Metabolic relations of inositol 3,4,5,6-tetrakisphosphate revealed by cell permeabilization. Identification of inositol 3,4,5, 6-tetrakisphosphate 1-kinase and inositol 3,4,5,6-tetrakisphosphate phosphatase activities in mesophyll cells

Protein pyrophosphorylation by inositol phosphates: a novel post-translational modification in plants?

Inositol pyrophosphates (PP-InsPs) are energy-rich molecules harboring one or more diphosphate moieties. PP-InsPs are found in all eukaryotes evaluated and their functional versatility is reflected in the various cellular events in which they take part. These include, among others, insulin signaling and intracellular trafficking in mammals, as well as innate immunity and hormone and phosphate signaling in plants. The molecular mechanisms by which PP-InsPs exert such functions are proposed to rely on the allosteric regulation via direct binding to proteins, by competing with other ligands, or by protein pyrophosphorylation. The latter is the focus of this review, where we outline a historical perspective surrounding the first findings, almost 20 years ago, that certain proteins can be phosphorylated by PP-InsPs in vitro. Strikingly, in vitro phosphorylation occurs by an apparent enzyme-independent but Mg2+-dependent transfer of the β-phosphoryl group of an inositol pyrophosphate to an already phosphorylated serine residue at Glu/Asp-rich protein regions. Ribosome biogenesis, vesicle trafficking and transcription are among the cellular events suggested to be modulated by protein pyrophosphorylation in yeast and mammals. Here we discuss the latest efforts in identifying targets of protein pyrophosphorylation, pointing out the methodological challenges that have hindered the full understanding of this unique post-translational modification, and focusing on the latest advances in mass spectrometry that finally provided convincing evidence that PP-InsP-mediated pyrophosphorylation also occurs in vivo. We also speculate about the relevance of this post-translational modification in plants in a discussion centered around the protein kinase CK2, whose activity is critical for pyrophosphorylation of animal and yeast proteins. This enzyme is widely present in plant species and several of its functions overlap with those of PP-InsPs. Until now, there is virtually no data on pyrophosphorylation of plant proteins, which is an exciting field that remains to be explored.

An ATP-responsive metabolic cassette comprised of inositol tris/tetrakisphosphate kinase 1 (ITPK1) and inositol pentakisphosphate 2-kinase (IPK1) buffers diphosphosphoinositol phosphate levels

Inositol polyphosphates are ubiquitous molecular signals in metazoans, as are their pyrophosphorylated derivatives that bear a so-called ‘high-energy’ phosphoanhydride bond. A structural rationale is provided for the ability of Arabidopsis inositol tris/tetrakisphosphate kinase 1 to discriminate between symmetric and enantiomeric substrates in the production of diverse symmetric and asymmetric myo-inositol phosphate and diphospho-myo-inositol phosphate (inositol pyrophosphate) products. Simple tools are applied to chromatographic resolution and detection of known and novel diphosphoinositol phosphates without resort to radiolabeling approaches. It is shown that inositol tris/tetrakisphosphate kinase 1 and inositol pentakisphosphate 2-kinase comprise a reversible metabolic cassette converting Ins(3,4,5,6)P₄ into 5-InsP₇ and back in a nucleotide-dependent manner. Thus, inositol tris/tetrakisphosphate kinase 1 is a nexus of bioenergetics status and inositol polyphosphate/diphosphoinositol phosphate metabolism. As such, it commands a role in plants that evolution has assigned to a different class of enzyme in mammalian cells. The findings and the methods described will enable a full appraisal of the role of diphosphoinositol phosphates in plants and particularly the relative contribution of reversible inositol phosphate hydroxykinase and inositol phosphate phosphokinase activities to plant physiology.

