Inositol phosphates in the duckweed Spirodela polyrhiza L

Crystal Structure and Enzymology of Solanum tuberosum Inositol Tris/Tetrakisphosphate Kinase 1 (StITPK1)

Inositol phosphates and their pyrophosphorylated derivatives are responsive to the phosphate supply and are agents of phosphate homeostasis and other aspects of physiology. It seems likely that the enzymes that interconvert these signals work against the prevailing milieu of mixed populations of competing substrates and products. The synthesis of inositol pyrophosphates is mediated in plants by two classes of ATP-grasp fold kinase: PPIP5 kinases, known as VIH, and members of the inositol tris/tetrakisphosphate kinase (ITPK) family, specifically ITPK1/2. A molecular explanation of the contribution of ITPK1/2 to inositol pyrophosphate synthesis and turnover in plants is incomplete: the absence of nucleotide in published crystal structures limits the explanation of phosphotransfer reactions, and little is known of the affinity of potential substrates and competitors for ITPK1. Herein, we describe a complex of ADP and StITPK1 at 2.26 Å resolution and use a simple fluorescence polarization approach to compare the affinity of binding of diverse inositol phosphates, inositol pyrophosphates, and analogues. By simple HPLC, we reveal the novel catalytic capability of ITPK1 for different inositol pyrophosphates and show Ins(3,4,5,6)P4 to be a potent inhibitor of the inositol pyrophosphate-synthesizing activity of ITPK1. We further describe the exquisite specificity of ITPK1 for the myo-isomer among naturally occurring inositol hexakisphosphates.

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.

A Short Historical Perspective of Methods in Inositol Phosphate Research

The multitudinous inositol phosphate family elicits a wide range of molecular effects that regulate countless biological responses. In this review, I provide a methodological viewpoint of the manner in which key advances in the field of inositol phosphate research were made. I also note some of the considerable challenges that still lie ahead.

Size exclusion chromatography with online ICP-MS enables molecular weight fractionation of dissolved phosphorus species in water samples

Phosphorus (P) is an important and often limiting element in terrestrial and aquatic ecosystem. A lack of understanding of its distribution and structures in the environment limits the design of effective P mitigation and recovery approaches. Here we developed a robust method employing size exclusion chromatography (SEC) coupled to an ICP-MS to determine the molecular weight (MW) distribution of P in environmental samples. The most abundant fraction of P varied widely in different environmental samples: (i) orthophosphate was the dominant fraction (93-100%) in one lake, two aerosols and DOC isolate samples, (ii) species of 400-600 Da range were abundant (74-100%) in two surface waters, and (iii) species of 150-350 Da range were abundant in wastewater effluents. SEC-DOC of the aqueous samples using a similar SEC column showed overlapping peaks for the 400-600 Da species in two surface waters, and for >20 kDa species in the effluents, suggesting that these fractions are likely associated with organic matter. The MW resolution and performance of SEC-ICP-MS agreed well with the time integrated results obtained using conventional ultrafiltration method. Results show that SEC in combination with ICP-MS and DOC has the potential to be a powerful and easy-to-use method in identifying unknown fractions of P in the environment.

Role of vacuoles in phosphorus storage and remobilization

Vacuoles play a fundamental role in storage and remobilization of various nutrients, including phosphorus (P), an essential element for cell growth and development. Cells acquire P primarily in the form of inorganic orthophosphate (Pi). However, the form of P stored in vacuoles varies by organism and tissue. Algae and yeast store polyphosphates (polyPs), whereas plants store Pi and inositol phosphates (InsPs) in vegetative tissues and seeds, respectively. In this review, we summarize how vacuolar P molecules are stored and reallocated and how these processes are regulated and co-ordinated. The roles of SYG1/PHO81/XPR1 (SPX)-domain-containing membrane proteins in allocating vacuolar P are outlined. We also highlight the importance of vacuolar P in buffering the cytoplasmic Pi concentration to maintain cellular homeostasis when the external P supply fluctuates, and present additional roles for vacuolar polyP and InsP besides being a P reserve. Furthermore, we discuss the possibility of alternative pathways to recycle Pi from other P metabolites in vacuoles. Finally, future perspectives for researching this topic and its potential application in agriculture are proposed.

