Inositol 1,4,5-trisphosphate releases Ca2+ from vacuolar membrane vesicles of oat roots

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

AtbZIP62 Acts as a Transcription Repressor to Positively Regulate ABA Responses in Arabidopsis

The basic region/leucine zipper (bZIP) transcription factor AtbZIP62 is involved in the regulation of plant responses to abiotic stresses, including drought and salinity stresses, NO₃ transport, and basal defense in Arabidopsis. It is unclear if it plays a role in regulating plant responses to abscisic acid (ABA), a phytohormone that can regulate plant abiotic stress responses via regulating downstream ABA-responsive genes. Using RT-PCR analysis, we found that the expression level of AtbZIP62 was increased in response to exogenously applied ABA. Protoplast transfection assays show that AtbZIP62 is predominantly localized in the nucleus and functions as a transcription repressor. To examine the roles of AtbZIP62 in regulating ABA responses, we generated transgenic Arabidopsis plants overexpressing AtbZIP62 and created gene-edited atbzip62 mutants using CRISPR/Cas9. We found that in both ABA-regulated seed germination and cotyledon greening assays, the 35S:AtbZIP62 transgenic plants were hypersensitive, whereas atbzip62 mutants were hyposensitive to ABA. To examine the functional mechanisms of AtbZIP62 in regulating ABA responses, we generated Arabidopsis transgenic plants overexpressing 35S:AtbZIP62-GR, and performed transcriptome analysis to identify differentially expressed genes (DEGs) in the presence and absence of DEX, and found that DEGs are highly enriched in processes including response to abiotic stresses and response to ABA. Quantitative RT-PCR results further show that AtbZIP62 may regulate the expression of several ABA-responsive genes, including USP, ABF2, and SnRK2.7. In summary, our results show that AtbZIP62 is an ABA-responsive gene, and AtbZIP62 acts as a transcription repressor to positively regulate ABA responses in Arabidopsis.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

Dissecting blue light signal transduction pathway in leaf epidermis using a pharmacological approach

Blue light signalling pathway in broad bean leaf epidermal cells includes key membrane transporters: plasma- and endomembrane channels and pumps of H (+) , Ca (2+) and K (+) ions, and plasma membrane redox system. Blue light signalling pathway in epidermal tissue isolated from the abaxial side of fully developed Vicia faba leaves was dissected by measuring the effect of inhibitors of second messengers on net K(+), Ca(2+) and H(+) fluxes using non-invasive ion-selective microelectrodes (the MIFE system). Switching the blue light on-off caused transient changes of the ion fluxes. The effects of seven groups of inhibitors were tested in this study: CaM antagonists, ATPase inhibitors, Ca(2+) anatagonists or chelators, agents affecting IP3 formation, redox system inhibitors, inhibitors of endomembrane Ca(2+) transport systems and an inhibitor of plasma membrane Ca(2+)-permeable channels. Most of the inhibitors had a significant effect on steady-state (basal) net fluxes, as well as on the magnitude of the transient ion flux responses to blue light fluctuations. The data presented in this study suggest that redox signalling and, specifically, plasma membrane NADPH oxidase and coupled Ca(2+) and K(+) fluxes play an essential role in blue light signal transduction.

Cytosolic calcium signals elicited by the pathogen-associated molecular pattern flg22 in stomatal guard cells are of an oscillatory nature

Changes in cytosolic free calcium ([Ca(2+)]cyt) are an early and essential element of signalling networks activated by the perception of pathogen-associated molecular patterns (PAMPs), such as flg22. The flg22-induced calcium signal has been described on whole-plant, but not on single-cell scale so far. Also, the Ca(2+) sources and channels contributing to its generation are still obscure. Ratiometric fluorescence imaging employing the calcium reporter Yellow Cameleon 3.6 was performed to analyse the flg22-induced calcium signature in single guard cells of Arabidopsis thaliana. Calcium stores and channel types involved in its generation were determined by a pharmacological approach. In contrast to the calcium signal determined on whole-plant level, the signature on single-cell level is not characterized by one sustained response, but by oscillations in [Ca(2+)]cyt. These oscillations were abolished by EGTA and lanthanum, as well as by U73122, neomycin and TMB-8, but only partially or not at all affected by inhibitors of glutamate receptor-like channels and cyclic nucleotide-gated channels. Our analyses suggest that the response observed on whole-plant level is the summary of oscillations occurring in single cells. Parallel to external calcium, influx via channels located at internal stores contributes to the signal.

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.

