Flux control of the malate valve in leaf cells

A cytosolic oxidation-reduction cycle in plant leaves

The viewpoint proposes a carbon-neutral biochemical cycle in the cytosol of plant leaves that is up-regulated by reactive oxygen species. Cycling provides NADPH and dissipates energy to counteract oxidative stress.

Modulating the activities of chloroplasts and mitochondria promotes adenosine triphosphate production and plant growth

Efficient photosynthesis requires a balance of ATP and NADPH production/consumption in chloroplasts, and the exportation of reducing equivalents from chloroplasts is important for balancing stromal ATP/NADPH ratio. Here, we showed that the overexpression of purple acid phosphatase 2 on the outer membranes of chloroplasts and mitochondria can streamline the production and consumption of reducing equivalents in these two organelles, respectively. A higher capacity of consumption of reducing equivalents in mitochondria can indirectly help chloroplasts to balance the ATP/NADPH ratio in stroma and recycle NADP⁺, the electron acceptors of the linear electron flow (LEF). A higher rate of ATP and NADPH production from the LEF, a higher capacity of carbon fixation by the Calvin-Benson-Bassham (CBB) cycle and a greater consumption of NADH in mitochondria enhance photosynthesis in the chloroplasts, ATP production in the mitochondria and sucrose synthesis in the cytosol and eventually boost plant growth and seed yields in the overexpression lines.

Elevated air temperature damage to photosynthetic apparatus alleviated by enhanced cyclic electron flow around photosystem I in tobacco leaves

High temperature can lead to increased production of excess light energy, thus reducing photosynthetic capacity in plants. Photosynthetic cyclic electron flow (CEF) in photosystem I (PSI) can effectively protect photosystems, but its physiological mechanism under high temperature is poorly understood. In this study, antimycin A (AA) and thenoyltrifluoroacetone (TTFA) were used to inhibit PGR5-and NDH-dependent CEF pathways, respectively, to reveal the photoprotective functions of CEF for PSII in tobacco leaves under high temperature stress (37 °C, HT). High temperatures caused decreases in maximal photochemistry efficiency (Fv/Fm) and damaged photosystem II (PSII) in tobacco leaves. Under AA inhibition of PGR5-dependent CEF, high temperature increased the fluorescence intensity of point O (Fo) in OJIP curves, i.e., the energy absorption per active reaction center (ABS/RC), the trapping rate of the reaction center (TRo/RC), and the electron transport efficiency per reaction center (ETo/RC) in tobacco leaves. High temperature induced an increase in the hydrogen peroxide content and a decrease in pigment content in tobacco leaves. Under the high temperature treatment, inhibition of PGR5-dependent CEF reduced the activities of the PSII reaction center significantly, destroyed the oxygen-evolving complex (OEC), and impeded photosynthetic electron transfer from PSII to the plastoquinone (PQ) pool in tobacco leaves. The TTFA treatment inhibited the NDH-dependent pathway under high temperature conditions, with the relative fluorescence intensity of point I (VI) decreased significantly, and the content of hydrogen peroxide and superoxide anion increased significantly. Additionally, Fo and the redox degree of the PSII donor side (Wk) increased, and pigment content decreased compared to the control, but with little change compared to high temperature treatment, indicating that the inhibition of the NDH-dependent pathway directly weakened the capacity of the PQ pool to lead to the accumulation of reactive oxygen species (ROS) in tobacco leaves. In conclusion, CEF alleviated damage to the photosynthetic apparatus in tobacco leaves by increasing PSII heat dissipation, reducing ROS production, and maintaining the stability of the PQ pool to accommodate photosynthetic electron flow.

Photosynthesis: basics, history and modelling

BACKGROUND

With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis and thus productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centres, primary photochemistry, electron and proton transport, NADPH and ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). Here we cover some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport 'chain' (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as 'state transitions' and 'non-photochemical quenching' of the excited state of chlorophyll a.

SCOPE

In this review, we emphasize that mathematical modelling is a highly valuable tool in understanding and making predictions regarding photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820) related to redox changes in P700, the reaction centre of photosystem I.

CONCLUSIONS

We highlight here the important role of modelling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.

In planta study of photosynthesis and photorespiration using NADPH and NADH/NAD+ fluorescent protein sensors

The challenge of monitoring in planta dynamic changes of NADP(H) and NAD(H) redox states at the subcellular level is considered a major obstacle in plant bioenergetics studies. Here, we introduced two circularly permuted yellow fluorescent protein sensors, iNAP and SoNar, into Arabidopsis thaliana to monitor the dynamic changes in NADPH and the NADH/NAD+ ratio. In the light, photosynthesis and photorespiration are linked to the redox states of NAD(P)H and NAD(P) pools in several subcellular compartments connected by the malate-OAA shuttles. We show that the photosynthetic increases in stromal NADPH and NADH/NAD+ ratio, but not ATP, disappear when glycine decarboxylation is inhibited. These observations highlight the complex interplay between chloroplasts and mitochondria during photosynthesis and support the suggestions that, under normal conditions, photorespiration supplies a large amount of NADH to mitochondria, exceeding its NADH-dissipating capacity, and the surplus NADH is exported from the mitochondria to the cytosol through the malate-OAA shuttle.

Citrate valve integrates mitochondria into photosynthetic metabolism

While in heterotrophic cells and in darkness mitochondria serve as main producers of energy, during photosynthesis this function is transferred to chloroplasts and the main role of mitochondria in bioenergetics turns to be the balance of the level of phosphorylation of adenylates and of reduction of pyridine nucleotides to avoid over-energization of the cell and optimize major metabolic fluxes. This is achieved via the establishment and regulation of local equilibria of the tricarboxylic acid (TCA) cycle enzymes malate dehydrogenase and fumarase in one branch and aconitase and isocitrate dehydrogenase in another branch. In the conditions of elevation of redox level, the TCA cycle is transformed into a non-cyclic open structure (hemicycle) leading to the export of the tricarboxylic acid (citrate) to the cytosol and to the accumulation of the dicarboxylic acids (malate and fumarate). While the buildup of NADPH in chloroplasts provides operation of the malate valve leading to establishment of NADH/NAD⁺ ratios in different cell compartments, the production of NADH by mitochondria drives citrate export by establishing conditions for the operation of the citrate valve. The latter regulates the intercompartmental NADPH/NADP⁺ ratio and contributes to the biosynthesis of amino acids and other metabolic products during photosynthesis.

