Inositol trisphosphate stimulates the release of calcium from intact vacuoles isolated fromAcercells

Calcium Signals from the Vacuole

The vacuole is by far the largest intracellular Ca(2+) store in most plant cells. Here, the current knowledge about the molecular mechanisms of vacuolar Ca(2+) release and Ca(2+) uptake is summarized, and how different vacuolar Ca(2+) channels and Ca(2+) pumps may contribute to Ca(2+) signaling in plant cells is discussed. To provide a phylogenetic perspective, the distribution of potential vacuolar Ca(2+) transporters is compared for different clades of photosynthetic eukaryotes. There are several candidates for vacuolar Ca(2+) channels that could elicit cytosolic [Ca(2+)] transients. Typical second messengers, such as InsP₃ and cADPR, seem to trigger vacuolar Ca(2+) release, but the molecular mechanism of this Ca(2+) release still awaits elucidation. Some vacuolar Ca(2+) channels have been identified on a molecular level, the voltage-dependent SV/TPC1 channel, and recently two cyclic-nucleotide-gated cation channels. However, their function in Ca(2+) signaling still has to be demonstrated. Ca(2+) pumps in addition to establishing long-term Ca(2+) homeostasis can shape cytosolic [Ca(2+)] transients by limiting their amplitude and duration, and may thus affect Ca(2+) signaling.

The plant vacuole: emitter and receiver of calcium signals

This review portrays the plant vacuole as both a source and a target of Ca(2+) signals. In plants, the vacuole represents a Ca(2+) store of enormous size and capacity. Total and free Ca(2+) concentrations in the vacuole vary with plant species, cell type, and environment, which is likely to have an impact on vacuolar function and the release of vacuolar Ca(2+). It is known that cytosolic Ca(2+) signals are often generated by release of the ion from internal stores, but in very few cases has a role of the vacuole been directly demonstrated. Biochemical and electrophysical studies have provided evidence for the operation of ligand- and voltage-gated Ca(2+)-permeable channels in the vacuolar membrane. The underlying molecular mechanisms are largely unknown with one exception: the slow vacuolar channel, encoded by TPC1, is the only vacuolar Ca(2+)-permeable channel cloned to date. However, due to its complex regulation and its low selectivity amongst cations, the role of this channel in Ca(2+) signalling is still debated. Many transport proteins at the vacuolar membrane are also targets of Ca(2+) signals, both by direct binding of Ca(2+) and by Ca(2+)-dependent phosphorylation. This enables the operation of feedback mechanisms and integrates vacuolar transport systems in the wider signalling network of the plant cell.

Phosphoenolpyruvate carboxylase kinase is controlled by a similar signaling cascade in CAM and C(4) plants

In Crassulacean acid metabolism (CAM) plants, phosphoenolpyruvate carboxylase (PEPC) is subject to day-night regulatory phosphorylation of a conserved serine residue in the plant enzyme's N-terminal domain. The dark increase in PEPC-kinase (PEPC-k) activity is under control of a circadian oscillator, via the enhanced expression of the corresponding gene (1). The signaling cascade leading to PEPC-k up-regulation was investigated in leaves and mesophyll cell protoplasts of the facultative, salt-inducible CAM species, Mesembryanthemum crystallinum. Mesophyll cell protoplasts had the same PEPC-k activity as leaves from which they were prepared (i.e., high at night, low during the day). However, unlike C(4) protoplasts (2), CAM protoplasts did not show marked PEPC-k up-regulation when isolated during the day and treated with a weak base such as NH(4)Cl. Investigations using various pharmacological reagents established the operation, in the darkened CAM leaf, of a PEPC-k cascade including the following components: a phosphoinositide-dependent phospholipase C (PI-PLC), inositol 1,4,5 P (IP(3))-gated tonoplast calcium channels, and a putative Ca(2+)/calmodulin protein kinase. These results suggest that a similar signaling machinery is involved in both C(4) (2, 3) and CAM plants to regulate PEPC-k activity, the phosphorylation state of PEPC, and, thus, carbon flux through this enzyme during CAM photosynthesis.

