Although DWD HYPERSENSITIVE TO UV-B1 (DHU1) is reported to be a negative regulator in UV-B mediated cellular responses, its detailed role in UV-B signaling is still elusive. To further understand the action mechanism of DHU1 in UV-B response, physical and genetic interactions of DHUI with various UV -B signaling components were investigated. Yeast two hybrid assay results suggested that DHU1 directly interacts with COP1 and RUP1, implying a functional connection with both COPI and RUP1. In spite of the physical association between DHU1 and COPI, loss of DHU1 did not affect protein stability of COPI. Epistatic analysis showed that the functional loss of both DHU1 and UVR8 leads to alleviation of UV -B hypersensitivity displayed in dhul-1. Moreover, phenotypic studies with dhul-1 copl-6 and dhul-1hy5-215 revealed that COP1 and HY5 are epistatic to DHU1, indicating that UV -B hypersensitivity of dhu-11 requires both COPI and HY5. In the case of dhul-1 rupl-1, UV-B responsiveness was similar to that of both dhul-1 and rupl-1, implying that DHU1 and RUP1 are required for each other's function. Collectively, these results show that the role of DHU1 as a negative regulator in UV-B response may be derived from its direct interaction with COP1 by sequestering COPI from the active UVR8-COP1 complex, resulting in a decrease in the COP1 population that positively participates in UV-B signaling together with UVR8. Furthermore, this inhibitory role of DHU1 in UV-B signaling is likely to be functionally connected to RUP1. This study will serve as a platform to further understand more detailed action mechanism of DHU1 in UV-B response and DHU1-mediated core UV-B signaling in Arabidopsis. (C) 2017 Elsevier Inc. All rights reserved.