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1.
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   

2.
The rate and extent of light activation of PEPC may be used as another criterion to distinguish C3 and C4 plants. Light stimulated phosphoenolypyruvate carboxylase (PEPC) in leaf discs of C4 plants, the activity being three times greater than that in the dark but stimulation of PEPC was limited about 30% over the dark-control in C3 species. The light activation of PEPC in leaves of C3 plants was complete within 10 min, while maximum activation in C4 plants required illumination for more than 20 min, indicating that the relative pace of PEPC activation was slower in C4 plants than in C3 plants. Similarly, the dark-deactivation of the enzyme was also slower in leaves of C4 than in C3 species. The extent of PEPC stimulation in the alkaline pH range indicated that the dark-adapted form of the C4 enzyme is very sensitive to changes in pH. The pH of cytosol-enriched cell sap extracted from illuminated leaves of C4 plants was more alkaline than that of dark-adapted leaves. The extent of such light-dependent alkalization of cell sap was three times higher in C4 leaves than in C3 plants. The course of light-induced alkalization and dark-acidification of cytosol-enriched cell sap was markedly similar to the pattern of light activation and dark-deactivation of PEPC in Alternanthera pungens, a C4 plant. Our report provides preliminary evidence that the photoactivation of PEPC in C4 plants may be mediated at least partially by the modulation of cytosolic pH.Abbreviations CAM Crassulacean acid metabolism - G-6-P glucose-6-phosphate - PMSF phenylmethylsulfonyl fluoride - PEPC phosphoenolpyruvate carboxylase - PEPC-PK phosphoenolpyruvate ca carboxylase-protein kinase  相似文献   

3.
The regulation of carbonic anhydrase (CA) activity in maize (Zea mays L.) leaves by light and nitrogen nutrition was determined. CA activity increased by more than 100-fold in illuminated leaves and decreased in leaves placed in the dark; low levels of CA activity were observed in leaves illuminated with low light intensities. CA activity was reduced in plants grown under nitrogen deficiency and recovered only slowly when supplemented with nitrate. Parallel studies were conducted to follow the levels of phosphoenolpyruvate carboxylase. Experiments indicate that the level of CA and phosphoenolpyruvate carboxylase present in leaves may be controlled by similar mechanisms.  相似文献   

4.
Phosphoenolpyruvate carboxylase (PEPC) was overproduced in the leaves of rice plants by introducing the intact maize C4-specific PEPC gene. Maize PEPC in transgenic rice leaves underwent activity regulation through protein phosphorylation in a manner similar to endogenous rice PEPC but contrary to that occurring in maize leaves, being downregulated in the light and upregulated in the dark. Compared with untransformed rice, the level of the substrate for PEPC (phosphoenolpyruvate) was slightly lower and the product (oxaloacetate) was slightly higher in transgenic rice, suggesting that maize PEPC was functioning even though it remained dephosphorylated and less active in the light. 14CO2 labeling experiments indicated that maize PEPC did not contribute significantly to the photosynthetic CO2 fixation of transgenic rice plants. Rather, it slightly lowered the CO2 assimilation rate. This effect was ascribable to the stimulation of respiration in the light, which was more marked at lower O2 concentrations. It was concluded that overproduction of PEPC does not directly affect photosynthesis significantly but it suppresses photosynthesis indirectly by stimulating respiration in the light. We also found that while the steady-state stomatal aperture remained unaffected over a wide range of humidity, the stomatal opening under non-steady-state conditions was destabilized in transgenic rice. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

5.
Control of C4 photosynthesis and Crassulacean acid metabolism (CAM) is, in part, mediated by the diel regulation of phosphoenolpyruvate carboxylase (PEPC) activity. The nature of this regulation of PEPC in the leaf cell cytoplasm of C4 and CAM plants is both metabolite-related and posttranslational. Specificially, the regulatory properties of the enzyme vary in accord with the physiological activity of C4 photosynthesis and CAM: PEPC is less sensitive to feedback inhibition by l-malate under light (C4 plants) or at night (CAM plants) than in darkness (C4) or during the day (CAM). While the view that a light-induced change in the aggregation state of the holoenzyme is a general mechanism for the diel regulation of PEPC activity in CAM plants is currently in dispute, there is no supportive in vivo evidence for such a tetramer/dimer interconversion in C4 plants. In contrast, a wealth of in vitro and in vivo data has accumulated in support of the view that the reversible phosphorylation of a specific, N-terminal regulatory serine residue in PEPC (e.g. Ser-15 or Ser-8 in the maize or sorghum enzymes, respectively) plays a key, if not cardinal, role in the posttranslational regulation of the carboxylase by light/dark or day/night transitions in both C4 and CAM plants, respectively.  相似文献   

