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1.
Photorespiration makes oxygenic photosynthesis possible by scavenging 2-phosphoglycolate. Hence, compromising photorespiration impairs photosynthesis. We examined whether facilitating photorespiratory carbon flow in turn accelerates photosynthesis and found that overexpression of the H-protein of glycine decarboxylase indeed considerably enhanced net-photosynthesis and growth of Arabidopsis thaliana. At the molecular level, lower glycine levels confirmed elevated GDC activity in vivo, and lower levels of the CO2 acceptor ribulose 1,5-bisphosphate indicated higher drain from CO2 fixation. Thus, the photorespiratory enzyme glycine decarboxylase appears as an important feed-back signaller that contributes to the control of the Calvin-Benson cycle and hence carbon flow through both photosynthesis and photorespiration.  相似文献   

2.
Mitochondrial dihydrolipoyl dehydrogenase (mtLPD; L-protein) is an integral component of several multienzyme systems involved in the tricarboxylic acid (TCA) cycle, photorespiration, and the degradation of branched-chain α-ketoacids. The majority of the mtLPD present in photosynthesizing tissue is used for glycine decarboxylase (GDC), necessary for the high-flux photorespiratory glycine-into-serine conversion. We previously suggested that GDC activity could be a signal in a regulatory network that adjusts carbon flux through the Calvin-Benson cycle in response to photorespiration. Here, we show that elevated GDC L-protein activity significantly alters several diagnostic parameters of cellular metabolism and leaf gas exchange in Arabidopsis thaliana. Overexpressor lines displayed markedly decreased steady state contents of TCA cycle and photorespiratory intermediates as well as elevated NAD(P)+-to-NAD(P)H ratios. Additionally, increased rates of CO2 assimilation, photorespiration, and plant growth were observed. Intriguingly, however, day respiration rates remained unaffected. By contrast, respiration was enhanced in the first half of the dark phase but depressed in the second. We also observed enhanced sucrose biosynthesis in the light in combination with a lower diel magnitude of starch accumulation and breakdown. These data thus substantiate our prior hypothesis that facilitating flux through the photorespiratory pathway stimulates photosynthetic CO2 assimilation in the Calvin-Benson cycle. They furthermore suggest that this regulation is, at least in part, dependent on increased light-capture/use efficiency.  相似文献   

3.
When the CO2 concentration to which Medicago sativa L. var. El Unico leaflets were exposed was increased from half-saturation to saturation (doubled rate of photosynthesis), glycolate and glycine production apparently decreased due to inhibition of a portion of the glycolate pathway. Serine and glycerate production was not inhibited. We conclude that serine and glycerate were made from 3-phosphoglycerate and not from glycolate and that the conversion of glycine to serine may not be the major source of photorespiratory CO2 in alfalfa. In investigations of glycolate and photorespiratory metabolism, separate labeling data should be obtained for glycine and serine as those two amino acids may be produced from different precursors and respond differently to environmental perturbations. The increased photosynthetic rate (at saturating CO2) resulted in greater labeling of both soluble and insoluble products. Sucrose labeling increased sharply, but there was no major shift of tracer carbon flow into sucrose relative to other metabolites. The flow of carbon from the reductive pentose phosphate cycle into the production of tricarboxylic acid cycle intermediates and amino acids increased. Only small absolute increases occurred in steady-state pool sizes of metabolites of the reductive pentose phosphate cycle at elevated CO2, providing further evidence that the cycle is well regulated.  相似文献   

4.
The effects of added glycine hydroxamate on the photosynthetic incorporation of 14CO2 into metabolites by isolated mesophyll cells of spinach (Spinacia oleracea L.) was investigated under conditions favorable to photorespiratory (PR) metabolism (0.04% CO2 and 20% O2) and under conditions leading to nonphotorespiratory (NPR) metabolism (0.2% CO2 and 2.7% O2). Glycine hydroxamate (GH) is a competitive inhibitor of the photorespiratory conversion of glycine to serine, CO2 and NH4+. During PR fixation, addition of the inhibitor increased glycine and decreased glutamine labeling. In contrast, labeling of glycine decreased under NPR conditions. This suggests that when the rate of glycolate synthesis is slow, the primary route of glycine synthesis is through serine rather than from glycolate. GH addition increased serine labeling under PR conditions but not under NPR conditions. This increase in serine labeling at a time when glycine to serine conversion is partially blocked by the inhibitor may be due to serine accumulation via the “reverse” flow of photorespiration from 3-P-glycerate to hydroxypyruvate when glycine levels are high. GH increased glyoxylate and decreased glycolate labeling. These observations are discussed with respect to possible glyoxylate feedback inhibition of photorespiration.  相似文献   

