Mitochondrial malate dehydrogenase lowers leaf respiration and alters photorespiration and plant growth in Arabidopsis

The function of glycine decarboxylase complex is optimized to maintain high photorespiratory flux via buffering of its reaction products

Oxidation of glycine in photorespiratory pathway is the major flux through mitochondria of C3 plants in the light. It sustains increased intramitochondrial concentrations of NADH and NADPH, which are required to engage the internal rotenone-insensitive NAD(P)H dehydrogenases and the alternative oxidase. We discuss here possible mechanisms of high photorespiratory flux maintenance in mitochondria and suggest that it is fulfilled under conditions where the concentrations of glycine decarboxylase reaction products NADH and CO2 achieve an equilibrium provided by malate dehydrogenase and carbonic anhydrase, respectively. This results in the removal of these products from the glycine decarboxylase multienzyme active sites and in the maintenance of their concentrations at levels sufficiently low to prevent substrate inhibition of the reaction.

Multiplex micro-respiratory measurements of Arabidopsis tissues

Researchers often want to study the respiratory properties of individual parts of plants in response to a range of treatments. Arabidopsis is an obvious model for this work; however, because of its size, it represents a challenge for gas exchange measurements of respiration. The combination of micro-respiratory technologies with multiplex assays has the potential to bridge this gap, and make measurements possible in this model plant species. We show the adaptation of the commercial technology used for mammalian cell respiration analysis to study three critical tissues of interest: leaf sections, root tips and seeds. The measurement of respiration in single leaf discs has allowed the age dependence of the respiration rate in Arabidopsis leaves across the rosette to be observed. The oxygen consumption of single root tips from plate-grown seedlings shows the enhanced respiration of root tips and their time-dependent susceptibility to salinity. The monitoring of single Arabidopsis seeds shows the kinetics of respiration over 48 h post-imbibition, and the effect of the phytohormones gibberellic acid (GA3 ) and abscisic acid (ABA) on respiration during seed germination. These studies highlight the potential for multiplexed micro-respiratory assays to study oxygen consumption in Arabidopsis tissues, and open up new possibilities to screen and study mutants and to identify differences in ecotypes or populations of different plant species.

The inter-relationship of ascorbate transport, metabolism and mitochondrial, plastidic respiration

SIGNIFICANCE

Ascorbate, this multifaceted small molecular weight carbohydrate derivative, plays important roles in a range of cellular processes in plant cells, from the regulation of cell cycle, through cell expansion and senescence. Beyond these physiological functions, ascorbate has a critical role in responses to abiotic stresses, such as high light, high salinity, or drought. The biosynthesis, recycling, and intracellular transport are important elements of the balancing of ascorbate level to the always-changing conditions and demands.

RECENT ADVANCES

A bidirectional tight relationship was described between ascorbate biosynthesis and the mitochondrial electron transfer chain (mETC), since L-galactono-1,4-lactone dehydrogenase (GLDH), the enzyme catalyzing the ultimate step of ascorbate biosynthesis, uses oxidized cytochrome c as the only electron acceptor and has a role in the assembly of Complex I. A similar bidirectional relationship was revealed between the photosynthetic apparatus and ascorbate biosynthesis since the electron flux through the photosynthetic ETC affects the biosynthesis of ascorbate and the level of ascorbate could affect photosynthesis.

CRITICAL ISSUES

The details of this regulatory network of photosynthetic electron transfer, respiratory electron transfer, and ascorbate biosynthesis are still not clear, as are the potential regulatory role and the regulation of intracellular ascorbate transport and fluxes.

FUTURE DIRECTIONS

The elucidation of the role of ascorbate as an important element of the network of photosynthetic, respiratory ETC and tricarboxylic acid cycle will contribute to understanding plant cell responses to different stress conditions.

