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

Optimized barley phytase gene expression by focused FIND-IT screening for mutations in cis-acting regulatory elements

Introduction

Induced modification of plant gene expression is of both fundamental and applied importance. Cis-acting regulatory elements (CREs) are major determinants of the spatiotemporal strength of gene expression. Yet, there are few examples where induced genetic variation in predetermined CREs has been exploited to improve or investigate crop plants.

Methods

The digital PCR based FIND-IT technology was applied to discover barley mutants with CRE variants in the promoter of the nutritional important barley grain phytase (PAPhy_a) gene.

Results and discussion

Mutants with higher or lower gene expression and ultimately higher or lower mature grain phytase activity (MGPA), respectively, were discovered. Field trials and inositol phosphate profiling during germination showed that PAPhy_a does not influence agronomic performance under the trial conditions but it does shorten the lag time of phosphate mobilization during germination. Higher endogenous MGPA is an improvement of grain quality for feed use as it improves the phosphate bioavailability for monogastric animals. Moreover, as the targeted CRE motifs of the PAPhy_a promoter are shared with a range of seed expressed genes like key cereal and legume storage genes, the current results demonstrates a concept for modulating individual gene expression levels of a range of seed genes.

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.

ITPK1 Regulates Jasmonate-Controlled Root Development in Arabidopsis thaliana

Jasmonic acid (JA) is a plant hormone that regulates a plethora of physiological processes including immunity and development and is perceived by the F-Box protein, Coronatine-insensitive protein 1 (COI1). The discovery of inositol phosphates (InsPs) in the COI1 receptor complex highlights their role in JAperception. InsPs are phosphate-rich signaling molecules that control many aspects of plant physiology. Inositol pyrophosphates (PP-InsPs) are diphosphate containing InsP species, of which InsP7 and InsP8 are the best characterized ones. Different InsP and PP-InsP species are linked with JA-related plant immunity. However, role of PP-InsP species in regulating JA-dependent developmental processes are poorly understood. Recent identification of ITPK1 kinase, responsible for the production of 5-InsP7 from InsP6in planta, provides a platform to investigate the possible involvement of ITPK-derived InsP species in JA-related plant development. Here, in this study, we report that ITPK1-defective plants exhibit increased root growth inhibition to bioactive JA treatment. The itpk1 plants also show increased lateral root density when treated with JA. Notably, JA treatment does not increase ITPK1 protein levels. Gene expression analyses revealed that JA-biosynthetic genes are not differentially expressed in ITPK1-deficient plants. We further demonstrate that genes encoding different JAZ repressor proteins are severely down-regulated in ITPK1-defective plants. Taken together, our study highlights the role of ITPK1 in regulating JA-dependent root architecture development through controlling the expression of different JAZ repressor proteins.

INOSITOL (1,3,4) TRIPHOSPHATE 5/6 KINASE1-dependent inositol polyphosphates regulate auxin responses in Arabidopsis

The combinatorial phosphorylation of myo-inositol results in the generation of different inositol phosphates (InsPs), of which phytic acid (InsP6) is the most abundant species in eukaryotes. InsP6 is also an important precursor of the higher phosphorylated inositol pyrophosphates (PP-InsPs), such as InsP7 and InsP8, which are characterized by a diphosphate moiety and are also ubiquitously found in eukaryotic cells. While PP-InsPs regulate various cellular processes in animals and yeast, their biosynthesis and functions in plants has remained largely elusive because plant genomes do not encode canonical InsP6 kinases. Recent work has shown that Arabidopsis (Arabidopsis thaliana) INOSITOL (1,3,4) TRIPHOSPHATE 5/6 KINASE1 (ITPK1) and ITPK2 display in vitro InsP6 kinase activity and that, in planta, ITPK1 stimulates 5-InsP7 and InsP8 synthesis and regulates phosphate starvation responses. Here we report a critical role of ITPK1 in auxin-related processes that is independent of the ITPK1-controlled regulation of phosphate starvation responses. Those processes include primary root elongation, root hair development, leaf venation, thermomorphogenic and gravitropic responses, and sensitivity to exogenously applied auxin. We found that the recombinant auxin receptor complex, consisting of the F-Box protein TRANSPORT INHIBITOR RESPONSE1 (TIR1), ARABIDOPSIS SKP1 HOMOLOG 1 (ASK1), and the transcriptional repressor INDOLE-3-ACETIC ACID INDUCIBLE 7 (IAA7), binds to anionic inositol polyphosphates with high affinity. We further identified a physical interaction between ITPK1 and TIR1, suggesting a localized production of 5-InsP7, or another ITPK1-dependent InsP/PP-InsP isomer, to activate the auxin receptor complex. Finally, we demonstrate that ITPK1 and ITPK2 function redundantly to control auxin responses, as deduced from the auxin-insensitive phenotypes of itpk1 itpk2 double mutant plants. Our findings expand the mechanistic understanding of auxin perception and suggest that distinct inositol polyphosphates generated near auxin receptors help to fine-tune auxin sensitivity in plants.

