Monitoring Phloem Unloading and Post-Phloem Transport by Microperfusion of Attached Wheat Grains

Phloem unloading and post-phloem transport in developing wheat (Triticum aestivum L.) grains were investigated by perfusing the endosperm cavities of attached grains. Relative unloading ratio (RUR) and the rate of sucrose release into the endosperm cavity (SRR) were calculated, respectively, from 14C import and from sucrose washout from the cavity. RUR and SRR continued at or near in vivo rates over a wide range of cavity sap osmolality (90 to approximately 500 milliosmolal) and sucrose concentration (14-430 mM) and for long times (29 h). These are much greater ranges than have been observed for the endosperm cavity in vivo (230-300 milliosmolal, and 40-120 mM, respectively), indicating that neither the cavity sap osmolality nor sucrose concentration are controlling factors for the rate of assimilate import into the cavity. The maintenance of in vivo transport rates over a wide range of conditions strongly implicates the role of transport processes within the maternal tissues of the wheat grain, rather than activities of the embryo or endosperm, in determining the rate of assimilate import into the grain. RUR was decreased by high concentrations of sucrose and sorbitol, but not of mannitol. By plasmolyzing some chalazal cells, sorbitol appeared to block symplastic transport across the crease tissues, but neither sucrose nor mannitol caused plasmolysis in maternal tissues of attached grains. The inhibition of RUR by KCN and carbonyl cyanide m-chlorophenyl (CCCP) and the continued import of sucrose into grains against its concentration gradient suggest that solute movement into the endosperm cavity might occur by active membrane transport. However, the evidence is weak, since KCN and CCCP appeared to act primarily on some aspect of symplastic (i.e. nonmembrane) transport. Also, sucrose could move from the endosperm cavity into the maternal tissues (i.e. opposite to the normal direction of sucrose movement), suggesting that transmembrane movement in the nucellus may be a reversible process. Pressure-driven flow into the grain could account for movement against a concentration gradient.     

Doubled sugar content in sugarcane plants modified to produce a sucrose isomer

Sucrose is the feedstock for more than half of the world's fuel ethanol production and a major human food. It is harvested primarily from sugarcane and beet. Despite attempts through conventional and molecular breeding, the stored sugar concentration in elite sugarcane cultivars has not been increased for several decades. Recently, genes have been cloned for bacterial isomerase enzymes that convert sucrose into sugars which are not metabolized by plants, but which are digested by humans, with health benefits over sucrose. We hypothesized that an appropriate sucrose isomerase (SI) expression pattern might simultaneously provide a valuable source of beneficial sugars and overcome the sugar yield ceiling in plants. The introduction of an SI gene tailored for vacuolar compartmentation resulted in sugarcane lines with remarkable increases in total stored sugar levels. The high-value sugar isomaltulose was accumulated in storage tissues without any decrease in stored sucrose concentration, resulting in up to doubled total sugar concentrations in harvested juice. The lines with enhanced sugar accumulation also showed increased photosynthesis, sucrose transport and sink strength. This remarkable step above the former ceiling in stored sugar concentration provides a new perspective into plant source–sink relationships, and has substantial potential for enhanced food and biofuel production.     

土壤水分亏缺对水稻茎秆贮藏碳水化合物向籽粒运转的调节

以两个茎秆贮藏物质利用效率不同的水稻(Oryza sativa)杂交组合(‘汕优63’和‘Pc311/早献党’)为材料,进行土壤水分亏缺处理(Water-deficit),以水层灌溉为对照(Well-watered),研究水分亏缺对水稻茎贮藏性碳水化合物运转及其关键酶活性的调节作用。结果表明,水分亏缺促进了水稻茎秆贮藏物质的运转和对籽粒产量的贡献,开花前茎秆贮藏的碳水化合物对产量贡献率分别提高了1.9~3.0倍(与水层灌溉相比)。土壤水分亏缺诱导了水稻茎节间α-淀粉酶、β-淀粉酶、α-葡萄糖苷酶、D-酶活性上升,但淀粉磷酸化酶受到了抑制,说明土壤水分亏缺加强水稻茎秆贮藏淀粉水解途径,而不是磷酸解途径。就蔗糖代谢而言,土壤水分亏缺提高了蔗糖磷酸合成酶的活性和活化状态,抑制蔗糖转化酶活性,促进蔗糖合成,加速贮藏物质快速降解和转移,从而调节稻株贮藏碳水化合物向籽粒的分配。     