Using solid 13C NMR coupled with solution 31P NMR spectroscopy to investigate molecular species and lability of organic carbon and phosphorus from aquatic plants in Tai Lake, China

Forms and labilities of plant-derived organic matters (OMs) including carbon (C) and phosphorus (P) were fundamental for understanding their release, degradation and environmental behaviour in lake ecosystems. Thus, solid ¹³C and solution ³¹P nuclear magnetic resonance (NMR) spectroscopy were used to characterize biomass of six aquatic plants in Tai Lake, China. The results showed that carbohydrates (61.2% of the total C) were predominant C functional group in the solid ¹³C NMR spectra of plant biomass, which may indicate high lability and bioavailability of aquatic plants-derived organic matter in lakes. There was 72.6-103.7% of the total P in aquatic plant biomass extracted by NaOH-EDTA extracts. Solution ³¹P NMR analysis of these NaOH-EDTA extracts further identified several molecular species of P including orthophosphate (50.1%), orthophosphate monoesters (46.8%), DNA (1.6%) and pyrophosphate (1.4%). Orthophosphate monoesters included β-glycerophosphate (17.7%), hydrolysis products of RNA (11.7%), α-glycerophosphate (9.2%) and other unknown monoesters (2.1%). Additionally, phytate, the major form of organic P in many lake sediments, was detected in floating plant water poppy. These inorganic P (e.g. orthophosphate and pyrophosphate) and organic P (e.g. diester and its degradation products) identified in plant biomass were all labile and bioavailable P, which would play an important role in recycling of P in lakes. These results increased knowledge of chemical composition and bioavailability of OMs derived from aquatic plants in lakes.

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.

Conformational changes in inositol 1,3,4,5,6-pentakisphosphate 2-kinase upon substrate binding: role of N-terminal lobe and enantiomeric substrate preference

Inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IP(5) 2-K) catalyzes the synthesis of inositol 1,2,3,4,5,6-hexakisphosphate from ATP and IP(5). Inositol 1,2,3,4,5,6-hexakisphosphate is implicated in crucial processes such as mRNA export, DNA editing, and phosphorus storage in plants. We previously solved the first structure of an IP(5) 2-K, which shed light on aspects of substrate recognition. However, failure of IP(5) 2-K to crystallize in the absence of inositide prompted us to study putative conformational changes upon substrate binding. We have made mutations to residues on a region of the protein that produces a clasp over the active site. A W129A mutant allowed us to capture IP(5) 2-K in its different conformations by crystallography. Thus, the IP(5) 2-K apo-form structure displays an open conformation, whereas the nucleotide-bound form shows a half-closed conformation, in contrast to the inositide-bound form obtained previously in a closed conformation. Both nucleotide and inositide binding produce large conformational changes that can be understood as two rigid domain movements, although local changes were also observed. Changes in intrinsic fluorescence upon nucleotide and inositide binding are in agreement with the crystallographic findings. Our work suggests that the clasp might be involved in enzyme kinetics, with the N-terminal lobe being essential for inositide binding and subsequent conformational changes. We also show how IP(5) 2-K discriminates between inositol 1,3,4,5-tetrakisphosphate and 3,4,5,6-tetrakisphosphate enantiomers and that substrate preference can be manipulated by Arg(130) mutation. Altogether, these results provide a framework for rational design of specific inhibitors with potential applications as biological tools for in vivo studies, which could assist in the identification of novel roles for IP(5) 2-K in mammals.