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.

Analyses of Inositol Phosphates and Phosphoinositides by Strong Anion Exchange (SAX)-HPLC

The phosphate esters of myo-inositol (Ins) occur ubiquitously in biology. These molecules exist as soluble or membrane-resident derivatives and regulate a plethora of cellular functions including phosphate homeostasis, DNA repair, vesicle trafficking, metabolism, cell polarity, tip-directed growth, and membrane morphogenesis. Phosphorylation of all inositol hydroxyl groups generates phytic acid (InsP₆), the most abundant inositol phosphate present in eukaryotic cells. However, phytic acid is not the most highly phosphorylated naturally occurring inositol phosphate. Specialized small molecule kinases catalyze the formation of the so-called myo-inositol pyrophosphates (PP-InsPs), such as InsP₇ and InsP₈. These molecules are characterized by one or several "high-energy" diphosphate moieties and are ubiquitous in eukaryotic cells. In plants, PP-InsPs play critical roles in immune responses and nutrient sensing. The detection of inositol derivatives in plants is challenging. This is particularly the case for inositol pyrophosphates because diphospho bonds are labile in plant cell extracts due to high amounts of acid phosphatase activity. We present two steady-state inositol labeling-based techniques coupled with strong anion exchange (SAX)-HPLC analyses that allow robust detection and quantification of soluble and membrane-resident inositol polyphosphates in plant extracts. These techniques will be instrumental to uncover the cellular and physiological processes controlled by these intriguing regulatory molecules in plants.

Globoids and Phytase: The Mineral Storage and Release System in Seeds

Phytate and phytases in seeds are the subjects of numerous studies, dating back as far as the early 20th century. Most of these studies concern the anti-nutritional properties of phytate, and the prospect of alleviating the effects of phytate with phytase. As reasonable as this may be, it has led to a fragmentation of knowledge, which hampers the appreciation of the physiological system at hand. In this review, we integrate the existing knowledge on the chemistry and biosynthesis of phytate, the globoid cellular structure, and recent advances on plant phytases. We highlight that these components make up a system that serves to store and-in due time-release the seed's reserves of the mineral nutrients phosphorous, potassium, magnesium, and others, as well as inositol and protein. The central component of the system, the phytate anion, is inherently rich in phosphorous and inositol. The chemical properties of phytate enable it to sequester additional cationic nutrients. Compartmentalization and membrane transport processes regulate the buildup of phytate and its associated nutrients, resulting in globoid storage structures. We suggest, based on the current evidence, that the degradation of the globoid and the mobilization of the nutrients also depend on membrane transport processes, as well as the enzymatic action of phytase.

Photo-releasable derivatives of inositol pyrophosphates

The specific non-invasive control of intracellular signaling events requires advanced tools that enter cells by diffusion and are controllable by light. Here, we detail the synthesis and application of membrane-permeant caged inositol pyrophosphates with respect to cell entry and cell distribution. We recently published the synthesis of these tools as well as their effect on PH-domain localization in HeLa cells and oscillations of the intracellular calcium concentration in β-cells, which are known to drive insulin secretion. In this chapter, we discuss the possibilities and limitations when using cell-penetrating inositol pyrophosphates. We provide a detailed protocol for the application in live mouse β-cells and we discuss the image analysis needed for following effects on calcium signaling.

