Sucrose uptake by sugar beet tap root tissue

Sugar beet PMT5a and STP13 carriers suitable for proton-driven plasma membrane sucrose and glucose import in taproots

Sugar beet (Beta vulgaris) is the major sugar-producing crop in Europe and Northern America, as the taproot stores sucrose at a concentration of around 20%. Genome sequence analysis together with biochemical and electrophysiological approaches led to the identification and characterization of the TST sucrose transporter driving vacuolar sugar accumulation in the taproot. However, the sugar transporters mediating sucrose uptake across the plasma membrane of taproot parenchyma cells remained unknown. As with glucose, sucrose stimulation of taproot parenchyma cells caused inward proton fluxes and plasma membrane depolarization, indicating a sugar/proton symport mechanism. To decipher the nature of the corresponding proton-driven sugar transporters, we performed taproot transcriptomic profiling and identified the cold-induced PMT5a and STP13 transporters. When expressed in Xenopus laevis oocytes, BvPMT5a was characterized as a voltage- and H+-driven low-affinity glucose transporter, which does not transport sucrose. In contrast, BvSTP13 operated as a high-affinity H+/sugar symporter, transporting glucose better than sucrose, and being more cold-tolerant than BvPMT5a. Modeling of the BvSTP13 structure with bound mono- and disaccharides suggests plasticity of the binding cleft to accommodate the different saccharides. The identification of BvPMT5a and BvSTP13 as taproot sugar transporters could improve breeding of sugar beet to provide a sustainable energy crop.

Uptake and transport of micro/nanoplastics in terrestrial plants: Detection, mechanisms, and influencing factors

The pervasive dispersion of micro/nanoplastics in various environmental matrices has raised concerns regarding their potential intrusion into terrestrial ecosystems and, notably, plants. In this comprehensive review, we focus on the interaction between these minute plastic particles and plants. We delve into the current methodologies available for detecting micro/nanoplastics in plant tissues, assess the accumulation and distribution of these particles within roots, stems, and leaves, and elucidate the specific uptake and transport mechanisms, including endocytosis, apoplastic transport, crack-entry mode, and stomatal entry. Moreover, uptake and transport of micro/nanoplastics are complex processes influenced by multiple factors, including particle size, surface charge, mechanical properties, and physiological characteristics of plants, as well as external environmental conditions. In conclusion, this review paper provided valuable insights into the current understanding of these mechanisms, highlighting the complexity of the processes and the multitude of factors that can influence them. Further research in this area is warranted to fully comprehend the fate of micro/nanoplastics in plants and their implications for environmental sustainability.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

Colocalization of sucrose synthase expression and sucrose storage in the sugarbeet taproot indicates a potential role for sucrose catabolism in sucrose accumulation

Sucrose metabolism is believed to have a central role in promoting sink strength and sucrose storage in the sugarbeet taproot. How sucrose accumulation is increased by sucrose-degrading enzymes, however, is a paradox. To elucidate roles for sucrose-degrading activities in sucrose accumulation, relationships between the intercellular location of sucrose-catabolizing enzymes and sites of sucrose accumulation were determined in the sugarbeet taproot. Sucrose storage was evident in parenchyma cells of the outer cortex, rays, and rings of parenchyma tissue, but was absent in phloem, the vascular cambium, cells surrounding these tissues, or cells surrounding xylem. Sucrose synthase, which was primarily responsible for sucrose catabolism throughout the taproot, was expressed in similar cell and tissue types to those accumulating sucrose. Colocalization of sucrose synthase with sucrose accumulation, as well as sucrose synthase localization near the tonoplast, suggests a role for the enzyme in generating metabolic energy to fuel sucrose sequestration in the vacuole. Localization near the plasma membrane also suggests a role for sucrose synthase in supplying substrates for cell wall biosynthesis. By utilizing sucrose for ATP or cell wall biosynthesis, sucrose synthase likely maintains the source-to-sink sucrose gradient that drives sucrose transport into the root, thereby promoting sugarbeet root sink strength.

Production of 5-ketofructose from fructose or sucrose using genetically modified Gluconobacter oxydans strains

The growing consumer demand for low-calorie, sugar-free foodstuff motivated us to search for alternative non-nutritive sweeteners. A promising sweet-tasting compound is 5-keto-D-fructose (5-KF), which is formed by membrane-bound fructose dehydrogenases (Fdh) in some Gluconobacter strains. The plasmid-based expression of the fdh genes in Gluconobacter (G.) oxydans resulted in a much higher Fdh activity in comparison to the native host G. japonicus. Growth experiments with G. oxydans fdh in fructose-containing media indicated that 5-KF was rapidly formed with a conversion efficiency of 90%. 5-KF production from fructose was also observed using resting cells with a yield of about 100%. In addition, a new approach was tested for the production of the sweetener 5-KF by using sucrose as a substrate. To this end, a two-strain system composed of the fdh-expressing strain and a G. oxydans strain that produced the sucrose hydrolyzing SacC was developed. The strains were co-cultured in sucrose medium and converted 92.5% of the available fructose units into 5-KF. The glucose moiety of sucrose was converted to 2-ketogluconate and acetate. With regard to the development of a sustainable and resource-saving process for the production of 5-KF, sugar beet extract was used as substrate for the two-strain system. Fructose as product from sucrose cleavage was mainly oxidized to 5-KF which was detected in a concentration of over 200 mM at the end of the fermentation process. In summary, the two-strain system was able to convert fructose units of sugar beet extract to 5-KF with an efficiency of 82 ± 5%.