Impact of Cross-Breeding of Low Phytic Acid MIPS1 and IPK1 Soybean ( Glycine max L. Merr.) Mutants on Their Contents of Inositol Phosphate Isomers

The knowledge on consequences of cross-breeding of induced low phytic acid ( lpa) soybean ( Glycine max L. Merr.) mutants on the contents of phytic acid (InsP6) and lower inositol phosphate isomers (InsP2-InsP5) in the resulting progenies is limited. Therefore, MIPS1 and IPK1 lpa soybean mutants were crossed with wild-type (WT) cultivars or among themselves to generate homozygous lpa and WT progenies and double lpa mutants. The lpa trait of the MIPS1 mutant was not altered by cross-breeding with a WT cultivar; lpa progenies had InsP6 reductions of about 44% compared to WT progenies. IPK1 progenies showed pronounced accumulations of specific InsP3-InsP5 isomers (up to 12.4 mg/g) compared to the progenitor lpa mutant (4.7 mg/g); the extent of InsP6 reduction (43-71%) was depending on the WT crossing parent. Double mutants exhibited the most pronounced InsP6 reductions (up to 87%), accompanied by moderate accumulations of InsP3-InsP5 (2.5 mg/g). Cross-breeding offers the potential to modulate the amounts of both InsP6 and InsP3-InsP5 contents in lpa soybean mutants and thus to improve their nutritional quality.

Arabidopsis inositol phosphate kinases IPK1 and ITPK1 constitute a metabolic pathway in maintaining phosphate homeostasis

Emerging studies have suggested that there is a close link between inositol phosphate (InsP) metabolism and cellular phosphate (Pi ) homeostasis in eukaryotes; however, whether a common InsP species is deployed as an evolutionarily conserved metabolic messenger to mediate Pi signaling remains unknown. Here, using genetics and InsP profiling combined with Pi -starvation response (PSR) analysis in Arabidopsis thaliana, we showed that the kinase activity of inositol pentakisphosphate 2-kinase (IPK1), an enzyme required for phytate (inositol hexakisphosphate; InsP6 ) synthesis, is indispensable for maintaining Pi homeostasis under Pi -replete conditions, and inositol 1,3,4-trisphosphate 5/6-kinase 1 (ITPK1) plays an equivalent role. Although both ipk1-1 and itpk1 mutants exhibited decreased levels of InsP6 and diphosphoinositol pentakisphosphate (PP-InsP5 ; InsP7 ), disruption of another ITPK family enzyme, ITPK4, which correspondingly caused depletion of InsP6 and InsP7 , did not display similar Pi -related phenotypes, which precludes these InsP species from being effectors. Notably, the level of d/l-Ins(3,4,5,6)P4 was concurrently elevated in both ipk1-1 and itpk1 mutants, which showed a specific correlation with the misregulated Pi phenotypes. However, the level of d/l-Ins(3,4,5,6)P4 is not responsive to Pi starvation that instead manifests a shoot-specific increase in the InsP7 level. This study demonstrates a more nuanced picture of the intersection of InsP metabolism and Pi homeostasis and PSRs than has previously been elaborated, and additionally establishes intermediate steps to phytate biosynthesis in plant vegetative tissues.

Molecular and biochemical identification of inositol 1,3,4,5,6-pentakisphosphate 2-kinase encoding mRNA variants in castor bean (Ricinus communis L.) seeds

During seed development, phytic acid (PA) associated with mineral cations is stored as phytin and mobilized following germination in support of seedling growth. Two parallel biosynthetic pathways for PA have been proposed; yet the pathway is still poorly understood in terms of its regulation and the enzymes involved. Here, the castor bean (Ricinus communis L.) gene for inositol 1,3,4,5,6-pentakisphosphate 2-kinase (RcIPK1) has been identified. This encodes the enzyme implicated in catalyzing the final reaction in PA biosynthesis, and its expression is enhanced in isolated germinated embryos by application of phosphate and myo-inositol (Ins). Even though only one copy of the RcIPK1 gene is present in the genome, numerous RNA variants are present, most likely due to alternative splicing. These are translated into six closely related protein isoforms according to in silico analysis. Functional analyses using yeast ipk1Δ revealed that only three of the mRNA variants can rescue a temperature-sensitive growth phenotype of this strain. High-performance liquid chromatography (HPLC) analysis of the synthesized inositol phosphates demonstrated that the ability to complement the missing yeast IPK1 enzyme is associated with the production of enzyme activity. The three active isoforms possess unique conserved motifs important for IPK1 catalytic activity.

Measurement of inositol (1,4,5) trisphosphate in plant tissues by a competitive receptor binding assay

The phosphoinositide signaling pathway is important for plant responses to many different stresses. As part of the responses to a stimulus, InsP3 levels may increase rapidly and transiently. The receptor binding assay for InsP3 described here is easy to use and an ideal method to monitor and compare InsP3 levels in multiple samples from large scale experiments. The method is based on competitive binding of InsP3 to the mammalian brain InsP3 specific receptor protein. This chapter describes a protocol for extracting and neutralizing plant samples and performing the receptor binding assay (using a commercially available kit). The protocol described has been used effectively to monitor InsP3 levels in plant tissues of different origin and in response to different stresses.

