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Matilda filled bars and Freja open bars. Seeds were harvested at different developmental stages after a 48 h incubation with 14 C-labelled sucrose. Proportions of 14 C accumulation in different lipid classes in seeds developed in vitro on detached oat panicles of cv. Freja open bars at different developmental stages. Matilda compared to cv. Matilda filled bars and Freja open bars at different developmental stages. Data sets are displayed as overview maps to illustrate the differences found in carbon partitioning into different components of oat seeds. The first map comparing 14 C accumulation in whole seeds of the two cultivars at the very early and at the late stage of development Fig.

Matilda compared with cv. The second map comparing 14 C accumulation in different parts of the seeds at stage E represents the latest stage of development where the two cultivars still show differences in 14 C accumulation in endosperm lipids before maturation Fig. Relative incorporation of 14 C-sucrose into components of oat seeds of high-oil cv. Freja at early C and late G stages of development.

Carbohydrate Reserves in Plants - Synthesis and Regulation (eBook)

Thicknesses of arrows represent sizes of incorporation. Values are mean values for three samples. Matilda a and medium-oil cv. Freja b at development stage E. Redirection of carbon flux from starch to oil in the cereal seed can provide for new high-yielding oil crops. Using oat as a model system, we would enhance our understanding of carbon partitioning from starch to oil in a cereal endosperm. Such studies require a method of studying these metabolic fluxes with radioactive and stable isotope labelling.

This research details the successful implementation of an in vitro culture system for oat seeds that allows the examination of metabolic flux throughout development. The in vitro system used in this study utilizes detached oat panicles fed with sucrose and nutrient solution through the transpiration stream. As a comparison, the in vivo sucrose concentration in the crease phloem of developing wheat grains is approximately — mM Fisher and Wang, The seed filling was somewhat less than that reported previously in similar growth systems for Oryza sativa Lee et al.

However, and more importantly, oat seeds in vitro displayed comparable developmental growth rates and cultivar differences in oil accumulation as those in planta.

1. Introduction

Oil concentrations, both in vitro and in planta, were also comparable to the levels detected under greenhouse and field conditions Banas et al. Cultivar differences in starch concentrations were not observed under our growth conditions in planta or on detached panicles. This is in contrast to previous data, where cv. Freja accumulated a higher starch level as compared to cv. Matilda under greenhouse Banas et al. This inverse relationship between starch and oil levels in oat seeds was also shown in the field in a selection of 25 oat lines with elevated oil levels Peterson and Wood, It is probable that the environmental conditions used in this study promoted a shift in sucrose import rate into the cv.

Matilda seeds compared to greenhouse or field conditions. However, the weight ratios between starch and oil in cvs Matilda and Freja are only marginally changed depending on growth condition cv. This finding clearly illustrates that the cultivar differences in partitioning of carbon between starch and oil are present under our experimental conditions.

A potential explanation on why the in vitro -grown seeds did not reach seed weight levels of those achieved in planta may be due to decreased starch deposition during the later stages of the in vitro development. Oil accumulation that peaks during the early stages of development was unchanged in vitro as compared to in planta conditions. However, the in vitro system can be regarded as a reasonable qualitative, though not quantitative, model system for storage filling throughout seed development.

The in vitro system of oat seed development on detached oat panicles was used for the first time in analyses of carbon partitioning between lipids and non-lipids in a cereal seed. Cultivar differences clearly show that the high-oil cv.

Hexose Kinases and Their Role in Sugar-Sensing and Plant Development

The proportion of 14 C recovered in lipids relative to total seed incorporation decreased in both cultivars with increasing maturity. This is in agreement with a study where oat plants were fed 14 CO 2 at different developmental stages Beringer, If a possible turnover of lipids is disregarded, the data suggest that the proportion of sugar channelled towards lipid synthesis contributes to the double amount of lipids observed in the mature seeds of cv.

Matilda as compared to cv. Whether cultivar differences in the turnover of endosperm- specific lipids also play a role in the final levels of oil stored in the endosperm remains to be elucidated, although turnover of lipids in oat seeds during development has been suggested Banas et al. The turnover of lipids has been characterized in dicot oil seeds during normal seed development Poirier et al. Nevertheless, the main part of 14 C accumulation in lipids is still carried out in the endosperm, but with differences between cultivars: Differences in lipid accumulation between cultivars were not only found at the level of carbon partitioning to total lipids, but also at the level of carbon partitioning to different lipid classes.