The Water to Solute Permeability Ratio Governs the Osmotic Volume Dynamics in Beetroot Vacuoles

Plant cell vacuoles occupy up to 90% of the cell volume and, beyond their physiological function, are constantly subjected to water and solute exchange. The osmotic flow and vacuole volume dynamics relies on the vacuole membrane -the tonoplast- and its capacity to regulate its permeability to both water and solutes. The osmotic permeability coefficient (P_f ) is the parameter that better characterizes the water transport when submitted to an osmotic gradient. Usually, P_f determinations are made in vitro from the initial rate of volume change, when a fast (almost instantaneous) osmolality change occurs. When aquaporins are present, it is accepted that initial volume changes are only due to water movements. However, in living cells osmotic changes are not necessarily abrupt but gradually imposed. Under these conditions, water flux might not be the only relevant driving force shaping the vacuole volume response. In this study, we quantitatively investigated volume dynamics of isolated Beta vulgaris root vacuoles under progressively applied osmotic gradients at different pH, a condition that modifies the tonoplast P_f . We followed the vacuole volume changes while simultaneously determining the external osmolality time-courses and analyzing these data with mathematical modeling. Our findings indicate that vacuole volume changes, under progressively applied osmotic gradients, would not depend on the membrane elastic properties, nor on the non-osmotic volume of the vacuole, but on water and solute fluxes across the tonoplast. We found that the volume of the vacuole at the steady state is determined by the ratio of water to solute permeabilites (P_f /P_s ), which in turn is ruled by pH. The dependence of the permeability ratio on pH can be interpreted in terms of the degree of aquaporin inhibition and the consequently solute transport modulation. This is relevant in many plant organs such as root, leaves, cotyledons, or stems that perform extensive rhythmic growth movements, which very likely involve considerable cell volume changes within seconds to hours.

Development and evaluation of a combined cultivator and band sprayer with a row-centering RTK-GPS guidance system

Typically, low-pressure sprayers are used to uniformly apply pre- and post-emergent herbicides to control weeds in crop rows. An innovative machine for weed control in inter-row and intra-row areas, with a unique combination of inter-row cultivation tooling and intra-row band spraying for six rows and an electro-hydraulic side-shift frame controlled by a GPS system, was developed and evaluated. Two weed management strategies were tested in the field trials: broadcast spraying (the conventional method) and band spraying with mechanical weed control using RTK-GPS (the experimental method). This approach enabled the comparison between treatments from the perspective of cost savings and efficacy in weed control for a sugar beet crop. During the 2010–2011 season, the herbicide application rate (112 L ha⁻¹) of the experimental method was approximately 50% of the conventional method, and thus a significant reduction in the operating costs of weed management was achieved. A comparison of the 0.2-trimmed means of weed population post-treatment showed that the treatments achieved similar weed control rates at each weed survey date. Sugar beet yields were similar with both methods (p = 0.92). The use of the experimental equipment is cost-effective on ≥20 ha of crops. These initial results show good potential for reducing herbicide application in the Spanish beet industry.

Role of metabolite transporters in source-sink carbon allocation

Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g., roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types developed according to their respective function, and cells of either tissue type are highly compartmentalized. Photoassimilates are allocated to distinct compartments of these tissues in all organs, requiring a set of metabolite transporters mediating this intercompartmental transfer. The general route of photoassimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triose phosphates either enter the cytosol for mainly sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil in oil bodies, or - in combination with nitrogen - as protein in protein storage vacuoles and protein bodies. Here, we focus on transport proteins known for either of these steps, and discuss the implications for yield increase in plants upon genetic engineering of respective transporters.

