Regulation of glycolysis and level of the Crassulacean acid metabolism

Hierarchical clustering reveals unique features in the diel dynamics of metabolites in the CAM orchid Phalaenopsis

Crassulacean acid metabolism (CAM) is a major adaptation of photosynthesis that involves temporally separated phases of CO2 fixation and accumulation of organic acids at night, followed by decarboxylation and refixation of CO2 by the classical C3 pathway during the day. Transitory reserves such as soluble sugars or starch are degraded at night to provide the phosphoenolpyruvate (PEP) and energy needed for initial carboxylation by PEP carboxylase. The primary photosynthetic pathways in CAM species are well known, but their integration with other pathways of central C metabolism during different phases of the diel light-dark cycle is poorly understood. Gas exchange was measured in leaves of the CAM orchid Phalaenopsis 'Edessa' and leaves were sampled every 2 h during a complete 12-h light-12-h dark cycle for metabolite analysis. A hierarchical agglomerative clustering approach was employed to explore the diel dynamics and relationships of metabolites in this CAM species, and compare these with those in model C3 species. High levels of 3-phosphoglycerate (3PGA) in the light activated ADP-glucose pyrophosphorylase, thereby enhancing production of ADP-glucose, the substrate for starch synthesis. Trehalose 6-phosphate (T6P), a sugar signalling metabolite, was also correlated with ADP-glucose, 3PGA and PEP, but not sucrose, over the diel cycle. Whether or not this indicates a different function of T6P in CAM plants is discussed. T6P levels were low at night, suggesting that starch degradation is regulated primarily by circadian clock-dependent mechanisms. During the lag in starch degradation at dusk, carbon and energy could be supplied by rapid consumption of a large pool of aconitate that accumulates in the light. Our study showed similarities in the diel dynamics and relationships between many photosynthetic metabolites in CAM and C3 plants, but also revealed some major differences reflecting the specialized metabolic fluxes in CAM plants, especially during light-dark transitions and at night.

A comparative study on diurnal changes in metabolite levels in the leaves of three crassulacean acid metabolism (CAM) species, Ananas comosus, Kalanchoë daigremontiana and K. pinnata

A comparative study on diurnal changes in metabolite levels associated with crassulacean acid metabolism (CAM) in the leaves of three CAM species, Ananas comosus (pineapple), a hexose-utilizing species, and Kalanchoë daigremontiana and K. pinnata, two starch-utilizing species, were made. All three CAM species showed a typical feature of CAM with nocturnal malate increase. In the two Kalanchoë species, isocitrate levels were higher than citrate levels; the reverse was the case in pineapple. In the two Kalanchoë species, a small nocturnal citrate increase was found and K. daigremontiana showed a small nocturnal isocitrate increase. Glucose 6-phosphate (G-6-P), fructose 6-phosphate (F-6-P) and glucose 1-phosphate (G-1-P) levels in the three CAM species rose rapidly during the first part of the dark period and decreased during the latter part of the dark period. The levels of the metabolites also decreased during the first 3 h of the light period, then, remained little changed through the rest of the light period. Absolute levels of G-6-P, F-6-P and G-1-P were higher in pineapple than in the two Kalanchoë species. Fructose 1,6-bisphosphate (F-1,6-P(2)) levels in the three CAM species increased during the dark period, then dramatically decreased during the first 3 h of the light period and remained unchanged through the rest of the light period. The extent of nocturnal F-1,6-P(2) increase was far greater in the two Kalanchoë species than in pineapple. Absolute levels of F-1,6-P(2) were higher in the two Kalanchoë species than in pineapple, especially during dark period. Diurnal changes in oxaloacetate (OAA), pyruvate (Pyr) and phosphoenolpyruvate (PEP) levels in the three CAM species were similar.

