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

Decoding Biosynthetic Pathways in Plants by Pulse-Chase Strategies Using (13)CO₂ as a Universal Tracer †

(13)CO₂ pulse-chase experiments monitored by high-resolution NMR spectroscopy and mass spectrometry can provide (13)C-isotopologue compositions in biosynthetic products. Experiments with a variety of plant species have documented that the isotopologue profiles generated with (13)CO₂ pulse-chase labeling are directly comparable to those that can be generated by the application of [U-(13)C₆]glucose to aseptically growing plants. However, the application of the (13)CO₂ labeling technology is not subject to the experimental limitations that one has to take into account for experiments with [U-(13)C₆]glucose and can be applied to plants growing under physiological conditions, even in the field. In practical terms, the results of biosynthetic studies with (13)CO₂ consist of the detection of pairs, triples and occasionally quadruples of (13)C atoms that have been jointly contributed to the target metabolite, at an abundance that is well above the stochastic occurrence of such multiples. Notably, the connectivities of jointly transferred (13)C multiples can have undergone modification by skeletal rearrangements that can be diagnosed from the isotopologue data. As shown by the examples presented in this review article, the approach turns out to be powerful in decoding the carbon topology of even complex biosynthetic pathways.

Metabolic flux analysis in plants: coping with complexity

Theory and experience in metabolic engineering both show that metabolism operates at the network level. In plants, this complexity is compounded by a high degree of compartmentation and the synthesis of a very wide array of secondary metabolic products. A further challenge to understanding and predicting plant metabolic function is posed by our ignorance about the structure of metabolic networks even in well-studied systems. Metabolic flux analysis (MFA) provides tools to measure and model the functioning of metabolism, and is making significant contributions to coping with their complexity. This review gives an overview of different MFA approaches, the measurements required to implement them and the information they yield. The application of MFA methods to plant systems is then illustrated by several examples from the recent literature. Next, the challenges that plant metabolism poses for MFA are discussed together with ways that these can be addressed. Lastly, new developments in MFA are described that can be expected to improve the range and reliability of plant MFA in the coming years.

Movement of water from old to young leaves in three species of succulents

A hypothetical adaptive response of succulent plants to drought-stress is the redistribution of water from old to young leaves. We examined the effects of possible movement of water from old to young leaves in three succulent species, Carpobrotus edulis (weak CAM-inducible), Kalanchoe tubiflora (CAM) and Sedum spectabile (possibly a CAM-cycler or CAM-inducible). Old leaves were removed from plants, and photosynthesis, transpiration, f. wt : d. wt ratios, diurnal acid fluctuations, stomatal conductance and internal CO2 concentrations of the remaining young leaves were measured during drought-stress. Comparison was made with plants retaining old leaves. There was no evidence that water moved from old to young leaves during drought-stress as previously hypothesized. Only in drought-stressed plants of K. tubiflora, were photosynthetic and transpiration rates of young leaves greater on shoots with old leaves removed compared with attached. There was a trend in all species for greater fluctuations in acidity in young leaves on shoots that lacked older leaves. For two of the three species studied, the f. wt : d. wt ratios of young leaves were greater under drought-stress, on shoots with old leaves removed than with them attached. Absence of old leaves may reduce competition for water with young leaves, which consequently have higher water content and greater photosynthetic rates.

PROBING PLANT METABOLISM WITH NMR

Analytical methods for probing plant metabolism are taking on new significance in the era of functional genomics and metabolic engineering. Among the available methods, nuclear magnetic resonance (NMR) spectroscopy is a technique that can provide insights into the integration and regulation of plant metabolism through a combination of in vivo and in vitro measurements. Thus NMR can be used to identify, quantify, and localize metabolites, to define the intracellular environment, and to explore pathways and their operation. We review these applications and their significance from a metabolic perspective. Topics of current interest include applications of NMR to metabolic flux analysis, metabolite profiling, and metabolite imaging. These and other areas are discussed in relation to NMR investigations of intermediary carbon and nitrogen metabolism. We conclude that metabolic NMR has a continuing role to play in the development of a quantitative understanding of plant metabolism and in the characterization of metabolic phenotypes.

