Control of photosynthetic sucrose synthesis in barley primary leaves: role of fructose 2,6-bisphosphate

Levels of fructose 2,6-bisphosphate (F2,6BP) and related metabolites were measured in 8- or 9-day-old barley (Hordeum vulgare L.) primary leaves throughout a 24 hour cycle. Young barley leaves contained about 0.4 nanomole F2,6BP per milligram chlorophyll at the end of a 12 hour dark period. F2,6BP levels increased rapidly following a dark-to-light transition and then decreased to about 0.1 nanomole per milligram chlorophyll after 5 or 10 minutes of light. Low levels of F2,6BP were detected in barley primary leaves throughout the day. A 10-fold increase in F2,6BP was observed during the first hour of the dark period and then levels of this metabolite decreased slowly for the next several hours. Only small diurnal fluctuations were noted in barley leaf glucose 6-phosphate and uridine 5′-diphosphoglucose levels. There were rapid changes in whole leaf F2,6BP levels when the light intensity was altered. High F2,6BP levels in the dark were not observed after short photosynthetic periods. Results obtained with barley primary leaves support the suggestion that F2,6BP is involved in regulating the flow of photosynthate from the chloroplast to sucrose. Extractable sucrose-phosphate synthase activity was inversely related to barley primary leaf F2,6BP levels. This finding may indicate that the activities of sucrose-phosphate synthase and cytosolic fructose 1,6-bisphosphatase in barley primary leaves are metabolically coordinated.     

Evidence for control of carbon partitioning by fructose 2,6-bisphosphate in spinach leaves

Excision of spinach (Spinacia oleracea L.) leaves had no effect on photosynthetic rates, but altered normal carbon partitioning to favor increased formation of starch and decreased formation of sucrose. The changes were evident within 2 hours after excision. Concurrently, leaf fructose-2,6-bisphosphate content increased about 5-fold (from 0.1 to 0.5 nanomoles per gram fresh weight). The activities of sucrose-P synthase and cytoplasmic fructose 1,6-bisphosphatase in leaf extracts remained constant during the time period tested. It is postulated that the rise in fructose 2,6-bisphosphate was responsible for the change in carbon partitioning.     

A Rapid Increase in Spinach Leaf Fructose 2,6-Bisphosphate Occurs during a Light to Dark Transition

Spinach leaf fructose 2,6-bisphosphate levels increase rapidly during the first 15 minutes of a normal dark period followed by a gradual decline during the next 5 hours. The regulatory mechanism responsible for the dark-induced rise in fructose 2,6-bisphosphate levels can be counteracted by a brief exposure to light intensities greater than 1 microeinstein per square meter per second.     

Control of Photosynthetic Sucrose Synthesis by Fructose 2,6-Bisphosphate : II. Partitioning between Sucrose and Starch

The role of fructose 2,6 bisphosphate in partitioning of photosynthate between sucrose and starch has been studied in spinach (Spinacia oleracea U.S. hybrid 424). Spinach leaf material was pretreated to alter the sucrose content, so that the rate of starch synthesis could be varied. The level of fructose 2,6-bisphosphate and other metabolites was then related to the accumulation of sucrose and the rate of starch synthesis. The results show that fructose 2,6-bisphosphate is involved in a sequence of events which provide a fine control of sucrose synthesis so that more photosynthate is diverted into starch in conditions when sucrose has accumulated to high levels in the leaf tissue. (a) As sucrose levels in the leaf rise, there is an accumulation of triose phosphates and hexose phosphates, implying an inhibition of sucrose phosphate synthase and cytosolic fructose 1,6-bisphosphatase. (b) In these conditions, fructose 2,6-bisphosphate increases. (c) The increased fructose 2,6-bisphosphate can be accounted for by the increased fructose 6-phosphate in the leaf. (d) Fructose 2,6-bisphosphate inhibits the cytosolic fructose 1,6-bisphosphatase so more photosynthate is retained in the chloroplast, and converted to starch.     