Plastidic metabolite transporters and their physiological functions in the inducible crassulacean acid metabolism plant Mesembryanthemum crystallinum

The inducible crassulacean acid metabolism (CAM) plant Mesembryanthemum crystallinum accumulates malic acid during the night and converts it to starch during the day via a pathway that, because it is located in different subcellular compartments, depends on specific metabolite transport across membranes. The chloroplast glucose transporter (pGlcT) and three members of the phosphate translocator (PT) family were isolated. After induction of CAM, transcript amounts of the phosphoenolpyruvate (PEP) phosphate translocator (PPT) and the glucose-6-phosphate (Glc6P) phosphate translocator (GPT) genes were increased drastically, while triose phosphate (TP) phosphate translocator (TPT) and the pGlcT transcripts remained unchanged. PPT- and GPT-specific transcripts and transporter activities exhibited a pronounced diurnal variation, displaying the highest amplitude in the light. pGlcT transcripts were elevated towards the end of the light period and at the beginning of the dark period. These findings, combined with diurnal variations of enzyme activities and metabolite contents, helped to elucidate the roles of the PPT, GPT, TPT and pGlcT in CAM. The main function of the PPT is the daytime export from the stroma of PEP generated by pyruvate orthophosphate:dikinase (PPDK). The increased transport activity of GPT in the light suggests a higher requirement for Glc6P import for starch synthesis rather than starch mobilization. Most likely, Glc6P rather than 3-phosphoglycerate or triose phosphates is the main substrate for daytime starch biosynthesis in M. crystallinum plants in which CAM has been induced (CAM-induced), similar to non-green plastids. In the dark, starch is mobilized both phosphorylytically and amylolytically and the products are exported by the GPT, TPT and pGlcT. The transport activities of all three phosphate translocators and the transcript amounts of the pGlcT adapt to changing transport requirements in order to maintain high metabolic fluxes during the diurnal CAM cycle.

Partial characterization of guard-cell phosphoenolpyruvate carboxylase: kinetic datum collection in real time from single-cell activities

Maximum velocity and Km(PEP.Mg) of phosphoenolpyruvate carboxylase (PEPC) from stomatal guard cells of Vicia faba L. were determined as a function of pH, presence of malate, and physiological state of guard cells. The biochemical rationale for these measurements is that (a) massive proton extrusion from guard cells, the primary event that drives stomatal movements, has been speculated to alkalinize the cell; (b) guard-cell malate concentration increases severalfold on stomatal opening, and malate, generally an inhibitor of PEPC's, affects the oligomeric state of some PEPC's; and (c) the apparent in vivo activity of guard-cell PEPC is greatly enhanced during stomatal opening, compared with that of other physiological states of these cells. As there are precedents for cell-specific expression of particular forms of PEPC and for labile reversible, post-translational modifications (which are manifested kinetically as distinct physiological-state isoforms), individual assays were initiated on the addition of a single stomatal complex directly to a microdroplet of assay cocktail. The stomatal complexes (each of which comprises a pair of guard cells having a mass of 6 x 10(-9) g) were dissected from lyophilized leaf tissue that had been freeze-quenched either before, during, or after a treatment to open stomata. Vmax at pH 7.0 was not significantly different from that at pH 8.5. Neither Vmax nor Km(PEP.Mg) was distinguished on the basis of the physiological state of the tissue from which the enzyme was extracted. However, Km(PEP.Mg) was greater than 4x lower at pH 8.5 than at pH 7.0. Malate inhibition was competitive at both pH's, but inhibition was greater than 3x greater at the lower pH. These data indicate that the combined effects of pH and malate over the range studied can produce changes in enzyme velocity of approximately 24-fold. Thus, the results are consistent with an interpretation that guard-cell PEPC is regulated by the cytoplasmic chemical environment and not by alternations between physiological-state isoforms.

Day-night changes in the levels of adenine nucleotides, phosphoenolpyruvate and inorganic pyrophosphate in leaves of plants having Crassulacean acid metabolism

The levels of phosphorylated compounds studied during the dark period of Crassulacean acid metabolism (CAM) in Kalanchoë leaves showed increases for ATP and pyrophosphate and decreases for ADP, AMP and phosphenolpyruvate; levels of inorganic phosphate remained constant. Changes in adenylate levels and the correlated nocturnal increase in adenylate-energycharge were closely related to changes in malate levels. The increase in ATP levels was much inhibited in CO2-free air and stimulated after induction of CAM in short-day-treated plants of K. blossfeldiana cv. Tom Thumb. Changes in levels of phosphoenolpyruvate and pyrophosphate were independent of the presence of CO2. The results show the operation of complex regulatory mechanisms in the energy metabolism of CAM plants during nocturnal malic-acid accumulation.

