Interaction between the Late Elongated hypocotyl (LHY) and Early flowering 3 (ELF3) genes in the Arabidopsis circadian clock

The circadian clock governs rhythms with 24 hours that allow organisms to anticipate daily changing environmental time cues. In Arabidopsis, the circadian clock is conceptually composed of three parts; input pathways for light and temperature signals, oscillators and output pathways for physiological processes including leaf movement, gene expression rhythms and flowering time. Oscillators consist of three interlocking loops, named morning, central and evening loops. Components of the central oscillator contain LHY, CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and TIMING OF CAB EXPRESSION (TOC1). The oscillator can be reset by light signals through input pathways. Genetic studies have revealed the components involved in light input pathways. The elf3 (early flowering 3) mutant was isolated by insensitivity to photoperiod showing long hypocotyls, elongated petioles and pale leaves characteristic of plants defective in light perception. Therefore the ELF3 has been proposed to act on light input pathways. The aim of this study is to test whether LHY and ELF3 encode interacting components of a circadian light input pathway. To address this possibility, lhy-1 elf3-1 (LHY overexpressing-mutant X ELF3 loss of function-mutant) and lhy-11 elf3-1 (LHY loss of function-mutant X ELF3 loss of function-mutant) double mutants were constructed. Their visual phenotypes and CAB (Chlorophyll a/b binding protein) expression patterns demonstrate that LHY may function downstream of ELF3 and that this interaction is disrupted when LHY expression is placed under the control of the 35S promoter. In addition, ELF3 is required for vigorous rhythms of LHY gene expression and LHY protein levels.     

What makes the Arabidopsis clock tick on time? A review on entrainment

Entrainment, the synchronization of a circadian clock with the external environment, is a crucial step in daily life. Although many signals contribute to entrainment, light and temperature are typically the strongest resetting cues. Much progress has been made concerning light resetting in the model plant Arabidopsis thaliana. Multiple photoreceptors (phytochromes, cryptochromes, LOV-domain proteins) are involved in light perception. The clock genes CCA1, LHY and TOC1 are all probable targets of light signalling, although the details of these pathways are not completely established. Temperature can entrain the clock, but little is known about the mechanism underlying this resetting; no obvious clock gene candidate for temperature resetting has been identified. Although circadian research has emphasized oscillations in free-running conditions, in the real world the circadian clock is entrained. During entrainment, short or long period mutants exhibit a 24-h period, but a mutant phenotype is often manifested as an altered phase relationship with the entraining cycle; short and long period mutants show leading and lagging phases, respectively, and this may be detrimental under some conditions. Arrhythmic CCA1-overexpressing plants display increased lethality under very short photoperiods, consistent with the circadian clock being of adaptive significance to life on a rotating world.     

Phylogenetic footprint of the plant clock system in angiosperms: evolutionary processes of <Emphasis Type="Italic">Pseudo-Response Regulator</Emphasis>s

Background  

Plant circadian clocks regulate many photoperiodic and diurnal responses that are conserved among plant species. The plant circadian clock system has been uncovered in the model plant, Arabidopsis thaliana, using genetics and systems biology approaches. However, it is still not clear how the clock system had been organized in the evolutionary history of plants. We recently revealed the molecular phylogeny of LHY/CCA1 genes, one of the essential components of the clock system. The aims of this study are to reconstruct the phylogenetic relationships of angiosperm clock-associated PRR genes, the partner of the LHY/CCA1 genes, and to clarify the evolutionary history of the plant clock system in angiosperm lineages.     

A complex genetic interaction between Arabidopsis thaliana TOC1 and CCA1/LHY in driving the circadian clock and in output regulation

It has been proposed that CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) together with TIMING OF CAB EXPRESSION 1 (TOC1) make up the central oscillator of the Arabidopsis thaliana circadian clock. These genes thus drive rhythmic outputs, including seasonal control of flowering and photomorphogenesis. To test various clock models and to disclose the genetic relationship between TOC1 and CCA1/LHY in floral induction and photomorphogenesis, we constructed the cca1 lhy toc1 triple mutant and cca1 toc1 and lhy toc1 double mutants and tested various rhythmic responses and circadian output regulation. Here we report that rhythmic activity was dramatically attenuated in cca1 lhy toc1. Interestingly, we also found that TOC1 regulates the floral transition in a CCA1/LHY-dependent manner while CCA1/LHY functions upstream of TOC1 in regulating a photomorphogenic process. This suggests to us that TOC1 and CCA1/LHY participate in these two processes through different strategies. Collectively, we have used genetics to provide direct experimental support of previous modeling efforts where CCA1/LHY, along with TOC1, drives the circadian oscillator and have shown that this clock is essential for correct output regulation.     

A modern circadian clock in the common angiosperm ancestor of monocots and eudicots

The circadian clock enhances fitness through temporal organization of plant gene expression, metabolism and physiology. Two recent studies, one in BMC Evolutionary Biology, demonstrate through phylogenetic analysis of the CCA1/LHY and TOC1/PRR gene families that the common ancestor of monocots and eudicots had components sufficient to construct a circadian clock consisting of multiple interlocked feedback loops.     

Genetic linkages of the circadian clock-associated genes, TOC1, CCA1 and LHY, in the photoperiodic control of flowering time in Arabidopsis thaliana

In Arabidopsis thaliana, the flowering time is regulated through the circadian clock that measures day-length and modulates the photoperiodic CO-FT output pathway in accordance with the external coincidence model. Nevertheless, the genetic linkages between the major clock-associated TOC1, CCA1 and LHY genes and the canonical CO-FT flowering pathway are less clear. By employing a set of mutants including an extremely early flowering toc1 cca1 lhy triple mutant, here we showed that CCA1 and LHY act redundantly as negative regulators of the photoperiodic flowering pathway. The partly redundant CCA1/LHY functions are largely, but not absolutely, dependent on the upstream TOC1 gene that serves as an activator. The results of examination with reference to the expression profiles of CO and FT in the mutants indicated that this clock circuitry is indeed linked to the CO-FT output pathway, if not exclusively. For this linkage, the phase control of certain flowering-associated genes, GI, CDF1 and FKF1, appears to be crucial. Furthermore, the genetic linkage between TOC1 and CCA1/LHY is compatible with the negative and positive feedback loop, which is currently believed to be a core of the circadian clock. The results of this study suggested that the circadian clock might open an exit for a photoperiodic output pathway during the daytime. In the context of the current clock model, these results will be discussed in connection with the previous finding that the same clock might open an exit for the early photomorphogenic output pathway during the night-time.