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1.
Engineering C4 photosynthesis into rice has been considered a promising strategy to increase photosynthesis and yield. A question that remains to be answered is whether expressing a C4 metabolic cycle into a C3 leaf structure and without removing the C3 background metabolism improves photosynthetic efficiency. To explore this question, we developed a 3D reaction diffusion model of bundle‐sheath and connected mesophyll cells in a C3 rice leaf. Our results show that integrating a C4 metabolic pathway into rice leaves with a C3 metabolism and mesophyll structure may lead to an improved photosynthesis under current ambient CO2 concentration. We analysed a number of physiological factors that influence the CO2 uptake rate, which include the chloroplast surface area exposed to intercellular air space, bundle‐sheath cell wall thickness, bundle‐sheath chloroplast envelope permeability, Rubisco concentration and the energy partitioning between C3 and C4 cycles. Among these, partitioning of energy between C3 and C4 photosynthesis and the partitioning of Rubisco between mesophyll and bundle‐sheath cells are decisive factors controlling photosynthetic efficiency in an engineered C3–C4 leaf. The implications of the results for the sequence of C4 evolution are also discussed.  相似文献   

2.
The aim of this work was to describe the photosynthetic carbon metabolism of the cooltemperate C4 grass Spartina anglica. With the exception of pyruvate, phosphate dikinase and pyruvate kinase, the maximum catalytic activities in leaves of putative enzymes of the C4 cycle of a phosphoenolpyruvate-carboxykinase C4 plant were considerably in excess of the observed, steady-state rate of photosynthesis, and were comparable with the maximum catalytic activities of key enzymes of the reductive pentose-phosphate pathway. Radioactive carbon from 14CO2 supplied to attached leaves during steady-state photosynthesis appeared first in malate and aspartate from which it moved to intermediates of the reductive pentose-phosphate pathway, and then to sucrose. These experiments show that photosynthetic carbon metabolism in this cool-temperate C4 plant is similar to that of C4 plants of hotter climates.  相似文献   

3.
Hydrogen and carbon isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from C3, C4, and Crassulacean acid metabolism (CAM) plants were determined for plants growing within a small area in Val Verde County, Texas. Plants having CAM had distinctly higher deuterium/hydrogen (D/H) ratios than plants having C3 and C4 metabolism. When hydrogen isotope ratios are plotted against carbon isotope ratios, each photosynthetic mode separates into a distinct cluster of points. C4 plants had many D/H ratios similar to those of C3 plants, so that hydrogen isotope ratios cannot be used to distinguish between these two photosynthetic modes. Portulaca mundula, which may have a modified photosynthetic mode between C4 and CAM, had a hydrogen isotope ratio between those of the C4 and CAM plants. When oxygen isotope ratios are plotted against carbon isotope ratios, no distinct clustering of the C4 and CAM plants occurs. Thus, oxygen isotope ratios are not useful in distinguishing between these metabolic modes. A plot of hydrogen isotope ratios versus oxygen isotope ratios for this sample set shows considerable overlap between oxygen isotope ratios of the different photosynthetic modes without a concomitant overlap in the hydrogen isotope ratios of CAM and the other two photosynthetic modes. This observation is consistent with the hypothesis that higher D/H ratios in CAM plants relative to C3 and C4 plants are due to isotopic fractionations occurring during biochemical reactions.  相似文献   

4.
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   

5.
The aim of this work was to investigate the fate of phosphoenolpyruvate (PEP) produced by decarboxylation of oxaloacetate during photosynthesis in the bundle sheaths of leaves of the PEP-carboxykinase C4 grass Spartina anglica Hubb. Mesophyll protoplasts and bundle sheath cells were separated enzymically and used to investigate activities and distributions of putative enzymes of the C4 cycle and the photosynthetic carbon metabolism of bundle sheath cells. The results indicate that neither conversion of PEP to pyruvate nor its conversion to 3-phosphoglycerate can account for all of the carbon flux through the C4 cycle during photosynthesis. It is likely, therefore, either that PEP moves directly from bundle sheath to mesophyll or that more than one pathway of regeneration of PEP is involved in the C4 cycle in this plant.Abbreviations Chl chlorophyll - PEP phosphoenolpyruvate - Pi phosphate - RuBP ribulose-1,5-bisphosphate  相似文献   