In and out of the plant storage vacuole

The plant storage vacuole is involved in a wide variety of metabolic functions a great many of which necessitate the transport of substances across the tonoplast. Some solutes, depending on the origin, have to cross the plasma membrane as well. The cell is equipped with a complex web of transport systems, cellular routes, and unique intracellular environments that support their transport and accumulation against a concentration gradient. These are capable of processing a diverse nature of substances of distinct sizes, chemical properties, and origins. In this review we describe the various mechanism involved in solute transport into the vacuole of storage cells with special emphasis placed on solutes arriving through the apoplast. Transport of solutes from the cytosol to the vacuole is carried out by tonoplast-bound ABC transporters, solute/H(+) antiporters, and ion channels whereas transport from the apoplast requires additional plasma membrane-bound solute/H(+) symporters and fluid-phase endocytosis. In addition, and based on new evidence accumulated within the last decade, we re-evaluate the current notion of extracellular solute uptake as partially based on facilitated diffusion, and offer an alternative interpretation that involves membrane bound transporters and fluid-phase endocytosis. Finally, we make several assertions in regards to solute export from the vacuole as predicted by the limited available data suggesting that both membrane-bound carriers and vesicle mediated exocytosis are involved during solute mobilization.

Fate of sucralose through environmental and water treatment processes and impact on plant indicator species

The degradation and partitioning of sucralose during exposure to a variety of environmental and advanced treatment processes (ATP) and the effect of sucralose on indicator plant species were systematically assessed. Bench scale experiments were used to reproduce conditions from environmental processes (microbial degradation, hydrolysis, soil sorption) and ATPs (chlorination, ozonation, sorption to activated carbon, and UV radiation). Degradation only occurred to a limited extent during hydrolysis, ozonation, and microbial processes indicating that breakdown of sucralose will likely be slow and incomplete leading to accumulation in surface waters. Further, the persistence of sucralose was compared to suggested human tracer compounds, caffeine and acesulfame-K. In comparison sucralose exhibits similar or enhanced characteristics pertaining to persistence, prevalence, and facile detection and can therefore be considered an ideal tracer for anthropogenic activity. Ecological effects of sucralose were assessed by measuring sucrose uptake inhibition in plant cotelydons and aquatic plant growth impairment. Sucralose did not inhibit plant cotelydon sucrose uptake, nor did it effect frond number, wet weight, or growth rate in aquatic plant, Lemna gibba. Though sucralose does not appear toxic to plant growth, the peristent qualities of sucralose may lead to chronic low-dose exposure with largely unknown consequences for human and environmental health.

Fluid phase endocytic uptake of artificial nano-spheres and fluorescent quantum dots by sycamore cultured cells: evidence for the distribution of solutes to different intracellular compartments

Fluid phase endocytic uptake of external solutes in plant cells was further substantiated using artificial polystyrene nano-spheres (40 nm) and CdSe/ZnS quantum dots (20 nm). Both types of artificial nano-particles were taken up by sycamore-cultured cells. However, whereas polystyrene nano-spheres were delivered to the central vacuole, CdSe/ZnS nano-dots were sequestered into cytoplasmic vesicular structures. Using dextran-Texas Red (m.w. 3,000; d-TR) as additional marker, confocal micrographs confirmed the distinct topographic distribution of CdSe/ZnS quantum dots within the cell. Initially, d-TR and CdSe/ZnS quantum dots colocalized within cytoplasmic vesicles. After 18 h incubation, d-TR was distinctly localized in the vacuole whereas CdSe/ZnS quantum dots remained sequestered in cytoplasmic membranous compartments. The data provide a first evidence for the rapid distribution of solutes taken up by endocytosis to distinct intracellular compartments.

In vitro sugar uptake by grapefruit (Citrus paradisi) juice-sac cells

To further our understanding of the mechanisms of sugar uptake and accumulation into grapefruit (Citrus paradisi Macf. cv. Marsh seedless), the patterns of uptake and utilisation of sucrose, glucose and fructose by Citrus juice cells was investigated. Analyses were conducted on sliced juice sacs that were incubated in radioactive [¹⁴C]-sugar solutions with unlabelled sugars, in the presence or absence of metabolic inhibitors. Both hexoses demonstrated an initial uptake peak in December and a second uptake peak in February-March. From March through April the rates of sucrose uptake increased to levels comparable to those of glucose and fructose. Sucrose and its moieties fructose and glucose entered the juice cells of Citrus juice fruit by an insaturable, and mostly by an independent, process. However, NaN₃ and carbonylcyanide m-chlorophenylhydrazone (CCCP) produced slight inhibition of these processes. Cells took up hexoses at a greater rate than sucrose, with accumulation reaching a plateau by 4-8 h, and then continuing unabated, in the case of glucose, for 42 h. Uptake of all three sugars increased linearly in the range of sugar concentrations tested, which extended from 0.01 to 320 mm, denoting an insaturable system for sugar uptake. ¹⁴CO₂ evolution was relatively low in all the experiments, the lowest evolution being recorded when the uptake of [¹⁴C]-sucrose was studied, while the highest ¹⁴CO₂ evolution was recorded when the uptake of [¹⁴C]-glucose was studied. The data demonstrate a preferential utilisation of glucose over fructose and sucrose. In all the experiments, the two metabolic inhibitors significantly inhibited the decarboxylation of the three sugars.