An increase in phosphoinositide-specific phospholipase C activity precedes induction of C4 phosphoenolpyruvate carboxylase phosphorylation in illuminated and NH4Cl-treated protoplasts from Digitaria sanguinalis

A Ca2+-dependent phosphoinositide-specific phospholipase C (PI-PLC) activity has been characterized in the microsomal fraction of Digitaria sanguinalis mesophyll cell protoplasts. Microsomal PI-PLC was found to be inhibited in vitro by a mammalian anti-PLC-delta1 antibody and by the aminosteroide U-73122, an inhibitor of PI-PLC activity in animal cells. In Western blot experiments, the antibody recognized an 85 kDa protein in both microsomal protein extracts from mesophyll protoplasts and rat brain protein extracts containing the authentic enzyme. The involvement of the microsomal PI-PLC in the light-dependent transduction pathway leading to the phosphorylation of C4 phosphoenolpyruvate carboxylase (PEPC) was investigated in D. sanguinalis protoplasts. A transient increase in the PI-PLC reaction product inositol-1,4,5-trisphosphate (Ins(1,4, 5)P3) was observed in situ during early induction of the C4 PEPC phosphorylation cascade. U-73122, but not the inactive analogue U-73343, efficiently blocked the transient accumulation of Ins(1,4, 5)P3, and both the increase in C4 PEPC kinase activity and C4 PEPC phosphorylation in illuminated and weak base-treated protoplasts. Taken together, these data suggest that PI-PLC-based signalling is a committed step in the cascade controlling the regulation of C4 PEPC phosphorylation in C4 leaves.

Signal transduction and ion channels in guard cells

Our understanding of the signalling mechanisms involved in the process of stomatal closure is reviewed. Work has concentrated on the mechanisms by which abscisic acid (ABA) induces changes in specific ion channels at both the plasmalemma and the tonoplast, leading to efflux of both K+ and anions at both membranes, requiring four essential changes. For each we need to identify the specific channels concerned, and the detailed signalling chains by which each is linked through signalling intermediates to ABA. There are two global changes that are identified following ABA treatment: an increase in cytoplasmic pH and an increase in cytoplasmic Ca2+, although stomata can close without any measurable global increase in cytoplasmic Ca2+. There is also evidence for the importance of several protein phosphatases and protein kinases in the regulation of channel activity. At the plasmalemma, loss of K+ requires depolarization of the membrane potential into the range at which the outward K+ channel is open. ABA-induced activation of a non-specific cation channel, permeable to Ca2+, may contribute to the necessary depolarization, together with ABA-induced activation of S-type anion channels in the plasmalemma, which are then responsible for the necessary anion efflux. The anion channels are activated by Ca2+ and by phosphorylation, but the precise mechanism of their activation by ABA is not yet clear. ABA also up-regulates the outward K+ current at any given membrane potential; this activation is Ca(2+)-independent and is attributed to the increase in cytoplasmic pH, perhaps through the marked pH-sensitivity of protein phosphatase type 2C. Our understanding of mechanisms at the tonoplast is much less complete. A total of two channels, both Ca(2+)-activated, have been identified which are capable of K+ efflux; these are the voltage-independent VK channel specific to K+, and the slow vacuolar (SV) channel which opens only at non-physiological tonoplast potentials (cytoplasm positive). The SV channel is permeable to K+ and Ca2+, and although it has been argued that it could be responsible for Ca(2+)-induced Ca2+ release, it now seems likely that it opens only under conditions where Ca2+ will flow from cytoplasm to vacuole. Although tracer measurements show unequivocally that ABA does activate efflux of Cl- from vacuole to cytoplasm, no vacuolar anion channel has yet been identified. There is clear evidence that ABA activates release of Ca2+ from internal stores, but the source and trigger for ABA-induced increase in cytoplasmic Ca2+ are uncertain. The tonoplast and another membrane, probably ER, have IP3-sensitive Ca2+ release channels, and the tonoplast has also cADPR-activated Ca2+ channels. Their relative contributions to ABA-induced release of Ca2+ from internal stores remain to be established. There is some evidence for activation of phospholipase C by ABA, by an unknown mechanism; plant phospholipase C may be activated by Ca2+ rather than by the G-proteins used in many animal cell signalling systems. A further ABA-induced channel modulation is the inhibition of the inward K+ channel, which is not essential for closing but will prevent opening. It is suggested that this is mediated through the Ca(2+)-activated protein phosphatase, calcineurin. The question of Ca(2+)-independent stomatal closure remains controversial. At the plasmalemma the stimulation of K+ efflux is Ca(2+)-independent and, at least in Arabidopsis, activation of anion efflux by ABA may also be Ca(2+)-independent. But there are no indications of Ca(2+)-independent mechanisms for K+ efflux at the tonoplast, and the appropriate anion channel at the tonoplast is still to be found. There is also evidence that ABA interferes with a control system in the guard cell, resetting its set-point to lower contents, suggesting that stretch-activated channels also feature in the regulation of guard cell ion channels, perhaps through interactions with cytoskeletal proteins. (ABSTRACT TRUN