Investigating the role of respiration in plant salinity tolerance by analyzing mitochondrial proteomes from wheat and a salinity-tolerant Amphiploid (wheat × Lophopyrum elongatum)

The effect of salinity on mitochondrial properties was investigated by comparing the reference wheat variety Chinese Spring (CS) to a salt-tolerant amphiploid (AMP). The octoploid AMP genotype was previously generated by combining hexaploid bread wheat (CS) with the diploid wild wheatgrass adapted to salt marshes, Lophopyrum elongatum. Here we used a combination of physiological, biochemical, and proteomic analyses to explore the mitochondrial and respiratory response to salinity in these two genotypes. The AMP showed greater growth tolerance to salinity treatments and altered respiration rate in both roots and shoots. A proteomic workflow of 2D-DIGE and MALDI TOF/TOF mass spectrometry was used to compare the protein composition of isolated mitochondrial samples from roots and shoots of both genotypes, following control or salt treatment. A large set of mitochondrial proteins were identified as responsive to salinity in both genotypes, notably enzymes involved in detoxification of reactive oxygen species. Genotypic differences in mitochondrial composition were also identified, with AMP exhibiting a higher abundance of manganese superoxide dismutase, serine hydroxymethyltransferase, aconitase, malate dehydrogenase, and β-cyanoalanine synthase compared to CS. We present peptide fragmentation spectra derived from some of these AMP-specific protein spots, which could serve as biomarkers to track superior protein variants.

Permeability of the peroxisomal membrane: lessons from the glyoxylate cycle

Glyoxylate serves as intermediate in various metabolic pathways, although high concentrations of this metabolite are toxic to the cell. In many organisms glyoxylate is fed into the glyoxylate cycle. Enzymes participating in this metabolism are located on both sides of the peroxisomal membrane. The permeability of this membrane for small metabolites paves the way for exchange of intermediates between proteins catalyzing consecutive reactions. A model, in which soluble enzymes accumulate in close proximity to both ends of pore-like structures forming a transmembrane metabolon could explain the rapid and targeted exchange of intermediates. The metabolites passing the membrane differ between the three model organisms Saccharomyces cerevisiae, Arabidopsis thaliana, and Candida albicans, which reflects the ease of evolutionary adaptation processes whenever specific transporter proteins are not involved. The atypical permeability properties of the peroxisomal membrane together with a flexible structural arrangement ensuring the swift and selective transport across the membrane might represent the molecular basis for the functional versatility of peroxisomes.

The anti-cancer property of proteins extracted from Gynura procumbens (Lour.) Merr

Gynura procumbens (Lour.) Merr. belongs to the Asteraceae Family. The plant is a well-known traditional herb in South East Asia and it is widely used to treat inflammation, kidney discomfort, high cholesterol level, diabetic, cancer and high blood pressure. Our earlier study showed the presence of valuable plant defense proteins, such as peroxidase, thaumatin-like proteins and miraculin in the leaf of G. procumbens. However, the effects of these defense proteins on cancers have never been determined previously. In the present study, we investigated the bioactivity of gel filtration fractionated proteins of G. procumbens leaf extract. The active protein fraction, SN-F11/12, was found to inhibit the growth of a breast cancer cell line, MDA-MB-231, at an EC50 value of 3.8 µg/mL. The mRNA expressions of proliferation markers, Ki67 and PCNA, were reduced significantly in the MDA-MB-23 cells treated with SN-F11/12. The expression of invasion marker, CCL2, was also found reduced in the treated MDA-MB-231 cells. All these findings highlight the anti-cancer property of SN-F11/12, therefore, the proteins in this fraction can be a potential chemotherapeutic agent for breast cancer treatment.

Towards closing the remaining gaps in photorespiration--the essential but unexplored role of transport proteins

Photorespiration is an essential prerequisite for all autotrophic organisms performing oxygenic photosynthesis. In contrast to the well-characterised enzymes accomplishing photorespiratory metabolism, current knowledge on the involved transport processes and the respective proteins is still quite limited. In this review, we focus on the status quo of translocators involved in photorespiratory metabolism. Although the transport of some of the photorespiratory intermediates could be characterised biochemically, using isolated organelles, the genes encoding these transporters have to date not been identified in most cases. Here, we describe the postulated transport processes, present information on established or hypothetical photorespiratory transporters, depict strategies on how to identify the transport proteins on the molecular level and, finally, discuss strategies for how to find the remaining candidates.