CARBON DIOXIDE AND WATER DEMAND: CRASSULACEAN ACID METABOLISM (CAM), A VERSATILE ECOLOGICAL ADAPTATION EXEMPLIFYING THE NEED FOR INTEGRATION IN ECOPHYSIOLOGICAL WORK

Plants having crassulacean acid metabolism (CAM) tend to occupy habitats where the prevailing environmental stress is scarcity of water. These are semi-arid or arid regions, salinas or epiphytic sites. CAM plants manage the dilemma of desiccation or starvation by nocturnal malic acid accumulation in the vacuoles. Malic acid serves as a form of CO₂ storage and as an osmoticum. In this way malic acid accumulation allows, firstly, separation of uptake and assimilation of atmospheric CO₂ with water-saving daytime stomatal closure and, secondly, osmotic acquisition of water. There is no very special trait which is specific for CAM. An array of biophysical and biochemical functional elements, which are also found in other plants, is integrated in CAM performance. This leads to a large diversity of behaviour which makes CAM plants highly versatile in their response to environmental variables. Besides CO₂ dark fixation, transport of malic acid across the tonoplast is one of the key elements in CAM function. This is examined in detail at the level of membrane biophysics and biochemistry. The versatility of CAM is illustrated by examples from field work, with comparisons involving different species, seasons, modes of photosynthesis (CAM vs C₃ ), kinds of stress and ways of stress imposition. Contents Summary 593 I. Studies of CAM: an example for the ecophysiological approach 594 II. Malic acid transport at the tonoplast 602 III. Regulation 605 IV. Desiccation or starvation 610 V. Comparative autecology 614 VI. Ecology: promise of integration 621 Acknowledgements 622 References 622.

TANSLEY REVIEW No 1.: VARIATION IN PHOTOSYNTHETIC ACID METABOLISM IN VASCULAR PLANTS: CAM AND RELATED PHENOMENA

The term, photosynthetic acid metabolism (PAM), encompasses processes which involve four-carbon (C₄ ) acids as carbon-carrying intermediates between the carboxylations catalysed by phosphoenolpyruvate (PEP) carboxylase and ribulose biphosphate (RuBP) carboxylase. These carboxylations may be separated either spatially, as in C₄ photosynthesis which is not covered here, or temporally. Separation in time is referred to as diel PAM (DPAM) and the variations on this particular theme are the subject of this review. DPAM includes stomatal and astomatal types. The former types, usually described as crassulacean acid metabolism (CAM), are referred to here as stomatal CAM (SCAM). Astomatal PAM includes both aquatic and terrestrial forms; aquatic acid metabolism (AAM) and terrestrial astomatal acid metabolism (TAAM). Physiological, biochemical, structural, ecological and evolutionary aspects of these mechanisms of acquisition of carbon dioxide are reviewed. CONTENTS Summary 3 I. General Introduction to Photosynthetic Acid Metabolism (PAM) 4 II. Stomatal Crassulacean (Diel Photosynthetic) Acid Metabolism (SCAM) 5 III. Astomatal Photosynthetic Acid Metabolism 13 IV. Idling 18 V. Structural Variation in Plants Exhibiting Diel PAM (DPAM) 18 VI. The Mechanism of the Switch between C₃ Photosynthesis and DPAM 19 VII. The Cost of DPAM 19 VIII. The Evolution of DPAM 20 IX. Concluding Comment 22 Acknowledgements 22 References 22.

