The bifunctional role of hexokinase in metabolism and glucose signaling

β-Glucan content increase in Waxy-mutated barley is closely associated with positive stress responses and is regulated by ASR1

Mixed-linkage (1,3; 1,4)-β-D-glucan (MLG) impacts the food and industrial end-uses of barley, but the molecular mechanism of variations in MLG content remains unclear. MLG content usually increases in Waxy-mutated barley. This study applied transcriptomic, proteomic, and metabolomic analyses to Waxy-mutated recombinant inbred lines with higher MLG content and wild-type lines with lower MLG content, and identified candidate genes and pathways regulating MLG content through combining preliminary gene function analysis. MLG biosynthesis differed significantly during late grain development in the Waxy-mutated and wild-type barley lines. The MLG increase was closely associated with strongly active sugar and starch metabolism and stress-responsive plant hormones, particularly abscisic acid (ABA) signaling process. Stress-responsive transcript factors ILR3, BTF3, RGGA, and PR13 protein bind to CslF6, which is critical for barley MLG biosynthesis, and the stress-responsive gene ASR1 also had a positive effect on MLG increase. Waxy mutation enhances barley stress responses by activating ABA- or other stress-responsive plant hormones signaling processes, which facilitates MLG biosynthesis. This study provides a new approach for elucidating the variations in MLG content of barley grains.

Optimizing d-mannose and glyceraldehyde concentrations as glucose preservatives without clinically affecting biochemical test results

Objectives: The objectives were to evaluate blood additives that combined lithium heparin (LH)-salt with glyceraldehyde (GLY) or d-mannose (MAN) for preserving glucose levels in plasma samples and to simultaneously determine the compatibility of these additives with 14 other biochemical tests.

Methods

Blood samples from 40 subjects, equally divided into healthy and diabetic groups, were collected using five different additives. The three most effective additives, LH/GLY, LH/MAN, and LH/GLY/MAN, were selected for ensuring the best preservation of glucose levels and compatibility with 14 biochemical tests. One-way analysis of variance was used to analyze the mean paired differences of glucose level and biochemical tests. Simultaneously, the clinical criteria from Johns Hopkins Hospital were used to guide the interpretation and set acceptable thresholds for measurements that exceeded the standards.

Results

The combination of 160 mmol/L GLY, 8.4 mmol/L MAN, and LH, maintained glucose levels at approximately 93.4-93.7 % for healthy subjects and 91.3-92.8% for subjects with diabetes mellitus over 8 h. The mean paired differences of glucose levels in preservation were statistically insignificant. The biases in 14 biochemical tests for LH/GLY/MAN and LH/MAN remained within the acceptable clinical criteria during the 8 h.

Conclusions

Combining 160 mmol/L GLY, 8.4 mmol/L MAN, and LH, proved more effective in maintaining glucose levels than individual additives or the conventional sodium fluoride preservative. It did not yield clinical discrepancies in the 14 biochemical tests during 8 h at room temperature.

Tomato SlSTK is involved in glucose response and regulated by the ubiquitin ligase SlSINA4

Glucose signaling plays an essential role in plant growth, development and stress response. Previous studies have shown that STOREKEEPER (STK) is a new class of DNA binding protein that regulates patatin expression in potato tubers and confers elevated sensitivity to glucose response in Arabidopsis thaliana. However, the biological functions of STK gene in tomato (Solanum lycopersicum) have not been studied. Here, we characterized the tomato SlSTK and determined its role in glucose signaling. The SlSTK protein was localized in the nucleus and the expression of the SlSTK gene was induced by the glucose treatment. Overexpression of SlSTK in tomato enhanced glucose sensitivity, as manifested by reduced seed germination rate and arrested growth at the early seedling stage. In contrast, the SlSTK-knockout plants generated via the clustered regularly interspaced short palindromic repeats (CRISPR) - CRISPR-associated protein 9 (CRISPR-Cas9) technique attenuated the sensitivity to glucose. In addition, SlSTK was ubiquitinated in plant cells and interacted with the tomato ubiquitin ligase SEVEN IN ABSENTIA4 (SlSINA4) that degrades SlSTK in a ligase-dependent manner. Taken together, these results suggest that SlSTK is involved in glucose signaling and its stability is regulated by the ubiquitin ligase SlSINA4.

Photosynthetic acclimation to changing environments

Plants are exposed to environments that fluctuate of timescales varying from seconds to months. Leaves that develop in one set of conditions optimise their metabolism to the conditions experienced, in a process called developmental acclimation. However, when plants experience a sustained change in conditions, existing leaves will also acclimate dynamically to the new conditions. Typically this process takes several days. In this review, we discuss this dynamic acclimation process, focussing on the responses of the photosynthetic apparatus to light and temperature. We briefly discuss the principal changes occurring in the chloroplast, before examining what is known, and not known, about the sensing and signalling processes that underlie acclimation, identifying likely regulators of acclimation.

