Chemical signaling under abiotic stress environment in plants

Many chemicals are critical for plant growth and development and play an important role in integrating various stress signals and controlling downstream stress responses by modulating gene expression machinery and regulating various transporters/pumps and biochemical reactions. These chemicals include calcium (Ca²⁺), cyclic nucleotides, polyphosphoinositides, nitric oxide (NO), sugars, abscisic acid (ABA), jasmonates (JA), salicylic acid (SA) and polyamines. Ca²⁺ is one of the very important ubiquitous second messengers in signal transduction pathways and usually its concentration increases in response to the stimuli including stress signals. Many Ca²⁺ sensors detect the Ca²⁺ signals and direct them to downstream signaling pathways by binding and activating diverse targets. cAMP or cGMP protects the cell with ion toxicity. Phosphoinositides are known to be involved both in transmission of signal across the plasma membrane and in intracellular signaling. NO activates various defense genes and acts as a developmental regulator in plants. Sugars affect the expression of many genes involved in photosynthesis, glycolysis, nitrogen metabolism, sucrose and starch metabolism, defense mechanisms and cell cycle regulation. ABA, JA, SA and polyamines are also involved in many stress responses. Cross-talk between these chemical signaling pathways is very common in plant responses to abiotic and bitotic factors. In this article we have described the role of these chemicals in initiating signaling under stress conditions mainly the abiotic stress.Key words: ABA, abiotic stress, Ca²⁺ binding proteins, calcium signaling, cyclic nucleotides, nitric oxide, phosphoinositides signaling, signal transduction, sugar signaling     

Early effect of steroids on 45calcium uptake by mouse thymocytes

The effects of various steroids on ⁴⁵calcium uptake were investigated in parallel with the viability of mouse thymocytes. Dexamethasone, a synthetic compound with glucocorticoid activity, induced a rapid increase in membrane permeability to calcium. This effect was still measurable using 10^?7 M dexamethasone and appeared specific for compounds with glucocorticoid potency. In addition, calcium efflux from prelabeled cells was not altered in the presence of dexamethasone, indicating an increased total cell concentration. It is therefore suggested that calcium ions play a role in steroid-induced cell lysis.     

High Concentration of Gadolinium Ion Modifying Isolated Rice Mitochondrial Biogenesis

Mitochondria play an important role in plant growth and development, cooperating with the endoplasmic reticulum and nucleus. Gadolinium, one of the rare earth elements, is an inhibitor of stretch-activated calcium channels located on the endoplasmic reticulum and plasma membrane and has no effect on nuclear calcium variation in plant cells. We analyzed the effects of Gd³⁺ on mitochondria function by monitoring mitochondrial swelling, changes of membrane fluidity, and transmembrane potential collapse and by observing mitochondrial ultrastructure. We found that high concentration of Gd³⁺ induces rice mitochondrial dysfunction through mitochondrial permeability transition (MPT). The protection of DTT and EDTA demonstrate that Gd³⁺ blocks the inner membrane ion channel through thiol chelation.     

Cross-Talk between Reactive Oxygen Species and Calcium in Living Cells

The results of many investigations have shown that calcium is essential for production of reactive oxygen species (ROS). Elevation of intracellular calcium level is responsible for activation of ROS-generating enzymes and formation of free radicals by the mitochondria respiratory chain. On the other hand, an increase in intracellular calcium concentration may be stimulated by ROS. H₂O₂ has been recently shown to accelerate the overall channel opening process in voltage-dependent calcium channels in plant and animal cells. The 1,4,5-inositol-triphosphate-receptors as well as the ryanodine receptors of sarcoplasmic reticulum have also been demonstrated to be redox-regulated. Activity of Ca²⁺-ATPases and Na²⁺/Ca²⁺ exchangers of animal cells are modulated by the intracellular redox state. Simultaneously, Ca²⁺ may activate antioxidant enzymes, such as plant catalase and glutathione reductase, and increase the level of superoxide dismutase in animal cells. Reviewed data support the speculation that Ca²⁺ and ROS are two cross-talking messengers in various cellular processes.     