Nonvascular, Symplasmic Diffusion of Sucrose Cannot Satisfy the Carbon Demands of Growth in the Primary Root Tip of Zea mays L

Nonvascular, symplasmic transport of sucrose (Suc) was investigated theoretically in the primary root tip of maize (Zea mays L. cv WF9 x Mo 17) seedlings. Symplasmic diffusion has been assumed to be the mechanism of transport of Suc to cells in the root apical meristem (R.T. Giaquinta, W. Lin, N.L. Sadler, V.R. Franceschi [1983] Plant Physiol 72: 362-367), which grow apical to the end of the phloem and must build all biomass with carbon supplied from the shoot or kernel. We derived an expression for the growth-sustaining Suc flux, which is the minimum longitudinal flux that would be required to meet the carbon demands of growth in the root apical meristem. We calculated this flux from data on root growth velocity, area, and biomass density, taking into account construction and maintenance respiration and the production of mucilage by the root cap. We then calculated the conductivity of the symplasmic pathway for diffusion, from anatomical data on cellular dimensions and the frequency and dimensions of plasmodesmata, and from two estimates of the diffusive conductance of a plasmodesma, derived from independent data. Then, the concentration gradients required to drive a growth-sustaining Suc flux by diffusion alone were calculated but were found not to be physiologically reasonable. We also calculated the hydraulic conductivity of the plasmodesmatal pathway and found that mass flow of Suc solution through plasmodesmata would also be insufficient, by itself, to satisfy the carbon demands of growth. However, much of the demand for water to cause cell expansion could be met by the water unloaded from the phloem while unloading Suc to satisfy the carbon demands of growth, and the hydraulic conductivity of plasmodesmata is high enough that much of that water could move symplasmically. Either our current understanding of plasmodesmatal ultrastructure and function is flawed, or alternative transport mechanisms must exist for Suc transport to the meristem.

The control of cell expansion in roots

The expansion of roots is considered at the level of the single cell. The water relations of cell expansion are discussed. Water entry, solute import and cell wall properties are considered as possible regulatory points. It is argued that root cell expansion can be understood in terms of cell turgor pressure and the physical properties of the cell wall, provided solute supply is not limiting. Various measurements of cell wall properties in roots are presented and the assumptions underlying their measurements are presented. It is concluded that cell wall properties must be measured over short time periods to prevent alterations in wall properties during the experiment. The radial location of the load-bearing layers is discussed and it is concluded that, unlike aerial tissue, growth is limited by the properties of the inner layer of the root cortex. Evidence is presented to show that cell wall properties can change both during development and following turgor perturbation. In general, however, turgor itself is tightly regulated, particularly towards the root tip. A number of environmental situations are presented in which root growth is altered. The mechanism of the alteration is discussed at the single cell level. These 'stresses'include osmotic stress, low temperature and soil compaction. In many cases the alteration of root growth is consistent with changes in the ceil wall properties of the growing ceils. Severe stress, resulting in near cessation of root cell extension, can result in a change (usually an increase) in turgor pressure. The change in turgor pressure of the cells in the growing zone is smaller than that which would be expected from a continuation of an unstressed solute import rate. This exemplifies both the change in cell wall properties and the tight turgor homeostasis of root tips. The biochemical processes which underlie the modulation of cell wall properties are presented as they are currently understood in roots. Measurements of the chemical composition of the wall have not revealed any useful differences which can explain the developmental or stress-induced changes in cell wall properties. Recent work on cell wall enzymes and proteins may provide information about control of cross-linkages within the wall. In the last section the relative importance of apoplastic and symplastic solute transport to the expanding cells is considered. At present the consensus appears to favour the symplastic route, but the apoplastic pathway may also operate, possibly as a scavenging mechanism for leaked ions. The regulation of turgor pressure by linking solute import with wall loosening is discussed. Contents Summary 3 I. Introduction 4 II. Factors controlling cell expansion 4 III. Wall extensibility and yield threshold in roots 6 IV. Environmental effects on root cell expansion 10 V. Modification of cell wall biochemistry 15 VI. Linkage of growth with solute import 18 VII. Future prospects 21 VIII. Acknowledgements 22 IX. References 22.

Sucrose Phosphate Is Not Transported into Vacuoles or Tonoplast Vesicles from Red Beet (Beta vulgaris) Hypocotyl

Tonoplast vesicles and vacuoles isolated from red beet (Beta vulgaris L.) hypocotyl accumulated externally supplied [(14)C]sucrose but not [(14)C]sucrose phosphate despite the occurrence of sucrose phosphate phosphohydrolytic activity in the vacuole. The activities of sucrose synthase and sucrose phosphate synthase in whole cell extracts were 960 and 30 nanomoles per milligram protein per minute, respectively; whereas, no sucrose synthesizing activity was measured in tonoplast preparations. The results obtained in this investigation are incompatible with the involvement of sucrose phosphate synthase in the process of sucrose synthesis and accumulation in the storage cells of red beet.