In fact, the basis for the different oil content between the two cultivars, if not taking turnover of lipids into account, is because cv. Matilda incorporates more carbon into the seeds and also channels a substantially higher proportion of that carbon into lipids. At the very early developmental stage cv. The ratio of carbon accumulation in TAG relative to other lipids decreased in both cultivars at the later stages of development, establishing altered oil metabolism in matured seeds.

However, proportions of carbon accumulation in TAG relative to other lipids in the endosperm still differed between cultivars at the earlier stage of development when seeds were still green. These differences were minimized to almost equal levels at the stage when seeds were almost yellow. It should be noted that the proportion of 14 C found in TAG compared to other lipids was considerably less than what was found by mass analysis at all stages Banas et al.

How plants manage food reserves at night: quantitative models and open questions

The most plausible explanation for this is that the 48 h labelling may not be long enough to generate a steady-state labelling pattern since there is a flux of fatty acids to TAG via polar lipids Stymne and Stobart, The regulation of oil synthesis in developing seeds can occur at multiple levels in the biochemical conversion of photosynthetically fixed carbon into TAG.

Sucrose transport in conjunction with the regulatory mechanisms in the endosperm play a central role in determining the final amount of oil in the endosperm. Sucrose from source organs is imported into the seed and degraded to pyruvate through glycolysis, both in the cytosol and in the plastid Plaxton and Podesta, Pyruvate subsequently enters de novo fatty acid synthesis in the plastid and finally is acylated to the glycerol backbone in the endoplasmatic reticulum to give rise to TAG.

Starch is synthesized in the plastid from hexose sugars upstream in glycolysis with less metabolic cost for the plant as compared to oil synthesis. This raises the following questions: Matilda exhibited increased sink strength under our experimental conditions as suggested by the higher sucrose incorporation together with the higher energy content contained in the accumulated oil plus starch as compared to cv.

This may be, in part, due to alterations in enzyme activities involved with sucrose cleavage in the endosperm. However, this difference in total energy content of oil plus starch is not seen in these cultivars grown under greenhouse conditions, regardless of those differences seen in oil content Banas et al. It is therefore likely that the most upstream biochemical component responsible for the difference in oil content between the cultivars is in the competition in sugar utilization between glycolysis and synthesis of starch and other sugar polymers in the endosperm cells.

Attempts to increase oil production of oil seeds include the following alternative approaches: ACC catalysis is the first committed step in fatty acid biosynthesis. However, this step is far downstream from the utilization of glucose for starch synthesis, which makes it unlikely to be the single determining enzyme for the switch from starch to oil synthesis in the endosperm cells. Transcription factors regulating oil synthesis in plants are promising targets to engineer crops to produce higher oil yields.

Similar results have been reported for A. Other examples are the dof-type transcription factors from soybean Glycince max GmDof4 and GmDof11 , that by overexpression in A. In this study, the high accumulation of carbon in oil found in the high-oil oat cv. Matilda compared to the medium-oil oat cv.

Global gene expression studies using novel methods for ultra-deep EST sequencing of oat endosperm Schuster, is likely to identify key events regarding gene expression as well as to identify enzymatic pathways regulating the carbon flux into oil in oat endosperm. These studies, in combination with metabolic flux analyses and the silencing of identified genes through antisense ODN Sun et al. It is this understanding that will enable us to engineer enhanced pathways into plants that have the potential to redirect fixed carbon from photosynthesis into oil.

Supplementary data are available at JXB online. Definitions of stages A-J of seed development in planta. We would also like to thank Mr Hassan Hartman for initial studies of the in vitro method for detached oat panicles. National Center for Biotechnology Information , U. This article has been corrected. See J Exp Bot. This article has been cited by other articles in PMC. Abstract Cereals accumulate starch in the endosperm as their major energy reserve in the grain. However, there are experimental clues on where they could be acting.