Calcium Signaling Network in Plants: An Overview

Calcium ion (Ca²⁺) is one of the very important ubiquitous intracellular second messenger molecules involved in many signal transduction pathways in plants. The cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) have been found to increased in response to many physiological stimuli such as light, touch, pathogenic elicitor, plant hormones and abiotic stresses including high salinity, cold and drought. This Ca²⁺ spikes normally result from two opposing reactions, Ca²⁺ influx through channels or Ca²⁺ efflux through pumps. The removal of Ca²⁺ from the cytosol against its electrochemical gradient to either the apoplast or to intracellular organelles requires energized ‘active’ transport. Ca²⁺-ATPases and H⁺/Ca²⁺ antiporters are the key proteins catalyzing this movement. The increased level of Ca²⁺ is recognised by some Ca²⁺-sensors or calcium-binding proteins, which can activate many calcium dependent protein kinases. These kinases regulate the function of many genes including stress responsive genes, resulted in the phenotypic response of stress tolerance. Calcium signaling is also involved in the regulation of cell cycle progression in response to abiotic stress. The regulation of gene expression by cellular calcium is also crucial for plant defense against various stresses. However, the number of genes known to respond to specific transient calcium signals is limited. This review article describes several aspects of calcium signaling such as Ca²⁺ requiremant and its role in plants, Ca²⁺ transporters, Ca²⁺-ATPases, H⁺/ Ca²⁺-antiporter, Ca²⁺-signature, Ca²⁺-memory and various Ca²⁺-binding proteins (with and without EF hand).Key Words: Calcium binding proteins, Ca²⁺ channel, Ca²⁺-dependent protein kinases, Ca²⁺/H⁺ antiport, calcium memory, calcium sensors, calcium signatures, Ca²⁺-transporters, EF hand motifs, plant signal transduction     

Relationship between calcium decoding elements and plant abiotic-stress resistance

Serving as an important second messenger, calcium ion has unique properties and universal ability to transmit diverse signals that trigger primary physiological actions in cells in response to hormones, pathogens, light, gravity, and stress factors. Being a second messenger of paramount significance, calcium is required at almost all stages of plant growth and development, playing a fundamental role in regulating polar growth of cells and tissues and participating in plant adaptation to various stress factors. Many researches showed that calcium signals decoding elements are involved in ABA-induced stomatal closure and plant adaptation to drought, cold, salt and other abiotic stresses. Calcium channel proteins like AtTPC1 and TaTPC1 can regulate stomatal closure. Recently some new studies show that Ca²⁺ is dissolved in water in the apoplast and transported primarily from root to shoot through the transpiration stream. The oscillating amplitudes of [Ca²⁺]_o and [Ca²⁺]_i are controlled by soil Ca²⁺ concentrations and transpiration rates. Because leaf water use efficiency (WUE) is determined by stomatal closure and transpiration rate, so there may be a close relationship between Ca²⁺ transporters and stomatal closure as well as WUE, which needs to be studied. The selection of varieties with better drought resistance and high WUE plays an increasing role in bio-watersaving in arid and semi-arid areas on the globe. The current paper reviews the relationship between calcium signals decoding elements and plant drought resistance as well as other abiotic stresses for further study.     

