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1.
Inhibition of chloroplastic respiration by osmotic dehydration   总被引:1,自引:1,他引:0       下载免费PDF全文
The respiratory capacity of isolated spinach (Spinacia oleracea L.) chloroplasts, measured as the rate of 14CO2 evolved from the oxidative pentose phosphate cycle in darkened chloroplasts exogenously supplied with [14C]glucose, was progressively diminished by escalating osmotic dehydration with betaine or sorbitol. Comparing the inhibitions of CO2 evolution generated by osmotic dehydration in chloroplasts given C-1 and C-6 labeled glucose, 54% and 84% respectively, indicates that osmotic dehydration effects to a greater extent the recycling of the oxidative pentose phosphate intermediates, fructose-6P and glyceraldehyde-3P. Respiratory inhibition in the darkened chloroplast could be alleviated by addition of NH4Cl (a stromal alkylating agent), iodoacetamide) an inhibitor of glyceraldehyde-3P dehydrogenase), or glycolate-2P (an inhibitor of phosphofructokinase). It is concluded that the site which primarily mediates respiratory inhibition in the darkened chloroplast occurs at the fructose 1,6-bisphosphatase/phosphofructokinase junction.  相似文献   

2.
Aerobic and anaerobic respiration in the intact spinach chloroplast   总被引:3,自引:3,他引:0       下载免费PDF全文
Aerobic and anaerobic chloroplastic respiration was monitored by measuring 14CO2 evolution from [14C]glucose in the darkened spinach (Spinacia oleracea) chloroplast and by estimating the conversion of fructose 1,6-bisphosphate to glycerate 3-phosphate in the darkened spinach chloroplast in air with O2 or in N2 with nitrite or oxaloacetate as electron acceptors. The pathway of 14CO2 evolution from labeled glucose in the absence and presence of the inhibitors iodoacetamide and glycolate 2-phosphate under air or N2 were those expected from the oxidative pentose phosphate cycle and glycolysis. Of the electron acceptors, O2 was the best (2.4 nanomoles CO2 per milligram chlorophyll per hour), followed by nitrite and oxaloacetate. With respect to glycerate 3-phosphate formation from fructose 1,6-bisphosphate, methylene blue increased the aerobic rate from 3.7 to 5.4 micromoles per milligram chlorophyll per hour. A rate of 4.8 micromoles per milligram chlorophyll per hour was observed under N2 with nitrite and oxaloacetate.  相似文献   

3.
The aim of this work was to study the pathway(s) of sugar phosphate metabolism in chloroplasts of the unicellular green alga, Dunaliella marina (Volvocales). Phosphofructokinase, detectable in crude cell extracts, copurifled with intact chloroplasts on sucrose density gradients. In isolated chloroplasts, phosphofructokinase activity displayed latency to the same degree as chloroplast marker enzymes. From the quantitative distribution of enzyme activities in fractionated cells, it is concluded that there is an exclusive localization of phosphofructokinase in chloroplasts. In addition, no separation into multiple forms could be achieved. For the study of regulatory properties, chloroplast phosphofructokinase was partially purified by ammonium sulfate fractionation followed by DEAE-cellulose chromatography. The pH optimum of the enzyme activity was 7.0 and was not altered with varying concentrations of substrates or low-molecular-weight effectors. Fructose 6-phosphate showed a sigmoidal saturation curve whose shape was further changed with varying protein concentrations of the preparation. The second substrate, ATP, gave a hyperbolic saturation curve with a Michaelis constant of 60 μm. At a Mg2+ concentration of 2.5 mm, ATP concentrations exceeding 1 mm inhibited the enzyme in a positive cooperative manner. The same type of inhibition was observed with other phosphorylated intermediates of carbon metabolism, the most efficient being phosphoenolpyruvate, glycolate 2-phosphate, glycerate 3-phosphate, and glycerate 2-phosphate. Inorganic phosphate was the only activator found for phosphofructokinase. With nonsaturating fructose 6-phosphate concentrations, Pi activated in a positive cooperative fashion, while no activation occurred with saturating fructose 6-phosphate concentrations. In the presence of either an activator or an inhibitor, the sigmoidal shape of the fructose 6-phosphate saturation curve was altered. Most notably, the activator Pi could relieve the inhibitory action of ATP, phosphoenolpyruvate, glycerate 3-phosphate, glycerate 2-phosphate, and glycolate 2-phosphate. Based on these experimental findings, the regulatory properties of D. marina chloroplast phosphofructokinase are discussed with respect to its playing a key role in the regulation of chloroplast starch metabolism during a light/dark transition. All available evidence is compatible with the interpretation that phosphofructokinase is active only in the dark thus channeling starch degradation products into glycolysis.  相似文献   