Molecular characterization, modeling, and docking analysis of late phytic acid biosynthesis pathway gene, inositol polyphosphate 6-/3-/5-kinase, a potential candidate for developing low phytate crops

The coding sequence of inositol polyphosphate 6-/3-/5-kinase (GmIPK2) gene was identified and cloned from popular Indian soybean cultivar Pusa-16. The clone was predicted to encode 279 amino acids long, 30.97 kDa protein. Multiple sequence alignment revealed an inositol phosphate-binding motif, PxxxDxKxG throughout the IPK2 sequences along with other motifs unique to inositol phosphate kinase superfamily. Eight α-helices and eight β-strands in antiparallel β-sheets arrangement were predicted in the secondary structure of GmIPK2. The temporal analysis of GmIPK2 revealed maximum expression in the seed tissues during later stages of development while spatially the transcript levels were lowest in leaf and stem tissues. Endosperm-specific cis-regulatory motifs (GCN4 and Skn_1) which support high levels of expression, as observed in the developing seeds, were detected in its promoter region. The protein structure of GmIPK2 was modeled based on the crystal structure of inositol polyphosphate multikinase from Arabidopsis thaliana (PDB:4FRF) and subsequently docked with inositol phosphate ligands (PDB: 5GUG-I3P and PDB: 4A69-I0P). Molecular dynamics (MD) simulation established the structural stability of both, modeled enzyme and ligand-bound complexes. Docking in combination with trajectory analysis for 50 ns MD run confirmed the participation of Lys105, Lys126 and Arg153 residues in the formation of a network of hydrogen bonds to stabilize the ligand-receptor interaction. Results of the present study thus provide valuable information on structural and functional aspects of GmIPK2 which shall assist in strategizing our long-term goal of achieving phytic acid reduction in soybean by genetic modification of its biosynthetic pathway to develop a nutritionally enhanced crop in the future.

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.

Inositol Polyphosphate Binding Specificity of the Jasmonate Receptor Complex

Inositol polyphosphate binding specificity of the jasmonate receptor is largely determined by the F-box protein COI1.

VIH2 Regulates the Synthesis of Inositol Pyrophosphate InsP8 and Jasmonate-Dependent Defenses in Arabidopsis

Diphosphorylated inositol polyphosphates, also referred to as inositol pyrophosphates, are important signaling molecules that regulate critical cellular activities in many eukaryotic organisms, such as membrane trafficking, telomere maintenance, ribosome biogenesis, and apoptosis. In mammals and fungi, two distinct classes of inositol phosphate kinases mediate biosynthesis of inositol pyrophosphates: Kcs1/IP6K- and Vip1/PPIP5K-like proteins. Here, we report that PPIP5K homologs are widely distributed in plants and that Arabidopsis thaliana VIH1 and VIH2 are functional PPIP5K enzymes. We show a specific induction of inositol pyrophosphate InsP8 by jasmonate and demonstrate that steady state and jasmonate-induced pools of InsP8 in Arabidopsis seedlings depend on VIH2. We identify a role of VIH2 in regulating jasmonate perception and plant defenses against herbivorous insects and necrotrophic fungi. In silico docking experiments and radioligand binding-based reconstitution assays show high-affinity binding of inositol pyrophosphates to the F-box protein COI1-JAZ jasmonate coreceptor complex and suggest that coincidence detection of jasmonate and InsP8 by COI1-JAZ is a critical component in jasmonate-regulated defenses.

Biosynthesis and possible functions of inositol pyrophosphates in plants

Inositol phosphates (InsPs) are intricately tied to lipid signaling, as at least one portion of the inositol phosphate signaling pool is derived from hydrolysis of the lipid precursor, phosphatidyl inositol (4,5) bisphosphate. The focus of this review is on the inositol pyrophosphates, which are a novel group of InsP signaling molecules containing diphosphate or triphosphate chains (i.e., PPx) attached to the inositol ring. These PPx-InsPs are emerging as critical players in the integration of cellular metabolism and stress signaling in non-plant eukaryotes. Most eukaryotes synthesize the precursor molecule, myo-inositol (1,2,3,4,5,6)-hexakisphosphate (InsP6), which can serve as a signaling molecule or as storage compound of inositol, phosphorus, and minerals (referred to as phytic acid). Even though plants produce huge amounts of precursor InsP6 in seeds, almost no attention has been paid to whether PPx-InsPs exist in plants, and if so, what roles these molecules play. Recent work has delineated that Arabidopsis has two genes capable of PP-InsP5 synthesis, and PPx-InsPs have been detected across the plant kingdom. This review will detail the known roles of PPx-InsPs in yeast and animal systems, and provide a description of recent data on the synthesis and accumulation of these novel molecules in plants, and potential roles in signaling.