Coupling of solute transport and cell expansion in pea stems

As cells expand and are displaced through the elongation zone of the epicotyl of etiolated pea (Pisum sativum L. var Alaska) seedlings, there is little net dilution of the cell sap, implying a coordination between cell expansion and solute uptake from the phloem. Using [14C] sucrose as a phloem tracer (applied to the hypogeous cotyledons), the pattern of label accumulation along the stem closely matched the growth rate pattern: high accumulation in the growing zone, little accumulation in nongrowing regions. Several results suggest that a major portion of phloem contents enters elongating cells through the symplast. We propose that the coordination between phloem transport and cell expansion is accomplished via regulatory pathways affecting both plasmodesmata conductivity and cell expansion.

Surcose transport in isolated plasma-membrane vesicles from sugar beet (Beta vulgaris L.) Evidence for an electrogenic sucrose-proton symport

An analysis of the molecular mechanism of sucrose transport across the plasmalemma was conducted with isolated plasma-membrane (PM) vesicles. Plasma membrane was isolated by aqueous two-phase partitioning from fully expanded sugar beet (Beta vulgaris L.) leaves. The isolated fraction was predominantly PM vesicles as determined by marker-enzyme analysis, and the vesicles were oriented right-side-out as determined by structurally linked latency of the PM enzyme, vanadate-sensitive Mg(2+)-ATPase. Sucrose uptake was investigated by equilibrating PM vesicles in pH 7.6 buffer and diluting them 20-fold into pH 6.0 buffer. Using this pH-jump technique, vesicles accumulated acetate in a pH-dependent, protonophore-sensitive manner, which demonstrated the presence of a pH gradient (ΔpH) across the vesicle membrane. Addition of sucrose to pH-jumped PM vesicles resulted in a pH-dependent, protonophoresensitive uptake of sucrose into the vesicles. Uptake was sucrose-specific in that a 10-fold excess of mannose, glucose, fructose, mannitol, melibiose, lactose or maltose did not inhibit sucrose accumulation. The rate of pH-dependent uptake was saturable with respect of sucrose concentration and had an apparent K m, of 0.45 mM. Sucrose uptake was stimulated approximately twofold by the addition of valinomycin and K(+), which indicated an electrogenic sucrose-H(+) symport. Membrane potentials (ΔΨ) were imposed across the vesicle membrane using valinomycin and K(+). A membrane potential, negative inside, stimulated pH-dependent sucrose uptake while a ΔΨ, positive inside, inhibited uptake. Conditions that produce a negative ΔΨ in the absence of a pH gradient supported, although weakly, sucrose uptake. These data support an electrogenic sucrose-H(+) symport as the mechanism of sucrose transport across the PM in Beta leaves.

Evidence for the presence of a sucrose carrier in immature sugar beet tap roots

The objectives of this work were to determine the path of phloem unloading and if a sucrose carrier was present in young sugar beet (Beta vulgaris L.) taproots. The approach was to exploit the characteristics of the sucrose analog, 1'-fluorosucrose (F-sucrose) which is a poor substrate for acid invertase but is a substrate for sucrose synthase. Ten millimolar each of [(3)H]sucrose and [(14)C]F-sucrose were applied in a 1:1 ratio to an abraded region of an attached leaf for 6 hours. [(14)C]F-sucrose was translocated and accumulated in the roots at a higher rate than [(3)H]sucrose. This was due to [(3)H]sucrose hydrolysis along the translocation path. Presence of [(3)H]hexose and [(14)C]F-sucrose in the root apoplast suggested apoplastic sucrose unloading with its subsequent hydrolysis. Labeled F-sucrose uptake by root tissue discs exhibited biphasic kinetics and was inhibited by unlabeled sucrose, indicating that immature roots have the ability for carrier-mediated sucrose transport from the apoplast. Collectively, in vivo and in vitro data indicate that despite sucrose hydrolysis by the wall-bound invertase, sucrose hydrolysis is not entirely essential for sugar accumulation in this tissue.

