Daily rhythm of phosphoenolpyruvate carboxylase in Crassulacean acid metabolism plants

Immunotitration of phosphoenolpyruvate carboxylase (EC 4.1.1.31) extracted from leaves of Kalanchoe blossfeldiana v. Poelln. cv. Tom Thumb. It was established that at different times of the day-night cycle the daily rhythm of enzyme capacity does not result from a rhythm in protein synthesis, but rather from changes in the specific activity of the enzyme.Abbreviations CAM Crassulacean acid metabolism - IgG immunoglobulin G - PEP phosphoenolpyruvate To whom correspondence should be addressed     

Properties of phosphoenolpyruvate carboxylase in rapidly prepared,desalted leaf extracts of the Crassulacean acid metabolism plant Mesembryanthemum crystallinum L.

Properties of phosphoenolpyruvate (PEP) carboxylase, obtained from leaves of Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism (CAM), were determined at frequent time points during a 12-h light/12-h dark cycle. Leaf extracts were rapidly desalted and PEP carboxylase activity as a function of PEP concentration, malate concentration, and pH was measured within 2 min after homogenization of the tissue. Maximum velocity of PEP carboxylase was similar in the light and dark at pH 7.5 and pH 8.0. However, PEP carboxylase had as much as a 12-fold lower K _m for PEP and as much as a 20-fold higher K _i for malate during the dark than during the light periods, the magnitude of these differences being dependent on the assay pH. Assuming that enzyme properties immediately after isolation reflect the approximate state of the enzyme in vivo, these differences in enzyme properties reduce the potential for CO₂ fixation via PEP carboxylase in the light. A small decrease in cytoplasmic pH in the light would greatly magnify the above differences in day/night properties of PEP carboxylase, because the sensitivity of PEP carboxylase to inhibition by malate increased with decreasing pH. Properties of PEP carboxylase were also studied in plants exposed to short-term perturbations of the normal 12-h light/12-h dark cycle (e.g., prolonged light period, prolonged dark period). Under all light/dark regimes, there was a close correlation between change in properties of PEP carboxylase and changes of the tissue from acidification to deacidification, and vice versa. Changes in properties of PEP carboxylase were not merely light/dark phenomena because they were also observed in plants exposed to continuous light or dark. the data indicate that, during CAM, PEP carboxylase exists in two stages which differ in their capacity for net malate synthesis. The physiologically-active state is distinguished by a low K _m for PEP and a high K _i for malate and favors malate synthesis. The physiologically-inactive state has a high K _m for PEP and a low K _i for malate and exists during periods of deacidification and other periods lacking synthesis of malic acid.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPC PEP carboxylase - RuBP ribulose 1,5-bisphosphate - RH relative humidity     

Changes in levels of phosphoenolpyruvate carboxylase with induction of Crassulacean acid metabolism (CAM)-like behavior in the C4 plant Portulaca oleracea

Changes in levels of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31, orthophosphate: oxaloacetate carboxy-lyase, phosphorylating) were followed in leaves and stems of CAM-expressing and non-expressing Portulaca oleracea L. plants. CAM expression was induced by growing plants under an 8-h photoperiod and water stress conditions (SD-WS). Leaves and stems of these plants (designated CAM) expressed nocturnal acidification with an oscillation pattern and an amplitude characteristic of CAM plants. Generally, PEPC activity increased by ca 3-fold during the period of CAM induction. Over the day/night cycle. PEPC activity oscillated in a pattern typical of CAM plants. Treatment of the other plant group (designated as non-CAM) by growth under a 16-h photoperiod and well-watered conditions (LD-WW) did not induce expression of the tested criteria of CAM in plants. In these plants, nocturnal acidification as well as changes in the magnitude of PEPC, activity and fluctuation pattern were undetectable. SDS-PAGE of leaf extracts of the CAM-expressing plants and the corresponding densitometric scans show progressive increase in the amount of PEPC subunit protein (ca 95 kDa) during the period of CAM induction. These results show that induction of CAM-like characteristics in the C₄ plant Portulaca oleracea is also accompanied by increased PEPC activity, which may be partly due to an increase in enzyme synthesis.     

