Antisense repression of hexokinase 1 leads to an overaccumulation of starch in leaves of transgenic potato plants but not to significant changes in tuber carbohydrate metabolism

The role of hexokinases in epigenetic regulation: altered hexokinase expression and chromatin stability in yeast

BACKGROUND

Human hexokinase 2 (HK2) plays an important role in regulating Warburg effect, which metabolizes glucose to lactate acid even in the presence of ample oxygen and provides intermediate metabolites to support cancer cell proliferation and tumor growth. HK2 overexpression has been observed in various types of cancers and targeting HK2-driven Warburg effect has been suggested as a potential cancer therapeutic strategy. Given that epigenetic enzymes utilize metabolic intermediates as substrates or co-factors to carry out post-translational modification of histones and nucleic acids modifications in cells, we hypothesized that altering HK2 expression could impact the epigenome and, consequently, chromatin stability in yeast. To test this hypothesis, we established genetic models with different yeast hexokinase 2 (HXK2) expression in Saccharomyces cerevisiae yeast cells and investigated the effect of HXK2-dependent metabolism on parental nucleosome transfer, a key DNA replication-coupled epigenetic inheritance process, and chromatin stability.

RESULTS

By comparing the growth of mutant yeast cells carrying single deletion of hxk1Δ, hxk2Δ, or double-loss of hxk1Δ hxk2Δ to wild-type cells, we firstly confirmed that HXK2 is the dominant HXK in yeast cell growth. Surprisingly, manipulating HXK2 expression in yeast, whether through overexpression or deletion, had only a marginal impact on parental nucleosome assembly, but a noticeable trend with decrease chromatin instability. However, targeting yeast cells with 2-deoxy-D-glucose (2-DG), a clinical glycolysis inhibitor that has been proposed as an anti-cancer treatment, significantly increased chromatin instability.

CONCLUSION

Our findings suggest that in yeast cells lacking HXK2, alternative HXKs such as HXK1 or glucokinase 1 (GLK1) play a role in supporting glycolysis at a level that adequately maintains epigenomic stability. While our study demonstrated an increase in epigenetic instability with 2-DG treatment, the observed effect seemed to occur dependent on non-glycolytic function of Hxk2. Thus, additional research is needed to identify the molecular mechanism through which 2-DG influences chromatin stability.

The Role of Hexokinases in Epigenetic Regulation: Altered Hexokinase Expression and Chromatin Stability in Yeast

Background . Human hexokinase 2 ( HK2 ) plays an important role in regulating Warburg effect, which metabolizes glucose to lactate acid even in the presence of ample oxygen and provides intermediate metabolites to support cancer cell proliferation and tumor growth. HK2 overexpression has been observed in various types of cancers and targeting HK2 -driven Warburg effect has been suggested as a potential cancer therapeutic strategy. Given that epigenetic enzymes utilize metabolic intermediates as substrates or co-factors to carry out post-translational modification of DNA and histones in cells, we hypothesized that altering HK2 expression-mediated cellular glycolysis rates could impact the epigenome and, consequently, genome stability in yeast. To test this hypothesis, we established genetic models with different yeast hexokinase 2 ( HXK2) expression in Saccharomyces cerevisiae yeast cells and investigated the effect of HXK2 -dependent metabolism on parental nucleosome transfer, a key DNA replication-coupled epigenetic inheritance process, and chromatin stability. Results . By comparing the growth of mutant yeast cells carrying single deletion of hxk1Δ , hxk2Δ , or double-loss of hxk1Δ hxk2Δ to wild-type cells, we demonstrated that HXK2 is the dominant HXK in yeast cell growth. Surprisingly, manipulating HXK2 expression in yeast, whether through overexpression or deletion, had only a marginal impact on parental nucleosome assembly, but a noticeable trend with decrease chromatin instability. However, targeting yeast cells with 2-deoxy-D-glucose (2-DG), a HK2 inhibitor that has been proposed as an anti-cancer treatment, significantly increased chromatin instability. Conclusion . Our findings suggest that in yeast cells lacking HXK2 , alternative HXK s such as HXK1 or glucokinase 1 ( GLK1 ) play a role in supporting glycolysis at a level that adequately maintain epigenomic stability. While our study demonstrated an increase in epigenetic instability with 2-DG treatment, the observed effect seemed to occur independently of Hxk2-mediated glycolysis inhibition. Thus, additional research is needed to identify the molecular mechanism through which 2-DG influences chromatin stability.

Suppression of the tonoplast sugar transporter, StTST3.1, affects transitory starch turnover and plant growth in potato

Transitory starch and vacuolar sugars function as highly dynamic pools of instantly accessible metabolites in plant leaf cells. Their metabolic regulation is critical for plant survival. The tonoplast sugar transporters (TSTs), responsible for sugar uptake into vacuoles, regulate cellular sugar partitioning and vacuolar sugar accumulation. However, whether TSTs are involved in leaf transient starch turnover and plant growth is unclear. Here, we found that suppressing StTST3.1 resulted in growth retardation and pale green leaves in potato plants. StTST3.1-silenced plants displayed abnormal chloroplasts and impaired photosynthetic performance. The subcellular localization assay and the oscillation expression patterns revealed that StTST3.1 encoded a tonoplast-localized protein and responded to photoperiod. Moreover, RNA-seq analyses identified that starch synthase (SS2 and SS6) and glucan water, dikinase (GWD), were downregulated in StTST3.1-silenced lines. Correspondingly, the capacity for starch synthesis and degradation was decreased in StTST3.1-silenced lines. Surprisingly, StTST3.1-silenced leaves accumulated exceptionally high levels of maltose but low levels of sucrose and hexose. Additionally, chlorophyll content was reduced in StTST3.1-silenced leaves. Analysis of chlorophyll metabolic pathways found that Non-Yellow Coloring 1 (NYC1)-like (NOL), encoding a chloroplast-localized key enzyme that catalyzes the initial step of chlorophyll b degradation, was upregulated in StTST3.1-silenced leaves. Transient overexpression of StNOL accelerated chlorophyll b degradation in tobacco leaves. Our results indicated that StTST3.1 is involved in transitory starch turnover and chlorophyll metabolism, thereby playing a critical role in normal potato plant growth.