5.
The nature of the inorganic carbon utilized during photosynthesis by Chlorella pyrenoidosa was investigated using three experimental techniques (open gas analysis system with “artificial leaf” or “aqueous” chambers and O2 electrode system) to measure carbon assimilation. Photosynthesis was studied as a function of pH and CO2 concentration. The CO2 concentration was inadequate to meet the requirements of photosynthesis only when HCO3 was added at high pH. Under all other conditions, the low and constant Km (CO2), in contrast to the highly variable Km (HCO3), suggested that CO2 was the major species utilized.  相似文献   

6.
Chemical inhibition of the glycolate pathway in soybean leaf cells   总被引:19,自引:15,他引:4       下载免费PDF全文
Isolated soybean (Glycine max [L.] Merr.) leaf cells were treated with three inhibitors of the glycolate pathway in order to evaluate the potential of such inhibitors for increasing photosynthetic efficiency. Preincubation of cells under acid conditions in α-hydroxypyridinemethanesulfonic acid increased 14CO2 incorporation into glycolate, but severely inhibited photosynthesis. Isonicotinic acid hydrazide (INH) increased the incorporation of 14CO2 into glycine and reduced label in serine, glycerate, and starch. Butyl 2-hydroxy-3-butynoate (BHB) completely and irreversibly inhibited glycolate oxidase and increased the accumulation of 14C into glycolate. Concomitant with glycolate accumulation was the reduction of label in serine, glycerate, and starch, and the elimination of label in glycine. The inhibitors INH and BHB did not eliminate serine synthesis, suggesting that some serine is synthesized by an alternate pathway. The per cent incorporation of 14CO2 into glycolate by BHB-treated cells or glycine by INH-treated cells was determined by the O2/CO2 ratio present during assay. Photosynthesis rate was not affected by INH or BHB in the absence of O2, but these compounds increased the O2 inhibition of photosynthesis. This finding suggests that the function of the photorespiratory pathway is to recycle glycolate carbon back into the Calvin cycle, so if glycolate metabolism is inhibited, Calvin cycle intermediates become depleted and photosynthesis is decreased. Thus, chemicals which inhibit glycolate metabolism do not reduce photorespiration and increase photosynthetic efficiency, but rather exacerbate the problem of photorespiration.  相似文献   

7.
Isotopic analyses of Candidatus “Brocadia anammoxidans,” a chemolithoautotrophic bacterium that anaerobically oxidizes ammonium (anammox), show that it strongly fractionates against 13C; i.e., lipids are depleted by up to 47‰ versus CO2. Similar results were obtained for the anammox bacterium Candidatus “Scalindua sorokinii,” which thrives in the anoxic water column of the Black Sea, suggesting that different anammox bacteria use identical carbon fixation pathways, which may be either the Calvin cycle or the acetyl coenzyme A pathway.  相似文献   

8.
C4 photosynthesis is nature’s most efficient answer to the dual activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the resulting loss of CO2 by photorespiration. Gly decarboxylase (GDC) is the key component of photorespiratory CO2 release in plants and is active in all photosynthetic tissues of C3 plants, but only in the bundle sheath cells of C4 plants. The restriction of GDC to the bundle sheath is assumed to be an essential and early step in the evolution of C4 photosynthesis, leading to a photorespiratory CO2 concentrating mechanism. In this study, we analyzed how the P-protein of GDC (GLDP) became restricted to the bundle sheath during the transition from C3 to C4 photosynthesis in the genus Flaveria. We found that C3 Flaveria species already contain a bundle sheath–expressed GLDP gene in addition to a ubiquitously expressed second gene, which became a pseudogene in C4 Flaveria species. Analyses of C3-C4 intermediate Flaveria species revealed that the photorespiratory CO2 pump was not established in one single step, but gradually. The knowledge gained by this study sheds light on the early steps in C4 evolution.  相似文献   

9.
10.

Background

The photorespiratory nitrogen cycle in C3 plants involves an extensive diversion of carbon and nitrogen away from the direct pathways of assimilation. The liberated ammonia is re-assimilated, but up to 25% of the carbon may be released into the atmosphere as CO2. Because of the loss of CO2 and high energy costs, there has been considerable interest in attempts to decrease the flux through the cycle in C3 plants. Transgenic tobacco plants were generated that contained the genes gcl and hyi from E. coli encoding glyoxylate carboligase (EC 4.1.1.47) and hydroxypyruvate isomerase (EC 5.3.1.22) respectively, targeted to the peroxisomes. It was presumed that the two enzymes could work together and compete with the aminotransferases that convert glyoxylate to glycine, thus avoiding ammonia production in the photorespiratory nitrogen cycle.