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.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate     

Induction of a Crassulacean acid like metabolism in the C4 succulent plant,Portulaca oleracea L.: physiological and morphological changes are accompanied by specific modifications in phosphoenolpyruvate carboxylase

The induction of a Crassulacean acid like metabolism (CAM) was evidenced after 21–23 days of drought stress in the C₄ succulent plant Portulaca oleracea L. by changes in the CO₂ exchange pattern, in malic acid content and in titratable acidity during the day–night cycle. Light microscopy studies also revealed differences in the leaf structure after the drought treatment. Following the induction of the CAM-like metabolism, the regulatory properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), the enzyme responsible for the diurnal fixation of CO₂ in C₄ plants but nocturnal in CAM plants, were studied. The enzyme from stressed plants showed different kinetic properties with respect to controls, notably its lack of cooperativity, higher sensitivity to L-malate inhibition, higher PEP affinity and lower enzyme content on a protein basis. In both conditions, PEPC's subunit mass was 110 kDa, although changes in the isoelectric point and electrophoretic mobility of the native enzyme were observed. In vivo phosphorylation and native isoelectrofocusing studies indicated variations in the phosphorylation status of the enzyme of samples collected during the night and day, which was clearly different for the control and stressed groups of plants. The results presented suggest that PEPC activity and regulation are modified upon drought stress treatment in a way that allows P. oleracea to perform a CAM-like metabolism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Discrimination in the dark. Resolving the interplay between metabolic and physical constraints to phosphoenolpyruvate carboxylase activity during the crassulacean acid metabolism cycle

A model defining carbon isotope discrimination (delta13C) for crassulacean acid metabolism (CAM) plants was experimentally validated using Kalanchoe daigremontiana. Simultaneous measurements of gas exchange and instantaneous CO2 discrimination (for 13C and 18O) were made from late photoperiod (phase IV of CAM), throughout the dark period (phase I), and into the light (phase II). Measurements of CO2 response curves throughout the dark period revealed changing phosphoenolpyruvate carboxylase (PEPC) capacity. These systematic changes in PEPC capacity were tracked by net CO2 uptake, stomatal conductance, and online delta13C signal; all declined at the start of the dark period, then increased to a maximum 2 h before dawn. Measurements of delta13C were higher than predicted from the ratio of intercellular to external CO2 (p(i)/p(a)) and fractionation associated with CO2 hydration and PEPC carboxylations alone, such that the dark period mesophyll conductance, g(i), was 0.044 mol m(-2) s(-1) bar(-1). A higher estimate of g(i) (0.085 mol m(-2) s(-1) bar(-1)) was needed to account for the modeled and measured delta18O discrimination throughout the dark period. The differences in estimates of g(i) from the two isotope measurements, and an offset of -5.5 per thousand between the 18O content of source and transpired water, suggest spatial variations in either CO2 diffusion path length and/or carbonic anhydrase activity, either within individual cells or across a succulent leaf. Our measurements support the model predictions to show that internal CO2 diffusion limitations within CAM leaves increase delta13C discrimination during nighttime CO2 fixation while reducing delta13C during phase IV. When evaluating the phylogenetic distribution of CAM, carbon isotope composition will reflect these diffusive limitations as well as relative contributions from C3 and C4 biochemistry.     

Salt stress increases the Ca2+-independent phosphoenolpyruvate carboxylase kinase activity in Sorghum leaves

In C4 plants, the photosynthetic enzyme phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) is subjected to a phosphorylation process via the light-dependent up-regulation of a Ca2+-independent PEPCase-kinase. The present work aimed to study the effect of salt stress on PEPCase phosphorylation in Sorghum vulgare Pers. leaves. The growth of salt-treated plants was reduced compared with that of the control plants. PEPCase activity modestly increased (around 20-40%) whereas PEPCase phosphorylation was markedly enhanced, on a protein basis, in extracts from illuminated leaves. The enhanced protein kinase activity was found to display a low molecular mass in the range 32-35 kDa, to be independent of Ca2+ and to be up-regulated by light. Furthermore, up-regulation was blocked in vivo by the cytosolic protein synthesis inhibitor cycloheximide. Collectively, these data demonstrated that salinity stress altered the Ca2+-independent PEPCase-kinase, presumably by increasing the mesophyll content of the enzyme. Potassium chloride, but not abscisic acid, mimicked the effect of NaCl on PEPCase-kinase activity.     

