Barley (Hordeum vulgare L.) inositol monophosphatase: gene structure and enzyme characteristics

Deciphering the structural basis of the broad substrate specificity of myo-inositol monophosphatase (IMP) from Cicer arietinum

Myo-inositol monophosphatase (IMP) is a crucial enzyme in the inositol biosynthetic pathway that dephosphorylates myo-inositol 1-phosphate and other inositol phosphate derivative compounds to maintain the homeostasis of cellular inositol pool. In our previous research, we have biochemically and functionally characterized IMP enzyme from chickpea (CaIMP), which was able to catalyze diverse substrates. We cloned, overexpressed recombinant CaIMP protein and purified it and further characterized the CaIMP with its three main substrates viz. galactose 1-P, inositol 6-P and fructose 1,6-bisP. Homology model of CaIMP was generated to elucidate the factors contributing to the broad substrate specificity of the protein. The active site of the CaIMP protein was analysed with respect to its interactions with the proposed substrates. Structural features such as, high B-factor and flexible loop regions in the active site, inspired further investigation into the static and dynamic behaviour of the active site of CaIMP protein. The electrostatic biding of each of the key substrates was assessed through molecular docking. Furthermore, molecular dynamics simulations showed that these interactions indeed were stable for extended periods of time under physiological conditions. These experiments conclusively allowed us to establish the primary factors contributing to the promiscuity in substrate binding by CaIMP protein.

Overexpression of the OsIMP Gene Increases the Accumulation of Inositol and Confers Enhanced Cold Tolerance in Tobacco through Modulation of the Antioxidant Enzymes' Activities

Inositol is a cyclic polyol that is involved in various physiological processes, including signal transduction and stress adaptation in plants. l-myo-inositol monophosphatase (IMPase) is one of the metal-dependent phosphatase family members and catalyzes the last reaction step of biosynthesis of inositol. Although increased IMPase activity induced by abiotic stress has been reported in chickpea plants, the role and regulation of the IMP gene in rice (Oryza sativa L.) remains poorly understood. In the present work, we obtained a full-length cDNA sequence coding IMPase in the cold tolerant rice landraces in Gaogonggui, which is named as OsIMP. Multiple alignment results have displayed that this sequence has characteristic signature motifs and conserved enzyme active sites of the phosphatase super family. Phylogenetic analysis showed that IMPase is most closely related to that of the wild rice Oryza brachyantha, while transcript analysis revealed that the expression of the OsIMP is significantly induced by cold stress and exogenous abscisic acid (ABA) treatment. Meanwhile, we cloned the 5' flanking promoter sequence of the OsIMP gene and identified several important cis-acting elements, such as LTR (low-temperature responsiveness), TCA-element (salicylic acid responsiveness), ABRE-element (abscisic acid responsiveness), GARE-motif (gibberellin responsive), MBS (MYB Binding Site) and other cis-acting elements related to defense and stress responsiveness. To further investigate the potential function of the OsIMP gene, we generated transgenic tobacco plants overexpressing the OsIMP gene and the cold tolerance test indicated that these transgenic tobacco plants exhibit improved cold tolerance. Furthermore, transgenic tobacco plants have a lower level of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and a higher content of total chlorophyll as well as increased antioxidant enzyme activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), when compared to wild type (WT) tobacco plants under normal and cold stress conditions.

Seed Biofortification and Phytic Acid Reduction: A Conflict of Interest for the Plant?

Most of the phosphorus in seeds is accumulated in the form of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate, InsP₆). This molecule is a strong chelator of cations important for nutrition, such as iron, zinc, magnesium, and calcium. For this reason, InsP₆ is considered an antinutritional factor. In recent years, efforts to biofortify seeds through the generation of low phytic acid (lpa) mutants have been noteworthy. Moreover, genes involved in the biosynthesis and accumulation of this molecule have been isolated and characterized in different species. Beyond its role in phosphorus storage, phytic acid is a very important signaling molecule involved in different regulatory processes during plant development and responses to different stimuli. Consequently, many lpa mutants show different negative pleitotropic effects. The strength of these pleiotropic effects depends on the specific mutated gene, possible functional redundancy, the nature of the mutation, and the spatio-temporal expression of the gene. Breeding programs or transgenic approaches aimed at development of new lpa mutants must take into consideration these different aspects in order to maximize the utility of these mutants.

Repeat length variation in the 5'UTR of myo-inositol monophosphatase gene is related to phytic acid content and contributes to drought tolerance in chickpea (Cicer arietinum L.)