Analysis of calcium signaling pathways in plants

BACKGROUND

Calcium serves as a versatile messenger in many adaptation and developmental processes in plants. Ca2+ signals are represented by stimulus-specific spatially and temporally defined Ca2+ signatures. These Ca2+ signatures are detected, decoded and transmitted to downstream responses by a complex toolkit of Ca2+ binding proteins that function as Ca2+ sensors.

SCOPE OF REVIEW

This review will reflect on advancements in monitoring Ca2+ dynamics in plants. Moreover, it will provide insights in the extensive and complex toolkit of plant Ca2+ sensor proteins that relay the information presented in the Ca2+ signatures into phosphorylation events, changes in protein-protein interaction or regulation of gene expression.

MAJOR CONCLUSIONS

Plants' response to signals is encoded by different Ca2+ signatures. The plant decoding Ca2+ toolkit encompasses different families of Ca2+ sensors like Calmodulins (CaM), Calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs). These Ca2+ sensors are encoded by complex gene families and form intricate signaling networks in plants that enable specific, robust and flexible information processing.

GENERAL SIGNIFICANCE

This review provides new insights about the biochemical regulation, physiological functions and of newly identified target proteins of the major plant Ca2+ sensor families. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Localization of Arabidopsis SYP125 syntaxin in the plasma membrane sub-apical and distal zones of growing pollen tubes

Tip growth in pollen tubes occurs by continuous vesicle secretion and delivery of new wall material, but the exact sub-cellular location of endocytic and exocytic domains remains unclear. Here we studied the localization of the Arabidopsis thaliana pollen specific syntaxin SYP125 using GFP-fusion constructs expressed in Nicotiana tobaccum pollen tubes. In agreement with the predicted role for syntaxins, SYP125 was found to be associated with the plasma membrane and apical vesicles in growing cells. At the plasma membrane, SYP125 was asymmetrically localized with a higher labeling 20-35 μm behind the apex, a distribution which is distinct from SYP124, another pollen-specific syntaxin. Competition with a related dominant negative mutant affected the specific distribution of SYP125 but not tip growth. Co-expression of the phosphatidylinositol-4-monophosphate-5-kinase 4 (PIP5K4) or of the small GTPase Rab11 perturbed polarity and the normal distribution of GFP-SYP but did not inhibit the accumulation in vesicles or at the plasma membrane. Taken together, our results corroborates previous observations that in normal growing pollen tubes, the asymmetric distribution of syntaxins helps to define exocytic sub-domains but requires the involvement of additional signaling and functional mechanisms, namely phosphoinositides and small GTPases. The localization of syntaxins at different membrane domains likely depends on the interaction with specific partners not yet identified.

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.

Asymmetric localization of Arabidopsis SYP124 syntaxin at the pollen tube apical and sub-apical zones is involved in tip growth

BACKGROUND

The continuous polarized vesicle secretion in pollen tubes is essential for tip growth but the location of endo- and exocytic sub-domains remains however controversial. In this report we aimed to show that Arabidopsis thaliana syntaxins are involved in this process and contribute to spatially define exocytosis and membrane recycling.

RESULTS

Using GFP-fusion constructs, we imaged the distribution of pollen-specific (AtSYP124) and non-pollen syntaxins (AtSYP121 and AtSYP122) in transiently transformed Nicotiana tabacum pollen tubes. All three proteins associate with the plasma membrane and with apical vesicles indicating a conserved action mechanism for all SYPs. However, the GFP tagged SYP124 showed a specific distribution with a higher labelling at the plasma membrane flanks, 10-25 mum behind the apex. This distribution is affected by Ca2+ fluxes as revealed by treatment with Gd3+ (an inhibitor of extracellular Ca2+ influx) and TMB-8 (an inhibitor of intracellular Ca2+ release). Both inhibitors decreased growth rate but the distribution of SYP124 at the plasma membrane was more strongly affected by Gd3+. Competition with a related dominant negative mutant affected the specific distribution of SYP124 but not tip growth. In contrast, co-expression of the phosphatidylinositol-4-monophosphate 5-kinase 4 (PIP5K4) or of the small GTPase Rab11 perturbed polarity and the normal distribution of GFP-SYP but did not inhibit the accumulation in vesicles or at the plasma membrane.