Elicitor-induced defence responses of a suspension-cultured woody plant (Larix decidua) and possible mechanisms of signal transduction

Treatment of suspension-cultured larch cells (Larix decidua Mill.) with an elicitor derived from the cell wall of Fusarium oxysporum Schlecht. triggers very rapid defence responses like an oxidative burst and an increased calcium influx from the medium into the cell, all occurring within minutes after elicitation. These rapid responses are followed by a much slower set of changes like increased activities of phenylalanine ammonia-lyase and peroxidases and enhanced lignin biosynthesis. This paper describes both rapid and slow reactions of a cell culture derived from a woody plant to an elicitor from a facultative pathogen. Experiments concerning the transduction of the elicitor signal showed that the presence of calcium in the medium is indispensable for all elicitor responses of larch cells. It can be demonstrated that H2O2 is not a part of the signal chain. The importance of inositol phosphates and protein phosphorylation were studied using inhibitors. Neomycin, an inhibitor of the phosphoinositol pathway, blocked only the slower responses whereas staurosporine, an inhibitor of protein kinases, blocked both rapid and all the slower reactions. These results support the hypothesis that phosphorylation plays an important role even in very early stages of the signal transduction. Key words: elicitor, Fusarium oxysporum, H2O2, Larix decidua, lignin.

Calcium channels in the vacuolar membrane of plants: multiple pathways for intracellular calcium mobilization