Identification of genes necessary for wild-type levels of seed phytic acid in Arabidopsis thaliana using a reverse genetics approach

The majority of phosphorus (P) in seeds is found in phytic acid (InsP(6)) which accumulates as the mixed salt phytate. InsP(6) is generally considered to be an anti-nutrient and the development of low phytic acid (lpa) seed crops is of significant interest. We have employed a reverse genetics approach to examine the impact of disrupting genes involved in inositol phosphate metabolism on Arabidopsis seed InsP(6) levels. Our analysis revealed that knockout mutations in three genes (AtITPK1, AtITPK4, and AtMIK/At5g58730) reduced seed InsP(6) in addition to knockouts of four previously reported genes (AtIPK1, AtIPK2β, AtMRP5, and At5g60760). Seeds of these lpa mutants also exhibited reduced germination under various stress conditions. The greatest reduction in InsP(6) (>70%) was observed in atmrp5 seeds which were also among the least sensitive to the stresses examined. Expression analysis of the lpa genes revealed three distinct patterns in developing siliques consistent with their presumed roles. Disruption of each lpa gene resulted in changes in the expression in some of the other lpa genes indicating that transcription of lpa genes is modulated by other constituents of InsP(6) metabolism. While all the lpa genes represent possible targets for genetic engineering of low phytate seed crops, mutations in AtMRP5, AtMIK, and At5g60760 may be most successful for conventional approaches such as mutation breeding.

Identification of genes necessary for wild-type levels of seed phytic acid in Arabidopsis thaliana using a reverse genetics approach

The majority of phosphorus (P) in seeds is found in phytic acid (InsP(6)) which accumulates as the mixed salt phytate. InsP(6) is generally considered to be an anti-nutrient and the development of low phytic acid (lpa) seed crops is of significant interest. We have employed a reverse genetics approach to examine the impact of disrupting genes involved in inositol phosphate metabolism on Arabidopsis seed InsP(6) levels. Our analysis revealed that knockout mutations in three genes (AtITPK1, AtITPK4, and AtMIK/At5g58730) reduced seed InsP(6) in addition to knockouts of four previously reported genes (AtIPK1, AtIPK2β, AtMRP5, and At5g60760). Seeds of these lpa mutants also exhibited reduced germination under various stress conditions. The greatest reduction in InsP(6) (>70%) was observed in atmrp5 seeds which were also among the least sensitive to the stresses examined. Expression analysis of the lpa genes revealed three distinct patterns in developing siliques consistent with their presumed roles. Disruption of each lpa gene resulted in changes in the expression in some of the other lpa genes indicating that transcription of lpa genes is modulated by other constituents of InsP(6) metabolism. While all the lpa genes represent possible targets for genetic engineering of low phytate seed crops, mutations in AtMRP5, AtMIK, and At5g60760 may be most successful for conventional approaches such as mutation breeding.

Genetic analysis of two OsLpa1-like genes in Arabidopsis reveals that only one is required for wild-type seed phytic acid levels

Phytic acid (inositol-1,2,3,4,5,6-hexakisphosphate or InsP(6)) is the primary storage form of phosphorus in plant seeds. The rice OsLpa1 encodes a novel protein required for wild-type levels of seed InsP(6) and was identified from a low phytic acid (lpa) mutant exhibiting a 45-50% reduction in seed InsP(6). OsLpa1 is highly conserved in plants and Arabidopsis contains two OsLpa1-like genes, At3g45090 and At5g60760. Analysis of homozygous T-DNA insertion mutants of At5g60760 revealed significantly reduced levels of seed InsP(6) while no changes were observed in seeds of At3g45090 mutants. A double knockout mutant of At5g60760 and At3g45090 was created and its seed InsP(6) content was similar to that of the At5g60760 mutant indicating that At3g45090 does not provide functional redundancy. OsLpa1 was confirmed to be the ortholog of At5g60760 by complementation of a knockout mutant with a cDNA clone corresponding to the largest of three alternative transcripts of OsLpa1. The spatial and temporal expression of At5g60760 during seed development is consistent with its involvement in seed InsP(6) biosynthesis.