Differential effects of turgor on sucrose and potassium transport at the tonoplast and plasma membrane of sugar beet storage root tissue

When turgor was increased, by decreasing the concentration of mannitol bathing discs of sugar beet storage root tissue, the rates of sucrose and potassium uptake into the vacuole were decreased. At all external mannitol concentrations the rate of sucrose and potassium uptake across the plasma membrane was an order of magnitude greater than the rate of quasi-steady uptake into the vacuole, implying a very large efflux. Efflux of both sucrose and potassium was increased at high turgor. However, while increasing turgor decreased the rate of K⁺ uptake, the rate of sucrose uptake at the plasma membrane increased with time. Compartmental analysis of tracer exchange kinetics was used to determine unidirectional K⁺ fluxes. From these results, it was estimated that the increase in K⁺ efflux accompanying a 1.5 MPa increase in turgor could lead to a net increase of 140mol^?3h^?1 in the external potassium concentration. It is suggested that the turgor-imposed increase in solute efflux is a means of regulating intracellular osmotic pressure and/or turgor in sugar beet storage roots, but that sucrose is preferentially retrieved from the apoplast, even under conditions of excessively high turgor. However, much of this sucrose is probably lost from the cell, implying a ‘futile’ sucrose transport cycle at the plasma membrane. The turgor-stimulated leak of potassium could play a major role in the regulation of turgor pressure in sugar beet storage root tissue.     

Sugar Preferences in Nectar‐ and Fruit‐Eating Birds: Behavioral Patterns and Physiological Causes1

Sucrose, glucose, and fructose are the three sugars that commonly occur in floral nectar and fruit pulp. The relative proportions of these three sugars in nectar and fruit in relation to the sugar preferences of pollinators and seed dispersers have received considerable attention. Based on the research of Herbert and Irene Baker and their collaborators, a dichotomy between sucrose‐dominant hummingbird‐pollinated flowers and hexose‐dominant passerine flowers and fruits was proposed. Data on sugar preferences of several hummingbird species (which prefer sucrose) vs. a smaller sample of passerines (which prefer hexoses) neatly fitted this apparent dichotomy. This hummingbird–passerine dichotomy was strongly emphasized until the discovery of South African plants with sucrose‐dominant nectars, which are pollinated by passerines that are able to digest, and prefer sucrose. Now we know that, with the exception of two clades, most passerines are able to assimilate sucrose. Most sugar preference studies have been conducted using a single, relatively high, sugar concentration in the nectar (ca 20%). Thus, we lack information about the role that sugar concentration might play in sugar selection. Because many digestive traits are strongly affected not only by sugar composition, but also by sugar concentration, we suggest that preferences for different sugar compositions are concentration‐dependent. Indeed, recent studies on several unrelated nectar‐feeding birds have found a distinct switch from hexose preference at low concentrations to sucrose preference at higher concentrations. Finally, we present some hypotheses about the role that birds could have played in molding the sugar composition of plant rewards.     

Sucrose metabolism during the growth of Camellia japonica pollen

The free sugar in the mature pollen grains of Camellia japonica is almost all sucrose and the sucrose content decreases rapidly during pollen growth. The activity of soluble invertase increases during culturing and a high constant activity was found at the later stages of pollen tube growth. By contrast, the level of sucrose synthetase activity remains constant during pollen growth and that of wall-bound invertase activity is very low. Cycloheximide has little effect on the activity of these enzymes. Exogenous sucrose or glucose was simultaneously incorporated into the pollen grains when they absorbed water and swelled. The free sugar levels in growing pollen depend on the nature of the exogenous sugar. The sugar metabolism in the pollen at the stage of germination differs from that during tube growth, the latter being particularly influenced by exogenous sugar.     