The SLIM1 transcription factor affects sugar signaling during sulfur deficiency in Arabidopsis

The homeostasis of major macronutrient metabolism needs to be tightly regulated, especially when the availability of one or more nutrients fluctuates in the environment. Both sulfur metabolism and glucose signaling are important processes throughout plant growth and development, as well as during stress responses. Still, very little is known about how these processes affect each other, although they are positively connected. Here, we showed in Arabidopsis that the crucial transcription factor of sulfur metabolism, SLIM1, is involved in glucose signaling during shortage of sulfur. The germination rate of the slim1_KO mutant was severely affected by high glucose and osmotic stress. The expression of SLIM1-dependent genes in sulfur deficiency appeared to be additionally induced by a high concentration of either mannitol or glucose, but also by sucrose, which is not only the source of glucose but another signaling molecule. Additionally, SLIM1 affects PAP1 expression during sulfur deficiency by directly binding to its promoter. The lack of PAP1 induction in such conditions leads to much lower anthocyanin production. Taken together, our results indicate that SLIM1 is involved in the glucose response by modulating sulfur metabolism and directly controlling PAP1 expression in Arabidopsis during sulfur deficiency stress.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

Polymorphisms of genes related to phase II metabolism and resistance to clopidogrel

Clopidogrel is an antiplatelet drug commonly used to prevent coagulation. This review aimed to investigate the effect of polymorphisms of G6PD, GCLC, GCLM, GSS, GST, GSR, HK and GLRX genes on clopidogrel during phase II metabolism through exploring previous studies. The results revealed that low glutathione plasma levels caused by several alleles related to these genes could affect the bioactivation process of the clopidogrel prodrug, making it unable to inhibit platelet aggregation perfectly and thus leading to severe consequences in patients with a high risk of blood coagulation. However, the study recommends platelet reactivity tests to predict clopidogrel efficacy rather than studying gene mutations, as most of these mutations are rare and other nongenetic factors could affect the drug's efficacy.

Spatio-temporally expressed sorbitol transporters cooperatively regulate sorbitol accumulation in pear fruit

Sorbitol is the primary substrate translocated from source to sink in pear species. Among the many sorbitol transporters (SOTs), some are known to be involved in sorbitol accumulation in fruit; however, their particular roles are unclear. In this study, we examined the transcriptome and metabolome of a variety of pear samples from six time points to identify those SOTs. Similar to previous studies, sorbitol and sucrose differed significantly between the leaf and fruit, and sorbitol was consistently observed at higher concentrations at all time points. Interestingly, we found that sorbitol accumulation in pear fruit was cooperatively mediated by SOT3, SOT6/20, SOT19/21, and SOT22. In particular, the up-regulated SOT6/20 and SOT19/21 in fruit under 1 mg L^-1 abscisic acid and 10 mg L^-1 indole acetic acid treatments, respectively, resulted in an increased sorbitol concentration. In addition, sorbitol concentration showed positive correlations to fructose and glucose concentrations, indicating a role for sorbitol in the determination of fruit sweetness. Together with the deduced process of sugar biosynthesis, transport, conversion, and accumulation in pear, our study provides a foundation for further research into sugar accumulation processes in pear fruit, contributing to the improvement of fruit quality.

Comparative phenotypic and transcriptomic analysis of Victoria and flame seedless grape cultivars during berry ripening

Grape berry development is a highly coordinated and intricate process. Herein, we analyzed the phenotypic and transcriptomic patterns of Victoria (VT) and Flame Seedless (FS) grape varieties during berry development. Physiological analysis and transcriptomic sequencing were performed at four berry developmental phases. VT berry size was comparatively larger to the FS variety. At maturity, 80 days postanthesis (DPA), the FS soluble solids were 61.8% higher than VT. Further, 4889 and 2802 differentially expressed genes were identified from VT and FS 40 DPA to 80 DPA development stages, respectively. VvSWEET15, VvHXK, and MYB44 genes were up‐regulated during the postanthesis period, while bHLH14, linked to glucose metabolism, was gradually down‐regulated during berry development. These genes may have significant roles in berry development, ripening, and sugar accumulation.

Glucose modulates proliferation in root apical meristems via TOR in maize during germination

The Glucose-Target of Rapamycin (Glc-TOR) pathway has been studied in different biological systems, but scarcely during early seed germination. This work examines its importance for cell proliferation, expression of cell cycle key genes, their protein levels, besides morphology and cellularization of the root apical meristem of maize (Zea mays) embryo axes during germination under the influence of two simple sugars, glucose and sucrose, and a specific inhibitor of TOR activity, AZD 8055. The two sugars promote germination similarly and to an extent, independently of TOR activity. However, the Glc-TOR pathway increases the number of cells committed to proliferation, increasing the expression of a cell cycle gene, ZmCycD4;2, a putative G1/S regulator. Also, Glc-TOR may have influence on the protein stability of another G1/S cyclin, ZmCycD3, but had no influence on ZmCDKA;1 or ZmKRP3 or their proteins. Results suggest that the Glc-TOR pathway participates in the regulation of proliferation through different mechanisms that, in the end, modify the timing of seed germination.

