Intracellular metabolite gradients and flow of carbon during photosynthesis of leaf protoplasts

Rapid in situ 13C tracing of sucrose utilization in Arabidopsis sink and source leaves

BACKGROUND

Conventional metabolomics approaches face the problem of hidden metabolic phenotypes where only fluxes are altered but pool sizes stay constant. Metabolic flux experiments are used to detect such hidden flux phenotypes. These experiments are, however, time consuming, may be cost intensive, and involve specialists for modeling. We fill the gap between conventional metabolomics and flux modeling. We present rapid stable isotope tracing assays and analysis strategies of ¹³C labeling data. For this purpose, we combine the conventional metabolomics approach that detects significant relative changes of metabolite pool sizes with analyses of differential utilization of ¹³C labeled carbon. As a test case, we use uniformly labeled ¹³C-sucrose.

RESULTS

We present petiole and hypocotyl feeding assays for the rapid in situ feeding (≤ 4 h) of isotopically labeled metabolic precursor to whole Arabidopsis thaliana rosettes. The assays are assessed by conventional gas chromatography-mass spectrometry based metabolite profiling that was extended by joined differential analysis of ¹³C-labeled sub-pools and of ¹³C enrichment of metabolites relative to the enrichment of ¹³C-sucrose within each sample. We apply these analyses to the sink to source transition continuum of leaves from single A. thaliana rosettes and characterize the associated relative changes of metabolite pools, as well as previously hidden changes of sucrose-derived carbon partitioning. We compared the contribution of sucrose as a carbon source in predominantly sink to predominantly source leaves and identified a set of primary metabolites with differential carbon utilization during sink to source transition.

CONCLUSION

The presented feeding assays and data evaluation strategies represent a rapid and easy-to-use tool box for enhanced metabolomics studies that combine differential pool size analysis with screening for differential carbon utilization from defined stable isotope labeled metabolic precursors.

Dynamic regulatory on/off minimization for biological systems under internal temporal perturbations

Background

Flux balance analysis (FBA) together with its extension, dynamic FBA, have proven instrumental for analyzing the robustness and dynamics of metabolic networks by employing only the stoichiometry of the included reactions coupled with adequately chosen objective function. In addition, under the assumption of minimization of metabolic adjustment, dynamic FBA has recently been employed to analyze the transition between metabolic states.

Results

Here, we propose a suite of novel methods for analyzing the dynamics of (internally perturbed) metabolic networks and for quantifying their robustness with limited knowledge of kinetic parameters. Following the biochemically meaningful premise that metabolite concentrations exhibit smooth temporal changes, the proposed methods rely on minimizing the significant fluctuations of metabolic profiles to predict the time-resolved metabolic state, characterized by both fluxes and concentrations. By conducting a comparative analysis with a kinetic model of the Calvin-Benson cycle and a model of plant carbohydrate metabolism, we demonstrate that the principle of regulatory on/off minimization coupled with dynamic FBA can accurately predict the changes in metabolic states.

Conclusions

Our methods outperform the existing dynamic FBA-based modeling alternatives, and could help in revealing the mechanisms for maintaining robustness of dynamic processes in metabolic networks over time.

A re-investigation of the path of carbon in photosynthesis utilizing GC/MS methodology. Unequivocal verification of the participation of octulose phosphates in the pathway

A GC/EIMS/SIM methodology has been developed to re-examine the path of carbon in photosynthesis. Exposing isolated spinach chloroplasts to 13CO2 on a solid support for a defined period followed by quenching and work-up provided a mixture of labelled sugar phosphates. After enzymatic dephosphorylation and derivatization, the Mox-TMS sugars were analysed using the above method. The purpose of the study was to try to calculate the atom% enrichment of 13C in as many of the individual carbons in each of the derivatized sugars as was practical using diagnostic fragment ions. In the event, only one 45 s experiment provided sufficient data to enable a range of enrichment values to be calculated. This confirmed that D-glycero-D-altro-octulose phosphate was present in the chloroplasts and was heavily labelled in the C4, C5 and C6 positions, in keeping with the hypothesis that it had an inclusive role and a labelling pattern consistent with a new modified pathway of carbon in photosynthesis.

