Monitoring circadian rhythms of individual cells in plants

The circadian clock is an endogenous timing system based on the self-sustained oscillation in individual cells. These cellular circadian clocks compose a multicellular circadian system working at respective levels of tissue, organ, plant body. However, how numerous cellular clocks are coordinated within a plant has been unclear. There was little information about behavior of circadian clocks at a single-cell level due to the difficulties in monitoring circadian rhythms of individual cells in an intact plant. We developed a single-cell bioluminescence imaging system using duckweed as the plant material and succeeded in observing behavior of cellular clocks in intact plants for over a week. This imaging technique quantitatively revealed heterogeneous and independent manners of cellular clock behaviors. Furthermore, these quantitative analyses uncovered the local synchronization of cellular circadian rhythms that implied phase-attractive interactions between cellular clocks. The cell-to-cell interaction looked to be too weak to coordinate cellular clocks against their heterogeneity under constant conditions. On the other hand, under light–dark conditions, the heterogeneity of cellular clocks seemed to be corrected by cell-to-cell interactions so that cellular clocks showed a clear spatial pattern of phases at a whole plant level. Thus, it was suggested that the interactions between cellular clocks was an adaptive trait working under day–night cycles to coordinate cellular clocks in a plant body. These findings provide a novel perspective for understanding spatio-temporal architectures in the plant circadian system.     

Picking out parallels: plant circadian clocks in context.

Molecular models have been described for the circadian clocks of representatives of several different taxa. Much of the work on the plant circadian system has been carried out using the thale cress, Arabidopsis thaliana, as a model. We discuss the roles of genes implicated in the plant circadian system, with special emphasis on Arabidopsis. Plants have an endogenous clock that regulates many aspects of circadian and photoperiodic behaviour. Despite the discovery of components that resemble those involved in the clocks of animals or fungi, no coherent model of the plant clock has yet been proposed. In this review, we aim to provide an overview of studies of the Arabidopsis circadian system. We shall compare these with results from different taxa and discuss them in the context of what is known about clocks in other organisms.     

肾上腺糖皮质激素与生物钟基因表达调控的相关研究进展

由生物体内源性生物钟所产生的昼夜节律是近年来生命科学的研究热点之一。哺乳动物中的昼夜节律系统由位于下丘脑SCN核内的主钟和位于多数外周细胞中的子钟组成。生物钟基因及其编码的蛋白质组成反馈回路,维持振荡系统持续进行并与环境周期保持同步。光照和食物是生物钟重要的授时因子, 光照刺激能引起肾上腺中基因表达变化以及糖皮质激素的分泌, 而肾上腺糖皮质激素能减缓由食物因子引起的外周生物钟时相的移动。可见, 肾上腺糖皮质激素与生物钟有着非常密切的关系。文章综述了两者的相互影响并对今后的研究方向做了展望。     

Modulation of circadian clocks by nutrients and food factors

Daily activity rhythms that are dominated by internal clocks are called circadian rhythms. A central clock is located in the suprachiasmatic nucleus of the hypothalamus, and peripheral clocks are located in most mammalian peripheral cells. The central clock is entrained by light/dark cycles, whereas peripheral clocks are entrained by feeding cycles. The effects of nutrients on the central and peripheral clocks have been investigated during the past decade and much interaction between them has come to light. For example, a high-fat diet prolongs the period of circadian behavior, a ketogenic diet advances the onset of locomotor activity rhythms, and a high-salt diet advances the phase of peripheral molecular clocks. Moreover, some food factors such as caffeine, nobiletin, and resveratrol, alter molecular and/or behavioral circadian rhythms. Here, we review nutrients and food factors that modulate mammalian circadian clocks from the cellular to the behavioral level.     

Casein kinase 2, circadian clocks, and the flight from mutagenic light

Circadian clocks play a fundamental role in biology and disease. Much has been learned about the molecular underpinnings of these biological clocks from genetic studies in model organisms, such as the fruit fly, Drosophila melanogaster. Here we review the literature from our lab and others that establish a role for the protein kinase CK2 in Drosophila clock timing. Among the clock genes described thus far, CK2 is unique in its involvement in plant, fungal, as well as animal circadian clocks. We propose that this reflects an ancient, conserved function for CK2 in circadian clocks. CK2 and other clock genes have been implicated in cellular responses to DNA damage, particularly those induced by ultraviolet (UV) light. The finding of a dual function of CK2 in clocks and in UV responses supports the notion that clocks evolved to assist organisms in avoiding the mutagenic effects of daily sunlight.     

哺乳动物卵巢生物钟的研究进展

近年来,越来越多的研究发现生物钟系统在许多生理活动中,包括心血管、内分泌、免疫、生殖等系统的生理,都起着重要作用。随着2006年卵巢生物钟的发现,生殖系统生物钟成为新的研究热点。研究发现卵巢生物钟不仅影响排卵,而且还控制类固醇激素的释放。卵巢生物钟属外围生物钟,受到中央生物钟(SCN)神经内分泌信号的调控。还发现下丘脑-垂体-卵巢(HPG)轴上各水平都存在生物钟,HPG轴上各生物钟失同步影响生殖能力,这可能导致一些疾病发生的病因。本文总结近十年的关于卵巢生物钟的研究,列举哺乳动物卵巢生物钟存在的证据,并阐述生物钟在雌鼠正常生殖生理过程,及在生殖系统疾病病理过程中的作用及其分子机制。     

The circadian clocks of plants and cyanobacteria

Classical research on the circadian rhythms of plants helped to demonstrate that all living organisms utilize circadian clocks to adapt their day–night cycles and that the clock is the basis for photoperiodic time measurements. Molecular models for the circadian oscillator have now been elucidated in Drosophila, Neurospora, mice and cyanobacteria. All share a similar feedback structure, but key proteins in each of the oscillators are different. A plant clock model has yet to be proposed, but clock mutants of Arabidopsis are expected to reveal key proteins in the mechanism. Here we discuss how a self-sustained oscillation is established in eukaryotic and prokaryotic models, and the polyphyletic evolution of these clock systems.     

Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus

This review presents a new perspective on the circadian regulation and functions of insect developmental hormones. In Rhodnius prolixus (Hemiptera), the brain neuropeptide prothoracicotropic hormone (PTTH) is released with a circadian rhythm that is controlled by paired photosensitive clocks in the brain. These clocks comprise the dorsal and lateral PER/TIM clock neurons known to regulate behavioral rhythms in Drosophila. Axons of PTTH and clock cells make close contact. Photosensitive PER/TIM clocks also reside in the paired prothoracic glands (PGs), which generate rhythmic synthesis and release of the ecdysteroid molting hormones. The PG clocks are entrained by both light and PTTH. These four clocks are coupled together by both nerves and hormones into a timing system whose primary regulated output is the circadian rhythm of ecdysteroids in the hemolymph. This complex timing system appears necessary to ensure circadian organization of the gene expression that is induced in target cells by ecdysteroids via circadian cycling of the nuclear ecdysteroid receptor (EcR). This multioscillator system serves to transduce 'the day outside' into endocrine rhythms that orchestrate 'the day inside'. It has many functional similarities with vertebrate circadian systems.     

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.     

Signalling entrains the peripheral circadian clock

In mammals, 24-h rhythms of behaviour and physiology are regulated by the circadian clock. The circadian clock is controlled by a central clock in the brain's suprachiasmatic nucleus (SCN) that synchronizes peripheral clocks in peripheral tissues. Clock genes in the SCN are primarily entrained by light. Increasing evidence has shown that peripheral clocks are also regulated by light and hormones independent of the SCN. How the peripheral clocks deal with internal signals is dependent on the relevance of a specific cue to a specific tissue. In different tissues, most genes that are under circadian control are not overlapping, revealing the tissue-specific control of peripheral clocks. We will discuss how different signals control the peripheral clocks in different peripheral tissues, such as the liver, gastrointestinal tract, and pancreas, and discuss the organ-to-organ communication between the peripheral clocks at the molecular level.     

Regulation of output from the plant circadian clock

Plants, like many other organisms, have endogenous biological clocks that enable them to organize their physiological, metabolic and developmental processes so that they occur at optimal times. The best studied of these biological clocks are the circadian systems that regulate daily (approximately 24 h) rhythms. At the core of the circadian system in every organism are oscillators responsible for generating circadian rhythms. These oscillators can be entrained (set) by cues from the environment, such as daily changes in light and temperature. Completing the circadian clock model are the output pathways that provide a link between the oscillator and the various biological processes whose rhythms it controls. Over the past few years there has been a tremendous increase in our understanding of the mechanisms of the oscillator and entrainment pathways in plants and many useful reviews on the subject. In this review we focus on the output pathways by which the oscillator regulates rhythmic plant processes. In the first part of the review we describe the role of the circadian system in regulation at all stages of a plant's development, from germination and growth to reproductive development as well as in multiple cellular processes. Indeed, the importance of a circadian clock for plants can be gauged by the fact that so many facets of plant development are under its control. In the second part of the review we describe what is known about the mechanisms by which the circadian system regulates these output processes.     

Beyond Arabidopsis: The circadian clock in non-model plant species

Circadian clocks allow plants to temporally coordinate many aspects of their biology with the diurnal cycle derived from the rotation of Earth on its axis. Although there is a rich history of the study of clocks in many plant species, in recent years much progress in elucidating the architecture and function of the plant clock has emerged from studies of the model plant, Arabidopsis thaliana. There is considerable interest in extending this knowledge of the circadian clock into diverse plant species in order to address its role in topics as varied as agricultural productivity and the responses of individual species and plant communities to global climate change and environmental degradation. The analysis of circadian clocks in the green lineage provides insight into evolutionary processes in plants and throughout the eukaryotes.     

Insect circadian clocks: is it all in their heads?

Circadian rhythms are ubiquitous in living organisms, synchronizing life functions at the biochemical, physiological, and behavioral levels. The rhythm-generating mechanisms, collectively known as circadian clocks, are not fully understood in any organism. Research in the fruit fly Drosophila has led to the identification of several clock genes that are involved in the function of the brain-centered clock, which controls behavioral rhythms of adult flies. With the use of clock genes as markers, putative circadian clocks were mapped in the fly peripheral organs and shown to be independent from clocks located in the brain. A homologue of fruit fly period gene has been identified in moths and other insects, allowing investigations of this gene's role in known insect rhythms. This approach may increase our understanding of how circadian clocks are organized into the circadian system that orchestrates temporal integration of life processess in insects.