The addition of phosphate groups is thought to allow access for starch degrading enzymes, with subsequent removal of the phosphate groups needed for full starch polymer degradation. This hypothesis is supported by evidence that the level of phosphate per unit mass of starch is not constant during the day, but rather reaches a maximum around dusk, in dynamics which resembles the behavior predicted for the S molecule concentration Scialdone et al.

Whether the starch phosphate content mirrors the total amount of starch, or the time to expected dawn, is a question that can be clarified using model predictions. According to the model, in plants that are subjected to an unexpected increase in light intensity, the starch phosphate levels would reach a higher value with respect to the control if the starch phosphate levels were proportional to starch reserves.

However, the phosphate levels would be unaltered relative to the control if starch phosphate levels were alternatively proportional to the time to expected dawn.


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Another interesting result concerns the phenotype of the mutant lacking PWD. Such a mutant maintains linear starch degradation with time during the night, but is not able to adjust the degradation rate to an unexpectedly early night. This result suggests that PWD could be part of the molecular machinery that sets the degradation rate Scialdone et al. Additional experiments will be required to test the behavior of the pwd mutant under different conditions e. The models could then help to interpret these results in order to uncover the exact role of this enzyme. Conversely, GWD has been shown to have little influence over the control of flux in starch degradation, and, in particular, its redox regulation is not necessary for the computation of the appropriate starch degradation rate Skeffington et al.

Alternative hypotheses have recently been investigated into how the circadian clock exerts control over starch turnover. The analysis combined models of the Arabidopsis circadian clock with the arithmetic division model described above Seaton et al. The first question posed was how information about the time to expected dawn is encoded in the dynamics of the T molecule, and, in turn, the role of the T molecule in the starch degradation process.

Another possibility is that the T concentration increases with time during the day, approximately tracking the time since last dawn. In this case, in order to implement arithmetic division between starch levels and the time to dawn, T must promote starch degradation Scialdone et al. Possible regulatory schemes capable of producing these T dynamics by using circadian clock components are shown in Seaton et al.

Models with a light-gated regulation of starch degradation, where the rate is set at each light-dark transition were also analyzed, as opposed to models with a continuous computation of the starch degradation rate throughout the night. These different scenarios were combined together in model variants, also considering the possibility that starch synthesis might be under the control of the circadian clock.

All the model variants explored in Seaton et al. The main differences between them consisted of the phenotypes predicted in the presence of circadian clock mutations. The general conclusion was that models with a light-gated control of starch degradation are more robust to circadian clock mutations than models based on a continuous calculation of the starch degradation rate.

On the other hand, if the degradation rate is fixed at dusk, it would not be possible to adjust for fluctuations in reaction rates that could occur at night and cause too rapid or too slow a starch turnover. Robustness to both circadian clock mutations and noise in reaction rates could be combined in a mechanism with an intermediate level of clock control, where the degradation rate is still continuously computed but the T molecule is controlled by the clock only during the day and decays linearly with time at night Seaton et al.

One possible way to rule out a pure light-gated response, would be to test for the presence of non-linear starch profiles at night in the wild-type. Even though some experiments hint at non-linear dynamics, especially around the time of expected dawn Scialdone et al. Several lines of evidence suggest concerted control of both starch degradation and synthesis. In long days, a comparatively slower starch synthesis rate is associated with more rapid starch degradation, with the opposite pattern occurring in short days Gibon et al.

Moreover, plants that run out of starch after an extended night, synthesize more starch during the next day Gibon et al. A recent phenomenological model has investigated this issue, employing sinusoidal cycles governed by the circadian clock for the rates of sucrose export, starch degradation and starch synthesis. This is the form generated by the arithmetic division model through the interaction between the S and T molecules.

Sucrose starvation was also analyzed when one or two among the three rates were constant, where it was shown that regulation of the starch degradation rate was the most important to achieve minimization of sucrose starvation Feugier and Satake, Although these conclusions are potentially interesting, the initial assumptions of this model may be too simplistic, as there is little good evidence that the starch synthesis rate is directly under the control of the circadian clock.