Dioscorea Alata Tuber Proteome Analysis Uncovers Differentially Regulated Growth-associated Pathways of Tuber Development

During its life cycle, the Dioscorea tuber undergoes multiple morphological and biochemical changes. To gain a better understanding of the metabolic changes associated with tuber growth, a stage-specific gel-free proteome analysis of four distinct morphological stages namely germinating tuber (S1), degrading tuber (S2), new tuber formation (S3) and tuber maturation (S4) was done and validated by principal component analysis. A comprehensive data set identifying 78.2% of the total 3,681 proteins was generated. PANTHER and KEGG MAPPER revealed both expected (carbohydrate metabolism and redox regulation) and novel biological processes (transcription factors and hormonal regulation) characteristic for each developmental stage. Higher abundance of the enzymes of ascorbate-glutathione cycle and carbohydrate metabolism was detected during tuber germination (S1) and tuber formation stages (S3) in comparison with the mature tuber. The presence of ethylene biosynthesis components during tuber formation hints toward its probable role in postharvest shelf life. The data set comprehensively describes the proteome of Dioscorea tuber and provides growth-specific markers for tuber germination (ascorbate peroxidase, monodehydroascorbate reductase, invertase) and tuber formation (sucrose synthase), which were validated by enzyme activity assays and Western blotting. The study provides information that may influence the direction of research for improving the productivity of this under-utilized and largely neglected crop.

Biochemical properties and subcellular localization of six members of the HXK family in maize and its metabolic contribution to embryo germination

BACKGROUND

Seed germination is a crucial process in the plant life cycle when a dramatic variation of type and sugar content occurs just as the seed is hydrated. The production of hexose 6 phosphate is a key node in different pathways that are required for a successful germination. Hexokinase (HXK) is the only plant enzyme that phosphorylates glucose (Glc), so it is key to fueling several metabolic pathways depending on their substrate specificity, metabolite regulatory responses and subcellular localization. In maize, the HXK family is composed of nine genes, but only six of them (ZmHXK4-9) putatively encode catalytically active enzymes. Here, we cloned and functionally characterized putative catalytic enzymes to analyze their metabolic contribution during germination process.

RESULTS

From the six HXKs analyzed here, only ZmHXK9 has minimal hexose phosphorylating activity even though enzymatic function of all isoforms (ZmHXK4-9) was confirmed using a yeast complementation approach. The kinetic parameters of recombinant proteins showed that ZmHXK4-7 have high catalytic efficiency for Glc, fructose (Fru) and mannose (Man), ZmHXK7 has a lower Km for ATP, and together with ZmHXK8 they have lower sensitivity to inhibition by ADP, G6P and N-acetylglucosamine than ZmHXK4-6 and ZmHXK9. Additionally, we demonstrated that ZmHXK4-6 and ZmHXK9 are located in the mitochondria and their location relies on the first 30 amino acids of the N-terminal domain. Otherwise, ZmHXK7-8 are constitutively located in the cytosol. HXK activity was detected in cytosolic and mitochondrial fractions and high Glc and Fru phosphorylating activities were found in imbibed embryos.

CONCLUSIONS

Considering the biochemical characteristics, location and the expression of ZmHXK4 at onset of germination, we suggest that it is the main contributor to mitochondrial activity at early germination times, at 24 h other ZmHXKs also contribute to the total activity. While in the cytosol, ZmHXK7 could be responsible for the activity at the onset of germination, although later, ZmHXK8 also contributes to the total HXK activity. Our observations suggest that the HXKs may be redundant proteins with specific roles depending on carbon and ATP availability, metabolic needs, or sensor requirements. Further investigation is necessary to understand their specific or redundant physiological roles.

The Ability of a Charophyte Alga Hexokinase to Restore Glucose Signaling and Glucose Repression of Gene Expression in a Glucose-Insensitive Arabidopsis Hexokinase Mutant Depends on Its Catalytic Activity

Hexokinases is a family of proteins that is found in all eukaryotes. Hexokinases play key roles in the primary carbon metabolism, where they catalyze the phosphorylation of glucose and fructose, but they have also been shown to be involved in glucose signaling in both yeast and plants. We have characterized the Klebsormidium nitens KnHXK1 gene, the only hexokinase-encoding gene in this charophyte alga. The encoded protein, KnHXK1, is a type B plant hexokinase with an N-terminal membrane anchor localizing the protein to the mitochondrial membranes. We found that KnHXK1 expressed in Arabidopsis thaliana can restore the glucose sensing and glucose repression defects of the glucose-insensitive hexokinase mutant gin2-1. Interestingly, both functions require a catalytically active enzyme, since an inactive double mutant was unable to complement gin2-1. These findings differ from previous results on Arabidopsis AtHXK1 and its orthologs in rice, where catalytic and glucose sensing functions could be separated, but are consistent with recent results on the rice cytoplasmic hexokinase OsHXK7. A model with both catalytic and non-catalytic roles for hexokinases in glucose sensing and glucose repression is discussed.

Cytosolic glyceraldehyde-3-phosphate dehydrogenases play crucial roles in controlling cold-induced sweetening and apical dominance of potato (Solanum tuberosum L.) tubers

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an important enzyme that functions in producing energy and supplying intermediates for cellular metabolism. Recent researches indicate that GAPDHs have multiple functions beside glycolysis. However, little information is available for functions of GAPDHs in potato. Here, we identified 4 putative cytosolic GAPDH genes in potato genome and demonstrated that the StGAPC1, StGAPC2, and StGAPC3, which are constitutively expressed in potato tissues and cold inducible in tubers, encode active cytosolic GAPDHs. Cosuppression of these 3 GAPC genes resulted in low tuber GAPDH activity, consequently the accumulation of reducing sugars in cold stored tubers by altering the tuber metabolite pool sizes favoring the sucrose pathway. Furthermore, GAPCs-silenced tubers exhibited a loss of apical dominance dependent on cell death of tuber apical bud meristem (TAB-meristem). It was also confirmed that StGAPC1, StGAPC2, and StGAPC3 interacted with the autophagy-related protein 3 (ATG3), implying that the occurrence of cell death in TAB-meristem could be induced by ATG3 associated events. Collectively, the present research evidences first that the GAPC genes play crucial roles in diverse physiological and developmental processes in potato tubers.

Plant Hexokinases are Multifaceted Proteins

Sugars are the main carbon and energy source in cells, but they can also act as signaling molecules that affect the whole plant life cycle. Certain tissues can produce sugars and supply them to others, and this plant tissue heterogeneity makes sugar signaling a highly complex process that requires elements capable of perceiving changes in sugar concentrations among different tissues, cell compartments and developmental stages. In plants, the regulatory effects of glucose (Glc) have been the most studied to date. The first Glc sensor identified in plants was hexokinase (HXK), which is currently recognized as a dual-function protein. In addition to its catalytic activity, this enzyme can also repress the expression of some photosynthetic genes in response to high internal Glc concentrations. Additionally, the catalytic activity of HXKs has a profound impact on cell metabolism and other sugar signaling pathways that depend on phosphorylated hexoses and intermediate glycolytic products. HXKs are the only proteins that are able to phosphorylate Glc in plants, since no evidence has been provided to date concerning the existence of a glucokinase. Moreover, the intracellular localization of HXKs seems to be crucial to their activity and sensor functions. Recently, two new and surprising functions have been described for HXKs. In this review, we discuss the versatility of HXKs in regard to their catalytic and glucose sensor activities, intracellular location, protein-protein and hormone interactions, as well as how these HXK characteristics influence plant growth and development, in an effort to understand this enzyme's role in improving plant productivity.