Transcript and metabolic adjustments triggered by drought in Ilex paraguariensis leaves

Abscisic acid is involved in the drought response of Ilex paraguariensis. Acclimation includes root growth stimulation, stomatal closure, osmotic adjustment, photoprotection, and regulation of nonstructural carbohydrates and amino acid metabolisms. Ilex paraguariensis (yerba mate) is cultivated in the subtropical region of South America, where the occurrence of drought episodes limit yield. To explore the mechanisms that allow I. paraguariensis to overcome dehydration, we investigated (1) how gene expression varied between water-stressed and non-stressed plants and (2) in what way the modulation of gene expression was linked to physiological status and metabolite composition. A total of 4920 differentially expressed transcripts were obtained through RNA-Seq after water deprivation. Drought induced the expression of several transcripts involved in the ABA-signalling pathway. Stomatal closure and leaf osmotic adjustments were promoted to minimize water loss, and these responses were accompanied by a high transcriptional remodeling of stress perception, signalling and transcriptional regulation, the photoprotective and antioxidant systems, and other stress-responsive genes. Simultaneously, significant changes in metabolite contents were detected. Glutamine, phenylalanine, isomaltose, fucose, and malate levels were shown to be positively correlated with dehydration. Principal component analysis showed differences in the metabolic profiles of control and stressed leaves. These results provide a comprehensive overview of how I. paraguariensis responds to dehydration at transcriptional and metabolomic levels and provide further characterization of the molecular mechanisms associated with drought response in perennial subtropical species.

Malate valves: old shuttles with new perspectives

Malate valves act as powerful systems for balancing the ATP/NAD(P)H ratio required in various subcellular compartments in plant cells. As components of malate valves, isoforms of malate dehydrogenases (MDHs) and dicarboxylate translocators catalyse the reversible interconversion of malate and oxaloacetate and their transport. Depending on the co-enzyme specificity of the MDH isoforms, either NADH or NADPH can be transported indirectly. Arabidopsis thaliana possesses nine genes encoding MDH isoenzymes. Activities of NAD-dependent MDHs have been detected in mitochondria, peroxisomes, cytosol and plastids. In addition, chloroplasts possess a NADP-dependent MDH isoform. The NADP-MDH as part of the 'light malate valve' plays an important role as a poising mechanism to adjust the ATP/NADPH ratio in the stroma. Its activity is strictly regulated by post-translational redox-modification mediated via the ferredoxin-thioredoxin system and fine control via the NADP⁺ /NADP(H) ratio, thereby maintaining redox homeostasis under changing conditions. In contrast, the plastid NAD-MDH ('dark malate valve') is constitutively active and its lack leads to failure in early embryo development. While redox regulation of the main cytosolic MDH isoform has been shown, knowledge about regulation of the other two cytosolic MDHs as well as NAD-MDH isoforms from peroxisomes and mitochondria is still lacking. Knockout mutants lacking the isoforms from chloroplasts, mitochondria and peroxisomes have been characterised, but not much is known about cytosolic NAD-MDH isoforms and their role in planta. This review updates the current knowledge on MDH isoforms and the shuttle systems for intercompartmental dicarboxylate exchange, focusing on the various metabolic functions of these valves.

Metabolic regulation of photosynthesis

Photosynthesis is fundamental to biomass production, but is a dynamic process sensitive to environmental constraints. In recent years, approaches to increase biomass and grain yield by altering photosynthetically related processes in the plant have received considerable attention. However, improving biomass yield requires a predictive understanding of the molecular mechanisms that allow photosynthesis to be adjusted. The important roles of metabolic reactions external to those directly involved in photosynthesis are highlighted in this review; however, our major focus is on the routes taken to improve photosynthetic carbon assimilation and to increase photosynthetic efficiency and consequently biomass yield.

Plants increase CO2 uptake by assimilating nitrogen via the photorespiratory pathway

Photorespiration is a major bioengineering target for increasing crop yields as it is often considered a wasteful process. Photorespiratory metabolism is integrated into leaf metabolism and thus may have certain benefits. Here, we show that plants can increase their rate of photosynthetic CO₂ uptake when assimilating nitrogen de novo via the photorespiratory pathway by fixing carbon as amino acids in addition to carbohydrates. Plants fed NO₃^- had higher rates of CO₂ assimilation under photorespiratory than low-photorespiratory conditions, while plants lacking NO₃^- nutrition exhibited lower stimulation of CO₂ uptake. We modified the widely used Farquhar, von Caemmerer and Berry photosynthesis model to include the carbon and electron requirements for nitrogen assimilation via the photorespiratory pathway. Our modified model improves predictions of photosynthetic CO₂ uptake and of rates of photosynthetic electron transport. The results highlight how photorespiration can improve photosynthetic performance despite reducing the efficiency of Rubisco carboxylation.

The fungal endophyte Epichloë typhina improves photosynthesis efficiency of its host orchard grass (Dactylis glomerata)

MAIN CONCLUSION

According to the results presented in this paper the fungal endophyte Epichloë typhina significantly improves the growth, PSII photochemistry and C assimilation efficiency of its host Dactylis glomerata. In this paper, we present a comprehensive study of the impact of the endophytic fungi Epichloë typhina on its plant hosts' photosynthesis apparatus. Chlorophyll a fluorescence, gas exchange, immuno-blotting and spectrophotometric measurements were employed to assess photosynthetic performance, changes in pigment content and mechanisms associated with light harvesting, carbon assimilation and energy distribution in Dactylis glomerata colonized with Epichloë typhina. According to the results presented in this study, colonization of D. glomerata results in improved photosynthesis efficiency. Additionally, we propose a new mechanism allowing plants to cope with the withdrawal of a significant fraction of its energy resources by the endophytic fungi. The abundance of LHCI, LHCII proteins as well as chlorophyll b was significantly higher in E+ plants. Malate export out of the chloroplast was shown to be increased in colonized plants. To our knowledge, we are the first to report this phenomenon. Epichloë colonization improved PSII photochemistry and C assimilation efficiency. Elevated energy demands of E+ D. glomerata plants are met by increasing the rate of carbon assimilation and PSII photochemistry.