CONCLUSIONS

The results presented suggest that in normal growing pollen tubes, a net exocytic flow occurs in the flanks of the tube apex mediated by SYP124. The specific distribution of SYP124 at the plasma membrane is affected by changes in Ca2+ levels in agreement with the importance of this ion for exocytosis. Apical growth and the specific localization of SYP124 were affected by regulators of membrane secretion (Ca2+, PIP5K4 and Rab11) but competition with a dominant negative mutant affected only SYP distribution. These data thus suggest that syntaxins alone do not provide the level of specificity that is required for apical growth and that additional signalling and functional mechanisms are required.

Increasing inositol (1,4,5)-trisphosphate metabolism affects drought tolerance, carbohydrate metabolism and phosphate-sensitive biomass increases in tomato

Inositol-(1,4,5)-trisphosphate (InsP(3)) is a second messenger in plants that increases in response to many stimuli. The metabolic consequences of this signalling pathway are not known. We reduced the basal level of InsP(3) in tomato (Solanum lycopersicum cv. Micro-Tom) by expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase) gene. Transgenic lines producing InsP 5-ptase protein had between 15% and 30% of the basal InsP(3) level of control plants. This increased hydrolysis of InsP(3) caused dramatic increases in drought tolerance, vegetative biomass and lycopene and hexose concentrations in the fruits. Transcript profiling of root, leaf and fruit tissues identified a small group of genes, including a cell-wall invertase inhibitor gene, that were differentially regulated in all tissues of the InsP 5-ptase expressing plants. Significant differences were found in the amounts of carbohydrates and organic phosphate in these plants. Plants with increased hydrolysis of InsP(3) in the cytosol also showed increased net CO(2)-fixation and sucrose export into sink tissue and storage of hexoses in the source leaves. The increase in biomass was dependent on the supply of inorganic phosphate in the nutrient medium. Uptake and storage of phosphate was increased in the transgene expressing lines. This suggests that in tomato, increased flux through the inositol phosphate pathway uncoupled phosphate sensing from phosphate metabolism. Altering the second messenger, InsP(3), revealed multiple coordinated changes in development and metabolism in tomato that have potential for crop improvement.

Shaping the calcium signature

In numerous plant signal transduction pathways, Ca2+ is a versatile second messenger which controls the activation of many downstream actions in response to various stimuli. There is strong evidence to indicate that information encoded within these stimulus-induced Ca2+ oscillations can provide signalling specificity. Such Ca2+ signals, or 'Ca2+ signatures', are generated in the cytosol, and in noncytosolic locations including the nucleus and chloroplast, through the coordinated action of Ca2+ influx and efflux pathways. An increased understanding of the functions and regulation of these various Ca2+ transporters has improved our appreciation of the role these transporters play in specifically shaping the Ca2+ signatures. Here we review the evidence which indicates that Ca2+ channel, Ca2+-ATPase and Ca2+ exchanger isoforms can indeed modulate specific Ca2+ signatures in response to an individual signal.

Phosphoinositides in plants: novel functions in membrane trafficking

Tight regulation of membrane trafficking is crucial to the proper maintenance of the endomembrane trafficking system of eukaryotic cells. Distinct organelles must maintain their identities while at the same time continuously accepting, sorting, and exchanging membrane and luminal cargo constituents. Additionally, many of these organelles differentiate specialized subdomains containing distinct sets of lipids and proteins and restrict certain aspects of membrane trafficking to these regions of the organelle. Phosphoinositides (PIs) are a class of membrane lipids that have emerged as key components in some of these membrane trafficking events. The ability of these lipids to be rapidly produced, modified, and hydrolyzed by distinct classes of phosphatidylinositol (PtdIns) kinases, phosphatases, and phospholipases, allows for their use as finely tuned spatial and temporal landmarks for organelle and sub-organelle domains. In this review we will attempt to highlight some of the recent studies of the roles of this class of lipids in plant membrane trafficking, particularly on their important roles in polarized membrane trafficking in plants.

Proteomic identification of putative plasmodesmatal proteins from Chara corallina

Plasmodesmata are channels that bridge the cell walls of plant cells, allowing regulated transport of molecules between neighbouring cells. We have used a proteomic strategy to identify putative plasmodesmata-associated proteins in the giant-celled green alga Chara corallina. Proteins were extracted from the plasmodesmata-rich nodal complexes and the middle of the long internodal cells, which do not contain plasmodesmata. Comparison of protein spot patterns generated by two-dimensional gel electrophoresis of both the soluble and cell wall fractions from the two cell types was done. Fifty-eight spots that were common to the nodal and internodal soluble fractions were analysed by matrix assisted laser desorption/ionisation-time of flight mass spectrometry, and peptide mass fingerprint data were used to search the database. Matches were made to four of these spots, in each case to housekeeping proteins. Further, a number of nodal specific spots were identified, 11 from the soluble fraction and nine from the wall fraction. These spots were excised from the gels and analysed by liquid chromatography tandem mass spectrometry to obtain peptide sequence. Database searches suggest that these spots include homologues to previously identified plasmodesmata-associated proteins cp-wap13 and heat shock cognate 70, as well as RNA-binding proteins, eukaryotic initiation factor 4A and a beta-1,3-glucanase. Several spots remained unidentified providing exciting new candidate plasmodesmata-associated proteins.