An increasing number of studies imply that Ca 2+ mobilization from intracellular stores plays an important role in stimulus evoked elevation of cytosolic free calcium during signal transduction in plants. It is believed that Ca 2+ is released mainly from the vacuole, which contains a high Ca 2+ concentration in a large volume, and can be regarded as the principal Ca 2+ pool in mature higher plant cells. The large size of the organelle confers unique experimental advantages to the study of endomembrane ion channels. The patch-clamp technique can be directly applied to isolated vacuoles to characterize Ca 2+ release pathways at the single channel level and confirm their membrane location. Using radiometric, ligand-binding and electrophysiological techniques we characterized two different pathways by which Ca 2+ can be mobilized from the vacuole of Beta vulgaris tap roots. Inositol 1,4,5 trisphosphate (Ins P 3 )-elicited Ca 2+ release from tonoplast enriched vesicles is dose-dependent, highly specific for Ins P 3 , and is competitively inhibited by low M r heparin ( K i = 34 nM). This striking resemblance to the animal counterpart which is probably located in the ER is further reflected by the binding properties of the solubilized Ins P 3 receptor from beet, which bears similarities to the Ins P 3 receptor of cerebellum. Thus, Ins P 3 and heparin bind to a single site with sub-micromolar K d s, whereas other inositol phosphates have affinities in the supra-micromolar range. The second Ca 2+ channel in the beet tonoplast is voltage-sensitive and channel openings are largely promoted by positive shifts in the vacuolar membrane potential over the physiological range. Channel activity is neither affected by Ins P 3 addition nor by alteration of cytosolic free calcium, and from a large range of Ca 2+ antagonists tested, only Zn 2+ and the lanthanide Gd 3+ proved to be effective inhibitors. With Ca 2+ as a charge carrier the maximum unitary slope conductance is about 12 pS and saturation occurs at < 5 mM vacuolar Ca 2+ . The channel has an approximately 20-fold higher selectivity for Ca 2+ over K + which is achieved by a Ca 2+ binding site in the channel pore. The unique properties of this novel Ca 2+ release pathway suggests that it is specific for plants. The presence of both Ins P 3 -gated and voltage-gated Ca 2+ channels at the vacuolar membrane implies flexibility in the mechanism of intracellular Ca 2+ mobilization in plant cells.

Calcium -regulated channels and their bearing on physiological activities in Characean cells

Calcium is involved in the regulation of cytoplasmic streaming, membrane excitation, turgor regulation and salt tolerance in the giant internodal cells of the Characeae. To analyse the mechanism of Ca 2+ action, model systems were used, namely, tonoplast-free and plasma m em brane-perm eabilized cells. In the former, the plasma membrane remained intact and its activity could be investigated by m anipulating the cytoplasmic and external media, whereas in the latter, the tonoplast remained intact and its activity could be studied by altering the bathing solution. Studies using these model systems have established the presence of voltage-dependent Ca 2+ -channels in the plasma membrane and Ca 2+ - dependent ion channels in both the plasma membrane and the tonoplast. To further analyse Ca 2+ action on the basis of single channel activities, patch-clam p techniques were applied to plasmolysed protoplasts and isolated cytoplasmic drops. Channel activities were measured using both cell-attached and excised membrane patch modes. A fresh-water member of the Characeae, Nitellopsis , becomes salt-tolerant if millimolar amounts of Ca 2+ are present in the external medium. Under these conditions, excised patches of the plasma membrane exhibit K + -channel activity with unitary conductances of 25-50 pS and a permeability ratio ( P Na / P K ) of 0.28. These K + channels were closed by external Ca 2+ when ATP was present on the cytoplasmic side of the membrane. ATP could be replaced with AMP, which suggests that ATP acted neither as an energy source nor as a substrate for protein phosphorylation, but rather as an effector. In the tonoplast, K + channels having a unitary conductance of 75 pS and a P Na / P K ratio of 0.2 were not activated by Ca 2+ when it was present on the cytoplasmic side of the excised patches, but these channels were activated by Ca 2+ injected into the cytoplasmic drop, which suggested the involvement of an unknown cytoplasmic factor(s) that mediates the Ca 2+ signal. The brackish water Characeae, Lamprothamnium , can regulate elevated turgor induced by hypotonic treatment only when millimolar amounts of Ca 2+ are present in the external medium. In this situation, elevated turgor may first activate Ca 2+ channels and the increased level of Ca 2+ in the cytoplasm may then activate K + and Cl - channels in both the plasma membrane and the tonoplast. In the cytoplasmic-drop-attached mode, single K + channel current-voltage measurements established that the K + channel exhibited a unitary conductance of 50 pS for negative shifts of the voltage, while under positive shifts in the voltage 100 pS channel conductance was observed. The channel with a P Na / P K of 0.02 is highly selective for K + against Na + and this channel is directly activated by Ca 2+ added to the cytoplasmic side of the excised patch. These results suggest that, in Nitellopsis and Lamprothamnium , Ca 2+ regulation of channels in both the plasma membrane and the tonoplast may form the molecular basis for Ca 2+ -regulated physiological functions such as salt tolerance and turgor regulation in characean cells. The mode of Ca 2+ regulation is discussed in light of current findings.