A role for inositol hexakisphosphate in the maintenance of basal resistance to plant pathogens

Phytic acid (myo-inositol hexakisphosphate, InsP6) is an important phosphate store and signal molecule in plants. However, low-phytate plants are being developed to minimize the negative health effects of dietary InsP6 and pollution caused by undigested InsP6 in animal waste. InsP6 levels were diminished in transgenic potato plants constitutively expressing an antisense gene sequence for myo-inositol 3-phosphate synthase (IPS, catalysing the first step in InsP6 biosynthesis) or Escherichia coli polyphosphate kinase. These plants were less resistant to the avirulent pathogen potato virus Y and the virulent pathogen tobacco mosaic virus (TMV). In Arabidopsis thaliana, mutation of the gene for the enzyme catalysing the final step of InsP6 biosynthesis (InsP5 2-kinase) also diminished InsP6 levels and enhanced susceptibility to TMV and to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae. Arabidopsis thaliana has three IPS genes (AtIPS1-3). Mutant atips2 plants were depleted in InsP6 and were hypersusceptible to TMV, turnip mosaic virus, cucumber mosaic virus and cauliflower mosaic virus as well as to the fungus Botrytis cinerea and to P. syringae. Mutant atips2 and atipk1 plants were as hypersusceptible to infection as plants unable to accumulate salicylic acid (SA) but their increased susceptibility was not due to reduced levels of SA. In contrast, mutant atips1 plants, which were also depleted in InsP6, were not compromised in resistance to pathogens, suggesting that a specific pool of InsP6 regulates defence against phytopathogens.

Metabolic and signaling properties of an Itpk gene family in Glycine max

We have cloned and characterized four Itpk genes from soybean. All four recombinant Itpk proteins showed canonical Ins(1,3,4)P3 5/6-kinase activity, but a kinetic analysis raised questions about its biological significance. Instead, we provide evidence that one alternative biological role for soybean Itpks is to interconvert the Cl(-) channel inhibitor, Ins(3,4,5,6)P4, and its metabolic precursor, Ins(1,3,4,5,6)P5, within a substrate cycle. The soybean Itpks also phosphorylated Ins(3,4,6)P3 to Ins(1,3,4,6)P4 which was further phosphorylated to Ins(1,3,4,5,6)P5 by soybean Ipk2. Thus, soybean Itpks may participate in an inositol lipid-independent pathway of InsP6 synthesis.

Expression pattern of inositol phosphate-related enzymes in rice (Oryza sativa L.): implications for the phytic acid biosynthetic pathway

Phytic acid, myo-inositol-hexakisphosphate (InsP(6)), is a storage form of phosphorus in plants. Despite many physiological investigations of phytic acid accumulation and storage, little is known at the molecular level about its biosynthetic pathway in plants. Recent work has suggested two pathways. One is an inositol lipid-independent pathway that occurs through the sequential phosphorylation of 1D-myo-inositol 3-phosphate (Ins(3)P). The second is a phospholipase C (PLC)-mediated pathway, in which inositol 1,4,5-tris-phosphate (Ins(1,4,5)P(3)) is sequentially phosphorylated to InsP(6). We identified 12 genes from rice (Oryza sativa L.) that code for the enzymes that may be involved in the metabolism of inositol phosphates. These enzymes include 1D-myo-inositol 3-phosphate synthase (MIPS), inositol monophosphatase (IMP), inositol 1,4,5-tris-phosphate kinase/inositol polyphosphate kinase (IPK2), inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1), and inositol 1,3,4-triskisphosphate 5/6-kinase (ITP5/6K). The quantification of absolute amounts of mRNA by real-time RT-PCR revealed the unique expression patterns of these genes. Outstanding up-regulation of the four genes, a MIPS, an IPK1, and two ITP5/6Ks in embryos, suggested that they play a significant role in phytic acid biosynthesis and that the lipid-independent pathway was mainly active in developing seeds. On the other hand, the up-regulation of a MIPS, an IMP, an IPK2, and an ITP5/6K in anthers suggested that a PLC-mediated pathway was active in addition to a lipid-independent pathway in the anthers.

