Wagnon, T. B. | Search bioRxiv

bioRxiv2021

BBX proteins promote HY5-mediated UVR8 signaling in Arabidopsis

Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), which binds substrates through their valine-proline (VP) motifs. The UV RESISTANCE LOCUS8 (UVR8) photoreceptor senses UV-B and inhibits COP1 through cooperative binding of its own VP motif mimicry and its photosensing core to COP1, thereby preventing COP1 binding to substrates, including the bZIP transcriptional regulator ELONGATED HYPOCOTYL5 (HY5). As a key promoter of visible light and UV-B photomorphogenesis, HY5 functions together with the B-box family transcription factors BBX20-22 that were recently described as HY5 rate-limiting coactivators under red light. Here we describe a hypermorphic bbx21-3D mutant with enhanced photomorphogenesis, which carries a proline-314 to leucine mutation in the VP motif that impairs interaction with and regulation through COP1. We show that BBX21 and BBX22 are UVR8-dependently stabilized after UV-B exposure, which is counteracted by a repressor induced by HY5/BBX activity. bbx20 bbx21 bbx22 mutants under UV-B are impaired in hypocotyl growth inhibition, photoprotective pigment accumulation, and expression of several HY5-dependent genes. We conclude that BBX20-22 importantly contribute to HY5 activity in a subset of UV-B responses, but that additional, presently unknown coactivators for HY5 are functional in early UVR8 signaling.

plant biology↗

bioRxiv2020

A constitutively monomeric UVR8 photoreceptor allele confers enhanced UV-B photomorphogenesis

The plant UV-B photoreceptor UVR8 plays an important role in UV-B acclimation and survival. UV-B absorption by homodimeric UVR8 induces its monomerization and interaction with the E3 ubiquitin ligase COP1, leading ultimately to gene expression changes. UVR8 is inactivated through redimerization, facilitated by RUP1 and RUP2. Here, we describe a novel semi-dominant, hyperactive allele, namely uvr8-17D, that harbors a glycine-101 to serine mutation. UVR8G101S-overexpression led to weak constitutive photomorphogenesis and extreme UV-B responsiveness. UVR8G101S was observed to be predominantly monomeric in vivo and, once activated by UV-B, was not efficiently inactivated. Analysis of a UVR8G101S crystal structure revealed the distortion of a loop region normally involved in stabilization of the UVR8 homodimer. Plants expressing a UVR8 variant combining G101S with the previously described W285A mutation exhibited robust constitutive photomorphogenesis. This work provides further insight into UVR8 activation and inactivation mechanisms, and describes a genetic tool for the manipulation of photomorphogenic responses.

plant biology↗