Transport of sugars across the plasma membrane of beetroot protoplasts

Protoplasts isolated from beetroot tissue took up glucose preferentially whereas sucrose was transported more slowly. The (14)C-label from [(14)C]glucose and [(14)C]sucrose taken up by the cells could be detected rapidly in phosphate esters and, after feeding of [(14)C]glucose was found also in sucrose. The temperature-dependent uptake process (activation energy EA about 50 kJ · mol(-1)) seems to be carrier mediated as indicated by its substrate saturation and, for glucose, by competition experiments which revealed positions C1, C5 and C6 of the D-glucose molecule as important for effective uptake. The apparent Km(20° C) for glucose (3-O-methylglucose) was about 1 mM whereas for sucrose a significantly lower apparent affinity was determined (Km about 10 mM). When higher concentrations of glucose (5 mM) or sucrose (20 mM) were administered, the uptake process followed first-order kinetics. Carrier-mediated transport was inhibited by N,N'-dicyclohexylcarbodiimide, Na-orthovanadate, p-chloromercuribenzenesulfonic acid, and by uncouplers and ionophores. The uptake system exhibited a distinct pH optimum at pH 5.0. The results indicate that generation of a proton gradient is a prerequisite for sugar uptake across the plasma membrane. Protoplasts from the bundle regions in the hypocotyl take up glucose at higher rates than those derived from bundle-free regions. The results favour the idea that apoplastic transport of assimilates en route of unloading might be restricted to distinct areas within the storage organ (i.e. the bundle region) whereas distribution in the storage parenchyma is symplastic.

Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with (14)CO2. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N2 atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N2 atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.

Turgor regulation of sucrose transport in sugar beet taproot tissue

Sink tissues that store osmotically active compounds must osmoregulate to prevent excessively high turgor. The ability to regulate turgor may be related to membrane transport of solutes and thus sink strength. To study this possibility, the kinetics of sugar uptake were determined in sugar beet (Beta vulgaris L.) taproot tissue discs over a range of cell turgors. Sucrose uptake followed biphasic kinetics with a high affinity saturating component below 20 millimolar and a low affinity linear component at higher concentrations. Glucose uptake exhibited only simple saturation type kinetics. The high affinity saturating component of sucrose and glucose uptake was inhibited by increasing cell turgor (decreasing external mannitol concentrations). The inhibition was evident as a decrease in V(max) but no effect on K(m). Sucrose uptake by tissue equilibrated in dilute buffer exhibited no saturating component. Ethylene glycol, a permeant osmoticum, had no effect on uptake kinetics, suggesting that the effect was due to changes in cell turgor and not due to decreased water potential per se. p-(Chloromercuri)benzene sulfonic acid (PCMBS) inhibited sucrose uptake at low but not high cell turgor. High cell turgor caused the tissue to become generally leaky to potassium, sucrose, amino acids, and reducing sugars. PCMBS had no effect on sucrose leakage, an indication that the turgor-induced leakage of sucrose was not via back flow through the carrier. The ability of the tissue to acidify the external media was turgor dependent with an optimum at 300 kilopascals. Acidification was sharply reduced at cell turgors above or below the optimum. The results suggest that the secondary transport of sucrose is reduced at high turgor as a result of inhibition of the plasma membrane ATPase. This inhibition of ATPase activity would explain the reduced V(max) and leakiness to low molecular weight solutes. Cell turgor is an important regulator of sucrose uptake in this tissue and thus may be an important determinant of sink strength in tissues that store sucrose.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot : II. Evidence for a Sucrose/H-Antiport

The process of sucrose transport was investigated in sealed putative tonoplast vesicles isolated from sugarbeet (Beta vulgaris L.) taproot. If the vesicles were allowed to develop a steady state pH gradient by the associated transport ATPase and 10 millimolar sucrose was added, a transient flux of protons out of the vesicles was observed. The presence of an ATPase produced pH gradient allowed [(14)C]sucrose transport into the vesicles to occur at a rate 10-fold higher than the rate observed in the absence of an imposed pH gradient. Labeled sucrose accumulated into the sealed vesicles could be released back to the external medium if the pH gradient was dissipated with carbonylcyanide-m-chlorophenyl hydrazone (CCCP). When the kinetics of ATP dependent [(14)C]sucrose uptake were examined, the kinetic profile followed the simple Michaelis-Menten relationship and a Michaelis constant of 12.1 millimolar was found. When a transient, inwardly directed sucrose gradient was imposed on the vesicles in the absence of charge compensating ions, a transient interior negative membrane potential was observed. This membrane potential could be prevented by the addition of CCCP prior to sucrose or dissipated by the addition of CCCP after sucrose was added. These results suggest that an electrogenic H(+)/sucrose antiport may be operating on the vesicle membrane.

Sugar transport in isolated corn root protoplasts

Isolated corn (Zea mays L.) root protoplasts were used to study sucrose and hexose uptake. It is found that glucose was preferentially taken up by the protoplasts over sucrose and other hexoses. Glucose uptake showed a biphasic dependence on external glucose concentration with saturable (K(m) of 7 millimolar) and linear components. In contrast, sucrose uptake only showed a linear kinetic curve. Sucrose and glucose uptake were linear over a minimum of 1 hour at pH 6.0 and 1 millimolar exogenous sugar concentration. Glucose uptake showed a sharp 42 degrees C temperature optimum, while sucrose uptake showed a lower temperature sensitivity which did not reach a maximum below 50 degrees C. Uptake of both sugars was sensitive to several metabolic inhibitors and external pH. Differences between sucrose and glucose uptake in two different sink tissue (i.e. protoplasts from corn roots and soybean cotyledons) are discussed.