Arabidopsis thaliana inositol 1,3,4-trisphosphate 5/6-kinase 4 (AtITPK4) is an outlier to a family of ATP-grasp fold proteins from Arabidopsis

The Arabidopsis genome encodes a family of inositol 1,3,4-trisphosphate 5/6-kinases which form a subgroup of a larger group of ATP-grasp fold proteins. An analysis of the inositol 1,3,4-trisphosphate 5/6-kinase family might, ultimately, be best rewarded by detailed comparison of related enzymes in a single genome. The enzyme encoded by At2G43980, AtITPK4; is an outlier to its family. At2G43980 is expressed in male and female organs of young and mature flowers. AtITPK4 differs from other family members in that it does not display inositol 3,4,5,6-tetrakisphosphate 1-kinase activity; rather, it displays inositol 1,4,5,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate isomerase activity.

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 myo-inositol hexakisphosphate deposits from larval Echinococcus granulosus

The abundant metabolite myo-inositol hexakisphosphate (InsP6) can form vesicular deposits with cations, a widespread phenomenon in plants also found in the cestode parasite, Echinococcus granulosus. In this organism, the deposits are exocytosed, accumulating in a host-exposed sheath of extracellular matrix termed the laminated layer. The formation and mobilization of InsP6 deposits, which involve precipitation and solubilization reactions, respectively, cannot yet be rationalized in quantitative chemical terms, as the solids involved have not been formally described. We report such a description for the InsP6 deposits from E. granulosus, purified as the solid residue left by mild alkaline digestion of the principal mucin component of the laminated layer. The deposits are largely composed of the compound Ca5H2L.16H2O (L representing fully deprotonated InsP6), and additionally contain Mg2+ (6-9% molar ratio with respect to Ca2+), but not K+. Calculations employing recently available chemical constants show that the precipitation of Ca5H2L.16H2O is predicted by thermodynamics in secretory vesicle-like conditions. The deposits appear to be similar to microcrystalline solids when analysed under the electron microscope; we estimate that each crystal comprises around 200 InsP6 molecules. We calculate that the deposits increase, by three orders of magnitude, the surface area available for adsorption of host proteins, a salient ability of the laminated layer. The major inositol phosphate in the deposits, other than InsP6, is myo-inositol (1,2,4,5,6) pentakisphosphate, or its enantiomer, inositol (2,3,4,5,6) pentakisphosphate. The compound appears to be a subproduct of the intracellular pathways leading to the synthesis and vesicular accumulation of InsP6, rather than arising from extracellular hydrolysis of InsP6.

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.

The concentrations and distribution of phytic acid-phosphorus and other mineral nutrients in wild-type and low phytic acid1-1 (lpa1-1) corn (Zea mays L.) grains and grain parts

A comparison of mineral nutrient and phytic acid-phosphorus (PA-P) distribution in the grains of wild-type (WT) and low phytic acid1-1 (lpa1-1) corn (Zea mays L.) was conducted to determine how the lpa1-1 mutation influences mineral element concentrations in different grain parts and impacts the structure of phosphorus-rich inclusions (globoids) in the grain cells. This is the first report regarding total phosphorus (P) and PA-P concentrations in scutellum and root-shoot axis portions of cereal embryos of WT in comparison to its matching lpa1-1 genotype. In WT, 95% of the grain PA-P was located in the embryo, mostly in the scutellum. The lpa1-1 mutation reduced whole-grain PA-P by 62% but influenced the scutella more than the root-shoot axes and rest-of-grain fractions. In spite of the lpa1-1 mutants containing greatly reduced PA-P, whole-grain amounts of Mg, Fe, and Mn were higher in lpa1-1 than in WT, K and Zn were similar, and Ca was lower. Iron was 1/3 higher in lpa1-1 grains than WT while Ca was 18% lower. Decreased phytic acid in lpa1-1 grains resulted in reduction in globoid size in both scutellum and aleurone layer cells. Most lpa1-1 aleurone globoids were non-spherical and scutellum globoids were clusters of small spheres while WT globoids were large discrete spheres. X-ray analyses of globoids in both grain types revealed major amounts of P, K, and Mg and traces of Ca, Fe, and Zn. Both grain types contained almost no mineral nutrient stores in the starchy endosperm.Key words: corn (Zea mays L.), phytic acid-phosphorus, low phytic acid1-1 (lpa1-1) grains, mineral nutrients, globoids, electron microscopy.