Synthesis of inositol phosphate ligands of plant hormone-receptor complexes: pathways of inositol hexakisphosphate turnover

Reduction of phytate is a major goal of plant breeding programs to improve the nutritional quality of crops. Remarkably, except for the storage organs of crops such as barley, maize and soybean, we know little of the stereoisomeric composition of inositol phosphates in plant tissues. To investigate the metabolic origins of higher inositol phosphates in photosynthetic tissues, we have radiolabelled leaf tissue of Solanum tuberosum with myo-[2-3H]inositol, undertaken a detailed analysis of inositol phosphate stereoisomerism and permeabilized mesophyll protoplasts in media containing inositol phosphates. We describe the inositol phosphate composition of leaf tissue and identify pathways of inositol phosphate metabolism that we reveal to be common to other kingdoms. Our results identify the metabolic origins of a number of higher inositol phosphates including ones that are precursors of cofactors, or cofactors of plant hormone-receptor complexes. The present study affords alternative explanations of the effects of disruption of inositol phosphate metabolism reported in other species, and identifies different inositol phosphates from that described in photosynthetic tissue of the monocot Spirodela polyrhiza. We define the pathways of inositol hexakisphosphate turnover and shed light on the occurrence of a number of inositol phosphates identified in animals, for which metabolic origins have not been defined.

Spatial and temporal integration of signalling networks regulating pollen tube growth

The overall function of a cell is determined by its contingent of active signal transduction cascades interacting on multiple levels with metabolic pathways, cytoskeletal organization, and regulation of gene expression. Much work has been devoted to analysis of individual signalling cascades interacting with unique cellular targets. However, little is known about how cells integrate information across hierarchical signalling networks. Recent work on pollen tube growth indicates that several key signalling cascades respond to changes in cell hydrodynamics and apical volume. Combined with known effects on cytoarchitecture and signalling from other cell systems, hydrodynamics has the potential to integrate and synchronize the function of the broader signalling network in pollen tubes. This review will explore recent work on cell hydrodynamics in a variety of systems including pollen, and discuss hydrodynamic regulation of cell signalling and function including exocytosis and endocytosis, actin cytoskeleton reorganization, cell wall deposition and assembly, phospholipid and inositol polyphosphate signalling, ion flux, small G-proteins, fertilization, and self-incompatibility. The combined data support a newly emerging model of pollen tube growth.

Functional and physiological characterization of Arabidopsis INOSITOL TRANSPORTER1, a novel tonoplast-localized transporter for myo-inositol

Arabidopsis thaliana INOSITOL TRANSPORTER1 (INT1) is a member of a small gene family with only three more genes (INT2 to INT4). INT2 and INT4 were shown to encode plasma membrane-localized transporters for different inositol epimers, and INT3 was characterized as a pseudogene. Here, we present the functional and physiological characterization of the INT1 protein, analyses of the tissue-specific expression of the INT1 gene, and analyses of phenotypic differences observed between wild-type plants and mutant lines carrying the int1.1 and int1.2 alleles. INT1 is a ubiquitously expressed gene, and Arabidopsis lines with T-DNA insertions in INT1 showed increased intracellular myo-inositol concentrations and reduced root growth. In Arabidopsis, tobacco (Nicotiana tabacum), and Saccharomyces cerevisiae, fusions of the green fluorescent protein to the C terminus of INT1 were targeted to the tonoplast membranes. Finally, patch-clamp analyses were performed on vacuoles from wild-type plants and from both int1 mutant lines to study the transport properties of INT1 at the tonoplast. In summary, the presented molecular, physiological, and functional studies demonstrate that INT1 is a tonoplast-localized H(+)/inositol symporter that mediates the efflux of inositol that is generated during the degradation of inositol-containing compounds in the vacuolar lumen.