Analysis of the control of photosynthesis in C 4 plants by changes in light and carbon dioxide

Parallel measurements of contents of photosynthetic intermediates, activities of enzymes of photosynthetic carbon assimilation, gas-exchange rates and components of chlorophyll-fluorescence quenching in leaves of C 4 plants are considered in relation to changes in photon flux density (PFD) and CO 2 . The influence of varying light and CO 2 concentration upon changes in the amounts of phosphoenolpyruvate (PEP) in leaves of C 4 plants during steady-state photosynthesis are interpreted in terms of the regulatory properties of PEP carboxylase and in terms of feedback interactions between the Calvin cycle and the C 4 cycle. Relations between electron transport and carbon assimilation are discussed in terms of the regulation of the supply of ATP and NADPH and the demands of carbon assimilation. In low light these relations differ in C 3 and C 4 plants. The lag in photosynthetic carbon assimilation in maize that follows a decrease in PFD has been analysed. The changes that occur in enzyme activities, metabolites and components of chlorophyll-fluorescence quenching following the transition from high to low light indicate that diminished production of ATP and NADPH is responsible for the lag in photosynthetic carbon assimilation and may reflect a stimulation of cyclic electron flow to make up a deficit in ATP.

Regulation of pea mitochondrial pyruvate dehydrogenase complex activity: inhibition of ATP-dependent inactivation

In contrast to the pyruvate dehydrogenase complex (PDC) from animal mitochondria, our in situ and in vitro studies indicate that the ATP:ADP ratio has little or no effect in regulating the mitochondrial pyruvate dehydrogenase complex from green pea seedlings. Pyruvate was a competitive inhibitor of ATP-dependent inactivation (Ki = 59 microM), while the PDC had a Km for pyruvate of microM. Thiamine pyrophosphate, the coenzyme for the pyruvate dehydrogenase (PDH) component of the complex, did not inhibit ATP-dependent inactivation when used alone but it enhanced inhibition by pyruvate. As such, thiamine pyrophosphate was a competitive inhibitor (Ki = 130 nM) of ATP-dependent inactivation. A model is proposed for the pyruvate plus thiamine pyrophosphate inhibition of ATP-dependent inactivation of the pyruvate dehydrogenase complex in which pyruvate exerts its inhibition of inactivation by altering or protecting the protein substrate from phosphorylation and not by directly inhibiting PDH kinase.

Photosynthetic carbon metabolism in isolated pea chloroplasts: metabolite levels and enzyme activities

We report here that enzyme activation precedes the rise in metabolite levels, which appear to limit photosynthetic CO2 fixation during induction in pea leaf chloroplasts. Therefore light activation may be required for the build-up of photosynthetic intermediates and hence for photosynthesis in isolated chloroplasts. Analysis of metabolite levels and the known kinetic properties of the chloroplast enzymes indicates that the reductive pentose phosphate cycle is subject to control which fluctuates between several points during induction and when CO2 fixation is maximal. The transketolase-aldolase-catalyzed reactions around sedoheptulose-biphosphatase appear to provide a simple and effective primary control for photosynthetic CO2 fixation. When substrate levels and enzyme active site concentrations are taken into account, there is insufficient glyceraldehyde 3-phosphate dehydrogenase, aldolase, and transketolase activity to support photosynthetic CO2 fixation at observed rates. These results suggest that there may be direct transfer of glyceraldehyde 3-phosphate among these enzymes in the pea chloroplast.

Inactivation of the photosynthetic carbon reduction cycle in isolated mesophyll protoplasts subjected to freezing stress

Isolated mesophyll protoplasts from Valerianella locusta L. were subjected to freeze-thaw cycles. Subsequently, steady-state pool sizes of (14)C-labeled intermediates of the photosynthetic carbon reduction cycle were determined by high performance liquid chromatography. Protoplasts in which CO2 fixation was inhibited by preceding freezing stress, showed a strong increase in the proportion of fructose-1,6-bisphosphate, sedoheptulose-1,7-bisphosphate and triose phosphates. These results indicate an inhibition of the activities of stromal fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase. Furthermore, freezing stress caused a slight increase in the proportion of labeled ribulose-1,5-bisphosphate, which may be based on an inhibition or ribulose bisphosphate carboxylase activity. It was shown earlier (Rumich-Bayer and Krause 1986) that freezing-thawing readily affects photosynthetic CO2 assimilation independently of thylakoid inactivation. The present results are interpreted in terms of an inhibition of the light-activation system of the photosynthetic carbon reduction cycle, caused by freezing stress.

Are sucrosyl-oligosaccharides synthesized in mesophyll protoplasts of mature leaves of Cucumis melo?