Engineering the expression level of cytosolic nucleoside diphosphate kinase in transgenic Solanum tuberosum roots alters growth, respiration and carbon metabolism

Nucleoside diphosphate kinase (NDPK) is a ubiquitous enzyme that catalyzes the transfer of the γ-phosphate from a donor nucleoside triphosphate to an acceptor nucleoside diphosphate. In this study we used a targeted metabolomic approach and measurement of physiological parameters to report the effects of the genetic manipulation of cytosolic NDPK (NDPK1) expression on physiology and carbon metabolism in potato (Solanum tuberosum) roots. Sense and antisense NDPK1 constructs were introduced in potato using Agrobacterium rhizogenes to generate a population of root clones displaying a 40-fold difference in NDPK activity. Root growth, O₂ uptake, flux of carbon between sucrose and CO₂ , levels of reactive oxygen species and some tricarboxylic acid cycle intermediates were positively correlated with levels of NDPK1 expression. In addition, NDPK1 levels positively affected UDP-glucose and cellulose contents. The activation state of ADP-glucose pyrophosphorylase, a key enzyme in starch synthesis, was higher in antisense roots than in roots overexpressing NDPK1. Further analyses demonstrated that ADP-glucose pyrophosphorylase was more oxidized, and therefore less active, in sense clones than antisense clones. Consequently, antisense NDPK1 roots accumulated more starch and the starch to cellulose ratio was negatively affected by the level of NDPK1. These data support the idea that modulation of NDPK1 affects the distribution of carbon between starch and cellulose biosynthetic pathways.

Transcriptome analysis of near-isogenic lines provides molecular insights into starch biosynthesis in maize kernel

Starch is the major component in maize kernels, providing a stable carbohydrate source for humans and livestock as well as raw material for the biofuel industry. Increasing maize kernel starch content will help meet industry demands and has the potential to increase overall yields. We developed a pair of maize near-isogenic lines (NILs) with different alleles for a starch quantitative trait locus on chromosome 3 (qHS3), resulting in different kernel starch content. To investigate the candidate genes for qHS3 and elucidate their effects on starch metabolism, RNA-Seq was performed for the developing kernels of the NILs at 14 and 21 d after pollination (DAP). Analysis of genomic and transcriptomic data identified 76 genes with nonsynonymous single nucleotide polymorphisms and 384 differentially expressed genes (DEGs) in the introgressed fragment, including a hexokinase gene, ZmHXK3a, which catalyzes the conversion of glucose to glucose-6-phosphate and may play a key role in starch metabolism. The expression pattern of all DEGs in starch metabolism shows that altered expression of the candidate genes for qHS3 promoted starch synthesis, with positive consequences for kernel starch content. These results expand the current understanding of starch biosynthesis and accumulation in maize kernels and provide potential candidate genes to increase starch content.

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.

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 futile cycling of hexose phosphates could account for the fact that hexokinase exerts a high control on glucose phosphorylation but not on glycolytic rate in transgenic potato (Solanum tuberosum) roots

The metabolism of potato (Solanum tuberosum) roots constitutively over- and underexpressing hexokinase (HK, EC 2.7.1.1) was examined. An 11-fold variation in HK activity resulted in altered root growth, with antisense roots growing better than sense roots. Quantification of sugars, organic acids and amino acids in transgenic roots demonstrated that the manipulation of HK activity had very little effect on the intracellular pools of these metabolites. However, adenylate and free Pi levels were negatively affected by an increase in HK activity. The flux control coefficient of HK over the phosphorylation of glucose was measured for the first time in plants. Its value varied with HK level. It reached 1.71 at or below normal HK activity value and was much lower (0.32) at very high HK levels. Measurements of glycolytic flux and O₂ uptake rates demonstrated that the differences in glucose phosphorylation did not affect significantly glycolytic and respiratory metabolism. We hypothesized that these results could be explained by the existence of a futile cycle between the pools of hexose-Ps and carbohydrates. This view is supported by several lines of evidence. Firstly, activities of enzymes capable of catalyzing these reactions were detected in roots, including a hexose-P phosphatase. Secondly, metabolic tracer experiments using ¹⁴C-glucose as precursor showed the formation of ¹⁴C-fructose and ¹⁴C-sucrose. We conclude that futile cycling of hexose-P could be partially responsible for the differences in energetic status in roots with high and low HK activity and possibly cause the observed alterations in growth in transgenic roots. The involvement of HK and futile cycles in the control of glucose-6P metabolism is discussed.

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.

Rethinking metabolic control

Modulation of metabolic fluxes in plants is usually not a successful business. The main reason is our limited understanding of metabolic plasticity and metabolic control, with the latter still largely influenced by the idea that each pathway has a rate limiting step controlling the flux. Not only is experimental evidence for such steps lacking for most pathways, despite intensive search, but there are also theoretical arguments against the idea that highly regulated enzymes catalyzing reactions far from equilibrium must be considered a priori rate limiting. Conversely, it is argued that reactions close to equilibrium need a lot of enzyme to be maintained close to equilibrium and, contrary to accepted wisdom, begin to limit flux when reduced. Using a few key examples of plant metabolic pathways as case studies, I draw some general conclusions. The approach of augmenting flux by pushing a pathway from above is well exemplified by the attempts at increasing starch content in potato tubers, where several different approaches failed. Also pulling at the other end (close to the end product) has yielded little improvement, while targeting a reaction close to equilibrium (ADP/ATP translocation at the plastid envelope) successfully increased starch content. Rethinking control is equally well applicable to photosynthesis, with prime examples of 'neglected', unregulated enzymes exerting significant control and overprized 'limiting' enzymes having little control in normal conditions like rubisco. In this new paradigm, the role of most control mechanisms is also challenged: feedback inhibition and post-translational modification of enzymes are relevant to metabolite homeostasis rather than flux control, with moiety conservation being a major reason for this constraint. I advocate a more extensive use of control circuitry elements (e.g. sensors like riboswitches), metabolic shortcuts and transcription factors in metabolic engineering.