PHYSIOLOGY OF ION TRANSPORT ACROSS THE TONOPLAST OF HIGHER PLANTS

The vacuole of plant cells plays an important role in the homeostasis of the cell. It is involved in the regulation of cytoplasmic pH, sequestration of toxic ions and xenobiotics, regulation of cell turgor, storage of amino acids, sugars and CO2 in the form of malate, and possibly as a source for elevating cytoplasmic calcium. All these activities are driven by two primary active transport mechanisms present in the vacuolar membrane (tonoplast). These two mechanisms employ high-energy metabolites to pump protons into the vacuole, establishing a proton electrochemical potential that mediates the transport of a diverse range of solutes. Within the past few years, great advances at the molecular and functional levels have been made on the characterization and identification of these mechanisms. The aim of this review is to summarize these studies in the context of the physiology of the plant cell.

Molecular cloning of a cDNA encoding diacylglycerol kinase (DGK) in Arabidopsis thaliana

Diacylglycerol kinase (DGK) synthesizes phosphatidic acid from diacylglycerol, an activator of protein kinase C (PKC), to resynthesize phosphatidylinositols. The structure of DGK has not been characterized in plants. We report the cloning of a cDNA, cATDGK1, encoding DGK from Arabidopsis thaliana. The cATDGK1 CDNA contains an open reading frame of 2184 bp, and encodes a putative protein of 728 amino acids with a predicted molecular mass of 79.4 kDa. The deduced ATDGK1 amino acid sequence exhibits significant similarity to that of rat, pig, and Drosophila DGKs. The ATDGK1 mRNA was detected in roots, shoots, and leaves. Southern blot analysis suggests that the ATDGK1 gene is a single-copy gene. The existence of DGK as well as phospholipase C suggests the existence of PKC in plants.

Inositol trisphosphate-dependent and -independent Ca2+ mobilization pathways at the vacuolar membrane of Candida albicans

Vacuolar membrane vesicles were isolated from Candida albicans protoplasts, and marker enzyme assays were employed to identify the membranes as vacuolar in origin. The mechanisms of Ca2+ uptake and Ca2+ release at the vacuolar membrane were investigated. Ca2+ accumulation by vacuolar membrane vesicles can be generated via H+/Ca2+ antiport. The inside-acid pH is in turn generated by a vacuolar-type H(+)-ATPase, as demonstrated by the sensitivity of Ca2+ uptake to ionophores and the vacuolar H(+)-ATPase inhibitor bafilomycin A1. Vacuolar membrane vesicles exhibit two Ca2+ release pathways: one induced by inositol 1,4,5-trisphosphate (InsP3) and the other by inside-positive voltage. These two pathways are distinct with respect to the amount of Ca2+ released, the nature of response to successive stimuli, and their respective pharmacological profiles. The InsP3-gated pathway exhibits a K0.5 for InsP3 of 2.4 microM but is not activated by inositol 4,5-bisphosphate or inositol 1,3,4,5-tetrakisphosphate at concentrations up to 50 microM. Ca2+ release by InsP3 is blocked partially by low molecular weight heparin. Ca2+ released by the voltage-sensitive pathway occurs at membrane potentials estimated to be over a physiological range from 0 to 80 mV. The voltage-sensitive Ca2+ release pathway can be blocked by lanthanide ions and organic channel blockers such as ruthenium red and verapamil. Furthermore, the voltage-sensitive Ca2+ release pathway exhibits Ca(2+)-induced Ca2+ release. These findings are discussed in relation to the mechanism of Ca(2+)-mediated cellular signaling in C. albicans and other fungi.