Subcellular dynamics of proteins and metabolites under abiotic stress reveal deferred response of the Arabidopsis thaliana hexokinase-1 mutant gin2-1 to high light

Stress responses in plants imply spatio-temporal changes in enzymes and metabolites, including subcellular compartment-specific re-allocation processes triggered by sudden changes in environmental parameters. To investigate interactions of primary metabolism with abiotic stress, the gin2-1 mutant, defective in the sugar sensor hexokinase 1 (HXK1) was compared with its wildtype Landsberg erecta (Ler) based on time resolved, compartment-specific metabolome and proteome data obtained over a full diurnal cycle. The high light sensitive gin2-1 mutant was substantially delayed in subcellular re-distribution of metabolites upon stress, and this correlated with a massive reduction in proteins belonging to the ATP producing electron transport chain under high light, while fewer changes occurred in the cold. In the wildtype, compounds specifically protecting individual compartments could be identified, e.g., maltose and raffinose in plastids, myo-inositol in mitochondria, but gin2-1 failed to recruit these substances to the respective compartments, or responded only slowly to high irradiance. No such delay was obtained in the cold. At the whole cell level, concentrations of the amino acids, glycine and serine, provided strong evidence for an important role of the photorespiratory pathway during stress exposure, and different subcellular allocation of serine may contribute to the slow growth of the gin2-1 mutant under high irradiance.

Mathematical modeling of diurnal patterns of carbon allocation to shoot and root in Arabidopsis thaliana

We developed a mathematical model to simulate dynamics of central carbon metabolism over complete diurnal cycles for leaves of Arabidopsis thaliana exposed to either normal (120 µmol m-2 s-1) or high light intensities (1200 µmol m- 2 s-1). The main objective was to obtain a high-resolution time series for metabolite dynamics as well as for shoot structural carbon formation (compounds with long residence time) and assimilate export of aerial organs to the sink tissue. Model development comprised a stepwise increment of complexity to finally approach the in vivo situation. The correct allocation of assimilates to either sink export or shoot structural carbon formation was a central goal of model development. Diurnal gain of structural carbon was calculated based on the daily increment in total photosynthetic carbon fixation, and this was the only parameter for structural carbon formation implemented in the model. Simulations of the dynamics of central metabolite pools revealed that shoot structural carbon formation occurred solely during the light phase but not during the night. The model allowed simulation of shoot structural carbon formation as a function of central leaf carbon metabolism under different environmental conditions without structural modifications. Model simulations were performed for the accession Landsberg erecta (Ler) and its hexokinase null-mutant gin2-1. This mutant displays a slow growth phenotype especially at increasing light intensities. Comparison of simulations revealed that the retarded shoot growth in the mutant resulted from an increased assimilate transport to sink organs. Due to its central function in sucrose cycling and sugar signaling, our findings suggest an important role of hexokinase-1 for carbon allocation to either shoot growth or assimilate export.

Endosperm sugar accumulation caused by mutation of PHS8/ISA1 leads to pre-harvest sprouting in rice

Pre-harvest sprouting (PHS) is an unfavorable trait in cereal crops that could seriously decrease grain yield and quality. Although some PHS-associated quantitative trait loci or genes in cereals have been reported, the molecular mechanism underlying PHS remains largely elusive. Here, we characterized a rice mutant, phs8, which exhibits PHS phenotype accompanied by sugary endosperm. Map-based cloning revealed that PHS8 encodes a starch debranching enzyme named isoamylase1. Mutation in PHS8 resulted in the phytoglycogen breakdown and sugar accumulation in the endosperm. Intriguingly, with increase of sugar contents, decreased expression of OsABI3 and OsABI5 as well as reduced sensitivity to abscisic acid (ABA) were found in the phs8 mutant. Using rice suspension cell system, we confirmed that exogenous sugar is sufficient to suppress the expression of both OsABI3 and OsABI5. Furthermore, overexpression of OsABI3 or OsABI5 could partially rescue the PHS phenotype of phs8. Therefore, our study presents important evidence supporting that endosperm sugar not only acts as an essential energy source for seed germination but also determines seed dormancy and germination by affecting ABA signaling.

Nitrogen Source Dependent Changes in Central Sugar Metabolism Maintain Cell Wall Assembly in Mitochondrial Complex I-Defective frostbite1 and Secondarily Affect Programmed Cell Death

For optimal plant growth, carbon and nitrogen availability needs to be tightly coordinated. Mitochondrial perturbations related to a defect in complex I in the Arabidopsis thalianafrostbite1 (fro1) mutant, carrying a point mutation in the 8-kD Fe-S subunit of NDUFS4 protein, alter aspects of fundamental carbon metabolism, which is manifested as stunted growth. During nitrate nutrition, fro1 plants showed a dominant sugar flux toward nitrogen assimilation and energy production, whereas cellulose integration in the cell wall was restricted. However, when cultured on NH₄⁺ as the sole nitrogen source, which typically induces developmental disorders in plants (i.e., the ammonium toxicity syndrome), fro1 showed improved growth as compared to NO₃⁻ nourishing. Higher energy availability in fro1 plants was correlated with restored cell wall assembly during NH₄⁺ growth. To determine the relationship between mitochondrial complex I disassembly and cell wall-related processes, aspects of cell wall integrity and sugar and reactive oxygen species signaling were analyzed in fro1 plants. The responses of fro1 plants to NH₄⁺ treatment were consistent with the inhibition of a form of programmed cell death. Resistance of fro1 plants to NH₄⁺ toxicity coincided with an absence of necrotic lesion in plant leaves.

