Abstract
The circadian clock is an endogenous oscillator with a period of approximately 24 h that allows organisms to anticipate, and respond to, changes in the environment. In Arabidopsis (Arabidopsis thaliana), the circadian clock regulates a wide variety of physiological processes, including hypocotyl elongation and flowering time. CIRCADIAN CLOCK ASSOCIATED1 (CCA1) is a central clock component, and CCA1 overexpression causes circadian dysfunction, elongated hypocotyls, and late flowering. EARLY FLOWERING3 (ELF3) modulates light input to the clock and is also postulated to be part of the clock mechanism. elf3 mutations cause light-dependent arrhythmicity, elongated hypocotyls, and early flowering. Although both genes affect similar processes, their relationship is not clear. Here, we show that CCA1 represses ELF3 by associating with its promoter, completing a CCA1-ELF3 negative feedback loop that places ELF3 within the oscillator. We also show that ELF3 acts downstream of CCA1, mediating the repression of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5 in the control of hypocotyl elongation. In the regulation of flowering, our findings show that ELF3 and CCA1 either cooperate or act in parallel through the CONSTANS/FLOWERING LOCUS T pathway. In addition, we show that CCA1 represses GIGANTEA and SUPPRESSOR OF CONSTANS1 by direct interaction with their promoters, revealing additional connections between the circadian clock and the flowering pathways.
Highlights
The circadian clock is an endogenous oscillator with a period of approximately 24 h that allows organisms to anticipate, and respond to, changes in the environment
To determine how EARLY FLOWERING3 (ELF3) affects CLOCK ASSOCIATED1 (CCA1) expression, CCA1 mRNA levels were examined over a time course in ELF3-OX and elf3-1 plants by quantitative reverse transcription-PCR (Fig. 1A)
Constitutive overexpression of ELF3 resulted in slightly elevated peak levels of CCA1 mRNA, whereas the peak levels of CCA1 mRNA were reduced in elf3-1 mutant plants
Summary
The circadian clock is an endogenous oscillator with a period of approximately 24 h that allows organisms to anticipate, and respond to, changes in the environment. EARLY FLOWERING3 (ELF3) accumulates in the evening and has dual roles in the clock; it modulates light input and operates as a core clock component to repress PRR9 (Hicks et al, 1996; McWatters et al, 2000; Covington et al, 2001; Thines and Harmon, 2010; Dixon et al, 2011; Kolmos et al, 2011). The circadian clock regulates the expression of two growthpromoting transcription factors, PHYTOCHROMEINTERACTING FACTOR4 (PIF4) and PIF5, through a complex of the evening-expressed proteins ELF3, ELF4, and LUX (Nusinow et al, 2011) This evening complex represses PIF4 and PIF5 during the first half of the night by binding to their promoters.
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