In vivo 31P and multilabel 13C NMR measurements for evaluation of plant metabolic pathways

Reliable measurements of intracellular metabolites are useful for effective plant metabolic engineering. This study explored the application of in situ 31P and 13C NMR spectroscopy for long-term measurements of intracellular pH and concentrations of several metabolites in glycolysis, glucan synthesis, and central carbon metabolic pathways in plant tissues. An NMR perfusion reactor system was designed to allow Catharanthus roseus hairy root cultures to grow for 3-6 weeks, during which time NMR spectroscopy was performed. Constant cytoplasmic pH (7.40+/-0.06), observed during the entire experiment, indicated adequate oxygenation. 13C NMR spectroscopy was performed on hairy root cultures grown in solutions containing 1-13C-, 2-13C-, and 3-13C-labeled glucose in separate experiments and the flow of label was monitored. Activities of pentose phosphate pathways, nonphotosynthetic CO2 fixation, and glucan synthesis pathways were evident from the experimental results. Scrambling of label in glucans also indicated recycling of triose phosphate and their subsequent conversion to hexose phosphates.

Estimation of Ammonium Ion Distribution between Cytoplasm and Vacuole Using Nuclear Magnetic Resonance Spectroscopy

Evidence is presented that intracellular ammonium is trapped in vacuoles of maize (Zea mays L.) root tips because of rapid movement of ammonia between cytoplasm and vacuoles. The concentration of cytoplasmic ammonium is estimated to be <15 mum at extracellular ammonium concentrations up to 1 mm. The implications for pathways of ammonium assimilation are discussed.

Cytoplasmic malate levels in maize root tips during K+ ion uptake determined by 13C-NMR spectroscopy

13C-NMR spectroscopy was used to determine the level of cytoplasmic malate in maize root tips that exhibited different rates of malate synthesis. Intracellular malate was 13C-labeled at carbons 1 and 4 by perfusing root tips with 5 nM H13CO3-. This labeling reflects the activities of phosphoenolpyruvate carboxylase and malate dehydrogenase (production of [4-13C]malate), and fumarase (scrambling of 13C-label between C1 and C4 of malate). In vivo 13C-NMR spectra contained a clearly resolved resonance from cytoplasmic [4-13C]malate, while the resonance from cytoplasmic [1-13C]malate overlapped with others. After 90 min of H13CO3- treatment, 13C-labeling of organic acid pools had reached steady-state. Thereafter, the ratios [13C]malate/[12C + 13C]malate and [1-13C]malate/[4-13C]malate in tissue extracts remained constant; evidence is presented that these ratios were the same for both cytoplasmic and total cellular malate. Hence, the intensity of the cytoplasmic [4-13C]malate signal was proportional to the amount of cytoplasmic malate in root tips. Potassium sulfate stimulate malate synthesis in maize root tips, relative to root tips perfused with HCO3- alone; total cellular malate doubled after approx. 1 h of 5 mM K2SO4-treatment. Cytoplasmic malate increased from approx. 3.5 mM to approx. 7.5 mM within 45 min of the onset of K2SO4-treatment, declining slightly thereafter. The possible effects of these changing cytoplasmic malate concentration on the enzymes involved in malate metabolism are discussed.

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.

Observation of Cytoplasmic and Vacuolar Malate in Maize Root Tips by C-NMR Spectroscopy

The accumulation of malate by maize (Zea mays L.) root tips perfused with KH(13)CO(3) was followed by (13)C nuclear magnetic resonance spectroscopy. In vivo nuclear magnetic resonance spectra contained distinct signals from two pools of malate in maize root tips, one at a pH approximately 5.3 (assigned to the vacuole) and one at a pH > 6.5 (assigned to the cytoplasm). The ratio of cytoplasmic to vacuolar malate was lower in 12 millimeter long root tips than in 2 millimeter root tips. The relatively broad width of the signals from C1- and C4-labeled vacuolar malate indicated heterogeneity in vacuolar pH. During the 3 hour KH(13)CO(3) treatment, (13)C-malate accumulated first primarily in the cytoplasm, increasing to a fairly constant level of approximately 6 millimolar by 1 hour. After a lag, vacuolar malate increased throughout the experiment.

