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.     

生物钟基因period的分子生物学

生物过程的昼夜节律是所有真核生物和部分原核生物的基本特征。自从ｐｅｒｉｏｄ基因被克隆以后,生物钟基因的研究已经取得长足的促进。本文回顾了ｐｅｒ基因及生物钟分子机制的研究历史,旨在为人类复杂行为的研究提供一条思路。     

The contributions of interlocking loops and extensive nonlinearity to the properties of circadian clock models

Background

Sensitivity and robustness are essential properties of circadian clock systems, enabling them to respond to the environment but resist noisy variations. These properties should be recapitulated in computational models of the circadian clock. Highly nonlinear kinetics and multiple loops are often incorporated into models to match experimental time-series data, but these also impact on model properties for clock models.

Methodology/Principal Findings

Here, we study the consequences of complicated structure and nonlinearity using simple Goodwin-type oscillators and the complex Arabidopsis circadian clock models. Sensitivity analysis of the simple oscillators implies that an interlocked multi-loop structure reinforces sensitivity/robustness properties, enhancing the response to external and internal variations. Furthermore, we found that reducing the degree of nonlinearity could sometimes enhance the robustness of models, implying that ad hoc incorporation of nonlinearity could be detrimental to a model''s perceived credibility.

Conclusion

The correct multi-loop structure and degree of nonlinearity are therefore critical in contributing to the desired properties of a model as well as its capacity to match experimental data.     

Cellular oscillators: rhythmic gene expression and metabolism

Many biological processes are driven by biological clocks that, depending on the frequency they generate, are classified into ultradian, circadian and infradian oscillators. In virtually all light-sensitive organisms from cyanobacteria to humans, a circadian timing system adapts cyclic physiology to geophysical time. Recent evidence suggests that even in mammals circadian oscillators function in a cell-autonomous manner. In yeast, an ultradian oscillator regulates cyclic respiratory activity and global gene expression. Circadian oscillators and the ultradian yeast respiratory clock share at least four properties: they follow limit-cycle kinetics, interweave with cellular metabolism, are temperature-compensated and influence the cell division clock.     

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。     

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.     

哺乳动物生物钟的遗传和表观遗传研究进展

生物钟对生物机体的生存与环境适应具有着重要意义,其相关研究近年来受到人们的广泛关注。生物钟的重要性质之一是内源节律的周期性,当前的研究认为这种周期性是由生物钟相关基因转录翻译的多反馈环路构成核心机制调控着近似24 h的节律振荡。哺乳动物的生物钟系统存在一个多层次的结构,包括位于视交叉上核的主时钟和外周器官和组织的子时钟。虽然主时钟和子时钟存在的组织不同,但是参与调节生物钟的分子机制是一致的。近年来,通过正向、反向遗传学方法和表观遗传学的研究方法,对生物钟的分子机制的解析和认知愈发深入。本文在简单回顾生物钟基因发现历史的基础上,重点从遗传学和表观遗传学两个方面,从振荡周期的角度,对哺乳动物生物钟分子机制的研究进展进行了综述性介绍,以期为靶向调节生物钟来改善机体的稳态系统的研究提供参考,同时希望能促进时间生物学领域与更多其他领域形成交叉研究。     

The Drosophila circadian clock: what we know and what we don't know.

Circadian rhythms are regulated by endogenous body clocks, which are formed by rhythmic cycles of clock gene expression. Almost all reviews of the Drosophila circadian clock state that the intracellular oscillator is based on a simple negative feedback loop. However, not many 'simple' feedback loops in biology last for 24 h. Instead, the Drosophila clock is a series of precisely timed steps that are deliberately slow. In this paper, I will discuss the current model for how the Drosophila clock is regulated, and ask what questions remain to be answered.