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.

Inositol phosphates in the environment

The inositol phosphates are a group of organic phosphorus compounds found widely in the natural environment, but that represent the greatest gap in our understanding of the global phosphorus cycle. They exist as inositols in various states of phosphorylation (bound to between one and six phosphate groups) and isomeric forms (e.g. myo, D-chiro, scyllo, neo), although myo-inositol hexakisphosphate is by far the most prevalent form in nature. In terrestrial environments, inositol phosphates are principally derived from plants and accumulate in soils to become the dominant class of organic phosphorus compounds. Inositol phosphates are also present in large amounts in aquatic environments, where they may contribute to eutrophication. Despite the prevalence of inositol phosphates in the environment, their cycling, mobility and bioavailability are poorly understood. This is largely related to analytical difficulties associated with the extraction, separation and detection of inositol phosphates in environmental samples. This review summarizes the current knowledge of inositol phosphates in the environment and the analytical techniques currently available for their detection in environmental samples. Recent advances in technology, such as the development of suitable chromatographic and capillary electrophoresis separation techniques, should help to elucidate some of the more pertinent questions regarding inositol phosphates in the natural environment.

Origin and seed phenotype of maize low phytic acid 1-1 and low phytic acid 2-1

Phytic acid (myo-inositol-1, 2, 3, 4, 5, 6-hexakisphosphate or Ins P(6)) typically represents approximately 75% to 80% of maize (Zea mays) seed total P. Here we describe the origin, inheritance, and seed phenotype of two non-lethal maize low phytic acid mutants, lpa1-1 and lpa2-1. The loci map to two sites on chromosome 1S. Seed phytic acid P is reduced in these mutants by 50% to 66% but seed total P is unaltered. The decrease in phytic acid P in mature lpa1-1 seeds is accompanied by a corresponding increase in inorganic phosphate (P(i)). In mature lpa2-1 seed it is accompanied by increases in P(i) and at least three other myo-inositol (Ins) phosphates (and/or their respective enantiomers): D-Ins(1,2,4,5,6) P(5); D-Ins (1,4,5,6) P(4); and D-Ins(1,2,6) P(3). In both cases the sum of seed P(i) and Ins phosphates (including phytic acid) is constant and similar to that observed in normal seeds. In both mutants P chemistry appears to be perturbed throughout seed development. Homozygosity for either mutant results in a seed dry weight loss, ranging from 4% to 23%. These results indicate that phytic acid metabolism during seed development is not solely responsible for P homeostasis and indicate that the phytic acid concentration typical of a normal maize seed is not essential to seed function.