Isolation, sequencing and analysis of the expression of Bryonia calmodulin after mechanical perturbation

A cDNA clone (Bc329) encoding calmodulin was isolated from a Bryonia cDNA library by screening with cloned Arabidopsis calmodulin cDNA. The cDNA Bc329 was 899 bp full-length clone. The predicted amino acid sequence consists of 149 residues and reveals a high homology with other known plant calmodulins (91 to 99% identity). Genomic southern blot suggests that Bryonia calmodulin is encoded by a single-copy gene. The Bc329 clone was used as a probe to study the expression of calmodulin mRNA after a mechanical stimulus applied on young Bryonia internodes. The steady-state of calmodulin mRNA reached a maximum 30 min after the treatment before it progressively decreased. The role of calcium and calmodulin as second messengers is discussed with regard to environmental changes.

Inositol 1,4,5-trisphosphate releases Ca2+ from vacuolar membrane vesicles of Saccharomyces cerevisiae

Inositol 1,4,5-trisphosphate (IP3) induces a release of Ca2+ from vacuolar membrane vesicles of Saccharomyces cerevisiae. The amount released is dependent on IP3 concentration (concentration for half maximal effect, Km, apparent = 0.4 microM). Myo-inositol, and inositol 1,4-bisphosphate up to 50 microM have no effect on Ca2+ levels in the vesicles. The IP3-induced Ca2+ release is blocked by dantrolene and 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8), which are known to block Ca2+ release from Ca2+ stores in animal cells. IP3-induced release of Ca2+ also occurs when Ca2+ is accumulated by means of an artificial pH gradient, indicating that the effect of IP3 is not due to an effect on the vacuolar H(+)-ATPase. The IP3-induced Ca2+ release is not accompanied by a change in the pH gradient, which indicates that it is not due to a reversal of the Ca2+/nH+ antiport or to a decrease in delta pH by IP3. The present results suggest that IP3 may act as a second messenger in the mobilization of Ca2+ in yeast cells. As in plant cells, the vacuolar membrane of yeast seems to contain a Ca2+ channel, which can be opened by IP3. In this respect the vacuole could function as an IP3-regulated intracellular Ca2+ store, equivalent to the endoplasmic- and sarcoplasmic reticulum in animal cells, and play a role in Ca(2+)-dependent signal transduction in yeast cells.

Evidence that generation of inositol 1,4,5-trisphosphate and hydrolysis of phosphatidylinositol 4,5-bisphosphate are rapid responses following addition of fungal elicitor which induces phytoalexin synthesis in lucerne (Medicago sativa) suspension culture cells

Treatment of lucerne suspension culture cells with glycoprotein elicitor from the phytopathogenic fungus Verticillium albo-atrum R & B triggers Ca(2+)-mediated induction of antimicrobial secondary metabolites termed phytoalexins. The present study investigated the possible role of polyphosphoinositide signal transduction in phytoalexin elicitation. Within 1 min of addition of elicitor to lucerne suspension culture cells we found a 100-160% (15-25 pmol/g fresh wt) increase in the level of compound with chromatographic and electrophoretic properties expected for an inositol trisphosphate (InsP3) and which was strongly bound by an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-specific binding protein; after 3 min the level of this compound had fallen below that observed prior to elicitor challenge. In 32P-prelabelled cells, the relative proportion of radioactivity which cochromatographed with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to have decreased by 48% 1 min after elicitor addition and that rapid depletion of membrane lipid radioactivity was specific to this lipid fraction. The rapid, transient increase in level of Ins(1,4,5)P3 and concomitant fall in PtdIns(4,5)P2 suggests that Ins(1,4,5)P3 generated by hydrolysis of PtdIns(4,5)P2 may provide a Ca(2+)-mobilizing signal in phytoalexin elicitation in lucerne.