Redox meets protein trafficking

After the engulfment of two prokaryotic organisms, the thus emerged eukaryotic cell needed to establish means of communication and signaling to properly integrate the acquired organelles into its metabolism. Regulatory mechanisms had to evolve to ensure that chloroplasts and mitochondria smoothly function in accordance with all other cellular processes. One essential process is the post-translational import of nuclear encoded organellar proteins, which needs to be adapted according to the requirements of the plant. The demand for protein import is constantly changing depending on varying environmental conditions, as well as external and internal stimuli or different developmental stages. Apart from long-term regulatory mechanisms such as transcriptional/translation control, possibilities for short-term acclimation are mandatory. To this end, protein import is integrated into the cellular redox network, utilizing the recognition of signals from within the organelles and modifying the efficiency of the translocon complexes. Thereby, cellular requirements can be communicated throughout the whole organism. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

A security network in PSI photoprotection: regulation of photosynthetic control, NPQ and O2 photoreduction by cyclic electron flow

Cyclic electron flow (CEF) around PSI regulates acceptor-side limitations and has multiple functions in the green alga, Chlamydomonas reinhardtii. Here we draw on recent and historic literature and concentrate on its role in Photosystem I (PSI) photoprotection, outlining causes and consequences of damage to PSI and CEF's role as an avoidance mechanism. We outline two functions of CEF in PSI photoprotection that are both linked to luminal acidification: firstly, its action on Photosystem II with non-photochemical quenching and photosynthetic control and secondly, its action in poising the stroma to overcome acceptor-side limitation by rebalancing NADPH and ATP ratios for carbon fixation.

Proton gradient regulation 5-mediated cyclic electron flow under ATP- or redox-limited conditions: a study of ΔATpase pgr5 and ΔrbcL pgr5 mutants in the green alga Chlamydomonas reinhardtii

The Chlamydomonas reinhardtii proton gradient regulation5 (Crpgr5) mutant shows phenotypic and functional traits similar to mutants in the Arabidopsis (Arabidopsis thaliana) ortholog, Atpgr5, providing strong evidence for conservation of PGR5-mediated cyclic electron flow (CEF). Comparing the Crpgr5 mutant with the wild type, we discriminate two pathways for CEF and determine their maximum electron flow rates. The PGR5/proton gradient regulation-like1 (PGRL1) ferredoxin (Fd) pathway, involved in recycling excess reductant to increase ATP synthesis, may be controlled by extreme photosystem I acceptor side limitation or ATP depletion. Here, we show that PGR5/PGRL1-Fd CEF functions in accordance with an ATP/redox control model. In the absence of Rubisco and PGR5, a sustained electron flow is maintained with molecular oxygen instead of carbon dioxide serving as the terminal electron acceptor. When photosynthetic control is decreased, compensatory alternative pathways can take the full load of linear electron flow. In the case of the ATP synthase pgr5 double mutant, a decrease in photosensitivity is observed compared with the single ATPase-less mutant that we assign to a decreased proton motive force. Altogether, our results suggest that PGR5/PGRL1-Fd CEF is most required under conditions when Fd becomes overreduced and photosystem I is subjected to photoinhibition. CEF is not a valve; it only recycles electrons, but in doing so, it generates a proton motive force that controls the rate of photosynthesis. The conditions where the PGR5 pathway is most required may vary in photosynthetic organisms like C. reinhardtii from anoxia to high light to limitations imposed at the level of carbon dioxide fixation.

Plastidial NADP-malic enzymes from grasses: unraveling the way to the C4 specific isoforms

Malic enzyme is present in many plant cell compartments such as plastids, cytosol and mitochondria. Particularly relevant is the plastidial isoform that participates in the C(4) cycle providing CO(2) to RuBisCO in C(4) species. This type of photosynthesis is more frequent among grasses where anatomical preconditioning would have facilitated the evolution of the C(4) syndrome. In maize (C(4) grass), the photosynthetic NADP dependent Malic enzyme (ZmC(4)-NADP-ME, l-malate:NADP oxidoreductase, E.C. 1.1.1.40) and the closest related non-photosynthetic isoform (ZmnonC(4)-NADP-ME, l-malate:NADP oxidoreductase, E.C. 1.1.1.40) are both plastidial but differ in expression pattern, kinetics and structure. Features like high catalytic efficiency, inhibition by high malate concentration at pH 7.0, redox modulation and tetramerization are characteristic of the photosynthetic NADP-ME. In this work, the proteins encoded by sorghum (C(4) grass) and rice (C(3) grass) NADP-ME genes, orthologues of the plastidial NADP-MEs from maize, were recombinantly expressed, purified and characterized. In a global comparison, we could identify a small group of residues which may explain the special features of C(4) enzymes. Overall, the present work presents biochemical and molecular data that helps to elucidate the changes that took place in the evolution of C(4) NADP-ME in grasses.

Characterization of the NADP-malic enzymes in the woody plant Populus trichocarpa

Plant NADP-malic enzyme (NADP-ME, EC 1.1.1.40) participates in a large number of metabolic pathways, but little is known about the NADP-ME family in woody plants or trees. Here, we characterized the tree Populus trichocarpa NADP-ME (PtNADP-ME) family and the properties of the family members. Five NADP-ME genes (PtNADP-ME1-PtNADP-ME5) were found in the genome of Populus. Semi-quantitative RT-PCR analysis show that the transcription levels of PtNADP-ME1 in lignified stems and roots are clearly higher than in other tissues, and PtNADP-ME2, PtNADP-ME3, PtNADP-ME4 and PtNADP-ME5 are broadly expressed in various tissues. PtNADP-ME gene expression was found to respond to salt and osmotic stresses, and NaCl salts upregulated the transcripts of putative plastidic ones (PtNADP-ME4 and PtNADP-ME5) significantly. Further, the NADP-ME activities of Populus seedlings increased at least two-fold under NaCl, mannitol and PEG treatments. Also, the expression of PtNADP-ME2 and PtNADP-ME3 increased during the course of leaf wounding. Each recombinant PtNADP-ME proteins were expressed and purified from Escherichia coli, respectively. Coomassie brilliant blue and NADP-ME activity staining on native polyacrylamide gels showed different oligomeric states of the recombinant PtNADP-MEs in vitro. Noticeably, the cytosolic PtNADP-ME2 aggregates as octamers and hexadecamers while the plastidic PtNADP-ME4 resembles hexamers and octamers. The four PtNADP-ME proteins except for PtNADP-ME1 have high activities on native polyacrylamide gels including different forms for PtNADP-ME2 (octamers and hexadecamers) or for PtNADP-ME4 (hexamers and octamers). High concentrations of NADP substrate decreased the activities of all PtNADP-MEs slightly, while the malate had no effect on them. The kinetic parameters (V (max), K (m), K (cat), and K (cat)/K (m)) of each isoforms were summarized. Our data show the different effects of metabolites (influx into tricarboxylic acid cycle or Calvin cycle) on the activity of the individual PtNADP-ME in vitro. According to phylogenetic analysis, five PtNADP-MEs are clustered into cytosolic dicot, plastidic dicot, and monocot and dicot cytosolic groups in a phylogenetic tree. These results suggest that woody Populus NADP-ME family have diverse properties, and possible roles are discussed.

