Arabidopsis COP1 shapes the temporal pattern of CO accumulation conferring a photoperiodic flowering response

Abstract

The transcriptional regulator CONSTANS (CO) promotes flowering of Arabidopsis under long summer days (LDs) but not under short winter days (SDs). Post-translational regulation of CO is crucial for this response by stabilizing the protein at the end of a LD, whereas promoting its degradation throughout the night under LD and SD. We show that mutations in CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a component of a ubiquitin ligase, cause extreme early flowering under SDs, and that this is largely dependent on CO activity. Furthermore, transcription of the CO target gene FT is increased in cop1 mutants and decreased in plants overexpressing COP1 in phloem companion cells. COP1 and CO interact in vivo and in vitro through the C-terminal region of CO. COP1 promotes CO degradation mainly in the dark, so that in cop1 mutants CO protein but not CO mRNA abundance is dramatically increased during the night. However, in the morning CO degradation occurs independently of COP1 by a phytochrome B-dependent mechanism. Thus, COP1 contributes to day length perception by reducing the abundance of CO during the night and thereby delaying flowering under SDs.

PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length

“…This domain was previously reported to be involved in mediating the binding of CO to DNA through its cognate binding site, defined as the CO response element (CORE) in the FT promoter (Tiwari et al , 2010). Moreover, the CCT domain was also defined as the region with which COP1 physically interacts (Jang et al , 2008; Liu et al , 2008). …”

“…The blue light photoreceptors cryptochrome 1 (cry1) and cryptochrome 2 (cry2) and the far‐red photoreceptor phytochrome A (phyA) are required to stabilize CO protein (Valverde et al , 2004; Zuo et al , 2011). A protein complex of an E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) and SUPPRESSOR OF PHYA1 (SPA1), which are repressors of A. thaliana photomorphogenesis, mediates between the photoreceptors and CO protein stabilization (Laubinger et al , 2006; Jang et al , 2008; Liu et al , 2008; Zuo et al , 2011; Lau & Deng, 2012; Sarid‐Krebs et al , 2015). During the night, the COP1/SPA1 protein complex physically interacts with CO in the nuclei to promote its proteasomal degradation, whereas during the day, the phyA and cry photoreceptors promote COP1 accumulation in the cytoplasm thus allowing CO to accumulate in the nucleus (Osterlund & Deng, 1998; Laubinger et al , 2006; Jang et al , 2008; Liu et al , 2008; Zuo et al , 2011; Sarid‐Krebs et al , 2015).…”

“…During the night, the COP1/SPA1 protein complex physically interacts with CO in the nuclei to promote its proteasomal degradation, whereas during the day, the phyA and cry photoreceptors promote COP1 accumulation in the cytoplasm thus allowing CO to accumulate in the nucleus (Osterlund & Deng, 1998; Laubinger et al , 2006; Jang et al , 2008; Liu et al , 2008; Zuo et al , 2011; Sarid‐Krebs et al , 2015). Since the COP1/SPA1 complex degrades CO protein during the night, it strongly reduces CO accumulation under SDs (Jang et al , 2008; Liu et al , 2008). However, in contrast to its clear biological function in the perception of SDs, COP1 also interferes with the recognition of LDs where a residual activity in the light further reduces CO protein levels during the day, perhaps because photoreceptor activity under LDs is insufficient to completely suppress COP1 function (Jang et al , 2008).…”

“…Since the COP1/SPA1 complex degrades CO protein during the night, it strongly reduces CO accumulation under SDs (Jang et al , 2008; Liu et al , 2008). However, in contrast to its clear biological function in the perception of SDs, COP1 also interferes with the recognition of LDs where a residual activity in the light further reduces CO protein levels during the day, perhaps because photoreceptor activity under LDs is insufficient to completely suppress COP1 function (Jang et al , 2008). FKF1 might overcome this effect because it has been demonstrated that FKF1 stabilizes CO under LDs (Song et al , 2012).…”

Differential effects of light-to-dark transitions on phase setting in circadian expression among clock-controlled genes in Pharbitis nil

“…9,10 Information on light exposure is thereby integrated at CO, leading to its accumulation under LDs and accomplishing measurement of day length. 7,8,10–12 Temporal regulation of CO transcription through the clock-controlled blue light photoreceptor FLAVIN-BINDING, KELCH REPEAT, F-BOX1 (FKF1), clock-controlled flowering regulators CYCLING DOF FACTORs (CDFs), and the clock protein GIGANTEA (GI) determines the proper daily accumulation pattern of CO transcript, ensuring that its accumulation coincides with light exposure only under LDs and CO accumulation occurs specifically under these conditions. 13-15 These proteins thereby link CO transcription and the circadian clock, which comprises transcriptional negative feedback-loops with particular clock genes such as LATE ELONGATED HYPOCOTYL ( LHY ) and TIMING OF CAB EXPRESSION 1 ( TOC1 ) in Arabidopsis.…”