Activation of the potato tuber ADP-glucose pyrophosphorylase by thioredoxin

A critical inter-subunit interaction for the transmission of the allosteric signal in the Agrobacterium tumefaciens ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase is a key regulatory enzyme involved in starch and glycogen synthesis in plants and bacteria, respectively. It has been hypothesized that inter-subunit communications are important for the allosteric effect in this enzyme. However, no specific interactions have been identified as part of the regulatory signal. The enzyme from Agrobacterium tumefaciens is a homotetramer allosterically regulated by fructose 6-phosphate and pyruvate. Three pairs of distinct subunit-subunit interfaces are present. Here we focus on an interface that features two symmetrical interactions between Arg11 and Asp141 from one subunit with residues Asp141 and Arg11 of the neighbor subunit, respectively. Previously, scanning mutagenesis showed that a mutation at the Arg11 position disrupted the activation of the enzyme. Considering the distance of these residues from the allosteric and catalytic sites, we hypothesized that the interaction between Arg11 and Asp141 is critical for allosteric signaling rather than effector binding. To prove our hypothesis, we mutated those two sites (D141A, D141E, D141N, D141R, R11D, and R11K) and performed kinetic and binding analysis. Mutations that altered the charge affected the regulation the most. To prove that the interaction per se (rather than the presence of specific residues) is critical, we partially rescued the R11D protein by introducing a second mutation (R11D/D141R). This could not restore the activator effect on kcat , but it did rescue the effect on substrate affinity. Our results indicate the critical functional role of Arg11 and Asp141 to relay the allosteric signal in this subunit interface.

Chlamydomonas mutants lacking chloroplast TRIOSE PHOSPHATE TRANSPORTER3 are metabolically compromised and light-sensitive

Modulation of photoassimilate export from the chloroplast is essential for controlling the distribution of fixed carbon in the cell and maintaining optimum photosynthetic rates. In this study we identified chloroplast TRIOSE PHOSPHATE/PHOSPHATE TRANSLOCATOR2 (CreTPT2) and CreTPT3 in the green alga Chlamydomonas (Chlamydomonas reinhardtii), which exhibit similar substrate specificities but whose encoding genes are differentially expressed over the diurnal cycle. We focused mostly on CreTPT3 because of its high level of expression and the severe phenotype exhibited by tpt3 relative to tpt2 mutants. Null mutants for CreTPT3 had a pleiotropic phenotype that affected growth, photosynthetic activities, metabolite profiles, carbon partitioning, and organelle-specific accumulation of H2O2. These analyses demonstrated that CreTPT3 is a dominant conduit on the chloroplast envelope for the transport of photoassimilates. In addition, CreTPT3 can serve as a safety valve that moves excess reductant out of the chloroplast and appears to be essential for preventing cells from experiencing oxidative stress and accumulating reactive oxygen species, even under low/moderate light intensities. Finally, our studies indicate subfunctionalization of the CreTPT transporters and suggest that there are differences in managing the export of photoassimilates from the chloroplasts of Chlamydomonas and vascular plants.

Comprehensive Expression Analyses of Plastidial Thioredoxins of Arabidopsis thaliana Indicate a Main Role of Thioredoxin m2 in Roots

Thioredoxins (TRXs) f and m are redox proteins that regulate key chloroplast processes. The existence of several isoforms of TRXs f and m indicates that these redox players have followed a specialization process throughout evolution. Current research efforts are focused on discerning the signalling role of the different TRX types and their isoforms in chloroplasts. Nonetheless, little is known about their function in non-photosynthetic plastids. For this purpose, we have carried out comprehensive expression analyses by using Arabidopsis thaliana TRXf (f1 and f2) and TRXm (m1, m2, m3 and m4) genes translationally fused to the green fluorescence protein (GFP). These analyses showed that TRX m has different localisation patterns inside chloroplasts, together with a putative dual subcellular localisation of TRX f1. Apart from mesophyll cells, these TRXs were also observed in reproductive organs, stomatal guard cells and roots. We also investigated whether photosynthesis, stomatal density and aperture or root structure were affected in the TRXs f and m loss-of-function Arabidopsis mutants. Remarkably, we immunodetected TRX m2 and the Calvin−Benson cycle fructose-1,6-bisphosphatase (cFBP1) in roots. After carrying out in vitro redox activation assays of cFBP1 by plastid TRXs, we propose that cFBP1 might be activated by TRX m2 in root plastids.

Starch metabolism in potato <i>Solanum tuberosum</i> L

Starch is a major storage carbohydrate in plants. It is an important source of calories in the human and animal diet. Also, it is widely used in various industries. Native starch consists of water-insoluble semicrystalline granules formed by natural glucose polymers amylose and amylopectin. The physicochemical properties of starch are determined by the amylose:amylopectin ratio in the granule and degrees of their polymerization and phosphorylation. Potato Solanum tuberosum L. is one of the main starch-producing crops. Growing industrial needs necessitate the breeding of plant varieties with increased starch content and specified starch properties. This task demands detailed information on starch metabolism in the producing plant. It is a complex process, requiring the orchestrated work of many enzymes, transporter and targeting proteins, transcription factors, and other regulators. Two types of starch are recognized with regard to their biological functions. Transitory starch is synthesized in chloroplasts of photosynthetic organs and degraded in the absence of light, providing carbohydrates for cell needs. Storage starch is synthesized and stored in amyloplasts of storage organs: grains and tubers. The main enzymatic reactions of starch biosynthesis and degradation, as well as carbohydrate transport and metabolism, are well known in the case of transitory starch of the model plant Arabidopsis thaliana. Less is known about features of starch metabolism in storage organs, in particular, potato tubers. Several issues remain obscure: the roles of enzyme isoforms and different regulatory factors in tissues at various plant developmental stages and under different environmental conditions; alternative enzymatic processes; targeting and transport proteins. In this review, the key enzymatic reactions of plant carbohydrate metabolism, transitory and storage starch biosynthesis, and starch degradation are discussed, and features specific for potato are outlined. Attention is also paid to the known regulatory factors affecting starch metabolism