An in vivo 1H and 31P NMR investigation of the effect of nitrate on hypoxic metabolism in maize roots

The effect of nitrate on the short-term hypoxic response and recovery of flooded mature maize roots has been investigated in vivo by 1H and 31P NMR and in vitro by 1H NMR and gas chromatography-mass spectrometry. Employing 1H NMR in addition to 31P NMR extended the number of identifiable compounds in vivo from 4 to 15, while in vitro two-dimensional NMR and gas chromatography-mass spectrometry aided rigorous in vivo 1H NMR resonance assignments and quantitation of 24 compounds. In the absence of nitrate, the concentrations of key metabolites including alanine, ethanol, gamma-aminobutyrate, lactate, succinate, and sucrose changed during 8 h of hypoxia in a manner consistent with reduced tricarboxylic acid cycle activity and diversion to glycolytic fermentation. The pH drop in the cytoplasm during hypoxia was rapid, about 0.2 unit, and diminished quickly upon recovery. Rapid recovery of ethanol, succinate, and sucrose levels was also observed, which indicates a return to normal aerobic metabolism. Although the hypoxic response itself, including pH, was not greatly affected by the presence of nitrate, nitrate reduced the amount of fermentation end products produced, helped maintain a higher free NTP concentration during hypoxia, and increased the rate of overall recovery from hypoxia. These findings suggest the presence of a nitrate-induced maintenance-level respiration in hypoxic maize roots, which helps explain the protection imparted by nitrate to flooded hypoxic maize plants.

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.

Growth inhibition by exogenous proline and its metabolism in saltgrass (Distichlis spicata) suspension cultures

The growth of Distichlis spicata suspension cultures in LS medium without NaCl was inhibited 54% by 2 mM proline. In medium containing 260 mM NaCl, 10 mM proline inhibited growth by only 22%. The uptake and metabolism of 10 mM L-[1-(13)C] proline was followed by (13)C NMR and ninhydrin analyses of suspensions cultured in the presence of 0 or 260 mM NaCl. Uptake of 85 to 92% of the exogenous proline occurred within 72 h in all media. In 10 mM proline and no NaCl, cellular proline reached a maximm of 51.5 μmoles/g FW compared to 1.9 μmoles/g FW in suspensions not grown on proline. In medium containing 260 mM NaCl and proline, cellular proline reached 59-65 μmoles/g FW compared to 30-40 μmoles/g FW in controls grown without proline. The (13)C-label in the proline-C1 was either retained in proline or disappeared, presumably released as carbon dioxide, by catabolism through the TCA cycle. Since no metabolite of (13)C-proline was detected by NMR, proline was considered to be the molecule which inhibited the suspension culture growth.

C nuclear magnetic resonance study of acetate incorporation into malate during ca-uptake by isolated leaf tissues

(13)C Nuclear magnetic resonance spectroscopy of leaflets of Gleditsia triacanthos and Albizia julibrisin was used to determine the fate of acetate taken up during the absorption of calcium from (13)C-labeled Ca-acetate solution. Small amounts of acetate accumulated temporarily in the leaf tissues, but the bulk of acetate was incorporated into malate. The initial rate of malate synthesis was very low, but increased rapidly during acetate treatment and reached its maximum after 8 hours; the enzymes involved in malate synthesis thus appear to be substrate induced. Use of acetate-2-(13)C yielded malate labeled in C-3, indicating that vacuolar malate accumulating during Ca-uptake might be synthesized via malate synthase from acetate and glyoxalate. However, a source of glyoxalate condensing with acetate during malate synthesis could not be identified. Glycolate produced in photorespiration is an unlikely source, because glycolate-2-(13)C was absorbed and metabolized by the leaf tissues into products of the glycolate pathway, but was not a major precursor in malate synthesis. Malate synthesis via the glyoxalate cycle is also unlikely, because no evidence for the recycling of a (13)C-labeled 4-carbon organic acid was found. Malate synthesis in the leaflets of Gleditsia and Albizia thus appears to involve the inducible condensation of acetate with a 2-carbon compound of unidentified nature and origin.

Use of Na-Nuclear Magnetic Resonance To Follow Sodium Uptake and Efflux in NaCl-Adapted and Nonadapted Millet (Panicum miliaceum) Suspensions

Cellular Na(+) transport was followed in vivo by (23)Na nuclear magnetic resonance (NMR) using anionic dysprosium-based shift reagents to resolve internal and external (23)Na(+) resonances. Proso millet (Panicum miliaceum) cell suspensions adapted for rapid growth on 130 mm NaCl had biphasic (23)Na efflux kinetics when shifted to low Na(+) medium, while nonadapted cells had little measurable Na(+) efflux after preloading with (23)NaCl. Uptake of (23)Na was also observed using (23)Na NMR. The resonance frequency of the external Na(+)-dysprosium (III) triphosphate, relative to that of the (23)Na in the cells, was sensitive to pH, permitting the pH of the external medium to be followed during the course of in vivo experiments.