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.

IP6K gene identification in plant genomes by tag searching

Background

Plants have played a special role in inositol polyphosphate (IP) research since in plant seeds was discovered the first IP, the fully phosphorylated inositol ring of phytic acid (IP6). It is now known that phytic acid is further metabolized by the IP6 Kinases (IP6Ks) to generate IP containing pyro-phosphate moiety. The IP6K are evolutionary conserved enzymes identified in several mammalian, fungi and amoebae species. Although IP6K has not yet been identified in plant chromosomes, there are many clues suggesting its presences in vegetal cells.

Results

In this paper we propose a new approach to search for the plant IP6K gene, that lead to the identification in plant genome of a nucleotide sequence corresponding to a specific tag of the IP6K family. Such a tag has been found in all IP6K genes identified up to now, as well as in all genes belonging to the Inositol Polyphosphate Kinases superfamily (IPK). The tag sequence corresponds to the inositol-binding site of the enzyme, and it can be considered as characterizing all IPK genes. To this aim we applied a technique based on motif discovery. We exploited DLSME, a software recently proposed, which allows for the motif structure to be only partially specified by the user. First we applied the new method on mitochondrial DNA (mtDNA) of plants, where such a gene could have been nested, possibly encrypted and hidden by virtue of the editing and/or trans-splicing processes. Then we looked for the gene in nuclear genome of two model plants, Arabidopsis thaliana and Oryza sativa.

Conclusions

The analysis we conducted in plant mitochondria provided the negative, though we argue relevant, result that IP6K does not actually occur in vegetable mtDNA. Very interestingly, the tag search in nuclear genomes lead us to identify a promising sequence in chromosome 5 of Oryza sativa. Further analyses are in course to confirm that this sequence actually corresponds to IP6K mammalian gene.

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.

RNAi-mediated silencing of the myo-inositol-1-phosphate synthase gene (GmMIPS1) in transgenic soybean inhibited seed development and reduced phytate content

Inositol plays a role in membrane trafficking and signaling in addition to regulating cellular metabolism and controlling growth. In plants, the myo-inositol-1-phosphate is synthesized from glucose 6-phosphate in a reaction catalyzed by the enzyme myo-inositol-1-phosphate synthase (EC 5.5.1.4). Inositol can be converted into phytic acid (phytate), the most abundant form of phosphate in seeds. The path to phytate has been suggested to proceed via the sequential phosphorylation of inositol phosphates, and/or in part via phosphatidylinositol phosphate. Soybean [Glycine max (L.) Merrill] lines were produced using interfering RNA (RNAi) construct in order to silence the myo-inositol-1-phosphate (GmMIPS1) gene. We have observed an absence of seed development in lines in which the presence of GmMIPS1 transcripts was not detected. In addition, a drastic reduction of phytate (InsP6) content was achieved in transgenic lines (up to 94.5%). Our results demonstrated an important correlation between GmMIPS1 gene expression and seed development.

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.

Culture Conditions and Characterizations of a New Phytase-Producing Fungal Isolate, Aspergillus sp. L117

A novel fungal strain Aspergillus sp. L117 that produced acid-stable and thermostable phytase was isolated on basis of the clearing zone on PSM plate and the ability of Na-phytate hydrolysis. The phytase of isolate showed a 3-fold higher activity than that of A. ficuun NRRL3135. The Aspergillus sp. L117 produced maximal level of phytase at initial pH of 5.0 and 30℃. The optimal pH and temperature for phytase activity were 5.5 and 50℃, respectively. The phytase showed totally stable activity after 20 min of exposure between 30 and 90℃, and even at 100℃. The highest level of residual phytase activity was obtained at pH 5.5, and still retained the stability at the broadest pH ranges (2.0 to 7.0) of all the aforementioned phytases. Storage stability of phytase was preserved over 96% of initial activities for 60 days at 4, -20, and -70℃ and to retain even 70% of the initial activity at room temperature.

