP1 and RUP2 which mediate the re-dimerization of the UVR8 photoreceptor back to its inactive state. Through the use of mutagenesis and genetic screening, we report in this study two mutants with an enhanced UV-B photomorphogenic phenotype, including a novel UVR8 allele and a new player in the UVR8 signaling pathway. The characterizations of those two mutants allow a better understanding of the mechanism and the major actors implicated in the UVR8 signaling pathway.