Calcium/calmodulin-mediated gravitropic response in plants

Calcium and calmodulin (CaM) play an important role in gravity signal transduction. However, the molecular and biochemical mechanisms involved in gravity signal transduction are not clearly understood. It is becoming evident that hydrogen peroxide is involved in gravity-induced response. Recent results indicate that Ca 2+/CaM is involved in hydrogen peroxide homeostasis by regulating catalase activity in plants (Yang and Poovaiah, 2002). It is well established that auxin controls differential growth during gravitropic bending. Results indicated that an auxin-responsive gene family (SAURs) encodes for Ca 2+ /CaM-binding proteins (Yang and Poovaiah, 2000a). To investigate the effects of gravity on the expression of genes involved in Ca 2+/CaM-mediated signaling, Arabidopsis and corn seedlings were subjected to simulated microgravity using the Random Positioning Machine (RPM), and hypergravity using the MidiCAR centrifuge. The changes in mRNA levels were studied. Selective and significant differences in gene expression were observed in simulated microgravity- and hypergravity- treated plants. The relevance of these genes in gravity signal perception and transduction is discussed.     

植物钙/钙调素介导的信号转导系统

钙离子(Ca²⁺)是一种重要的第二信使, 参与调节植物的生长发育和对环境的适应。钙调素(CaM)和类钙调蛋白(CML)是一类最主要的Ca²⁺感受器, 虽然其自身没有催化活性, 但可通过调节下游靶蛋白的活性, 进而调控细胞的各种生理活动。该文总结了植物体内CaM结合蛋白(CBP)的生理功能、鉴定方法和调控机理, 以及CaM介导的信号转导途径, 包括蛋白磷酸化与去磷酸化、基因转录、离子运输、活性氧代谢、激素和磷脂信号等, 并对今后的研究方向进行了展望。     

Calcium and signal transduction in plants

Environmental and hormonal signals control diverse physiological processes in plants. The mechanisms by which plant cells perceive and transduce these signals are poorly understood. Understanding biochemical and molecular events involved in signal transduction pathways has become one of the most active areas of plant research. Research during the last 15 years has established that Ca2+ acts as a messenger in transducing external signals. The evidence in support of Ca2+ as a messenger is unequivocal and fulfills all the requirements of a messenger. The role of Ca2+ becomes even more important because it is the only messenger known so far in plants. Since our last review on the Ca2+ messenger system in 1987, there has been tremendous progress in elucidating various aspects of Ca(2+) -signaling pathways in plants. These include demonstration of signal-induced changes in cytosolic Ca2+, calmodulin and calmodulin-like proteins, identification of different Ca2+ channels, characterization of Ca(2+) -dependent protein kinases (CDPKs) both at the biochemical and molecular levels, evidence for the presence of calmodulin-dependent protein kinases, and increased evidence in support of the role of inositol phospholipids in the Ca(2+) -signaling system. Despite the progress in Ca2+ research in plants, it is still in its infancy and much more needs to be done to understand the precise mechanisms by which Ca2+ regulates a wide variety of physiological processes. The purpose of this review is to summarize some of these recent developments in Ca2+ research as it relates to signal transduction in plants.     

Calmodulin and calmodulin-mediated processes in plants

Abstract. The Ca²⁺ -binding protein calmodulin is found in all plants investigated so far. The comparison of the biochemical and functional properties reveals that it is structurally conserved and functionally preserved throughout the plant and animal kingdom. Among the plant enzymes so far known to be dependent on the Ca²⁺ -calmodulin complex are NAD kinase(s), Ca²⁺ -transport ATPase, quinate: NAD⁺ oxidoreductase, soluble and membrane bound protein kinases, and H⁺ -transport ATPase. Calmodulin may play also an important role in the regulation of other cellular reactions, such as hormone-mediated processes, secretion of enzymes, and contractile mechanisms. On the basis of the NAD kinase and its regulation by light and Ca²⁺ -calmodulin, it is suggested that changes in the cellular, free Ca²⁺ concentration following stimulation may alter the metabolism of a plant cell. According to this suggestion free Ca²⁺ may act as a second messenger in plants much as it does in animal cells.     

A coupled equilibrium shift mechanism in calmodulin-mediated signal transduction

We used nuclear magnetic resonance data to determine ensembles of conformations representing the structure and dynamics of calmodulin (CaM) in the calcium-bound state (Ca(2+)-CaM) and in the state bound to myosin light chain kinase (CaM-MLCK). These ensembles reveal that the Ca(2+)-CaM state includes a range of structures similar to those present when CaM is bound to MLCK. Detailed analysis of the ensembles demonstrates that correlated motions within the Ca(2+)-CaM state direct the structural fluctuations toward complex-like substates. This phenomenon enables initial ligation of MLCK at the C-terminal domain of CaM and induces a population shift among the substates accessible to the N-terminal domain, thus giving rise to the cooperativity associated with binding. Based on these results and the combination of modern free energy landscape theory with classical allostery models, we suggest that a coupled equilibrium shift mechanism controls the efficient binding of CaM to a wide range of ligands.     