Dynamics of inositol phosphate pools (tris-, tetrakis- and pentakisphosphate) in relation to the rate of phytate synthesis during seed development in common bean (Phaseolus vulgaris)

Four cultivars of Phaseolus vulgaris were grown in a greenhouse and each flower was Labeled with date of anthesis. Seeds were collected at six different stages of development and inositol phosphates (InsPs) were analyzed by ion-pair reversed-phase HPLC. Phytate accumulation was similar in all cultivars, and the specific rate of phytate synthesis (Rs) peaked at about 22 days after flowering (DAF). Variations in the concentrations of the InsP3 and InsP4 pools matched changes in Rs in cultivars Una and Aruã. These results suggest mass-action effects. Thus, the rates of conversion of InsP3 to InsP5 appeared to be at least partly dependent on substrate concentration. Proportional increases in size of all InsP pools up to 21 DAF are also consistent with Little regulation in this part of the pathway. However, this did not appear to be the case in cv. Diamante Negro or with the conversion of InsP5 to InsP6 in all cultivars, where concentrations of the InsP precursor pools peaked earlier or even dropped as Rs peaked, suggesting activation of enzyme activity. Therefore, the evidence is consistent with a control point regulating this metabolic route upstream of InsP3 and possibly in the conversion of InsP5 to InsP6.

Seed phosphorus and inositol phosphate phenotype of barley low phytic acid genotypes

myo-Inositol-1,2,3,4,5,6-hexakisphosphate (Ins P(6) or "phytic acid") typically represents approximately 75% of the total phosphorus and >80% of soluble myo-inositol (Ins) phosphates in seeds. The seed phosphorus and Ins phosphate phenotypes of four non-lethal barley (Hordeum vulgare L.) low phytic acid mutations are described. In seeds homozygous for M 635 and M 955 reductions in Ins P(6), approximately 75 and >90% respectively, are accompanied by reductions in other Ins phosphates and molar-equivalent increases in Pi. This phenotype suggests a block in supply of substrate Ins. In seeds homozygous for barley low phytic acid 1-1 (lpa1-1), a 45% decrease in Ins P(6) is mostly matched by an increase in Pi but also accompanied by small increases in Ins(1,2,3,4,6)P(5). In seeds homozygous for barley lpa2-1, reductions in seed Ins P(6) are accompanied by increases in both Pi and in several Ins phosphates, a phenotype that suggests a lesion in Ins phosphate metabolism, rather than Ins supply. The increased Ins phosphates in barley lpa2-1 seed are: Ins(1,2,3,4,6)P(5); Ins(1,2,4,6)P(4) and/or its enantiomer Ins(2,3,4,6)P(4); Ins(1,2,3,4)P(4) and/or its enantiomer Ins(1,2,3,6)P(4); Ins(1,2,6)P(3) and/or its enantiomer Ins(2,3,4)P(3); Ins(1,5,6)P(3) and/or its enantiomer Ins(3,4,5)P(3) (the methods used here cannot distinguish between enantiomers). This primarily "5-OH" series of Ins phosphates differs from the "1-/3-OH" series observed at elevated levels in seed of the maize lpa2 genotype, but previous chromosomal mapping data indicated that the maize and barley lpa2 loci might be orthologs of a single ancestral gene. Therefore one hypothesis that might explain the differing lpa2 phenotypes is that their common ancestral gene encodes a multi-functional, Ins phosphate kinase with both "1-/-3-" and "5-kinase" activities. A putative pyrophosphate-containing Ins phosphate, possibly an Ins P(7), was also observed in the mature seed of all barley genotypes except lpa2-1. Barley M 955 indicates that at least for this species, the ability to accumulate Ins P(6) can be nearly abolished while retaining at least short-term ( approximately 1.0 years) viability.

The maize low-phytic acid mutant lpa2 is caused by mutation in an inositol phosphate kinase gene