4.
Pathways of glucose catabolism, potentially operational in six strains of obligately aerobic, acidophilic bacteria, including Acidiphilium cryptum strain Lhet2, were investigated by short-term radiorespirometry and enzyme assays. Short-term radiorespirometry was conducted at pH 3.0 with specifically labeled [14C]glucose. The high rate and yield of C-1 oxidized to CO2 indicated that the Entner-Doudoroff, pentose phosphate, or both pathways were operational in all strains. Apparent nonequivalent yields of CO2 from C-1 and estimated CO2 from C-4 (C-1 > C-4) were suggestive of simultaneous glucose catabolism by both pathways in all strains tested. Variation in the relative contribution of the two pathways of glucose catabolism appears to account for observed strain differences. Calculation of the actual percent pathway participation was not feasible. Enzyme assays were completed with crude extracts of glucose-grown cells to substantiate the results obtained by radiorespirometry. The key enzymes of the pentose phosphate pathway (6-phosphogluconate dehydrogenase) and the Entner-Doudoroff pathway (2-keto-3-deoxy-6-phosphogluconate aldolase and 6-phosphogluconate dehydrase) were present in all strains examined (PW2, Lhet2, KLB, OP, and QBP). However, none of the strains exhibited detectable levels of the key enzyme of the Embden-Meyerhof-Parnas pathway, 6-phosphofructokinase. All strains contained glucose-6-phosphate dehydrogenase and fructose bisphosphate aldolase. The results of the enzyme study supported the contention that the pentose phosphate and Entner-Doudoroff pathways are operational for glucose catabolism in the acidophilic heterotrophs, and that the Embden-Meyerhof-Parnas pathway is apparently absent.  相似文献   

5.
The assimilation of NH4+ causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoenolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH4+ assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.  相似文献   

6.
Measurements have been made of the activities of enzymes of the pentose phosphate pathway, the glucuronate-xylulose pathway, hexokinase and phosphofructokinase in kidney of diabetic and normal rats. The activities of these enzymes keep pace with kidney growth, remaining constant per gram tissue but showing a marked increase on the basis of total activity per 100 g body wt. The formation of 14CO2 from [1-14C]glucose and [6-14C]glucose by kidney slices from diabetic rats was decreased to approximately half the control value; evidence was obtained for an equivalent dilution of the glucose 6-phosphate pool. Correction of the 14CO2 yields for the change in specific activity of glucose 6-phosphate yielded values consistent with the enzyme profile. Calculations from specific yields of 14CO2 provided evidence for an increased flux of glucose via the pentose phosphate pathway in the kidney in diabetes. The results are discussed in relation to kidney hypertrophy in diabetes and the requirement for ribose 5-phosphate and NADPH for biosynthetic reactions and in relation to the thickening of the basement membrane in diabetes. These results are in accord with the concept of glucose overutilization by non-insulin-requiring tissues.  相似文献   

7.
Heath RL 《Plant physiology》1984,75(4):964-967
Using differentially labeled glucose as a substrate to probe the operation of the hexose monophosphate shunt (pentose cycle) in Chlorella sorokiniana, we found that the labeling patterns for the release of 14CO2 over the first 5 minutes are compatible with the operation of the recently described L-type pentose shunt. Experimentally, this L-type differs from the F-type or `textbook' variety in that no radioactivity is obtained from C-2 labeled glucose, and the small amount derived from C-6 labeled glucose is due to a second pass of the glucose molecule (derived from the pentose cycle) through the pentose cycle.  相似文献   

