Putting plant hexokinases in their proper place

The oxidative pentose phosphate pathway in photosynthesis: a tale of two shunts

CO2 release in the light (RL) and its presumed source, oxidative pentose phosphate pathways, were found to be insensitive to CO2 concentration. The oxidative pentose phosphate pathways form glucose 6-phosphate (G6P) shunts that bypass the nonoxidative pentose phosphate reactions of the Calvin-Benson cycle. Using adenosine diphosphate glucose and uridine diphosphate glucose as proxies for labeling of G6P in the stroma and cytosol respectively, it was found that only the cytosolic shunt was active. Uridine diphosphate glucose, a proxy for cytosolic G6P, and 6-phosphogluconate (6PG) were significantly less labeled than Calvin-Benson cycle intermediates in the light. But ADP glucose, a proxy for stromal G6P, is labeled to the same degree as Calvin-Benson cycle intermediates and much greater than 6PG. A metabolically inert pool of sedoheptulose bisphosphate can slowly equilibrate keeping the label in sedoheptulose lower than in other stromal metabolites. Finally, phosphorylation of fructose 6-phosphate (F6P) in the cytosol can allow some unlabeled carbon in cytosolic F6P to dilute label in phosphenolpyruvate. The results clearly show that there is oxidative pentose phosphate pathway activity in the cytosol that provides a shunt around the nonoxidative pentose phosphate pathway reactions of the Calvin-Benson cycle and is not strongly CO2-sensitive.

Function of a non-enzymatic hexokinase LcHXK1 as glucose sensor in regulating litchi fruit abscission

Fruit abscission is a severe hindrance to commercial crop production, and a lack of carbohydrates causes fruit abscission to intensify in a variety of plant species. However, the precise mechanism by which carbohydrates affect fruit setting potential has yet to be determined. In the current study, we noticed negative correlation between hexose level and fruit setting by comparing different cultivars, bearing shoots of varying diameters, and girdling and defoliation treatments. The cumulative fruit-dropping rate was significantly reduced in response to exogenous glucose dipping. These results suggested that hexose, especially glucose, is the key player in lowering litchi fruit abscission. Moreover, five putative litchi hexokinase genes (LcHXKs) were isolated and the subcellular localization as well as activity of their expressed proteins in catalyzing hexose phosphorylation were investigated. LcHXK2 was only found in mitochondria and expressed catalytic protein, whereas the other four HXKs were found in both mitochondria and nuclei and had no activity in catalyzing hexose phosphorylation. LcHXK1 and LcHXK4 were found in the same cluster as previously reported hexose sensors AtHXK1 and MdHXK1. Furthermore, VIGS-mediated silencing assay confirms that LcHXK1 suppression increases fruit abscission. These findings revealed that LcHXK1 functions as hexose sensor, negatively regulating litchi fruit abscission.

Analysis of Physiological Indicators Associated with Drought Tolerance in Wheat under Drought and Re-Watering Conditions

Wheat (Triticum aestivum L.) production is severely threatened by an increase in the frequency of drought events. It is crucial to determine stable and effective morphological, physiological, and associated oxidative stress indicators, to evaluate the drought tolerance of wheat for breeding and cultivation. Therefore, the cultivars Luohan 22 (LH 22, drought-tolerant) and Zhengmai 366 (ZM 366, drought-sensitive) were used as experimental materials to analyze the changes in 12 physiological and biochemical indicators, as well as the yield, when the stress was prolonged to different times. Re-watering after 6 days of drought can effectively alleviate the associated oxidative stress of drought to wheat. The physiological responses of plants were reversible when they were re-watered in the range of 6 to 12 days after drought. The degree of recovery of LH 22 was higher than that of ZM 366. Afterwards, seven indicators, including stomatal conductance, proline, malondialdehyde, soluble sugar, hexokinase, glucose, and the non-photochemical quenching parameter, were screened out to characterize tolerance of wheat to drought using the multivariate statistical analytical method. This study further investigated the method of evaluating and indexing tolerance of wheat to drought, from the physiological and biochemical levels. This study can provide a theoretical basis and reference for the selection of wheat cultivars to breed and cultivate against drought stress.

Tomato growth promotion by the fungal endophytes Serendipita indica and Serendipita herbamans is associated with sucrose de-novo synthesis in roots and differential local and systemic effects on carbohydrate metabolisms and gene expression

Plant growth-promoting and stress resilience-inducing root endophytic fungi represent an additional carbohydrate sink. This study aims to test if such root endophytes affect the sugar metabolism of the host plant to divert the flow of resources for their purposes. Fresh and dry weights of roots and shoots of tomato (Solanum lycopersicum) colonised by the closely related Serendipita indica and Serendipita herbamans were recorded. Plant carbohydrate metabolism was analysed by measuring sugar levels, by determining activity signatures of key enzymes of carbohydrate metabolism, and by quantifying mRNA levels of genes involved in sugar transport and turnover. During the interaction with the tomato plants, both fungi promoted root growth and shifted shoot biomass from stem to leaf tissues, resulting in increased leaf size. A common effect induced by both fungi was the inhibition of phosphofructokinase (PFK) in roots and leaves. This glycolytic-pacing enzyme shows how the glycolysis rate is reduced in plants and, eventually, how sugars are allocated to different tissues. Sucrose phosphate synthase (SPS) activity was strongly induced in colonised roots. This was accompanied by increased SPS-A1 gene expression in S. herbamans-colonised roots and by increased sucrose amounts in roots colonised by S. indica. Other enzyme activities were barely affected by S. indica, but mainly induced in leaves of S. herbamans-colonised plants and decreased in roots. This study suggests that two closely related root endophytic fungi differentially influence plant carbohydrate metabolism locally and systemically, but both induce a similar increase in plant biomass. Notably, both fungal endophytes induce an increase in SPS activity and, in the case of S. indica, sucrose resynthesis in roots. In leaves of S. indica-colonised plants, SWEET11b expression was enhanced, thus we assume that excess sucrose was exported by this transporter to the roots. ‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬.

Barley Genotypes Vary in Stomatal Responsiveness to Light and CO2 Conditions

Changes in stomatal conductance and density allow plants to acclimate to changing environmental conditions. In the present paper, the influence of atmospheric CO₂ concentration and light intensity on stomata were investigated for two barley genotypes-Barke and Bojos, differing in their sensitivity to oxidative stress and phenolic acid profiles. A novel approach for stomatal density analysis was used-a pair of convolution neural networks were developed to automatically identify and count stomata on epidermal micrographs. Stomatal density in barley was influenced by genotype, as well as by light and CO₂ conditions. Low CO₂ conditions resulted in increased stomatal density, although differences between ambient and elevated CO₂ were not significant. High light intensity increased stomatal density compared to low light intensity in both barley varieties and all CO₂ treatments. Changes in stomatal conductance were also measured alongside the accumulation of pentoses, hexoses, disaccharides, and abscisic acid detected by liquid chromatography coupled with mass spectrometry. High light increased the accumulation of all sugars and reduced abscisic acid levels. Abscisic acid was influenced by all factors-light, CO₂, and genotype-in combination. Differences were discovered between the two barley varieties: oxidative stress sensitive Barke demonstrated higher stomatal density, but lower conductance and better water use efficiency (WUE) than oxidative stress resistant Bojos at saturating light intensity. Barke also showed greater variability between treatments in measurements of stomatal density, sugar accumulation, and abscisic levels, implying that it may be more responsive to environmental drivers influencing water relations in the plant.