Cytosolic calcium and pH signaling in plants under salinity stress

Calcium is one of the essential nutrients for growth and development of plants. It is an important component of various structures in cell wall and membranes. Besides some fundamental roles under normal condition, calcium functions as a major secondary-messenger molecule in plants under different developmental cues and various stress conditions including salinity stress. Also changes in cytosolic pH, pH_cyt, either individually, or in coordination with changes in cytosolic Ca²⁺ concentration, [Ca²⁺]_cyt, evoke a wide range of cellular functions in plants including signal transduction in plant-defense responses against stresses. It is believed that salinity stress, like other stresses, is perceived at cell membrane, either extra cellular or intracellular, which then triggers an intracellular-signaling cascade including the generation of secondary messenger molecules like Ca²⁺ and protons. The variety and complexity of Ca²⁺ and pH signaling result from the nature of the stresses as well as the tolerance level of the plant species against that specific stress. The nature of changes in [Ca²⁺]_cyt concentration, in terms of amplitude, frequency and duration, is likely very important for decoding the specific downstream responses for salinity stress tolerance in planta. It has been observed that the signatures of [Ca²⁺]_cyt and pH differ in various studies reported so far depending on the techniques used to measure them, and also depending on the plant organs where they are measured, such as root, shoot tissues or cells. This review describes the recent advances about the changes in [Ca²⁺]_cyt and pH_cyt at both cellular and whole-plant levels under salinity stress condition, and in various salinity-tolerant and -sensitive plant species.Key words: cytosolic calcium, ionic toxicity, osmotic stress, pH, salinity stress, salt tolerance, signaling     

Active Calcium Transport by Plant Cell Membranes

The cytosolic free calcium concentration of higher plant cellsis maintained at about 01 µM by the action of membranecalcium transporters. These act to remove calcium from the cytosoland expel it to the apoplast or accumulate it in intracellularstores. In this review, the properties and subcellular localizationsof these systems are described. The major calcium transporterof the plasma membrane is a calcium pumping ATPase which showsmany similarities to its equivalent in mammalian cells. Thetransporter has been purified from maize coleoptiles and isof M_r 140 000, binds (and is activated by) calmodulin and showscommon antigenicity with the mammalian protein. Higher plantendoplasmic reticulum also contains a calcium pumping ATPasewhich transports calcium from the cytoplasm and its role andproperties, together with those of the tonoplast calcium/protonantiporter are presented. Evidence for calcium accumulationby chloroplasts and mitochondria is considered. The review alsodeals with the regulation of plant cell membrane calcium transportand its role in providing intracellular pools of calcium forsignal transduction. Key words: Plant, calcium transport, ATPase, cell membrane, calmodulin     

植物体内钙信号及其在调节干旱胁迫中的作用

钙作为植物体内第二信使广泛参与了植物响应的各种非生物和生物胁迫的信号传导。胁迫信号通过激活位于细胞质膜上的钙离子通道,产生胞质内特异性的钙信号,传递至钙信号感受蛋白,如钙调素(calmodulin,CaM)、钙依赖蛋白激酶(Ca2+-dependent protein kinases,CDPK)和类钙调磷酸酶B蛋白(calcineurin B-like protein,CBL)等,进而引起胞内一系列生理生化变化,最终对胁迫做出响应。钙信号在植物响应干旱胁迫信号系统中起枢纽作用,主要通过调节气孔运动,水通道蛋白(aquaporin,AQP)和抗氧化酶活性来减少水分流失,提高水分利用率,最终降低干旱对植物细胞的伤害,并具有一定的生态学功能。该文对近年来国内外有关植物体内钙信号的研究进展以及在干旱逆境中的调节作用进行综述,并对今后的研究做了展望。     

Fungal hypaphorine reduces growth and induces cytosolic calcium increase in root hairs of Eucalyptus globulus

Summary. Root hairs are tubular cells resulting from a tip-localized growth in which calcium ions play a key role. Hypaphorine, an indole alkaloid secreted by the fungus Pisolithus microcarpus during the formation of ectomycorrhizae with the host plant Eucalyptus globulus, inhibits root hair tip growth. Hypaphorine-induced inhibition is linked to a transient depolarization of the plasma membrane and a reorganization of the actin and microtubule cytoskeletons. Here we investigated the activity of hypaphorine on calcium distribution in E. globulus root hairs with the ratiometric fluorochrome calcium indicator Indo-1. In 85% of actively growing root hairs, a significant but modest calcium gradient between the apex and the base was observed due to an elevated cytoplasmic calcium concentration at the apical tip. Following exposure to 1 mM hypaphorine, the apical and basal cytoplasmic Ca²⁺ concentration increased in 70 and 77% of the hairs, respectively, 10 min after treatment. This led to a reduced calcium gradient in 81% of the cells. The hypothetical links between calcium concentration elevation, regulation of actin cytoskeleton dynamics, and root hair growth inhibition in response to hypaphorine treatment are discussed. Correspondence and reprints: UMR 1136 Interactions Arbres–Microorganismes, Faculté des Sciences, Université Nancy I, BP 239, 54506 Vandoeuvre Cedex, France.     