Sulphur limitation provokes physiological and leaf proteome changes in oilseed rape that lead to perturbation of sulphur, carbon and oxidative metabolisms

BACKGROUND

The decline in industrial emissions of sulphur (S) has led to a sulphate depletion in soil resulting in an alteration of crop performance. In oilseed rape, an S deficiency dramatically reduced the seed yield and/or quality. Paradoxically, little is known about the impact of sulphate limitation on oilseed rape leaf metabolism, despite it being a key determinant of growth. In order to identify the metabolic processes involved in the oilseed rape response to S restriction, an analysis of the young leaf proteome combined with a physiological study was carried out at the vegetative stage.

RESULTS

S limitation does not significantly reduce the total shoot biomass but inhibits growth and photosynthesis of young leaves. This photosynthesis decline is not due to a decrease in chlorophyll content, which remains similar to Control. The increase in anthocyanins and H(2)O(2) content in young leaves of S-limited plants suggests that S restriction leads to an oxidative stress. Proteomic analysis at 35 d of S limitation also revealed the induction of 12-oxophitodienoate reductase and ACC synthase, respectively involved in jasmonate and ethylene biosynthesis, two phytohormones that could be implicated in oxidative stress. Proteins involved in photosynthesis and carbon metabolism were also modulated by S restriction. In particular, the decrease in plastocyanin and ferredoxin-NADP reductase suggests that H(2)O(2) accumulation is associated with perturbation of the photosynthetic electron transport chain. The accumulation of chloroplastic Cu-Zn SOD reinforces the idea that an oxidative stress probably occurs in the chloroplast. Proteomic results suggest that the maintenance of chlorophyll in S-limited conditions is related to an accumulation of Water Soluble Chlorophyll binding Proteins, involved in the protection of chlorophyll against ROS. The accumulation of the catalytic α-subunit of chloroplastic ATP synthase suggests that energy production is maintained.

CONCLUSION

S limitation leads to photosynthesis and carbon metabolism disturbances that could be responsible for the oxidative stress observed in the young leaves of oilseed rape. Despite this, induction of proteins involved in oxidative stress resistance and energy production shows that the leaf capacity to capture and use photosynthetic active radiations for ATP production remains efficient for as long as possible.

Recent advances in the composition and heterogeneity of the Arabidopsis mitochondrial proteome

Mitochondria are important organelles for providing the ATP and carbon skeletons required to sustain cell growth. While these organelles also participate in other key metabolic functions across species, they have a specialized role in plants of optimizing photosynthesis through participating in photorespiration. It is therefore critical to map the protein composition of mitochondria in plants to gain a better understanding of their regulation and define the uniqueness of their metabolic networks. To date, <30% of the predicted number of mitochondrial proteins has been verified experimentally by proteomics and/or GFP localization studies. In this mini-review, we will provide an overview of the advances in mitochondrial proteomics in the model plant Arabidopsis thaliana over the past 5 years. The ultimate goal of mapping the mitochondrial proteome in Arabidopsis is to discover novel mitochondrial components that are critical during development in plants as well as genes involved in developmental abnormalities, such as those implicated in mitochondrial-linked cytoplasmic male sterility.