Crop Productivity and Photoassimilate Partitioning

The photosynthetic basis for increasing the yield of major field crops is examined in terms of improving the interception of seasonal solar radiation by crop foliage, the efficiency of conversion of intercepted light to photosynthetic assimilates, and the partitioning of photoassimilates to organs of economic interest. It is concluded that, in practice, genetic and chemical manipulation of light interception over the season and of partitioning offer the most potential for achieving further increases in yield. During the history of improvement of genetic yield potential of crops, increase in the partitioning of photoassimilates to harvested organs has been of primary importance.

Reserve carbohydrate in maize stem : [C]glucose and [C]sucrose uptake characteristics

Maize (Zea mays L.) stem is thought to function alternately as a net importing and net exporting organ during ontogeny, depending on whole plant photosynthetic source and sink status. The [(14)C]sucrose and [(14)C]glucose uptake capacity of stem tissues was investigated to increase our understanding of the transport factors which may influence sink status.Uptake from solutions containing up to 200 millimolar radiolabeled sugar showed that d-glucose uptake consisted of saturable and nonsaturable components, while sucrose uptake was primarily nonsaturable during the kernel-fill stages. l-Glucose uptake lacked the saturable component but both d and l isomers apparently had similar slopes for the nonsaturable component. Uptake was sensitive to inhibitors and temperature, and was increased slightly by lowered pH.The seasonal chronology for saturable uptake by isolated vascular bundles and associated pith revealed highest rates between anthesis and early kernel growth, corresponding with the stage when net sugar accumulation rates were highest. For isolated pith, the rates increased at the final stages of plant development.The rate of labeled l-glucose movement from vascular bundles into pith in isolated stem segments was greater at the silking stage than at later developmental stages, suggesting a lower resistance to diffusive transport from vascular bundles into pith at silking. Studies with stem plus ear explants showed that the capability for sugar transport from pith to vascular bundles and for phloem loading and export from the stem region was present throughout the developmental period from early kernel fill (milk) to late kernel fill (dent).

Energetics of threonine uptake by pod wall tissues of Vicia faba L

Kjeldahl assays showed that the pod wall of Vicia faba fruits behaves as a transitory reservoir of nitrogen. We have studied the properties and energetics of amino-acid uptake during the accumulating stage of pod wall development. A comparative analysis using various inhibitors or activators of the proton pump has been carried out i) on threonine uptake, ii) on the acidifying activity of the tissues, and iii) on the transmembrane potential difference of mesocarp cells. Except for the effect of dicyclohexylcarbodiimide which could not be satisfactorily explained, all other results obtained with ATPase inhibitors, uncouplers and fusicoccin were consistent with the view of a transport energized by the proton-motive force. Adding threonine to a medium containing fragments of pericarp or of endocarp induced a pH change (to-wards more alkaline values) of the medium and a membrane depolarization of the storage cells which depended on the amino-acid concentration added. These data indicate H(+)-threonine cotransport in the pod wall of broad bean. Moreover, because p-chloromercuribenzenesulphonic acid inhibits threonine uptake without affecting the transmembrane potential difference, it is concluded that the threonine carrier possesses a functional SH-group located at the external side of the plasmalemma.

Effect of plant hormones on sucrose uptake by sugar beet root tissue discs

Abscisic acid (ABA), auxins, cytokinins, gibberellic acid, alone or in combination were tested for their effects on short-term sucrose uptake in sugar beet (Beta vulgaris cv USH-20) roots. The effect of ABA on active sucrose uptake varied from no effect to the more generally observed 1.4-to 3.0-fold stimulation. A racemic mixture of ABA and its trans isomer were more stimulatory than ABA alone. Pretreating and/or simultaneously treating the tissue with K(+) or IAA prevented the ABA response while cytokinins and gibberellic acid did not. While the variable sensitivities of beet root to ABA may somehow be related to the auxin and alkali cation status of the tissue, tissue sensitivity to ABA was not correlated with ABA uptake, accumulation, or metabolic patterns. In contrast to ABA, indoleacetic acid (IAA) and other auxins strongly inhibited active sucrose uptake in beet roots. Cytokinins enhanced the auxin-induced inhibition of sucrose uptake but ABA and gibberellic acid did not modify or counteract the auxin effect. Trans-zeatin, benzyladenine, kinetin, and gibberellins had no effect on active sucrose uptake. None of the hormones or hormone mixtures tested had any significant effect on passive sucrose uptake. The effects of IAA and ABA on sucrose uptake were detectable within 1 h suggesting a rather close relationship between the physiological activities of IAA and ABA and the operation of the active transport system.