Pharmacological evidence that multiple phospholipid signaling pathways link Rhizobium nodulation factor perception in Medicago truncatula root hairs to intracellular responses, including Ca2+ spiking and specific ENOD gene expression

Rhizobium nodulation (Nod) factors are specific lipochito-oligosaccharide signals essential for initiating in root hairs of the host legume developmental responses that are required for controlled entry of the microsymbiont. In this article, we focus on the Nod factor signal transduction pathway leading to specific and cell autonomous gene activation in Medicago truncatula cv Jemalong in a study making use of the Nod factor-inducible MtENOD11 gene. First, we show that pharmacological antagonists that interfere with intracellular ion channel and Ca2+ pump activities are efficient blockers of Nod factor-elicited pMtENOD11-beta-glucuronidase (GUS) expression in root hairs of transgenic M. truncatula. These results indicate that intracellular Ca2+ release and recycling activities, essential for Ca2+ spiking, are also required for specific gene activation. Second, pharmacological effectors that inhibit phospholipase D and phosphoinositide-dependent phospholipase C activities are also able to block pMtENOD11-GUS activation, thus underlining a central role for multiple phospholipid signaling pathways in Nod factor signal transduction. Finally, pMtENOD11-GUS was introduced into all three Nod-/Myc- dmi M. truncatula mutant backgrounds, and gene expression was evaluated in response to the mastoparan peptide agonist Mas7. We found that Mas7 elicits root hair MtENOD11 expression in dmi1 and dmi2 mutants, but not in the dmi3 mutant, suggesting that the agonist acts downstream of DMI1/DMI2 and upstream of DMI3. In light of these results and the recently discovered identities of the DMI gene products, we propose an integrated cellular model for Nod factor signaling in legume root hairs in which phospholipids play a key role in linking the Nod factor perception apparatus to downstream components such as Ca2+ spiking and ENOD gene expression.

Don't shoot the (second) messenger: endomembrane transporters and binding proteins modulate cytosolic Ca2+ levels

Ca(2+) signal transduction requires the meticulous regulation of cytosolic Ca(2+) levels. Endomembrane Ca(2+) transporters and binding proteins are important components in partitioning these Ca(2+) signals to mediate cellular activity. Recently, many of these proteins have been characterized and mutant analysis suggests that these transporters form a network. Future attempts to manipulate plant Ca(2+) signaling must address all aspects of this complex.

Salt tolerance

Studying salt stress is an important means to the understanding of plant ion homeostasis and osmo-balance. Salt stress research also benefits agriculture because soil salinity significantly limits plant productivity on agricultural lands. Decades of physiological and molecular studies have generated a large body of literature regarding potential salt tolerance determinants. Recent advances in applying molecular genetic analysis and genomics tools in the model plant Arabidopsis thaliana are shading light on the molecular nature of salt tolerance effectors and regulatory pathways.

Pharmacological analysis of nod factor-induced calcium spiking in Medicago truncatula. Evidence for the requirement of type IIA calcium pumps and phosphoinositide signaling

Bacterial Nod factors trigger a number of cellular responses in root hairs of compatible legume hosts, which include periodic, transient increases in cytosolic calcium levels, termed calcium spiking. We screened 13 pharmaceutical modulators of eukaryotic signal transduction for effects on Nod factor-induced calcium spiking. The purpose of this screening was 2-fold: to implicate enzymes required for Nod factor-induced calcium spiking in Medicago sp., and to identify inhibitors of calcium spiking suitable for correlating calcium spiking to other Nod factor responses to begin to understand the function of calcium spiking in Nod factor signal transduction. 2-Aminoethoxydiphenylborate, caffeine, cyclopiazonic acid (CPA), 2,5-di-(t-butyl)-1,4-hydroquinone, and U-73122 inhibit Nod factor-induced calcium spiking. CPA and U-73122 are inhibitors of plant type IIA calcium pumps and phospholipase C, respectively, and implicate the requirement for these enzymes in Nod factor-induced calcium spiking. CPA and U-73122 inhibit Nod factor-induced calcium spiking robustly at concentrations with no apparent toxicity to root hairs, making CPA and U-73122 suitable for testing whether calcium spiking is causal to subsequent Nod factor responses.

