Functional interaction of the circadian clock and UV RESISTANCE LOCUS 8‐controlled UV‐B signaling pathways in Arabidopsis thaliana

Abstract

SUMMARYCircadian clocks regulate many molecular and physiological processes in Arabidopsis (Arabidopsis thaliana), allowing the timing of these processes to occur at the most appropriate time of the day in a 24-h period. The accuracy of timing relies on the synchrony of the clock and the environmental day/night cycle. Visible light is the most potent signal for such synchronization, but light-induced responses are also rhythmically attenuated (gated) by the clock. Here, we report a similar mutual interaction of the circadian clock and non-damaging photomorphogenic UV-B light. We show that low-intensity UV-B radiation acts as entraining signal for the clock. UV RESISTANCE LOCUS 8 (UVR8) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) are required, but ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG (HYH) are dispensable for this process. UV-B responsiveness of clock gene expression suggests that photomorphogenic UV-B entrains the plant clock through transcriptional activation. We also demonstrate that UV-B induction of gene expression under these conditions is gated by the clock in a HY5/HYH-independent manner. The arrhythmic early flowering 3-4 mutant showed non-gated, high-level gene induction by UV-B, yet displayed no increased tolerance to UV-B stress. Thus, the temporal restriction of UV-B responsiveness by the circadian clock can be considered as saving resources during acclimation without losing fitness.

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

“…For protein gel blot analysis, total cellular proteins (15 μg) or immunoprecipitates were separated by electrophoresis in 8% (wt/vol) SDS-polyacrylamide gels and electrophoretically transferred to PVDF membranes according to the manufacturer's instructions (Bio-Rad). We used anti-COP1 (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) , anti-UVR8 (426-440) , anti-actin (A0480; Sigma-Aldrich), and anti-chalcone synthase (sc-12620; Santa Cruz Biotechnology) as primary antibodies, with HRPconjugated protein A (Pierce) or anti-rabbit, anti-goat, and anti-mouse immunoglobulins (DAKO) used as secondary antibodies, as required. We used anti-UVR8 (426-440) from guinea pig and HRP-conjugated anti-guinea pig secondary antibodies (ab7139; Abcam) to detect UVR8 in immunoprecipitates generated using anti-UVR8 (426-440) from rabbits.…”

“…Rabbit polyclonal antibodies were generated against synthetic peptides derived from the UVR8 protein sequence [amino acids 410-424 + C: VPDETGLTDGSSKGNC; anti-UVR8 (410-424) ] and the COP1 protein sequence [amino acids C + 13-26: CVKPDPRTSSVGEGA; anti-COP1 (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) ] and were affinity-purified against the peptide (Eurogentec). Guinea pig polyclonal antibodies were generated against a synthetic peptide [amino acids C + 426-440: CGDISVPQTDVKRVRI; anti-UVR8 (426-440) ] already previously used to generate polyclonal UVR8 antibodies in rabbits (4) and were affinity-purified against the peptide (Eurogentec).…”

“…One of the main molecular outcomes of this interaction is an increase in protein level of the bZIP transcription factor ELONGATED HYPOCOTYL 5 (HY5), which may be the result of reduced HY5 ubiquitination by COP1 (4). HY5 together with its homolog HYH induce expression of the majority but not all genes included in the UVR8-dependent UV-B response (15)(16)(17)(18). In a negative feedback loop, the light-regulated SALT TOLERANCE/B-BOX DOMAIN PROTEIN 24 (STO/BBX24) was shown to fine-tune the UV-B response by impinging on HY5 (19).…”

Blue Light- and Low Temperature-Regulated COR27 and COR28 Play Roles in the Arabidopsis Circadian Clock

“…CRYs transduce blue light to the clock, while phytochromes transduce red/far-red light (Somers et al, 1998). UVR8 acts in mediating low-intensity UV-B light input to the clock (Feher et al, 2011). ZTL and FKF are circadian photoreceptors.…”

UV-B-Responsive Association of the Arabidopsis bZIP Transcription Factor ELONGATED HYPOCOTYL5 with Target Genes, Including Its Own Promoter

“…However, the molecular events and the identity of the components mediating the transcriptional regulation of target genes by UVR8 remained elusive. It is known that the bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) mediates UV-B-induced gene expression changes downstream of UVR8, in partial redundancy with its homolog HYH (Ulm et al, 2004;Brown et al, 2005;Oravecz et al, 2006;Brown and Jenkins, 2008;Stracke et al, 2010;Fehér et al, 2011;Huang et al, 2012). Indeed, HY5 and HYH are thought to govern the majority of the UVR8-mediated UV-B transcriptional responses (Tilbrook et al, 2013;Jenkins, 2014).…”