Mass-spectrometric evidence for the double-carboxylation pathway of malate synthesis by Crassulacean acid metabolism plants in light

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO₂ uptake, via phosphoenolpyruvate carboxylase (PEPC, C₄ carboxylation), from the diurnal refixation by Rubisco (C₃ carboxylation). At the end of the day (CAM-Phase IV), when nocturnally stored CO₂ has depleted, stomata reopen and allow additional CO₂ uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO₂ uptake via C₃ and C₄ carboxylation in phase IV in the CAM species Phalaenopsis "Sacramento" and Kalanchoe blossfeldiana "Saja." Short blackout periods during phase IV caused a sharp drop in CO₂ uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O₂ indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.

A comparative study on the regulation of C(3) and C (4) carboxylation processes in the constitutive crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana and the C(3)-CAM intermediate Clusia minor

A comparison of carbon metabolism in the constitutive crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. and the C(3)-CAM intermediate Clusia minor L. was undertaken under controlled environmental conditions where plants experience gradual changes in light intensity, temperature and humidity at the start and end of the photoperiod. The magnitude of CAM activity was manipulated by maintaining plants in ambient air or by enclosing leaves overnight in an atmosphere of N(2) to suppress C(4) carboxylation. Measurements of diel changes in carbonisotope discrimination and organic acid content were used to quantify the activities of C(3) and C(4) carboxylases in vivo and to indicate the extent to which the activities of phosphoenolpyruvate carboxylase (PEPCase), ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and decarboxylation processes overlap at the start and end of the photoperiod. These measurements in vivo were compared with measurements in vitro of changes in the diel sensitivity of PEPCase to malate inhibition. The results demonstrate fundamental differences in the down-regulation of PEPCase during the day in the two species. While PEPCase is inactivated within the first 30 min of the photoperiod in K. daigremontiana, the enzyme is active for 4 h at the start and 3 h at the end of the photoperiod in C. minor. Enclosing leaves in N(2) overnight resulted in a two-to threefold increase in PEPCase-mediated CO(2) uptake during Phase II of CAM in both species. However, futile cycling of CO(2) between malate synthesis and decarboxylation does not occur during Phase II in either species. In terms of overall carbon balance, C(4) carboxylation accounted for approximately 20% of net daytime assimilation in both species under control conditions, increasing to 30-34% after a night in N(2). Although N(2)-treated leaves of K. daigremontiana took up 25% more CO(2) than control leaves during the day this was insufficient to compensate for the loss of CO(2) taken up by CAM the previous night. In contrast, in N(2)-treated leaves of C. minor, the twofold increase in daytime PEPCase activity and the increase in net CO(2) uptake by Rubisco during Phase III compensated for the inhibition of C(4) carboxylation at night in terms of diel carbon balance.