Biosynthesis of sucrosyl-oligosaccharides (raffinose, stachyose) was traced in source leaves of Cucumis melo after (14)C-photoassimilation. The main carbon compound exported was (14)C-labeled stachyose. No oligosaccharide synthesis was detected in young, importing leaves. Mesophyll protoplasts, isolated from mature leaves which had previously photosynthesized (14)CO2, did not contain (14)C-oligosaccharides but contained [(14)C]-sucrose and (14)C-hexoses. Isolated minor-vein-enriched fractions from the same leaves, however, showed nearly 30% of the (14)C of the neutral fraction to be in oligosaccharides. Isolated, viable mesophyll protoplasts incubated with NaH(14)CO3 also failed to incorporate radioactivity into oligosaccharides, although sucrose and galactinol synthesis was unimpaired. Galactinolsynthase activity in leaf extracts and in mesophyll protoplasts was 16.8 μmol·h(-1)·mg(-1) protein and 13.8 μmol·h(-1)·mg(-1) protein, respectively. Galactosyltransferase (EC 2.4.1.67), which synthesizes stachyose from raffinose and galactinol, had an activity of 50 nmol·h(-1)·mg(-1) protein in leaf extracts and was also present in the minor-vein-enriched fraction, but could not be detected in mesophyll protoplast lysates. The results indicate that mesophyll cells may not be the site of stachyose synthesis although precursor compounds like sucrose and galactinol are synthesized there.

Oscillatory response of photosynthesis in leaves to environmental perturbations: a mathematical model

Oscillations in the yield of chlorophyll fluorescence, in oxygen evolution, and in CO2 uptake observed with leaves upon perturbation of steady-state conditions are suggested to be due to the interdependence of turnover of adenylates and Calvin cycle intermediates. This suggestion is quantified in a mathematical model; the behavior of the model system in the neighborhood of the singular point of the system is analyzed. The linearized system is solved analytically, a condition for the occurrence of oscillations is given, and explicit expressions for the oscillation period and the damping constant are derived. The model is shown to be capable of exhibiting oscillations with the period observed with algae or leaves, whereas calculated values of the damping constant are higher than those measured for leaves or algae.

A comparative study of metabolite levels in plant leaf material in the dark

Metabolite levels have been compared in the dark and during photosynthesis in leaves and protoplasts from spinach, pea, wheat and barley. In protoplasts the subcellular distribution was also studied. The levels of triose phosphates and sugar bisphosphates were high in the light and low in the dark. The hexose phosphates and 3-phosphoglycerate levels in the dark were very variable depending on the plant material. In most conditions, hexose phosphates and triose phosphates were mainly in the extrachloroplast compartment, while 3-phosphoglycerate and the sugar bisphosphates were mainly in the chloroplast compartment. Leaves always had a very low triose phosphate: 3-phosphoglycerate ratio in the dark, but in protoplasts this ratio was higher. Detailed studies with spinach showed that metabolite levels were very dependent on the availability of carbohydrate in the leaf, particularly starch. Starch mobilisation is not controlled just by the availability of inorganic phosphate and accumulation of phosphorylated intermediates. Hydrolysis of starch may provide precursors for sucrose synthesis while phosphorolysis leads to provision of substrates for respiration. Starch breakdown generates high enough levels of hexose phosphate to support substantial rates of sucrose synthesis in the dark. Respiration is not greatly increased when metabolite levels are high during starch mobilisation. Higher levels of metabolites shorten the length of the induction phase of photosynthesis.

Reactivity of glyoxylate with hydrogen perioxide and simulation of the glycolate pathway of C3 plants and Euglena

The nonenzymatic reaction of glyoxylate and H2O2 was measured under physiological conditions of the pH and concentrations of reactants. The reaction of glyoxylate and H2O2 was secondorder, with a rate constant of 2.27 l mol(-1) s(-1) at pH 8.0 and 25° C. The rate constant increased by 4.4 times in the presence of Zn(2+) and doubled at 35°C. We propose a mechanism for the reaction between glyoxylate and H2O2. From a comparison of the rates of H2O2 decomposition by catalase and the reaction with glyoxylate, we conclude that H2O2 produced during glycolate oxidation in peroxisomes is decomposed by catalase but not by the reaction with glyoxylate, and that photorespiratory CO2 originates from glycine, but not from glyoxylate, in C3 plants. Simulation using the above rate constant and reported kinetic parameters leads to the same conclusion, and also makes it clear that alanine is a satisfactory amino donor in the conversion of glyoxylate to glycine. Some serine might be decomposed to give glycine and methylene-tetrahydrofolate; the latter is ultimately oxidized to CO2. In the simulation of the glycolate pathway of Euglena, the rate constant was high enough to ensure the decarboxylation of glyoxylate by H2O2 to produce photorespiratory CO2 during the glycolate metabolism of this organism.