Putative regulatory role of hexokinase and fructokinase in terpenoid aroma biosynthesis in Lilium 'Siberia'

Lily is one of the most economically important flowers worldwide due to its elegant appearance and appealing scent, which is mainly composed of monoterpene ocimene, linalool and benzenoids. Sugars are the primary products of plants, with fructose and hexose sugars being the substrate material for most organic compounds and metabolic pathways in plants. Herein, we isolated and functionally characterized hexokinase (LoHXK) and fructokinase (LoFRK) from Lilium 'Siberia' flower, which indicated their potential roles in floral aroma production. Real-time PCR analysis showed that LoHXK and LoFRK were highly expressed in the flower filament. Overexpression and virus-induced gene silencing (VIGS) assays revealed that LoHXK and LoFRK significantly modified the emission of β-ocimene and linalool contents via regulation of expression of key structural volatile synthesis genes (LoTPS1 and LoTPS3). Under exogenous glucose and fructose application, the volatile contents of β-ocimene and linalool were increased and the expression levels of key structural genes were upregulated. The emission of β-ocimene and linalool followed a diurnal circadian rhythm. Determination of carbon fluxes via ¹³C-labeled glucose and ¹³C-labeled fructose experiments showed that the mass spectra of ocimene and linalool significantly increased, however, the m/z ratio of ethyl benzoate did not change. Furthermore, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that LoFRK interacted with LoMYB1 and LoMYB2 proteins. Together, these results suggest that hexokinase and fructokinase may play significant roles in the regulation of ocimene and linalool biosynthesis in Lilium 'Siberia'.

Guard cells control hypocotyl elongation through HXK1, HY5, and PIF4

The hypocotyls of germinating seedlings elongate in a search for light to enable autotrophic sugar production. Upon exposure to light, photoreceptors that are activated by blue and red light halt elongation by preventing the degradation of the hypocotyl-elongation inhibitor HY5 and by inhibiting the activity of the elongation-promoting transcription factors PIFs. The question of how sugar affects hypocotyl elongation and which cell types stimulate and stop that elongation remains unresolved. We found that overexpression of a sugar sensor, Arabidopsis hexokinase 1 (HXK1), in guard cells promotes hypocotyl elongation under white and blue light through PIF4. Furthermore, expression of PIF4 in guard cells is sufficient to promote hypocotyl elongation in the light, while expression of HY5 in guard cells is sufficient to inhibit the elongation of the hy5 mutant and the elongation stimulated by HXK1. HY5 exits the guard cells and inhibits hypocotyl elongation, but is degraded in the dark. We also show that the inhibition of hypocotyl elongation by guard cells' HY5 involves auto-activation of HY5 expression in other tissues. It appears that guard cells are capable of coordinating hypocotyl elongation and that sugar and HXK1 have the opposite effect of light on hypocotyl elongation, converging at PIF4.

PdGNC confers drought tolerance by mediating stomatal closure resulting from NO and H2 O2 production via the direct regulation of PdHXK1 expression in Populus

Drought is one of the primary abiotic stresses, seriously implicating plant growth and productivity. Stomata play a crucial role in regulating drought tolerance. However, the molecular mechanism on stomatal movement-mediated drought tolerance remains unclear. Using genetic, molecular and biochemical techniques, we identified that the PdGNC directly activating the promoter of PdHXK1 by binding the GATC element, a hexokinase (HXK) synthesis key gene. Here, PdGNC, a member of the GATA transcription factor family, was greatly induced by abscisic acid and dehydration. Overexpressing PdGNC in poplar (Populus clone 717) resulted in reduced stomatal aperture with greater water-use efficiency and increased water deficit tolerance. By contrast, CRISPR/Cas9-mediated poplar mutant gnc exhibited increased stomatal aperture and water loss with reducing drought resistance. PdGNC activates PdHXK1 (a hexokinase synthesis key gene), resulting in a remarkable increase in hexokinase activity in poplars subjected to water deficit. Furthermore, hexokinase promoted nitric oxide (NO) and hydrogen peroxide (H₂ O₂ ) production in guard cells, which ultimately reduced stomatal aperture and increased drought resistance. Together, PdGNC confers drought stress tolerance by reducing stomatal aperture caused by NO and H₂ O₂ production via the direct regulation of PdHXK1 expression in poplars.

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.

Suppression of a hexokinase gene, SlHXK1, leads to accelerated leaf senescence and stunted plant growth in tomato

Sugars are the key regulatory molecules that impact diverse biological processes in plants. Hexokinase, the key rate-limiting enzyme in hexose metabolism, takes part in the first step of glycolytic pathway. Acting as a sensor that mediates sugar regulation, hexokinase has been proved to play significant roles in regulating plant growth and development. Here, we isolated a hexokinase gene SlHXK1 from tomato. Its transcript levels were higher in flowers and leaves than in other organs and decreased during leaf and petiole development. SlHXK1-RNAi lines displayed advanced leaf senescence and stunted plant growth. Physiological features including plant height, leaf length, thickness and size, the contents of chlorophyll, starch and MDA, and hexokinase activity were dramatically altered in SlHXK1-RNAi plants. Dark-induced leaf senescence were advanced and the transcripts of senescence-related genes after darkness treatment were markedly increased in SlHXK1-RNAi plants. RNA-seq and qRT-PCR analyses showed that the transcripts of genes related to plant hormones, photosynthesis, chloroplast development, chlorophyll synthesis and metabolism, cellular process, starch and sucrose metabolism, and senescence were significantly altered in SlHXK1-RNAi plants. Taken together, our data demonstrate that SlHXK1 is a significant gene involved in leaf senescence and plant growth and development in tomato through affecting starch turnover.