A comparative analysis of fructose 2,6-bisphosphate levels and photosynthate partitioning in the leaves of some agronomically important crop species

Starch, sucrose, and fructose 2,6-bisphosphate (F2, 6BP) levels were measured in pea (Pisum sativum L.), maize (Zea mays L.), onion (Allium cepa L.) and soybean (Glycine max L.) leaves throughout a light/dark cycle. Leaf starch accumulated in pea, maize, and soybean but not in onion. Sucrose was a major leaf storage reserve in pea, maize, and onion but was only found at low levels in soybean. In all species examined, the most dramatic changes in F2,6BP concentration coincided with light/dark transitions. During the light period F2,6BP levels were about 0.1 nanomole/milligram chlorophyll in soybean source leaves and there was a small increase in effector concentration in the dark. Levels of F2,6BP were also low in pea and maize leaves during the light period but then increased 10- or 20-fold in the dark. Dark onion leaf F2,6BP levels were about 1.1 to 1.3 nanomole/milligram chlorophyll and these values decreased by 20 to 30% in the light. Thus, three different patterns were identified that describe diurnal F2,6BP levels in source leaves. These results support the suggestion that F2,6BP is involved in the regulation of sucrose biosynthesis. However, it was not possible to demonstrate that high levels of F2,6BP are essential for starch synthesis in the chloroplast.     

A role for fructose 2,6-bisphosphate in the regulation of sucrose synthesis in spinach leaves

The subcellular distribution of fructose 2,6-bisphosphate in spinach (Spinacia oleracea) leaves was studied using nonaqueous fractionation, showing that all, or almost all, is located in the cytosol. The amount of fructose 2,6-bisphosphate present in leaves during the diurnal cycle was measured and compared to the accumulation of starch and sucrose, and the amounts of selected phosphorylated intermediates in the leaf. Upon illumination, the level of fructose 2,6-bisphosphate decreases, but prolonged illumination leads to an increase in the level to above that found in the dark, which accompanies the onset of rapid accumulation of starch in the leaf.     

Control of Photosynthetic Sucrose Synthesis by Fructose 2,6-Bisphosphate : I. Coordination of CO(2) Fixation and Sucrose Synthesis

A mechanism is proposed for a feed-forward control of photosynthetic sucrose synthesis, which allows withdrawal of carbon from the chloroplast for sucrose synthesis to be coordinated with the rate of carbon fixation. (a) Decreasing the rate of photosynthesis of spinach (Spinacia oleracea, U.S. hybrid 424) leaf discs by limiting light intensities or CO₂ concentrations leads to a 2-to 4-fold increase in fructose 2,6-bisphosphate. (b) This increase can be accounted for by lower concentrations of metabolites which inhibit the synthesis of fructose 2,6-bisphosphate, such as dihydroxyacetone phosphate and 3-phosphoglycerate. (c) Thus, as photosynthesis decreases, lower levels of dihydroxyacetone phosphate should inhibit the cytosolic fructose bisphosphatase via simultaneously lowering the concentration of the substrate fructose 1,6-bisphosphate, and raising the concentration of the inhibitor fructose 2,6-bisphosphate.     

Regulation of climacteric respiration in ripening avocado fruit

Ripening of avocado fruit is associated with a dramatic increase in respiration. In vivo³¹P nuclear magnetic resonance spectroscopy revealed large increases in ATP levels accompanying the increase in respiration. Both glycolytic enzymes, phosphofructokinase, and pyrophosphate: fructose-6-phosphate phosphotransferase were present in avocado fruit with the latter activity being highly stimulated by fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate levels increased approximately 90% at the onset of ripening, suggesting that the respiratory increase in ripening avocado fruit may be regulated by the activation of pyrophosphate:fructose-6-phosphate phosphotransferase by an increase in fructose 2,6-bisphosphate.     