Tansley Review No. 37 Circadian rhythms: their origin and control

This article reviews the circadian rhythm of carbon dioxide metabolism in leaves of the Crassulacean plant Bryophyllum (Kalanchoë) fedtsckenkoi which persists both in continuous darkness and a CO₂ -free atmosphere, and in continuous light and normal air. Under both conditions the rhythm is due to the periodic activity of the enzyme phosphoenolpyruvate carboxylase (PEPc). The physiological characteristics of the rhythm are described in detail and, from these characteristics, hypotheses are advanced to account for both the generation of the rhythm and the regulation of its phase and period by environmental factors. The periodic activity of PEPc is ascribed to the periodic accumulation of an allosteric inhibitor, malate, in the cytoplasm and its subsequent removal either to the vacuole in continuous darkness, or by metabolism in continuous light. Also involved in the generation of the rhythm is a periodic change in the sensitivity of PEPc to malate inhibition due to the periodic phosphorylation and dephosphorylation of PEPc which changes its K₁ by a factor of 10 from 30 to 0.3 mM and vice versa. This periodic phosphorylation of PEPc is apparently achieved by the periodic synthesis and breakdown of a PEPc kinase which phosphorylates the enzyme on a serine residue; dephosphorylation is achieved by a type 2A phosphatase which shows no rhythmic variation. The induction of phase shifts in the rhythm in continuous darkness and CO₂ -free air has been explained in terms of light and high-temperature activated gates or channels in the tonoplast which, when open, allow malate to diffuse between the vacuole and cytoplasm. For the rhythm in continuous light and normal air phase, control by environmental signals can be attributed to changes in the malate levels in critical cell compartments, or in particular cell populations such as the stomatal guard cells, due to regulation of the malate synthesizing enzyme system involving PEPc, and malic enzyme which is responsible for malate metabolism. The role of the stomata in the generation of the rhythm is also discussed. The biochemical events which appear to give rise to the well-studied circadian rhythms in leaf movement in Samanea and Albizza, in luminescence in Gonyaulax polyedra and in the synthesis of the chlorophyll a/b binding protein are also reviewed in an attempt to identify similarities between these events and those involved in the Bryophyllum rhythm. Finally, the somewhat similar nature of the genes apparently responsible for circadian rhythmicity in Neurospora and Drosophila are discussed, and suggestions made for utilizing anti-sense nucleic acid technology in the further elucidation of the critical biochemical events involved in the basic, temperature-compensated circadian oscillator in living organisms. CONTENTS Summary 347 I. Introduction 348 II. Occurrence of circadian rhythms 348 III. Physiological characteristics of circadian rhythms 349 IV. Biochemical and molecular events involved in the circadian rhythm in Bryophyllum leaves 362 V. Biochemical and molecular events involved in the origin and control of circadian rhythmicity in other organisms 366 VI. Genetic studies 370 VII. Conclusion 371 References 372.

Ecophysiological comportment of the tropical CAM-tree Clusia in the field: II. Modes of photosynthesis in trees and seedlings

Measurements were performed on leaves of Clusia rosea Jacq. trees in the moist central mountains (330 to 365 m above sea level) and at the dry south coast of St John Island (US Virgin Islands, Lesser Antilles). Seedlings of C. rosea were also studied in the central hills. During the study period (March 1989) all trees showed crassulacean acid metabolism (CAM), in which net CO₂ uptake extended for a remarkably long time in the morning (phase II of CAM: until about 11 to 12 h) and contributed about 1/3 of total net CO₂ -uptake. During the night (phase I of CAM) malic acid and citric acid were accumulated concurrently at a molar ratio of malic: citric acid of about 1.6. Internal recycling of respiratory CO₂ was 20% of total CO₂ fixed during the night. Water-use-efficiency (mol CO₂ taken up: mol H₂ O transpired) was 0.014 to 0.022. The pH of leaf-cell sap at the end of the dark period was 2.85. This would still allow an H⁺ -ATPase at the tonoplast to transport 2H⁺ into the vacuole per ATP hydrolysed when operating near thermodynamic equilibrium. Free sugars, glucose and fructose, and starch were used as precursors for the CO₂ -acceptor phosphoenolpyruvate during the dark period; contributions of the two hexoses were about equal and together four-times that of starch. Xylem tensions showed increases of up to 8 bar during day-time. Leaf-sap osmotic pressures did not change significantly; the trend was a small decline during day-time. Among the seedlings, three different modes of photosynthesis were encountered, namely C₃ -photosynthesis in terrestrial and in epiphytic seedlings, continuous stomatal opening and CO₂ -uptake day and night in epiphytic seedlings, and CAM in seedlings growing in the tanks of Aechmea lingulata (L.) Baker.

