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. 2019 Feb 20;116(10):4722–4731. doi: 10.1073/pnas.1816268116

Fig. 6.

Fig. 6.

Working model for the coordinated regulation of photomorphogenesis by COP1, HY5, RUP1/RUP2, and CUL4-DDB1. During their emergence from the soil to establish photomorphogenesis, seedlings encounter increasing levels of UV-B light. In darkness, COP1, together with CUL4-DDB1, represses photomorphogenesis by mediating HY5 degradation. In a prolonged response to low light with UV-B, the E3 ligase responsible for HY5 degradation switches from CUL4-DDB1-COP1 to CUL4-DDB1-RUP1/RUP2. Meanwhile, COP1 directly targets RUP1/RUP2 for degradation, leading to balanced accumulation of RUP1/RUP2, alleviation of the COP1–HY5 interaction, and stabilization of HY5. Once UV-B light is removed, RUP1 and RUP2 function redundantly to mediate UVR8 redimerization to halt UVR8 signaling. As a result, RUP1/RUP2, CUL4-DDB1, COP1, and HY5 constitute the repression and derepression machinery by which plants develop photomorphogenesis.