Influence of long photoperiods on plant development and expression of Crassulacean acid metabolism in Mesembryanthemum crystallinum

Abstract. In the natural habitat plants of Mesembryanthemum crystallinum are induced to perform Crassulacean acid metabolism (CAM) after 3 months, and reproductive growth begins after 5 months (Winter, Liittge & Winter, 1978, Oecologia (Berlin), 34, 225-237). The life cycle of M. crystallinum and the extent of growth required prior to induction of enzymes of Crassulacean acid metabolism (CAM) are dramatically shortened by growing seedlings with a long photoperiod (3=16h/8h light/dark). Reproductive growth begins as soon as five weeks after germination when plants are grown in continuous light (under 600μmol quanta m⁻² s⁻¹, 30°C). In plants grown under well-watered conditions, the activities of PEP carboxylase and NADP-malic enzyme begin increasing markedly 2 weeks after germination, with plants grown under longer photoperiods having higher enzyme activities. After 3 weeks of growth, leaves accumulated a large amount of malate, but the microequivalents of malate present were up to nine times greater than the total titratable acidities. Interestingly, plants from a 24h/0h or a 20h/4h photo-period showed no diurnal fluctuation of malate, but did produce malate in the light as a major photosynthetic end product. That is, under these environmental conditions, principal enzymes of CAM can be induced without the plants performing CAM. However, plants grown in a 16h/8h photoperiod did exhibit nocturnal accumulation of malate after 3 weeks of growth. In plants of all three growth conditions, the activities of NADP-malic enzyme and PEP carboxylase were further increased two- to live-fold by irrigating 3-week-old-plants with 350mol m⁻³ NaCl. Such early enhancement of these enzymes by salt and the shortened life cycle may be due to an accelerated development under the long photoperiods.     

Isolation and characterization of mutants of common ice plant deficient in crassulacean acid metabolism

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that improves water use efficiency by shifting part or all of net atmospheric CO2 uptake to the night. Genetic dissection of regulatory and metabolic attributes of CAM has been limited by the difficulty of identifying a reliable phenotype for mutant screening. We developed a novel and simple colorimetric assay to measure leaf pH to screen fast neutron-mutagenized populations of common ice plant (Mesembryanthemum crystallinum), a facultative CAM species, to detect CAM-deficient mutants with limited nocturnal acidification. The isolated CAM-deficient mutants showed negligible net dark CO2 uptake compared with wild-type plants following the imposition of salinity stress. The mutants and wild-type plants accumulated nearly comparable levels of sodium in leaves, but the mutants grew more slowly than the wild-type plants. The mutants also had substantially reduced seed set and seed weight relative to wild type under salinity stress. Carbon-isotope ratios of seed collected from 4-month-old plants indicated that C3 photosynthesis made a greater contribution to seed production in mutants compared to wild type. The CAM-deficient mutants were deficient in leaf starch and lacked plastidic phosphoglucomutase, an enzyme critical for gluconeogenesis and starch formation, resulting in substrate limitation of nocturnal C4 acid formation. The restoration of nocturnal acidification by feeding detached leaves of salt-stressed mutants with glucose or sucrose supported this defect and served to illustrate the flexibility of CAM. The CAM-deficient mutants described here constitute important models for exploring regulatory features and metabolic consequences of CAM.     

Crassulacean acid metabolism in the ZZ plant, Zamioculcas zamiifolia (Araceae)

Zamioculcas zamiifolia (Araceae), a terrestrial East African aroid, with two defining attributes of crassulacean acid metabolism (CAM) (net CO(2) uptake in the dark and diel fluctuations of titratable acidity) is the only CAM plant described within the Araceae, a mainly tropical taxon that contains the second largest number of epiphytes of any vascular plant family. Within the Alismatales, the order to which the Araceae belong, Z. zamiifolia is the only documented nonaquatic CAM species. Zamioculcas zamiifolia has weak CAM that is upregulated in response to water stress. In well-watered plants, day-night fluctuations in titratable acidity were 2.5 μmol H(+)·(g fresh mass)(-1), and net CO(2) uptake in the dark contributed less than 1% to daily carbon gain. Following 10 d of water stress, net CO(2) uptake in the light fell 94% and net CO(2) uptake in the dark increased 7.5-fold, such that its contribution increased to 19% of daily carbon gain. Following rewatering, dark CO(2) uptake returned to within 5% of prestressed levels. We postulate that CAM assists survival of Z. zamiifolia by reducing water loss and maintaining carbon gain during seasonal droughts characteristic of its natural habitat.