Functional coupling of ion channels in cellular mechanotransduction

The major players in the processes of cellular mechanotransduction are considered to be mechanosensitive (MS) or mechano-gated ion channels. Non-selective Ca²⁺-permeable channels, whose activity is directly controlled by membrane stretch (stretch-activated channels, SACs) are ubiquitously present in mammalian cells of different origin. Ca²⁺ entry mediated by SACs presumably has a significant impact on various Ca²⁺-dependent intracellular and membrane processes. It was proposed that SACs could play a crucial role in the different cellular reactions and pathologies, including oncotransformation, increased metastatic activity and invasion of malignant cells. In the present work, coupling of ion channels in transformed fibroblasts in course of stretch activation was explored with the use of patch-clamp technique. The combination of cell-attached and inside-out single-current experiments showed that Ca²⁺ influx via SACs triggered the activity of Ca²⁺-sensitive K⁺ channels indicating functional compartmentalization of different channel types in plasma membrane. Importantly, the analysis of single channel behavior demonstrated that K⁺ currents could be activated by the rise of intracellular calcium but displayed no direct mechanosensitivity. Taken together, our data imply that local changes in Ca²⁺ concentration due to SAC activity may provide a functional link between various Ca²⁺-dependent molecules in the processes of cellular mechanotransduction.     

Arabidopsis SOS3 plays an important role in salt tolerance by mediating calcium-dependent microfilament reorganization

Key message

SOS3 mediates calcium dependent actin filament reorganization that plays important roles in plant responses to salt stress.

Abstract

Arabidopsis salt overly sensitive 3 (SOS3) plays an important role in plant salt tolerance by regulation of Na⁺/K⁺ homeostasis. Plants lacking SOS3 are hypersensitive to salt stress and this phenomenon can be partially rescued by the addition of calcium. However the mechanism underlying remains elusive. We here report that the organization of actin filaments in sos3 mutant differs from that in wild-type plant. Under salt stress abnormal actin assembly and arrangement in sos3 are more pronounced, which can be partially complemented by addition of external calcium or low concentration of latrunculin A, an actin monomer-sequestering agent. The effects of calcium and Lat A on actin filament organization of sos3 mutant are accordant with their effects on sos3 salt sensitivity under salt stress. These findings indicate that the salt-hypersensitivity of sos3 mutant partially results from its disordered actin filaments, and SOS3 mediated actin filament reorganization plays important roles in plant responses to salt stress.     

Characterization of vacuolar calcium-binding proteins

The vacuole plays a major structural and biochemical role in the higher plant cell. Among the most studied properties of the vacuole have been transport activities. One important aspect of vacuolar function is its participation in the regulation of cytosolic calcium levels. To identify the molecular entities involved in calcium regulation, a study of vacuole-associated, calcium-binding proteins (CaBs) was initiated. A competition assay was used, and it was observed that the majority of the total cellular membrane-associated, calcium-binding activity resided in low-density fractions enriched in vacuole membranes. Much of that calcium-binding activity was inactivated by a 0.5 m KI wash, and of the remaining activity, 77% was estimated to be peripherally associated with vacuolar membranes, whereas 23% was integrally associated with the vacuolar membrane. Calcium-ligand blots were used, and four major CaBs, with apparent molecular masses of 64, 58, 55, and 42 kD, were detected in purified vacuole membrane fractions. Two of these, the 58- and the 55-kD polypeptide, also appear to be present in significant amounts in endoplasmic reticulum-enriched fractions. However, the 64- and the 42-kD polypeptide are found primarily in vacuolar fractions. It is interesting that expression of the 42-kD polypeptide appears to be restricted to the heavily vacuolated cortical tissues (i.e. it is not found in vascular tissues). The localization of CaBs in the vacuole is consistent with the presence of calcium uptake (H⁺/Ca²⁺ antiport) and release mechanisms (inositol trisphosphate sensitive) on vacuolar membranes. These vacuole-associated CaBs, which may play a role in calcium buffering, together with the calcium transport systems, could mediate the vacuolar component of cellular calcium homeostasis.     

