The pathway of carbon dioxide fixation in crassulacean plants

The mechanism of oxalate biosynthesis in higher plants: investigations with the stable isotopes 18 O and 13 C

Substantial incorporation of 18 O 2 into photorespiratory carbon oxidation cycle intermediates in illuminated Spinacia oleracea leaves confirms that oxygenase activity of the enzyme ribulose biphosphate carboxylase–oxygenase is a major source of glycollate in illuminated leaves. No 18 O 2 incorporation into oxalate was detected in these experiments, although 13 C incorporation from 13 CO 2 shows that oxalate synthesis is occurring under the experimental conditions. This result tends to minimize the role of a direct oxidation of glyoxylate derived (via phosphoglycollate and glycollate) from ribulose biphosphate oxygenase activity in oxalate synthesis in Spinacia . Measurements of δ 13 C show (in confirmation of earlier reports) that oxalate from Spinacia is less depleted in 13 C than is bulk organic C in the plant; it is possible the phosphoenolpyruvate carboxylase is involved in the production of the oxalate precursor. Of the plants tested, Mercurialis and Pelargonium shared with Spinacia the high δ 13 C value, while Chenopodium (closely related to Spinacia ), Oxalis (more distantly related to Pelargonium ) and two members of the Polygonaceae had oxalate δ 13 C values close to the whole-leaf δ 13 C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase–oxygenase.

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

This paper relates the ¹³ C/¹² C ratio of C₃ plant material relative to that of source CO₂ to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The ¹³ C/¹² C ratio is quantified as Δ, defined as (δ¹³ C substrate -δ¹³ C product)/(1+δ¹³ C product), where δ¹³ C values of substrate or product (i.e. the samples) are defined as [¹³ C/¹² C]_sample ]/[(¹³ C/¹² C)_standard ]-1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO₂ ). Application of this approach to a 'typical' C₃ land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0-24 %, for NH₄ ⁺ assimilation (which always occurs in the roots) to 2-80%, for NO₃ ^- assimilation in shoots with the organic acid salt which results from acid-base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N₂ , or where NO₃ ^- assimilation in root or shoot is accompanied by some acid-base regulation via OH- loss to the root medium. Comparison with published data on the difference in Δ of Ricinus communis cultured with NH₄ ⁺ or NO₃ ^- shows that the measured influence of nitrogen source is in the right direction (NO₃ ^- grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non-Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ¹³ C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH₄ ⁺ -grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C₃ terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.

Malate Metabolism in the Dark After CO(2) Fixation in the Crassulacean Plant Kalanchoë tubiflora

The metabolism of [(13)C]malate was studied in the Crassulacean plant Kalanchoë tubiflora following exposure to (13)CO(2) for 2 hour intervals during a 16 hour dark cycle. Nuclear magnetic resonance spectroscopy of [(13)C]malate extracted from labeled tissue revealed that the transient flux of malate to the mitochondria, estimated by the randomization of [4-(13)C]malate to [1- (13)C]malate by fumarase, varied substantially during the dark period. At both 15 and 25 degrees C, the extent of malate label randomization in the mitochondria was greatest during the early and late parts of the dark period and was least during the middle of the night, when the rate of (13)CO(2) uptake was highest. Randomization of labeled malate continued for many hours after malate synthesis had initially occurred. Internally respired (12)CO(2) also served as a source of carbon for malate formation. At 15 degrees C, 15% of the total malate was formed from respired (12)CO(2), while at 25 degrees C, 49% of the accumulated malate was derived from respired (12)CO(2). Some of the malate synthesized from external (13)CO(2) was also respired during the night. The proportion of the total [(13)C]malate respired during the dark period was similar at 15 and 25 degrees C, and respiration of newly formed [(13)C]malate increased as the night period progressed. These data are discussed with regard to the relative fluxes of malate to the mitochondria and the vacuole during dark CO(2) fixation.

Temperature effects on malic-acid efflux from the vacuoles and on the carboxylation pathways in crassulacean-acid-metabolism plants

The studies described in the paper were conducted with tissue slices of Crassulacean acid metabolism (CAM) plants floating in isotonic buffer. In a first series of experiments, temperature effects on the efflux of [(14)C]malate and(14)CO2 were studied. An increase of temperature increased the efflux from the tissue in a non-linear manner. The efflux was markedly influenced also by the temperatures applied during the pretreatment. The rates of label export in response to the temperature and the relative contributions of(14)CO2 and [(14)C]malate to the label export were different in the two studied CAM plants (Kalanchoë daigremontiana, Sempervivum montanum). In further experiments, temperature response of the labelling patterns produced by(14)CO2 fixation and light and darkness were studied. In tissue which had accumulated malate (acidified state) an increase of temperature decreased the rates of dark CO2 fixation whilst the rates of CO2 fixation in light remained largely unaffected. An increase of temperature shifted the labelling patterns from a C4-type (malate being the mainly labelled compound) into a C3-type (label in carbohydrates). No such shift in the labelling patterns could be observed in the tissue which had depleted the previously stored malate (deacidified state). The results indicate that in the acidified tissue the increase of temperature increases the efflux of malate from the vacuole by changing the properties of the tonoplast. It is assumed that the increased export of malic acid lowers the in-vivo activity of phosphoenol pyruvate carboxylase by feedback inhibition.