Redox-shuttling between chloroplast and cytosol: integration of intra-chloroplast and extra-chloroplast metabolism

Reducing equivalents produced in the chloroplast are essential for many key cellular metabolic enzyme reactions. Two redox shuttle systems transfer reductant out of the chloroplast; these systems consist of metabolite transporters, coupled with stromal and cytosolic dehydrogenase isozymes. The transporters function in the redox shuttle and also operate as key enzymes in carbon/nitrogen metabolism. To maintain adequate levels of reductant and proper metabolic balance, the shuttle systems are finely controlled. Also, in the leaves of C(4) plants, cell-specific division of carbon and nitrogen assimilation includes cell-specific localization of the redox shuttle systems. The redox shuttle systems are tightly linked to cellular metabolic pathways and are essential for maintaining metabolic balance between energy and reducing equivalents.

Photosynthetic control of electron transport and the regulation of gene expression

The term 'photosynthetic control' describes the short- and long-term mechanisms that regulate reactions in the photosynthetic electron transport (PET) chain so that the rate of production of ATP and NADPH is coordinated with the rate of their utilization in metabolism. At low irradiances these mechanisms serve to optimize light use efficiency, while at high irradiances they operate to dissipate excess excitation energy as heat. Similarly, the production of ATP and NADPH in ratios tailored to meet demand is finely tuned by a sophisticated series of controls that prevents the accumulation of high NAD(P)H/NAD(P) ratios and ATP/ADP ratios that would lead to potentially harmful over-reduction and inactivation of PET chain components. In recent years, photosynthetic control has also been extrapolated to the regulation of gene expression because mechanisms that are identical or similar to those that serve to regulate electron flow through the PET chain also coordinate the regulated expression of genes encoding photosynthetic proteins. This requires coordinated gene expression in the chloroplasts, mitochondria, and nuclei, involving complex networks of forward and retrograde signalling pathways. Photosynthetic control operates to control photosynthetic gene expression in response to environmental and metabolic changes. Mining literature data on transcriptome profiles of C(3) and C(4) leaves from plants grown under high atmospheric carbon dioxide (CO(2)) levels compared with those grown with ambient CO(2) reveals that the transition to higher photorespiratory conditions in C(3) plants enhances the expression of genes associated with cyclic electron flow pathways in Arabidopsis thaliana, consistent with the higher ATP requirement (relative to NADPH) of photorespiration.

Lessons from engineering a single-cell C(4) photosynthetic pathway into rice

The transfer of C(4) plant traits into C(3) plants has long been a strategy for improving the photosynthetic performance of C(3) plants. The introduction of a pathway mimicking the C(4) photosynthetic pathway into the mesophyll cells of C(3) plants was only a realistic approach when transgenic technology was sufficiently well developed and widely adopted. Here an attempt to introduce a single-cell C(4)-like pathway in which CO(2) capture and release occur in the mesophyll cell, such as the one found in the aquatic plant Hydrilla verticillata (L.f.) Royle, into rice (Oryza sativa L.) is described. Four enzymes involved in this pathway were successfully overproduced in the transgenic rice leaves, and 12 different sets of transgenic rice that overproduce these enzymes independently or in combination were produced and analysed. Although none of these transformants has yet shown dramatic improvements in photosynthesis, these studies nonetheless have important implications for the evolution of C(4) photosynthetic genes and their metabolic regulation, and have shed light on the unique aspects of rice physiology and metabolism. This article summarizes the lessons learned during these attempts to engineer single-cell C(4) rice.

Regulation of cyclic electron flow in C₃ plants: differential effects of limiting photosynthesis at ribulose-1,5-bisphosphate carboxylase/oxygenase and glyceraldehyde-3-phosphate dehydrogenase

Cyclic electron flow around photosystem I (CEF1) is thought to augment chloroplast ATP production to meet metabolic needs. Very little is known about the induction and regulation of CEF1. We investigated the effects on CEF1 of antisense suppression of the Calvin-Benson enzymes glyceraldehyde-3-phosphate dehydrogenase (gapR), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit (SSU), in tobacco (Nicotiana tabacum cv. Wisconsin 38). The gapR, but not ssuR, mutants showed substantial increases in CEF1, demonstrating that specific intermediates, rather than slowing of assimilation, induce CEF1. Both types of mutant showed increases in steady-state transthylakoid proton motive force (pmf) and subsequent activation of the photoprotective q(E) response. With gapR, the increased pmf was caused both by up-regulation of CEF1 and down-regulation of the ATP synthase. In ssuR, the increased pmf was attributed entirely to a decrease in ATP synthase activity, as previously seen in wild-type plants when CO₂ levels were decreased. Comparison of major stromal metabolites in gapR, ssuR and hcef1, a mutant with decreased fructose 1,6-bisphosphatase activity, showed that neither the ATP/ADP ratio, nor major Calvin-Benson cycle intermediates can directly account for the activation of CEF1, suggesting that chloroplast redox status or reactive oxygen species regulate CEF1.

Optimization of CO₂fixation in photosynthetic cells via thermodynamic buffering

Stable operation of photosynthesis is based on the establishment of local equilibria of metabolites in the Calvin cycle. This concerns especially equilibration of stromal contents of adenylates and pyridine nucleotides and buffering of CO₂ concentration to prevent its depletion at the sites of Rubisco. Thermodynamic buffering that controls the homeostatic flux in the Calvin cycle is achieved by equilibrium enzymes such as glyceraldehyde phosphate dehydrogenase, transaldolase and transketolase. Their role is to prevent depletion of ribulose-1,5-bisphosphate, even at high [CO₂], and to maintain conditions where the only control is exerted by the CO₂ supply. Buffering of adenylates is achieved mainly by chloroplastic adenylate kinase, whereas NADPH level is maintained by mechanisms involving alternative sinks for electrons both within the chloroplast (cyclic phosphorylation, chlororespiration, etc.) and shuttling of reductants outside chloroplast (malate valve). This results in optimization of carbon fixation in chloroplasts, illustrating the principle that the energy of light is used to support stable non-equilibrium which drives all living processes in plants.