The Glucose Sensor MdHXK1 Phosphorylates a Tonoplast Na+/H+ Exchanger to Improve Salt Tolerance

Glc regulates many vital processes, including plant growth, development, metabolism, and responses to biotic and abiotic stress. However, the molecular mechanism by which Glc acts as a signal to regulate salinity tolerance remains unclear. In this study, we found that the apple (Malus domestica Borkh.) Glc sensor hexokinase1 (MdHXK1) contributes to Glc-mediated salinity tolerance. A combination of split ubiquitin system, pull-down, co-immunoprecipitation, and bimolecular fluorescence complementation assays demonstrated that MdHXK1 interacts with and phosphorylates the Na⁺/H⁺ exchanger MdNHX1 at its Ser-275 residue. Phosphorylation improved the stability of MdNHX1 and enhanced its Na⁺/H⁺ transport activity in MdNHX1 overexpression transgenic apple and yeast complementation cells. Furthermore, Ser-275 of MdNHX1 was found to be crucial for MdHXK1-mediated phosphorylation. Finally, a series of transgenic analyses demonstrated that salt tolerance mediated by MdHXK1 partially depended on MdNHX1. Overall, our findings provide insights into how sugar recruits and regulates MdNHX1 in response to high salinity in plants.

Hexokinase plays a critical role in deoxynivalenol (DON) production and fungal development in Fusarium graminearum

Fusarium graminearum, the causal agent of Fusarium head blight, is a common pathogen on small grain cereals worldwide and produces various trichothecenes [deoxynivalenol (DON) is predominant] during infection. A previous study has revealed that DON production is positively correlated with the occurrence of carbendazim (MBC) resistance. Here, we identified and characterized two putative genes encoding hexokinase in F. graminearum (FgHXK1 and FgHXK2), which is a rate-limiting enzyme in DON biosynthesis. The expression level of hexokinase genes and the production of pyruvate, which is the precursor of DON, were up-regulated in the MBC-resistant strain, indicating that hexokinase genes might be involved in increased DON production. Phylogenetic and comparative analyses indicated that FgHXK1 was the predominant hexokinase gene. Gene disruption showed that ΔFgHXK1 severely affected DON production, indicating that FgHXK1 played a role in the regulation of DON biosynthesis. Morphological characterization showed that ΔFgHXK1 led to inhibited vegetative growth and conidiation. Sensitivity tests to MBC and various stresses indicated that both ΔFgHXK1 and ΔFgHXK2 mutants showed no significant difference from parental strains. Pathogencity assays showed that ΔFgHXK1 mutants lost virulence on wheat head and corn stigma; however, they showed no change in sexual reproduction. The FgHXK1-overexpressing transformants were obtained subsequently. Their pyruvate and DON production was confirmed to be increased, indicating that FgHXK1 positively regulated DON biosynthesis. Although additional defects appeared in overexpression mutants, MBC sensitivity showed no change. All of the results indicated that the transcriptional level of FgHXK1 regulated DON biosynthesis, but showed no direct relationship with MBC resistance.

The overexpression of the pine transcription factor PpDof5 in Arabidopsis leads to increased lignin content and affects carbon and nitrogen metabolism

PpDof 5 is a regulator of the expression of glutamine synthetase (GS; EC 6.3.1.2) genes in photosynthetic and non-photosynthetic tissues of maritime pine. We have used Arabidopsis thaliana as a model system to study PpDof 5 function in planta, generating transgenic lines overexpressing the pine transcription factor. The overexpression of PpDof 5 resulted in a substantial increase of lignin content with a simultaneous regulation of carbon and nitrogen key genes. In addition, partitioning in carbon and nitrogen compounds was spread via various secondary metabolic pathways. These results suggest pleiotropic effects of PpDof 5 expression on various metabolic pathways of carbon and nitrogen metabolism. Plants overexpressing PpDof 5 exhibited upregulation of genes encoding enzymes for sucrose and starch biosynthesis, with a parallel increase in the content of soluble sugars. When the plants were grown under nitrate as the sole nitrogen source, they exhibited a significant regulation of the expression of genes involved mainly in signaling, but similar growth rates to wild-type plants. However, plants grown under ammonium exhibited major induction of the expression of photosynthetic genes and differential expression of ammonium and nitrate transporters. All these data suggest that in addition to controlling ammonium assimilation, PpDof 5 could be also involved in the regulation of other pathways in carbon and nitrogen metabolism in pine trees.

Sugars, signalling, and plant development

Like all organisms, plants require energy for growth. They achieve this by absorbing light and fixing it into a usable, chemical form via photosynthesis. The resulting carbohydrate (sugar) energy is then utilized as substrates for growth, or stored as reserves. It is therefore not surprising that modulation of carbohydrate metabolism can have profound effects on plant growth, particularly cell division and expansion. However, recent studies on mutants such as stimpy or ramosa3 have also suggested that sugars can act as signalling molecules that control distinct aspects of plant development. This review will focus on these more specific roles of sugars in development, and will concentrate on two major areas: (i) cross-talk between sugar and hormonal signalling; and (ii) potential direct developmental effects of sugars. In the latter, developmental mutant phenotypes that are modulated by sugars as well as a putative role for trehalose-6-phosphate in inflorescence development are discussed. Because plant growth and development are plastic, and are greatly affected by environmental and nutritional conditions, the distinction between purely metabolic and specific developmental effects is somewhat blurred, but the focus will be on clear examples where sugar-related processes or molecules have been linked to known developmental mechanisms.