Sugar-mediated semidian oscillation of gene expression in the cassava storage root regulates starch synthesis

Starch branching enzyme (SBE) activity in the cassava storage root exhibited a diurnal fluctuation, dictated by a transcriptional oscillation of the corresponding SBE genes. The peak of SBE activity coincided with the onset of sucrose accumulation in the storage, and we conclude that the oscillatory mechanism keeps the starch synthetic apparatus in the storage root sink in tune with the flux of sucrose from the photosynthetic source. When storage roots were uncoupled from the source, SBE expression could be effectively induced by exogenous sucrose. Turanose, a sucrose isomer that cannot be metabolized by plants, mimicked the effect of sucrose, demonstrating that downstream metabolism of sucrose was not necessary for signal transmission. Also glucose and glucose-1-P induced SBE expression. Interestingly, induction by sucrose, turanose and glucose but not glucose-1-P sustained an overt semidian (12-h) oscillation in SBE expression and was sensitive to the hexokinase (HXK) inhibitor glucosamine. These results suggest a pivotal regulatory role for HXK during starch synthesis. Abscisic acid (ABA) was another potent inducer of SBE expression. Induction by ABA was similar to that of glucose-1-P in that it bypassed the semidian oscillator. Both the sugar and ABA signaling cascades were disrupted by okadaic acid, a protein phosphatase inhibitor. Based on these findings, we propose a model for sugar signaling in regulation of starch synthesis in the cassava storage root.

Overriding the co-limiting import of carbon and energy into tuber amyloplasts increases the starch content and yield of transgenic potato plants

Transgenic potato (Solanum tuberosum) plants simultaneously over-expressing a pea (Pisum sativum) glucose-6-phosphate/phosphate translocator (GPT) and an Arabidopsis thaliana adenylate translocator (NTT1) in tubers were generated. Double transformants exhibited an enhanced tuber yield of up to 19%, concomitant with an additional increased starch content of up to 28%, compared with control plants. The total starch content produced in tubers per plant was calculated to be increased by up to 44% in double transformants relative to the wild-type. Single over-expression of either gene had no effect on tuber starch content or tuber yield, suggesting that starch formation within amyloplasts is co-limited by the import of energy and the supply of carbon skeletons. As total adenosine diphosphate-glucose pyrophosphorylase and starch synthase activities remained unchanged in double transformants relative to the wild-type, they cannot account for the increased starch content found in tubers of double transformants. Rather, an optimized supply of amyloplasts with adenosine triphosphate and glucose-6-phosphate seems to favour increased starch synthesis, resulting in plants with increased starch content and yield of tubers.

Role of tomato hexose kinases

Hexose phosphorylation is an essential step of sugar metabolism. Only two classes of glucose and fructose phosphorylating enzymes, hexokinases (HXK) and fructokinases (FRK), have been found in plants. Tomato (Lycopersicon esculentum Mill.) is the only plant species from which four HXK and four FRK genes have been identified and characterised. One HXK and one FRK isozyme are located within plastids. The other three HXK isozymes are associated with the mitochondria, and the other three FRK isozymes are dispersed in the cytosol. These differences in location suggest that the cytoplasmic HXK and FRK have distinct roles to play in sugar metabolism. The specific roles of each of the HXK and FRK genes have been investigated using transgenic plants with modified expression of the genes. Sugar signalling effects were obtained with modified expression of the mitochondria associated HXK. In contrast, modified expression of the cytosolic FRK affected fructose metabolism rather than sugar signalling. Future research efforts will aim to determining the roles of specific hexose phosphorylating enzymes in tomato plants, the source of the hexose monomers to be phosphorylated, and their intracellular trafficking route.

The monosaccharide transporter(-like) gene family inArabidopsis

The availability of complete plant genomes has greatly influenced the identification and analysis of phylogenetically related gene clusters. In Arabidopsis, this has revealed the existence of a monosaccharide transporter(-like) gene family with 53 members, which play a role in long-distance sugar partitioning or sub-cellular sugar distribution and catalyze the transport of hexoses, but also polyols and in one case also pentoses and tetroses. An update on the currently available information on these Arabidopsis monosaccharide transporters, on their sub-cellular localization and physiological function will be given.

Phosphate starvation responses are mediated by sugar signaling in Arabidopsis

Phosphate (Pi) is one of the least available plant nutrients in soils. It is associated with dynamic changes in carbon fluxes and several crucial processes that regulate plant growth and development. Pi levels regulate the expression of large number of genes including those involved in photosynthesis and carbon metabolism. Herein we show that sugar is required for Pi starvation responses including changes in root architecture and expression of phosphate starvation induced (PSI) genes in Arabidopsis. Active photosynthesis or the supplementation of sugar in the medium was essential for the expression of PSI genes under Pi limiting conditions. Expression of these genes was not only induced by sucrose but also detected, albeit at reduced levels, with other metabolizable sugars. Non-metabolizable sugar analogs did not induce the expression of PSI genes. Although sugar input appears to be down-stream of initial Pi sensing, it is absolutely required for the completion of the PSI signaling pathway. Altered expression of PSI genes in the hexokinase signaling mutant gin2 indicates that hexokinase-dependent signaling is involved in this process. The study provides evidence for requirement of sugars in PSI signaling and evokes a role for hexokinase in some components of Pi response mechanism.

Evidence for intracellular spatial separation of hexokinases and fructokinases in tomato plants

Four hexokinase (LeHXK1-4) and four fructokinase (LeFRK1-4) genes were identified in tomato plants. Previous GFP fusion studies indicate that the gene product of LeHXK3 is associated with the mitochondria while that of LeHXK4 is located within plastids. In this study we found that the enzyme encoded by the fructokinase gene LeFRK3 is also located within plastids. The presence of LeFrk3 enzyme in plastids raises the question of the origin of fructose in these organelles. The other three FRKs enzymes, LeFrk1&2&4, are located in the cytosol. Unlike LeFrk1&2&4, the two additional HXKs, LeHxk1&2, share a common membrane anchor domain and are associated with the mitochondria similar to LeHxk3. The difference in the locations of the cytoplasmic FRK and HXK isozymes suggests that glucose phosphorylation is confined to defined special intracellular localizations while fructose phosphorylation is less confined.