Effects of osmotic shock and calcium on growth and solute composition of Phaseolus vulgaris plants

Identification of the primary response to salinity is of great importance in order to develop salt tolerant species. In this work the effect of a NaCl osmotic shock on leaf elongation of Phaseolus vulgaris L. cv. Contender has been studied. After establishing an osmotic shock by adding NaCl to the root medium, three consecutive events could be distinguished. First, there was a sudden interruption of leaf growth, followed by a period of retardation of growth, and then by a restoration of growth to reach the steady-state growth rate. High calcium (5 mM) in the growth medium diminished the initial loss of water in the roots and kept the water content higher upon restoration of leaf growth, as compared to low calcium levels (0.5 mM). During the low calcium treatment, leaf malate started to increase already 3 h after the start of the osmotic treatment, showing a maximum concentration at around 9 h, then decreasing and approaching the value of control plants. At high calcium, malate concentration remained unchanged with time. Plants grown in the presence of low calcium showed an increase in the concentration of total amino acids upon NaCl shock. It is estimated that organic metabolites, if confined to the cell cytoplasm, contribute significantly to the osmotic adjustment, together with inorganic ions. Our results support the hypothesis that water shortage in the roots is responsible for the initial inhibition of leaf elongation. The beneficial effect of calcium on restoration of growth after the NaCl shock is a consequence of short-term, energetically expensive osmotic adjustment, in which mainly organic metabolites are involved. Under steady state conditions, high calcium treatment results in a faster growth rate than low calcium. In a process of osmotic adjustment, in which inorganic ions are principally involved, this is the result of a decrease in leaf Na⁺ concentration in expanding leaves, together with an increase in K⁺ and Ca²⁺.     

Plant calcium signaling and monitoring: pros and cons and recent experimental approaches

Summary This review focusses on Ca²⁺-mediated plant cell signaling and optical methods for in vivo [Ca²⁺] monitoring and imaging in plants. The cytosolic free calcium concentration has long been considered the central cellular key in plants. However, more and more data are turning up that critically question this view. Conflicting arguments show that there are still many open questions. One conclusion is that the cytosolic free Ca²⁺ concentration is just one of many cellular network parameters orchestrating complex cellular signaling. Novel experimental strategies which unveil interference of cellular parameters and communication of transduction pathways are required to understand this network. To date only optical methods are able to provide both kinetic and spatial information about cellular key parameters simultaneously. Focussing on calcium there are currently three classes of calcium indicators employed (i.e., chemical fluorescent dyes, luminescent indicators, and green-fluorescent-protein-based indicators). Properties and capabilities as well as advantages and disadvantages of these indicators when used in plant systems are discussed. Finally, general experimental strategies are mentioned which are able to answer open questions raised here.Abbreviations CTZ coelenterazine - GFP green-fluorescent protein - FRET fluorescence resonance energy transfer - [Ca²⁺] calcium ion concentration - CaM calmodulin - CDPKs calmodulindomain protein kinases - IP₃ inositol 1,4,5-trisphosphate     

Sulfhydryl modification induces calcium entry through IP₃-sensitive store-operated pathway in activation-dependent human neutrophils