Metabolism of free sugars in relation to starch synthesis in the developing Sorghum vulgare grain

Accumulation of starch at expense of its free-sugar precursors was studied in the developing grains of the ‘SL-44’variety of Sorghum vulgare Pers. The content of starch gradually increased with the maturation of the grain and this increase was relatively fast until 18 days after anthesis. The daily rate of starch accumulation was at a maximum 15 days after anthesis. The content of total free sugars, reducing sugars, non-reducing sugars other than sucrose, total and non-sucrosyl fructose, and glucose also increased, reaching maximum values at 18 days after anthesis. Sucrose content gradually increased with a concomitant decrease in the activity of invertase, and sucrose was the major non-reducing sugar in the matured grains. Detached heads incubated in labelled sugars indicated that, compared to sucrose and fructose. ¹⁴C was more efficiently incorporated from glucose into grain starch, which was maximally synthesized at the mid-milky stage of grain development. Exogenous supply of NAD⁺ plus ATP stimulated the in vivo incorporation of ¹⁴C from sucrose to starch. The decline in the rate of starch accumulation did not synchronise with that of protein synthesis.     

Sugar content and activity of sucrose metabolism enzymes in milled rice grain

Most rice (Oryza sativa L.) cultivars grown in the United States were selected for endosperm starch properties and not soluble sugar content. The minor pool of soluble sugar may affect the qualities of rice as a food. Some cultivar variation in soluble sugar content was detected in milled grain, essentially the starchy endosperm, of long grain varieties. Milled grain of cultivars Lemont and Texmati had a soluble sugar content of 0.21 and 0.35% (w/w), respectively, on a fresh weight basis. The dorsal portion of the milled grain contained the greatest amount of soluble sugar, approximately tenfold the amount found in the central core of the grain. Extracts of the milled grain contained sucrose-phosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities, which were separated by anion exchange chromatography. The presence of sucrose-phosphate synthase in the rice endosperm suggested a mechanism for sucrose accumulation which might be involved in carbon partitioning during grain development.     

Sugar uptake and transport in rice embryo. Expression of companion cell-specific sucrose transporter (OsSUT1) induced by sugar and light

We investigated sugar uptake and transport in rice (Oryza sativa) embryo during grain germination. Endogenous sugar levels, accumulation of starch granules, and gene expression of a rice sucrose transporter (OsSUT1) were examined using rice embryos germinated with or without exogenous sugar supply. Starch granules remarkably accumulated in the cells around vascular bundles as a consequence of the sugar taken up by the embryos, indicating that the taken-up sugars are transiently converted into starch. In situ detection for OsSUT1 mRNA indicated its localization in the phloem companion cells. Furthermore, northern-blot and in situ hybridization analyses showed that OsSUT1 expression is not detectable in embryos subjected to sugar starvation conditions, whereas its expression is enhanced by an increased endogenous sugar level. Overall results indicate that the expression of companion cell-specific sucrose transporter, OsSUT1 is regulated by the endogenous sugar status as well as light exposure.     

Cultivar differences in soluble sugar content of mature rice grain

Mature grain from 31 rice ( Oryza sativa L.) cultivars grown in the same location was analyzed for soluble sugar content to determine quantitative differences in this parameter. Cultivar variation in caryopsis sucrose content was 4-fold ranging from 15 to 59 μmol sucrose (g fresh weight)⁻¹. Soluble reducing sugar ranged from 7 to 15 μmol hexose (g fresh weight)⁻¹. Soluble sugar content was much greater in the outer part of the grain than in the endosperm. Caryopsis enzyme activities were measured in 8 cultivars having a range of grain sucrose content. No relationship between grain sucrose content of these cultivars and the level of enzymes of sucrose metabolism, glycolysis, and oxidative pentose pathway was established. Caryopsis respiration after 1 h of imbibition also was not related to the amount of soluble sugars available in the grain among these 8 cultivars. The results show that there are significant differences in the sugar-accumulating capacity of the caryopsis of different rice cultivars. The detection of a fructose 2,6-bisphosphate-sensitive, PP_i: fructose 6-phosphate phosphototransferase (EC 2.7.1.90) in the endosperm suggests the presence of a regulatory mechanism for sucrose/starch partitioning established in other plant tissues.     