Salt and drought stress signal transduction in plants

Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

FIERY1 encoding an inositol polyphosphate 1-phosphatase is a negative regulator of abscisic acid and stress signaling in Arabidopsis

The plant hormone abscisic acid (ABA) plays a wide range of important roles in plant growth and development, including embryogenesis, seed dormancy, root and shoot growth, transpiration, and stress tolerance. ABA and various abiotic stresses also activate the expression of numerous plant genes through undefined signaling pathways. To gain insight into ABA and stress signal transduction, we conducted a genetic screen based on ABA- and stress-inducible gene transcription. Here we report the identification of an Arabidopsis mutation, fiery1 (fry1), which results in super-induction of ABA- and stress-responsive genes. Seed germination and postembryonic development of fry1 are more sensitive to ABA or stress inhibition. The mutant plants are also compromised in tolerance to freezing, drought, and salt stresses. Map-based cloning revealed that FRY1 encodes an inositol polyphosphate 1-phosphatase, which functions in the catabolism of inositol 1, 4, 5-trisphosphate (IP(3)). Upon ABA treatment, fry1 mutant plants accumulated more IP(3) than did the wild-type plants. These results provide the first genetic evidence indicating that phosphoinositols mediate ABA and stress signal transduction in plants and their turnover is critical for attenuating ABA and stress signaling.

Unravelling response-specificity in Ca2+ signalling pathways in plant cells

Considerable advances have been made, both in the technologies available to study changes in intracellular cytosolic free Ca²⁺ ([Ca²⁺ ]_i ), and in our understanding of Ca²⁺ signalling cascades in plant cells, but how specificity can be generated from such a ubiquitous component as Ca²⁺ is questionable. Recently the concept of 'Ca²⁺ signatures' has been formulated; tight control of the temporal and spatial characteristics of alterations in [Ca²⁺ ]_i signals is thought to be responsible, at least in part, for the specificity of the response. However, the way in which Ca²⁺ signatures are decoded, which depends on the nature and location of the targets of the Ca²⁺ signals, has received little attention. In a few key systems, progress is being made on how diverse Ca²⁺ signatures might be transduced within cells in response to specific signals. Valuable pieces of the signal-specificity puzzle are being put together and this is illustrated here using some key examples; these emphasize the global importance of Ca²⁺ -mediated signal-transduction cascades in the responses of plants to a wide diversity of extracellular signals. However, the way in which signal specificity is encoded and transduced is still far from clear. Contents Summary 7 I. Introduction: Ca²⁺ as a signal transducer 8 II. Alterations in intracellular [Ca²⁺ ] 8 1. Measuring alterations in [Ca²⁺ ] 8 Imaging [Ca²⁺ ]_i using Ca²⁺ -sensitive dyes 8 Measuring [Ca²⁺ ]_i using aequorin 9 Imaging [Ca²⁺ ]_i using cameleon 10 2. The concept of the 'Ca²⁺ signature 10 3. How might specific Ca²⁺ signatures be generated? 11 Control of intracellular Ca²⁺ release 11 Control of influx of extracellular Ca²⁺ 12 4. Examples of Ca²⁺ signatures and cellular responses to increases in [Ca²⁺ ] 13 Ca²⁺ signatures in stomatal guard cells in response to abscisic acid signals 14 Ca²⁺ signals in response to abiotic stimuli1 8 Ca²⁺ signatures involved in plant-pathogen responses 19 Ca²⁺ signatures in control of plant reproduction 20 Ca²⁺ signatures in root hairs in response to nodulation signals 23 III. Decoding the [Ca²⁺ ]_i signatures 24 1. Coupling Ca²⁺ signals to responses through CaM 26 2. Coupling Ca²⁺ signals to responses through CDPK 27 3. Novel Ca²⁺ binding proteins as primary Ca²⁺ sensors 28 Conclusions and Perspective 28 References 29.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