GPCR和CaCC信号传导中的钙信号网络

G蛋白偶联受体(G protein-coupled receptor,GPCR)作为最大的一类人膜蛋白受体家族和最重要的药物靶标而倍受关注,其中钙离子在细胞内信号传导级联放大中起了关键的作用。阐述了GPCR和钙激活的氯离子通道蛋白(calcium-activated chloride channel,CaCC)中的钙信号网络与生理功能以及如何干扰阻断该网络,为药物设计和很多疾病的治疗提供了依据。     

Calcium in plants

Calcium is an essential plant nutrient. It is required for various structural roles in the cell wall and membranes, it is a counter-cation for inorganic and organic anions in the vacuole, and the cytosolic Ca2+ concentration ([Ca2+]cyt) is an obligate intracellular messenger coordinating responses to numerous developmental cues and environmental challenges. This article provides an overview of the nutritional requirements of different plants for Ca, and how this impacts on natural flora and the Ca content of crops. It also reviews recent work on (a) the mechanisms of Ca2+ transport across cellular membranes, (b) understanding the origins and specificity of [Ca2+]cyt signals and (c) characterizing the cellular [Ca2+]cyt-sensors (such as calmodulin, calcineurin B-like proteins and calcium-dependent protein kinases) that allow plant cells to respond appropriately to [Ca2+]cyt signals.     

Calcium signal compartmentalization

Cytosolic calcium signals are produced by suddenly increasing the concentration of free calcium ions (Ca2+). This can occur by opening channels permeable to Ca2+ either in the surface cell membrane or in the membranes of intracellular organelles containing high Ca2+ concentrations. Ca2+ signals can control several different processes, even in the same cell. In pancreatic acinar cells, for example, Ca2+ signals do not only control the normal secretion of digestive enzymes, but can also activate autodigestion and programmed cell death. Recent technical advances have shown that different patterns of Ca2+ signals can be created, in space and time, which allow specific cellular responses to be elicited. The mechanisms responsible for Ca2+ signal compartmentalization are now largely known and will be described on the basis of recent studies of Ca2+ transport pathways and their regulation in pancreatic acinar cells. It turns out that the Ca2+ handling as well as the structural characteristics of the endoplasmic reticulum (ER) and the mitochondria are of particular importance. Using a variety of Ca(2+)-sensitive fluorescent probes placed in different sub-cellular compartments in combination with local uncaging of caged Ca2+, many new insights into Ca2+ signal generation, compartmentalization and termination have recently been obtained.     

Calcium/calmodulin up-regulates a cytoplasmic receptor-like kinase in plants

Calcium/calmodulin-dependent kinases play an important role in protein phosphorylation in eukaryotes. However, not much is known about calcium/calmodulin-dependent protein phosphorylation and its role in signal transduction in plants. By using a protein-protein interaction-based approach, we have isolated a novel plant-specific calmodulin-binding receptor-like cytoplasmic kinase (CRCK1) from Arabidopsis thaliana, as well as its ortholog from Medicago sativa (alfalfa). CRCK1 does not show high homology to calcium/calmodulin-dependent protein kinases in animals. In contrast, it shows high homology in the kinase domain to serine/threonine receptor-like kinases in plants. However, it contains neither a transmembrane domain nor an extracellular domain. Calmodulin binds to CRCK1 in a calcium-dependent manner with an affinity of approximately 20.5 nm. The calmodulin-binding site in CRCK1 is located in amino acids 160-183, which overlap subdomain II of the kinase domain. CRCK1 undergoes autophosphorylation in the presence of Mg2+ at the threonine residue(s). The Km and Vmax values of CRCK1 for ATP are 1 microm and 33.6 pmol/mg/min, respectively. Calcium/calmodulin stimulates the kinase activity of CRCK1, which increases the Vmax of CRCK1 approximately 9-fold. The expression of CRCK1 is increased in response to stresses such as cold and salt and stress molecules such as abscisic acid and hydrogen peroxide. These results indicate the presence of a calcium/calmodulin-regulated receptor-like cytoplasmic kinase in plants. Furthermore, these results also suggest that calcium/calmodulin-regulated protein phosphorylation involving CRCK1 plays a role in stress signal transduction in plants.     

Calcium oscillations in higher plants

There is considerable interest in the possibility that stimulus-induced oscillations in cytosolic free calcium encode information that is used to specify the outcome of the final response in calcium-based signalling pathways in plants. Recent results provide conclusive evidence that plant cells can decipher complex calcium signatures.     

Calcium messenger system in plants

The purpose of this review is to delineate the ubiquitous and pivotal role of Ca2+ in diverse physiological processes. Emphasis will be given to the role of Ca2+ in stimulus-response coupling. In addition to reviewing the present status of research, our intention is to critically evaluate the existing data and describe the newly developing areas of Ca2+ research in plants.     