6.
Reversible seryl-phosphorylation contributes to the light/dark regulation of C4-leaf phosphoenolpyruvate carboxylase (PEPC) activity in vivo. The specific regulatory residue that, upon in vitro phosphorylation by a maize-leaf protein-serine kinase(s), leads to an increase in catalytic activity and a decrease in malate-sensitivity of the target enzyme has been recently identified as Ser-15 in 32P-phosphorylated/activated dark-form maize PEPC (J-A Jiao, R Chollet [1990] Arch Biochem Biophys 283: 300-305). In order to ascertain whether this N-terminal seryl residue is, indeed, the in vivo regulatory phosphorylation site, [32P]phosphopeptides were isolated and purified from in vivo 32P-labeled maize and sorghum leaf PEPC and subjected to automated Edman degradation analysis. The results show that purified light-form maize PEPC contains 14-fold more 32P-radioactivity than the corresponding dark-form enzyme on an equal protein basis and, more notably, only a single N-terminal serine residue (Ser-15 in maize PEPC and its structural homolog, Ser-8, in the sorghum enzyme) was found to be 32P-phosphorylated in the light or dark. These in vivo observations, combined with the results from our previous in vitro phosphorylation studies (J-A Jiao, R Chollet [1989] Arch Biochem Biophys 269: 526-535; [1990] Arch Biochem Biophys 283: 300-305), demonstrate that an N-terminal seryl residue in C4 PEPC is, indeed, the regulatory site that undergoes light/dark changes in phosphorylation-status and, thus, plays a major, if not cardinal role in the light-induced changes in catalytic and regulatory properties of this cytoplasmic C4-photosynthesis enzyme in vivo.  相似文献   

7.
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   

8.
A peptide containing the N-terminal phosphorylation site (Ser-8) of Sorghum C4-phospho enolpyruvate carboxylase (PEPC) was synthesized, purified and used to raise an antiserum in rabbits. Affinity-purified IgGs prevented PEPC phosphorylation in a reconstituted in vitro assay and reacted with both the phosphorylated and dephosphorylated forms of either native or denatured PEPC in immunoblotting experiments. Saturation of dephospho-PEPC with these specific IgGs resulted in a marked alteration of its functional and regulatory properties that mimicked phosphorylation of Ser-8. A series of recombinant C4 PEPCs mutated in the N-terminal phosphorylation domain and a C3-like PEPC isozyme from Sorghum behaved similarly to their C4 counterpart with respect to these phosphorylation-site antibodies.Abbreviations PEPC phospho enolpyruvate carboxylase - PKA catalytic subunit of the cAMP-dependent protein kinase - KLH Keyhole Limpet Haemocyanin - IgG immunoglobulin G - PEP phospho enolpyruvate - SDS-PAGE sodium dodecyl sulfate, polyacrylamide gel electrophoresis - MDH malate deshydrogenase  相似文献   

9.
The photosynthetic characteristics of four transgenic rice lines over-expressing rice NADP-malic enzyme (ME), and maize phosphoenolpyruvate carboxylase (PC), pyruvate,orthophosphate dikinase (PK), and PC+PK (CK) were investigated using outdoor-grown plants. Relative to untransformed wild-type (WT) rice, PC transgenic rice exhibited high PC activity (25-fold increase) and enhanced activity of carbonic anhydrase (more than two-fold increase), while the activity of ribulose-bisphosphate carboxylase/oxygenase (Rubisco) and its kinetic property were not significantly altered. The PC transgenic plants also showed a higher light intensity for saturation of photosynthesis, higher photosynthetic CO2 uptake rate and carboxylation efficiency, and slightly reduced CO2 compensation point. In addition, chlorophyll a fluorescence analysis indicates that PC transgenic plants are more tolerant to photo-oxidative stress, due to a higher capacity to quench excess light energy via photochemical and non-photochemical means. Furthermore, PC and CK transgenic rice produced 22–24% more grains than WT plants. Taken together, these results suggest that expression of maize C4 photosynthesis enzymes in rice, a C3 plant, can improve its photosynthetic capacity with enhanced tolerance to photo-oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

10.
Calcium-dependent phosphoenolpyruvate carboxylase protein kinasewas copurified with C4 phosphoenolpyruvate carboxylase (C4 PEPC)from illuminated Sorghum leaves during purification by variousprocedures. Isolated mesophyll cell protoplasts contained bothcalcium-dependent and -independent protein kinases. The latterwas induced by light and weak bases and was found to be themajor protein kinase phosphorylating C4 PEPC in the mesophyll. (Received July 29, 1997; Accepted November 28, 1997)  相似文献   

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