Transport processes of solutes across the vacuolar membrane of higher plants

The central vacuole is the largest compartment of a mature plant cell and may occupy more than 80% of the total cell volume. However, recent results indicate that beside the large central vacuole, several small vacuoles may exist in a plant cell. These vacuoles often belong to different classes and can be distinguished either by their contents in soluble proteins or by different types of a major vacuolar membrane protein, the aquaporins. Two vacuolar proton pumps, an ATPase and a PPase energize vacuolar uptake of most solutes. The electrochemical gradient generated by these pumps can be utilized to accumulate cations by a proton antiport mechanism or anions due to the membrane potential difference. Uptake can be catalyzed by channels or by transporters. Growing evidence shows that for most ions more than one transporter/channel exist at the vacuolar membrane. Furthermore, plant secondary products may be accumulated by proton antiport mechanisms. The transport of some solutes such as sucrose is energized in some plants but occurs by facilitated diffusion in others. A new class of transporters has been discovered recently: the ABC type transporters are directly energized by MgATP and do not depend on the electrochemical force. Their substrates are organic anions formed by conjugation, e.g. to glutathione. In this review we discuss the different transport processes occurring at the vacuolar membrane and focus on some new results obtained in this field.

Subcellular localization of a high affinity binding site for D-myo-inositol 1,4,5-trisphosphate from Chenopodium rubrum

It is now generally accepted that a phosphoinositide cycle is involved in the transduction of a variety of signals in plant cells. In animal cells, the binding of D-myo-inositol 1,4,5-trisphosphate (InsP(3)) to a receptor located on the endoplasmic reticulum (ER) triggers an efflux of calcium release from the ER. Sites that bind InsP(3) with high affinity and specificity have also been described in plant cells, but their precise intracellular locations have not been conclusively identified. In contrast to animal cells, it has been suggested that in plants the vacuole is the major intracellular store of calcium involved in signal induced calcium release. The aim of this work was to determine the intracellular localization of InsP(3)-binding sites obtained from 3-week-old Chenopodium rubrum leaves. Microsomal membranes were fractionated by sucrose density gradient centrifugation in the presence and absence of Mg(2+) and alternatively by free-flow electrophoresis. An ER-enriched fraction was also prepared. The following enzymes were employed as specific membrane markers: antimycin A-insensitive NADH-cytochrome c reductase for ER, cytochrome c oxidase for mitochondrial membrane, pyrophosphatase for tonoplast, and 1,3-beta-D-glucansynthase for plasma membrane. In all membrane separations, InsP(3)-binding sites were concentrated in the fractions that were enriched with ER membranes. These data clearly demonstrate that the previously characterized InsP(3)-binding site from C. rubrum is localized on the ER. This finding supports previous suggestions of an alternative non-vacuolar InsP(3)-sensitive calcium store in plant cells.

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.

Calcium homeostasis and signaling in the blood-stage malaria parasite

The nature of the mechanisms underlying Ca2+ homeostasis in malaria parasites has puzzled investigators for almost two decades. This review summarizes the current knowledge about Ca2+ homeostasis in Plasmodium spp and highlights some key aspects of this process that are specific to this parasite. Plasmodium spp are exposed, during their intracellular stage, not to the usual millimolar concentrations of Ca2+ found in body fluids, but rather to the very low Ca2+ environment of the host cell cytoplasm. Two crucial questions then arise: (1) how is Ca2+ homeostasis achieved by these protozoa; and (2) do they use Ca2+-based signaling pathways? By critically reviewing the recent literature in the field, Célia Garcia here provides at least some partial answers to these questions.

Calcium-induced calcium release mediated by a voltage-activated cation channel in vacuolar vesicles from red beet

Little is known about the mechanisms underlying calcium-induced Ca2+ release (CICR) in plants. The slow-activating vacuolar (SV) channel is both permeable to, and activated by Ca2+, and is therefore a prime candidate for a role in CICR. Cytosol-side-out vacuolar membrane vesicles loaded with 45Ca2+ showed voltage- and Ca(2+)-dependent Ca2+ release, which was sensitive to the SV channel modulators DIDS, protein phosphatase 2B and calmodulin. Significantly, voltage-dependent Ca2+ release strongly depended on cytoplasmic Ca2+ concentrations. The results support the notion that CICR occurs in plant cells and that the process can be catalysed by the SV channel on the vacuolar membrane.