Subcellular compartmentation of sugar signaling: links among carbon cellular status, route of sucrolysis, sink-source allocation, and metabolic partitioning

Recent findings suggest that both subcellular compartmentation and route of sucrolysis are important for plant development, growth, and yield. Signaling effects are dependent on the tissue, cell type, and stage of development. Downstream effects also depend on the amount and localization of hexoses and disaccharides. All enzymes of sucrose metabolism (e.g., invertase, hexokinase, fructokinase, sucrose synthase, and sucrose 6-phosphate synthase) are not produced from single genes, but from paralog families in plant genomes. Each paralog has unique expression across plant organs and developmental stages. Multiple isoforms can be targeted to different cellular compartments (e.g., plastids, mitochondria, nuclei, and cytosol). Many of the key enzymes are regulated by post-transcriptional modifications and associate in multimeric protein complexes. Some isoforms have regulatory functions, either in addition to or in replacement of their catalytic activity. This explains why some isozymes are not redundant, but also complicates elucidation of their specific involvement in sugar signaling. The subcellular compartmentation of sucrose metabolism forces refinement of some of the paradigms of sugar signaling during physiological processes. For example, the catalytic and signaling functions of diverse paralogs needs to be more carefully analyzed in the context of post-genomic biology. It is important to note that it is the differential localization of both the sugars themselves as well as the sugar-metabolizing enzymes that ultimately led to sugar signaling. We conclude that a combination of subcellular complexity and gene duplication/subfunctionalization gave rise to sugar signaling as a regulatory mechanism in plant cells.

Predicting the open conformations of protein kinases using molecular dynamics simulations

Protein kinases (PK) control phosphorylation in eukaryotic cells, and thereby regulate metabolic pathways, cell cycle progression, apoptosis, and transcription. Consequently, there is significant interest in manipulating PK activity and treat diseases by using small-molecule drugs. All PK catalytic domains undergo large conformational changes as a result of substrate binding and phosphorylation. The "closed" state of a PK catalytic domain is the only state able to phosphorylate the target substrate, which makes the two other observed states (the "open" and the "intermediate" states) interesting drug targets. We investigate whether molecular dynamics (MD) simulations starting from the closed state of the catalytic domain of protein kinase A (C-PKA) can be used to produce realistic structures representing the intermediate and/or open conformation of C-PKA, because this would allow for drug docking calculations and drug design using MD snapshots. We perform 36 ten-nanosecond MD simulations starting from the closed conformation [PDB ID: ATP] of C-PKA in various liganded and phosphorylated states. The results show that MD simulations are capable of reproducing the open conformation of C-PKA with good accuracy within 1 ns of simulation as measured by Cα root mean square deviations (RMSDs) and RMSDs of atoms defining the ATP-binding pocket. Importantly, we are able to show that even without knowledge of the structure of the open form of C-PKA, we can identify the MD snapshots resembling the open conformation most using the open structure of a different PK displaying only 23% sequence identity to C-PKA.

The function of MoGlk1 in integration of glucose and ammonium utilization in Magnaporthe oryzae

Hexokinases are conserved proteins functioning in glucose sensing and signaling. The rice blast fungus Magnaporthe oryzae contains several hexokinases, including MoHxk1 (hexokinase) and MoGlk1 (glucokinase) encoded respectively by MoHXK1 and MoGLK1 genes. The heterologous expression of MoGlk1 and MoHxk1 in Saccharomyces cerevisiae confirmed their conserved functions. Disruption of MoHXK1 resulted in growth reduction in medium containing fructose as the sole carbon source, whereas disruption of MoGLK1 did not cause the similar defect. However, the ΔMoglk1 mutant displayed decreased proton extrusion and a lower biomass in the presence of ammonium, suggesting a decline in the utilization of ammonium. Additionally, the MoGLK1 allele lacking catalytic activity restored growth to the ΔMoglk1 mutant. Moreover, the expression of MoPMA1 encoding a plasma membrane H⁺-ATPase decreased in the ΔMoglk1 mutant that can be suppressed by glucose and G-6-P. Thus, MoGlk1, but not MoHxk1, regulates ammonium utilization through a mechanism that is independent from its catalytic activity.

