Arabidopsis B‐box transcription factors BBX20‐22 promote UVR8 photoreceptor‐mediated UV‐B responses

Abstract

SUMMARY Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which binds to substrates through their valine–proline (VP) motifs. The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor senses UV‐B and inhibits COP1 through the cooperative binding of its own VP motif and photosensing core to COP1, thereby preventing COP1 binding to substrates, including the basic leucine zipper (bZIP) transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). As a key promoter of visible light and UV‐B photomorphogenesis, HY5 requires coregulators for its function. The B‐box family transcription factors BBX20–BBX22 were recently described as HY5 rate‐limiting coactivators under red light, but their role in UVR8 signaling was unknown. Here we describe a hypermorphic bbx21‐3D mutant with enhanced photomorphogenesis, carrying a proline‐to‐leucine mutation at position 314 in the VP motif that impairs the interaction with and regulation by 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 the expression of several HY5‐dependent genes under continuous UV‐B, but the immediate induction of marker genes after exposure to UV‐B remains surprisingly rather unaffected. We conclude that BBX20–BBX22 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.

Shoot‐to‐root communication via GmUVR8‐GmSTF3 photosignaling and flavonoid biosynthesis fine‐tunes soybean nodulation under UV‐B light

“…Approximately 7–9% of the sunlight is composed of UV light (wavelength range 100–400 nm), including UV‐A (315–400 nm), UV‐B (280–315 nm), and UV‐C (100–280 nm). Low levels of UV‐B light induce photomorphogenesis, including the inhibition of hypocotyl elongation, promotion of cotyledon expansion, and accumulation of flavonoids (Jenkins, 2009; Podolec et al ., 2022). The perception of UV‐B signals relies on the UV‐B photoreceptor UVR8, whose intrinsic tryptophan residues serve as UV‐B chromophores (Rizzini et al ., 2011; Tilbrook et al ., 2013).…”

Arabidopsis BBX14 negatively regulates nitrogen starvation‐ and dark‐induced leaf senescence

“…BBX18 (also called Double B‐box 1a, DBB1a) controls hypocotyl elongation by elevating the levels of bioactive gibberellins under blue light illumination (Wang et al., 2011). BBX20‐BBX22 promote responses to ultraviolet light B (UV‐B) via photoreceptor UVR8 (Podolec et al., 2022). BBX32 appears to control the acclimation of Arabidopsis leaves to high‐light conditions, as revealed by time‐series transcriptomic studies (Alvarez‐Fernandez et al., 2021).…”

Photoreceptor–induced sinapate synthesis contributes to photoprotection in Arabidopsis

“…Our data show that, in addition to the induction of flavonoid biosynthesis, UVR8, cryptochromes and phytochromes indeed activate FAH1 expression through the HY5 and HYH transcription factors, promoting sinapate ester accumulation and photoprotection. HY5 functions often in concert with BBX factors, such as BBX20–22, BBX29, and BBX31, that were shown to promote accumulation of soluble phenylpropanoid derivatives and may thus contribute to UV-B acclimation (Yadav et al, 2019; Bursch et al, 2020; Xu, 2020; Podolec et al, 2022; Medina-Fraga et al, 2023). Whether FAH1 expression can be modulated by other transcriptional regulators associated with UVR8 signalling remains to be determined (Liang et al, 2018; Liang et al, 2019; Liang et al, 2020; Qian et al, 2020; Tavridou et al, 2020a; Tavridou et al, 2020b; Yang et al, 2020; Podolec et al, 2021a).…”