6.
Crassulacean acid metabolism (CAM), an advanced photosynthetic pathway conferring water conservation to plants in arid habitats, has enigmatically been reported in some species restricted to extremely wet tropical forests. Of these, epiphytic Bromeliaceae may possess absorbent foliar trichomes that hinder gas‐exchange when wetted, imposing further limitations on carbon dioxide (CO2) uptake. The hypothesis that the metabolic plasticity inherent to CAM confers an ecological advantage over conventional C3 plants, when constant rainfall and mist might inhibit gas‐exchange was investigated. Gas‐exchange, fluorometry and organic acid and mineral nutrient contents were compared for the bromeliads Aechmea dactylina (CAM) and Werauhia capitata (C3) in situ at the Cerro Jefe cloud forest, Panama (annual rainfall > 4 m). Daily carbon gain and photosynthetic nutrient use efficiencies were consistently higher for A. dactylina, due to a greater CO2 uptake period, recycling of CO2 from respiration and a dynamic response of CO2 uptake to wetting of leaf surfaces. During the dry season CAM also had water conserving and photoprotective roles. A paucity of CAM species at Cerro Jefe suggests a recent radiation of this photosynthetic pathway into the wet cloud forest, with CAM extending diversity in form and function for epiphytes.  相似文献   

7.
8.
The potential of control analysis to aid our understanding of regulation and control of photosynthetic carbon metabolism is investigated. Methods of metabolic control analysis are used to determine flux control coefficients of photosynthetic reactions from enzyme elasticities. Equations expressing control coefficients symbolically by enzyme elasticities are derived, and general properties of these expressions are analysed. Suggestions for experimental determination of flux control coefficients from enzyme elasticities are given. A simplified model of the Calvin-Benson cycle is used to illustrate interrelations between patterns of photosynthetic metabolites and that of control coefficients.Abbreviations GAPDH glyceraldehyde phosphate dehydrogenase - PGA 3-phosphoglycerate - PGK 3-phosphoglycerate kinase - Pi inorganic phosphate - PRK phosphoribulokinase - RuBP ribulose-1,5-bisphosphate(total, free) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Ru5P ribulose-5-phosphate  相似文献   

9.
The most prominent role of peroxisomes in photosynthetic plant tissues is their participation in photorespiration, a process also known as the oxidative C2 cycle or the oxidative photosynthetic carbon cycle. Photorespiration is an essential process in land plants, as evident from the conditionally lethal phenotype of mutants deficient in enzymes or transport proteins involved in this pathway. The oxidative C2 cycle is a salvage pathway for phosphoglycolate, the product of the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to the Calvin cycle intermediate phosphoglycerate. The pathway is highly compartmentalized and involves reactions in chloroplasts, peroxisomes, and mitochondria. The H2O2-producing enzyme glycolate oxidase, catalase, and several aminotransferases of the photorespiratory cycle are located in peroxisomes, with catalase representing the major constituent of the peroxisomal matrix in photosynthetic tissues. Although photorespiration is of major importance for photosynthesis, the identification of the enzymes involved in this process has only recently been completed. Only little is known about the metabolite transporters for the exchange of photorespiratory intermediates between peroxisomes and the other organelles involved, and about the regulation of the photorespiratory pathway. This review highlights recent developments in understanding photorespiration and identifies remaining gaps in our knowledge of this important metabolic pathway.  相似文献   

10.
Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO2 and O2 concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO2 is suggested to explain the discrepancy. The model suggests that resistance to CO2 fixation is mainly in the carboxylation reactions, not in CO2 transport. Using the steady state model, the kinetics of 14C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of 14C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO2 released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.  相似文献   