Inositol 1,3,4,5,6-pentakisphosphate 2-kinase from maize: molecular and biochemical characterization

Inositol 1,3,4,5,6-pentakisphosphate 2-kinase, an enzyme encoded by the gene IPK1, catalyzes the terminal step in the phytic acid biosynthetic pathway. We report here the isolation and characterization of IPK1 cDNA and genomic clones from maize (Zea mays). DNA Southern-blot analysis revealed that ZmIPK1 in the maize genome constitutes a small gene family with two members. Two nearly identical ZmIPK1 paralogs, designated as ZmIPK1A and ZmIPK1B, were identified. The transcripts of ZmIPK1A were detected in various maize tissues, including leaves, silks, immature ears, seeds at 12 d after pollination, midstage endosperm, and maturing embryos. However, the transcripts of ZmIPK1B were exclusively detected in roots. A variety of alternative splicing products of ZmIPK1A were discovered in maize leaves and seeds. These products are derived from alternative acceptor sites, alternative donor sites, and retained introns in the transcripts. Consequently, up to 50% of the ZmIPK1A transcripts in maize seeds and leaves have an interrupted open reading frame. In contrast, only one type of splicing product of ZmIPK1B was detected in roots. When expressed in Escherichia coli and subsequently purified, the ZmIPK1 enzyme catalyzes the conversion of myo-inositol 1,3,4,5,6-pentakisphosphate to phytic acid. In addition, it is also capable of catalyzing the phosphorylation of myo-inositol 1,4,6-trisphosphate, myo-inositol 1,4,5,6-tetrakisphosphate, and myo-inositol 3,4,5,6-tetrakisphosphate. Nuclear magnetic resonance spectroscopy analysis indicates that the phosphorylation product of myo-inositol 1,4,6-trisphosphate is inositol 1,2,4,6-tetrakisphosphate. Kinetic studies showed that the K(m) for ZmIPK1 using myo-inositol 1,3,4,5,6-pentakisphosphate as a substrate is 119 microm with a V(max) at 625 nmol/min/mg. These data describing the tissue-specific accumulation and alternative splicing of the transcripts from two nearly identical ZmIPK1 paralogs suggest that maize has a highly sophisticated regulatory mechanism controlling phytic acid biosynthesis.

Characterization of an Arabidopsis inositol 1,3,4,5,6-pentakisphosphate 2-kinase (AtIPK1)

The metabolic pathway(s) by which plants synthesize InsP6 (inositol 1,2,3,4,5,6-hexakisphosphate) remains largely undefined [Shears (1998) Biochim. Biophys. Acta 1436, 49-67], while the identities of the genes that encode enzymes catalysing individual steps in these pathways are, with the notable exception of myo-inositol phosphate synthase and ZmIpk [Shi, Wang, Wu, Hazebroek, Meeley and Ertl (2003) Plant Physiol. 131, 507-515], unidentified. A yeast enzyme, ScIPK1, catalyses the synthesis of InsP6 by 2-phosphorylation of Ins(1,3,4,5,6)P5 (inositol 1,3,4,5,6-pentakisphosphate). A human orthologue, HsIPK1, is able to substitute for yeast ScIPK1, restoring InsP6 production in a Saccharomyces cerevisiae mutant strain lacking the ScIPK1 open reading frame (ScIpk1Delta). We have identified an Arabidopsis genomic sequence, AtIPK1, encoding an Ins(1,3,4,5,6)P5 2-kinase. Inclusion of the AtIPK1 protein in alignments of amino acid sequences reveals that human and Arabidopis kinases are more similar to each other than to the S. cerevisiae enzyme, and further identifies an additional motif. Recombinant AtIPK1 protein expressed in Escherichia coli catalysed the synthesis of InsP6 from Ins(1,3,4,5,6)P5. The enzyme obeyed Michaelis-Menten kinetics with an apparent V(max) of 35 nmol x min(-1) x (mg of protein)(-1) and a K(m) for Ins(1,3,4,5,6)P5 of 22 microM at 0.4 mM ATP. RT (reverse transcriptase)-PCR analysis of AtIPK1 transcripts revealed that AtIPK1 is expressed in siliques, leaves and cauline leaves. In situ hybridization experiments further revealed strong expression of AtIPK1 in male and female organs of flower buds. Expression of AtIPK1 protein in an ScIpk1Delta mutant strain restored InsP6 production and rescued the temperature-sensitive growth phenotype of the yeast.