A dynamic computer model of the metabolic and regulatory processes in Crassulacean acid metabolism

The paper describes a computer model which is capable of simulating the typical phenomena of Crassulacean acid metabolism (CAM). The model is based on a simplified scheme of the metabolic processes of CAM described earlier in the literature. The evolution of the model proceeded in the following steps, namely i) a verbal description of CAM in the form of a scheme integrating the metabolic and regulatory CAM processes at the cellular level of the cell, and transcription of the scheme into a block diagram; ii) the stepwise transformation of the block diagram into a structural model, represented by a system of differential equations; this was later used as the dynamic model. In the first attempt to construct the dynamic model, it appeared to be useful to accept the following simplifications: i) All reactions involved were considered to be of the first order. ii) Sequences of reactions, in which the intermediary products appeared to be of minor importance, were summarized in a single step. iii) All reactions were considered to proceed irreversibly in the main direction. iv) The mathematical formulations, usually used in describing enzyme regulations (for instance, competitive or allosteric behaviour), were replaced in the model by a uniformly simplified equation which independent of the actual mechanism, described activation by the multiplication of the velocity constant with an activating factor, and inhibition by division of the velocity constant by an inhibiting factor. v) From the manifold interactions between the plants and their environment, at present, only two factors have been selected to act as input parameters of the model, namely, the CO2 concentration in the air and light. Our studies showed that the model was capable of simulating not only some basic phenomena of CAM such as the diurnal rhythms of malic acid and starch, and the diurnal pattern of net CO2 exchange, but also alterations in the pool sizes of phosphoenolpyruvate, glucose-6-phosphate and internal CO2. The latter were of particular interest since the experimental findings were not made known to the model-building coauthors prior to the formulation of the model. Thus, the results could not influence the structure and behaviour of the model. It was also possible to simulate alterations of CAM behaviour as occurring in vivo in response to environmental signals. In all tested cases, the simulation was in very good agreement with the in-vivo behaviour of the plants documented by experiments or observations. This close agreement between the in-vivo behaviour of CAM and the simulation by the model indicated that the basic scheme of CAM contained all the major metabolic and regulatory interrelationships operating in vivo to bring about CAM.

Activity of enzymes of carbon metabolism during the induction of Crassulacean acid metabolism in Mesembryanthemum crystallinum L

The maximum extractable activities of twenty-one photosynthetic and glycolytic enzymes were measured in mature leaves of Mesembryanthemum crystallinum plants, grown under a 12 h light 12 h dark photoperiod, exhibiting photosynthetic characteristics of either a C3 or a Crassulacean acid metabolism (CAM) plant. Following the change from C3 photosynthesis to CAM in response to an increase in the salinity of in the rooting medium from 100 mM to 400 mM NaCl, the activity of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) increased about 45-fold and the activities of NADP malic enzyme (EC 1.1.1.40) and NAD malic enzyme (EC 1.1.1.38) increased about 4- to 10-fold. Pyruvate, Pi dikinase (EC 2.7.9.1) was not detected in the non-CAM tissue but was present in the CAM tissue; PEP carboxykinase (EC 4.1.1.32) was detected in neither tissue. The induction of CAM was also accompanied by large increases in the activities of the glycolytic enzymes enolase (EC 4.2.1.11), phosphoglyceromutase (EC 2.7.5.3), phosphoglycerate kinase (EC 2.7.2.3), NAD glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), and glucosephosphate isomerase (EC 2.6.1.2). There were 1.5- to 2-fold increases in the activities of NAD malate dehydrogenase (EC 1.1.1.37), alanine and aspartate aminotransferases (EC 2.6.1.2 and 2.6.1.1 respectively) and NADP glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). The activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39), fructose-1,6-bisphosphatase (EC 3.1.3.11), phosphofructokinase (EC 2.7.1.11), hexokinase (EC 2.7.1.2) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) remained relatively constant. NADP malate dehydrogenase (EC 1.1.1.82) activity exhibited two pH optima in the non-CAM tissue, one at pH 6.0 and a second at pH 8.0. The activity at pH 8.0 increased as CAM was induced. With the exceptions of hexokinase and glucose-6-phosphate dehydrogenase, the activities of all enzymes examined in extracts from M. crystallinum exhibiting CAM were equal to, or greater than, those required to sustain the maximum rates of carbon flow during acidification and deacidification observed in vivo. There was no day-night variation in the maximum extractable activities of phosphoenolpyruvate carboxylase, NADP malic enzyme, NAD malic enzyme, fructose-1,6-bisphosphatase and NADP malate dehydrogenase in leaves of M. crystallinum undergoing CAM.