Regulation of malic-acid metabolism in Crassulacean-acid-metabolism plants in the dark and light: In-vivo evidence from (13)C-labeling patterns after (13)CO 2 fixation

The labeling patterns in malic acid from dark (13)CO2 fixation in seven species of succulent plants with Crassulacean acid metabolism were analysed by gas chromatography-mass spectrometry and (13)C-nuclear magnetic resonance spectrometry. Only singly labeled malic-acid molecules were detected and on the average, after 12-14 h dark (13)CO2 fixation the ratio of [4-(13)C] to [1-(13)C] label was 2:1. However the 4-C carboxyl contained from 72 to 50% of the label depending on species and temperature. The (13)C enrichment of malate and fumarate was similar. These data confirm those of W. Cockburn and A. McAuley (1975, Plant Physiol. 55, 87-89) and indicate fumarase randomization is responsible for movement of label to 1-C malic acid following carboxylation of phosphoenolpyruvate. The extent of randomization may depend on time and on the balance of malic-acid fluxes between mitochondria and vacuoles. The ratio of labeling in 4-C to 1-C of malic acid which accumulated following (13)CO2 fixation in the dark did not change during deacidification in the light and no doubly-labeled molecules of malic acid were detected. These results indicate that further fumarase randomization does not occur in the light, and futile cycling of decarboxylation products of [(13)C] malic acid ((13)CO2 or [1-(13)C]pyruvate) through phosphoenolpyruvate carboxylase does not occur, presumably because malic acid inhibits this enzyme in the light in vivo. Short-term exposure to (13)CO2 in the light after deacidification leads to the synthesis of singly and multiply labeled malic acid in these species, as observed by E.W. Ritz et al. (1986, Planta 167, 284-291). In the shortest times, only singly-labeled [4-(13)C]malate was detected but this may be a consequence of the higher intensity and better detection statistics of this ion cluster during mass spectrometry. We conclude that both phosphoenolpyruvate carboxylase (EC 4.1.1.32) and ribulose-1,5-biphosphate carboxylase (EC 4.1.1.39) are active at this time.

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.

Persistent circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi leaves and in its sensitivity to inhibition by malate

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPCase) from Bryophyllum fedtschenkoi leaves has previously been shown to exist in two forms in vivo. During the night the enzyme is phosphorylated and relatively insensitive to feedback inhibition by malate whereas during the day the enzyme is dephosphorylated and more sensitive to inhibition by malate. These properties of PEPCase have now been investigated in leaves maintained under constant conditions of temperature and lighting. When leaves were maintained in continuous darkness and CO2-free air at 15°C, PEPCase exhibited a persistent circadian rhythm of interconversion between the two forms. There was a good correlation between periods during which the leaves were fixing respiratory CO2 and periods during which PEPCase was in the form normally observed at night. When leaves were maintained in continuous light and normal air at 15°C, starting at the end of a night or the end of a day, a circadian rhythm of net uptake of CO2 was observed. Only when these constant conditions were applied at the end of a day was a circadian rhythm of interconversions between the two forms of PEPCase observed and the rhythms of enzyme interconversion and CO2 uptake did not correlate in phase or period.

Characterization of carbon metabolism in Opuntia ficus-indica Mill. exhibiting the idling mode of Crassulacean acid metabolism

Upon transfer from well-watered conditions to total drought, long-day-grown cladodes of Opuntia ficus-indica Mill. shift from full Crassulacean acid metabolism (CAM) to CAM-idling. Experiments using (14)C-tracers were conducted in order to characterize the carbon-flow pattern in cladodes under both physiological situations. Tracer was applied by (14)CO2 fumigations and NaH(14)CO3 injections during the day-night cycle. The results showed that behind the closed stomata, mesophyll cells of CAM-idling plants retained their full capacity to metabolize CO2 in light and in darkness. Upon the induction of CAM-idling the level of the capacity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) was maintained. By contrast, malate pools decreased, displaying finally only a small or no day-night oscillation. The capacity of NADP-malic enzyme (EC 1.1.1.40) decreased in parallel with the reduction in malate pools. Differences in the labelling patterns, as influenced by the mode of tracer application, are discussed.