Myo-inositol metabolism plays a significant role in plant growth and development, and is also used as a precursor for many important metabolites, such as ascorbate, pinitol, and phytate. Phytate (inositol hexakisphosphate) is the major storage pool for phosphate in the seeds. It is utilized during seed germination and growth of the developing embryo. In addition, it is implicated in protection against oxidative stress. In the present study, a panel of chickpea accessions was used for an association analysis. Association analysis accounting for population structure and relative kinship identified alleles of a simple sequence repeat marker, NCPGR90, that are associated with both phytic acid content and drought tolerance. These alleles varied with respect to the dinucleotide CT repeats present within the marker. NCPGR90 located to the 5'UTR of chickpea myo-inositol monophosphatase gene (CaIMP) and showed transcript length variation in drought-tolerant and drought-susceptible accessions. CaIMP from a drought-tolerant accession with a smaller repeat was almost 2-fold upregulated as compared to a susceptible accession having a longer repeat, even under control non-stressed conditions. This study suggests an evolution of simple sequence repeat length variation in CaIMP, which might be regulating phytic acid levels to confer drought tolerance in natural populations of chickpea.

Differentially expressed myo-inositol monophosphatase gene (CaIMP) in chickpea (Cicer arietinum L.) encodes a lithium-sensitive phosphatase enzyme with broad substrate specificity and improves seed germination and seedling growth under abiotic stresses

myo-Inositol monophosphatase (IMP) is an essential enzyme in the myo-inositol metabolic pathway where it primarily dephosphorylates myo-inositol 1-phosphate to maintain the cellular inositol pool which is important for many metabolic and signalling pathways in plants. The stress-induced increased accumulation of inositol has been reported in a few plants including chickpea; however, the role and regulation of IMP is not well defined in response to stress. In this work, it has been shown that IMP activity is distributed in all organs in chickpea and was noticeably enhanced during environmental stresses. Subsequently, using degenerate oligonucleotides and RACE strategy, a full-length IMP cDNA (CaIMP) was cloned and sequenced. Biochemical study revealed that CaIMP encodes a lithium-sensitive phosphatase enzyme with broad substrate specificity, although maximum activity was observed with the myo-inositol 1-phosphate and l-galactose 1-phosphate substrates. Transcript analysis revealed that CaIMP is differentially expressed and regulated in different organs, stresses and phytohormones. Complementation analysis in Arabidopsis further confirmed the role of CaIMP in l-galactose 1-phosphate and myo-inositol 1-phosphate hydrolysis and its participation in myo-inositol and ascorbate biosynthesis. Moreover, Arabidopsis transgenic plants over-expressing CaIMP exhibited improved tolerance to stress during seed germination and seedling growth, while the VTC4/IMP loss-of-function mutants exhibited sensitivity to stress. Collectively, CaIMP links various metabolic pathways and plays an important role in improving seed germination and seedling growth, particularly under stressful environments.

The inositol monophosphatase All2917 (IMPA1) is involved in osmotic adaptation in Anabaena sp. PCC7120

Inositol monophosphatase (IMPase; EC 3.1.3.25) acts at the last step in the inositol biosynthesis pathway by hydrolysing inositol-1-phosphate into inositol. In this study, an IMPase encoding gene, all2917 from Anabaena sp. PCC7120, was characterized. We found that All2917 exhibits a specific activity on inositol-1-phosphate, in a typical Mg(2+) -dependent, Li(+) -sensitive manner. The deletion of all2917 in Anabaena made the cells more sensitive to osmotic stress caused by sucrose or sorbitol, while its overexpression led to an increased resistance to such stress. Consistent with these phenotypes, the transcription of all2917 was significantly upregulated upon the sucrose-mediated osmotic stress. Phylogenic analysis using 134 IMPase homologues from 36 cyanobacterial strains shows that members of IMPase family form three major distinct clades, suggesting that multiple copies of IMPase family proteins have been maintained in Cyanobacteria during a long history of evolution, and they may play important roles in cyanobacterial physiology.

Molecular cloning and characterization of L-galactose-1-phosphate phosphatase from tobacco (Nicotiana tabacum)

L-Galactose-1-phosphate phosphatase (GPPase) is an enzyme involved in ascorbate biosynthesis in higher plants. We isolated a cDNA encoding GPPase from tobacco, and named it NtGPPase. The putative amino acid sequence of NtGPPase contained inositol monophosphatase motifs and metal binding sites. Recombinant NtGPPase hydrolyzed not only L-galactose-1-phosphate, but also myo-inositol-1-phosphate. The optimum pH for the GPPase activity of NtGPPase was 7.5. Its enzyme activity required Mg2+, and was inhibited by Li+ and Ca2+. Its fluorescence, fused with green fluorescence protein in onion cells and protoplasts of tobacco BY-2 cells, was observed in both the cytosol and nucleus. The expression of NtGPPase mRNA and protein was clearly correlated with L-ascorbic acid (AsA) contents of BY-2 cells during culture. The AsA contents of NtGPPase over expression lines were higher than those of empty lines at 13 d after subculture. This suggests that NtGPPase contributes slightly to AsA biosynthesis.

cDNA-AFLP analysis of salt-inducible genes expression in Chrysanthemum lavandulifolium under salt treatment