8.
Singh KK  Chen C  Gibbs M 《Plant physiology》1992,100(1):327-333
The role of an electron transport pathway associated with aerobic carbohydrate degradation in isolated, intact chloroplasts was evaluated. This was accomplished by monitoring the evolution of 14CO2 from darkened spinach (Spinacia oleracea) and Chlamydomonas reinhardtii chloroplasts externally supplied with [14C]fructose and [14C]glucose, respectively, in the presence of nitrite, oxaloacetate, and conventional electron transport inhibitors. Addition of nitrite or oxaloacetate increased the release of 14CO2, but it was shown that O2 continued to function as a terminal electron acceptor. 14CO2 evolution was inhibited up to 30 and 15% in Chlamydomonas and spinach, respectively, by 50 μm rotenone and by amytal, but at 500- to 1000-fold higher concentrations, indicating the involvement of a reduced nicotinamide adenine dinucleotide phosphate-plastoquinone oxidoreductase. 14CO2 release from the spinach chloroplast was inhibited 80% by 25 μm 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone. 14CO2 release was sensitive to propylgallate, exhibiting approximately 50% inhibition in Chlamydomonas and in spinach chloroplasts of 100 and 250 μm concentrations, respectively. These concentrations were 20- to 50-fold lower than the concentrations of salicylhydroxamic acid (SHAM) required to produce an equivalent sensitivity. Antimycin A (100 μm) inhibited approximately 80 to 90% of 14CO2 release from both types of chloroplast. At 75 μm, sodium azide inhibited 14CO2 evolution about 50% in Chlamydomonas and 30% in spinach. Sodium azide (100 mm) combined with antimycin A (100 μm) inhibited 14CO2 evolution more than 90%. 14CO2 release was unaffected by uncouplers. These results are interpreted as evidence for a respiratory electron transport pathway functioning in the darkened, isolated chloroplast. Chloroplast respiration defined as 14CO2 release from externally supplied [1-14C]glucose can account for at least 10% of the total respiratory capacity (endogenous release of CO2) of the Chlamydomonas reinhardtii cell.  相似文献   

9.
Purified pea root plastids were supplied with glutamine, 2-oxoglutarate and phosphorylated sugars. Formation of glutamate was linear for 75 min and dependent upon the intactness of the organelle. Glucose-6-phosphate and ribose-5-phosphate were the most effective substrates in supporting glutamate synthesis. Flux through the oxidative pentose phosphate pathway during glutamate synthesis in purified plastids was followed by monitoring the release of 14CO2 from [1-14C]glucose-6-phosphate. 14CO2 evolution from C-1 was dependent upon the presence of both glutamine and 2-oxoglutarate and could be inhibited by the application of azaserine. The data are discussed in view of the role of the oxidative pentose phosphate pathway in non-photosynthetic plastids.  相似文献   