Reduced phytic acid content in seeds is a desired goal for genetic improvement in several crops. Low-phytic acid mutants have been used in genetic breeding, but it is not known what genes are responsible for the low-phytic acid phenotype. Using a reverse genetics approach, we found that the maize (Zea mays) low-phytic acid lpa2 mutant is caused by mutation in an inositol phosphate kinase gene. The maize inositol phosphate kinase (ZmIpk) gene was identified through sequence comparison with human and Arabidopsis Ins(1,3,4)P(3) 5/6-kinase genes. The purified recombinant ZmIpk protein has kinase activity on several inositol polyphosphates, including Ins(1,3,4)P(3), Ins(3,5,6)P(3), Ins(3,4,5,6)P(4), and Ins(1,2,5,6)P(4). The ZmIpk mRNA is expressed in the embryo, the organ where phytic acid accumulates in maize seeds. The ZmIpk Mutator insertion mutants were identified from a Mutator F(2) family. In the ZmIpk Mu insertion mutants, seed phytic acid content is reduced approximately 30%, and inorganic phosphate is increased about 3-fold. The mutants also accumulate myo-inositol and inositol phosphates as in the lpa2 mutant. Allelic tests showed that the ZmIpk Mu insertion mutants are allelic to the lpa2. Southern-blot analysis, cloning, and sequencing of the ZmIpk gene from lpa2 revealed that the lpa2-1 allele is caused by the genomic sequence rearrangement in the ZmIpk locus and the lpa2-2 allele has a nucleotide mutation that generated a stop codon in the N-terminal region of the ZmIpk open reading frame. These results provide evidence that ZmIpk is one of the kinases responsible for phytic acid biosynthesis in developing maize seeds.

Biochemical and molecular characterization of a mutation that confers a decreased raffinosaccharide and phytic acid phenotype on soybean seeds

A single, recessive mutation in soybean (Glycine max L. Merr.), which confers a seed phenotype of increased inorganic phosphate, decreased phytic acid, and a decrease in total raffinosaccharides, has been previously disclosed (S.A. Sebastian, P.S. Kerr, R.W. Pearlstein, W.D. Hitz [2000] Soy in Animal Nutrition, pp 56-74). The genetic lesion causing the multiple changes in seed phenotype is a single base change in the third base of the codon for what is amino acid residue 396 of the mature peptide encoding a seed-expressed myo-inositol 1-phospate synthase gene. The base change causes residue 396 to change from lysine to asparagine. That amino acid change decreases the specific activity of the seed-expressed myo-inositol 1-phosphate synthase by about 90%. Radio tracer experiments indicate that the supply of myo-inositol to the reaction, which converts UDP-galactose and myo-inositol to galactinol is a controlling factor in the conversion of total carbohydrate into the raffinosaccharides in both wild-type and mutant lines. That same decrease in myo-inositol 1-phosphate synthetic capacity leads to a decreased capacity for the synthesis of myo-inositol hexaphosphate (phytic acid) and a concomitant increase in inorganic phosphate.

Inositol phosphates from barley low-phytate grain mutants analysed by metal-dye detection HPLC and NMR

Inositol phosphates from barley low-phytate grain mutants and their parent variety were analysed by metal-dye detection HPLC and NMR. Compound assignment was carried out by comparison of retention times using a chemical hydrolysate of phytate [Ins(1,2,3,4,5,6)P(6)] as a reference. Co-inciding retention times indicated the presence of phytate, D/L-Ins(1,2,3,4,5)P(5), Ins(1,2,3,4,6)P(5), D/L-(1,2,4,5,6)P(5), D/L-(1,2,3,4)P(4), D/L-Ins(1,2,5,6)P(4) and D/L-Ins(1,4,5,6)P(4) in PLP1B mutants as well as the parent variety. In grain extracts from mutant lines PLP1A, PLP2A and PLP3A unusual accumulations of D/L-Ins(1,3,4,5)P(4) were observed whereas phytate and the above-mentioned inositol phosphates were present in relatively small amounts. Assignment of D/L-Ins(1,3,4,5)P(4) was corroborated by precise co-chromatography with a commercial Ins(1,3,4,5)P(4) standard and by NMR spectroscopy. Analysis of inositol phosphates during grain development revealed accumulation of phytate and D/L-Ins(1,3,4,5)P(4), which suggested the tetrakisphosphate compound to be an intermediate of phytate synthesis. This assumption was strengthened further by phytate degradation assays showing that D/L-Ins(1,3,4,5)P(4) did not belong to the spectrum of degradation products generated by endogenous phytase activity. Metabolic scenarios leading to accumulation of D/L-Ins(1,3,4,5)P(4) in barley low-phytate mutants are discussed.

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.

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.

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.

