COLD-REGULATED GENE27 Integrates Signals from Light and the Circadian Clock to Promote Hypocotyl Growth in Arabidopsis

Abstract

Light and the circadian clock are two essential external and internal cues affecting seedling development. COLD-REGULATED GENE27 (COR27), which is regulated by cold temperatures and light signals, functions as a key regulator of the circadian clock. Here, we report that COR27 acts as a negative regulator of light signaling. COR27 physically interacts with the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)-SUPPRESSOR OF PHYTOCHROME A1 (SPA1) E3 ubiquitin ligase complex and undergoes COP1-mediated degradation via the 26S proteasome system in the dark. cor27 mutant seedlings exhibit shorter hypocotyls, while transgenic lines overexpressing COR27 show elongated hypocotyls in the light. In addition, light induces the accumulation of COR27. On one hand, accumulated COR27 interacts with ELONGATED HYPOCOTYL5 (HY5) to repress HY5 DNA binding activity. On the other hand, COR27 associates with the chromatin at the PHYTOCHROME INTERACTING FACTOR4 (PIF4) promoter region and upregulates PIF4 expression in a circadian clock-dependent manner. Together, our findings reveal a mechanistic framework whereby COR27 represses photomorphogenesis in the light and provide insights toward how light and the circadian clock synergistically control hypocotyl growth.

How COR27 and COR28 Promote Hypocotyl Growth: Bind to COP1 and Suppress HY5 Activity

“…Both studies set out to test whether impairment of COR27 function affected PIF4 mRNA levels. Zhu et al (2020) detected reduced PIF4 mRNA levels in the cor27 mutant in the afternoon, but not at other time-points over the course of a 12 h light/12 h dark diurnal cycle. By contrast, Li et al (2020) observed lower PIF4 expression in the cor27 cor28 mutant at night in seedlings grown in short days (8 h light/ 16 h dark), but did not test other time-points.…”

“…The studies by Li et al (2020) and Zhu et al (2020) present strong evidence for a COP1dependent role of COR27 and COR28 in the promotion of hypocotyl growth. How this effect is executed through downstream targets, such as HY5 and potentially other transcription factors, will be the next pieces to place in the puzzle of temperature, light and circadian clock signaling.…”

“…As ELONGATED HYPOCOTYL 5 (HY5) is a major transcriptional regulator of light responses that functions directly downstream of the COP1-SPA1 complex, both sets of authors then tested whether COR27 interacted with HY5; indeed they do. COR27 inhibited HY5 binding to DNA, thereby suppressing its transcriptional activation activity (Zhu et al, 2020) and highlighting a mechanism by which COR27 and COR28 may affect hypocotyl elongation (see Figure). COR27 may also exhibit additional functions in hypocotyl growth besides suppressing HY5 activity, based on hypocotyl length differences between the hy5 mutant and seedlings overexpressing or lacking COR27 in the hy5 background.…”

“…Likewise, COR27 and COR28 modulate the pace of the circadian clock (Li et al, 2016). In the current issue, Li et al (2020) and Zhu et al (2020) identify a new role for COR27 and COR28 in promoting hypocotyl growth via their interaction with the COP1-SPA1 complex and regulation of downstream gene expression.…”

“…The two independent studies started with yeast two-hybrid screens to identify interaction partners for COR27 and COR28 (Li et al, 2020) or COP1 (Zhu et al, 2020). Both discovered that COR27 and COR28 interacted with COP1 and its partner, SPA1.…”