Sugar uptake by cotton tissues: leaf disc versus cultured roots

The tissue accumulation of sucrose, glucose, and fructose has been studied in cultured cotton (Gossypium hirsutum L.) roots and leaf discs. Sucrose uptake by both tissues from high apoplastic concentrations was independent of pH but has a slightly acidic pH optimum from low concentrations. Like other higher plant tissues, cotton root cells accumulate sucrose via a ;saturable,' inhibitor-sensitive mechanism and a linear, inhibitor-resistant mechanism. The linear mechanism of sucrose uptake is not as pronounced in leaf disc data as it is in root data. Further, sucrose uptake by cotton leaf discs is more resistant than uptake by root cells to pH alterations, inhibitors, and monosaccharides in the uptake medium. The saturable phase of sucrose influx into cotton root is eliminated by glucose, fructose, and high pH. Sucrose influx into both tissues is not altered by osmotica up to 200 milliOsmolar. Sucrose accumulated by both tissues is rapidly converted to other chemical forms, especially in root tissue where only approximately 50% remains as neutral sugars 1 hour following the start of radiolable exposure. Although the entry of radiolabeled sucrose is faster in abraded leaf discs, they give the same response patterns to pH, inhibitors, and monosaccharide as do unabraded discs.The sucrose accumulation kinetics of cotton roots and leaf discs differ. These differences may be related to the physiological roles (source versus sink) of the two tissues in the intact plant.

Pathway of Phloem unloading of sucrose in corn roots

The pathway of phloem unloading and the metabolism of translocated sucrose were determined in corn (Zea mays) seedling roots. Several lines of evidence show that exogenous sucrose, unlike translocated sucrose, is hydrolyzed in the apoplast prior to uptake into the root cortical cells. These include (a) presence of cell wall invertase activity which represents 20% of the total tissue activity; (b) similarity in uptake and metabolism of [(14)C]sucrose and [(14)C]hexoses; and (c) randomization of (14)C within the hexose moieties of intracellular sucrose following accumulation of [(14)C] (fructosyl)sucrose. Conversely, translocated sucrose does not undergo apoplastic hydrolysis during unloading. Asymmetrically labeled sucrose ([(14)C](fructose)sucrose), translocated from the germinating kernels to the root, remained intact indicating a symplastic pathway for unloading. In addition, isolated root protoplasts and vacuoles were used to demonstrate that soluble invertase activity (V(max) = 29 micromoles per milligram protein per hour, K(m) = 4 millimolar) was located mainly in the vacuole, suggesting that translocated sucrose entered via the symplasm and was hydrolyzed at the vacuole prior to metabolism.

A Reanalysis of the Two-Component Phloem Loading System in Beta vulgaris

Kinetic analysis of [(14)C]sucrose loading into sugar beet leaf discs revealed the presence of two transport components. At low exogenous sucrose concentrations, a saturable component, which exhibited Michaelis-Menten characteristics, was the main mode of transport. At concentrations greater than 50 millimolar, phloem loading was dominated by a linear component which appeared to operate as a first order kinetic transport process. Over the exogenous sucrose concentrations employed, influx could be described by the equation v = V(max)S/(S + K(m)) + kS. Influx via both processes was strongly pH-dependent. Evidence is presented that the linear component was not explicable in terms of simple diffusion, or exchange diffusion, into either mesophyll or minor vein phloem tissue. Extensive metabolic conversion of sucrose was not a factor contributing to influx at high external sucrose concentrations. At present, it is believed that both components operate in parallel at the membrane bounding the sieve element-companion cell complex. The saturable component is identified with sucrose-H(+) cotransport. While the significance of the linear component has been established, its nature remains to be elucidated.

Kinetics of C-photosynthate uptake by developing soybean fruit

By pulse-labeling field-grown soybean leaves for 60 seconds at midday with (14)CO(2) and then sequentially harvesting, dissecting, and extracting the radioactive fruit tissues (of pod and seeds), the route, uptake kinetics, and metabolic fate of (14)C-photosynthate as it was imported by 35- to 40-day-old pods were determined. As the [(14)C]sucrose pulse entered the pods, the seeds became radioactive immediately but a lag of nearly 30 minutes occurred before label could be detected in the pod wall pericarp.Import of the (14)C-pulse by the seeds was exclusively via the seed coats, where rapid unloading occurred. Maximum accumulation of label in the seed coat occurred in about 60 minutes at which time 59% of the total radioactivity in the fruit was in the three seed coats, whereas only 7% was in the cotyledons they enclosed. The photosynthate remained as [(14)C]sucrose as it passed through the seed coat, but appeared to be hydrolyzed relatively soon after uptake by the cotyledons. By 2.5 hours, 60% of the (14)C-photosynthate pulse had passed into the cotyledons with only 27% remaining in the seed coats. Inasmuch as there is no vascular connection between maternal seed coat and the developing embryo, cotyledonary uptake of sucrose released from the inner seed coat surface may require specialized transport mechanisms.