Phytic acid synthesis and vacuolar accumulation in suspension-cultured cells of Catharanthus roseus induced by high concentration of inorganic phosphate and cations

We have established a new system for studying phytic acid, myo-inositol hexakisphosphate (InsP(6)) synthesis in suspension-cultured cells of Catharanthus. InsP(6) and other intermediates of myo-inositol (Ins) phosphate metabolism were measured using an ion chromatography method. The detection limit for InsP(6) was less than 50 nM, which was sufficient to analyze Ins phosphates in living cells. Synthesis of Ins phosphates was induced by incubation in high inorganic phosphate medium. InsP(6) was mainly accumulated in vacuoles and was enhanced when cells were grown in high concentration of inorganic phosphates with the cations K(+), Ca(2+), or Zn(2+). However, there was a strong tendency for InsP(6) to accumulate in the vacuole in the presence of Ca(2+) and in nonvacuolar compartments when supplied with Zn(2+), possibly due to precipitation of InsP(6) with Zn(2+) in the cytosol. A vesicle transport inhibitor, brefeldin A, stimulated InsP(6) accumulation. The amounts of both Ins(3)P(1) myo-inositol monophosphate synthase, a key enzyme for InsP(6) synthesis, and Ins(1,4,5)P(3) kinase were unrelated to the level of accumulation of InsP(6). The mechanisms for InsP(6) synthesis and localization into vacuoles in plant cells are discussed.

Diastereoselective Synthesis of D- and L-myo-Inositol 3,4,5,6-Tetrakisphosphates from D-Glucose via Dihydroxylation of (+)-Conduritol B Derivatives

Syntheses of D- and L-myo-inositol 3,4,5,6-tetrakisphosphates were achieved via diastereoselective 1,2-addition of vinylcopper reagent with the chiral aldehyde prepared from 1,2,5,6-diisopropylidene-D-glucose, ring-closing metathesis of 1,7-diene with Grubbs catalyst followed by catalytic OsO(4) dihydroxylation of (+)-conduritol B derivatives.

State and spectral properties of chloride oscillations in pollen

Pollen tube growth is a dynamic system expressing a number of oscillating circuits. Our recent work identified a new circuit, oscillatory efflux of Cl(-) anion from the pollen tube apex. Cl(-) efflux is the first ion signal found to be coupled in phase with growth oscillations. Functional analyses indicate an active role for Cl(-) flux in pollen tube growth. In this report the dynamical properties of Cl(-) efflux are examined. Phase space analysis demonstrates that the system trajectory converges on a limit cycle. Fourier analysis reveals that two harmonic frequencies characterize normal growth. Cl(-) efflux is inhibited by the channel blocker DIDS, is stimulated by hypoosmotic treatment, and is antagonized by the signal encoded in inositol 3,4,5,6-tetrakisphosphate. These perturbations induce transitions of the limit cycle to new metastable states or cause system collapse to a static attractor centered near the origin. These perturbations also transform the spectral profile, inducing subharmonic frequencies, transitions to period doubling and tripling, superharmonic resonance, and chaos. These results indicate that Cl(-) signals in pollen tubes display features that are characteristic of active oscillators that carry frequency-encoded information. A reaction network of the Cl(-) oscillator coupled to two nonlinear feedback circuits that may drive pollen tube growth oscillations is considered.

Arabidopsis inositol polyphosphate 6-/3-kinase is a nuclear protein that complements a yeast mutant lacking a functional ArgR-Mcm1 transcription complex

Inositol 1,4,5-trisphosphate 3-kinase, and more generally inositol polyphosphate kinases (Ipk), play important roles in signal transduction in animal cells; however, their functions in plant cells remain to be elucidated. Here, we report the molecular cloning of a cDNA (AtIpk2beta) from a higher plant, Arabidopsis. Arabidopsis AtIpk2beta is a 33-kD protein that exhibits weak homology ( approximately 25% identical amino acids) with Ipk proteins from animals and yeast and lacks a calmodulin binding site, as revealed by sequence analysis and calmodulin binding assays. However, recombinant AtIpk2beta phosphorylates inositol 1,4,5-trisphosphate to inositol 1,4,5,6-tetrakisphosphate and also converts it to inositol 1,3,4,5,6-pentakisphosphate [Ins(1,3,4,5,6)P(5)]. AtIpk2beta also phosphorylates inositol 1,3,4,5-tetrakisphosphate to Ins(1,3,4,5,6)P(5). Thus, the enzyme is a D3/D6 dual-specificity inositol phosphate kinase. AtIpk2beta complements a yeast ARG82/IPK2 mutant lacking a functional ArgR-Mcm1 transcription complex. This complex is involved in regulating Arg metabolism-related gene expression and requires inositol polyphosphate kinase activity to function. AtIpk2beta was found to be located predominantly in the nucleus of plant cells, as demonstrated by immunolocalization and fusion to green fluorescent protein. RNA gel blot analysis and promoter-beta-glucuronidase reporter gene studies demonstrated AtIpk2beta gene expression in various organs tested. These data suggest a role for AtIpk2beta as a transcriptional control mediator in plants.