HXK1 regulates carbon catabolism, sporulation, fumonisin B₁ production and pathogenesis in Fusarium verticillioides

In Fusarium verticillioides, a ubiquitous pathogen of maize, virulence and mycotoxigenesis are regulated in response to the types and amounts of carbohydrates present in maize kernels. In this study, we investigated the role of a putative hexokinase-encoding gene (HXK1) in growth, development and pathogenesis. A deletion mutant (Δhxk1) of HXK1 was not able to grow when supplied with fructose as the sole carbon source, and growth was impaired when glucose, sucrose or maltotriose was provided. Additionally, the Δhxk1 mutant produced unusual swollen hyphae when provided with fructose, but not glucose, as the sole carbon source. Moreover, the Δhxk1 mutant was impaired in fructose uptake, although glucose uptake was unaffected. On maize kernels, the Δhxk1 mutant was substantially less virulent than the wild-type, but virulence on maize stalks was not impaired, possibly indicating a metabolic response to tissue-specific differences in plant carbohydrate content. Finally, disruption of HXK1 had a pronounced effect on fungal metabolites produced during colonization of maize kernels; the Δhxk1 mutant produced approximately 50 % less trehalose and 80 % less fumonisin B₁ (FB₁) than the wild-type. The reduction in trehalose biosynthesis likely explains observations of increased sensitivity to osmotic stress in the Δhxk1 mutant. In summary, this study links early events in carbohydrate sensing and glycolysis to virulence and secondary metabolism in F. verticillioides, and thus provides a new foothold from which the genetic regulatory networks that underlie pathogenesis and mycotoxigenesis can be unravelled and defined.

Cloning, biochemical characterization and expression of a sunflower (Helianthus annuus L.) hexokinase associated with seed storage compounds accumulation

A full-length hexokinase cDNA, HaHXK1, was cloned and characterized from Helianthus annuus L. developing seeds. Based on its sequence and phylogenetic relationships, HaHXK1 is a membrane-associated (type-B) hexokinase. The predicted structural model resembles known hexokinase structures, folding into two domains of unequal size: a large and a small one separated by a deep cleft containing the residues involved in the enzyme active site. A truncated version, without the 24 N-terminal residues, was heterologously expressed in Escherichia coli, purified to electrophoretic homogeneity using immobilized metal ion affinity chromatography and biochemically characterized. The purified enzyme behaved as a monomer on size exclusion chromatography and had a specific activity of 19.3 μmol/min/mg protein, the highest specific activity ever reported for a plant hexokinase. The enzyme had higher affinity for glucose and mannose relative to fructose, but the enzymatic efficiency was higher with glucose. Recombinant HaHXK1 was inhibited by ADP and was insensitive either to glucose-6-phosphate or to trehalose-6-phosphate. Its expression profile showed higher levels in heterotrophic tissues, developing seeds and roots, than in photosynthetic ones. A time course of HXK activity and expression in seeds showed that the highest HXK levels are found at the early stages of reserve compounds, lipids and proteins accumulation.

Role of the rice hexokinases OsHXK5 and OsHXK6 as glucose sensors

The Arabidopsis (Arabidopsis thaliana) hexokinase 1 (AtHXK1) is recognized as an important glucose (Glc) sensor. However, the function of hexokinases as Glc sensors has not been clearly demonstrated in other plant species, including rice (Oryza sativa). To investigate the functions of rice hexokinase isoforms, we characterized OsHXK5 and OsHXK6, which are evolutionarily related to AtHXK1. Transient expression analyses using GFP fusion constructs revealed that OsHXK5 and OsHXK6 are associated with mitochondria. Interestingly, the OsHXK5DeltamTP-GFP and OsHXK6DeltamTP-GFP fusion proteins, which lack N-terminal mitochondrial targeting peptides, were present mainly in the nucleus with a small amount of the proteins seen in the cytosol. In addition, the OsHXK5NLS-GFP and OsHXK6NLS-GFP fusion proteins harboring nuclear localization signals were targeted predominantly in the nucleus, suggesting that these OsHXKs retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays using promoterluciferase fusion constructs, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the Glc-dependent repression of the maize (Zea mays) Rubisco small subunit (RbcS) and rice alpha-amylase genes in mesophyll protoplasts of maize and rice. Notably, the expression of OsHXK5, OsHXK6, or their mutant alleles complemented the Arabidopsis glucose insensitive2-1 mutant, thereby resulting in wild-type characteristics in seedling development, Glc-dependent gene expression, and plant growth. Furthermore, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the photosynthetic gene RbcS in response to Glc treatment. These results provide evidence that rice OsHXK5 and OsHXK6 can function as Glc sensors.

Changes in photosynthetic rates and gene expression of leaves during a source-sink perturbation in sugarcane

BACKGROUND AND AIMS

In crops other than sugarcane there is good evidence that the size and activity of carbon sinks influence source activity via sugar-related regulation of the enzymes of photosynthesis, an effect that is partly mediated through coarse regulation of gene expression.

METHODS

In the current study, leaf shading treatments were used to perturb the source-sink balance in 12-month-old Saccharum spp. hybrid 'N19' (N19) by restricting source activity to a single mature leaf. Changes in leaf photosynthetic gas exchange variables and leaf and culm sugar concentrations were subsequently measured over a 14 d period. In addition, the changes in leaf gene response to the source-sink perturbation were measured by reverse northern hybridization analysis of an array of 128 expressed sequence tags (ESTs) related to photosynthetic and carbohydrate metabolism.

KEY RESULTS

Sucrose concentrations in immature culm tissue declined significantly over the duration of the shading treatment, while a 57 and 88% increase in the assimilation rate (A) and electron transport rate (ETR), respectively, was observed in the source leaf. Several genes (27) in the leaf displayed a >2-fold change in expression level, including the upregulation of several genes associated with C(4) photosynthesis, mitochondrial metabolism and sugar transport. Changes in gene expression levels of several genes, including Rubisco (EC 4.1.1.39) and hexokinase (HXK; EC 2.7.1.1), correlated with changes in photosynthesis and tissue sugar concentrations that occurred subsequent to the source-sink perturbation.