11.
Ecotypic differences in the photosynthetic carbon metabolism of Mollugo verticillata were studied. Variations in C3 and C4 cycle activity are apparently due to differences in the activities of enzymes associated with each pathway. Compared to C4 plants, the activities of C4 pathway enzymes were generally lower in M. verticillata, with the exception of the decarboxylase enzyme, NAD malic enzyme. The combined total carboxylase enzyme activity of M. verticillata was greater than that of C3 plants, possibly accounting for the high photosynthetic rates of this species. Unlike either C3 or C4 plants, ribulose bisphosphate carboxylase was present in both mesophyll and bundle sheath cell chloroplasts in M. verticillata. The localization of this enzyme in both cells in this plant, in conjunction with an efficient C4 acid decarboxylation mechanism most likely localized in bundle sheath cell mitochondria, may account for intermediate photorespiration levels previously observed in this species.  相似文献   

12.
13.
Gracilaria tenuistipitata Zhang et Xia was cultured for 15 d at low, normal and high inorganic carbon concentrations under constant light, temperature and nutrient conditons. Carbonic anhydrase (CA; EC 4.2.1.1.) activity, ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco; EC 4.1.1.39) content, pigment content and C/N ratio were measured, and also the photosynthesis and growth rates. Both Rubisco content and CA activity increased under conditions of low inorganic carbon (Ci) but decreased at high Ci with respect to the control. The amount of pigments declined considerably at high Ci and was slightly higher at low Ci. The maximum rate of photosynthesis and the photosynthetic efficiency increased in low Ci and the opposite was found at high Ci concentration. The effects of Ci concentration on maximum rate of photosynthesis and photosynthetic efficiency are discussed in relation to the variation in pigment and Rubisco contents and CA activity. The data indicate that Ci may be an important factor controlling the photosynthetic physiology of G. tenuistipitata with regard, not only to the enzymes of Ci metabolism, but also to the pigment content.Abbreviations APSmax maximum apparent photosynthetic rate - CA carbonic anhydrase - Chl chlorophyll - Ci inorganic carbon - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work has been supported by grants No. PB91-0962 and No. MAR90-0365 from Spanish Direction for Science and Technology (DIGICYT). M.J. G-S holds a fellowship from the DIGICYT.  相似文献   

14.
Variations in the natural abundance of 18O and 2H in plant cellulose are influenced by the isotopic composition of the water directly involved in metabolism—the metabolic water fraction. The isotopic distinction between the metabolic source water and total tissue water must reflect the formation of isotopic gradients within the tissue that are influenced by the rate of water turnover, by properties of the water conducting system and by environmental conditions. It seems that the 18O abundance in the metabolic water is conserved in cellulose with a relatively constant isotope effect. The relationship of the 2H abundance between metabolic water and cellulose is more complex. Hydrogen incorporated into photosynthetic products during primary reduction steps is highly depleted in 2H. However, a large proportion of these hydrogens are subsequently replaced by exchange with water, leading to 2H enrichment during heterotrophic metabolism. Deciphering the oxygen isotope ratio of cellulose could help in providing insights into the carbon and oxygen fluxes exchanged between plants and the atmosphere. This is because the 18O abundance in cellulose records the 18O abundance in the metabolic water, which in turn, controls the oxygen isotopic signatures of the CO2 and O2 released by plants into the atmosphere. The hydrogen isotope effects associated with carbohydrate metabolism provide insights into the autotrophic state of a plant tissue. This is because the hydrogen isotope ratio of carbohydrates must reflect the net effects of the two opposing isotope effects associated with photosynthesis and heterotrophic metabolism.  相似文献   

