Relationships Between Behavioral Rhythms,Plasma Corticosterone and Hypothalamic Circadian Rhythms

Circadian rhythms in physiological processes and behaviors were compared with hypothalamic circadian rhythms in norepinephrine (NE) metabolites, adrenergic transmitter receptors, cAMP, cGMP and suprachiasmatic nucleus (SCN) arginine vasopressin (AVP) in a single population of rats under D: D conditions. Eating, drinking and locomotor activity were high during the subjective night (the time when lights were out in L: D) and low during the subjective day (the time when lights were on in L: D). Plasma corticosterone concentration rose at subjective dusk and remained high until subjective dawn. Binding to hypothalamic α₁- and β-adrenergic receptors also peaked during the subjective night. Cyclic cGMP concentration was elevated throughout the 24-hr period except for a trough at dusk, whereas DHPG concentration peaked at dawn. Arginine vasopressin levels in the suprachiasmatic nucleus peaked in the middle of the day. No rhythm was found either in binding to the α₂-adrenergic receptor, or in MHPG or cAMP concentration. Behavioral and corticosterone rhythms, therefore, are parallel to rhythms in hypothalamic α₁-and β-receptor binding and NE-release. Cyclic GMP falls only at dusk, suggesting the possibility that cGMP inhibits activity much of the day and that at dusk the inhibition of nocturnal activity is removed. SCN AVP, on the other hand, peaking at 1400 hr, may play a role in the pacemaking function of the SCN that drives these other rhythms.     

Circadian Rhythms in Stomatal Responsiveness to Red and Blue Light

Stomata of many plants have circadian rhythms in responsiveness to environmental cues as well as circadian rhythms in aperture. Stomatal responses to red light and blue light are mediated by photosynthetic photoreceptors; responses to blue light are additionally controlled by a specific blue-light photoreceptor. This paper describes circadian rhythmic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to "free run" as if in continuous light. The rhythm in the stomatal conductance established during the 1.5-h light periods was caused both by a rhythm in sensitivity to light and by a rhythm in the stomatal conductance established during the preceding 2.5-h dark periods. Both rhythms peaked during the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitude of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light, small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attributable largely to changes in internal CO2 availability governed by stomatal conductance.     

植物中的光敏色素

光敏色素是植物体内的光受体。本介绍了光敏色素的结构、特征及由光敏色素引发的昼夜节律生物钟,着重介绍了在信号转导和昼夜节律系统中的光敏色素作用因子。     

光敏色素分子及其信号转导途径

作为植物体内的一种光受体,光敏色素在植物的光形态建成过程中意义重大。植物光敏色素及由它介导的信号传导途径是目前细胞生物学、发育生物学和分子生物学研究的热点之一。本文介绍了光敏色素的分子特性、生理功能和信号转导途径等方面的研究进展。     

Phytochromes and photomorphogenesis in Arabidopsis.

Plants have evolved exquisite sensory systems for monitoring their light environment. The intensity, quality, direction and duration of light are continuously monitored by the plant and the information gained is used to modulate all aspects of plant development. Several classes of distinct photoreceptors, sensitive to different regions of the light spectrum, mediate the developmental responses of plants to light signals. The red-far-red light-absorbing, reversibly photochromic phytochromes are perhaps the best characterized of these. Higher plants possess a family of phytochromes, the apoproteins of which are encoded by a small, divergent gene family. Arabidopsis has five apophytochrome-encoding genes, PHYA-PHYE. Different phytochromes have discrete biochemical and physiological properties, are differentially expressed and are involved in the perception of different light signals. Photoreceptor and signal transduction mutants of Arabidopsis are proving to be valuable tools in the molecular dissection of photomorphogenesis. Mutants deficient in four of the five phytochromes have now been isolated. Their analysis indicates considerable overlap in the physiological functions of different phytochromes. In addition, mutants defining components acting downstream of the phytochromes have provided evidence that different members of the family use different signalling pathways.     