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.

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

Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix (13)CO2 in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After (13)CO2 fixation in darkness, only singly labelled [(13)C]malate molecules were found. Fixation of (13)CO2 under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during (13)CO2 fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the (13)CO2 application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.

C Nuclear Magnetic Resonance Studies of Crassulacean Acid Metabolism in Intact Leaves of Kalanchoë tubiflora

(13)C nuclear magnetic resonance spectroscopy of intact leaves of Kalanchoë tubiflora was used to observe Crassulacean acid metabolism in vivo. (13)C signals from C-4 of malate were observed after overnight exposure of leaves to (13)CO(2). Illumination of the labeled leaves resulted in a gradual decrease in the malate signals. After a period of darkness in normal air, (13)C signals were detected in all four carbons of malate in the previously labeled leaves. The (13)C nuclear magnetic resonance spectrum of malate in solution was pH dependent, which allowed an estimation of the vacuolar pH from the whole leaf spectrum. The pH was 4.0 following a 14-hour dark period, but rose to greater than 6.0 after 6 hours of illumination.

Effect of Varying CO(2) Partial Pressure on Photosynthesis and on Carbon Isotope Composition of Carbon-4 of Malate from the Crassulacean Acid Metabolism Plant Kalanchoë daigremontiana Hamet et Perr

Intact leaves of Kalanchoë daigremontiana were exposed to CO(2) partial pressures of 100, 300, and 1000 microbars. Malic acid was extracted, purified, and degraded in order to obtain isotopic composition of carbon-1 and carbon-4. From these data, it is possible to calculate the carbon isotope composition of newly fixed carbon in malate. In all three treatments, the isotopic composition of newly introduced carbon is the same as that of the CO(2) source and is independent of CO(2) partial pressures over the range tested. Comparison with numerical models described previously (O'Leary 1981 Phytochemistry 20: 553-567) indicates that we would expect carbon 4 of malate to be 4 per thousand more negative than source CO(2) if diffusion is totally limiting or 7 per thousand more positive than source CO(2) if carboxylation is totally limiting. Our results demonstrate that stomatal aperture adjusts to changing CO(2) partial pressures and maintains the ratio of diffusion resistance to carboxylation resistance approximately constant. In this study, carboxylation and diffusion resistances balance so that essentially no fractionation occurs during malate synthesis. Gas exchange studies of the same leaves from which malate was extracted show that the extent of malate synthesis over the whole night is nearly independent of CO(2) partial pressure, although there are small variations in CO(2) uptake rate. Both the gas exchange and the isotope studies indicate that the ratio of external to internal CO(2) partial pressure is the same in all three treatments. Inasmuch as a constant ratio will result in constant isotope fractionation, this observation may explain why plants in general have fairly invariable (13)C contents, despite growing under a variety of environmental conditions.

Pathway of malic Acid synthesis in response to ion uptake in wheat and lupin roots: evidence from fixation of C and C

Malate synthesis by CO(2) fixation in wheat (Triticum aestivum L.) and lupin (Lupinus luteus) roots was investigated by labeling with NaH(13)CO(3) as well as with NaH(14)CO(3). The distribution of (14)C label in the malate was examined, using enzymic degradation methods (malic enzyme, pyruvate decarboxylase) and, in the case of (13)C, gas chromatography-mass spectrometry. In long-term experiments (2 to 12 hours), both methods showed that the [1-C] and [4-C] positions of malic acid are approximately equally labeled, in agreement with former findings. Short-term experiments (15, 30 seconds) showed that (14)C is confined initially to the [4-C] position of malate but then is distributed quickly to the [1-C] atom. Neither labeling pattern nor rate of randomization was influenced by salt treatment. Analysis of malate from roots by gas chromatography-mass spectrometry, a procedure which was tested against in vitro-prepared [1-(13)C]-, [4-(13)C]-, and [1,4-(13)C] malate, gave strong evidence for the existence of only singly labeled malate molecules. These data suggest that only one carboxylation step, catalyzed by phosphoenolpyruvate carboxylase and/or phosphoenolpyruvate carboxykinase, is responsible for malic acid synthesis in roots and that malate label is randomized by a fumarase-like reaction, presumably in mitochondria.