15.
Cyanobacteria are environmentally important photosynthetic microorganisms attracting a growing attention in various areas of basic and applied researches. To better understand their metabolism, we presently report on the development of a robust and simple protocol for facile extraction and high throughput analysis of the metabolites of the widely-used strain Synechocystis PCC6803 through liquid chromatography coupled to high resolution mass spectrometry (LC/MS). Our analytical method was developed and tested with 102 reference compounds representative of the chemical diversity of polar cell metabolites, and Synechocystis cell extracts spiked with 37 reference compounds. These samples were analyzed with two chromatographic systems, each coupled to a LTQ-Orbitrap mass spectrometer: a liquid chromatographic system equipped with a pentafluorophenylpropyl column (the PFPP-LC/MS system), and an ultra-high performance liquid chromatographic system with a C18-reversed phase column (the C18-UHPLC/MS system). We showed that the PFPP-LC/MS method performs better than the C18-UHPLC/MS method in terms of retention, separation and detection of metabolites. Consequently, we applied the PFPP-LC/MS method to analyze the metabolome of Synechocystis growing under various conditions of light and glucose, which strongly influence cell growth. We found that glucose increases glucose storage (synthesis of glycogen-like polysaccharide) and catabolism (oxidative pentose phosphate pathway and glycolysis), while it decreases the Calvin–Benson cycle that consumes photosynthetic electrons for CO2 assimilation. Depending on light and glucose availabilities, this global metabolic reprogramming can generate an oxidative stress, likely through the recombination of the glucose-spared electrons with the photosynthetic oxygen thereby producing toxic reactive oxygen species.  相似文献   

16.
The effect of N-assimilation on the partitioning of carbon fixation between phosphoenolpyruvate carboxylase (PEPcase) and ribulose bisphosphate carboxylase/oxygenase (Rubisco) was determined by measuring stable carbon isotope discrimination during photosynthesis by an N-limited green alga, Selenastrum minutum (Naeg.) Collins. This was facilitated by a two process model accounting for simultaneous CO2 fixation and respiratory CO2 release. Discrimination by control cells was consistent with the majority of carbon being fixed by Rubisco. During nitrogen assimilation however, discrimination was greatly reduced indicating an enhanced flux through PEPcase which accounted for upward of 70% of total carbon fixation. This shift toward anaplerotic metabolism supports a large increase in tricarboxylic acid cycle activity primarily between oxaloacetate and α-ketoglutarate thereby facilitating the provision of carbon skeletons for amino acid synthesis. This provides an example of a unique set of conditions under which anaplerotic carbon fixation by PEPcase exceeds photosynthetic carbon fixation by Rubisco in a C3 organism.  相似文献   

17.
The purple nonsulfur bacterium Rhodospirillum rubrum has been employed to study physiological adaptation to limiting oxygen tensions (microaerophilic conditions). R. rubrum produces maximal levels of photosynthetic membranes when grown with both succinate and fructose as carbon sources under microaerophilic conditions in comparison to the level (only about 20% of the maximum) seen in the absence of fructose. Employing a unique partial O2 pressure (pO2) control strategy to reliably adjust the oxygen tension to values below 0.5%, we have used bioreactor cultures to investigate the metabolic rationale for this effect. A metabolic profile of the central carbon metabolism of these cultures was obtained by determination of key enzyme activities under microaerophilic as well as aerobic and anaerobic phototrophic conditions. Under aerobic conditions succinate and fructose were consumed simultaneously, whereas oxygen-limiting conditions provoked the preferential breakdown of fructose. Fructose was utilized via the Embden-Meyerhof-Parnas pathway. High levels of pyrophosphate-dependent phosphofructokinase activity were found to be specific for oxygen-limited cultures. No glucose-6-phosphate dehydrogenase activity was detected under any conditions. We demonstrate that NADPH is supplied mainly by the pyridine-nucleotide transhydrogenase under oxygen-limiting conditions. The tricarboxylic acid cycle enzymes are present at significant levels during microaerophilic growth, albeit at lower levels than those seen under fully aerobic growth conditions. Levels of the reductive tricarboxylic acid cycle marker enzyme fumarate reductase were also high under microaerophilic conditions. We propose a model by which the primary “switching” of oxidative and reductive metabolism is performed at the level of the tricarboxylic acid cycle and suggest how this might affect redox signaling and gene expression in R. rubrum.  相似文献   