Indeed, the partitioning of photoassimilate between starch and sucrose is likely to be controlled by multiple mechanisms that may be quite distinct from those ensuring a constant supply of sucrose from starch at night Mugford et al. Moreover, it seems unlikely that, as speculated in Feugier and Satake , plants can adjust to different photoperiods through alteration of the phases of clock-dependent metabolic rates, given observations suggesting that there is little change in clock components' phases with photoperiod Edwards et al.

A model exploring the molecular mechanisms that could couple starch degradation to other processes in the plant's metabolism has recently been proposed Pokhilko et al. This model includes several modules describing metabolism, the circadian clock and interactions between the two. While starch degradation is described by the arithmetic division model, the existence of a molecule I is hypothesized, which controls the fraction of photosynthate allocated to starch synthesis.

The levels of this molecule I are assumed to increase with increasing levels of carbon starvation. In this picture, since regulation of the starch degradation rate imposes a depletion of carbon reserves at the time of expected dawn, an unexpectedly prolonged night would be accompanied by a period of starvation and a consequent increase in the levels of the I molecule. This would then induce a higher starch synthesis rate during the following light period, as experimentally observed see above and Gibon et al.

The model predicts the characteristics and behaviors of I and of the other presumptive molecular regulators under a number of conditions. Comparisons with experimental data then allowed identification of candidates for some of these molecules Pokhilko et al.

SnRK1, a kinase that plays an important role in the sucrose synthesis pathway by modulating activity of sucrose-phosphate synthase and F26P phosphatase, was identified as a candidate for I. This prediction could be validated experimentally by, e. However, how the levels of these proteins and not just transcripts change, and the role of their post-translational modifications, would need to be assessed in order to confirm these putative functions.

Regulation of carbohydrate availability at night is a crucial aspect of plant metabolism. This is particularly true for annual species like Arabidopsis , which complete their life cycle in a year and are under strong selective pressure to optimize carbon utilization for the maximization of seed production.

In such a mechanism a central role is played by the circadian clock, and there are suggestions that the regulation of starch degradation is the clock's major contribution to the optimization of plant productivity Graf and Smith, Mathematical modeling is helping to unravel the molecular mechanisms that control starch degradation by generating hypotheses and testable predictions. This approach has been particularly useful in providing a starting point for future experiments for a process whose underlying mechanism was previously completely mysterious.

The arithmetic division computation that produces the appropriate starch degradation rate is thought to occur at a post-translational level. Hence it will probably be necessary to analyse the dynamics of the relevant post-translational modifications in order to shed light on the identity of the molecules that implement the computation Skeffington et al. Models could then be refined to incorporate additional biochemical details to aid the interpretation and design of new experiments. However, some model predictions are already supported by experiments, such as the response to previously untested environmental perturbations.

Mutants that accumulate abnormally high e. In particular, the starch degradation pattern of some starch-excess mutants was used to confirm predictions from the models see above and Scialdone et al. Other predictions yet to be tested will help to elucidate important questions, such as whether the computation of the starch degradation rate is a one-off event occurring at dusk or whether it happens continuously during the night, and what is the role for cycles of starch phosphate content. Models have also helped to identify molecules that could have important regulatory functions e.

There have also been efforts to combine models for starch degradation and other aspects of plant metabolism with models at higher scales e. The trehalose metabolic pathway has emerged as an important regulatory mechanism in plants, affecting sugar metabolism and plant growth Paul et al. Trehalose cleavage as a source of glucose sensed by HXK might seem plausible as the involvement of a trehalose pathway in sensing sucrose level and sugar status of the cell has been suggested Paul et al.

Yet, the effects of trehalose on plant growth and sugar metabolism occur independently of the expression level of AtHXK1 , perhaps eliminating trehalose as a potential source of the glucose sensed by HXK Ramon et al. The use of a high concentration of glucose to obtain sugar-sensing effects has raised concerns about the physiological relevance of these assays Leon and Sheen, ; Rook and Bevan, Unlike what was observed in the Arabidopsis study, tomato plants expressing AtHXK1 exhibited sugar-sensing effects when grown in soil under natural growth conditions independent of exogenous sugar Dai et al.