Genome-Wide Identification and Characterization of Hexokinase Genes in Moso Bamboo (Phyllostachys edulis)

Plant hexokinases (HXKs) are a class of multifunctional proteins that not only act as the enzymes required for hexose phosphorylation but also serve as sugar sensors that repress the expression of some photosynthetic genes when internal glucose level increases and regulators of cell metabolism and some sugar-related signaling pathways independent on their catalytic actives. The HXKs have been studied in many plants; however, limited information is available on HXKs of moso bamboo (Phyllostachys edulis). In this study, we identified and characterized 12 hexokinase genes in moso bamboo. Phylogenetic analysis revealed that the moso bamboo hexokinases (PeHXKs) were classifiable into five subfamilies which represented the three types of hexokinases in plants. Gene structure and conserved motif analysis showed that the PeHXK genes contained diverse numbers of introns and exons and that the encoded proteins showed similar motif organization within each subfamily. Multiple sequence alignment revealed that the PeHXK proteins contained conserved domains, such as phosphate 1 (P1), phosphate 2 (P2), adenosine, and a sugar-binding domain. Evolutionary divergence analysis indicated that the PeHXK, OsHXK, and BdHXK families underwent negative selection and experienced a large-scale duplication event approximately 19-319 million years ago. Expression analysis of the PeHXK genes in the leaf, stem, root, and rhizome of moso bamboo seedlings indicated that the PeHXKs perform pivotal functions in the development of moso bamboo. A protein subcellular localization assay showed that PeHXK5a, PeHXK8, and PeHXK3b were predominantly localized in mitochondria, and PeHXK8 protein was also detected in the nucleus. The HXK activity of the PeHXK5a, PeHXK8, and PeHXK3b was verified by a functional complementation assay using the HXK-deficient triple-mutant yeast strain YSH7.4-3C (hxk1, hxk2, and glk1), and the results showed that the three PeHXKs had the plant HXK-specific enzyme traits. The present findings would provide a foundation for further functional analysis of the PeHXK gene family.

Expression of Hexokinase in Stomata of Citrus Fruit Reduces Fruit Transpiration and Affects Seed Development

The temporal formation and spatial distribution of stomata on the surface of citrus floral organs and, specifically, on the ovule from which the fruit develops, were analyzed using citrus plants that express green fluorescent protein (GFP) under the guard cell-specific KST1 promoter. Stomata are found on the style, sepal, and anther of the closed flower and on ovules from the stage of anthesis. It has previously been shown that hexokinase (HXK) mediates sugar-sensing in leaf guard cells and stimulates stomatal closure. The activity and response of citrus fruit stomata to sugar-sensing by HXK was examined using plants that express HXK under the KST1 promoter. Those plants are referred to as GCHXK plants. The transpiration of young green GCHXK citrus fruits was significantly reduced, indicating that their stomata respond to sugar similar to leaf stomata. Toward fruit maturation, fruit stomata are plugged and stop functioning, which explains why WT and GCHXK mature yellow fruits exhibited similar water loss. Seeds of the GCHXK plants were smaller and germinated more slowly than the WT seeds. We suggest that the stomata of young green citrus fruits, but not mature yellow fruits, respond to sugar levels via HXK and that fruit stomata are important for proper seed development.

Expression of Arabidopsis Hexokinase in Tobacco Guard Cells Increases Water-Use Efficiency and Confers Tolerance to Drought and Salt Stress

Abiotic stresses such as drought and saline water impose major limitations on plant growth. Modulation of stomatal behavior may help plants cope with such stresses by reducing both water loss and salt uptake. Hexokinase (HXK) is a sugar-phosphorylating enzyme involved in guard cells' sugar-sensing, mediating stomatal closure and coordinating photosynthesis with transpiration. We generated transgenic tobacco lines expressing the Arabidopsis hexokinase1 (AtHXK1) under the guard cell-specific promoter KST1 and examined those plants using growth room and greenhouse experiments. The expression of AtHXK1 in tobacco guard cells reduced stomatal conductance and transpiration by about 25% with no negative effects on photosynthesis or growth, leading to increased water-use efficiency. In addition, these plants exhibited tolerance to drought and salt stress due to their lower transpiration rate, indicating that improved stomatal function has the potential to improve plant performance under stress conditions.

The role of HEXOKINASE1 in Arabidopsis leaf growth

Here, we used a hxk1 mutant in the Col-0 background. We demonstrated that HXK1 regulates cell proliferation and expansion early during leaf development, and that HXK1 is involved in sucrose-induced leaf growth stimulation independent of GPT2. Furthermore, we identified KINγ as a novel HXK1-interacting protein. In the last decade, extensive efforts have been made to unravel the underlying mechanisms of plant growth control through sugar availability. Signaling by the conserved glucose sensor HEXOKINASE1 (HXK1) has been shown to exert both growth-promoting and growth-inhibitory effects depending on the sugar levels, the environmental conditions and the plant species. Here, we used a hxk1 mutant in the Col-0 background to investigate the role of HXK1 during leaf growth in more detail and show that it is affected in both cell proliferation and cell expansion early during leaf development. Furthermore, the hxk1 mutant is less sensitive to sucrose-induced cell proliferation with no significant increase in final leaf growth after transfer to sucrose. Early during leaf development, transfer to sucrose stimulates expression of GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSPORTER2 (GPT2) and represses chloroplast differentiation. However, in the hxk1 mutant GPT2 expression was still upregulated by transfer to sucrose although chloroplast differentiation was not affected, suggesting that GPT2 is not involved in HXK1-dependent regulation of leaf growth. Finally, using tandem affinity purification of protein complexes from cell cultures, we identified KINγ, a protein containing four cystathionine β-synthase domains, as an interacting protein of HXK1.

Proteomic analysis of melatonin-mediated osmotic tolerance by improving energy metabolism and autophagy in wheat (Triticum aestivum L.)

Melatonin-mediated osmotic tolerance was attributed to increased antioxidant capacity, energy metabolism, osmoregulation and autophagy in wheat (Triticum aestivum L.). Melatonin is known to play multiple roles in plant abiotic stress tolerance. However, its role in wheat has been rarely investigated. In this study, 25% polyethylene glycol 6000 (PEG 6000) was used to simulate osmotic stress, and wheat seeds and seedlings were treated with different concentrations of melatonin under PEG stress. Isobaric tag for relative and absolute quantification (iTRAQ)-based proteomic techniques were used to identify the differentially accumulated proteins from melatonin-treated and non-treated seedlings. Seeding priming with melatonin significantly increased the germination rate, coleoptile length, and primary root number of wheat under PEG stress, as well as the fresh weight, dry weight, and water content of wheat seedlings. Under PEG stress, melatonin significantly improved reactive oxygen species homeostasis, as revealed by lower H₂O₂ and O ₂^· content; and the expression of antioxidant enzymes at the transcription and translation levels was increased. Melatonin maintained seedling growth by improving photosynthetic rates and instantaneous and intrinsic water use efficiencies, as well as carbon fixation and starch synthesis at the protein level. Melatonin treatment significantly affected the expression of glycolytic proteins, including fructose-1,6-bisphosphate aldolase, hexokinase, glyceraldehyde-3-phosphate dehydrogenase, and enolase, and remarkably increased the expression of the nicotinamide adenine dinucleotide transporter and nicotinamide adenine dinucleotide binding protein, thereby indirectly modulating electron transport in the respiratory chain. This indicated that melatonin improved energy production in PEG-stressed seedlings. Further, melatonin played a regulatory role in autophagy, protease expression, and ubiquitin-mediated protein degradation by significantly upregulating rab-related protein, fused signal recognition particle receptor, aspartyl protease, serine protease, ubiquitin-fold modifier 1, and ubiquitin at the mRNA or protein level. These findings suggested that melatonin might activate a metabolic cascade related to autophagy under PEG stress in wheat seedlings.