Phospholipid-based signaling in plants

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.

Up-regulation of phosphoinositide metabolism in tobacco cells constitutively expressing the human type I inositol polyphosphate 5-phosphatase

To evaluate the impact of suppressing inositol 1,4,5-trisphosphate (InsP(3)) in plants, tobacco (Nicotiana tabacum) cells were transformed with the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme which specifically hydrolyzes InsP(3). The transgenic cell lines showed a 12- to 25-fold increase in InsP 5-ptase activity in vitro and a 60% to 80% reduction in basal InsP(3) compared with wild-type cells. Stimulation with Mas-7, a synthetic analog of the wasp venom peptide mastoparan, resulted in an approximately 2-fold increase in InsP(3) in both wild-type and transgenic cells. However, even with stimulation, InsP(3) levels in the transgenic cells did not reach wild-type basal values, suggesting that InsP(3) signaling is compromised. Analysis of whole-cell lipids indicated that phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)), the lipid precursor of InsP(3), was greatly reduced in the transgenic cells. In vitro assays of enzymes involved in PtdInsP(2) metabolism showed that the activity of the PtdInsP(2)-hydrolyzing enzyme phospholipase C was not significantly altered in the transgenic cells. In contrast, the activity of the plasma membrane PtdInsP 5 kinase was increased by approximately 3-fold in the transgenic cells. In vivo labeling studies revealed a greater incorporation of (32)P into PtdInsP(2) in the transgenic cells compared with the wild type, indicating that the rate of PtdInsP(2) synthesis was increased. These studies show that the constitutive expression of the human type I InsP 5-ptase in tobacco cells leads to an up-regulation of the phosphoinositide pathway and highlight the importance of PtdInsP(2) synthesis as a regulatory step in this system.

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.

Inositol hexakisphosphate is a physiological signal regulating the K+-inward rectifying conductance in guard cells

(RS)-2-cis, 4-trans-abscisic acid (ABA), a naturally occurring plant stress hormone, elicited rapid agonist-specific changes in myo-inositol hexakisphosphate (InsP(6)) measured in intact guard cells of Solanum tuberosum (n = 5); these changes were not reproduced by (RS)-2-trans, 4-trans-abscisic acid, an inactive stereoisomer of ABA (n = 4). The electrophysiological effects of InsP(6) were assessed on both S. tuberosum (n = 14) and Vicia faba (n = 6) guard cell protoplasts. In both species, submicromolar concentrations of InsP(6), delivered through the patch electrode, mimicked the inhibitory effects of ABA and internal calcium (Ca(i)(2+)) on the inward rectifying K(+) current, I(K,in), in a dose-dependent manner. Steady state block of I(K,in) by InsP(6) was reached much more quickly in Vicia (3 min at approximately 1 microM) than Solanum (20-30 min). The effects of InsP(6) on I(K,in) were specific to the myo-inositol isomer and were not elicited by other conformers of InsP(6) (e.g., scyllo- or neo-). Chelation of Ca(2+) by inclusion of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or EGTA in the patch pipette together with InsP(6) prevented the inhibition of I(K,in), suggesting that the effect is Ca(2+) dependent. InsP(6) was approximately 100-fold more potent than Ins(1,4,5)P(3) in modulating I(K,in). Thus ABA increases InsP(6) in guard cells, and InsP(6) is a potent Ca(2+)-dependent inhibitor of I(K,in). Taken together, these results suggest that InsP(6) may play a major role in the physiological response of guard cells to ABA.