Chrysanthemum lavandulifolium (Fisch. ex Trautv.) Makino is a halophyte species that belongs to the Asteraceae family, and the genus Chrysanthemum. It is one of the ancestors of C.×morifolium Ramatella. Understanding the tolerance mechanism associated with salt stress in C. lavandulifolium could provide important information for explaining the salt tolerance of higher plants and could also help enhancing breeding programs of cultivated Chrysanthemum. In this study, cDNA amplified fragment length polymorphism (cDNA-AFLP) was used to detect differential gene expression in leaves of C. lavandulifolium in response to NaCl treatment. The determination of membrane permeablility, peroxidase activity (POD), malon-dialdehyde (MDA), as well as proline and leaf chlorophyll contents under different NaCl concentrations showed that a 200 mM NaCl treatment was an optimal condition for the cDNA-AFLP experiment. Using this concentration during different times (0, 3 h, 12 h, 24 h and 48 h), we obtained 1930 cDNA fragments using 64 primers. After sequencing 234 randomly chosen cDNA clones and BLASTx analyzing, we got 129 expressed sequence tags (ESTs) which had no significant homology with other sequences, 85 ESTs were homologous to genes with known functions, whereas the rest of ESTs showed homology to unclassified or putative proteins. 25 ESTs that were similar to known functional genes involved in several abiotic and biotic stresses were confirmed by semi-quantitative RT-PCR and qRT-PCR. The expression patterns of these salt-responsive genes not only responded to salt stress but also to plant hormones, such as abscisic acid (ABA), and to other abiotic stresses such as drought and cold. These results indicate an extensive cross-talk among several stresses. Our results provide interesting information for further understanding the molecular mechanisms of salt tolerance in C. lavandulifolium.

Expression and functions of myo-inositol monophosphatase family genes in seed development of Arabidopsis

Myo-inositol monophosphatase (IMP) catalyzes the dephosphorylation of myo-inositol 3-phosphate in the last step of myo-inositol biosynthesis. IMP is also important in phosphate metabolism and is required for the biosynthesis of cell wall polysaccharides, phytic acid, and phosphatidylinositol. In Arabidopsis, IMP is encoded by VTC4. There are, however, two additional IMP candidate genes, IMPL1 and IMPL2, which have not yet been elucidated. In our genetic studies of Arabidopsis IMP genes, only the loss-of-function mutant impl2 showed embryonic lethality at the globular stage. All IMP genes were expressed in a similar manner both in the vegetative and reproductive organs. In developing seeds, expression of IMP genes was not coupled with the expression of the genes encoding myo-inositol phosphate synthases, which supply the substrate for IMPs in the de novo synthesis pathway. Instead, expression of IMP genes was correlated with expression of the gene for myo-inositol polyphosphate 1-phosphatase (SAL1), which is involved in the myo-inositol salvage pathway, suggesting a possible salvage pathway role in seed development. Moreover, the partial rescue of the impl2 phenotype by histidine application implies that IMPL2 is also involved in histidine biosynthesis during embryo development.

Mutations in genes controlling the biosynthesis and accumulation of inositol phosphates in seeds

Most of the phosphorus in the resting seed is stored inside protein storage vacuoles as PA (phytic acid; InsP(6)). The biosynthesis and accumulation of PA can be detected beginning from a few days after anthesis and seem to continue during seed development until maturation. The first step in PA biosynthesis is the formation of Ins3P by conversion of glucose 6-phosphate. This is then followed by a sequential and ordered phosphorylation of the remaining five positions of the inositol ring by a number of kinases, resulting in PA. Identification of low-PA mutants in cereals, legumes and Arabidopsis is instrumental for resolving the biosynthetic pathway and identification of genes controlling the accumulation of PA. Mutations in seven genes involved in the metabolism of PA have been identified and characterized among five plant species using induced mutagenesis and insertion elements. Understanding the biosynthetic pathway and genes controlling the accumulation of PA in plant seeds and how PA may balance the free phosphate is of importance for molecular breeding of crop plants, particularly cereals and legumes.

VTC4 is a bifunctional enzyme that affects myoinositol and ascorbate biosynthesis in plants

Myoinositol synthesis and catabolism are crucial in many multiceullar eukaryotes for the production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic noncellulosic polysaccharides, and several other molecules including ascorbate. Myoinositol monophosphatase (IMP) is a major enzyme required for the synthesis of myoinositol and the breakdown of myoinositol (1,4,5)trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit (Actinidia deliciosa) has similarity to IMP and can hydrolyze l-galactose 1-phosphate (l-Gal 1-P), suggesting that this enzyme may be bifunctional and linked with two potential pathways of plant ascorbate synthesis. We describe here the kinetic comparison of the Arabidopsis (Arabidopsis thaliana) recombinant VTC4 with d-myoinositol 3-phosphate (d-Ins 3-P) and l-Gal 1-P. Purified VTC4 has only a small difference in the V(max)/K(m) for l-Gal 1-P as compared with d-Ins 3-P and can utilize other related substrates. Inhibition by either Ca(2+) or Li(+), known to disrupt cell signaling, was the same with both l-Gal 1-P and d-Ins 3-P. To determine whether the VTC4 gene impacts myoinositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in abscisic acid, salt, and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myoinositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myoinositol and ascorbate synthesis pathways.