Phytochromes and shade-avoidance responses in plants

BACKGROUND AND AIMS: The ability to detect and respond to the impending threat of shade can confer significant selective advantage to plants growing in natural communities. This Botanical Briefing highlights (a) the regulation of shade-avoidance responses by endogenous and exogenous factors and (b) current understanding of the molecular components involved in red to far-red ratio signal transduction. SCOPE: The Briefing covers: (a) the shade-avoidance syndrome in higher plants; (b) the adaptive significance of shade avoidance in natural light environments; (c) phytochrome regulation of shade-avoidance responses; (d) the role of blue light signals in shade avoidance; (e) gating of rapid shade-avoidance responses by the circadian clock; (f) potential signalling components and future perspectives.     

Phytochromes as light-modulated protein kinases

Many phytochrome responses in plants are induced by red light and inhibited by far-red light. To explain the biochemical basis of these observations, it was speculated that plant phytochromes are light-regulated enzymes more than 40 years ago. The search for such an enzymatic activity has a long and rather tumultuous history. Biochemical data in the late 1980s had suggested that oat phytochrome might be a light-regulated protein kinase. The topic was the subject of intense debate, but solid experimental data backing the kinase model has been published recently. Two lines of research played a key role in this finding: the production of biologically active highly purified recombinant phytochrome and the discovery of phytochromes in prokaryotes. This review discusses the key steps of this discovery, and suggests some hypotheses for the role of protein kinase activity in photomorphogenesis.     

Phytochromes are Pr-ipatetic kinases.

A series of new studies reveal how the red/far-red light photoreceptors called phytochromes act. Phytochrome A and phytochrome B each move to the nucleus when activated by light, and phytochrome A is a kinase. Phytochrome-interacting proteins provide candidate signal transduction components and a recent physiological study suggests how phyA may mediate responses to far-red light. Regulation of phytochrome nuclear localization and kinase activities creates multiple phytochrome species, which may each have different regulatory activities.     

Phytochromes: An Atomic Perspective on Photoactivation and Signaling

The superfamily of phytochrome (Phy) photoreceptors regulates a wide array of light responses in plants and microorganisms through their unique ability to reversibly switch between stable dark-adapted and photoactivated end states. Whereas the downstream signaling cascades and biological consequences have been described, the initial events that underpin photochemistry of the coupled bilin chromophore and the ensuing conformational changes needed to propagate the light signal are only now being understood. Especially informative has been the rapidly expanding collection of 3D models developed by x-ray crystallographic, NMR, and single-particle electron microscopic methods from a remarkably diverse array of bacterial Phys. These structures have revealed how the modular architecture of these dimeric photoreceptors engages the buried chromophore through distinctive knot, hairpin, and helical spine features. When collectively viewed, these 3D structures reveal complex structural alterations whereby photoisomerization of the bilin drives nanometer-scale movements within the Phy dimer through bilin sliding, hairpin reconfiguration, and spine deformation that ultimately impinge upon the paired signal output domains. When integrated with the recently described structure of the photosensory module from Arabidopsis thaliana PhyB, new opportunities emerge for the rational redesign of plant Phys with novel photochemistries and signaling properties potentially beneficial to agriculture and their exploitation as optogenetic reagents.     