Differential protein expression reveals salt tolerance mechanisms of Desmostachya bipinnata at moderate and high levels of salinity

A proteomics approach was used to investigate salt tolerance mechanisms of Desmostachya bipinnata (L.) Stapf. Plants were subjected to 0mM (control), 100mM (moderate) and 400mM (high) NaCl. Proteins were separated by two-dimensional gel electrophoresis and identified with available databases. Optimal plant fresh weight was found at moderate salinity but declined at high salinity. Water potential, osmotic potential, Na+/K+ ratio, leaf electrolyte leakage, sugars and proline were altered at high salinity. However, water potential, proline content and electrolyte leakage were maintained at moderate salinity; Na+ and K+ concentrations increased, whereas sugars and osmotic potential decreased. Comparative proteome analysis revealed 103 salt responsive proteins. At moderate salinity, most of the proteins involved in energy metabolism, transport, antioxidative defence and cell growth were either unchanged or increased. Proteins related to amino-acid metabolism were decreased while those associated with secondary metabolism were accumulated. At high salinity, amino-acid metabolism and dehydration responses were evident; proteins of energy metabolism, transport and stress defence were downregulated. These results suggest that an efficient defence system, improved transport of water and metabolites, increased cell wall lignification and regulation of energy and carbohydrate metabolism allowed better potential for plant growth under moderately saline conditions.

The regulator of G-protein signalling protein mediates D-glucose-induced stomatal closure via triggering hydrogen peroxide and nitric oxide production in Arabidopsis

2-Deoxy-D-glucose, 3-O-methyl-D-glucose and D-mannose are all non-metabolisable D-glucose analogues. Among these, 2-deoxy-D-glucose and D-mannose are substrates for hexokinase (HXK). D-sorbitol and D-mannitol are reduced forms of D-glucose and are typically used as comparable osmotic solutes. Similar to 2-deoxy-D-glucose and D-mannose, D-glucose induced stomatal closure in Arabidopsis, whereas 3-O-methyl-D-glucose, D-sorbitol and D-mannitol did not. The data show that the effect of D-glucose on stomata is metabolism-independent, HXK-dependent and irrelevant to osmotic stress. Additionally, the D-glucose induced closure of stomata in wild-type Arabidopsis, but did not in rgs1-1 and rgs1-2 or gpa1-3 and gpa1-4 mutants, indicating that the regulator of G-protein signalling protein (RGS1) and heterotrimeric guanine nucleotide-binding proteins (G proteins)-α subunit (Gα) also mediate the stomatal closure triggered by D-glucose. Furthermore, the effects of D-glucose on hydrogen peroxide (H2O2) or nitric oxide (NO) production and stomatal closure were more significant in AtrbohD or Nia2-1 mutants than in AtrbohF and AtrbohD/F or Nia1-2 and Nia2-5/Nia1-2. The data indicate that H2O2 sourced from AtrbohF and NO generated by Nia1 are essential for D-glucose-mediated stomatal closure. D-glucose-induced H2O2 and NO production in guard cells were completely abolished in rgs1-1 and rgs1-2, which suggests that RGS1 stimulates H2O2 and NO production in D-glucose-induced stomatal closure. Collectively, our data reveal that both HXK and RGS1 are required for D-glucose-mediated stomatal closure. In this context, D-glucose can be sensed by its receptor RGS1, thereby inducing AtrbohF-dependent H2O2 production and Nia1-catalysed NO accumulation, which in turn stimulates stomatal closure.

Sugar and hexokinase suppress expression of PIP aquaporins and reduce leaf hydraulics that preserves leaf water potential

Sugars affect central aspects of plant physiology, including photosynthesis, stomatal behavior and the loss of water through the stomata. Yet, the potential effects of sugars on plant aquaporins (AQPs) and water conductance have not been examined. We used database and transcriptional analyses, as well as cellular and whole-plant functional techniques to examine the link between sugar-related genes and AQPs. Database analyses revealed a high level of correlation between the expression of AQPs and that of sugar-related genes, including the Arabidopsis hexokinases 1 (AtHXK1). Increased expression of AtHXK1, as well as the addition of its primary substrate, glucose (Glc), repressed the expression of 10 AQPs from the plasma membrane-intrinsic proteins (PIP) subfamily (PIP-AQPs) and induced the expression of two stress-related PIP-AQPs. The osmotic water permeability of mesophyll protoplasts of AtHXK1-expressing plants and the leaf hydraulic conductance of those plants were significantly reduced, in line with the decreased expression of PIP-AQPs. Conversely, hxk1 mutants demonstrated a higher level of hydraulic conductance, with increased water potential in their leaves. In addition, the presence of Glc reduced leaf water potential, as compared with an osmotic control, indicating that Glc reduces the movement of water from the xylem into the mesophyll. The production of sugars entails a significant loss of water and these results suggest that sugars and AtHXK1 affect the expression of AQP genes and reduce leaf water conductance, to coordinate sugar levels with the loss of water through transpiration.

The buffering capacity of stems: genetic architecture of nonstructural carbohydrates in cultivated Asian rice, Oryza sativa

Harnessing stem carbohydrate dynamics in grasses offers an opportunity to help meet future demands for plant-based food, fiber and fuel production, but requires a greater understanding of the genetic controls that govern the synthesis, interconversion and transport of such energy reserves. We map out a blueprint of the genetic architecture of rice (Oryza sativa) stem nonstructural carbohydrates (NSC) at two critical developmental time-points using a subpopulation-specific genome-wide association approach on two diverse germplasm panels followed by quantitative trait loci (QTL) mapping in a biparental population. Overall, 26 QTL are identified; three are detected in multiple panels and are associated with starch-at-maturity, sucrose-at-maturity and NSC-at-heading. They tag OsHXK6 (rice hexokinase), ISA2 (rice isoamylase) and a tandem array of sugar transporters. This study provides the foundation for more in-depth molecular investigation to validate candidate genes underlying rice stem NSC and informs future comparative studies in other agronomically vital grass species.