Photosynthetic carbohydrate metabolism in wheat (Triticum aestivum L.) leaves: optimization of methods for determination of fructose 2, 6-bisphosphate

The accurate measurement of fructose 2,6-bisphosphate from plants such as wheat is fraught with difficulty. Extraction and assay methods for fructose 2,6-bisphosphate that give near 100% recovery of the metabolite, and a linear response with volume have therefore been developed for extracts prepared from wheat leaves of different ages. Amounts of fructose 2,6-bisphosphate in different regions of leaves generally showed a positive correlation with chlorophyll content. Measurements of sucrose and starch in third leaves harvested at different times of the diurnal cycle demonstrated that sucrose is the major form in which photosynthate is stored in the leaf, but starch can account for up to about 30% of the stored carbohydrate. Virtually all of the carbohydrate accumulated as starch and sucrose during the day was degraded at night. Amounts of fructose 2,6-bisphosphate were generally lower in extracts prepared from leaves harvested in the light than in the dark. Additionally, there was no change in either the amount of fructose 2, 6-bisphosphate or the ratio of sucrose to starch in samples prepared from leaves harvested at different times of the day. These results are broadly consistent with a role for fructose 2,6-bisphosphate in the regulation of sucrose synthesis and the partitioning of carbohydrate between sucrose and starch in wheat leaves.     

Effects of Elevated CO(2) Concentrations on Glycolysis in Intact ;Bartlett' Pear Fruit

Mature intact `Bartlett' pear fruit (Pyrus communis L.) were stored under a continuous flow of air or air + 10% CO₂ for 4 days at 20°C. Fruit kept under elevated CO₂ concentrations exhibited reduced respiration (O₂ consumption) and ethylene evolution rates, and remained firmer and greener than fruit stored in air. Protein content, fructose 1,6-bisphosphate levels, and ATP:phosphofructokinase and PPi:phosphofructokinase activities declined, while levels of fructose 6-phosphate and fructose 2,6-bisphosphate increased in fruit exposed to air + 10% CO₂. These results are discussed in light of a possible inhibitory effect of CO₂ at the site of action of both phosphofructokinases in the glycolytic pathway, which could account, at least in part, for the observed reduction in respiration.     

Diurnal Changes in Maize Leaf Photosynthesis : II. Levels of Metabolic Intermediates of Sucrose Synthesis and the Regulatory Metabolite Fructose 2,6-Bisphosphate

Diurnal changes in the regulatory metabolite, fructose-2,6-bisphosphate (F26BP), and key metabolic intermediates of sucrose biosynthesis were studied in maize (Zea mays L. cv Pioneer 3184) during a day-night cycle. Whole leaf concentrations of dihydroxyacetonephosphate (DHAP) and fructose 1,6-bisphosphate changed markedly during the photoperiod. DHAP concentration was correlated positively with the rate of sucrose formation in vivo (assimilate export plus sucrose accumulation) and extractable activity of sucrose phosphate synthase (SPS). The changes closely followed net photosynthetic rate, which tracked irradiance. The other metabolic intermediates measured (glucose 6-phosphate, fructose 6-phosphate, and UDP-glucose) were either relatively constant over the 24 hour period or changed in a different pattern. Diurnal changes in leaf F26BP concentrations were pronounced, and fundamentally different than the pattern reported with other species. F26BP concentration decreased at the beginning of the day and remained low and constant; a 3- to 4-fold increase occurred with darkness, and slowly declined thereafter. In general, leaf F26BP concentration was negatively correlated with net photosynthetic rate, and also leaf DHAP concentration. Consequently, co-ordination of the regulation of cytosolic fructose 1,6-bisphosphatase and SPS was apparent. The results support the postulate that in maize leaves the activation state of SPS may be dependent on availability of DHAP and possibly other metabolites.     