Photoperiodism and Crassulacean acid metabolism : III. Different characteristics of the photoperiod-sensitive and non-sensitive isoforms of phosphoenolpyruvate carboxylase and Crassulacean acid metabolism operation

Sensitivity to glucose-6-P, inorganic phosphate and malate, Km phosphoenolpyruvate (PEP), and the effect of pH were comparatively investigated for phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) extracted along the day-night cycle from leaves of Kalanchoe blossfeldiana v. Poelln. Tom Thumb, grown under different photoperiodic conditions. Under long days, sensitivity to effectors is weak and varies only slightly during the 24h cycle, together with small variations of Km (PEP), ranging from 1.2 to 1.8 mM. The response-curve to pH shows two peaks for pH 7.4 and 8.4. Transfer of the plants to short days established an increase in the sensitivity of the enzyme to the effectors together with the appearance of a day-night variation of this effect, maximum during the day. A clear diurnal oscillation of Km (PEP) is observed from 3.1 mM at the beginning of day (09.00 h) to 0.9 mM at midnight (00.00 h), at pH 7.4. These results complement previous electrophoretic and immunological data by affording enzyme kinetic evidence that short days induce a PEP carboxylase form responsible for full-Crassulacean acid metabolism (CAM) operation which is kinetically different from the photoperiod-sensitive, C3-like form prevailing in young leaves under long days. The results indicate that diurnal enzymic rhythms would improve the efficiency of the metabolic regulatory mechanisms and act as a coordinating factor for the daily and seasonal adaptive operation of CAM.

Activity and quantity of ribulose bisphosphate carboxylase-and phosphoenolpyruvate carboxylase-protein in two Crassulacean acid metabolism plants in relation to leaf age, nitrogen nutrition, and point in time during a day/night cycle

Activity of ribulose 1,5-bisphosphate (RuBP) carboxylase in leaf extracts of the constitutive Crassulacean acid metabolism (CAM) plant Kalanchoe pinnata (Lam.) Pers. decreased with increasing leaf age, whereas the activity of phosphoenolpyruvate (PEP) carboxylase increased. Changes in enzyme activities were associated with changes in the amount of enzyme proteins as determined by immunochemical analysis, sucrose density gradient centrifugation, and SDS gel electrophoresis of leaf extracts. Young developing leaves of plants which received high amounts of NO 3 (-) during growth contained about 30% of the total soluble protein in the form of RuBP carboxylase; this value declined to about 17% in mature leaves. The level of PEP carboxylase in young leaves of plants at high NO 3 (-) was an estimated 1% of the total soluble protein and increased to approximately 10% in mature leaves, which showed maximum capacity for dark CO2 fixation. The growth of plants at low levels of NO 3 (-) decreased the content of soluble protein per unit leaf area as well as the extractable activity and the percentage contribution of both RUBP carboxylase and PEP carboxylase to total soluble leaf protein. There was no definite change in the ratio of RuBP carboxylase to PEP carboxylase activity with a varying supply of NO 3 (-) during growth. It has been suggested (e.g., Planta 144, 143-151, 1978) that a rhythmic pattern of synthesis and degradation of PEP carboxylase protein is involved in the regulation of β-carboxylation during a day/night cycle in CAM. No such changes in the quantity of PEP carboxylase protein were observed in the leaves of Kalanchoe pinnata (Lam.) Pers. or in the leaves of the inducible CAM plant Mesembryanthemum crystallinum L.

Crassulacean acid metabolism (CAM) in Kalanchoë daigremontiana: Temperature response of phosphoenolpyruvate (PEP)-carboxylase in relation to allosteric effectors

Net CO2 dark fixation of Kalanchoë daigremontiana varies with night temperature. We found an optimum of fixation at about 15° C; with increasing night temperature fixation decreased. We studied the temperature dependence of the activity of phosphoenolpyruvate (PEP)-carboxylase, the key enzyme for CO2 dark fixation. We varied the pH, the substrate concentration (PEP), and the L-malate and glucose-6-phosphate (G-6-P) concentration in the assay. Generally, lowering the pH and reducing the amount of substrate resulted in an increase in activation by G-6-P and in an increase in malate inhibition of the enzyme. Furthermore, malate inhibition and G-6-P activation increased with increasing temperature. Activity measurements between 10° C and 45°C at a given concentration of the effectors revealed that the temperature optimum and maximum activities at that optimum varied with the effector applied. Under the influence of 5 mol m(-3) L-malate the temperature optimum and maximum activity dropped drastically, especially when the substrate level was low (at 0.5 mol m(-3) PEP from 32° C to 20° C). G-6-P raised the temperature optimum and maximum activity when the substrate level was low. If both malate and G-6-P were present, intermediate values were measured. We suggest that changes in metabolite levels in K. daigremontiana leaves can alter the temperature features of PEP-carboxylase so that the observed in vivo CO2 dark fixation can be explained on the basis of PEP-carboxylase activity.