Bathy Phytochromes in Rhizobial Soil Bacteria

Phytochromes are biliprotein photoreceptors that are found in plants, bacteria, and fungi. Prototypical phytochromes have a Pr ground state that absorbs in the red spectral range and is converted by light into the Pfr form, which absorbs longer-wavelength, far-red light. Recently, some bacterial phytochromes have been described that undergo dark conversion of Pr to Pfr and thus have a Pfr ground state. We show here that such so-called bathy phytochromes are widely distributed among bacteria that belong to the order Rhizobiales. We measured in vivo spectral properties and the direction of dark conversion for species which have either one or two phytochrome genes. Agrobacterium tumefaciens C58 contains one bathy phytochrome and a second phytochrome which undergoes dark conversion of Pfr to Pr in vivo. The related species Agrobacterium vitis S4 contains also one bathy phytochrome and another phytochrome with novel spectral properties. Rhizobium leguminosarum 3841, Rhizobium etli CIAT652, and Azorhizobium caulinodans ORS571 contain a single phytochrome of the bathy type, whereas Xanthobacter autotrophicus Py2 contains a single phytochrome with dark conversion of Pfr to Pr. We propose that bathy phytochromes are adaptations to the light regime in the soil. Most bacterial phytochromes are light-regulated histidine kinases, some of which have a C-terminal response regulator subunit on the same protein. According to our phylogenetic studies, the group of phytochromes with this domain arrangement has evolved from a bathy phytochrome progenitor.Phytochromes are biological photoreceptors that were discovered in plants, where they control development throughout the life cycle in manifold ways (21, 33). Today, a large number of homologs are known also from cyanobacteria, other bacteria, and fungi, which are termed cyanobacterial phytochromes (Cphs), bacteriophytochromes (BphPs), and fungal phytochromes (Fphs), respectively (20, 24). The chromophore is autocatalytically assembled within the N-terminal part of the protein, the photosensory core module (PCM), which contains the PAS, GAF, and PHY domains (30). Typically, phytochromes are converted by light between two spectrally different forms, the red-absorbing Pr and the far-red-absorbing Pfr forms. Photoconversion is initiated by an isomerization of the covalently bound bilin chromophore (32).Plant and cyanobacterial phytochromes incorporate phytochromobilin (PΦB) and phycocyanobilin (PCB) as natural chromophores, respectively, which are covalently bound to Cys residues in the GAF domains. All characterized phytochromes that belong to these groups have a Pr ground state. Plant phytochromes can undergo dark conversion of Pfr to Pr (5), whereas the Pfr form of typical cyanobacterial phytochromes is stable in darkness (26).Bacteriophytochromes utilize biliverdin (BV) instead as a natural chromophore (1), which is covalently attached to a Cys residue in the N terminus of the PAS domain (26). Since the conjugated system of BV is longer than that of PΦB or PCB, the absorption maxima of bacteriophytochromes are found at higher wavelengths than those of cyanobacterial or plant homologs.With the discovery of a bacterial phytochrome from Bradyrhizobium sp. strain ORS278, termed BrBphP1, the first phytochrome with a Pfr ground state and dark conversion from Pr to Pfr was found (10). Thereafter, five more phytochromes with dark conversion of Pr to Pfr were described: Rhodopseudomonas palustris BphP1 (RpBphP1) from strain CEA001, RpBphP5, and RpBphP6 from strain CGA009 (11); Agrobacterium tumefaciens Agp2 (or AtBphP2) from strain C58 (18); and Pseudomonas aeruginosa BphP1 (PaBphP1) (40). These phytochromes are now termed bathy phytochromes because the absorption maxima of their ground states are bathochromically (to longer wavelengths) shifted compared to those of all other phytochromes.Moreover, some other bacterial phytochromes with unusual properties have been described. In the Ppr from Rhodospirillum centenum, a photoactive yellow protein (PYP) domain is fused to the N terminus of a phytochrome homolog. The phytochrome part of Ppr assembles with BV to form a Pr adduct. However, irradiation does not result in the formation of Pfr but in a bleaching of the Pr spectrum (23). The BV adduct of RpBphP3 from R. palustris, which has a Pr ground state, photoconverts to the so-called Pnr form with a blue-shifted absorption maximum (12). RpBphP4 from R. palustris strains Ha2 and BisB5 and Bradyrhizobium BphP3 (BrBphP3) from Bradyrhizobium BTAi1, both with a Pr ground state, photoconvert into a long-lived MetaR form (8, 42). MetaRa and MetaRc are intermediates in the photoconversion from Pr to Pfr of prototypical phytochromes (3). BphP3 from the Bradyrhizobium strain ORS 278 is an exception among bacteriophytochromes as it binds PCB as a natural chromophore. This phytochrome adopts a so-called Po (P-orange) ground state with an absorbance maximum in the orange range (11, 15). Upon irradiation, this phytochrome converts into the Pr form. RpBphP4 from R. palustris CGA009 lacks the biliverdin binding cysteine and does not bind a chromophore (42).With the rapidly growing number of bacterial genome sequences, many new bacterial phytochromes are being discovered. Thus, a large and increasing number of newly identified phytochromes remain spectroscopically uncharacterized. We established an in vivo photometry approach which allowed the rapid acquisition of spectral information about phytochromes from intact bacterial cells. In the beginning period of plant phytochrome research, in vivo photometry was extensively applied (4, 6, 29, 34). This method, in fact, allowed the identification of phytochromes for the first time in plant tissues (6), which led to the purification of phytochromes from plant extracts (37). Here, we apply in vivo photometry for the first time to organisms outside the plant kingdom. This method is especially useful for studying species with single phytochrome genes. The approach is also helpful for comparing properties of native phytochromes in vivo and of their recombinant proteins in vitro.In the present study, we concentrate on nonphotosynthetic species of the order Rhizobiales which belongs to the Alphaproteobacteria. The family Rhizobiaceae comprises plant-interacting soil bacteria. A. tumefaciens and Agrobacterium vitis can transfer genes into plants to induce plant tumors, whereas many other Rhizobiaceae can live as plant symbionts in nodules of stems or roots in which they assimilate molecular nitrogen to produce NH₄⁺, which is used by the plant for synthesis of amino acids and other nitrogen-containing molecules. A. tumefaciens C58 contains two phytochromes, termed Agp1 (or AtBphP1) and Agp2 (or AtBphP2), that have been characterized as recombinant proteins (14, 18, 26, 35) and whose spectral activities have been measured in extracts of wild-type and knockout mutants (31). A large number of phytochromes from photosynthetic Bradyrhizobium and Rhodopseudomonas species, which also belong to the order Rhizobiales, have been characterized as recombinant proteins (11), some of which have already been noted above.It turned out that most of our analyzed phytochromes undergo dark conversion of Pr to Pfr and thus belong to the group of bathy phytochromes. Such phytochromes, which absorb at around 750 nm, clearly dominate among Rhizobiales. We propose that this specific property reflects an adaptation to the light regime in the soil. Our studies also suggest that bacterial phytochromes with a C-terminal response regulator have evolved from a bathy phytochrome progenitor.     