18.
Investigations of a wide range of methane- and methanol-utilizers showed a striking versatility of their metabolism dependent on the genotype and growth conditions. A correlation between pathways of carbon and nitrogen metabolism was found. It was most stringent in obligate methane-utilizers: the hexulosephosphate pathway bacteria assimilated NH3 by the reductive amination of α-ketoglutarate or pyruvate whereas the serine pathway bacteria used the glutamate cycle (glutamine synthetase + glutamate-oxoglutarate aminotransferase). Multiple enzymic lesions were found in central metabolism of obligate methylotrophs, i.e. the absence of the enzymes of glycolytic and pentosephosphate pathways, gluconeogenesis, citric acid cycle and glyoxylate shunt. These metabolic blocks were not so profound and could be compensated in restricted and facultative methylotrophs during heterotrophic growth. The average levels of exogenous CO2 fixation in methylotrophic bacteria with the hexulosephosphate, serine and ribulosebisphosphate pathways were found to be 10, 30 and 80% of their total cell carbon, respectively. These results served as a basis for biotechnological applications of metabolic potential of methylotrophs (production of biomass, polysaccharides and enzymes as well as for microbiological treatment of industrial waters containing toxic C1- and Cn-compounds).  相似文献   

19.
Glycolate is produced in autotrophic cells under high temperatures and Ci‐limitation via oxygenation of ribulose‐1,5‐bisphosphate. In unicellular algae, glycolate is lost via excretion or metabolized via the C2 cycle by consuming reductants, ATP and CO2 emission (photorespiration). Therefore, photorespiration is an inhibitory process for biomass production. However, cells can be manipulated in a way that they become glycolate‐producing ‘cell factories’, when the ratio carboxylation/oxygenation is 2. If under these conditions the C2 cycle is blocked, glycolate excretion becomes the only pathway of photosynthetic carbon flow. The study aims to proof the biotechnological applicability of algal‐based glycolate excretion as a new biotechnological platform. It is shown that cells of Chlamydomonas can be cultivated under specific conditions to establish a constant and long‐term stable glycolate excretion during the light phase. The cultures achieved a high efficiency of 82% of assimilated carbon transferred into glycolate biosynthesis without losses of function in cell vitality. Moreover, the glycolate accumulation in the medium is high enough to be directly used for microbial fermentation but does not show toxic effects to the glycolate‐producing cells.  相似文献   

20.
The results of the numerous measurements obtained during the last 40 years on gas exchange rate, photosynthetic carbon metabolism by exposition in 14CO2 and activities of primary carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBPC/O), in various wheat and soybean genotypes grown over a wide area in the field and contrasting in photosynthetic traits and productivity are presented in this article. It was established that high productive wheat genotypes (7–9 t ha?1) with the optimal architectonics possess higher rate of CO2 assimilation during the leaf ontogenesis. Along with the high rate of photosynthesis, high values of photorespiration are characteristic for the high productive genotypes. Genotypes with moderate (4–5 t ha?1) and low (3 t ha?1) grain yield are characterized by relatively low rates of both CO2 assimilation and photorespiration. A value of photorespiration constitutes 28–35% of photosynthetic rate in contrasting genotypes. The activities of RuBPC and RuBPO were changing in a similar way in the course of the flag leaf and ear elements development. High productive genotypes are also characterized by a higher rate of biosynthesis and total value of glycine–serine and a higher photosynthetic rate. Therefore, contrary to conception arisen during many years on the wastefulness of photorespiration, taking into account the versatile investigations on different aspects of photorespiration, it was proved that photorespiration is one of the evolutionarily developed vital metabolic processes in plants and the attempts to reduce this process with the purpose of increasing the crop productivity are inconsistent.  相似文献   

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