A kinetic model describes metabolic response to perturbations and distribution of flux control in the benzenoid network of Petunia hybrida

In recent years there has been much interest in the genetic enhancement of plant metabolism; however, attempts at genetic modification are often unsuccessful due to an incomplete understanding of network dynamics and their regulatory properties. Kinetic modeling of plant metabolic networks can provide predictive information on network control and response to genetic perturbations, which allow estimation of flux at any concentration of intermediate or enzyme in the system. In this research, a kinetic model of the benzenoid network was developed to simulate whole network responses to different concentrations of supplied phenylalanine (Phe) in petunia flowers and capture flux redistributions caused by genetic manipulations. Kinetic parameters were obtained by network decomposition and non-linear least squares optimization of data from petunia flowers supplied with either 75 or 150 mm(2)H(5)-Phe. A single set of kinetic parameters simultaneously accommodated labeling and pool size data obtained for all endogenous and emitted volatiles at the two concentrations of supplied (2)H(5)-Phe. The generated kinetic model was validated using flowers from transgenic petunia plants in which benzyl CoA:benzyl alcohol/phenylethanol benzoyltransferase (BPBT) was down-regulated via RNAi. The determined in vivo kinetic parameters were used for metabolic control analysis, in which flux control coefficients were calculated for fluxes around the key branch point at Phe and revealed that phenylacetaldehyde synthase activity is the primary controlling factor for the phenylacetaldehyde branch of the benzenoid network. In contrast, control of flux through the beta-oxidative and non-beta-oxidative pathways is highly distributed.

The onset of NPQ and Deltamu(H)+ upon illumination of tobacco plants studied through the influence of mitochondrial electron transport

The relationship between the development of photoprotective mechanisms (non-photochemical quenching, NPQ), the generation of the electrochemical proton gradient in the chloroplast and the capacity to assimilate CO(2) was studied in tobacco dark-adapted leaves at the onset of illumination with low light. These conditions induce the generation of a transient NPQ, which relaxes in the light in parallel with the activation of the Calvin cycle. Wild-type plants were compared with a CMSII mitochondrial mutant, which lacks the respiratory complex I and shows a delayed activation of photosynthesis. In the mutant, a slower onset of photosynthesis was mirrored by a decreased capacity to develop NPQ. This correlates with a reduced efficiency to reroute electrons at the PSI reducing side towards cyclic electron flow around PSI and/or other alternative acceptor pools, and with a smaller ability to generate a proton motive force in the light. Altogether, these data illustrate the tight relationship existing between the capacity to evacuate excess electrons accumulated in the intersystem carriers and the capacity to dissipate excess photons during a dark to light transition. These data also underline the essential role of respiration in modulating the photoprotective response in dark-adapted leaves, by poising the cellular redox state.

Nicotiana tabacum NADP-malic enzyme: cloning, characterization and analysis of biological role

NADP-malic enzyme (NADP-ME) catalyzes the oxidative decarboxylation of L-malate, producing pyruvate, CO2 and NADPH. The photosynthetic role of this enzyme in C(4) and Crassulacean acid metabolism (CAM) plants has been well established; however, the biological role of several non-photosynthetic isoforms described in C(3), C(4) and CAM plants is still speculative. In this study, the characterization of the NADP-ME isoforms from Nicotiana tabacum was performed. Three different nadp-me transcripts were identified in this C(3) plant, two of which encode for putative cytosolic isoforms (DQ923118 and EH663836), while the third encodes for a plastidic counterpart (DQ923119). Although the three transcripts are expressed in vegetative as well as in reproductive tissues, they display different levels of expression. With regards to enzyme activity, root is the tissue that displays the highest NADP-ME activity. Recombinant NADP-MEs encoded by DQ923118 and DQ923119 were expressed in Escherichia coli and their kinetic parameters and response to different metabolic effectors were analyzed. Studies carried out with crude extracts and with the recombinant proteins indicate that the cytosolic and plastidic isoforms aggregate as tetramers of subunits of 65 and 63 kDa, respectively. Real-time reverse transcription-PCR studies show that the three nadp-me tobacco transcripts respond differently to several biotic and abiotic stress stimuli. Finally, the physiological role of each isoform is discussed in terms of the occurrence, kinetic properties and response to stress. The structure of the NADP-ME family in tobacco is compared with those of other C(3) species.

Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases

We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O(2) concentrations in N(2) (9-1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O(2) concentration and the increase in CO(2) concentration as result of leaf respiration were measured by a zirconium cell O(2) analyser and infrared-absorption CO(2) analyser, respectively. The low O(2) concentrations little influenced the rate of CO(2) evolution during the 60-s exposure. The initial slope of the O(2) uptake curve on the dissolved O(2) concentration basis was relatively constant in leaves of a single species, 1.5 mm s(-1) in sunflower and 1.8 mm s(-1) in aspen. The apparent K(0.5)(O(2)) values ranged from 0.33 to 0.67 microM in sunflower and from 0.33 to 1.1 microM in aspen, mainly because of the variation of the maximum rate, V(max) (leaf temperature 22 degrees C). The initial slope of the O(2) response of respiration characterizes the catalytic efficiency of terminal oxidases, an important parameter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria.

Determining the limitations and regulation of photosynthetic energy transduction in leaves

The light-dependent production of ATP and reductants by the photosynthetic apparatus in vivo involves a series of electron and proton transfers. Consideration is given as to how electron fluxes through photosystem I (PSI), using absorption spectroscopy, and through photosystem II (PSII), using chlorophyll fluorescence analyses, can be estimated in vivo. Measurements of light-induced electrochromic shifts using absorption spectroscopy provide a means of analyzing the proton fluxes across the thylakoid membranes in vivo. Regulation of these electron and proton fluxes is required for the thylakoids to meet the fluctuating metabolic demands of the cell. Chloroplasts exhibit a wide and flexible range of mechanisms to regulate electron and proton fluxes that enable chloroplasts to match light use for ATP and reductant production with the prevailing metabolic requirements. Non-invasive probing of electron fluxes through PSI and PSII, and proton fluxes across the thylakoid membranes can provide insights into the operation of such regulatory processes in vivo.