Results

When grown in ambient air, but not in elevated CO2, the transgenic tobacco lines had a distinctive phenotype of necrotic lesions on the leaves. Three of the six lines chosen for a detailed study contained single copies of the gcl gene, two contained single copies of both the gcl and hyi genes and one line contained multiple copies of both gcl and hyi genes. The gcl protein was detected in the five transgenic lines containing single copies of the gcl gene but hyi protein was not detected in any of the transgenic lines. The content of soluble amino acids including glycine and serine, was generally increased in the transgenic lines growing in air, when compared to the wild type. The content of soluble sugars, glucose, fructose and sucrose in the shoot was decreased in transgenic lines growing in air, consistent with decreased carbon assimilation.

Conclusions

Tobacco plants have been generated that produce bacterial glyoxylate carboligase but not hydroxypyruvate isomerase. The transgenic plants exhibit a stress response when exposed to air, suggesting that some glyoxylate is diverted away from conversion to glycine in a deleterious short-circuit of the photorespiratory nitrogen cycle. This diversion in metabolism gave rise to increased concentrations of amino acids, in particular glutamine and asparagine in the leaves and a decrease of soluble sugars.  相似文献   

11.
Chen C  Gibbs M 《Plant physiology》1992,100(3):1361-1365
The oxyhydrogen reaction (the reduction of O2 to water by H2) in the presence of CO2 was studied in the isolated Chlamydomonas reinhardtii chloroplast by monitoring the rate of 14CO2 incorporation into acid-stable products in the dark. The endogenous rate of CO2 uptake (50-125 nmol/mg chlorophyll per h) was increased about 3- to 4-fold by ATP and additionally when combined with glucose, ribose-5-phosphate, and glycerate-3-phosphate. The rate was diminished 50 to 75%, respectively, when H2 was replaced by N2 or by air. Decrease in CO2 uptake by dl-glyceraldehyde was taken to indicate that the regenerative phase and complete Calvin cycle turnover were involved. Diminution of CO2 incorporation by rotenone, antimycin A, and 2,5-dibromo-3-methyl-6-isopropanol-p-benzoquinone was attributed to an inhibition of the oxyhydrogen reaction, resulting in an elevated NADPH/NADP ratio. If so, then the diminished CO2 uptake could have been by “product inhibition” of the carbon metabolic network. Our data are consistent with the proposal (H. Gaffron [1942] J Gen Physiol 26: 241-267) that CO2 fixation coupled to the oxyhydrogen reaction is dependent to some extent on exchloroplastic metabolism. This support is primarily ATP provided by mitochondrial respiration.  相似文献   

12.
Responses of succulents to plant water stress   总被引:19,自引:16,他引:3       下载免费PDF全文
Experiments were performed to test the hypothesis that succulents “shift” their method of photosynthetic metabolism in response to environmental change. Our data showed that there were at least three different responses of succulents to plant water status. When plant water status of Portulacaria afra (L.) Jacq. was lowered either by withholding water or by irrigating with 2% NaCl, a change from C3-photosynthesis to Crassulacean acid metabolism (CAM) occurred. Fluctuation of titratable acidity and nocturnal CO2 uptake was induced in the stressed plants. Stressed Peperomia obtusifolia A. Dietr. plants showed a change from C3-photosynthesis to internal cycling of CO2. Acid fluctuation commenced in response to stress but exogenous CO2 uptake did not occur. Zygocactus truncatus Haworth plants showed a pattern of acid fluctuation and nocturnal CO2 uptake typical of CAM even when well irrigated. The cacti converted from CAM to an internal CO2 cycle similar to Peperomia when plants were water-stressed. Reverse phase gas exchange in succulents results in low water loss to carbon gain. Water is conserved and low levels of metabolic activity are maintained during drought periods by complete stomatal closure and continual fluctuation of organic acids.  相似文献   

13.
A mendelian mutant of the unicellular green alga Chlamydomonas reinhardii has been isolated that is deficient in inorganic carbon transport. This mutant strain, designated pmp-1-16-5K (gene locus pmp-1), was selected on the basis of a requirement of elevated CO2 concentration for photoautrophic growth. Inorganic carbon accumulation in the mutant was considerably reduced in comparison to wild type, and the CO2 response of photosynthesis indicated a reduced affinity for CO2 in the mutant. At air levels of CO2 (0.03-0.04%), O2 inhibited photosynthesis and stimulated the synthesis of photorespiratory intermediates in the mutant but not in wild type. Neither strain was significantly affected by O2 at saturating CO2 concentration. Thus, the primary consequence of inorganic carbon transport deficiency in the mutant was a much lower internal CO2 concentration compared to wild type. From these observations, we conclude that enzyme-mediated transport of inorganic carbon is an essential component of the CO2 concentrating system in C. reinhardii photosynthesis.  相似文献   

