Plant Carbohydrates I: Intracellular Carbohydrates

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It appears that transport here is against a concentration gradient, with the process powered via the ATP-dependent establishment of a pH gradient that is linked to sucrose uptake via a sucrose-proton antiporter Bush The situation in leaves is much less clear.

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In many species, chloroplast starch is the major stored form of carbohydrate, and kinetic analysis suggests that much of the soluble sugar mainly sucrose is rapidly translocated out of the leaf. In others, such as temperate C3 grasses and cereals, sucrose is both stored and further metabolised in leaves, and starch is a relatively minor component Cairns et al. Studies on rapidly isolated vacuoles from cereal leaves showed that sucrose was quickly partitioned between cytosol and vacuole, with kinetics similar to the rate of synthesis Kaiser et al. This model has, however, been challenged by Heldt and co-workers for both barley and spinach Winter et al.

A summary of some of these data is given in table 1 , suggesting that there is active retention of sucrose in the cytosol of both sucrose- and starch-storing leaves.

Plant Carbohydrates I: Intracellular Carbohydrates - Google книги

However, their cytosolic fraction will include phloem contents, which will artefactually increase its concentration. Unfortunately, the technique of single-cell sampling, which gives almost pure samples of vacuolar sap from a number of plant tissues Tomos et al. Currently, therefore, it is not possible to assess with certainty the significance of the vacuole in the active storage of sucrose and hexose in leaves. However, the large volume of the vacuole when compared with cytosol means that a significant proportion of the low molecular weight soluble sugars in the leaf will reside in this compartment, even if the cytosolic concentrations are higher.

The vacuole is, however, undoubtedly involved in the storage of higher molecular weight soluble sugars based on sucrose, particularly fructans and the raffinose-family oligosaccharides RFO; Pollock et al. Analysis of enzymatically-prepared vacuoles from a range of species and tissues where these compounds accumulate has indicated both the presence of the compounds themselves and, in some cases, the enzymes thought to be involved in their synthesis tables 2 and 3.

In the case of RFO synthesis in leaves of Ajuga reptans , the enzymes that catalyse the formation of galactinol and stachyose processes thought to occur in the cytosol could not be found in purified vacuoles, whereas the reversible galactosyl transferase that catalysed chain elongation was present apparently exclusively in vacuoles.

This is consistent with the first stages of synthesis being cytosolic, followed by transport of stachyose into the vacuole and subsequent chain elongation Bachmann et al. Unfortunately, the situation in leaves of temperate gramineae that accumulate fructans is less clear-cut. The majority of the fructans in barley leaves, together with the measurable fructosyl transferase activities thought to be responsible for their synthesis, can be isolated from vacuoles Wagner et al. There is, however, a considerable discrepancy between the conditions within the vacuole and the conditions needed for in vitro fructan synthesis Cairns et al.

These do not equate to measurements of concentrations in isolated vacuoles, which are considerably lower Cairns et al. Although it is possible that some form of metabolic channelling exists Dixon et al. Such a suggestion has been made by Kaeser on the basis of ultrastructural studies of Jerusalem artichoke tubers. Measurements of hydrolytic activity table 2 indicate that these enzymes are also in the vacuole. Here affinities for the substrate are much higher and there is less need to consider alternative patterns of compartmentation. It would appear, therefore, that there is good evidence for a role for the vacuole in carbohydrate storage, but some doubt over the involvement of vacuolar processes in the biosynthesis of these polymers.

Direct demonstration of partitioning, however, is not straightforward because of the difficulties involved in mechanical dissection and the almost universal presence in leaves of hydrolytic enzymes that can degrade sucrose, starch and fructans whenever tissues are damaged. Histochemical analysis of starch granule accumulation in barley chloroplasts showed differences in both appearance and diel kinetics between mesophyll cells and those of the photosynthetic parenchymatous bundle sheath Williams et al. The samples of tissue sap are collected into an oil-filled glass microcapillary mounted on a micromanipulator.

The capillary is inserted through the cell wall whilst the tissue is observed under a stereomicroscope. The turgor pressure within the cell forces the cell sap into the capillary and this sample can then be expelled onto the surface of a glass slide under a layer of water-saturated paraffin oil, sub-sampled by picolitre constriction pipette and the sub-samples analysed. Assay of sugars and malate is via enzyme-linked reduction of pyridine nucleotides that are then measured fluorimetrically Koroleva et al.

The experimental challenge was to be able to measure accurately fructans in the presence of sucrose. This was overcome by using sucrose phosphorylase specifically to cleave sucrose Harrison et al. The resultant hexoses could be assayed directly. Barley plants were treated to stimulate sucrose accumulation, and hence to induce fructan accumulation Wagner et al. In separate experiments, roots were cooled to reduce sink activity, and carbon fixation was increased by increasing the irradiance Koroleva et al. The results obtained for measurements on epidermal, mesophyll and parenchymatous bundle sheath cells are summarised in table 4.

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Significant differences between the three cell types were observed. Epidermal cells contained low concentrations of sucrose, glucose and fructose, and no fructan. Experimental treatments that promoted sucrose and fructan accumulation did not affect the sugar contents of these cells. The majority of the soluble carbon in epidermal cells was as malate Frike et al. By contrast, mesophyll and parenchymatous bundle sheath cells accumulated sucrose after both environmental treatments, together with significant amounts of fructan.