In Vitro Sugar Transport in Zea mays L. Kernels : II. Characteristics of Sugar Absorption and Metabolism by Isolated Developing Embryos

In vitro sugar transport into developing isolated maize embryos was studied. Embryo fresh and dry weight increased concomitantly with endogenous sucrose concentration and glucose uptake throughout development. However, endogenous glucose and fructose concentration and sucrose uptake remained constant. The uptake kinetics of radiolabeled sucrose, glucose, and fructose showed a biphasic dependence on exogenous substrate concentration. Hexose uptake was four to six times greater than sucrose uptake throughout development. Carbonylcyanide-m-chlorophenylhydrazone and dinitrophenol inhibited sucrose and glucose uptake significantly, but 3-O-methyl glucose uptake was less affected. The uptake of 1 millimolar sucrose was strongly pH dependent while glucose was not. Glucose and fructose were readily converted to sucrose and insoluble products soon after absorption into the embryo. Thus, sucrose accumulated, while glucose pools remained low. Based on the findings of this and other studies a model for sugar transport in the developing maize kernel is presented.     

Manipulation of xylem transport affects Zn and Mn transport into developing wheat grains of cultured ears

Zinc and manganese loading into developing wheat grain is little understood at present. The objective of this work was to investigate factors that may affect the rate of transport of Zn and Mn into developing wheat grain of cultured ears. Ears 18-22 days post-anthesis were cultured in solutions containing labelled Zn and Mn. The effect of additions of Cu, Fe, citrate, malate and EDTA to the culture solution was observed. The effect of humidity and awn removal as well as the sucrose status of the ears on Zn and Mn transport was also investigated. The effect of high concentration of Zn and Mn on [¹⁴C]-sucrose transport was determined. High humidity almost completely blocked transport of Zn and Mn into the grain. Awn removal reduced the transport of Zn and Mn to the lemma but not the grain. When the ears were depleted of sucrose (by maintaining them in the dark prior to labelling) transport of Zn and Mn to the grain was reduced compared to ears cultured with sucrose. The presence of Cu reduced the loading of Zn into the grain. There was little effect of Cu on Mn transport or Fe on either Zn or Mn transport. High concentrations of Zn and Mn in the culture solution did not affect [¹⁴C]-sucrose loading into the grain but loading of Zn and Mn was limited at high concentrations suggesting membrane saturation. This study demonstrates that sucrose status and humidity clearly influence the transport of Zn and Mn into the grain, and that other ions may influence Zn and Mn transport.     

Effects of cold‐girdling on flows in the transport phloem in Ricinus communis: is mass flow inhibited?

The effects of cold girdling of the transport phloem at the hypocotyl of Ricinus communis on solute and water transport were investigated. Effects on the chemical composition of saps of phloem and xylem as well as of stem tissue were studied by conventional techniques and the water flow in the phloem was investigated by NMR imaging. Cold girdling reduced the concentration of sucrose but not that of inorganic solutes or amino acids in phloem saps. The possibility that cold treatment inhibited the retrieval of sucrose into the phloem, following leaching from the sieve tubes along a chemical gradient is discussed. Leaching of other solutes did not occur, as a result of missing promoting gradients in stem tissue. Following 3 d of cold girdling, sugar concentration increased and starch was synthesized and accumulated in stem tissue above the cold girdling region and along the cold-treated phloem pathway due to leaching of sugars from the phloem. Only in the very first period of cold girdling (<15-30 min) was mass flow inhibited, but recovered in the rest of cold treatment period to values similar to the control period before and the recovery period after the cold treatment. It is concluded that cold treatment affected phloem transport through two independent and reversible processes: (1) a permanent leaching of sucrose from the phloem stem without normal retrieval during cold treatment, and (2) a short-term inhibition of mass flow at the beginning of cold treatment, possibly involving P proteins. Possible further mechanisms for reversible inhibition of water flow are discussed.     