Two newly identified membrane-associated and plastidic tomato HXKs: characteristics, predicted structure and intracellular localization

Two new tomato hexokinase genes, LeHXK3 and LeHXK4, were cloned and characterized, placing tomato as the first plant with four characterized HXK genes. Based on their sequence, LeHXK3 is the third membrane-associated (type-B) and LeHXK4 is the first plastidic (type-A) HXK identified in tomato. Expression of HXK-GFP fusion proteins in protoplasts indicated that the LeHxk3 enzyme is associated with the mitochondria while LeHxk4 is localized in plastids. Furthermore, LeHxk4::GFP fusion protein is found within stromules, suggesting transport of LeHxk4 between plastids. Structure prediction of the various plant HXK enzymes suggests that unlike the plastidic HXKs, the predicted membrane-associated HXKs are positively charged near their putative N-terminal membrane anchor domain, which might enhance their association with the negatively charged membranes. LeHxk3 and LeHxk4 were analyzed following expression in yeast. Both enzymes have higher affinity for glucose relative to fructose and are inhibited by ADP. Yet, unlike the other HXKs, the stromal HXK has higher Vmax with glucose than with fructose. Expression analysis of the four HXK genes in tomato tissues demonstrated that LeHXK1 and LeHXK4 are the dominant HXKs in all tissues examined. Notably, the plastidic LeHXK4 is expressed in all tissues including starchless, non-photosynthetic sink tissues, such as pink and red fruits, implying phosphorylation of imported hexoses in plastids. It has been suggested that trehalose 6-phosphate (T6P) might inhibit HXK activity. However, none of the yeast-expressed tomato HXK genes was sensitive either to T6P or to trehalose, suggesting that unlike fungi HXKs, plant HXKs are not regulated by T6P.

Subcellular pyrophosphate metabolism in developing tubers of potato (Solanum tuberosum)

PPi has previously been implicated specifically in the co-ordination of the sucrose-starch transition and in the broader context of its role as co-factor in heterotrophic plant metabolism. In order to assess the compartmentation of pyrophosphate (PPi) metabolism in the potato tuber we analysed the effect of expressing a bacterial pyrophosphatase in the amyloplast of wild type tubers or in the cytosol or amyloplast of invertase-expressing tubers. The second and third approaches were adopted since we have previously characterized the invertase expressing lines to both exhibit highly altered sucrose metabolism and to contain elevated levels of PPi (Farré et al. (2000a) Plant Physiol 123:681) and therefore this background rendered questions concerning the level of communication between the plastidic and cytosolic pyrophosphate pools relatively facile. In this study we observed that the increase in PPi in the invertase expressing lines was mainly confined to the cytosol. Accordingly, the expression of a bacterial pyrophosphatase in the plastid of either wild type or invertase-expressing tubers did not lead to a decrease in total PPi content. However, the expression of the heterologous pyrophosphatase in the cytosol of cytosolic invertase-expressing tubers led to strong metabolic changes. These results are discussed both with respect to our previous hypotheses and to current models of the compartmentation of potato tuber metabolism.

Subcellular pyrophosphate metabolism in developing tubers of potato (Solanum tuberosum)

PPi has previously been implicated specifically in the co-ordination of the sucrose-starch transition and in the broader context of its role as co-factor in heterotrophic plant metabolism. In order to assess the compartmentation of pyrophosphate (PPi) metabolism in the potato tuber we analysed the effect of expressing a bacterial pyrophosphatase in the amyloplast of wild type tubers or in the cytosol or amyloplast of invertase-expressing tubers. The second and third approaches were adopted since we have previously characterized the invertase expressing lines to both exhibit highly altered sucrose metabolism and to contain elevated levels of PPi (Farré et al. (2000a) Plant Physiol 123:681) and therefore this background rendered questions concerning the level of communication between the plastidic and cytosolic pyrophosphate pools relatively facile. In this study we observed that the increase in PPi in the invertase expressing lines was mainly confined to the cytosol. Accordingly, the expression of a bacterial pyrophosphatase in the plastid of either wild type or invertase-expressing tubers did not lead to a decrease in total PPi content. However, the expression of the heterologous pyrophosphatase in the cytosol of cytosolic invertase-expressing tubers led to strong metabolic changes. These results are discussed both with respect to our previous hypotheses and to current models of the compartmentation of potato tuber metabolism.

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.

Modulation of fructokinase activity of potato (Solanum tuberosum) results in substantial shifts in tuber metabolism

Potato plants (Solanum tuberosum L. cvs Desiree and Record) transformed with sense and antisense constructs of a cDNA encoding the potato fructokinase StFK1 exhibited altered transcription of this gene, altered amount of protein and altered enzyme activities. Measurement of the maximal catalytic activity of fructokinase revealed a 2-fold variation in leaf (from 90 to 180% of wild type activity) and either a 10- or 30-fold variation in tuber (from 10 or 30% to 300% in Record and Desiree, respectively) activity. The comparative effect of the antisense construct in leaf and tuber tissue suggests that this isoform is only a minor contributor to the total fructokinase activity in the leaf but the predominant isoform in the tuber. Antisense inhibition of the fructokinase resulted in a reduced tuber yield; however, its overexpression had no impact on this parameter. The modulation of fructokinase activity had few, consistent effects on carbohydrate levels, with the exception of a general increase in glucose content in the antisense lines, suggesting that this enzyme is not important for the control of starch synthesis. However, when metabolic fluxes were estimated, it became apparent that the transgenic lines display a marked shift in metabolism, with the rate of redistribution of radiolabel to sucrose markedly affected by the activity of fructokinase. These data suggest an important role for fructokinase, acting in concert with sucrose synthase, in maintaining a balance between sucrose synthesis and degradation by a mechanism independent of that controlled by the hexose phosphate-mediated activation of sucrose phosphate synthase.

Leaf starch degradation comes out of the shadows

During the day, plants accumulate starch in their leaves as an energy source for the coming night. Based on recent findings, the prevailing view of how the transitory starch is remobilized needs considerable revision. Analyses of transgenic and mutant plants demonstrate that plastidic glucan phosphorylase is not required for normal starch breakdown and cast doubt on the presumed essential role of alpha-amylase but do show that beta-amylase is important. Repression of the activity of a plastidic beta-amylase, the export of its product (maltose) or further metabolism of maltose by a newly identified transglucosidase impairs starch degradation. Breakdown of particulate starch also depends on the activity of glucan-water dikinase, which phosphorylates glucosyl residues within the polymer.

Control of plant development and gene expression by sugar signaling

Coordination of development with the availability of nutrients, such as soluble sugars, may help ensure an adequate supply of building materials and energy with which to carry out specific developmental programs. For example, in-vivo and in-vitro experiments suggest that increasing sugar levels delay seed germination and stimulate the induction of flowering and senescence in at least some plant species. Higher sugar concentrations can also increase the number of tubers formed by potatoes and can stimulate the formation of adventitious roots by Arabidopsis. New insights into the mechanisms by which sugar-response pathways interact with other response pathways have been provided by microarray experiments examining sugar-regulated gene expression under different light and nitrogen conditions.

Application of Petri net theory for modelling and validation of the sucrose breakdown pathway in the potato tuber

MOTIVATION

Because of the complexity of metabolic networks and their regulation, formal modelling is a useful method to improve the understanding of these systems. An essential step in network modelling is to validate the network model. Petri net theory provides algorithms and methods, which can be applied directly to metabolic network modelling and analysis in order to validate the model. The metabolism between sucrose and starch in the potato tuber is of great research interest. Even if the metabolism is one of the best studied in sink organs, it is not yet fully understood.