Structure, Expression, and Functional Analysis of the Hexokinase Gene Family in Cassava

Hexokinase (HXK) proteins play important roles in catalyzing hexose phosphorylation and sugar sensing and signaling. To investigate the roles of HXKs in cassava tuber root development, seven HXK genes (MeHXK1-7) were isolated and analyzed. A phylogenetic analysis revealed that the MeHXK family can be divided into five subfamilies of plant HXKs. MeHXKs were clearly divided into type A (MeHXK1) and type B (MeHXK2-7) based on their N-terminal sequences. MeHXK1-5 all had typical conserved regions and similar protein structures to the HXKs of other plants; while MeHXK6-7 lacked some of the conserved regions. An expression analysis of the MeHXK genes in cassava organs or tissues demonstrated that MeHXK2 is the dominant HXK in all the examined tissues (leaves, stems, fruits, tuber phloems, and tuber xylems). Notably, the expression of MeHXK2 and the enzymatic activity of HXK were higher at the initial and expanding tuber stages, and lower at the mature tuber stage. Furthermore, the HXK activity of MeHXK2 was identified by functional complementation of the HXK-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). The gene expression and enzymatic activity of MeHXK2 suggest that it might be the main enzyme for hexose phosphorylation during cassava tuber root development, which is involved in sucrose metabolism to regulate the accumulation of starch.

Expression of Arabidopsis Hexokinase in Citrus Guard Cells Controls Stomatal Aperture and Reduces Transpiration

Hexokinase (HXK) is a sugar-phosphorylating enzyme involved in sugar-sensing. It has recently been shown that HXK in guard cells mediates stomatal closure and coordinates photosynthesis with transpiration in the annual species tomato and Arabidopsis. To examine the role of HXK in the control of the stomatal movement of perennial plants, we generated citrus plants that express Arabidopsis HXK1 (AtHXK1) under KST1, a guard cell-specific promoter. The expression of KST1 in the guard cells of citrus plants has been verified using GFP as a reporter gene. The expression of AtHXK1 in the guard cells of citrus reduced stomatal conductance and transpiration with no negative effect on the rate of photosynthesis, leading to increased water-use efficiency. The effects of light intensity and humidity on stomatal behavior were examined in rooted leaves of the citrus plants. The optimal intensity of photosynthetically active radiation and lower humidity enhanced stomatal closure of AtHXK1-expressing leaves, supporting the role of sugar in the regulation of citrus stomata. These results suggest that HXK coordinates photosynthesis and transpiration and stimulates stomatal closure not only in annual species, but also in perennial species.

Constitutively overexpressing a tomato fructokinase gene (LeFRK1) in cotton (Gossypium hirsutum L. cv. Coker 312) positively affects plant vegetative growth, boll number and seed cotton yield

Increasing fructokinase (FRK) activity in cotton (Gossypium hirsutum L.) plants may reduce fructose inhibition of sucrose synthase (Sus) and lead to improved fibre yield and quality. Cotton was transformed with a tomato (Solanum lycopersicum L.) fructokinase gene (LeFRK1) under the control of the CMV 35S promoter. In a greenhouse, the LeFRK1 plants had increased fibre and leaf FRK activity over nonexpressing nulls, but not improved fibre length and strength. Compared with the nulls, LeFRK1 plants yielded 13-100% more seed-cotton mass per boll and more bolls per plant, and therefore more seed cotton and fibre yield per plant. The enhanced yield was related to a greater seed number per boll for LeFRK1 plants. Photosynthetic rates were not appreciably different among genotypes. However, more area per leaf and leaf number (in some instances) for LeFRK1 plants than for nulls enhanced the capacity for C gain. Larger leaf areas for LeFRK1 plants were associated with larger stem diameters. Lower sucrose levels in developing leaves of LeFRK1 plants suggest that LeFRK1 overexpression leads to improved in vivo Sus activity in developing leaves and possibly in developing seeds. The improvement in yield for LeFRK1 plants may also be the result of improvements in photosynthate supply as a consequence of greater leaf area.

The guard cell metabolome: functions in stomatal movement and global food security

Guard cells represent a unique single cell-type system for the study of cellular responses to abiotic and biotic perturbations that affect stomatal movement. Decades of effort through both classical physiological and functional genomics approaches have generated an enormous amount of information on the roles of individual metabolites in stomatal guard cell function and physiology. Recent application of metabolomics methods has produced a substantial amount of new information on metabolome control of stomatal movement. In conjunction with other "omics" approaches, the knowledge-base is growing to reach a systems-level description of this single cell-type. Here we summarize current knowledge of the guard cell metabolome and highlight critical metabolites that bear significant impact on future engineering and breeding efforts to generate plants/crops that are resistant to environmental challenges and produce high yield and quality products for food and energy security.

Response of nitrate reductase activity and NIA genes expression in roots of Arabidopsis hxk1 mutant treated with selected carbon and nitrogen metabolites

In plants sugar sensing and signal transduction involves pathways dependent or independent on HXK1 as a glucose sensor. Research was conducted to determine which pathway is responsible for regulation of the nitrate reduction. The effect of selected carbon and nitrogen metabolites on nitrate reductase (NR) activity in Arabidopsis thaliana wild type (WT) and hxk1 mutant roots was studied. Exogenously supplied sugar, sucrose (Suc) and organic acid, 2-oxoglutarate (2-OG) led to an increase in the total and actual activity of NR. It was due to both the increase in expression of NIA genes and NR activation state. The stimulatory effect of Suc and 2-OG on nitrate reduction was less pronounced in hxk1 mutant roots with T-DNA insertion in the AtHXK1 gene encoding hexokinase1 (HXK1) and characterized by reduced hexokinase activity and root level of G6P and F6P. On the other hand, it was shown that exogenous glucose did not mimic Suc-mediated NR activation in Arabidopsis roots. Taken together, this data suggest that the Suc signaling pathway might be independent from hexose's sensor dependent mechanism.

Isolation, structural analysis, and expression characteristics of the maize (Zea mays L.) hexokinase gene family

Hexokinases (HXKs, EC 2.7.1.1) play important roles in metabolism, glucose (Glc) signaling, and phosphorylation of Glc and fructose and are ubiquitous in all organisms. Despite their physiological importance, the maize HXK (ZmHXK) genes have not been analyzed systematically. We isolated and characterized nine members of the ZmHXK gene family which were distributed on 3 of the 10 maize chromosomes. A multiple sequence alignment and motif analysis revealed that the maize ZmHXK proteins share three conserved domains. Phylogenetic analysis revealed that the ZmHXK family can be divided into four subfamilies. We identified putative cis-elements in the ZmHXK promoter sequences potentially involved in phytohormone and abiotic stress responses, sugar repression, light and circadian rhythm regulation, Ca(2+) responses, seed development and germination, and CO2-responsive transcriptional activation. To study the functions of maize HXK isoforms, we characterized the expression of the ZmHXK5 and ZmHXK6 genes, which are evolutionarily related to the OsHXK5 and OsHXK6 genes from rice. Analysis of tissue-specific expression patterns using quantitative real time-PCR showed that ZmHXK5 was highly expressed in tassels, while ZmHXK6 was expressed in both tassels and leaves. ZmHXK5 and ZmHXK6 expression levels were upregulated by phytohormones and by abiotic stress.