Experimental and mathematical approaches to modeling plant metabolic networks

To support their sessile and autotrophic lifestyle higher plants have evolved elaborate networks of metabolic pathways. Dynamic changes in these metabolic networks are among the developmental forces underlying the functional differentiation of organs, tissues and specialized cell types. They are also important in the various interactions of a plant with its environment. Further complexity is added by the extensive compartmentation of the various interconnected metabolic pathways in plants. Thus, although being used widely for assessing the control of metabolic flux in microbes, mathematical modeling approaches that require steady-state approximations are of limited utility for understanding complex plant metabolic networks. However, considerable progress has been made when manageable metabolic subsystems were studied. In this article, we will explain in general terms and using simple examples the concepts underlying stoichiometric modeling (metabolic flux analysis and metabolic pathway analysis) and kinetic approaches to modeling (including metabolic control analysis as a special case). Selected studies demonstrating the prospects of these approaches, or combinations of them, for understanding the control of flux through particular plant pathways are discussed. We argue that iterative cycles of (dry) mathematical modeling and (wet) laboratory testing will become increasingly important for simulating the distribution of flux in plant metabolic networks and deriving rational experimental designs for metabolic engineering efforts.

Light regulation of stomatal movement

Stomatal pores, each surrounded by a pair of guard cells, regulate CO2 uptake and water loss from leaves. Stomatal opening is driven by the accumulation of K+ salts and sugars in guard cells, which is mediated by electrogenic proton pumps in the plasma membrane and/or metabolic activity. Opening responses are achieved by coordination of light signaling, light-energy conversion, membrane ion transport, and metabolic activity in guard cells. In this review, we focus on recent progress in blue- and red-light-dependent stomatal opening. Because the blue-light response of stomata appears to be strongly affected by red light, we discuss underlying mechanisms in the interaction between blue-light signaling and guard cell chloroplasts.

Calibration of simultaneous measurements of photosynthetic carbon dioxide uptake and oxygen evolution in leaves

The stoichiometric ratio of O2 evolution to CO2 uptake during photosynthesis reveals information about reductive metabolism, including the reduction of alternative electron acceptors, such as nitrite and oxaloacetate. Recently we reported that in simultaneous measurements of CO2 uptake and O2 evolution in a sunflower leaf, O2 evolution changed by 7% more than CO2 uptake when light intensity was varied. Since the O2/CO2 exchange ratio is approximately 1, small differences are important. Thus, these gas exchange measurements need precise calibration. In this work, we describe a new calibration procedure for such simultaneous measurements, based on the changes of O2 concentration caused by the addition of pure CO2 or O2 into a flow of dry air (20.95% O2) through one and the same capillary. The relative decrease in O2 concentration during the addition of CO2 and the relative increase in O2 concentration during the addition of O2 allowed us to calibrate the CO2 and O2 scales of the measurement system with an error (relative standard deviation, RSD) of <1%. Measurements on a sunflower leaf resulted in an O2/CO2 ratio between 1.0 and 1.03 under different CO2 concentrations and light intensities, in the presence of an ambient O2 concentration of 20-50 micromol mol(-1). This shows that the percentage use of reductive power from photochemistry in synthesis of inorganic or organic matter other than CO2 assimilation in the C3 cycle is very low in mature leaves and, correspondingly, the reduction of alternative acceptors is a weak source of coupled ATP synthesis.

Identification of domains involved in tetramerization and malate inhibition of maize C4-NADP-malic enzyme

C(4) photosynthetic NADP-malic enzyme (ME) has evolved from non-C(4) isoforms and gained unique kinetic and structural properties during this process. To identify the domains responsible for the structural and kinetic differences between maize C(4) and non-C(4)-NADP-ME several chimeras between these isoforms were constructed and analyzed. By using this approach, we found that the region flanked by amino acid residues 102 and 247 is critical for the tetrameric state of C(4)-NADP-ME. In this way, the oligomerization strategy of these NADP-ME isoforms differs markedly from the one that present non-plant NADP-ME with known crystal structures. On the other hand, the region from residue 248 to the C-terminal end of the C(4) isoform is involved in the inhibition by high malate concentrations at pH 7.0. The inhibition pattern of the C(4)-NADP-ME and some of the chimeras suggested an allosteric site responsible for such behavior. This pH-dependent inhibition could be important for regulation of the C(4) isoform in vivo, with the enzyme presenting maximum activity while photosynthesis is in progress.

Photosystem II cycle and alternative electron flow in leaves

Sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.) were grown in the laboratory and leaves were taken from field-grown birch trees (Betula pendula Roth). Chlorophyll fluorescence, CO2 uptake and O2 evolution were measured and electron transport rates were calculated, J(C) from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O2 evolution rate, and J(F) from Chl fluorescence parameters. Mesophyll diffusion resistance, r(md), used for the calculation of J(C), was determined such that the in vivo Rubisco kinetic curve with respect to the carboxylation site CO2 concentration became a rectangular hyperbola with Km(CO2) of 10 microM at 22.5 degrees C. In sunflower, in the absence of external O2, J(O) = 1.07 J(C) when absorbed photon flux density (PAD) was varied, showing that the O2-independent components of the alternative electron flow to acceptors other than CO2 made up 7% of J(C). Under saturating light, J(F), however, was 20-30% faster than J(C), and J(F)-J(C) depended little on CO2 and O2 concentrations. The inter-relationship between J(F)-J(C) and non-photochemical quenching (NPQ) was variable, dependent on the CO2 concentration. We conclude that the relatively fast electron flow J(F)-J(C) appearing at light saturation of photosynthesis contains a minor component coupled with proton translocation, serving for nitrite, oxaloacetate and oxygen reduction, and a major component that is mostly cyclic electron transport around PSII. The rate of the PSII cycle is sufficient to release the excess excitation pressure on PSII significantly. Although the O2-dependent Mehler-type alternative electron flow appeared to be under the detection threshold, its importance is discussed considering the documented enhancement of photosynthesis by oxygen.

The polyphasic chlorophyll a fluorescence rise measured under high intensity of exciting light

Chlorophyll a fluorescence rise caused by illumination of photosynthetic samples by high intensity of exciting light, the O-J-I-P (O-I₁-I₂-P) transient, is reviewed here. First, basic information about chlorophyll a fluorescence is given, followed by a description of instrumental set-ups, nomenclature of the transient, and samples used for the measurements. The review mainly focuses on the explanation of particular steps of the transient based on experimental and theoretical results, published since a last review on chlorophyll a fluorescence induction [Lazár D (1999) Biochimica et Biophysica Acta 1412, 1-28]. In addition to 'old' concepts (e.g. changes in redox states of electron acceptors of photosystem II (PSII), effect of the donor side of PSII, fluorescence quenching by oxidised plastoquinone pool), 'new' approaches (e.g. electric voltage across thylakoid membranes, electron transport through the inactive branch in PSII, recombinations between PSII electron acceptors and donors, electron transport reactions after PSII, light gradient within the sample) are reviewed. The K-step, usually detected after a high-temperature stress, and other steps appearing in the transient (the H and G steps) are also discussed. Finally, some applications of the transient are also mentioned.