High-performance thin-layer chromatography method for inositol phosphate analysis

A simple and inexpensive high-performance thin-layer chromatography (HPTLC) method for the analysis of inositol mono- to hexakisphosphates on cellulose precoated plates is described. Plates were developed in 1-propanol-25% ammonia solution-water (5:4:1) and substance quantities as low as 100-200 pmol were detected by molybdate staining. Chromatographic mobilities of nucleotides and phosphorylated carbohydrates were also characterized. Charcoal treatment was employed to separate nucleotides from inositol phosphates with similar R(F) values prior to HPTLC analysis. Practical application of the HPTLC system is demonstrated by analysis of grain extracts from wild type and low-phytate mutant barley as well as phytate degradation products resulting from barley phytase activity.

Site-directed mutagenesis of diphosphoinositol polyphosphate phosphohydrolase, a dual specificity NUDT enzyme that attacks diadenosine polyphosphates and diphosphoinositol polyphosphates

Diphosphoinositol polyphosphate phosphohydrolase (DIPP) hydrolyzes diadenosine 5',5"'-P(1),P(6)-hexaphosphate (Ap(6)A), a Nudix (nucleoside diphosphate attached-moiety "x") substrate, and two non-Nudix compounds: diphosphoinositol pentakisphosphate (PP-InsP(5)) and bis-diphosphoinositol tetrakisphosphate ((PP)(2)-InsP(4)). Guided by multiple sequence alignments, we used site-directed mutagenesis to obtain new information concerning catalytically essential amino acid residues in DIPP. Mutagenesis of either of two conserved glutamate residues (Glu(66) and Glu(70)) within the Nudt (Nudix-type) catalytic motif impaired hydrolysis of Ap(6)A, PP-InsP(5), and (PP)(2)-InsP(4) >95%; thus, all three substrates are hydrolyzed at the same active site. Two Gly-rich domains (glycine-rich regions 1 and 2 (GR1 and GR2)) flank the Nudt motif with potential sites for cation coordination and substrate binding. GR1 comprises a GGG tripeptide, while GR2 is identified as a new functional motif (GX(2)GX(6)G) that is conserved in yeast homologues of DIPP. Mutagenesis of any of these Gly residues in GR1 and GR2 reduced catalytic activity toward all three substrates by up to 95%. More distal to the Nudt motif, H91L and F84Y mutations substantially decreased the rate of Ap(6)A and (PP)(2)-InsP(4) metabolism (by 71 and 96%), yet PP-InsP(5) hydrolysis was only mildly reduced (by 30%); these results indicate substrate-specific roles for His(91) and Phe(84). This new information helps define DIPP's structural, functional, and evolutionary relationships to Nudix hydrolases.

A novel context for the 'MutT' module, a guardian of cell integrity, in a diphosphoinositol polyphosphate phosphohydrolase

Diphosphoinositol pentakisphosphate (PP-InsP5 or 'InsP7') and bisdiphosphoinositol tetrakisphosphate ([PP]2-InsP4 or 'InsP8') are the most highly phosphorylated members of the inositol-based cell signaling family. We have purified a rat hepatic diphosphoinositol polyphosphate phosphohydrolase (DIPP) that cleaves a beta-phosphate from the diphosphate groups in PP-InsP5 (Km = 340 nM) and [PP]2-InsP4 (Km = 34 nM). Inositol hexakisphophate (InsP6) was not a substrate, but it inhibited metabolism of both [PP]2-InsP4 and PP-InsP5 (IC50 = 0.2 and 3 microM, respectively). Microsequencing of DIPP revealed a 'MutT' domain, which in other contexts guards cellular integrity by dephosphorylating 8-oxo-dGTP, which causes AT to CG transversion mutations. The MutT domain also metabolizes some nucleoside phosphates that may play roles in signal transduction. The rat DIPP MutT domain is conserved in a novel recombinant human uterine DIPP. The nucleotide sequence of the human DIPP cDNA was aligned to chromosome 6; the candidate gene contains at least four exons. The dependence of DIPP's catalytic activity upon its MutT domain was confirmed by mutagenesis of a conserved glutamate residue. DIPP's low molecular size, Mg2+ dependency and catalytic preference for phosphoanhydride bonds are also features of other MutT-type proteins. Because overlapping substrate specificity is a feature of this class of proteins, our data provide new directions for future studies of higher inositol phosphates.

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.