Sucrose transport into vacuoles isolated from barley mesophyll protoplasts

Sucrose transport has been investigated in vacuoles isolated from barley mesophyll protoplasts. Rates of sucrose transfer across the tonoplast were even higher in vitro than in vivo indicating that the sucrose transport system had not suffered damage during isolation of the vacuoles. Sucrose transport is carrier-mediated as shown by substrate saturation of transport and sensitivity to a metabolic inhibitor and to competitive substrates. A number of sugars, in particular maltose and raffinose, decreased uptake of sucrose. Sorbitol was slowly taken up but had no effect on sucrose transport. The SH-reagent p-chloromercuribenzene sulfonate inhibited sucrose uptake completely. The apparent Km of the carrier for sucrose uptake was 21 mM. Transport was neither influenced by ATP and pyrophosphate, with or without Mg(2+) present, nor by protonophores and valinomycin (with K(+) present). Apparently uptake was not energy dependent. Efflux experiments with preloaded vacuoles indicated that sucrose unloading from the isolated vavuoles is mediated by the same carrier which catalyses uptake. The vacuole of mesophyll cells appears to represent an intermediary storage compartment. Uptake of photosynthetic products into the vacuole during the light apparently minimizes osmotic swelling of the small cytosolic compartment of vacuolated leaf cells when photosynthetic productivity exceeds the capacity of the phloem for translocation of sugars.

Regulation of fructose 2,6-bisphosphate concentration in spinach leaves

Fructose-6-phosphate 2-kinase and fructose-2,6-bisphosphatase have been partially purified from spinach leaves and their regulatory properties studied. Fructose-6-phosphate 2-kinase was activated by phosphate and fructose 6-phosphate, and inhibited by 3-phosphoglycerate and dihydroxyacetone phosphate. Fructose-2,6-bisphosphatase was inhibited by fructose 6-phosphate and phosphate. The interaction between these effectors was studied when they were varied, alone or in combination, over a range of concentrations representative of those in the cytosol of spinach leaf cells. In conditions when dihydroxyacetone phosphate or 3-phosphoglycerate rise, as is typical during photosynthesis, the fructose 2,6-bisphosphate level will decrease, which will favour sucrose synthesis. In conditions when fructose 6-phosphate accumulates, fructose 2,6-bisphosphate should rise, which will favour a restriction of sucrose synthesis and promotion of starch synthesis.

Influence of glycerate on photosynthesis by wheat chloroplasts

Glycerate was found to effect photosynthetic O2 evolution in wheat chloroplasts by its conversion to triose phosphate and by influencing the rate of photosynthesis through the reductive pentose phosphate pathway. In the absence of bicarbonate, the photosynthetic O2 evolution with glycerate was low (10 to 25 mumol mg chlorophyll-1 h-1), and only about 15% of the rate of bicarbonate-dependent O2 evolution under optimum conditions. This corresponds to a rate of glycerate conversion to triose phosphate of 20 to 50 mumol mg chlorophyll-1 h-1, which appears sufficient to accommodate flux through the glycolate pathway in vivo. Pi was required for this glycerate-dependent O2 evolution; rates remained relatively constant between 0.1 and 40 mM Pi, and proceeded with little lag upon illumination (less than 0.5 min). Evidence for O2 evolution due to glycerate conversion to triose phosphate could be conclusively demonstrated by addition of glycolaldehyde, an inhibitor of the regenerative phase of photosynthesis, which prevents CO2 fixation. The effect of glycerate on photosynthesis in the presence of bicarbonate was determined by measuring both photosynthetic O2 evolution and 14CO2 fixation at varying Pi concentrations. Low concentrations of glycerate (micro- to millimolar levels) prevented inhibition of photosynthesis by Pi. With 1 mM bicarbonate and pH 8.2, which is favorable for glycolate synthesis, maximum rates of photosynthesis were obtained at low Pi (25 microM), whereas strong inhibition of photosynthesis occurred at only 0.2 mM Pi. Addition of glycerate relieved the inhibition of photosynthesis by Pi, indicating the possible importance of glycerate metabolism in the chloroplast under photorespiratory conditions. The initiation of photosynthesis by glycerate at inhibitory Pi levels occurred with little reduction in the ratio of CO2 fixed/O2 evolved, and the main effect of glycerate was on carbon assimilation. While the basis for the beneficial effect of glycerate on CO2 assimilation under moderate to high Pi levels is uncertain, it may increase the concentration of 3-phosphoglycerate (PGA) in the chloroplast, and thus make conditions more favorable for induction of photosynthesis and reduction of PGA to triose phosphate.