Decreasing the mitochondrial synthesis of malate in potato tubers does not affect plastidial starch synthesis, suggesting that the physiological regulation of ADPglucose pyrophosphorylase is context dependent

Modulation of the malate content of tomato (Solanum lycopersicum) fruit by altering the expression of mitochondrially localized enzymes of the tricarboxylic acid cycle resulted in enhanced transitory starch accumulation and subsequent effects on postharvest fruit physiology. In this study, we assessed whether such a manipulation would similarly affect starch biosynthesis in an organ that displays a linear, as opposed to a transient, kinetic of starch accumulation. For this purpose, we used RNA interference to down-regulate the expression of fumarase in potato (Solanum tuberosum) under the control of the tuber-specific B33 promoter. Despite displaying similar reductions in both fumarase activity and malate content as observed in tomato fruit expressing the same construct, the resultant transformants were neither characterized by an increased flux to, or accumulation of, starch, nor by alteration in yield parameters. Since the effect in tomato was mechanistically linked to derepression of the reaction catalyzed by ADP-glucose pyrophosphorylase, we evaluated whether the lack of effect on starch biosynthesis was due to differences in enzymatic properties of the enzyme from potato and tomato or rather due to differential subcellular compartmentation of reductant in the different organs. The results are discussed in the context both of current models of metabolic compartmentation and engineering.

Searching for resistance genes to Bursaphelenchus xylophilus using high throughput screening

BACKGROUND

Pine wilt disease (PWD), caused by the pinewood nematode (PWN; Bursaphelenchus xylophilus), damages and kills pine trees and is causing serious economic damage worldwide. Although the ecological mechanism of infestation is well described, the plant's molecular response to the pathogen is not well known. This is due mainly to the lack of genomic information and the complexity of the disease. High throughput sequencing is now an efficient approach for detecting the expression of genes in non-model organisms, thus providing valuable information in spite of the lack of the genome sequence. In an attempt to unravel genes potentially involved in the pine defense against the pathogen, we hereby report the high throughput comparative sequence analysis of infested and non-infested stems of Pinus pinaster (very susceptible to PWN) and Pinus pinea (less susceptible to PWN).

RESULTS

Four cDNA libraries from infested and non-infested stems of P. pinaster and P. pinea were sequenced in a full 454 GS FLX run, producing a total of 2,083,698 reads. The putative amino acid sequences encoded by the assembled transcripts were annotated according to Gene Ontology, to assign Pinus contigs into Biological Processes, Cellular Components and Molecular Functions categories. Most of the annotated transcripts corresponded to Picea genes-25.4-39.7%, whereas a smaller percentage, matched Pinus genes, 1.8-12.8%, probably a consequence of more public genomic information available for Picea than for Pinus. The comparative transcriptome analysis showed that when P. pinaster was infested with PWN, the genes malate dehydrogenase, ABA, water deficit stress related genes and PAR1 were highly expressed, while in PWN-infested P. pinea, the highly expressed genes were ricin B-related lectin, and genes belonging to the SNARE and high mobility group families. Quantitative PCR experiments confirmed the differential gene expression between the two pine species.

CONCLUSIONS

Defense-related genes triggered by nematode infestation were detected in both P. pinaster and P. pinea transcriptomes utilizing 454 pyrosequencing technology. P. pinaster showed higher abundance of genes related to transcriptional regulation, terpenoid secondary metabolism (including some with nematicidal activity) and pathogen attack. P. pinea showed higher abundance of genes related to oxidative stress and higher levels of expression in general of stress responsive genes. This study provides essential information about the molecular defense mechanisms utilized by P. pinaster and P. pinea against PWN infestation and contributes to a better understanding of PWD.

Modulation of genes related to specific metabolic pathways in response to cytosolic ascorbate peroxidase knockdown in rice plants

As a central component of the hydrogen peroxide detoxifying system in plant cells, ascorbate peroxidases (APX) play an essential role in the control of intracellular reactive oxygen species (ROS) levels. To characterise the function of cytosolic APX isoforms (OsAPX1 and OsAPX2) in the mechanisms of plant defence, OsAPX1/2 knockdown rice plants were previously obtained. OsAPX1/2 knockdown plants (APx1/2s) exhibited a normal phenotype and development, even though they showed a global reduction of APX activity and increased hydrogen peroxide accumulation. To understand how rice plants compensate for the deficiency of cytosolic APX, expression and proteomic analyses were performed to characterise the global expression pattern of the APx1/2s mutant line compared with non-transformed plants. Our results strongly suggest that deficiencies in cytosolic APX isoforms markedly alter expression of genes associated with several key metabolic pathways, especially of genes involved in photosynthesis and antioxidant defence. These metabolic changes are compensatory because central physiological processes such as photosynthesis and growth were similar to non-transformed rice plants. Our analyses showed modulation of groups of genes and proteins related to specific metabolic pathways. Among the differentially expressed genes, the largest number corresponded to those with catalytic activity. Genes related to oxidative stress, carbohydrate metabolism, photosynthesis and transcription factor-encoding genes were also modulated. These results represent an important step toward understanding of the role played by cytosolic APX isoforms and hydrogen peroxide in the regulation of metabolism by redox modulation in monocots.