10.
Phosphoenolpyruvate phosphatase from Brassica nigra leaf petiole suspension cells has been purified 1700-fold to apparent homogeneity and a final specific activity of 380 micromole pyruvate produced per minute per milligram protein. Purification steps included: ammonium sulfate fractionation, S-Sepharose, chelating Sepharose, concanavalin A Sepharose, and Superose 12 chromatography. The native protein was monomeric with a molecular mass of 56 kilodaltons as estimated by analytical gel filtration. The enzyme displayed a broad pH optimum of about pH 5.6 and was relatively heat stable. Western blots of microgram quantities of the final preparation showed no cross-reactivity when probed with rabbit polyclonal antibodies prepared against either castor bean endosperm cytosolic pyruvate kinase, or sorghum leaf phosphoenolpyruvate carboxylase. The final preparation exhibited a broad substrate selectivity, showing high activity toward p-nitrophenyl phosphate, adenosine diphosphate, adenosine triphosphate, gluconate 6-phosphate, and phosphoenolpyruvate, and moderate activity toward several other organic phosphates. Phosphoenolpyruvate phosphatase possessed at least a fivefold and sixfold greater affinity and specificity constant, respectively, for phosphoenolpyruvate (apparent Michaelis constant = 50 micromolar) than for any other nonartificial substrate. The enzyme was activated 1.7-fold by 4 millimolar magnesium, but was strongly inhibited by molybdate, fluoride, zinc, copper, iron, and lead ions, as well as by orthophosphate, ascorbate, glutamate, aspartate, and various organic phosphate compounds. It is postulated that phosphoenolpyruvate phosphatase functions to bypass the adenosine diphosphate dependent pyruvate kinase reaction during extended periods of orthophosphate starvation.  相似文献   

11.
Bradyrhizobium japonicum bacteroids were isolated anaerobically and were supplied with 14C-labeled trehalose, sucrose, UDP-glucose, glucose, or fructose under low O2 (2% in the gas phase). Uptake and conversion of 14C to CO2 were measured at intervals up to 90 minutes. Of the five compounds studied, UDP-glucose was most rapidly absorbed but it was very slowly metabolized. Trehalose was the sugar most rapidly converted to CO2, and fructose was respired at a rate at least double that of glucose. Sucrose and glucose were converted to CO2 at a very low but measurable rate (<0.1 nanomoles per milligram protein per hour). Carbon Number 1 of glucose appeared in CO2 at a rate 30 times greater than the conversion of carbon Number 6 to CO2, indicating high activity of the pentose phosphate pathway. Enzymes of the Entner-Doudoroff pathway were not detected in bacteroids, but very low activities of sucrose synthase and phosphofructokinase were demonstrated. Although metabolism of sugars by B. japonicum bacteroids was clearly demonstrated, the rate of sugar uptake was only 1/30 to 1/50 the rate of succinate uptake. The overall results support the view that, although bacteroids metabolize sugars, the rates are very low and are inadequate to support nitrogenase.  相似文献   

12.
The activity of the pentose phosphate pathway in isolated liver cells   总被引:2,自引:0,他引:2  
Isolated liver cells have been used to assess the relative contribution of the pentose phosphate pathway to glucose metabolism. The incorporation of carbon from specifically labelled glucose into 14CO2 by isolated cells gave values (μg.atoms/g.cells/hr) of: C-1, 7.9; C-6, 1.3; C-U, 3.4. The corresponding figures for liver slices were: C-1, 2.3; C-6, 1.6; C-U, 3.0. The most striking difference was the 3.5-fold increase in the oxidation of C-1 of glucose. Isolated cells retain more than 50% of ATP and have a content of intermediates of the glycolytic pathway closely similar to freeze-clamped liver. The relative importance of the pentose phosphate pathway in isolated liver cells, approximately 16% of glucose catabolised, is consistent with the enzyme profile of liver and the reductive synthetic reactions of the tissue.  相似文献   

13.
The aim of this work was to establish the pathways of carbohydrate oxidation used in the dark by leaves of Pisum sativum and Triticum aestivum. Segments of young and mature leaves of pea released the carbons of glucose-[14C] as 14CO2 in the order 3,4 > 1 > 2 > 6 whereas in segments of young and mature leaves of wheat the order was 3,4 > 1 > 6 > 2. The detailed labelling of the constituents of mature leaves of wheat by glucose-[1-14C], -[2-14C], -[3,4-14C], and -[6-14C] was determined and showed that the high yield of CO2 from C-6 relative to that from C-2 was due to release of C-6 during pentan synthesis. Estimates were made of the maximum catalytic activities of phosphofructokinase and glucose-6-phosphate dehydrogenase in pea and wheat leaves of three ages. The results of all the above investigations strongly indicate that both pea and wheat leaves in the dark oxidize carbohydrate via glycolysis and the pentose phosphate pathway with the latter accounting for no more than a third of the total. No evidence was obtained of any major change in the relative activities of the two pathways during the development of either type of leaf.  相似文献   