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

Using a permeabilization strategy to introduce Ins(3,4,5,6) P(4) into mesophyll protoplasts of Commelina communis, we have identified Ins(3,4,5,6) P(4) 1-kinase activity in mesophyll cells. Multiple InsP(3) isomers were identified in Spirodela polyrhiza and Arabidopsis. Only two of these, Ins(1,2,3) P(3) and Ins(3,4,6) P(3), have previously been identified in plants and only in monocots. The isomers detected in S. polyrhiza included D- and/or L-Ins(3,4,5) P(3), D- and/or L-Ins(3,5,6) P(3), and D- and/or L-Ins(2,4,5) P(3). Ins(1,4,5) P(3), if present, was only a tiny fraction of total InsP(3) species. We have also identified inositol polyphosphate phosphatase activities, Ins(3,4,5,6) P(4) 6-phosphatase and Ins(3,4, 5, 6) P(4) 4-phosphatase, whose action on endogenous inositol polyphosphates explains the presence of D- and/or L-Ins(3,4,5) P(3) and D- and/or L-Ins(3,5,6) P(3) in mesophyll cells. Inositol trisphosphates identified in Arabidopsis include Ins(1,2,3) P(3) and D- and/or L-Ins(3,4,6) P(3), suggesting that dicots may share pathways of InsP(6) biosynthesis and breakdown in common with monocots.

PiUS (Pi uptake stimulator) is an inositol hexakisphosphate kinase

A cDNA cloned from its ability to stimulate inorganic phosphate uptake in Xenopus oocytes (phosphate uptake stimulator (PiUS)) shows significant similarity with inositol 1,4,5-trisphosphate 3-kinase. However, the expressed PiUS protein showed no detectable activity against inositol 1,4,5-trisphosphate, nor the 1,3,4,5- or 3,4,5, 6-isomers of inositol tetrakisphosphate, whereas it was very active in converting inositol hexakisphosphate (InsP(6)) to inositol heptakisphosphate (InsP(7)). PiUS is a member of a family of enzymes found in many eukaryotes and we discuss the implications of this for the functions of InsP(7) and for the evolution of inositol phosphate kinases.

Molecular and enzymatic characterization of three phosphoinositide-specific phospholipase C isoforms from potato

Many cellular responses to stimulation of cell-surface receptors by extracellular signals are transmitted across the plasma membrane by hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2), which is cleaved into diacylglycerol and inositol-1,4,5-trisphosphate by phosphoinositide-specific phospholipase C (PI-PLC). We present structural, biochemical, and RNA expression data for three distinct PI-PLC isoforms, StPLC1, StPLC2, and StPLC3, which were cloned from a guard cell-enriched tissue preparation of potato (Solanum tuberosum) leaves. All three enzymes contain the catalytic X and Y domains, as well as C2-like domains also present in all PI-PLCs. Analysis of the reaction products obtained from PIP2 hydrolysis unequivocally identified these enzymes as genuine PI-PLC isoforms. Recombinant StPLCs showed an optimal PIP2-hydrolyzing activity at 10 microM Ca2+ and were inhibited by Al3+ in equimolar amounts. In contrast to PI-PLC activity in plant plasma membranes, however, recombinant enzymes could not be activated by Mg2+. All three stplc genes are expressed in various tissues of potato, including leaves, flowers, tubers, and roots, and are affected by drought stress in a gene-specific manner.

Metabolic evidence for PtdIns(4,5)P2-directed phospholipase C in permeabilized plant protoplasts

Comparison of the sequences of the genes encoding phospholipase C (PLC) which have been cloned to date in plants with their mammalian counterparts suggests that plant PLC is similar to PLCdelta of mammalian cells. The physiological role and mechanism of activation of PLCdelta is unclear. It has recently been shown that Ins(1,4,5)P3 may not solely be the product of PtdIns(4,5)P2-directed PLC activity. Enzyme activities capable of producing Ins(1,4,5)P3 from endogenous inositol phosphates are present in Dictyostelium and also in rat liver. Significantly it has not been directly determined whether Ins(1,4,5)P3 present in higher plants is the product of a PtdIns(4, 5)P2-directed PLC activity. Therefore we have developed an experimental strategy for the identification of d-Ins(1,4,5)P3 in higher plants. By the use of a short-term non-equilibrium labelling strategy in permeabilized plant protoplasts, coupled to the use of a 'metabolic trap' to prevent degradation of [32P]Ins(1,4,5)P3, we were able to determine the distribution of 32P in individual phosphate esters of Ins(1,4,5)P3. The [32]Ins(1,4,5)P3 identified showed the same distribution of label in individual phosphate esters as that of [32P]PtdIns(4,5)P2 isolated from the same tissue. We thus provide in vivo evidence for the action of a PtdIns(4,5)P2-directed PLC activity in plant cells which is responsible for the production of Ins(1,4,5)P3 observed here. This observation does not, however, exclude the possibility that in other cells or under different conditions Ins(1,4,5)P3 can be generated by alternative routes.