CONCLUSIONS

These results are consistent with the notion that sink demand may limit source activity through a kinase-mediated sugar signalling mechanism that correlates to a decrease in source hexose concentrations, which, in turn, correlate with increased expression of genes involved in photosynthesis and metabolite transport. The signal feedback system reporting sink sufficiency and regulating source activity may be a potentially valuable target for future genetic manipulation to increase sugarcane sucrose yield.

Interorganellar communication

Signals originating from chloroplasts and mitochondria modulate nuclear gene expression (retrograde signalling). Relevant signals are derived from the pool of reactive oxygen species or generated by changes in redox state, flux through the tetrapyrrole biosynthetic pathway, or rates of organelle protein synthesis. In addition, multiple interactions of these four pathways with sugar and hormone signalling occur. Although the nature of the molecules that relay information through the cytosol to the nucleus is still unknown, the first putative signalling components in the chloroplast have recently been identified, and give tentative hints of overlaps between the different pathways. Retrograde signalling-dependent modulation of nuclear gene expression seems to involve multilayered transcriptional control and the transcription factor ABI4.

Regulatory functions of nuclear hexokinase1 complex in glucose signaling

Arabidopsis hexokinase1 (HXK1) is a glucose sensor that integrates nutrient and hormone signals to govern gene expression and plant growth in response to environmental cues. How the metabolic enzyme mediates glucose signaling remains a mystery. By coupling proteomic and binary-interaction screens, we discover two nuclear-specific HXK1 unconventional partners: the vacuolar H(+)-ATPase B1 (VHA-B1) and the 19S regulatory particle of proteasome subunit (RPT5B). Remarkably, vha-B1 and rpt5b mutants uniquely share a broad spectrum of glucose response defects with the HXK1 mutant gin2 (glucose-insensitive2). Genetic and chromatin immunoprecipitation analyses suggest that the nuclear HXK1 forms a glucose signaling complex core with VHA-B1 and RPT5B that directly modulates specific target gene transcription independent of glucose metabolism. The findings support a model in which conserved metabolic enzymes and proteins of well-established activities may perform previously unrecognized nuclear functions.

Sugar signalling and gene expression in relation to carbohydrate metabolism under abiotic stresses in plants

Sucrose is required for plant growth and development. The sugar status of plant cells is sensed by sensor proteins. The signal generated by signal transduction cascades, which could involve mitogen-activated protein kinases, protein phosphatases, Ca 2+ and calmodulins, results in appropriate gene expression. A variety of genes are either induced or repressed depending upon the status of soluble sugars. Abiotic stresses to plants result in major alterations in sugar status and hence affect the expression of various genes by down- and up-regulating their expression. Hexokinase-dependent and hexokinase-independent pathways are involved in sugar sensing. Sucrose also acts as a signal molecule as it affects the activity of a proton-sucrose symporter. The sucrose trans-porter acts as a sucrose sensor and is involved in phloem loading. Fructokinase may represent an additional sensor that bypasses hexokinase phosphorylation especially when sucrose synthase is dominant. Mutants isolated on the basis of response of germination and seedling growth to sugars and reporter-based screening protocols are being used to study the response of altered sugar status on gene expression. Common cis-acting elements in sugar signalling pathways have been identified. Transgenic plants with elevated levels of sugars/sugar alcohols like fructans, raffinose series oligosaccharides, trehalose and mannitol are tolerant to different stresses but have usually impaired growth. Efforts need to be made to have transgenic plants in which abiotic stress responsive genes are expressed only at the time of adverse environmental conditions instead of being constitutively synthesized.

Structure, expression, and functional analysis of the hexokinase gene family in rice (Oryza sativa L.)

Hexokinase (HXK) is a dual-function enzyme that both phosphorylates hexose to form hexose 6-phosphate and plays an important role in sugar sensing and signaling. To investigate the roles of hexokinases in rice growth and development, we analyzed rice sequence databases and isolated ten rice hexokinase cDNAs, OsHXK1 (Oryza sativa Hexokinase 1) through OsHXK10. With the exception of the single-exon gene OsHXK1, the OsHXKs all have a highly conserved genomic structure consisting of nine exons and eight introns. Gene expression profiling revealed that OsHXK2 through OsHXK9 are expressed ubiquitously in various organs, whereas OsHXK10 expression is pollen-specific. Sugars induced the expression of three OsHXKs, OsHXK2, OsHXK5, and OsHXK6, in excised leaves, while suppressing OsHXK7 expression in excised leaves and immature seeds. The hexokinase activity of the OsHXKs was confirmed by functional complementation of the hexokinase-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). OsHXK4 was able to complement this mutant only after the chloroplast-transit peptide was removed. The subcellular localization of OsHXK4 and OsHXK7, observed with green fluorescent protein (GFP) fusion constructs, indicated that OsHXK4 is a plastid-stroma-targeted hexokinase while OsHXK7 localizes to the cytosol.