Substantial roles of hexokinase and fructokinase in the effects of sugars on plant physiology and development

The basic requirements for plant growth are light, CO2, water, and minerals. However, the absorption and utilization of each of these requires investment on the part of the plant. The primary products of plants are sugars, and the hexose sugars glucose and fructose are the raw material for most of the metabolic pathways and organic matter in plants. To be metabolized, hexose sugars must first be phosphorylated. Only two families of enzymes capable of catalysing the essential irreversible phosphorylation of glucose and fructose have been identified in plants, hexokinases (HXKs) and fructokinases (FRKs). These hexose-phosphorylating enzymes appear to coordinate sugar production with the abilities to absorb light, CO2, water, and minerals. This review describes the long- and short-term effects mediated by HXK and FRK in various tissues, as well as the role of these enzymes in the coordination of sugar production with the absorption of light, CO2, water, and minerals.

Regulation of nitrate reduction in Arabidopsis WT and hxk1 mutant under C and N metabolites

As in plants sugar sensing and signal transduction involve pathways dependent or independent on hexokinase 1 (HXK1) as a glucose sensor, research was conducted to determine which pathway is responsible for regulation of the nitrate reduction. An Arabidopsis mutant with T-DNA insertion in the AtHXK1 gene and defects in glucose signaling (hxk1) was used to determine nitrate reductase (NR) activity, NIA genes expression in leaves after 8-h treatment with sugars (glucose and sucrose), organic acids [2-oxoglutarate (2OG)] and amino acids (glutamine and glutamate). Sugars, especially sucrose, caused induction of NR actual activity accompanied by an increase of the NR activation state, indicating the posttranslational nature of the modifications. Those modifications were observed in wild-type (WT) and hxk1 leaves, suggesting that regulation of NR activity by sugars does not involve HXK1 as a glucose sensor. Moreover, sugars enhanced expression of NIA genes. However, a higher level of NIA transcripts did not lead to an increase of total NR activity in sugar-treated plants. This may suggest that posttranslational modification of NR is fundamental regulatory mechanisms controlling NR activity in response to C metabolites. Treatment of plants with 2-OG also modified NR through the posttranslational modifications. Elevation of actual NR activity and the enzyme activation state in WT and hxk1 leaves was observed. Amino acids caused a decrease of NIA gene expression and NR activities in WT and hxk1 leaves indicating that mutation in the hexokinase-dependent glucose signaling pathway did not interrupt the amino acid feedback regulation of NR.

Hexokinase mediates stomatal closure

Stomata, composed of two guard cells, are the gates whose controlled movement allows the plant to balance the demand for CO2 for photosynthesis with the loss of water through transpiration. Increased guard-cell osmolarity leads to the opening of the stomata and decreased osmolarity causes the stomata to close. The role of sugars in the regulation of stomata is not yet clear. In this study, we examined the role of hexokinase (HXK), a sugar-phosphorylating enzyme involved in sugar-sensing, in guard cells and its effect on stomatal aperture. We show here that increased expression of HXK in guard cells accelerates stomatal closure. We further show that this closure is induced by sugar and is mediated by abscisic acid. These findings support the existence of a feedback-inhibition mechanism that is mediated by a product of photosynthesis, namely sucrose. When the rate of sucrose production exceeds the rate at which sucrose is loaded into the phloem, the surplus sucrose is carried toward the stomata by the transpiration stream and stimulates stomatal closure via HXK, thereby preventing the loss of precious water.

A dual role of tobacco hexokinase 1 in primary metabolism and sugar sensing

Hexokinase (HXK) is present in all virtually living organisms and is central to carbohydrate metabolism catalysing the ATP-dependent phosphorylation of hexoses. In plants, HXKs are supposed to act as sugar sensors and/or to interact with other enzymes directly supplying metabolic pathways such as glycolysis, the nucleotide phosphate monosaccharide (NDP-glucose) pathway and the pentose phosphate pathway. We identified nine members of the tobacco HXK gene family and observed that among RNAi lines of these nine NtHXKs, only RNAi lines of NtHXK1 showed an altered phenotype, namely stunted growth and leaf chlorosis. NtHXK1 was also the isoform with highest relative expression levels among all NtHXKs. GFP-tagging and immunolocalization indicated that NtHXK1 is associated with mitochondrial membranes. Overexpression of NtHXK1 resulted in elevated glucose phosphorylation activity in leaf extracts or chloroplasts. Moreover, NtHXK1 was able to complement the glucose-insensitive Arabidopsis mutant gin2-1 suggesting that NtHXK1 can take over glucose sensing functions. RNAi lines of NtHXK1 showed severely damaged leaf and chloroplast structure, coinciding with an excess accumulation of starch. We conclude that NtHXK1 is not only essential for maintaining glycolytic activity during respiration but also for regulating starch turnover, especially during the night.

ATP produced by oxidative phosphorylation is channeled toward hexokinase bound to mitochondrial porin (VDAC) in beetroots (Beta vulgaris)

Mitochondrial porins or voltage-dependent anion channels (VDAC) are the main route for solute transport through outer mitochondrial membranes (OMM). In mammals, hexokinase (HK) binds to VDAC, which allows the channeling of ATP synthesized by oxidative phosphorylation toward HK. In plants, although HK has been found associated with OMM, evidence for an interaction with VDAC is scarce. Thus, in this work, we studied the physical and functional interaction between these proteins in beetroot mitochondria. To observe a physical interaction between HK and VDAC, OMM presenting HK activity were prepared from purified mitochondria. Protein complexes were solubilized from OMM with mild detergents and separated by centrifugation in glycerol gradients. Both HK activity and immunodetected VDAC were found in small (9S-13S) and large (>40S) complexes. OMM proteins were also separated according to their hydropathy by serial phase partitioning with Triton X-114. Most of HK activity was found in hydrophobic fractions where VDAC was also present. These results indicated that HK could be bound to VDAC in beetroot mitochondria. The functional interaction of HK with VDAC was demonstrated by observing the effect of apyrase on HK-catalyzed glucose phosphorylation in intact mitochondria. Apyrase, which hydrolyzes freely soluble ATP, competed efficiently with hexokinase for ATP when it was produced outside mitochondria (with PEP and pyruvate kinase), but not when it was produced inside mitochondria by oxidative phosphorylation. These results suggest that HK closely interacts with VDAC in beetroot mitochondria, and that this interaction allows the channeling of respiratory ATP toward HK through VDAC.