It has been hypothesized that due to the growth inhibition effects of AtHXK1 , transgenic Arabidopsis plants with high levels of AtHXK1 expression were discriminated against throughout the transformation selection procedure, in favor of plants with lower levels of AtHXK1 expression. As a result, only transgenic plants with moderate expression of AtHXK1 were selected and, therefore, a high level of exogenous sugar was required to obtain a sugar-sensing response.

To examine this hypothesis, Arabidopsis plants were transformed with AtHXK1 and poorly growing kanamycin-resistant transformants were isolated. Indeed, several independent new transformants with high levels of AtHXK1 expression exhibited classical sugar-sensing effects independent of exogenous sugar, alleviating the concern about the physiological relevance of these assays Kelly et al.

These new transgenic lines, together with the tomato lines that express high levels of AtHXK1 , provide a way to study the role of AtHXK1 at all developmental stages and in all plant organs and tissues. The growth-arrest phenotype of Arabidopsis seedlings observed in the presence of exogenous sugars enabled the isolation of a number of sugar-insensitive and sugar-hypersensitive mutants.

Characterization of these mutants revealed connections between sugar and plant hormone signaling pathways reviewed in Leon and Sheen, ; Rolland et al. In addition, exogenous glucose increased the expression of ABA-synthesis and signaling genes, as well as endogenous ABA levels Cheng et al.

Introduction

Yet, it is not known whether ABA biosynthesis is directly involved in the sugar signal transduction cascade or indirectly stimulated by sugars, modulating sugar-responsiveness Ramon et al. Unlike ABA, ethylene was shown to act in an antagonistic manner to glucose responses. The ethylene precursor 1-aminocyclopropanecarboxylic acid ACC prevented inhibition of cotyledon greening and seedling development at high concentrations of glucose in wild type seedlings Zhou et al.

A glucose insensitive gin phenotype was also displayed in constitutive ethylene biosynthesis eto1 and constitutive ethylene signaling ctr1 mutants Zhou et al. Lastly, the ethylene-insensitive mutants etr , ein2 , ein3 as well as mkk9 exhibit glucose hypersensitivity Ramon et al. Thus, ethylene acts as an antagonist of the glucose response while ABA promotes it. Therefore, it has been suggested that early seedling developmental arrest in glucose is mediated by AtHXK1 independent of ABA and ethylene. The molecular mechanism of the AtHXK1 -mediated sugar-sensing is not known.

This complex can bind the promoters of specific genes, such as CAB2 , and may modulate their expression independent of glucose metabolism. It is likely that a conserved glucose binding site on AtHXK1 acts as a sensor and responds directly to the presence of glucose. Whether glucose promotes the transport of AtHXK1 to the nucleus or facilitates the complex formation has not been examined. In addition to the original sugar-sensing roles of AtHXK1 e. These effects were observed when AtHXK1 was expressed under the global promoter 35S; whether native expression levels of AtHXK1 in various plants parts and tissues also regulate these physiological responses remains to be addressed.

Yet, transgenic Arabidopsis plants expressing the two catalytically inactive AtHXK1 mutant alleles GD and SA in the gin2 null mutant background displayed substantial leaf expansion when grown in soil in the presence of intense light. Therefore, it has been suggested that catalytically inactive AtHXK1 mutants support both growth-inhibiting and growth-promoting roles of AtHXK1 under different growth conditions Moore et al. However, these results differ from those presented of recent work performed with mature Arabidopsis plants grown in soil, in which high levels of AtHXK1 expression suppressed auxin-response genes Kelly et al.

These opposite effects might be due to the different developmental stages examined in the two experiments. Transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited growth inhibition and reduced expression of photosynthetic genes in response to glucose treatment Cho et al. This is the first evidence of a non-mitochondria-associated HXK that is involved in sugar-sensing.

It would be interesting to test whether other cytosolic HXKs in monocots and mosses play similar roles. In Physcomitrella , the plastidic PpHXK1 was found to regulate development by controlling the type of filamentous gametophyte formed. Mosses have two types of filaments, chloronemata cells, which are photosynthetically active, and caulonemata cells, which spread the colony.