Antisense repression reveals a crucial role of the plastidic 2-oxoglutarate/malate translocator DiT1 at the interface between carbon and nitrogen metabolism

Ammonia assimilation by the plastidic glutamine synthetase/glutamate synthase system requires 2-oxoglutarate (2-OG) as a carbon precursor. Plastids depend on 2-OG import from the cytosol. A plastidic dicarboxylate translocator 1-[2-OG/malate translocator (DiT1)] has been identified and its substrate specificity and kinetic constants have been analyzed in vitro. However, the role of DiT1 in intact plants and its significance for ammonia assimilation remained uncertain. Here, to study the role of DiT1 in intact plants, its expression was antisense-repressed in transgenic tobacco plants. This resulted in a reduced transport capacity for 2-OG across the plastid envelope membrane. In consequence, allocation of carbon precursors to amino acid synthesis was impaired, organic acids accumulated and protein content, photosynthetic capacity and sugar pools in leaves were strongly decreased. The phenotype was consistent with a role of DIT1 in both, primary ammonia assimilation and the re-assimilation of ammonia resulting from the photorespiratory carbon cycle. Unexpectedly, the in situ rate of nitrate reduction was extremely low in alpha-DiT1 leaves, although nitrate reductase (NR) expression and activity remained high. We hypothesize that this discrepancy between extractable NR activity and in situ nitrate reduction is due to substrate limitation of NR. These findings and the severe phenotype of the antisense plants point to a crucial role of DiT1 at the interface between carbon and nitrogen metabolism.

Co-expression of cytochrome b561 and ascorbate oxidase in leaves of wild watermelon under drought and high light conditions

Despite carrying out C3 photosynthesis, wild watermelon (Citrullus lanatus sp.) exhibits exceedingly good tolerance to severe drought at high light intensities. However, the mechanism(s) by which this plant protects itself from photodamage has yet to be elucidated. In this study, we characterized wild watermelon cytochrome b561 (cyt b561), which potentially mediates regeneration of apoplastic ascorbate by transferring electrons from cytosolic ascorbate across the plasma membrane. Two cDNA species for wild watermelon cyt b561, designated CLb561A and CLb561B, were isolated. Levels of both CLb561A mRNA and protein were significantly elevated in the leaves during drought at a light intensity of 700 micromol photons m(-2) s(-1). The transcript of CLb561B was detected to a much lesser extent, but no CLb561B protein was produced under any condition used in this study. A transient expression assay with the CLb561A::green fluorescent protein fusion construct showed clear fluorescence on the plasma membrane of onion epidermal cells. The CLb561A protein was enriched in the plasma membrane fraction in leaves of transgenic tobacco expressing CLb561A. Moreover, the high activity of apoplastic ascorbate oxidase (AO), which was able to dispose of cyt b561-transferred reducing equivalents, increased in leaves of wild watermelon grown at high light intensity, but not lower light intensities. Taken together, these observations suggest the occurrence of a novel pathway for excess light energy dissipation in wild watermelon leaves, where excessive energy absorbed by chloroplasts can be transported to and dissipated safely in the apoplasts through the cooperative action of cyt b561 and AO.

A comprehensive analysis of the NADP-malic enzyme gene family of Arabidopsis

The Arabidopsis (Arabidopsis thaliana) genome contains four genes encoding putative NADP-malic enzymes (MEs; AtNADP-ME1-ME4). NADP-ME4 is localized to plastids, whereas the other three isoforms do not possess any predicted organellar targeting sequence and are therefore expected to be cytosolic. The plant NADP-MEs can be classified into four groups: groups I and II comprising cytosolic and plastidic isoforms from dicots, respectively; group III containing isoforms from monocots; and group IV composed of both monocots and dicots, including AtNADP-ME1. AtNADP-MEs contained all conserved motifs common to plant NADP-MEs and the recombinant isozymes showed different kinetic and structural properties. NADP-ME2 exhibits the highest specific activity, while NADP-ME3 and NADP-ME4 present the highest catalytic efficiency for NADP and malate, respectively. NADP-ME4 exists in equilibrium of active dimers and tetramers, while the cytosolic counterparts are present as hexamers or octamers. Characterization of T-DNA insertion mutant and promoter activity studies indicates that NADP-ME2 is responsible for the major part of NADP-ME activity in mature tissues of Arabidopsis. Whereas NADP-ME2 and -ME4 are constitutively expressed, the expression of NADP-ME1 and NADP-ME3 is restricted by both developmental and cell-specific signals. These isoforms may play specific roles at particular developmental stages of the plant rather than being involved in primary metabolism.

A theoretical study on effect of the initial redox state of cytochrome b559 on maximal chlorophyll fluorescence level (F(M)): implications for photoinhibition of photosystem II

In this work, we extended the reversible radical pair model which describes energy utilization and electron transfer up to the first quinone electron acceptor (Q(A)) in photosystem II (PSII), by redox reactions involving cytochrome (cyt) b559. In the model, cyt b559 accepts electrons from the reduced primary electron acceptor in PSII, pheophytin, and donates electrons to the oxidized primary electron donor in PSII (P680+). Theoretical simulations of chlorophyll fluorescence rise based on the model show that the maximal fluorescence, F(M), increases with an increasing amount of initially reduced cyt b559. In this work we applied, the first to our knowledge, metabolic control analysis (MCA) to a model of reactions in PSII. The MCA was used to determine to what extent the reactions occurring in the model control the F(M) level and how this control depends on the initial redox state of cyt b559. The simulations also revealed that increasing the amount of initially reduced cyt b559 could protect PSII against photoinhibition. Also experimental data, which might be used to validate our theory, are presented and discussed.

Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions

Plant photosynthesis performs the remarkable feat of converting light energy into usable chemical forms, which involves taming highly reactive intermediates without harming plant cells. This requires an apparatus that is not only efficient and robust but also flexible in its responses to changing environmental conditions. It also requires that the output of the energy-storing reactions be matched with the demands of metabolism. This article addresses the mechanisms by which this flexibility is achieved for short-term environmental changes. We argue that chloroplasts need two types of flexible mechanisms: one for modulating the output ratio of ATP:NADPH, which involves cyclic electron flux around photosystem I; and another for changing the regulatory sensitivity of the light-harvesting antenna to electron (and proton) flow.