Conversion of sucrose to starch in the developing Pennisetum typhoides grain

Developing grains of pearl millet ( Pennisetum typhoides Burm. S & H cv. PIB 155) were sampled and analyzed for starch and its free-sugar precursors. The activities of invertase, sucrose-ADP (UDP) glucosyl transferase and of α-amylase and β-amylase in relation to the rate of starch accumulation in the developing grain were assayed. By culturing detached ears, the incorporation of ¹⁴C from free sugar precursors to starch was studied. The starch content gradually increased until grain maturity. The rate of starch accumulation was maximum around 12 days after anthesis. Around this period, the activities of sucrose-ADP(UDP) glucosyl transferase and α-amylase, β-amylase were also at a peak. Invertase activity was high during the early period of grain development but gradually declined as the grains matured. In the most actively metabolising milky grains, incorporation of ¹⁴C from [¹⁴C]-sugars to starch was maximum in the mid mid-milky grains. Addition of 20 m M K⁺ to the culture solution did not affect the incorporation of ¹⁴C from supplied sucrose to the free sugar pool and to the starch of the grain, but Mg²⁺ supply at 20 m M concentration lowered ¹⁴C incorporation from exogenous sucrose to grain free sugars, although the utilization of the latter for starch synthesis was enhanced.     

Sugar assimilation and digestive efficiency in Wahlberg's epauletted fruit bat (Epomophorus wahlbergi)

Fruit- and nectar-feeding bats have high energy demands because of the cost of flight, and sugar is a good fuel because it is easily digested and absorbed. This study investigated the digestive efficiency of different sugars at different concentrations in Wahlberg's epauletted fruit bat (Epomophorus wahlbergi). We predicted that the sugar type and concentration would affect the total amount of solution consumed, while the total energy gained and the apparent assimilation efficiency would be high, irrespective of sugar type or concentration. Equicaloric solutions of two sugar types, glucose and sucrose, at low (10%), medium (15%) and high (25%) concentrations were offered in separate trials to bats. Total amount of solution consumed, total energy gained from each solution, and apparent assimilation efficiency, were measured. Bats had higher total volumetric intake of glucose and sucrose at the low concentrations than at the higher concentrations. However, bats maintained similar total energy intake on the respective glucose and sucrose concentrations. Bats were found to have high assimilation efficiencies on both glucose and sucrose irrespective of concentration. As bats used both sugars efficiently to maximize and maintain energy gain, it is expected that they feed opportunistically on fruit in the wild depending on temporal and spatial availability to obtain their energy requirements. Furthermore, fruit with high sucrose or glucose content will be consumed.     

Kinetics of the lamellar gel–fluid transition in phosphatidylcholine membranes in the presence of sugars

Phase diagrams are presented for dipalmitoylphosphatidylcholine (DPPC) in the presence of sugars (sucrose) over a wide range of relative humidities (RHs). The phase information presented here, determined by small angle X-ray scattering (SAXS), is shown to be consistent with previous results achieved by differential scanning calorimetry (DSC). Both techniques show a significant effect of sucrose concentration on the phase behaviour of this phospholipid bilayer. An experimental investigation into the effect of sugars on the kinetic behaviour of the gel to fluid transition is also presented showing that increasing the sugar content appears to slightly increase the rate at which the transition occurs.     

The Influence of Sucrose and Abscisic Acid on the Determination of Grain Number in Wheat

Using detached ears of wheat cultured in nutrient solution theeffect of sucrose and ABA supply on grain number was examined.In ears detached prior to ear emergence, to correspond withthe time of pollen mother cell meiosis, higher levels of sucroseuptake were associated with higher grain number per ear. ABAindirectly caused a reduction in grain number by decreasinguptake of sucrose through decreasing stomatal conductance andtranspiration. There was possibly also a direct effect of ABAon grain number. At higher levels of ABA the negative effecton grain number was decreased by increasing the sucrose concentration.In ears detached one to three days after anthesis grain numberwas also influenced by the amount of sucrose taken up. althoughgrain number was not as sensitive to the sucrose supply at thistime as it was at meiosis. The influence of ABA following anthesiswas limited to an indirect effect on grain number through reductionsin sucrose uptake. Key words: Wheat, Sucrose, ABA, Grain number