Diffusional Contribution to Carbon Isotope Fractionation during Dark CO(2) Fixation in CAM Plants

A mathematical model is developed which can be used to predict in vivo carbon isotope fractionations associated with carbon fixation in plants in terms of diffusion, CO(2) hydration, and carboxylation components. This model also permits calculation of internal CO(2) concentration for comparison with results of gas-exchange experiments. The isotope fractionations associated with carbon fixation in Kalanchoë daigremontiana and Bryophyllum tubiflorum have been measured by isolation of malic acid following dark fixation and enzymic determination of the isotopic composition of carbon-4 of this material. Corrections are made for residual malic acid, fumarase activity, and respiration. Comparison of these data with calculations from the model indicates that the rate of carbon fixation is limited principally by diffusion, rather than by carboxylation. Processes subsequent to the initial carboxylation also contribute to the over-all isotopic composition of the plant.

Dark Carbon Dioxide Fixation under Aerobic and Anaerobic Conditions in Maize Leaves after Preillumination in the Absence of Oxygen: Ribulose 1,5-Bisphosphate Can Serve as a Primary Acceptor of Carbon Dioxide

When dark (14)CO(2) fixation in maize leaves was carried out under anaerobic conditions after preillumination in the absence of O(2), the (14)C incorporation in aspartic acid was transient; its maximum level was very low compared with that of malic acid. The addition of 5% O(2) during the dark fixation period increased the total uptake of (14)CO(2) and the (14)C incorporation into aspartic acid.A study of the intramolecular distribution of radioactivity showed that 71 to 76% of the (14)C was located in the C(4) (beta-carboxyl) of malate and aspartate and the remainder in the C(1). This intramolecular labeling pattern did not change during the 5- to 60-second dark (14)CO(2) fixation period and was scarcely altered by the presence of O(2). Three degradation techniques led to similar data.The significance of these results is discussed taking into account the known possible carboxylation pathways. It is concluded that ribulose 1,5-bisphosphate can be a primary acceptor of CO(2) when maize leaves are preilluminated in the absence of O(2).

Malate synthesis by dark carbon dioxide fixation in leaves

The rates of dark CO(2) fixation and the label distribution in malate following dark (14)CO(2) fixation in a C-4 plant (maize), a C-3 plant (sunflower), and two Crassulacean acid metabolism plants (Bryophyllum calycinum and Kalanchoë diagremontianum leaves and plantlets) are compared. Within the first 30 minutes of dark (14)CO(2) fixation, leaves of maize, B. calycinum, and sunflower, and K. diagremontianum plantlets fix CO(2) at rates of 1.4, 3.4, 0.23, and 1.0 mumoles of CO(2)/mg of chlorophyll. hour, respectively. Net CO(2) fixation stops within 3 hours in maize and sunflower, but Crassulaceans continue fixing CO(2) for the duration of the 23-hour experiment.A bacterial procedure using Lactobacillus plantarum ATCC No. 8014 and one using malic enzyme to remove the beta-carboxyl (C(4)) from malate are compared. It is reported that highly purified malic enzyme and the bacterial method provide equivalent results. Less purified malic enzyme may overestimate the label in C(4) as much as 15 to 20%.The contribution of carbon atom 1 of malate is between 18 and 21% of the total carboxyl label after 1 minute of dark CO(2) fixation. Isotopic labeling in the two carboxyls approached unity with time. The rate of increase is greatest in sunflower leaves and Kalanchoë plantlets. In addition, Kalanchoë leaves fix (14)CO(2) more rapidly than Kalanchoë plantlets and the equilibration of the malate carboxyls occurs more slowly. The rates of fixation and the randomization are tissue-specific. The rate of fixation does not correlate with the rate of randomization of isotope in the malate carboxyls.

Changes in Metabolite Levels in Kalanchoë daigremontiana and the Regulation of Malic Acid Accumulation in Crassulacean Acid Metabolism

Changes in glucose-6-P, fructose-6-P, fructose-1,6-diP, 6-phospho-gluconate, phosphoenolpyruvate, 3-phosphoglycerate, and pyruvate levels in the leaves of the Crassulacean plant Kalanchoë daigremontiana Hammet et Perrier were measured enzymically during transitions from CO(2)-free air to air, air to CO(2)-free air, and throughout the course of acid accumulation in darkness. The data are discussed in terms of the involvement of phosphoenolpyruvate carboxylase in malic acid synthesis and in terms of the regulation of the commencement of malic acid synthesis and accumulation through the effects of CO(2) on storage carbohydrate mobilization and its termination through the effects of malic acid on phosphoenolpyruvate carboxylase activity.

Determination of the isotope distribution in malate-14C with fumarase-negative lactic acid bacteria

No fumarase activity could be found in whole cells or in cell-free crude extracts from Leuconostoc mesenteroides or Lactobacillus curvatus. The degradation of L-malate-4-14C by these organisms yielded more than 95% of the label as 14CO2. It is therefore recommended that these organisms, rather than Lactobacillus plantarum, should be used in the determination of isotope distribution in L-malate-14C, since L. plantarum exhibits a significant fumarase activity and thus randomizes malate prior to the decarboxylation of this substance by the malolactic enzyme.