The role of mitochondria in leaf nitrogen metabolism

For optimal plant growth and development, cellular nitrogen (N) metabolism must be closely coordinated with other metabolic pathways, and mitochondria are thought to play a central role in this process. Recent studies using genetically modified plants have provided insight into the role of mitochondria in N metabolism. Mitochondrial metabolism is linked with N assimilation by amino acid, carbon (C) and redox metabolism. Mitochondria are not only an important source of C skeletons for N incorporation, they also produce other necessary metabolites and energy used in N remobilization processes. Nitric oxide of mitochondrial origin regulates respiration and influences primary N metabolism. Here, we discuss the changes in mitochondrial metabolism during ammonium or nitrate nutrition and under low N conditions. We also describe the involvement of mitochondria in the redistribution of N during senescence. The aim of this review was to demonstrate the role of mitochondria as an integration point of N cellular metabolism.

Characterization of a NADH-dependent glutamate dehydrogenase mutant of Arabidopsis demonstrates the key role of this enzyme in root carbon and nitrogen metabolism

The role of NADH-dependent glutamate dehydrogenase (GDH) was investigated by studying the physiological impact of a complete lack of enzyme activity in an Arabidopsis thaliana plant deficient in three genes encoding the enzyme. This study was conducted following the discovery that a third GDH gene is expressed in the mitochondria of the root companion cells, where all three active GDH enzyme proteins were shown to be present. A gdh1-2-3 triple mutant was constructed and exhibited major differences from the wild type in gene transcription and metabolite concentrations, and these differences appeared to originate in the roots. By placing the gdh triple mutant under continuous darkness for several days and comparing it to the wild type, the evidence strongly suggested that the main physiological function of NADH-GDH is to provide 2-oxoglutarate for the tricarboxylic acid cycle. The differences in key metabolites of the tricarboxylic acid cycle in the triple mutant versus the wild type indicated that, through metabolic processes operating mainly in roots, there was a strong impact on amino acid accumulation, in particular alanine, γ-aminobutyrate, and aspartate in both roots and leaves. These results are discussed in relation to the possible signaling and physiological functions of the enzyme at the interface of carbon and nitrogen metabolism.

Respiratory carbon fluxes in leaves

Leaf respiration is a major metabolic process that drives energy production and growth. Earlier works in this field were focused on the measurement of respiration rates in relation to carbohydrate content, photosynthesis, enzymatic activities or nitrogen content. Recently, several studies have shed light on the mechanisms describing the regulation of respiration in the light and in the dark and on associated metabolic flux patterns. This review will highlight advances made into characterizing respiratory fluxes and provide a discussion of metabolic respiration dynamics in relation to important biological functions.

REDOX regulation of mitochondrial function in plants

Mitochondrial components dynamically change in response to environmental and developmental cues. However, the regulatory pathways that underlie these changes are largely unknown. A global analysis of changes in mitochondrial components at the transcript, protein and metabolite levels was undertaken, to gain a greater insight into how mitochondrial functions are regulated and respond to various internal or external cues. At the transcript level, large-scale changes in groups of genes suggest the presence of co-regulatory mechanisms for these components. Furthermore, the pathways that regulate these changes appear to be integrated into regulatory pathways that alter a variety of functions in cells. However, the changes in transcripts are not always observed at the protein or the metabolite level. This is likely to be due to post-transcriptional levels of regulation and also the fact that in-depth profiles, which have been obtained for transcripts from a variety of studies, are currently not available for proteins and metabolites. Thus, while transcripts for genes give us a picture of what the cells are 'thinking' in relation to mitochondrial components, some of these responses may be lost in translation.