As the first line of host defense, neutrophils are stimulated by pro-inflammatory cytokines from resting state, facilitating the execution of immunomodulatory functions in activation state. Sulfhydryl modification has a regulatory role in a wide variety of physiological functions through mediation of signaling transductions in various cell types. Recent research suggested that two kinds of sulfhydryl modification, S-nitrosylation by exogenous nitric oxide (NO) and alkylation by N-ethylmaleimide (NEM), could induce calcium entry through a non-store-operated pathway in resting rat neutrophils and DDT₁MF-2 cells, while in active human neutrophils a different process has been observed by us. In the present work, data showed that NEM induced a sharp rising of cytosolic calcium concentration ([Ca²⁺]_c) without external calcium, followed by a second [Ca²⁺]_c increase with readdition of external calcium in phorbol 12-myristate 13-acetate (PMA)-activated human neutrophils. Meanwhile, addition of external calcium did not cause [Ca²⁺]_c change of Ca²⁺-free PMA-activated neutrophils before application of NEM. These data indicated that NEM could induce believable store-operated calcium entry (SOCE) in PMA-activated neutrophils. Besides, we found that sodium nitroprusside (SNP), a donor of exogenous NO, resulted in believable SOCE in PMA-activated human neutrophils via S-nitrosylation modification. In contrast, NEM and SNP have no effect on [Ca²⁺]_c of resting neutrophils which were performed in suspension. Furthermore, 2-Aminoethoxydiphenyl borate, a reliable blocker of SOCE and an inhibitor of inositol 1,4,5-trisphosphate (IP₃) receptor, evidently abolished SNP and NEM-induced calcium entry at 75 µM, while preventing calcium release in a concentration-dependent manner. Considered together, these results demonstrated that NEM and SNP induced calcium entry through an IP₃-sensitive store-operated pathway of human neutrophils via sulfhydryl modification in a PMA-induced activation-dependent manner.     

Staphylococcal serine proteinase increases intracellular free calcium concentration in human polymorphonuclear leukocytes

Staphylococcal serine proteinase (SSP) can influence various functions of human polymorphonuclear leukocytes (PMNL) including chemotaxis and phagocytosis. Since the rise in intracellular free calcium concentration is an important step in signal transduction leading to phagocyte activation, we tested the ability of SSP to increase the intracellular free calcium concentration in human PMNL using the fluorescent calcium indicator Fura-2AM. PMNL isolated from healthy donors responded to SSP in the concentration range of 10 to 100 µg/ml. The highest Ca²⁺ rise (104 ± 47 nM) was observed for 10 µg/ml SSP. It was mainly dependent (81 ± 11%) on extracellular calcium influx, however, SSP mobilized 68 ± 7% of Ca²⁺ from intracellular calcium stores. Boiling of SSP or preincubation with phenylmethylsulphonylfluoride (an serine proteinase inhibitor) did not change its ability to increase intracellular free calcium concentration in PMNL. It suggests that active center of SSP is not responsible for Ca²⁺ mobilization. Finally, PMNL responded to each of three consecutive stimulations with SSP independently of the presence of high or low extracellular Ca² concentration. This may be an additional mechanism responsible for activation of human PMNL and degradation of alveolar walls during the staphylococcal infection in the lower airways.     

Little evidence for a major role of Ca2+ in cold-induced injury of liver cells

We have previously shown that cold-induced injury to hepatocytes and liver endothelial cells occurs predominantly via an iron-dependent pathway. However, other groups have reported evidences suggesting that Ca²⁺ ions could be involved in the process of cold-induced injury of liver cells. We here assessed the relative importance and potential interaction of both pathways in cultured primary hepatocytes and cultured liver endothelial cells. The sequence cold incubation/rewarming of hepatocytes and endothelial cells led to an increase in the cytosolic calcium concentration during the early rewarming phase, but the increased cytosolic calcium concentration did not correlate with cell injury. A partial protection from cold-induced cell injury was achieved by the intracellular calcium chelators Quin-2 and BAPTA. However, additional experiments showed that the ability of these chelators to bind iron was probably responsible for a major part of this protection. Incubation in calcium-free media led to an increased cell injury and a physiological calcium concentration (2.5 mM) was protective. In addition, targeting suggested downstream pathways of calcium-dependent cold-induced injury, i.e. by the addition of Ruthenium Red, an inhibitor of mitochondrial Ca²⁺ uniporter, or by inhibiting Bax translocation to the mitochondria, did not provide protection from cold-induced injury in both cell types. Taken together, our data suggest that calcium increases but does not play a major role in cold-induced cell injury to hepatocytes and liver endothelial cells.