Phosphatidylinositol 3,5-bisphosphate defines a novel PI 3-kinase pathway in resting mouse fibroblasts

PtdIns(3,5)P2 is identified as the product of an agonist-independent, wortmannin-sensitive pathway in resting mouse cells. Results are presented here to indicate that PtdIns(3,5)P2 is formed by phosphorylation of PtdIns3P at the D-5 position, and they suggest that relatively constant cellular levels of PtdIns3P and PtdIns(3, 5)P2 are maintained by the concerted action of PtdIns3P 5-kinase and PtdIns(3,5)P2 5-phosphatase. These studies imply a novel mechanism for the action of PtdIns-specific phosphoinositide 3-hydroxykinases in mammalian cells.

Inositol phosphates in barley (Hordeum vulgare L.) aleurone tissue are stereochemically similar to the products of breakdown of InsP6 in vitro by wheat-bran phytase

Partisphere SAX HPLC analysis of endogenous inositol phosphates in [3H]inositol-labelled barley aleurone tissue revealed a range of isomers, including D- and/or L-Ins3P, D- and/or L-Ins(1,4)P2, D- and/or L-Ins(1,2)P2, a third unidentified InsP2, Ins(1,2,3)P3, D- and/or L-Ins(1,2,6)P3, D-and/or L-Ins(1,2,3,4)P4, D- and/or L-Ins(1,2,5,6)P4, Ins(1,3,4,5,6)P5, D- and/or L-Ins(1,2,3,4,5)P5, Ins(1,2,3,4,6)P5, InsP6 and a molecule with the chromatographic properties of an inositol pyrophosphate. The striking match between the identities of the stereoisomers, and in some cases enantiomers, detected in vivo and those stereoisomers produced in vitro by the action of wheat-bran phytase on InsP6 [Cosgrove (1980) Inositol Phosphates: Their Chemistry, Bio-chemistry and Physiology. Elsevier, Amsterdam] strongly suggests that most of the inositol phosphates identified are products of the breakdown of InsP4 by endogenous phytase(s) with stereospecificity similar to that of the wheat-bran enzyme(s).

Metabolic evidence for the order of addition of individual phosphate esters in the myo-inositol moiety of inositol hexakisphosphate in the duckweed Spirodela polyrhiza L

The aquatic monocotyledonous plant Spirodela polyrhiza was labelled with [33P]Pi for short periods under non-equilibrium conditions. An InsP6 fraction was obtained and dissected by using enantiospecific (enzymic) and non-enantiospecific (chemical) means to determine the relative labelling of individual phosphate substituents on the inositol ring of InsP6. Phosphates in positions D-1, -2, -3, -4, -5 and -6 contained approx. 21%, 32-39%, 9-10%, 14-16%, 19-23% and 16-18% of the label respectively. We conclude from the foregoing, together with identities [described in the preceding paper, Brearley and Hanke (1996) Biochem. J. 314, 215-225] of inositol phosphates found in this plant at a developmental stage associated with massive accumulation of InsP6, that synthesis of InsP6 from myo-inositol proceeds according to the sequence Ins3P-->Ins(3,4)P2-->Ins(3,4,6)P3-->Ins(3,4,5,6)P4-->Ins(1,3,4,5,6 ) P5-->InsP6 in Spirodela polyrhiza. These results represent the first description of the synthetic sequence to InsP6 in the plant kingdom and the only comprehensive description of endogenous inositol phosphates in any plant tissue. The sequence described differs from that reported in the slime mould Dictyostelium discoideum.

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.