Multiple strategies to prevent oxidative stress in Arabidopsis plants lacking the malate valve enzyme NADP-malate dehydrogenase

The nuclear-encoded chloroplast NADP-dependent malate dehydrogenase (NADP-MDH) is a key enzyme controlling the malate valve, to allow the indirect export of reducing equivalents. Arabidopsis thaliana (L.) Heynh. T-DNA insertion mutants of NADP-MDH were used to assess the role of the light-activated NADP-MDH in a typical C(3) plant. Surprisingly, even when exposed to high-light conditions in short days, nadp-mdh knockout mutants were phenotypically indistinguishable from the wild type. The photosynthetic performance and typical antioxidative systems, such as the Beck-Halliwell-Asada pathway, were barely affected in the mutants in response to high-light treatment. The reactive oxygen species levels remained low, indicating the apparent absence of oxidative stress, in the mutants. Further analysis revealed a novel combination of compensatory mechanisms in order to maintain redox homeostasis in the nadp-mdh plants under high-light conditions, particularly an increase in the NTRC/2-Cys peroxiredoxin (Prx) system in chloroplasts. There were indications of adjustments in extra-chloroplastic components of photorespiration and proline levels, which all could dissipate excess reducing equivalents, sustain photosynthesis, and prevent photoinhibition in nadp-mdh knockout plants. Such metabolic flexibility suggests that the malate valve acts in concert with other NADPH-consuming reactions to maintain a balanced redox state during photosynthesis under high-light stress in wild-type plants.

Functional genomics tools applied to plant metabolism: a survey on plant respiration, its connections and the annotation of complex gene functions

The application of post-genomic techniques in plant respiration studies has greatly improved our ability to assign functions to gene products. In addition it has also revealed previously unappreciated interactions between distal elements of metabolism. Such results have reinforced the need to consider plant respiratory metabolism as part of a complex network and making sense of such interactions will ultimately require the construction of predictive and mechanistic models. Transcriptomics, proteomics, metabolomics, and the quantification of metabolic flux will be of great value in creating such models both by facilitating the annotation of complex gene function, determining their structure and by furnishing the quantitative data required to test them. In this review, we highlight how these experimental approaches have contributed to our current understanding of plant respiratory metabolism and its interplay with associated process (e.g., photosynthesis, photorespiration, and nitrogen metabolism). We also discuss how data from these techniques may be integrated, with the ultimate aim of identifying mechanisms that control and regulate plant respiration and discovering novel gene functions with potential biotechnological implications.

Metabolic control and regulation of the tricarboxylic acid cycle in photosynthetic and heterotrophic plant tissues

The tricarboxylic acid (TCA) cycle is a crucial component of respiratory metabolism in both photosynthetic and heterotrophic plant organs. All of the major genes of the tomato TCA cycle have been cloned recently, allowing the generation of a suite of transgenic plants in which the majority of the enzymes in the pathway are progressively decreased. Investigations of these plants have provided an almost complete view of the distribution of control in this important pathway. Our studies suggest that citrate synthase, aconitase, isocitrate dehydrogenase, succinyl CoA ligase, succinate dehydrogenase, fumarase and malate dehydrogenase have control coefficients flux for respiration of -0.4, 0.964, -0.123, 0.0008, 0.289, 0.601 and 1.76, respectively; while 2-oxoglutarate dehydrogenase is estimated to have a control coefficient of 0.786 in potato tubers. These results thus indicate that the control of this pathway is distributed among malate dehydrogenase, aconitase, fumarase, succinate dehydrogenase and 2-oxoglutarate dehydrogenase. The unusual distribution of control estimated here is consistent with specific non-cyclic flux mode and cytosolic bypasses that operate in illuminated leaves. These observations are discussed in the context of known regulatory properties of the enzymes and some illustrative examples of how the pathway responds to environmental change are given.