Mitochondrial contribution to the anoxic Ca2+ signal in maize suspension-cultured cells

Anoxia induces a rapid elevation of the cytosolic Ca2+ concentration ([Ca2+]cyt) in maize (Zea mays L.) cells, which is caused by the release of the ion from intracellular stores. This anoxic Ca2+ release is important for gene activation and survival in O2-deprived maize seedlings and cells. In this study we examined the contribution of mitochondrial Ca2+ to the anoxic [Ca2+]cyt elevation in maize cells. Imaging of intramitochondrial Ca2+ levels showed that a majority of mitochondria released their Ca2+ in response to anoxia and took up Ca2+ upon reoxygenation. We also investigated whether the mitochondrial Ca2+ release contributed to the increase in [Ca2+]cyt under anoxia. Analysis of the spatial association between anoxic [Ca2+]cyt changes and the distribution of mitochondrial and other intracellular Ca2+ stores revealed that the largest [Ca2+]cyt increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoromethoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca2+]cyt under aerobic conditions but prevented a [Ca2+]cyt increase in response to a subsequent anoxic pulse. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca2+]cyt and participate in the signaling of O2 deprivation.

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

Phosphatidylinositol-4-phosphate 5-kinase localized on the plasma membrane is essential for yeast cell morphogenesis

Phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2), an important element in eukaryotic signal transduction, is synthesized either by phosphatidylinositol-4-phosphate 5-kinase (PtdIns(4)P 5K) from phosphatidylinositol 4-phosphate (PtdIns(4)P) or by phosphatidylinositol-5-phosphate 4-kinase (PtdIns(5)P 4K) from phosphatidylinositol 5-phosphate (PtdIns(5)P). Two Saccharomyces cerevisiae genes, MSS4 and FAB1, are homologous to mammalian PtdIns(4)P 5Ks and PtdIns(5)P 4Ks. We show here that MSS4 is a functional homolog of mammalian PtdIns(4)P 5K but not of PtdIns(5)P 4K in vivo. We constructed a hemagglutinin epitope-tagged form of Mss4p and found that Mss4p has PtdIns(4)P 5K activity. Immunofluorescent and fractionation studies of the epitope-tagged Mss4p suggest that Mss4p is localized on the plasma membrane, whereas Fab1p is reportedly localized on the vacuolar membrane. A temperature-sensitive mss4-1 mutant was isolated, and its phenotypes at restrictive temperatures were found to include increased cell size, round shape, random distribution of actin patches, and delocalized staining of cell wall chitin. Thus, biochemical and genetic analyses on Mss4p indicated that yeast PtdIns(4)P 5K localized on the plasma membrane is required for actin organization.

Strontium-induced repetitive calcium spikes in a unicellular green alga

The divalent cation Sr2+ induced repetitive transient spikes of the cytosolic Ca2+ activity [Ca2+]cy and parallel repetitive transient hyperpolarizations of the plasma membrane in the unicellular green alga Eremosphaera viridis. [Ca2+]cy measurements, membrane potential measurements, and cation analysis of the cells were used to elucidate the mechanism of Sr2+-induced [Ca2+]cy oscillations. Sr2+ was effectively and rapidly compartmentalized within the cell, probably into the vacuole. The [Ca2+]cy oscillations cause membrane potential oscillations, and not the reverse. The endoplasmic reticulum (ER) Ca2+-ATPase blockers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr2+-induced repetitive [Ca2+]cy spikes, whereas the compartmentalization of Sr2+ was not influenced. A repetitive Ca2+ release and Ca2+ re-uptake by the ER probably generated repetitive [Ca2+]cy spikes in E. viridis in the presence of Sr2+. The inhibitory effect of ruthenium red and ryanodine indicated that the Sr2+-induced Ca2+ release from the ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca2+ channel. The blockage of Sr2+-induced repetitive [Ca2+]cy spikes by La3+ or Gd3+ indicated the necessity of a certain influx of divalent cations for sustained [Ca2+]cy oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca2+]cy oscillations in E. viridis.

Inositol 1,4,5-trisphosphate induced Ca2+ release from chloroquine-sensitive and -insensitive intracellular stores in the intraerythrocytic stage of the malaria parasite P. chabaudi