RESULTS

We provide an approach for model validation of metabolic networks using Petri net theory, which we demonstrate for the sucrose breakdown pathway in the potato tuber. We start with hierarchical modelling of the metabolic network as a Petri net and continue with the analysis of qualitative properties of the network. The results characterize the net structure and give insights into the complex net behaviour.

Temporally regulated expression of a yeast invertase in potato tubers allows dissection of the complex metabolic phenotype obtained following its constitutive expression

The constitutive cytosolic expression of a yeast ( Saccharomyces cerevisiae ) invertase within potato ( Solanum tuberosum ) tubers has previously been documented to produce a dramatic metabolic phenotype in which glycolysis, respiration and amino acid synthesis are markedly enhanced at the cost of starch synthesis. These transgenic lines were further characterised by a massive cycle of sucrose degradation and resynthesis via sucrose-phosphate synthase. We have recently developed a B33 patatin driven alc gene construct allowing tight chemical control of gene expression following supply of acetaldehyde with minimal pleiotropic effects of the inducing agent on metabolism. This construct was used for chemical induction of the yeast invertase gene after 10-weeks growth to dissect the complex metabolic phenotype obtained after constitute expression. Inducible expression led to increased invertase activity within 24 h in well-defined areas within growing tubers. Although the sucrose levels were reduced, there was no effect on the levels of starch whilst levels of many amino acids decreased. Labelling experiments revealed that these lines exhibited increased rates of sucrose cycling, whereas rates of glycolysis and of starch synthesis were not substantially changed. From these results we conclude that sucrose cycling is stimulated in response to a short-term increase in the rate of sucrose mobilisation, providing evidence for a role of sucrose cycling as a buffering capacity that regulates the net rate of sucrose usage. In contrast, the dramatic increase in hexose-phosphate levels and the switch from starch synthesis to respiration seen on the constitutive expression of the invertase was not observed in the inducible lines, suggesting that this is the result of cumulative pleiotropic effects that occurred when the transgene was expressed throughout development.

EST-analysis of the thermo-acidophilic red microalga Galdieria sulphuraria reveals potential for lipid A biosynthesis and unveils the pathway of carbon export from rhodoplasts

When we think of extremophiles, organisms adapted to extreme environments, prokaryotes come to mind first. However, the unicellular red micro-alga Galdieria sulphuraria (Cyanidiales) is a eukaryote that can represent up to 90% of the biomass in extreme habitats such as hot sulfur springs with pH values of 0-4 and temperatures of up to 56 degrees C. This red alga thrives autotrophically as well as heterotrophically on more than 50 different carbon sources, including a number of rare sugars and sugar alcohols. This biochemical versatility suggests a large repertoire of metabolic enzymes, rivaled by few organisms and a potentially rich source of thermo-stable enzymes for biotechnology. The temperatures under which this organism carries out photosynthesis are at the high end of the range for this process, making G. sulphuraria a valuable model for physical studies on the photosynthetic apparatus. In addition, the gene sequences of this living fossil reveal much about the evolution of modern eukaryotes. Finally, the alga tolerates high concentrations of toxic metal ions such as cadmium, mercury, aluminum, and nickel, suggesting potential application in bioremediation. To begin to explore the unique biology of G. sulphuraria , 5270 expressed sequence tags from two different cDNA libraries have been sequenced and annotated. Particular emphasis has been placed on the reconstruction of metabolic pathways present in this organism. For example, we provide evidence for (i) a complete pathway for lipid A biosynthesis; (ii) export of triose-phosphates from rhodoplasts; (iii) and absence of eukaryotic hexokinases. Sequence data and additional information are available at http://genomics.msu.edu/galdieria.

Heterologous expression of a ketohexokinase in potato plants leads to inhibited rates of photosynthesis, severe growth retardation and abnormal leaf development

In the present paper we investigated the effect of heterologous expression of a rat liver ketohexokinase in potato (Solanum tuberosum L.) plants with the aim of investigating the role of fructose 1-phosphate in plant metabolism. Plants were generated that contained appreciable activity of ketohexokinase but did not accumulate fructose 1-phosphate. They were, however, characterised by a severe growth retardation and abnormal leaf development. Studies of (14)CO(2) assimilation and metabolism, and of the levels of photosynthetic pigments, revealed that these lines exhibited restricted photosynthesis. Despite this fact, the levels of starch and soluble sugars remained relatively constant. Analysis of intermediates of starch and sucrose biosynthesis revealed large increases in the triose phosphate and fructose 1,6-bisphosphate pools but relatively unaltered levels of inorganic phosphate and 3-phosphoglycerate, and these lines were also characterised by an accumulation of glyceraldehyde. The transformants neither displayed consistent changes in the activities of Calvin cycle enzymes nor in enzymes of sucrose synthesis but displayed a metabolic profile partially reminiscent of that brought about by end-product limitation, but most likely caused by an inhibition of photosynthesis brought about by the accumulation of glyceraldehyde. Analysis of the metabolite contents in lamina and vein fractions of the leaf, and of the enzymes of carbohydrate oxidation indicate that the phloem-enriched veins of ketohexokinase-expressing leaves tend toward hypoxia and indicate a problem of phloem transport.

Expression of a bacterial xylose isomerase in potato tubers results in an altered hexose composition and a consequent induction of metabolism

Here we investigate the role of hexoses in the metabolism of the developing potato (Solanum tuberosum) tuber by the expression of a bacterial xylose isomerase which catalyzes the interconversion of glucose and fructose. Previously, we found that glycolysis was induced in transgenic tubers expressing a yeast invertase in the cytosol and postulated that this was due either to the decreased levels of sucrose or to effects downstream of the sucrose cleavage. In the present study xylose isomerase was expressed under the control of the tuber-specific patatin promoter. Selected transformants exhibited minor changes in the levels of tuber glucose and fructose but not in sucrose. Analysis of the enzyme activities of the glycolytic pathway revealed minor yet significant increases in the maximal catalytic activities of aldolase and glyceraldehyde 3-phosphate dehydrogenase but no increase in the activities of other enzymes of glycolysis. These lines were also characterized by an elevated tuber number, glycolytic and sucrose synthetic fluxes and in some metabolite levels downstream of glycolysis. When considered together these data suggest that the perturbation of hexose levels can result in increased glycolytic and sucrose (re)synthetic fluxes in the potato tuber even in the absence of changes in the level of sucrose. The consequences of altering hexose levels in the tuber are, however, not as severe as those observed following perturbation of the level of tuber sucrose.