An engineered pathway for glyoxylate metabolism in tobacco plants aimed to avoid the release of ammonia in photorespiration

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.

Single cystathionine β-synthase domain-containing proteins modulate development by regulating the thioredoxin system in Arabidopsis

Plant thioredoxins (Trxs) participate in two redox systems found in different cellular compartments: the NADP-Trx system (NTS) in the cytosol and mitochondria and the ferredoxin-Trx system (FTS) in the chloroplast, where they function as redox regulators by regulating the activity of various target enzymes. The identities of the master regulators that maintain cellular homeostasis and modulate timed development through redox regulating systems have remained completely unknown. Here, we show that proteins consisting of a single cystathionine β-synthase (CBS) domain pair stabilize cellular redox homeostasis and modulate plant development via regulation of Trx systems by sensing changes in adenosine-containing ligands. We identified two CBS domain-containing proteins in Arabidopsis thaliana, CBSX1 and CBSX2, which are localized to the chloroplast, where they activate all four Trxs in the FTS. CBSX3 was found to regulate mitochondrial Trx members in the NTS. CBSX1 directly regulates Trxs and thereby controls H(2)O(2) levels and regulates lignin polymerization in the anther endothecium. It also affects plant growth by regulating photosynthesis-related [corrected] enzymes, such as malate dehydrogenase, via homeostatic regulation of Trxs. Based on our findings, we suggest that the CBSX proteins (or a CBS pair) are ubiquitous redox regulators that regulate Trxs in the FTS and NTS to modulate development and maintain homeostasis under conditions that are threatening to the cell.

Fumarate: Multiple functions of a simple metabolite

Although much is now known about fumarate metabolism, our knowledge of some aspects of its biological function remain far from comprehensive. In this short review we begin with an introductory overview of the role of fumarate in both plant and non-plant systems. We next highlight the relative importance of fumarate in relation to cell type and circumstance in contrast to other chemically similar organic acids. Considerable cumulative evidence is suggestive of a role for fumarate in pH regulation during nitrate assimilation and that fumarate has similar effects as malate during stomatal movement. Indeed it is currently difficult to separate the biological function of fumarate from malate under certain circumstances. However, in other cases this can be easily performed. This physiological complexity notwithstanding it remains possible that the engineering of fumarate metabolism may provide opportunities to improve plant growth and performance.

Energy efficiency and energy homeostasis as genetic and epigenetic components of plant performance and crop productivity

The importance of energy metabolism in plant performance and plant productivity is conceptually well recognized. In the eighties, several independent studies in Lolium perenne (ryegrass), Zea mays (maize), and Festuca arundinacea (tall fescue) correlated low respiration rates with high yields. Similar reports in the nineties largely confirmed this correlation in Solanum lycopersicum (tomato) and Cucumis sativus (cucumber). However, selection for reduced respiration does not always result in high-yielding cultivars. Indeed, the ratio between energy content and respiration, defined here as energy efficiency, rather than respiration on its own, has a major impact on the yield potential of a crop. Besides energy efficiency, energy homeostasis, representing the balance between energy production and consumption in a changing environment, also contributes to an enhanced plant performance and this happens mainly through an increased stress tolerance. Although a few single gene approaches look promising, probably whole interacting networks have to be modulated, as is done by classical breeding, to improve the energy status of plants. Recent developments show that both energy efficiency and energy homeostasis have an epigenetic component that can be directed and stabilized by artificial selection (i.e. selective breeding). This novel approach offers new opportunities to improve yield potential and stress tolerance in a wide variety of crops.