14.
Badiorespirometric experiments with glucose labelled in positions 1; 2; 3,4 and 6 were carried out inClaviceps purpurea strain pepty 695. The pattern of14CO2 evolution from 5 – 50 mM glucose indicated the operation of the pentose phosphate cycle during alkaloid production. The authors thank Mrs. H. Pechfelder for her skilled technical assistance and Prof. Dr. H. Reinbothe and Dr. C. Wasternack for helpful discussions.  相似文献   

15.
《Insect Biochemistry》1987,17(2):347-352
Glucose catabolism in overwintering larvae Eurosta solidaginis was examined to determine the relative contributions of glycolysis and the pentose phosphate pathway to polyol synthesis at different temperatures. Rates of 14CO2 evolution were determined after injection of [14C]1-glucose, [14C]6-glucose, and [14C]3,4-glucose. In addition incorporation of label from each isotope into sorbitol and glycerol was monitored. The respirometric studies showed a relative increase in pentose phosphate activity between 10 and 5°C. Similar results were obtained from the changes of radioactivities incorporated into glycerol, although the activation of the pentose phosphate pathway was low. The conversion of [14C]glucose to glycerol was highest at 10°C, suggesting that maximum glycerol synthesis may occur at this temperature. Radioactivity appeared in the sorbitol fraction of larvae incubated at temperatures below 5°C. Late autumn larvae converted more [14C]glucose than did early autumn larvae.  相似文献   

16.
The aim of this work was to determine which of the two reactions (i.e. phosphorylation or dephosphorylation) involved in the establishment of the phosphorylated status of the wheat leaf phosphoenolpyruvate carboxylase and sucrose phosphate synthase protein responds in vivo to NO3 uptake and assimilation. Detached mature leaves of wheat (Triticum aestivum L. cv Fidel) were fed with N-free (low-NO3 leaves) or 40 mm NO3 solution (high-NO3 leaves). The specific inhibition of the enzyme-protein kinase or phosphatase activities was obtained in vivo by addition of mannose or okadaic acid, respectively, in the uptake solution. Mannose at 50 mm, by blocking the kinase reaction, inhibited the processes of NO3-dependent phosphoenolpyruvate carboxylase activation and sucrose phosphate synthase deactivation. Following the addition of mannose, the deactivation of phosphoenolpyruvate carboxylase and the activation of sucrose phosphate synthase, both due to the enzyme-protein dephosphorylation, were at the same rate in low-NO3 and high-NO3 leaves, indicating that NO3 had no effect per se on the enzyme-protein phosphatase activity. Upon treatment with okadaic acid, the higher increase of phosphoenolpyruvate carboxylase and decrease of sucrose phosphate synthase activities observed in high NO3 compared with low NO3 leaves showed evidence that NO3 enhanced the protein kinase activity. These results support the concept that NO3, or a product of its metabolism, favors the activation of phosphoenolpyruvate carboxylase and deactivation of sucrose phosphate synthase in wheat leaves by promoting the light activation of the enzyme-protein kinase(s) without affecting the phosphatase(s).  相似文献   

17.
A mass spectrometric method combining 16O/18O and 12C/13C isotopes was used to quantify the unidirectional fluxes of O2 and CO2 during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O2 uptake and CO2 evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O2 (61 micromoles of O2 per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O2 per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO2 in darkness at a rate of 27 micromoles of CO2 per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O2 evolution and CO2 fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phosphoenolpyruvate carboxylase activity. Under light, CO2 evolution by guard cell protoplasts was sharply decreased (37%), while O2 uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO2 assimilation and may be dissipated for other purposes such as ion uptake.  相似文献   