Isolated P. chabaudi parasites were permeabilized with digitonin and the function of intracellular Ca2+ stores was studied using the Ca2+ indicators arsenazo III or Fluo 3-acid in the medium. Addition of the second messenger InsP3 (5 microM) to permeabilized parasites leads to Ca2+ release into the medium, with the mean extent of release being 40 nmol Ca2+/10(8) cells. This Ca2+ release was completely abolished in the presence of heparin, an InsP3 receptor antagonist. The amount of Ca2+ released was approximately 50% reduced when InsP3 was added subsequent to the discharge of the endoplasmic reticulum (ER) Ca2+ pool with the SERCA (sarcoplasmic ER Ca2+ ATPase) inhibitors thapsigargin and tBHQ (2,5-di(ter-butyl)-1,4 benzohydroquinone). The thapsigargin- and tBHQ-sensitive pool account for 20 nmol of Ca2+/10(8) cells. If InsP3 was added after the discharge of the residual Ca2+ by addition of either the K+/H+ uncoupler nigericin or the antimalarial drug chloroquine, no further Ca2+ release was observed. This is the first report of InsP3-induced Ca2+ release in a parasite protozoa. In addition our finding that chloroquine depletes an InsP3-sensitive Ca2+ compartment, raises the possibility that the InsP3-dependent Ca2+ release from this store might be important for the regulation of growth and differentiation of the parasite.

Mastoparan induces an increase in cytosolic calcium ion concentration and subsequent activation of protein kinases in tobacco suspension culture cells

Mastoparan induced a transient elevation of cytosolic free Ca2+ concentration ([Ca2+]cyt) in tobacco suspension culture cells. The mastoparan-induced [Ca2+]cyt elevation was inhibited by 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl and neomycin but not by depletion of extracellular Ca2+, suggesting that the elevation was the result of Ca2+ release from the intracellular stores caused by stimulation of phosphoinositide turnover. Hydrogen peroxide which has been shown to induce an oxidative burst in soybean cells by mastoparan treatment [L. Legendre, P.F. Heinstein, P.S. Low, Evidence for participation of GTP-binding proteins in elicitation of the rapid oxidative burst in cultured soybean cells, J. Biol. Chem., 267 (1992) 20140-20147], also induced a transient [Ca2+]cyt elevation in the tobacco cells. However, mastoparan did not induce an oxidative burst in the tobacco cells. Activation of a 50, a 75 and a 80 kDa protein kinases after the mastoparan-induced [Ca2+]cyt elevation was shown by an in-gel protein kinase assay. This activation was inhibited by neomycin, suggesting that the [Ca2+]cyt elevation is necessary for the mastoparan-induced activation of the protein kinases. The activation was inhibited also by pretreatment with staurosporine and was sustained by pretreatment with calyculin A, suggesting that the protein kinase activity is regulated by protein phosphorylation/dephosphorylation. The present report shows that mastoparan induces an increase in [Ca2+]cyt without oxidative burst and subsequent activation of protein kinases in tobacco cells.

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.

Tansley Review No. 86 Accumulation of phytoalexins: defence mechanism and stimulus response system

Phytoalexin synthesis is a defence-response- that is characterized by a requirement for a number of distinct elements, all of which must be present for the response to be expressed fully. These same elements: a signal, a cellular receptor, a signal transduction system and a responsive metabolic system, are also used to describe a stimulus-response system. A number of molecular species can function as signal molecules or elicitors of phytoalexin synthesis, including poly- and oligosaccharides, proteins and polypeptides, and fatty acids. Few receptors for elicitors have been identified but those that have been are proteins located on the plasma membrane of the plant. Induction of phytoalexin synthesis involves selective and co-ordinated activation of specific defence response genes, including those encoding the enzymes of phytoalexin synthesis, and these genes constitute the responsive metabolic system. The separate, and distant, locations of the receptor and the responsive genes means that the event in which the signal is perceived by the receptor must be relayed to the genes by means of a second messenger system. Several second messengers are candidates for such a coupling- or signal transduction-system, including udenosine-3',5'-cyclic monophosphate, Ca²⁺ , diacylglycerol and inositol 1,4,5-trisphosphate, active oxygen species and jasmonic acid. Each has been examined as a possible component of the signal transduction system mediating between the elicitor receptor interaction and the phytoalexin synthesis it induces. Analysis of the signalling events is made complex by the simultaneous solicitation by the invading micro-organism of several defence responses, each of which might involve elements of a different signal system. The same complexity is evident which the role of phytoalexin accumulation in resistance is analysed. Evaluation of the contribution made by phytoalexin accumulation towards resistance has been attempted by the use of various inhibitors and enhancers of the process. Transgenic and mutant plants with specific alterations in one or more ot those elements necessary for the plant to respond to the signals for phytoalexin synthesis and other defence responses, are beginning to aid resolution of the complex pattern ot signalling events and the respective roles of the inducible defence mechanisms in resistance. CONTENTS Summary 1 I. Introduction 2 II. Chemistry of phytoalexins 3 III. Phytoalexin accumulation as a determinant of resistance 6 IV. Elicitation of phytoalexin accumulation 11 References 34.

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.