The role of proteins in C(3) plants prior to their recruitment into the C(4) pathway

Our most productive crops and native vegetation use a modified version of photosynthesis known as the C(4) pathway. Leaves of C(4) crops have increased nitrogen and water use efficiencies compared with C(3) species. Although the modifications to leaves of C(4) plants are complex, their faster growth led to the proposal that C(4) photosynthesis should be installed in C(3) crops in order to increase yield potential. Typically, a limited set of proteins become restricted to mesophyll or bundle sheath cells, and this allows CO(2) to be concentrated around the primary carboxylase RuBisCO. The role that these proteins play in C(3) species prior to their recruitment into the C(4) pathway is addressed here. Understanding the role of these proteins in C(3) plants is likely to be of use in predicting how the metabolism of a C(3) leaf will alter as components of the C(4) pathway are introduced as part of efforts to install characteristics of C(4) photosynthesis in leaves of C(3) crops.

Resolving the compartmentation and function of C4 photosynthesis in the single-cell C4 species Bienertia sinuspersici

Bienertia sinuspersici is a land plant known to perform C(4) photosynthesis through the location of dimorphic chloroplasts in separate cytoplasmic domains within a single photosynthetic cell. A protocol was developed with isolated protoplasts to obtain peripheral chloroplasts (P-CP), a central compartment (CC), and chloroplasts from the CC (C-CP) to study the subcellular localization of photosynthetic functions. Analyses of these preparations established intracellular compartmentation of processes to support a NAD-malic enzyme (ME)-type C(4) cycle. Western-blot analyses indicated that the CC has Rubisco from the C(3) cycle, the C(4) decarboxylase NAD-ME, a mitochondrial isoform of aspartate aminotransferase, and photorespiratory markers, while the C-CP and P-CP have high levels of Rubisco and pyruvate, Pidikinase, respectively. Other enzymes for supporting a NAD-ME cycle via an aspartate-alanine shuttle, carbonic anhydrase, phosophoenolpyruvate carboxylase, alanine, and an isoform of aspartate aminotransferase are localized in the cytosol. Functional characterization by photosynthetic oxygen evolution revealed that only the C-CP have a fully operational C(3) cycle, while both chloroplast types have the capacity to photoreduce 3-phosphoglycerate. The P-CP were enriched in a putative pyruvate transporter and showed light-dependent conversion of pyruvate to phosphoenolpyruvate. There is a larger investment in chloroplasts in the central domain than in the peripheral domain (6-fold more chloroplasts and 4-fold more chlorophyll). The implications of this uneven distribution for the energetics of the C(4) and C(3) cycles are discussed. The results indicate that peripheral and central compartment chloroplasts in the single-cell C(4) species B. sinuspersici function analogous to mesophyll and bundle sheath chloroplasts of Kranz-type C(4) species.

Plant D-2-hydroxyglutarate dehydrogenase participates in the catabolism of lysine especially during senescence

D-2-Hydroxyglutarate dehydrogenase (D-2HGDH) catalyzes the specific and efficient oxidation of D-2-hydroxyglutarate (D-2HG) to 2-oxoglutarate using FAD as a cofactor. In this work, we demonstrate that D-2HGDH localizes to plant mitochondria and that its expression increases gradually during developmental and dark-induced senescence in Arabidopsis thaliana, indicating an enhanced demand of respiration of alternative substrates through this enzymatic system under these conditions. Using loss-of-function mutants in D-2HGDH (d2hgdh1) and stable isotope dilution LC-MS/MS, we found that the D-isomer of 2HG accumulated in leaves of d2hgdh1 during both forms of carbon starvation. In addition to this, d2hgdh1 presented enhanced levels of most TCA cycle intermediates and free amino acids. In contrast to the deleterious effects caused by a deficiency in D-2HGDH in humans, d2hgdh1 and overexpressing lines of D-2HGDH showed normal developmental and senescence phenotypes, indicating a mild role of D-2HGDH in the tested conditions. Moreover, metabolic fingerprinting of leaves of plants grown in media supplemented with putative precursors indicated that D-2HG most probably originates during the catabolism of lysine. Finally, the L-isomer of 2HG was also detected in leaf extracts, indicating that both chiral forms of 2HG participate in plant metabolism.