Cytological indication of the involvement of calcium and calcium-related proteins in the early responses of Bryonia dioica to mechanical stimulus

The distribution of membrane-bound calcium, activated calmodulin, and callose synthesis was visualized in Bryonia dioica internodes before and after mechanical stimulus, using fluorescent probes, respectively, chlorotetracycline, fluphenazine, and aniline blue. Bright chlorotetracycline fluorescence remains localized in the plasma membrane of control cells, 30 s after stimulation calcium left the plasmalemma. A delocalization of activated calmodulin was observed after wounding and deposition of callose, which could not be detected before, appeared in the same times in most cells. The callose formation and the decrease in membrane-associated calcium suggest a rapid influx of calcium in the cytosol and an intervention of this ion in the cascade of the early events underlying Bryonia dioica thigmomorphogenesis.

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.

Ca2+ regulation of phosphatidylinositol turnover in the plasma membrane of tobacco suspension culture cells

The biochemical properties of the enzymes involved in phosphatidylinositol (PI) turnover in higher plants were investigated using the plasma membrane isolated from tobacco suspension culture cells by aqueous two-phase partitioning. Submicromolar concentrations of Ca2+ inhibited PI kinase and phosphatidylinositol 4-phosphate (PIP) kinase and stimulated phospholipase C. Diacylglycerol (DG) kinase was inhibited by Ca2+, but required a higher concentration than the physiological level. From the above results we postulate the following scheme: signal coupled activation of phospholipase C produces IP3 which induces Ca2+ release from the intracellular Ca2+ compartment, the increased cytoplasmic Ca2+ in turn activates phospholipase C and causes a further increase of the cytoplasmic Ca2+ level. This inhibits PI kinase and PIP kinase and brings about a limited supply of PIP2, the substrate of phospholipase C. Consequently, IP3 production decreases and Ca2+ mobilization ceases. Then cytosolic Ca2+ returns to the stationary level by the Ca2+ pump at the plasma membrane and at the endoplasmic reticulum and Ca2+/H+ antiporter at the plasma membrane and at the tonoplast.

Reversible inactivation of K+ channels of Vicia stomatal guard cells following the photolysis of caged inositol 1,4,5-trisphosphate

RECENT investigations suggest that cytoplasmic D-myo-inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger in plants, as in animals, coupling environmental and other stimuli to intracellular Ca2+ release. Cytoplasmic levels of InsP3 and the turnover of several probable precursors in plants are affected by physiological stimuli--including light, osmotic stress and the phytohormone indoleacetic acid--and InsP3 activates Ca2+ channels and Ca2+ flux across plant vacuolar and microsomal membranes. Complementary data also link changes in cytoplasmic free Ca2+ to several physiological responses, notably in guard cells which regulate gas exchange through the stomatal pores of higher plant leaves. Recent evidence indicates that guard cell K+ channels and, hence, K+ flux for stomatal movements may be controlled by cytoplasmic Ca2+. So far, however, direct evidence of a role for InsP3 in signalling in plants has remained elusive. Here we report that InsP3 released from an inactive, photolabile precursor, the P5-1-(2-nitrophenyl)ethyl ester of InsP3 (caged InsP3) reversibly inactivates K+ channels thought to mediate K+ uptake by guard cells from Vicia faba L. while simultaneously activating an apparently time-independent, inward current to depolarize the membrane potential and promote K+ efflux through a second class of K+ channels. The data are consistent with a transient rise in cytoplasmic free Ca2+ and demonstrate that intact guard cells are competent to use InsP3 in signal cascades controlling ion flux through K+ channels.