A novel type of chloroplast stromal hexokinase is the major glucose-phosphorylating enzyme in the moss Physcomitrella patens

Hexokinase catalyzes the first step in the metabolism of glucose but has also been proposed to be involved in sugar sensing and signaling both in yeast and in plants. We have cloned a hexokinase gene, PpHXK1, in the moss Physcomitrella patens where gene function can be studied directly by gene targeting. PpHxk1 is a novel type of chloroplast stromal hexokinase that differs from previously studied membrane-bound plant hexokinases. Enzyme assays on a knock-out mutant revealed that PpHxk1 is the major glucose-phosphorylating enzyme in Physcomitrella, accounting for 80% of the total activity in protonemal tissue. The mutant is deficient in the response to glucose, which in wild type moss induces the formation of caulonemal filaments that protrude from the edge of the colony. Growth on glucose in the dark is strongly reduced in the mutant. Sequence data suggest that most plants including Physcomitrella and Arabidopsis have both chloroplast-imported hexokinases similar to PpHxk1 and traditional membrane-bound hexokinases. We propose that the two types of plant hexokinases have distinct physiological roles.

Metabolic profiling of transgenic tomato plants overexpressing hexokinase reveals that the influence of hexose phosphorylation diminishes during fruit development

We have conducted a comprehensive metabolic profiling on tomato (Lycopersicon esculentum) leaf and developing fruit tissue using a recently established gas chromatography-mass spectrometry profiling protocol alongside conventional spectrophotometric and liquid chromatographic methodologies. Applying a combination of these techniques, we were able to identify in excess of 70 small-M(r) metabolites and to catalogue the metabolite composition of developing tomato fruit. In addition to comparing differences in metabolite content between source and sink tissues of the tomato plant and after the change in metabolite pool sizes through fruit development, we have assessed the influence of hexose phosphorylation through fruit development by analyzing transgenic plants constitutively overexpressing Arabidopsis hexokinase AtHXK1. Analysis of the total hexokinase activity in developing fruits revealed that both wild-type and transgenic fruits exhibit decreasing hexokinase activity with development but that the relative activity of the transgenic lines with respect to wild type increases with development. Conversely, both point-by-point and principal component analyses suggest that the metabolic phenotype of these lines becomes less distinct from wild type during development. In summary, the data presented in this paper demonstrate that the influence of hexose phosphorylation diminishes during fruit development and highlights the importance of greater temporal resolution of metabolism.

Evidence that SNF1-related kinase and hexokinase are involved in separate sugar-signalling pathways modulating post-translational redox activation of ADP-glucose pyrophosphorylase in potato tubers

We recently discovered that post-translational redox modulation of ADP-glucose pyrophosphorylase (AGPase) is a powerful new mechanism to adjust the rate of starch synthesis to the availability of sucrose in growing potato tubers. A strong correlation was observed between the endogenous levels of sucrose and the redox-activation state of AGPase. To identify candidate components linking AGPase redox modulation to sugar supply, we used potato tuber discs as a model system. When the discs were cut from growing wild-type potato tubers and incubated for 2 h in the absence of sugars, redox activation of AGPase decreased because of a decrease in internal sugar levels. The decrease in AGPase redox activation could be prevented when glucose or sucrose was supplied to the discs. Both sucrose uptake and redox activation of AGPase were increased when EDTA was used to prepare the tuber discs. However, EDTA treatment of discs had no effect on glucose uptake. Feeding of different glucose analogues revealed that the phosphorylation of hexoses by hexokinase is an essential component in the glucose-dependent redox activation of AGPase. In contrast to this, feeding of the non-metabolisable sucrose analogue, palatinose, leads to a similar activation as with sucrose, indicating that metabolism of sucrose is not necessary in the sucrose-dependent AGPase activation. The influence of sucrose and glucose on redox activation of AGPase was also investigated in discs cut from tubers of antisense plants with reduced SNF1-related protein kinase activity (SnRK1). Feeding of sucrose to tuber discs prevented AGPase redox inactivation in the wild type but not in SnRK1 antisense lines. However, feeding of glucose leads to a similar activation of AGPase in the wild type and in SnRK1 transformants. AGPase redox activation was also increased in transgenic tubers with ectopic overexpression of invertase, containing high levels of glucose and low sucrose levels. Expression of a bacterial glucokinase in the invertase-expressing background led to a decrease in AGPase activation state and tuber starch content. These results show that both sucrose and glucose lead to post-translational redox activation of AGPase, and that they do this by two different pathways involving SnRK1 and an endogenous hexokinase, respectively.

Overexpression of the sucrose transporter SoSUT1 in potato results in alterations in leaf carbon partitioning and in tuber metabolism but has little impact on tuber morphology

The aim of this work was to examine the consequences of the heterologous expression of a spinach ( Spinacia oleracea L.) sucrose transporter ( SoSUT1) in potato ( Solanum tuberosum L.). Many studies have indicated that reduction of the expression of this class of sucrose transporter has deleterious effects on plant growth and development; however, until now the possibility of improving plant performance by enhancing the expression of this sucrose transporter has not been reported. With this intention we constructed a chimeric construct in which SoSUT1 was cloned in-frame with the myc epitope. We confirmed that this construct, SoSUT1m, was able to mediate sucrose transport by expression in the yeast strain SUSY7. SoSUT1m was expressed in wild-type potato in the sense orientation under the control of the cauliflower mosaic virus 35S promoter to evaluate the effect of an increased constitutive expression of a class-I sucrose transporter. We confirmed that these plants displayed expression of SoSUT1 at both the transcript and protein level and that microsomal fragments isolated from selected lines had an increased sucrose uptake capacity. Analysis of metabolism of these lines indicated that the leaves were characterised by a reduced sucrose level yet exhibited little change in photosynthetic rate. Furthermore, despite the observed increase in sugar (and reduction in amino acid) levels within the tubers, there was little change in either starch content or tuber yield in the transformants. In summary, the genetic manipulation described in this paper resulted in a shift in carbon partitioning in both leaves and tubers and an increased sucrose uptake rate in plasma-membrane vesicles isolated from these lines, but had little impact on tuber metabolism or morphology.

Potato hexokinase 2 complements transgenic Arabidopsis plants deficient in hexokinase 1 but does not play a key role in tuber carbohydrate metabolism

Potato plants (Solanum tuberosum L. cv. Désirée) transformed with sense and antisense constructs of a cDNA encoding the potato hexokinase 2 exhibited altered enzyme activities and expression of hexokinase 2 mRNA. Measurements of the maximum catalytic activity of hexokinase revealed an 11-fold variation in leaf (from 48% of the wild-type activity in antisense transformants to 446% activity in sense transformants) and an 8-fold variation in developing tubers (from 35% of the wild-type activity in antisense transformants to 212% activity in sense transformants). Despite the wide range of hexokinase activities, no substantial change was found in the fresh weight yield, starch, sugar and metabolite levels of transgenic tubers. However, both potato hexokinases 1 and 2 were able to complement the hyposensitivity of antisense hexokinase 1 Arabidopsis transgenic plants to glucose. In an in vitro bioassay of seed germination in a medium with high glucose levels, double transformants showed the same sensitivity to glucose as that of the wild-type ecotype, displaying a stunted phenotype in hypocotyls, cotyledons and roots.