Dr Leonard Arthur: his trial and its implications

Oscillatory growth of pollen tubes has been correlated with oscillatory influxes of the cations Ca(2+), H(+), and K(+). Using an ion-specific vibrating probe, a new circuit was identified that involves oscillatory efflux of the anion Cl(-) at the apex and steady influx along the tube starting at 12 mum distal to the tip. This spatial coupling of influx and efflux sites predicts that a vectorial flux of Cl(-) ion traverses the apical region. The Cl(-) channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid completely inhibited tobacco pollen tube growth at 80 and 20 muM, respectively. Cl(-) channel blockers also induced increases in apical cell volume. The apical 50 mum of untreated pollen tubes had a mean cell volume of 3905 +/- 75 mum(3). DIDS at 80 muM caused a rapid and lethal cell volume increase to 6206 +/- 171 mum(3), which is at the point of cell bursting at the apex. DIDS was further demonstrated to disrupt Cl(-) efflux from the apex, indicating that Cl(-) flux correlates with pollen tube growth and cell volume status. The signal encoded by inositol 3,4,5,6-tetrakisphosphate [Ins(3,4,5,6)P(4)] antagonized pollen tube growth, induced cell volume increases, and disrupted Cl(-) efflux. Ins(3,4,5,6)P(4) decreased the mean growth rate by 85%, increased the cell volume to 5997 +/- 148 mum(3), and disrupted normal Cl(-) efflux oscillations. These effects were specific for Ins(3,4,5,6)P(4) and were not mimicked by either Ins(1,3,4,5)P(4) or Ins(1,3,4,5,6)P(5). Growth correlation analysis demonstrated that cycles of Cl(-) efflux were coupled to and temporally in phase with cycles of growth. A role for Cl(-) flux in the dynamic cellular events during growth is assessed. Differential interference contrast microscopy and kymographic analysis of individual growth cycles revealed that vesicles can advance transiently to within 2 to 4 mum of the apex during the phase of maximally increasing Cl(-) efflux, which temporally overlaps the phase of cell elongation during the growth cycle. In summary, these investigations indicate that Cl(-) ion dynamics are an important component in the network of events that regulate pollen tube homeostasis and growth.

Dr Leonard Arthur: his trial and its implications

Oscillatory growth of pollen tubes has been correlated with oscillatory influxes of the cations Ca(2+), H(+), and K(+). Using an ion-specific vibrating probe, a new circuit was identified that involves oscillatory efflux of the anion Cl(-) at the apex and steady influx along the tube starting at 12 mum distal to the tip. This spatial coupling of influx and efflux sites predicts that a vectorial flux of Cl(-) ion traverses the apical region. The Cl(-) channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid completely inhibited tobacco pollen tube growth at 80 and 20 muM, respectively. Cl(-) channel blockers also induced increases in apical cell volume. The apical 50 mum of untreated pollen tubes had a mean cell volume of 3905 +/- 75 mum(3). DIDS at 80 muM caused a rapid and lethal cell volume increase to 6206 +/- 171 mum(3), which is at the point of cell bursting at the apex. DIDS was further demonstrated to disrupt Cl(-) efflux from the apex, indicating that Cl(-) flux correlates with pollen tube growth and cell volume status. The signal encoded by inositol 3,4,5,6-tetrakisphosphate [Ins(3,4,5,6)P(4)] antagonized pollen tube growth, induced cell volume increases, and disrupted Cl(-) efflux. Ins(3,4,5,6)P(4) decreased the mean growth rate by 85%, increased the cell volume to 5997 +/- 148 mum(3), and disrupted normal Cl(-) efflux oscillations. These effects were specific for Ins(3,4,5,6)P(4) and were not mimicked by either Ins(1,3,4,5)P(4) or Ins(1,3,4,5,6)P(5). Growth correlation analysis demonstrated that cycles of Cl(-) efflux were coupled to and temporally in phase with cycles of growth. A role for Cl(-) flux in the dynamic cellular events during growth is assessed. Differential interference contrast microscopy and kymographic analysis of individual growth cycles revealed that vesicles can advance transiently to within 2 to 4 mum of the apex during the phase of maximally increasing Cl(-) efflux, which temporally overlaps the phase of cell elongation during the growth cycle. In summary, these investigations indicate that Cl(-) ion dynamics are an important component in the network of events that regulate pollen tube homeostasis and growth.