The ratio between sucrose and fructan differed markedly, however, between the two cell types. The fructan to sucrose ratio was much higher under both treatments for parenchymatous bundle sheath cells, which is consistent with a gradient of sucrose concentration between the mesophyll cells and the site of active vein loading in the phloem parenchyma Koroleva et al. By contrast, bundle sheath cells had a lower specific activity, suggesting that there were significant unlabelled pools of sucrose, and perhaps that these cells were involved in refixation of carbon dioxide released by respiration from within the vascular bundles Koroleva et al.

It has been suggested Labhart et al. If this is so, then the observations summarised above indicate that the process must also be considered in conjunction with the sequestration of sucrose both within the vasculature and within individual cell types. There are two lines of evidence to suggest that this latter process may be significant.

The second line of evidence relates to the distribution of invertase protein and activity. We used measurements of the rate of sucrose hydrolysis in single-cell samples to estimate soluble invertase activity and then compared it to extractable activity in whole-leaf extracts. Based on these observations, we propose that there is extensive compartmentation of primary carbon metabolism within leaves of temperate gramineae.

Epidermal cells appear to be buffered from environmentally-induced changes in assimilate abundance, with mesophyll and parenchymatous bundle sheath cells showing differential responses of starch, sucrose and fructan metabolism. There is also strong circumstantial evidence that significant elements of carbon metabolism are localised in the vasculature, although technical limitations make it difficult to measure these directly. Recently we have developed techniques to extract, amplify and assess the relative abundance of specific mRNA species from single cell samples.

In initial experiments, we have shown that transcript abundance for a barley fructosyl transferase sucrose-fructan 6-fructosyltransferase; Sprenger et al. Further development of this approach should enable us to pinpoint the location and timing of gene expression changes in response to alterations in assimilate abundance and to extend this to analysis of possible down-regulation of key photosynthetic processes. Specialisation of function is a key element in the evolution of higher multicellular organisms.

Morphological analysis of vascular plants provides ample examples of such specialisation at the organ, tissue, cellular and sub-cellular level.

Components of Plasma Membranes

It is, perhaps, a consequence of the resolution of "traditional" biochemical techniques that studies on metabolic specialisation have tended to emphasise the physical location of specific pathways or enzymes rather than spatial differences in flux or metabolic control. Unfortunately, plant responses to edaphic or biotic changes are often exhibited, at least initially, precisely at the latter level. The development of techniques such as image analysis, reporter genes, in situ hybridisation and single cell analysis offers new opportunities to analyse the dynamics of metabolism at the tissue level.

The leaves of temperate C 3 grasses and cereals function in an environment where marked changes in the supply of and demand for fixed carbon occur on an annual, daily and moment-by moment basis. The flux through primary carbon metabolism is large and the effects of environmental changes on such metabolism are equally significant. As such, they offer an excellent model system for the application of these approaches and our understanding of the central importance of compartmentation of metabolism is likely to grow rapidly over the next few years.

Acknowledgements - We acknowledge the Biotechnology and Biological Sciences Research Council UK for financial support for our work in the area of this review and the University of Wales Bangor for financial support for O. We are grateful to Jo Spikes for assistance in the preparation of the manuscript. Respiration and crop productivity. Springer, Berlin. Metabolism of the raffinose family oligosaccharides in leaves of Ajuga reptans L.

Inter- and intracellular compartmentation. Plant Physiology Cold acclimation, translocation, and sink to source transition: discovery of chain elongation enzyme. Proton-coupled sugar and amino-acid transporters in plants. Effects of enzyme concentration of oligofructan synthesis from sucrose.

Phytochemistry Fructan biosynthesis in excised leaves of Lolium temulentum L. Changes in fructosyltransferase activity following excision and application of inhibitors of gene expression. New Phytologist A comparison of the in vitro properties of fructosyl transferase activities with the characteristics on in vivo fructan accumulation. Fructans: Synthesis and regulation. In Photosynthesis: physiology and metabolism R.

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Plant Carbohydrates I

Portland Press, London. The essential features of constitution, configuration, and conformation in carbo- hydrate chemistry, so well established in the. Two outstanding developments, discovery of sugar nucleotides and the advent of chromatography, brought together the insight and a means of probing complexities inherent in plant carbohydrates. These advances, combined with a modern knowledge of enzymes and cellular metabolism, have provided new horizons of investigation for the student of plant physiology.

This volume and its companion Vol. To accommodate the extensive amount of information to be presented, subject matter has been divided, somewhat arbitrarily, into intracellular and extracellular carbohydrates, with the latter defined as carbohydrates occurring in space out- side the plasma membrane plasmalemma. This classification is not exclusive; rather it is intended to lend a degree of flexibility to the way in which subject matter is arranged between volumes.

The first section of this volume addresses the occurrence, metabolism, and function of monomeric and higher saccharides of fungi, algae, and higher plants.

Plant carbohydrates 1; intracellular carbohydrates [1982]

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Carbohydrates Part 1: Simple Sugars and Fischer Projections

Ships in 15 business days. Link Either by signing into your account or linking your membership details before your order is placed. Description Table of Contents Product Details Click on the cover image above to read some pages of this book! Macromolecular Carbohydrates - Occurrence, Metabolism, Function. Requirements for Activity. Glycolipids and Other Glycosides. Physiological Processes. More Books in Biochemistry See All.