Glucose inhibits the expression of triose phosphate/phosphate translocator gene in wheat via hexokinase-dependent mechanism

Although triose phosphate/phosphate translocator is known to play an important role in regulating the distribution of assimilates in wheat chloroplasts, the mechanism of triose phosphate/phosphate translocator gene control has not yet been clearly elucidated. We first showed that glucose inhibited the expression of triose phosphate/phosphate translocator gene in wheat by reverse transcription-polymerase chain reaction and Western blotting. The triose phosphate/phosphate translocator expression was seriously impaired by 5 mmol/L glucose, and it responded slowly, more than 48 h, to level as low as 1 mmol/L glucose. Both glucose and 2-deoxyglucose inhibited the expression of triose phosphate/phosphate translocator gene, but 2-deoxyglucose-6-P, product of phosphorylated 2-deoxyglucose, cannot be further metabolized, therefore the further metabolism of phosphorylated glucose by hexokinase is not a prerequisite for triggering glucose-regulated expression of triose phosphate/phosphate translocator gene. Glucose had little inhibitory effect on the expression of triose phosphate/phosphate translocator gene when hexokinase activity was reduced or eliminated by transforming wheat protoplasts with a hexokinase antisense construct or treating protoplasts with glucosamine, an inhibitor of hexokinase. Therefore, it appears essential for hexokinase to retain phosphorylation activity for glucose to regulate the expression of triose phosphate/phosphate translocator gene. The treatment of protoplasts with glucose-6-phosphate resulting in no inhibition of triose phosphate/phosphate translocator expression demonstrated that phosphorylation via hexokinase is necessary for glucose inhibiting triose phosphate/phosphate translocator expression. All the data suggest that triose phosphate/phosphate translocator is regulated by glucose via a hexokinase-dependent pathway.

Cloning, expression, purification, and properties of a putative plasma membrane hexokinase from Solanum chacoense

A full-length hexokinase cDNA was cloned from Solanum chacoense, a wild relative of the cultivated potato. Analysis of the predicted primary sequence suggested that the protein product, ScHK2, may be targeted to the secretory pathway and inserted in the plant plasma membrane, facing the cytosol. ScHK2 was expressed as a hexahistidine-tagged protein in Escherichia coli. Expression conditions for this construct were optimized using a specific anti-hexokinase polyclonal anti-serum raised against a truncated version of ScHK2. The full-length recombinant protein was purified to electrophoretic homogeneity using immobilized metal ion affinity chromatography followed by anion exchange chromatography on Fractogel EMD DEAE-650 (S). The purified enzyme had a specific activity of 5.3 micromol/min/mg protein. Its apparent Kms for glucose (23 microM), mannose (30 microM), fructose (5.2 mM), and ATP (61 microM) were in good agreement with values found in the literature for other plant hexokinases. Hexahistidine-tagged ScHK2 was highly sensitive to pH variations between 7.7 and 8.7. It was inhibited by ADP and insensitive to glucose-6-phosphate. These findings constitute the first kinetic characterization of a homogeneous plant hexokinase preparation. The relevance of ScHK2 kinetic properties is discussed in relation to the regulation of hexose metabolism in plants.

Light, the circadian clock, and sugar perception in the control of lignin biosynthesis

Experiments were undertaken to investigate some of the mechanisms that may function to regulate lignin biosynthesis (lignification) in Arabidopsis thaliana. Northern blot analyses revealed that several genes encoding enzymes involved in the synthesis of lignin monomers displayed significant changes in transcript abundance over a diurnal cycle. Northern blot analysis also suggested that some of the changes in diurnal transcript abundance were likely to be attributable to circadian regulation, whereas others were likely to be attributable to light perception. Comparison of circadian changes in transcript abundance of lignin biosynthetic genes between wild-type plants and the sex1 mutant, which is impaired in starch turnover, suggested that carbon availability related to starch turnover might determine the capacity to synthesize lignins. This hypothesis was supported by the observation that the sex1 mutant accumulated fewer lignins than wild-type plants. Consistent with the relationship between carbon availability and lignin accumulation, analysis of dark-grown wild-type A. thaliana seedlings uncovered a role for sugars in the regulation of lignin biosynthesis. Analysis of lignin accumulation, as determined by qualitative changes in phloroglucinol staining, suggested that metabolizable sugars positively influence the abundance of lignins. Transcriptome analysis supports the hypothesis that sugars are not merely a source of carbon skeletons for lignification, but they also function as a signal to enhance the capacity to synthesize lignins.

Alteration of biomass and artemisinin production in Artemisia annua hairy roots by media sterilization method and sugars

Transformed root cultures of Artemisia annua grown in autoclaved medium show large variations in biomass and artemisinin production regardless of the culture conditions or clonal type. However, using filter-sterilized sugars singly or in combination while holding the carbon level in the medium constant resulted in an unexpected variability in biomass production and artemisinin yield. Autoclaving results in variable hydrolysis of sucrose in the culture medium. Subsequent experiments using combinations of filter-sterilized sugars at a constant total carbon level in the medium showed a stimulation of artemisinin production by glucose. Growth in sucrose was equivalent to growth in fructose and significantly better than in glucose. These results suggest that sugars may be affecting terpenoid metabolism not only as carbon sources, but also as signal molecules.