Injected inositol 1,4,5-trisphosphate activates Ca2(+)-sensitive K+ channels in the plasmalemma of Eremosphaera viridis

InsP3, and established mediator of intracellular Ca2+ signals in animal cells, is microinjected into the cytoplasm of Eremosphaera viridis. InsP3, but not Ins, InsP1, InsP2 or F2,6-P2 induce a transient opening of Ca2(+)-dependent K+ channels in the plasmalemma of this alga. This effect is indicated by a transient polarization (TP) with a simultaneous increase of membrane conductance. The TP is inhibited by TMB8 (2 mM), an intracellular Ca2+ antagonist or by BAPTA (20 mM), microinjected together with InsP3. The results suggest that InsP3 initiates an increase in the cytoplasmic Ca2+ activity and an activation of Ca2(+)-dependent membrane currents, hence, opening of K+ channels.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

cis,cis-cyclohexane 1,3,5-triol polyphosphates release calcium from Neurospora crassa via an unspecific Ins 1,4,5-P3 receptor

We investigated the effects of new inositol 1,4,5-trisphosphate analogues on the release of Ca2+ from isolated vacuoles of Neurospora crassa. Tri-O-butyryl-inositol 1,4,5-trisphosphate and a set of cis,cis-cyclohexane 1,3,5-triol bis-(CHT-P2) and trisphosphates (CHT-P3) gave an increase in free Ca2+ as measured directly with fura-2, a Ca2(+)-chelator. However, inositol 1,4-bisphosphate, 6-O-palmitoyl-inositol 4,5-bisphosphate and trans-cyclohexane 1,2-diol bisphosphate (trans CHD-P2) did not induce Ca2(+)-release. These results suggest that the 1,5-bisphosphate position in inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) is the only essential arrangement for receptor binding to vacuoles of Neurospora crassa. The structures of these analogues are discussed on the basis of a general concept for the design of new Ins 1,4,5-P3 analogues.

Inositol trisphosphate induces calcium release from Neurospora crassa vacuoles

Inositol 1,4,5-trisphosphate is known to release calcium ions from intracellular stores thought to be parts of endoplasmic reticulum in animal cells. In Neurospora crassa, however, inositol 1,4,5-trisphosphate acts on vacuoles stimulating a calcium efflux with a Km of 5.28 microM. The calcium release is inhibited effectively by dantrolene. These results were obtained by applying two independent methods, measuring calcium binding to fura-2 and loading vacuoles with 45Ca.

Evidence of an auxin-mediated phosphoinositide turnover and an inositol (1,4,5)trisphosphate effect on isolated membranes of Daucus carota L

Microsomal membranes from carrot suspension cells were phosphorylated in vitro with [gamma-32P]ATP. In the presence of submicromolar concentrations of the natural auxin indoleacetic acid (IAA), a rapid, but transient decrease of the [32P] label could be detected in the phospholipid extracts of the membranes. The phytohormone effect was not the result of an inhibition of the lipid phosphorylation reactions, but was caused by a simultaneous release of water-soluble compounds, which, according to their chromatographic properties, were assumed to contain inositol polyphosphates. Although the [32P]-labeled lipids, as well as the inositol polyphosphates, were not identified unequivocally by chemical analysis, these findings point to an auxin-mediated control of a phosphoinositidase C-like reaction similar to the hormone-stimulated phosphoinositide response in animals. Exogenously applied inositol (1,4,5)trisphosphate [(1,4,5)IP3] was found to release 45Ca2+ from preloaded membrane vesicles of carrot cells. Both the detection of the auxin-stimulated phosphoinositide response and the (1,4,5)IP3-mediated Ca2+ release on isolated cell membranes offer new experimental approaches for the identification of the putative auxin receptor and its signal transduction pathway.

Identification of GTP-binding proteins in the plasma membrane of higher plants

Antisera raised against a highly conserved amino acid sequence (G alpha-common peptide) of animal Gs alpha, Gi alpha, Go alpha and Gt alpha recognize, in plasma membranes of several higher plants, sets of proteins of Mr = 37 and 31 kDa (Vicia faba), 36 and 31 kDa (Arabidopsis thaliana) and 38 and 34 kDa (Commelina communis). The A. thaliana proteins were solubilized and partially purified. They bind [35S]GTP gamma S with high affinity (apparent Kd approximately 10 nM) and, with lower affinity, GTP but not the other nucleotides tested (ATP, CTP, ITP, UTP).