Hexose kinases and their role in sugar-sensing and plant development

Hexose sugars, such as glucose and fructose produced in plants, are ubiquitous in most organisms and are the origin of most of the organic matter found in nature. To be utilized, hexose sugars must first be phosphorylated. The central role of hexose-phosphorylating enzymes has attracted the attention of many researchers, leading to novel discoveries. Only two families of enzymes capable of phosphorylating glucose and fructose have been identified in plants; hexokinases (HXKs), and fructokinases (FRKs). Intensive investigations of these two families in numerous plant species have yielded a wealth of knowledge regarding the genes number, enzymatic characterization, intracellular localization, and developmental and physiological roles of several HXKs and FRKs. The emerging picture indicates that HXK and FRK enzymes found at specific intracellular locations play distinct roles in plant metabolism and development. Individual HXKs were shown for the first time to be dual-function enzymes - sensing sugar levels independent of their catalytic activity and controlling gene expression and major developmental pathways, as well as hormonal interactions. FRK, on the other hand, seems to play a central metabolic role in vascular tissues, controlling the amounts of sugars allocated for vascular development. While a clearer picture of the roles of these two types of enzymes is emerging, many questions remain unsolved, such as the specific tissues and types of cells in which these enzymes function, the roles of individual HXK and FRK genes, and how these enzymes interact with hormones in the regulation of developmental processes. It is anticipated that ongoing efforts will broaden our knowledge of these important plant enzymes and their potential uses in the modification of plant traits.

The SlFRK4 promoter is active only during late stages of pollen and anther development

Carbohydrates are essential for male gametophyte development. However, our understanding of the mechanism by which the sugar supply is controlled in the stamen is still in its infancy. We previously reported on the stamen-specific expression of the tomato (Solanum lycopersicum) sugar metabolic gene, fructokinase 4 (SlFRK4). Here, we present the cloning and the characterization of the SlFRK4 promoter and show its differential activation during anther development. We also show that the tissue-specific expression of SlFRK4 promoter is maintained in Arabidopsis thaliana. By histochemical analyses of the GUS reporter gene and DTA toxin driven by the SlFRK4 promoter, we show that the SlFRK4 promoter is gradually activated in pollen grains throughout the later stages of anther development and upon pollen germination. In addition, we analyzed the expression profile of SlFRK4 and other sugar metabolic genes and found that SlFRK4 and the invertase LIN7 are co-expressed in mature and germinated pollen. These findings point to the existence of a specialized mechanism in which carbohydrates are provided to the male gametophyte during the later stages of its development and suggest a valuable tool for manipulating the development of male gametophytes in crop species.

Hexokinase-mediated sugar signaling controls expression of the calcineurin B-like interacting protein kinase 15 gene and is perturbed by oxidative phosphorylation inhibition

Calcineurin B-like (CBL) interacting protein kinase 15 (CIPK15) is a newly identified positive regulator which is critical to directing the O(2) deficiency signal to the sugar signaling cascade as part of Amy3D (representative Amy3 gene) regulation in rice. It is located upstream and probably contributes to reserve mobilization under anoxia. In isolated starving embryos, the temporal pattern of accumulation of CIPK15 transcripts and leaky suppression of this gene suggests that factors other than CIPK15 may also be involved in the regulation of Amy3D expression. Probing of a variety of sugars and sugar analogs has shown that hexokinase mediates the sugar regulation of CIPK15. For example, hexokinase substrates, such as mannose, 2-deoxyglucose, and other metabolizable sugars, repressed CIPK15 expression, whereas 3-O-methylglucose and 6-deoxyglucose did not. By using glucosamine, a hexokinase inhibitor, to release glucose-dependent CIPK15 suppression, we confirmed that hexokinase mediates regulation of this gene. Chemical inhibitors of mitochondrial electron transfer, proton separation or ATP synthase also effectively abolished sugar-induced repression of CIPK15. This type of interference, the release from glucose-induced repression of gene expression by inhibition of oxidative phosphorylation, was previously identified for the Amy3D gene, which suggests that hexokinase-mediated sugar signaling may be coordinated with the cellular energy status. Analysis of a transgenic rice cell line harboring the GUS reporter gene under the control of the CIPK15 promoter, and transient expression assay for 3' UTR of the CIPK15 gene indicate that sugar regulation of the rice CIPK15 gene is likely mediated by 2548-bp 5'-flanking region, with no additional post-transcriptional control.

Fructokinase is required for carbon partitioning to cellulose in aspen wood

Sucrose is the main transported form of carbon in several plant species, including Populus species. Sucrose metabolism in developing wood has therefore a central role in carbon partitioning to stem biomass. Half of the sucrose-derived carbon is in the form of fructose, but metabolism of fructose has received little attention as a factor in carbon partitioning to walls of wood cells. We show that RNAi-mediated reduction of FRK2 activity in developing wood of hybrid aspen (Populus tremula × tremuloides) led to the accumulation of soluble neutral sugars and a decrease in hexose phosphates and UDP-glucose, indicating that carbon flux to cell-wall polysaccharide precursors is decreased. Reduced FRK2 activity also led to thinner fiber cell walls with a reduction in the proportion of cellulose. No pleiotropic effects on stem height or diameter were observed. The results establish a central role for FRK2 activity in carbon flux to wood cellulose.

An Arabidopsis T-DNA insertion mutant for galactokinase (AtGALK, At3g06580) hyperaccumulates free galactose and is insensitive to exogenous galactose

Galactokinase (GALK, EC 2.7.1.6) is a cytosolic enzyme with a wide occurrence across the taxonomic kingdoms. It catalyzes the phosphorylation of α-d-galactose (Gal) to α-d-Gal-1-P. The cytotoxicity of free (unphosphorylated) Gal is well documented in plants and causes marked defects. An Arabidopsis GALK (AtGALK, At3g06580) was previously identified, cloned and functionally characterized in Escherichia coli and was suggested to occur as a single copy gene in Arabidopsis. We identified an AtGALK T-DNA insertion mutant (atgalk) that (i) is AtGALK transcript deficient; (ii) displays no GALK activity in vegetative tissues; and (iii) accumulates Gal up to 6.8 mg g(-1) FW in vegetative tissues, in contrast to wild-type plants. By constitutively overexpressing the AtGALK cDNA, atgalk was functionally rescued. Three independent transformed lines showed restored AtGALK transcripts and GALK activity and had low leaf Gal concentrations comparable with those observed in wild-type plants. Surprisingly, in vitro grown atgalk plants were largely insensitive to the exogenous application of up to 100 mM free Gal, while wild-type plants exhibited sensitivity to low Gal concentrations (10 mM). Furthermore, atgalk seedlings retained the capacity for uptake of exogenously supplied Gal (100 mM), accumulating up to 57 mg g(-1) FW in leaves. Leaves from soil-grown atgalk plants that exhibited no growth or morphological defects were used to demonstrate that the accumulating Gal occurred exclusively in the vacuoles of mesophyll protoplasts. Collectively, these findings suggest a novel Gal detoxification pathway that targets free Gal to the vacuole and is active in the atgalk mutant background.