光敏色素分子特性及其信号转导机制

结合生物物理、分子遗传学和细胞生物学的方法已证实,光敏色素信号转导是一个空间分布的、非线形信号传递链。尤其是最近又发现了不同种类的光敏色素分子及其它们在Pr、Pfr光转换中产生的中间体,不仅说明了光敏色素信号转导链是一个多维的信号网络,而且这也暗示着光转换中产生的中间体也直接参与了早期的信号转导。在此,综述了光敏色素分子光转换及其早期信号转导的若干新进展,讨论光敏色素原初光反应及其信号转导的机制。     

Dinocrocuta gigantea头骨的发现

本文记述了甘肃和政县晚中新世地层中发现的一个完整的巨鬣狗头骨;讨论了它的分类地位,修正了 Schlosser 1903年建种时的一些鉴定错误.巨鬣狗是鬣狗科中一个十分特化的成员,应代表一个独立的属.根据命名规则,我们采用 N. schmidt-Kittler 1976年所创的 Dinocrocuta. 因此,这个种应订正为 Dinocrocuta gigantea (schlosser, 1903).     

Chaos and Ultradian Rhythms

Systems in a chaotic state have apparently random outputs despite a simple underlying kinetic mechanism. For instance, the interaction of two coupled oscillators (the mitotic oscillator and the ultradian clock) can produce chaotic behaviour over a limited range of parameter values. Mathematical modelling shows that physiologically realistic characteristics are thereby exhibited. Cell division cycles of lower eukaryotes (protozoa and yeasts) show both deterministic and stochastic properties. Both dispersion of cell cycle times and quantized values can be generated, as a deterministic chaotic consequence of oscillator interaction rather than from noisy limit cycles. Advantages may stem from chaotic operation; a controlled chaotic attractor could provide multifrequency outputs that determine rhythmic behaviour on different time scales (e.g. ultradian and circadian) with the facility for rapid state changes from one periodicity to another.     

Melatonin and Human Rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines ‘biological night.’ It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6‐sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase‐shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a ‘chronobiotic.’ Acutely, it increases sleep propensity during ‘biological day.’ These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.