Calcium oxalate crystals in plants

Calcium (Ca) oxalate crystals occur in many plant species and in most organs and tissues. They generally form within cells although extracellular crystals have been reported. The crystal cells or idioblasts display ultrastructural modifications which are related to crystal precipitation. Crystal formation is usually associated with membranes, chambers, or inclusions found within the cell vacuole(s). Tubules, modified plastids and enlarged nuclei also have been reported in crystal idioblasts. The Ca oxalate crystals consist of either the monohydrate whewellite form, or the dihydrate weddellite form. A number of techniques exist for the identification of calcium oxalate. X-ray diffraction, Raman microprobe analysis and infrared spectroscopy are the most accurate. Many plant crystals assumed to be Ca oxalate have never been positively identified as such. In some instances, crystals have been classified as whewellite or weddellite solely on the basis of their shape. Certain evidence indicates that crystal shape may be independent of hydration form of Ca oxalate and that the vacuole crystal chamber membranes may act to mold crystal shape; however, the actual mechanism controlling shape is unknown. Oxalic acid is formed via several major pathways. In plants, glycolate can be converted to oxalic acid. The oxidation occurs in two steps with glyoxylic acid as an intermediate and glycolic acid oxidase as the enzyme. Glyoxylic acid may be derived from enzymatic cleavage of isocitric acid. Oxaloacetate also can be split to form oxalate and acetate. Another significant precursor of oxalate in plants is L-ascorbic acid. The intermediate steps in the conversion of L-ascorbic acid to oxalate are not well defined. Oxalic acid formation in animals occurs by similar pathways and Ca oxalate crystals may be produced under certain conditions. Various functions have been attributed to plant crystal idioblasts and crystals. There is evidence that oxalate synthesis is related to ionic balance. Plant crystals thus may be a manifestation of an effort to maintain an ionic equilibrium. In many plants oxalate is metabolized very slowly or not at all and is considered to be an end product of metabolism. Plant crystal idioblasts may function as a means of removing the oxalate which may otherwise accumulate in toxic quantities. Idioblast formation is dependent on the availability of both Ca and oxalate. Under Ca stress conditions, however, crystals may be reabsorbed indicating a storage function for the idioblasts for Ca. In addition, it has been suggested that the crystals serve purely as structural supports or as a protective device against foraging animals. The purpose of this review is to present an overview of plant crystal idioblasts and Ca oxalate crystals and to include the most recent literature.     

Calcium decoding mechanisms in plants

Ca²⁺ is a crucial second messenger that is involved in mediating responses to various biotic and abiotic environmental cues and in the regulation of many developmental processes in plants. Intracellular Ca²⁺ signals are realized by spatially and temporally defined changes in Ca²⁺ concentration that represent stimulus-specific Ca²⁺ signatures. These Ca²⁺ signatures are sensed, decoded and transmitted to downstream responses by a complex tool kit of Ca²⁺ binding proteins that function as Ca²⁺ sensors. Plants possess an extensive and complex array of such Ca²⁺ sensors that convey the information presented in the Ca²⁺ signatures into phosphorylation events, changes in protein-protein interactions or regulation of gene expression. Prominent Ca²⁺ sensors like, Calmodulins (CaM), Calmodulin-like proteins (CMLs), calcium dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs) exist in complex gene families and form intricate signaling networks in plants that are capable of robust and flexible information processing. In this review we reflect on the recently gained knowledge about the mechanistic principles of these Ca²⁺ sensors, their biochemical properties, physiological functions and newly identified targets proteins. These aspects will be discussed in the context of emerging functional principles that govern the information processing via these signaling modules.     

Calcium channels in higher plants

Calcium channels are involved principally in signal transduction. Their opening results in increased cytoplasmic Ca(2+) concentration, and the spatial and temporal variations in this are thought to elicit specific physiological responses to diverse biotic and abiotic stimuli. Calcium-permeable channels have been recorded in the plasma membrane, tonoplast, endoplasmic reticulum, chloroplast and nuclear membranes of plant cells. This article reviews their electrophysiological properties and discusses their physiological roles. Emphasis is placed on the voltage-dependent and elicitor-activated Ca(2+) channels of the plasma membrane and the depolarisation-activated (SV), hyperpolarisation-activated, IP(3)- and cADPR-dependent Ca(2+) channels of the tonoplast. The closing of stomatal guard cells in the presence of abscisic acid (ABA) is used to illustrate the co-ordination of Ca(2+) channel activities during a physiological response.     

Light signal transduction in plants

Light signal-transduction pathways are a central component of the mechanisms that regulate plant development. These pathways provide the means by which information from specific wavelengths of light may be amplified and coordinated, resulting in complex physiological and developmental responses. This review focuses upon recent approaches towards establishing the intermediates that transmit signals from photoreceptors, phytochromes in particular, to target elements in the promoters of light-regulated genes.     

Drought signal transduction in plants

Water deficit is one of the most common environmental limitations of crop productivity by affecting growth through alterations in metabolism and gene expression. The mechanisms involved in drought perception and signal transduction pathways are poorly understood. The participation of the plant hormone abscisic acid (ABA) has been well established. ABA levels increase when there are changes in the environment that result in cellular dehydration. Different approaches have been taken to understanding the molecular responses to desiccation and how ABA regulates gene expression. Recent efforts have identified particular topics of importance in the dissection of the signal transduction pathway which are summarized as follows: physiological approaches: identification of signalling molecules. Genetic approaches: the use of mutants, and Molecular approaches: promoter analysis.