Reversion of the Arabidopsis UV-B photoreceptor UVR8 to the homodimeric ground state

Plants require the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) for acclimation and survival in sunlight. Upon UV-B perception, UVR8 switches instantaneously from a homodimeric to monomeric configuration, which leads to interaction with the key signaling protein CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and induction of UV-B–protective responses. Here, we show that UVR8 monomerization is reversible in vivo, restoring the homodimeric ground state. We also demonstrate that the UVR8-interacting proteins REPRESSOR OF UV-B PHOTOMORPHOGENESIS (RUP)1 and RUP2 mediate UVR8 redimerization independently of COP1. UVR8 redimerization consequently disrupts the UVR8–COP1 interaction, which halts signaling. Our results identify a key role of RUP1- and RUP2-mediated UVR8 redimerization in photoreceptor inactivation, a crucial process that regenerates reactivatable UVR8 homodimers.