18.
d-Glucose catabolism of a phosphofructokinase-deficient yeast Rhodotorula gracilis has been studied. By using d-glucose specifically 14C-labelled at different positions and measuring the distribution of the label in various fractions of cell metabolism, the following results were found. 1. The pentose phosphate pathway, being the main pathway of d-glucose catabolism, simultaneously converts glucose molecules into pentose phosphates oxidatively by using two NADP-linked dehydrogenases and via the non-oxidative transketolase–transaldolase pathway. 2. From the correlation of the 14CO2 liberation and the d-glucose consumption and from the fact that the pentose phosphate moiety in nucleic acids is almost equally labelled from d-[1-14C]- and d-[6-14C]-glucose, it is concluded that of the glucose utilized about 80% undergoes transformation via the non-oxidative pentose phosphate pathway. Only about 20% of glucose is directly decarboxylated to pentose phosphate. 3. For further degradation it is postulated that the pentose phosphates are split into C2 fragments and glyceraldehyde 3-phosphates. 4. All three loci of oxidative decarboxylation appear to be effective in Rh. gracilis, the oxidative part of the pentose phosphate pathway, the decarboxylation of pyruvate in the later part of the glycolytic pathway as well as the oxidation in the tricarboxylic acid cycle. 5. d-Glucose molecules taken up are only partially oxidized to CO2: about four-fifths of each glucose molecule metabolized is incorporated into cell constituents. 6. The quantitative interrelations of the fluxes of d-glucose subunits along the catabolic pathways have been estimated and are discussed.  相似文献   

19.
Values of δ13C and levels of phosphoenolpyruvate carboxylase and ribulose 1,5-bisphosphate carboxylase/oxygenase were analyzed in segments from the fourth leaf of young maize (Zea mays L.) plants. The δ13C values became significantly more negative from the base to the tip of the leaves. Phosphoenolpyruvate carboxylase levels and ribulose bisphosphate carboxylase levels both increased from the base to the tip. The principal effect of phosphoenolpyruvate carboxylase levels or δ13C should arise through its effect on the carboxylation/diffusion balance in the mesophyll. In this case, δ13C values should become more negative as phosphoenolpyruvate carboxylase levels increase, unless there are offsetting changes in stomatal aperture. The principal effect of ribulose bisphosphate carboxylase/oxygenase on δ13C should occur through its effect on the extent of leakage of CO2 from the bundle sheath cells. In this case, δ13C values should become more positive as ribulose bisphosphate carboxylase levels increase. Accordingly, the variation in δ13C values seen in maize leaves appears to be the result of variations in the level of phosphoenolpyruvate carboxylase.  相似文献   

20.
In Mesembryanthemum crystallinum, phosphoenolpyruvate carboxylase is synthesized de novo in response to osmotic stress, as part of the switch from C3-photosynthesis to Crassulacean acid metabolism. To better understand the environmental signals involved in this pathway, we have investigated the effects of light on the induced expression of phosphoenolpyruvate carboxylase mRNA and protein in response to stress by 400 millimolar NaCl or 10 micromolar abscisic acid in hydroponically grown plants. When plants were grown in high-intensity fluorescent or incandescent light (850 microeinsteins per square meter per second), NaCl and abscisic acid induced approximately an eightfold accumulation of phosphoenolpyruvate carboxylase mRNA when compared to untreated controls. Levels of phosphoenolpyruvate carboxylase protein were high in these abscisic acid- and NaCl-treated plants, and detectable in the unstressed control. Growth in high-intensity incandescent (red) light resulted in approximately twofold higher levels of phosphoenolpyruvate carboxylase mRNA in the untreated plants when compared to control plants grown in high-intensity fluorescent light. In low light (300 microeinsteins per square meter per second fluorescent), only NaCl induced mRNA levels significantly above the untreated controls. Low light grown abscisic acid- and NaCl-treated plants contained a small amount of phosphoenolpyruvate carboxylase protein, whereas the (untreated) control plants did not contain detectable amounts of phosphoenolpyruvate carboxylase. Environmental stimuli, such as light and osmotic stress, exert a combined effect on gene expression in this facultative halophyte.  相似文献   

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