14.
Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration. The reduction of hydroxypyruvate to glycerate catalyzed by hydroxypyruvate reductase (HPR) in the peroxisomes is thought to be facilitated by the production of NADH by peroxisomal malate dehydrogenase (PMDH). PMDH, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana), reduces NAD+ to NADH via the oxidation of malate supplied from the cytoplasm to oxaloacetate. A double mutant lacking the expression of both PMDH genes was viable in air and had rates of photosynthesis only slightly lower than in the wild type. This is in contrast to other photorespiratory mutants, which have severely reduced rates of photosynthesis and require high CO2 to grow. The pmdh mutant had a higher O2-dependent CO2 compensation point than the wild type, implying that either Rubisco specificity had changed or that the rate of CO2 released per Rubisco oxygenation was increased in the pmdh plants. Rates of gross O2 evolution and uptake were similar in the pmdh and wild-type plants, indicating that chloroplast linear electron transport and photorespiratory O2 uptake were similar between genotypes. The CO2 postillumination burst and the rate of CO2 released during photorespiration were both greater in the pmdh mutant compared with the wild type, suggesting that the ratio of photorespiratory CO2 release to Rubisco oxygenation was altered in the pmdh mutant. Without PMDH in the peroxisome, the CO2 released per Rubisco oxygenation reaction can be increased by over 50%. In summary, PMDH is essential for maintaining optimal rates of photorespiration in air; however, in its absence, significant rates of photorespiration are still possible, indicating that there are additional mechanisms for supplying reductant to the peroxisomal HPR reaction or that the HPR reaction is altogether circumvented.  相似文献   

15.
Abstract

Ecological aspects of C3, C4 and CAM photosynthetic pathways. - Three different photosynthetic CO2 fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO2 release. Both C4 and CAM pathways behave as a CO2 concentrating mechanism which prevent photorespiration. The CO2-concentrating mechanism in C4 plants is based on intracellular symplastic transport of C4 dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO2-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C4 photosynthetic pathway as compared to the C3 pathway permits higher rates of CO2 fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO2.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C4 plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C3; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.  相似文献   

16.
17.
Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m−2s−1). The CO2 assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO2 assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO2 assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m−2 s−1 PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m−2 s−1, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m−2 s−1), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m−2 s−1) or mean irradiance (200-300-200 μmol m−2 s−1).  相似文献   

18.
Light-activated hydrogen and oxygen evolution as a function of CO2 concentration in helium were measured for the unicellular green alga Chlamydomonas reinhardtii. The concentrations were 58, 30, 0.8 and 0 ppm CO2. The objective of these experiments was to study the differential affinity of CO2/HCO 3 - for their respective Photosystem II and Calvin cycle binding sites vis-à-vis photoevolution of molecular oxygen and the competitive pathways of hydrogen photoevolution and CO2 photoassimilation. The maximum rate of hydrogen evolution occurred at 0.8 ppm CO2, whereas the maximum rate of oxygen evolution occurred at 58 ppm CO2. The key result of this work is that the rate of photosynthetic hydrogen evolution can be increased by, at least partially, satisfying the Photosystem II CO2/HCO 3 - binding site requirement without fully activating the Calvin-Benson CO2 reduction pathway. Data are presented which plot the rates of hydrogen and oxygen evolution as functions of atmospheric CO2 concentration in helium and light intensity. The stoichiometric ratio of hydrogen to oxygen changed from 0.1 at 58 ppm to approximately 2.5 at 0.8 ppm. A discussion of partitioning of photosynthetic reductant between the hydrogen/hydrogenase and Calvin-Benson cycle pathways is presented.Abbreviations PET photosynthetic electron transport - PS Photosystem  相似文献   

19.
The influence of abscisic acid (ABA) on carbon metabolism, rate of photorespiration, and the activity of the photorespiratory enzymes ribulose bisphosphate oxygenase and glycolate oxidase in 7-day-old barley seedlings (Hordeum vulgare L. var. Alfa) was investigated. Plants treated with ABA had enhanced incorporation of labeled carbon from 14CO2 into glycolic acid, glycine, and serine, while 14C incorporation into 3-phosphoglyceric acid and sugarphosphate esters was depressed. Parallel with this effect, treated plants showed a rise in activity of RuBP oxygenase and glycolic acid oxidase. The rate of photorespiration was increased twofold by ABA treatment at IO−6 molar while the CO2-compensation point increased 46% and stomatal resistance increased more than twofold over control plants.  相似文献   

20.
Current models of C3 photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO2 and O2, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO2 and O2, i.e., increasing O2 stimulates assimilation and increasing CO2 inhibits assimilation. We propose a model which explains reversed sensitivity to CO2 and O2 by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO2, O2, and light is demonstrated.Abbreviations PFD photon flux density - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - TPU triose phosphate utilization  相似文献   

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