Malate valves to balance cellular energy supply

In green parts of the plant, during illumination ATP and NAD(P)H act as energy sources that are generated mainly in photosynthesis and respiration, whereas in darkness, glycolysis, respiration and the oxidative pentose-phosphate pathway (OPP) generate the required energy forms. In non-green parts, sugar oxidation in glycolysis, respiration and OPP are the only means of producing energy. For energy-consuming reactions, the delivery of NADPH, NADH, reduced ferredoxin and ATP has to take place at the required rates and in the specific compartments, since the pool sizes of these energy carriers are rather limited and, in general, they are not directly transported across biomembranes. Indirect transport of reducing equivalents can be achieved by malateoxaloacetate shuttles, involving malate dehydrogenase (MDH) for the interconversion. Isoenzymes of MDH are present in each cellular compartment. Chloroplasts contain the redox-controlled NADP-MDH that is only active in the light. In addition, a plastid NAD-MDH that is permanently active and is present in all plastid types has been found. Export of excess NAD(P)H through the malate valves will allow for the continued production of ATP (1) in photosynthesis, and (2) in oxidative phosphorylation. In the latter case, the coupled production of NADH is catalysed by the bispecific NAD(P)-GAPDH (GapAB) in chloroplasts that is active with NAD even in darkness, or by the specific plastid NAD-GAPDH (GapCp) in non-green tissues. When plants are subjected to conditions such as high light, high CO(2), NH(4) (+) nutrition, cold stress, which require changed activities of the enzymes of the malate valves, changed expression levels of the MDH isoforms can be observed. In nodules, the induction of a nodule-specific plastid NAD-MDH indicates the changed requirements for energy supply during N(2) fixation. Furthermore, the induction of glucose 6-phosphate dehydrogenase isoforms by ammonium and of ferredoxin and ferredoxin-NADP reductase by nitrate has been described. All these findings are in line with the assumption that a changed redox state caused by metabolic variability leads to the induction of enzymes involved in redox poise.

The Arabidopsis mutant dct is deficient in the plastidic glutamate/malate translocator DiT2

The Arabidopsis mutant dicarboxylate transport (dct) is one of the classic mutants in the photorespiratory pathway. It requires high CO2 levels for survival. Physiologic and biochemical characterization of dct indicated that dct is deficient in the transport of dicarboxylates across the chloroplast envelope membrane. Hence, re-assimilation of ammonia generated by the photorespiratory cycle is blocked. However, the defective gene in dct has not been identified at the molecular level. Here, we report on the molecular characterization of the defective gene in dct, on the complementation of the mutant phenotype with a wild-type cDNA, and on the functional characterization of the gene product, DiT2, in a recombinant reconstituted system. Furthermore, we provide the kinetic constants of recombinant DiT1 and DiT2, and we discuss these data with respect to their functions in ammonia assimilation. Moreover, an analysis of the transcript levels of DiT1 and DiT2 in different C3- and C4-type plant species is presented, and we demonstrate that the substrate specificity of DiT2 from the C4-plant Flaveria bidentis is similar to its counterpart from C3 plants.

Redox control, redox signaling, and redox homeostasis in plant cells

Redox chemistry is a key feature of life. Oxidized substrates are reduced to synthesize functional molecules; reduced substrates are oxidized for energy supply. In addition, cells must fight against uncontrolled oxidation of essential constituents, a process that continuously occurs in an atmosphere of 21% O2. The redox situation is further complicated in plants with their highly reactive photosynthetic metabolism. To this end it is now well established that redox regulation is a central element in adjusting plant metabolism and development to the prevailing environmental conditions. This review introduces general redox chemistry and the main components of the cellular redox network, namely pyridine nucleotides, ascorbate, glutathione, lipoic acid, tocopherol, thioredoxins, glutaredoxins, peroxiredoxins, and other thiol proteins. Examples for redox sensing, transduction, redox-regulated enzymes and transcription, and the function of regulatory circuits are presented. Emphasis is placed on redox regulation of photosynthesis, which is the best understood metabolism governed by redox control on essentially all levels, ranging from gene transcription to translation, assembly and turnover, as well as short-term adaptation by state transition and enzyme activity. Increasing evidence shows the importance of redox regulation in the context of transport, plant development, and programmed cell death.

Mathematical modeling of plant metabolic pathways

The understanding of the control of metabolic flux in plants requires integrated mathematical formulations of gene and protein expression, enzyme kinetics, and developmental biology. Plants have a large number of metabolically active compartments, and non-steady-state conditions are frequently encountered. Consequently steady-state metabolic flux balance and isotopic flux balance modeling approaches have limited utility in probing plant metabolic systems. Transient isotopic flux analysis and kinetic modeling are powerful proven techniques for the quantification of metabolic fluxes in compartmentalized, dynamic metabolic systems. These tools are now widely used to address metabolic flux responses to environmental and genetic perturbations in plant metabolism. Continued developments in isotopic and kinetic modeling, quantifying metabolite exchange between compartments, and transcriptional and posttranscriptional regulatory mechanisms governing enzyme level and activity will enable simulation of large sections of plant metabolism under non-steady-state conditions. Metabolic control analysis will continue to make substantial contributions to the understanding of quantitative distribution of control of flux. From the synergy between mathematical models and experiments, creative methods for controlling the distribution of flux by genetic or environmental means will be discovered and rationally implemented.

High level expression of C4-specific NADP-malic enzyme in leaves and impairment of photoautotrophic growth in a C3 plant, rice

The chloroplastic NADP-malic enzyme (NADP-ME) is a key enzyme of the C4 photosynthesis pathway in NADP-ME type C4 plants such as maize. To express the chloroplastic NADP-ME in leaves of a C3 plant, rice, full-length cDNAs encoding the rice C3-specific isoform and the maize C4-specific isoform of the enzyme were expressed under the control of the rice CAB: promoter. Transformants carrying the rice cDNA showed the NADP-ME activities in the leaves less than several-fold that of non-transformants, while those carrying the maize cDNA showed activities up to 30-fold that of non-transformants or about 60% of the NADP-ME activity of maize leaves. These results indicate that expression of the rice C3-specific NADP-ME is suppressed at co- and/or post-transcriptional levels by some regulation mechanisms intrinsic to rice, while that of the foreign C4-specific isoform can escape from such suppression. The accumulation of the maize C4-specific NADP-ME led to bleaching of leaf color and growth hindrance in rice plants under natural light. These deteriorative effects resulted from enhanced photoinhibition of photosynthesis due to an increase in the level of NADPH inside the chloroplast by the action of the maize enzyme.