The relationship between steady-state gas exchange of bean leaves and the levels of carbon-reduction-cycle intermediates

The relationship between the gas-exchange characteristics of attached leaves of Phaseolus vulgaris L. and the pool sizes of several carbon-reduction-cycle intermediates was examined. After determining the rate of CO2 assimilation at known intercellular CO2 pressure, O2 pressure and light, the leaf was rapidly killed (<0.1 s) and the levels of ribulose-1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), fructose-1,6-bisphosphate, fructose-6-phosphate, glucose-6-phosphate, glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate were measured. In 210 mbar O2, photosynthesis appeared RuBP-saturated at low CO2 pressure and RuBP-limited at high CO2 pressure. In 21 mbar (2%) O2, the level of RuBP always appeared saturating. Very high levels of PGA and other phosphate-containing compounds were found with some conditions, especially under low oxygen.

Compartmentation of labeled fixation products in intact mesophyll protoplasts from Avena sativa L. after in-situ inhibition of the chloroplast phosphate translocator

Leaf mesophyll protoplasts of oat (Avena sativa L.) were allowed to fix (14)C-labeled bicarbonate in the absence or presence of pyridoxal phosphate (PLP), a specific inhibitor of the phosphate translocator of the inner envelope membrane of chloroplasts. The incubation was terminated by a method of rapid integrated protoplast homogenization and fractionation, and compartmented levels of label contained in sugars, phosphate esters, amino acids and organic acids were determined. The results show that the addition of PLP to a suspension of intact protoplasts causes an accumulation of phosphate esters in the chloroplasts stroma for up to 2.5 min of incubation, with a corresponding decrease in the cytosol. Prolonged treatment of protoplasts with PLP in the light resulted in a decrease of starch-associated label, combined with higher levels of labeled sugars in the cytosol, indicating a switch from phosphorolytic to hydrolytic starch degradation. Together with the determination of pool sizes of triose phosphates and of inorganic phosphate, the results demonstrate that the method employed is an important tool in investigating processes of intracellular regulation. They are discussed with respect to the permeability and possible side reactions of PLP, as well as in the light of reports on PLP action on isolated chloroplasts.

Photosynthesis of leaf cell protoplasts and permeability of the plasmalemma to some solutes

Photosynthesis was measured in mesophyll protoplasts isolated from spinach leaves. Under high intensity illumination and in the presence of 21% O2, half-saturation of photosynthesis by CO2 required CO2 concentrations between 8 and 12 μm at different pH values of the suspending medium. Concentrations of HCO 3 (-) needed for half-saturation increased correspondingly with the pH of the media. The pH profile of protoplast photosynthesis was much broader than that of CO2 assimilation by isolated chloroplasts. The data indicate that leaf cells possess mechanisms to maintain considerable differences between external and internal pH over prolonged periods of time. Protoplast photosynthesis was inhibited by nitrite, acetate and bicarbonate; inhibition was more pronounced at low than at high pH and was attributed to stroma acidification. Nitrite was reduced in the light by protoplasts and chloroplasts. At pH 7.6, the apparent Km NO 2 (-) was about 0.6 mM for chloroplasts and 25 mM for protoplasts. Approximate permeability coefficients for NO 2 (-) and HNO2 were calculated from nitrite-dependent oxygen evolution at low nitrite concentrations, known nitrite or HNO2 gradients, data on the surface area of protoplasts and chloroplasts and the pH profile of nitrite inhibition of photosynthesis. The membrane potential was assumed to be-100 mV. For the chloroplast envelope, permeability coefficients were 1.5·10(-3) ms(-1) (HNO2) and 2·10(-8) ms(-1) (NO 2 (-) ) and for the plasmalemma 4·10(-5) ms(-1) (HNO2) and 5·10(-10) ms(-1) (NO 2 (-) ). The values calculated for anion penetration probably represent upper limits of permeability. The protoplasts appeared to be largely impermeable to phosphate and phosphate esters. A rapid metabolic response of cells or cellular strands to added anionic substrates such as phosphate esters as reported in the literature appears to be possible only in damaged cells. It requires the presence of open channels between the cytosol and external medium.