Diverse functional roles of monosaccharide transporters and their homologs in vascular plants: a physiological perspective

Vascular plants contain two gene families that encode monosaccharide transporter proteins. The classical monosaccharide transporter(-like) gene superfamily is large and functionally diverse, while the recently identified SWEET transporter family is smaller and, thus far, only found to transport glucose. These transporters play essential roles at many levels, ranging from organelles to the whole plant. Many family members are essential for cellular homeostasis and reproductive success. Although most transporters do not directly participate in long-distance transport, their indirect roles greatly impact carbon allocation and transport flux to the heterotrophic tissues of the plant. Functional characterization of some members from both gene families has revealed their diverse roles in carbohydrate partitioning, phloem function, resource allocation, plant defense, and sugar signaling. This review highlights the broad impacts and implications of monosaccharide transport by describing some of the functional roles of the monosaccharide transporter(-like) superfamily and the SWEET transporter family.

Metabolism of soluble sugars in developing melon fruit: a global transcriptional view of the metabolic transition to sucrose accumulation

The sweet melon fruit is characterized by a metabolic transition during its development that leads to extensive accumulation of the disaccharide sucrose in the mature fruit. While the biochemistry of the sugar metabolism pathway of the cucurbits has been well studied, a comprehensive analysis of the pathway at the transcriptional level allows for a global genomic view of sugar metabolism during fruit sink development. We identified 42 genes encoding the enzymatic reactions of the sugar metabolism pathway in melon. The expression pattern of the 42 genes during fruit development of the sweet melon cv Dulce was determined from a deep sequencing analysis performed by 454 pyrosequencing technology, comprising over 350,000 transcripts from four stages of developing melon fruit flesh, allowing for digital expression of the complete metabolic pathway. The results shed light on the transcriptional control of sugar metabolism in the developing sweet melon fruit, particularly the metabolic transition to sucrose accumulation, and point to a concerted metabolic transition that occurs during fruit development.

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.

Two novel types of hexokinases in the moss Physcomitrella patens

BACKGROUND

Hexokinase catalyzes the phosphorylation of glucose and fructose, but it is also involved in sugar sensing in both fungi and plants. We have previously described two types of hexokinases in the moss Physcomitrella. Type A, exemplified by PpHxk1, the major hexokinase in Physcomitrella, is a soluble protein that localizes to the chloroplast stroma. Type B, exemplified by PpHxk2, has an N-terminal membrane anchor. Both types are found also in vascular plants, and localize to the chloroplast stroma and mitochondrial membranes, respectively.

RESULTS

We have now characterized all 11 hexokinase encoding genes in Physcomitrella. Based on their N-terminal sequences and intracellular localizations, three of the encoded proteins are type A hexokinases and four are type B hexokinases. One of the type B hexokinases has a splice variant without a membrane anchor, that localizes to the cytosol and the nucleus. However, we also found two new types of hexokinases with no obvious orthologs in vascular plants. Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus. Type D hexokinases, encoded by three genes, have membrane anchors and localize to mitochondrial membranes, but their sequences differ from those of the type B hexokinases. Interestingly, all moss hexokinases are more similar to each other in overall sequence than to hexokinases from other plants, even though characteristic sequence motifs such as the membrane anchor of the type B hexokinases are highly conserved between moss and vascular plants, indicating a common origin for hexokinases of the same type.

CONCLUSIONS

We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants. In particular, the presence of a cytosolic and nuclear hexokinase (type C) sets Physcomitrella apart from vascular plants, and instead resembles yeast, where all hexokinases localize to the cytosol. The fact that all moss hexokinases are more similar to each other than to hexokinases from vascular plants, even though both type A and type B hexokinases are present in all plants, further suggests that the hexokinase gene family in Physcomitrella has undergone concerted evolution.

Sugar signalling and antioxidant network connections in plant cells

Sugars play important roles as both nutrients and regulatory molecules throughout plant life. Sugar metabolism and signalling function in an intricate network with numerous hormones and reactive oxygen species (ROS) production, signalling and scavenging systems. Although hexokinase is well known to fulfil a crucial role in glucose sensing processes, a scenario is emerging in which the catalytic activity of mitochondria-associated hexokinase regulates glucose-6-phosphate and ROS levels, stimulating antioxidant defence mechanisms and the synthesis of phenolic compounds. As a new concept, it can be hypothesized that the synergistic interaction of sugars (or sugar-like compounds) and phenolic compounds forms part of an integrated redox system, quenching ROS and contributing to stress tolerance, especially in tissues or organelles with high soluble sugar concentrations.

Evolutionary lineages and functional diversification of plant hexokinases

Sequencing data from 10 species show that a plant hexokinase (HXK) family contains 5-11 genes. Functionally, a given family can include metabolic catalysts, glucose signaling proteins, and non-catalytic, apparent regulatory enzyme homologs. This study has two goals. The first aim is to develop a predictive method to determine which HXK proteins within a species have which type of function. The second aim is to determine whether HXK-dependent glucose signaling proteins occur among more primitive plants, as well as among angiosperms. Using a molecular phylogeny approach, combined with selective experimental testing, we found that non-catalytic HXK homologs might occur in all plants, including the relatively primitive Selaginella moellendorffi. We also found that different lineages of angiosperm HXKs have apparent conserved features for catalytic activity and for sub-cellular targeting. Most higher-plant HXKs are expressed predominantly at mitochondria, with HXKs of one lineage occurring in the plastid, and HXKs of one monocot lineage occurring in the cytosol. Using protoplast transient expression assays, we found that HXK glucose signaling proteins occur likely in all higher plants and in S. moellendorffi as well. Thus, the use of glucose by plant HXK isoforms in metabolism and/or as a regulatory metabolite occurs as widespread, conserved processes.

Evidence for a role of hexokinases as conserved glucose sensors in both monocot and dicot plant species

The role of the hexokinases (HXKs) as glucose (Glc) sensors has been mainly demonstrated for Arabidopsis (Arabidopsis thaliana) HXK1 (AtHXK1) but has yet to be shown in other plant species. In our recent publication, we reported that two rice (Oryza sativa) HXKs, OsHXK5 and OsHXK6, also function as Glc sensors. These two enzymes harbor both mitochondrial targeting peptides (mTPs) and nuclear localization signals (NLSs), and we confirmed their dual-targeting ability to nuclei and mitochondria using GFP fusion experiments. Consistently, it has been previously known that AtHXK1 is predominantly associated with mitochondria but is also present in nuclei in vivo at appreciable levels. Notably, the expression of OsHXK5, OsHXK6, or their catalytically inactive mutant alleles complemented the Arabidopsis glucose insensitive2 (gin2) mutant. In addition, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the Rubisco small subunit (RbcS) gene in response to glucose treatment. Our results thus provided evidence that OsHXK5 and OsHXK6 can function as glucose sensors in rice. Hence, the available current data suggest that the role of the HXKs as Glc sensors may be conserved in both monocot and dicot plant species, and that the nuclear localization of AtHXK1, OsHXK5 and OsHXK6 may be critical for Glc sensing and signaling.