Using a knockout hxk1 mutant, Olsson et al. This shift is reversed when the energy supply is limited. It has also been suggested that HXKs may prevent programmed cell death Kim et al. HXKs may also be involved in biotic and abiotic stress responses Claeyssen and Rivoal, ; Sarowar et al. Yet the question of which of the above roles are related to sugar-sensing and which are the result of the metabolic catalytic functions of HXK remains to be studied. The lack of FRK plant mutants may suggest that FRK genes are either essential or have highly redundant functions under normal growth conditions.

Consequently, information about the function of FRK isozymes has been gathered mostly from transgenic plants and through the examination of gene expression profiles under different growth conditions. It has been proposed that FRKs affect starch accumulation in different plant species, including tomato Schaffer and Petreikov, b. Instead, FRK2 was found to be essential for vascular development German et al.

Reduced expression of FRK2 in antisense plants resulted in deformed vasculature, smaller cell size in the xylem and phloem, reduced cambium activity and secondary walls in vessels, and small sieve elements with low levels of callose deposition Damari-Weissler et al. UDP-G may be used for cellulose and cell wall synthesis, while phosphorylated fructose can be utilized for energy production or fed into other metabolic pathways.

SUS is feedback-inhibited by its product, fructose, when the concentration of fructose exceeds 0. Hence, the phosphorylation of fructose by FRK2 might be necessary for the sucrose cleavage, sugar metabolism, and cell wall synthesis that are essential for proper development of the vascular tissues German et al. Although that report did not include a detailed analysis of the vasculature system of the affected plants, it is tempting to speculate that deformation of the vascular system is responsible for the reduced tuber yield.

The involvement of FRK in the vasculature of potato tubers is further supported by the results of in situ staining of FRK activity showing localization in vascular bundles Sergeeva and Vreugdenhil, Although both FRK1 - and FRK2 -antisense transgenic tomato plants exhibited reduced carbohydrate content, the transition to flowering was delayed only in the FRK1 -suppressed plants Odanaka et al. Sugar involvement in flowering transition has been reported for Arabidopsis and tobacco. In those species, impaired sugar translocation resulted in delayed flowering while an increase in sugar synthesis caused plants to flower early Burkle et al.

Although the mechanism by which carbohydrates control the transition to flowering is not yet clear, the different phenotypes of antisense FRK1 and FRK2 suggest that the transition to flowering is not simply affected by carbohydrate status, but rather that FRK1 might mediate a signal promoting this process. Still, an additional line of evidence is required to support this hypothesis. Fructokinases has also been shown to play a role in anther development. The developing anther is a photosynthetically inactive organ and thus requires sucrose as a source of energy for its development.

It has been suggested that FRK plays a central role in providing fructose 6-phosphate and thus facilitating the production of UDP-G to support the synthesis of cellulose for the elongating cell wall Karni and Aloni, The further biological significance of FRK has been demonstrated by the specific expression of FRK4 in tomato anthers during late stages of pollen development and during pollen germination German et al. Interestingly, the Arabidopsis FRK ortholog At4g displays a similar restricted expression pattern.

In a study that aimed to elucidate the protein interaction network underlying the process of pollen development in rice, Kerim et al. These isoforms were co-regulated with two isoforms of vacuolar acid INV. Hence, FRK may be implicated in both the accumulation of starch during pollen development and the degradation of starch during pollen tube elongation. The involvement of FRK in plant responses to abiotic stress has been reported recently in sunflower, maize, and rice. In sunflower, proteins related to basic carbon metabolism, including an ortholog of the plastidic SlFRK3 , are up-regulated in response to drought stress Fulda et al.

Up-regulation of OsFK2 was reported in rice under anoxic conditions, implying that this gene plays a role in anaerobic energy production. In contrast, OsFK1 is expressed under aerobic conditions Guglielminetti et al. Another example of a specific FRK isozyme that is expressed in response to abiotic stress was found in maize, in which FRK2 is up-regulated in response to short-term salt stress Zorb et al.

This isozyme, together with other carbohydrate-metabolism enzymes, may serve as a marker for early signs of salt stress Zorb et al. While the cytoplasmic FRKs are located within the cytosol, all of the cytoplasmic HXKs in eudicots and most of the cytoplasmic HXKs in monocots are associated with the mitochondria.

