生物谷：大多数植物和动物的活动以24小时为1周期。然而，美国和瑞士研究人员表示，他们通过研究发现，有一种植物能将自己内部生物钟信号同周围环境的信号结合起来控制自己每天的生长节奏。
采用间隔照像技术，加州大学UCDavis大学从事博士后研究的卡尊纳芮·诺祖和同事以及瑞士洛桑大学的同行发现，拟南芥幼苗每天快速生长一次，猛长的时间由植物内部的生物钟和阳光来控制。而阳光作用主要体现在名为PIF4和PIF5这两种基因上。
研究人员表示，当拟南芥幼苗处于恒定阳光环境中时，每天的最快生长期发生在下午。然而，当将其转移到更接近自然的昼夜周期性环境中时，生长最快期推迟了几个小时，接近黎明。在自然界，通常黎明前植物周围会有更多的水分。
研究人员证实，PIF4和PIF5两种基因同植物的生长相关，同时受生物钟的控制。这两种基因在白天会“接通”生产蛋白质，天黑后“关闭”，然后在后半夜再“接通”。但是，暴露在阳光中时，PIF4和PIF5生产的蛋白质会发生断裂，于是当植物内部生物钟在促使基因产生蛋白质的同时，外部的阳光则在“消耗”蛋白质。
据悉，美国国家科学基金会和瑞士国家科学基金会是这项研究工作的主要资助机构。相关研究报告发表在《自然》杂志网站上。(援引科技日报)
原始出处:
Nature advance online publication 24 June 2007 | doi:10.1038/nature05946; Received 20 January 2007; Accepted 16 May 2007; Published online 24 June 2007
Rhythmic growth explained by coincidence between internal and external cues
Kazunari Nozue1, Michael F. Covington1, Paula D. Duek2,3, Séverine Lorrain2, Christian Fankhauser2, Stacey L. Harmer1 & Julin N. Maloof1
Section of Plant Biology, College of Biological Sciences, University of California, Davis, One Shields Avenue, Davis, California 95616, USA
Center for Integrative Genomics, University of Lausanne, Genopode Building, CH-1015 Lausanne, Switzerland
Present address: Swiss Institute of Bioinformatics, 1 rue Michel Servet, CH-1211 Geneva 4, Switzerland.
Correspondence to: Julin N. Maloof1 Correspondence and requests for materials should be addressed to J.N.M. (Email: jnmaloof@ucdavis.edu).
Most organisms use circadian oscillators to coordinate physiological and developmental processes such as growth with predictable daily environmental changes like sunrise and sunset. The importance of such coordination is highlighted by studies showing that circadian dysfunction causes reduced fitness in bacteria1 and plants2, as well as sleep and psychological disorders in humans3. Plant cell growth requires energy and water—factors that oscillate owing to diurnal environmental changes. Indeed, two important factors controlling stem growth are the internal circadian oscillator4, 5, 6 and external light levels7. However, most circadian studies have been performed in constant conditions, precluding mechanistic study of interactions between the clock and diurnal variation in the environment. Studies of stem elongation in diurnal conditions have revealed complex growth patterns, but no mechanism has been described8, 9, 10. Here we show that the growth phase of Arabidopsis seedlings in diurnal light conditions is shifted 8–12 h relative to plants in continuous light, and we describe a mechanism underlying this environmental response. We find that the clock regulates transcript levels of two basic helix–loop–helix genes, phytochrome-interacting factor 4 (PIF4) and PIF5, whereas light regulates their protein abundance. These genes function as positive growth regulators; the coincidence of high transcript levels (by the clock) and protein accumulation (in the dark) allows them to promote plant growth at the end of the night. Thus, these two genes integrate clock and light signalling, and their coordinated regulation explains the observed diurnal growth rhythms. This interaction may serve as a paradigm for understanding how endogenous and environmental signals cooperate to control other processes.
Figure 1: Diurnal rhythms of hypocotyl elongation require light and the circadian clock.
Plants were entrained for three days under short-day conditions and then switched to continuous light (a) or 4L:4D (d). Alternatively, plants were entrained for four days under short-day conditions and then switched to continuous darkness (b) or kept in short-day conditions (c). We used infrared imaging to monitor seedling growth (see Methods). Growth rate is plotted as a function of time; zero indicates dawn of the fourth day. The vertical scale bar indicates 0.1 mm h–1. Measurements were started when hypocotyls were easily discernible, typically t = 8. The mean s.e.m. of at least two independent experiments is shown; n 6 seedlings. In all plot areas, times of true light and darkness are indicated by clear and grey rectangles, respectively; see below for meaning of x axis rectangles. a, Rhythmic elongation of wild-type Col and Wassilewskija (Ws) hypocotyls in continuous light. White and grey bars on the x axis indicate subjective day and night, respectively. b, Continuous hypocotyl elongation of wild-type (WT) Col in continuous darkness. Grey and black bars on the x axis indicate subjective day and night, respectively. c, Hypocotyl elongation in short-day conditions. Col is the wild-type background for CCA1-OX, elf3 and hy2; Landsberg erecta (Ler) is the wild-type background for hy5. d, Growth in 4L:4D conditions is altered in clock mutants. Grey and black rectangles on the x axis indicate subjective day and night, respectively.
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