Fructose 2,6-bisphosphate levels in mature leaves of tobacco (Nicotiana tabacum) and potato (Solanum tuberosum)

Here we show that fructose 2,6-bisphosphate cannot be reliably measured in mature leaves of tobacco (Nicotiana tabacum L.), potato (Solanum tuberosum L.), or stinging nettle (Urtica dioica L.) using conventional extraction techniques, since the recoveries of fructose 2,6-bisphosphate added during extraction are poor. However, fructose 2,6-bisphosphate could be extracted by boiling leaves in ethanol and aqueous buffer. Evidence for the reliability of this technique is provided by high recovery measurements of fructose 2,6-bisphosphate added to the leaves before extraction. This extraction method was used to measure changes in the level of fructose 2,6-bisphosphate throughout the photoperiod in tobacco and potato leaves. These changes are compared with the rate of accumulation of sucrose and starch in the leaf samples. Variations in the levels of fructose 2,6-bisphosphate, and the relationship between this metabolite and sucrose and starch accumulation in these leaves during the photoperiod are similar to the pattern observed in leaves of other plant species.Abbreviations BSA bovine serum albumin - Fru-2,6-P₂ fructose 2,6-bisphosphate This research was supported by the Agricultural and Food Research Council (Grant no. PG43/531), and the Royal Society.     

Kinetic properties of pyrophosphate:fructose-6-phosphate phosphotransferase from germinating castor bean endosperm

Pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) was purified over 500-cold from endosperm of germinating castor bean (Ricinus commiunis L. var. Hale). The kinetic properties of the purified enzyme were studied. PFP was specific for pyrophosphate and had a requirement for a divalent metal ion. The pH optimum for activity was 7.3 to 7.7. The enzyme had similar activities in the forward and reverse directions and exhibited hyperbolic kinetics with all substrates. Kinetic constants were determined in the presence of fructose 2,6-bisphosphate, which stimulated activity about 20-fold and increased the affinity of the enzyme for fructose 6-phosphate, fructose 1,6-bisphosphate, and pyrophosphate up to 10-fold. Half-maximum activation of PFP by fructose 2,6-bisphosphate was obtained at 10 nanomolar. The affinity of PFP for this activator was reduced by decreasing the concentration of fructose 6-phosphate or increasing that of phosphate. Phosphate inhibited PFP when the reaction was measured in the reverse direction, i.e. fructose 6-phosphate production. In the presence of fructose 2,6-bisphosphate, phosphate was a mixed inhibitor with respect to both fructose 6-phosphate and pyrophosphate when the reaction was measured in the forward direction, i.e. fructose 1,6-bisphosphate production. The possible roles of fructose 2,6-bisphosphate, fructose 6-phosphate, and phosphate in the control of PFP are discussed.     

Effect of ethylene treatment on the concentration of fructose-2,6-bisphosphate and on the activity of phosphofructokinase 2/fructose-2,6-bisphosphatase in banana

Preclimacteric bananas fruits were treated for 12 h with ethylene to induce the climacteric rise in respiration. One day after the end of the hormonal treatment, the two activities of the bifunctional enzyme, phosphofructokinase 2/fructose-2,6-bisphosphatase started to increase to reach fourfold their initial value 6 days later. By contrast, the activities of the pyrophosphate-dependent and of the ATP-dependent 6-phosphofructo-1-kinases remained constant during the whole experimental period, the first one being fourfold greater than the second. The concentrations of fructose 2,6-bisphosphate and of fructose 1,6-bisphosphate increased in parallel during 4 days and then slowly decreased, the second one being always about 100-fold greater than the first. The change in fructose 2,6-bisphosphate concentration can be partly explained by the rise of the bifunctional enzyme, but also by an early increase in the concentration of fructose 6-phosphate, the substrate of all phosphofructokinases, and also by the decrease in the concentration of glycerate 3-phosphate, a potent inhibitor of phosphofructokinase 2. The burst in fructose 2,6-bisphosphate and the activity of the pyrophosphate-dependent phosphofructokinase, which is in banana the only enzyme known to be sensitive to fructose 2,6-bisphosphate, can explain the well-known increase in fructose 1,6-bisphosphate which occurs during ripening.     