HXKs may also appear in the nucleus. They not only support the theory that HXKs and FRKs play different roles, but also raise questions about the intracellular trafficking of glucose and fructose. Sugar-sensing roles in addition to the metabolic function with regard to several physiological processes, primarily related to photosynthesis and photosynthetic tissues, have been documented for a few HXKs. However, the molecular mechanisms of these processes and the roles of the various HXKs in sink and other tissues are not yet understood.

Specifically, the molecular and physiological nature of the interactions between HXKs and plant hormones remain unclear. Rather, FRK may function primarily in the regulation of sugar metabolism in sink and vascular tissues. Nevertheless, the roles of HXK and FRK may be dictated by the specific tissues and cell types in which they are found. Therefore, it is essential to explore in which tissues and types of cells the various HXK and FRK genes are expressed.

Later on, tissue-specific modulated expression of the corresponding genes using mutants or gene expression under specific promoters may help clarify the roles of these enzymes in different tissues. For example, FRK may control the amount of sugar allocated for vascular tissues and may be used to enhance xylem and vascular development in woody plants. Similarly, HXK may be used to control transpiration in agricultural crops. We believe that these discoveries and the potential uses of these enzymes will encourage further exploration of these gene families. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

We wish to thank Shlomo Goren for his very insightful comments on this manuscript. This research was supported by research grant No. National Center for Biotechnology Information , U. Journal List Front Plant Sci v. Published online Mar Prepublished online Jan 6. Box 6, Bet Dagan , Israel. Received Nov 26; Accepted Feb This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.

This article has been cited by other articles in PMC. Abstract 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. The Origin of Hexoses and Other Sugars Found in Source and Sink Plant Tissues and Their Intracellular Localization Sugars such as the disaccharide sucrose and the hexoses glucose and fructose are the primary products of photosynthesis and the initial building blocks of most natural organic matter. Open in a separate window. Table 1 Hexokinase genes and their physiological function.

Table 2 Fructokinase genes and isozymes and their physiological functions. Physiological Roles of HXK and FRK It has been proposed that sugar levels and metabolism in the plant are monitored and regulated to ensure optimal growth. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments We wish to thank Shlomo Goren for his very insightful comments on this manuscript. Application of rice nuclear proteome analysis to the identification of evolutionarily conserved and glucose-responsive nuclear proteins. A membrane-associated form of sucrose synthase and its potential role in synthesis of cellulose and callose in plants.

Analysis of Arabidopsis glucose insensitive mutants, gin5 and gin6, reveals a central role of the plant hormone ABA in the regulation of plant vegetative development by sugar. A role for F-actin in hexokinase-mediated glucose signaling.

Actin-based cellular framework for glucose signaling by Arabidopsis hexokinase1. Sucrose synthase catalyzes the de novo production of ADPglucose linked to starch biosynthesis in heterotrophic tissues of plants. Plant nucleotide sugar formation, interconversion, and salvage by sugar recycling. Partial purification and characterisation of fructokinase activity from barley leaves.

Evidence for multiple sites of glucosyl transfer in the synthase complex. A putative plastidic glucose translocator is expressed in heterotrophic tissues that do not contain starch, during olive Olea europea L. Suppression of kinetic cooperativity of hexokinase D glucokinase by competitive inhibitors. A slow transition model. Evidence for plasma membrane-associated forms of sucrose synthase in maize. Cloning and functional expression of alkaline alpha-galactosidase from melon fruit: Partial purification and characterization of fructokinase from developing taproots of sugar beet Beta vulgaris.

Sweet sensor, surprising partners. STKE , 7. Subcellular localization of rice hexokinase in the mesophyll protoplasts of tobacco. Plant Cell 14 , — Evidence for a role of hexokinases as conserved glucose sensors in both monocot and dicot plant species. Structure, expression, and functional analysis of the hexokinase gene family in rice Oryza sativa L. Planta , — Regulatory functions of nuclear hexokinase1 complex in glucose signaling. Cell , — Low glucose uncouples hexokinase1-dependent sugar signaling from stress and defense hormone abscisic acid and C2H4 responses in Arabidopsis.

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