Kinetic analysis of effectors of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi

The activity of phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxy-lyase (phosporylating) EC 4.1.1.31) purified from Bryophyllum fedtschenkoi has been measured in the presence of various concentrations of phosphoenolpyruvate, L-malate and glucose 6-phosphate. At high pH, the enzyme is competitively inhibited by L-malate and activated by glucose 6-phosphate. A reaction scheme describing the interaction of enzyme, substrate and effectors is proposed. Values for the appropriate equilibrium constants have been calculated for the enzyme acting at pH 7.8, which is one of its two pH optima. The kinetics are more complicated at low pH, partly because of non-linear reaction rates and partly because inhibition by L-malate is not competitive. Activation by glucose 6-phosphate is similar at high and low pH values. The behaviour of a wide range of other possible effectors is described briefly.

Carbon isotope composition of intermediates of the starch-malate sequence and level of the crassulacean acid metabolism in leaves of Kalanchoe blossfeldiana Tom Thumb

Isotype analyses were performed on biochemical fractions isolated from leaves of Kalanchoe blossfeldiana Tom Thumb. during aging under long days or short days. Irrespective of the age or photoperiodic conditions, the intermediates of the starch-malate sequence (starch, phosphorylated compounds and organic acids) have a level of (13)C higher than that of soluble sugars, cellulose and hemicellulose. In short days, the activity of the crassulacean acid metabolism pathway is predominant as compared to that of C3 pathway: leaves accumulate organic acids, rich in (13)C. In long days, the activity of the crassulacean acid metabolism pathway increases as the leaves age, remaining, however, relatively low as compared to that of C3 pathway: leaves accumulate soluble sugars, poor in (13)C. After photoperiodic change (long days→short days), isotopic modifications of starch and organic acids suggest evidence for a lag phase in the establishment of the crassulacean acid metabolism pathway specific to short days. The relative proportions of carbon from a C3-origin (RuBPC acitivity as strong discriminating step, isotope discrimination in vivo=20‰) or C4-origin (PEPC activity as weak discriminating step, isotope discrimination in vivo=4‰) present in the biochemical fractions were calculated from their δ(13)C values. Under long days, 30 to 70% versus 80 to 100% under short days, of the carbon of the intermediates linked to the starch-malate sequence, or CAM pathway (starch, phosphorylated compounds and organic acids), have a C4-origin. Products connected to the C3 pathway (free sugars, cellulose, hemicellulose) have 0 to 50% of their carbon, arising from reuptake of the C4 from malate, under long days versus 30 to 70% under short days.

Changes in the isozymic pattern of phosphoenolpyruvate : An early step in photoperiodic control of crassulacean acid metabolism level

Two major isofunctional forms of phosphoenolpyruvate carboxylase (EC 4.1.1.31) have been separated from the leaves of Kalanchoe blossfeldiana Poelln. Tom Thumb by acrylamide gel electrophoresis and diethylaminoethyl cellulose techniques: one of the forms prevails under long-day treatment (low crassulacean acid metabolism level), the other develops under short-day treatment (high Crassulacean acid metabolism level). Molecular weights are significantly different: 175·10(3) and 186·10(3), respectively. These results indicate that two populations of phosphoenolyruvate carboxylase are present in the plant, one of which is responsible for Crassulacean acid metabolism activity under the control of photoperiod.The Crassulacean acid metabolism appears to depend on the same endogenous clock that governs other photoperiodically controlled events (e.g. flowering). The metabolic and energetic significance of this feature is discussed. It is suggested that modification in isozymic composition could be an early step in the response to photoperiodism at the metabolic level.