Differential inhibition and activation of two leaf dihydroxyacetone phosphate reductases : role of fructose 2,6-bisphosphate

The chloroplastic and cytosolic forms of spinach (Spinacia oleracea cv Long Standing Bloomsdale) leaf NADH:dihydroxyacetone phosphate (DHAP) reductase were separated and partially purified. The chloroplastic form was stimulated by dithiothreitol, reduced thioredoxin, dihydrolipoic acid, 6-phosphogluconate, and phosphate; the cytosolic isozyme was stimulated by fructose 2,6-bisphosphate but not by reduced thioredoxin. End product components that severely inhibited both forms of the reductase included lipids and free fatty acids, membranes, and glycerol phosphate. In addition, two groups of inhibitory peptides were obtained from the fraction precipitated by 70 to 90% saturation with (NH₄)₂SO₄. Chromatography of this fraction on Sephadex G-50 revealed a peptide peak of about 5 kilodaltons which inhibited the chloroplastic DHAP reductase and a second peak containing peptides of about 2 kilodaltons which inhibited the cytosolic form of the enzyme. Regulation of the reduction of dihydroxyacetone phosphate from the C₃ photosynthetic carbon cycle or from glycolysis is a complex process involving activators such as thioredoxin or fructose 2,6-bisphosphate, peptide and lipid inhibitors, and intermediary metabolites. It is possible that fructose 2,6-bisphosphate increases lipid production by stimulating DHAP reductase for glycerol phosphate production as well as inhibiting fructose 1,6-bisphosphatase to stimulate glycolysis.     

Fructose 2,6-bisphosphate and the climacteric in bananas

This work was done to test the view that there is a marked rise in the content of fructose 2,6-bisphosphate during the climacteric of the fruit of banana (Musa cavendishii Lamb ex. Paxton). Bananas were ripened in the dark in a continuous stream of air in the absence of exogenous ethylene. CO2 production and the contents of fructose 2,6-bisphosphate and sucrose were monitored over a 15-day period. A range of extraction procedures for fructose 2,6-bisphosphate were compared. Recovery of fructose 2,6-bisphosphate added to samples of unripe fruit varied from poor to unmeasurable. Recoveries from samples of ripe fruit were high. It is argued that this differential recovery of fructose 2,6-bisphosphate undermines claims that the amount of this compound increases at the climacteric. When recoveries are taken into account, our data suggest that there is no major change in fructose 2,6-bisphosphate content during the onset of the climacteric in bananas.     

Effect of fructose 2,6-bisphosphate on the kinetic properties of cytoplasmic fructose 1,6-bisphosphatase from germinating castor bean endosperm

The cytoplasmic form of fructose 1,6-bisphosphatase (FBPase) was purified over 60-fold from germinating castor bean endosperm (Ricinus communis). The kinetic properties of the purified enzyme were studied. The preparation was specific for fructose 1,6-bisphosphate and exhibited optimum activity at pH 7.5. The affinity of the enzyme for fructose 1,6-bisphosphate was reduced by AMP, which was a mixed linear inhibitor. Fructose 2,6-bisphosphate also inhibited FBPase and induced a sigmoid response to fructose 1,6-bisphosphate. The effects of fructose 2,6-bisphosphate were enhanced by low levels of AMP. The latter two compounds interacted synergistically in inhibiting FBPase, and their interaction was enhanced by phosphate which, by itself, had little effect. The enzyme was also inhibited by ADP, ATP, UDP and, to a lesser extent, phosphoenolpyruvate. There was no apparent synergism between UDP, a mixed inhibitor, and fructose 2,6-bisphosphate. Similarly ADP, a predominantly competitive inhibitor, did not interact with fructose 2,6-bisphosphate. Possible roles for fructose 2,6-bisphosphate and the other effectors in regulating FBPase are discussed.     