Distinct physiological roles of fructokinase isozymes revealed by gene-specific suppression of Frk1 and Frk2 expression in tomato

There are two divergent fructokinase isozymes, Frk1 and Frk2 in tomato (Lycopersicon esculentum Mill.) plants. To investigate the physiological functions of each isozyme, the expression of each fructokinase mRNA was independently suppressed in transgenic tomato plants, and the respective phenotypes were evaluated. Suppression of Frk1 expression resulted in delayed flowering at the first inflorescence. Suppression of Frk2 did not effect flowering time but resulted in growth inhibition of stems and roots, reduction of flower and fruit number, and reduction of seed number per fruit. Localization of Frk1 and Frk2 mRNA accumulation by in situ hybridization in wild-type tomato fruit tissue indicated that Frk2 is expressed specifically in early tomato seed development. Fruit hexose and starch content were not effected by the suppression of either Frk gene alone. The results collectively indicate that flowering time is specifically promoted by Frk1 and that Frk2 plays specific roles in contributing to stem and root growth and to seed development. Because Frk1 and Frk2 gene expression was suppressed individually in transgenic plants, other significant metabolic roles of fructokinases may not have been observed if Frk1 and Frk2 play, at least partially, redundant metabolic roles.

Distinct physiological roles of fructokinase isozymes revealed by gene-specific suppression of Frk1 and Frk2 expression in tomato

There are two divergent fructokinase isozymes, Frk1 and Frk2 in tomato (Lycopersicon esculentum Mill.) plants. To investigate the physiological functions of each isozyme, the expression of each fructokinase mRNA was independently suppressed in transgenic tomato plants, and the respective phenotypes were evaluated. Suppression of Frk1 expression resulted in delayed flowering at the first inflorescence. Suppression of Frk2 did not effect flowering time but resulted in growth inhibition of stems and roots, reduction of flower and fruit number, and reduction of seed number per fruit. Localization of Frk1 and Frk2 mRNA accumulation by in situ hybridization in wild-type tomato fruit tissue indicated that Frk2 is expressed specifically in early tomato seed development. Fruit hexose and starch content were not effected by the suppression of either Frk gene alone. The results collectively indicate that flowering time is specifically promoted by Frk1 and that Frk2 plays specific roles in contributing to stem and root growth and to seed development. Because Frk1 and Frk2 gene expression was suppressed individually in transgenic plants, other significant metabolic roles of fructokinases may not have been observed if Frk1 and Frk2 play, at least partially, redundant metabolic roles.

Downregulation of a chloroplast-targeted beta-amylase leads to a starch-excess phenotype in leaves

A functional screen in Escherichia coli was established to identify potato genes coding for proteins involved in transitory starch degradation. One clone isolated had a sequence very similar to a recently described chloroplast-targeted beta-amylase of Arabidopsis. Expression of the gene in E. coli showed that the protein product was a functional beta-amylase that could degrade both starch granules and solubilized amylopectin, while import experiments demonstrated that the beta-amylase was imported and processed into pea chloroplasts. To study the function of the protein in transitory starch degradation, transgenic potato plants were generated where its activity was reduced using antisense techniques. Analysis of plants reduced in the presence of this beta-amylase isoform showed that their leaves had a starch-excess phenotype, indicating a defect in starch degradation. In addition, it was shown that the antisense plants degraded only 8-30% of their total starch, in comparison with 50% in the wild type, over the dark period. This is the first time that a physiological role for a beta-amylase in plants has been demonstrated.

Characterization of native and yeast-expressed tomato fruit fructokinase enzymes

Three fructokinase isozymes (FKI, FKII, FKIII) were separated from both immature and ripe tomato fruit pericarp. All three isozymes were specific for fructose with undetectable activity towards glucose or mannose. The three isozymes could be distinguished from one another with respect to response to fructose, Mg and nucleotide donor concentrations and this allowed the comparison of the fruit enzymes with the gene products of the two known cloned tomato fructokinase genes, LeFRK1 and LeFRK2. FKI was characterized by both substrate (fructose), as well as Mg, inhibition; FKII was inhibited by neither fructose nor Mg; and FKIII was inhibited by fructose but not by Mg. ATP was the preferred nucleotide donor for all three FKs and FKI showed inhibition by CTP and GTP above 1 mM. All three FKs showed competitive inhibition by ADP. During the maturation of the tomato fruit total FK activity decreased dramatically. There were decreases in activity of all three FKs, nevertheless, all were still observed in the ripe fruit. The two tomato LeFRK genes were expressed in yeast and the gene products were characterized with respect to the distinguishing characteristics of fructose, Mg and nucleotide inhibition. Our results indicate that FKI is the gene product of LeFRK2 and FKII is probably the gene product of LeFRK1.

Analysis of the compartmentation of glycolytic intermediates, nucleotides, sugars, organic acids, amino acids, and sugar alcohols in potato tubers using a nonaqueous fractionation method

The compartmentation of metabolism in heterotrophic plant tissues is poorly understood due to the lack of data on metabolite distributions and fluxes between subcellular organelles. The main reason for this is the lack of suitable experimental methods with which intracellular metabolism can be measured. Here, we describe a nonaqueous fractionation method that allows the subcellular distributions of metabolites in developing potato (Solanum tuberosum L. cv Desiree) tubers to be calculated. In addition, we have coupled this fractionation method to a recently described gas chromatography-mass spectrometry procedure that allows the measurement of a wide range of small metabolites. To calculate the subcellular metabolite concentrations, we have analyzed organelle volumes in growing potato tubers using electron microscopy. The relative volume distributions in tubers are very similar to the ones for source leaves. More than 60% of most sugars, sugar alcohols, organic acids, and amino acids were found in the vacuole, although the concentrations of these metabolites is often higher in the cytosol. Significant amounts of the substrates for starch biosynthesis, hexose phosphates, and ATP were found in the plastid. However, pyrophosphate was located almost exclusively in the cytosol. Calculation of the mass action ratios of sucrose synthase, UDP-glucose pyrophosphorylase, phosphoglucosisomerase, and phosphoglucomutase indicate that these enzymes are close to equilibrium in developing potato tubers. However, due to the low plastidic pyrophosphate concentration, the reaction catalyzed by ADP-glucose pyrophosphorylase was estimated to be far removed from equilibrium.