Seasonal Changes in the Photosynthetic Rate in Apple Trees : A Comparison between Fruiting and Nonfruiting Trees

Seasonal changes in photosynthesis of apple trees (Malus domestica Borkh.) were monitored to examine the effect of source-sink interactions on photosynthesis and photorespiration. Elevated photosynthetic rates were observed during two periods of the growing season and correlated with the fruiting process. The first period of increased photosynthetic rates was during the bloom period, when spur leaves on flowering shoots exhibited up to 25% higher photosynthetic rates than vegetative spur leaves on a leaf area basis. CO₂ assimilation rates were also higher in fruiting trees than nonfruiting trees during the period of rapid fruit growth from July to September. Photorespiration, dark respiration, leaf resistance, and transpiration exhibited no seasonal changes which correlated to the presence or absence of fruit. These data represent the first comprehensive examination of the effects of flowering/fruit formation on photosynthesis and photorespiration in perennial plants.     

Fructose 2,6-bisphosphate in Dictyostelium discoideum. Independence of cyclic AMP production and inhibition of fructose-1,6-bisphosphatase

The occurrence of fructose 2,6-bisphosphate was detected in Dictyostelium discoideum. The levels of this compound were compared with those of cyclic AMP and several glycolytic intermediates during the early stages of development. Removal of the growth medium and resuspension of the organism in the differentiation medium decreased the content of fructose 2,6-bisphosphate to about 20% within 1 h, remaining low when starvation-induced development was followed for 8 h. The content of cyclic AMP exhibited a transient increase that did not correlate with the change in fructose 2,6-bisphosphate. If after 1 h of development 2% glucose was added to the differentiation medium, fructose 2,6-bisphosphate rapidly rose to similar levels to those found in the vegetative state, while the increase in cyclic AMP was prevented. The contents of hexose 6-phosphates, fructose 1,6-bisphosphate and triose phosphates changed in a way that was parallel to that of fructose 2,6-bisphosphate, and addition of sugar resulted in a large increase in the levels of these metabolites. The content of fructose 2,6-bisphosphate was not significantly modified by the addition of the 8-bromo or dibutyryl derivatives of cyclic AMP to the differentiation medium. These results provide evidence that the changes in fructose 2,6-bisphosphate levels in D. discoideum development are not related to a cyclic-AMP-dependent mechanism but to the availability of substrate. Fructose 2,6-bisphosphate was found to inhibit fructose-1,6-bisphosphatase activity of this organism at nanomolar concentrations, while it does not affect the activity of phosphofructokinase in the micromolar range. The possible physiological implications of these phenomena are discussed.     

Cyclic AMP regulation of fructose metabolism in germinatingPilobolus longipes spores

DormantPilobolus longipes spores metabolized fructose primarily to ethanol, CO₂, and trehalose. Cyclic AMP-induced spore activation was accompanied by a large stimulation of glycolytic activity. Mobilization of reserves, which was cyclic AMP dependent, accounted for a portion of the glycolytic product. The remaining product was derived from exogenous fructose. Increases in both fructose transport activity and hexose 6-phosphate levels were associated with 6-deoxyglucose-induced spore activation. Phosphofructokinase-1 activity in spore extracts was almost totally dependent upon fructose, 2,6-bisphosphate. High fructose 2,6-bisphosphate levels were correlated with rapid fructose metabolism. However, fructose alone caused a rise in fructose 2,6-bisphosphate content (sufficient to fully stimulate phosphofructokinase-1 activity) but there was no concurrent stimulation of glycolysis. These results suggest that glycolytic rates are determined mainly by hexose 6-phosphate levels and that cyclic AMP regulation of transport is an important determinant of hexose 6-phosphate concentration.