Structure, function, and evolution of plant ADP-glucose pyrophosphorylase

This review outlines research performed in the last two decades on the structural, kinetic, regulatory and evolutionary aspects of ADP-glucose pyrophosphorylase, the regulatory enzyme for starch biosynthesis. ADP-glucose pyrophosphorylase (ADP-Glc PPase) catalyzes the first committed step in the pathway of glycogen and starch synthesis in bacteria and plants, respectively. Plant ADP-Glc PPase is a heterotetramer allosterically regulated by metabolites and post-translational modifications. In this review, we focus on the three-dimensional structure of the plant enzyme, the amino acids that bind the regulatory molecules, and the regions involved in transmitting the allosteric signal to the catalytic site. We provide a model for the evolution of the small and large subunits, which produce heterotetramers with distinct catalytic and regulatory properties. Additionally, we review the various post-translational modifications observed in ADP-Glc PPases from different species and tissues. Finally, we discuss the subcellular localization of the enzyme found in grain endosperm from grasses, such as maize and rice. Overall, this work brings together research performed in the last two decades to better understand the multiple mechanisms involved in the regulation of ADP-Glc PPase. The rational modification of this enzyme could improve the yield and resilience of economically important crops, which is particularly important in the current scenario of climate change and food shortage.

Nucleotide-sugar metabolism in plants: the legacy of Luis F. Leloir

This review commemorates the 50th anniversary of the Nobel Prize in Chemistry awarded to Luis F. Leloir 'for his discovery of sugar-nucleotides and their role in the biosynthesis of carbohydrates'. He and his co-workers discovered that activated forms of simple sugars, such as UDP-glucose and UDP-galactose, are essential intermediates in the interconversion of sugars. They elucidated the biosynthetic pathways for sucrose and starch, which are the major end-products of photosynthesis, and for trehalose. Trehalose 6-phosphate, the intermediate of trehalose biosynthesis that they discovered, is now a molecule of great interest due to its function as a sugar signalling metabolite that regulates many aspects of plant metabolism and development. The work of the Leloir group also opened the doors to an understanding of the biosynthesis of cellulose and other structural cell wall polysaccharides (hemicelluloses and pectins), and ascorbic acid (vitamin C). Nucleotide-sugars also serve as sugar donors for a myriad of glycosyltransferases that conjugate sugars to other molecules, including lipids, phytohormones, secondary metabolites, and proteins, thereby modifying their biological activity. In this review, we highlight the diversity of nucleotide-sugars and their functions in plants, in recognition of Leloir's rich and enduring legacy to plant science.

Recent Advances in Carbon and Nitrogen Metabolism in C3 Plants

C and N are the most important essential elements constituting organic compounds in plants. The shoots and roots depend on each other by exchanging C and N through the xylem and phloem transport systems. Complex mechanisms regulate C and N metabolism to optimize plant growth, agricultural crop production, and maintenance of the agroecosystem. In this paper, we cover the recent advances in understanding C and N metabolism, regulation, and transport in plants, as well as their underlying molecular mechanisms. Special emphasis is given to the mechanisms of starch metabolism in plastids and the changes in responses to environmental stress that were previously overlooked, since these changes provide an essential store of C that fuels plant metabolism and growth. We present general insights into the system biology approaches that have expanded our understanding of core biological questions related to C and N metabolism. Finally, this review synthesizes recent advances in our understanding of the trade-off concept that links C and N status to the plant's response to microorganisms.

Phosphorylation of ADP-Glucose Pyrophosphorylase During Wheat Seeds Development

Starch is the dominant reserve polysaccharide accumulated in the seed of grasses (like wheat). It is the most common carbohydrate in the human diet and a material applied to the bioplastics and biofuels industry. Hence, the complete understanding of starch metabolism is critical to design rational strategies to improve its allocation in plant reserve tissues. ADP-glucose pyrophosphorylase (ADP-Glc PPase) catalyzes the key (regulated) step in the synthetic starch pathway. The enzyme comprises a small (S) and a large (L) subunit forming an S₂L₂ heterotetramer, which is allosterically regulated by orthophosphate, fructose-6P, and 3P-glycerate. ADP-Glc PPase was found in a phosphorylated state in extracts from wheat seeds. The amount of the phosphorylated protein increased along with the development of the seed and correlated with relative increases of the enzyme activity and starch content. Conversely, this post-translational modification was absent in seeds from Ricinus communis. In vitro, the recombinant ADP-Glc PPase from wheat endosperm was phosphorylated by wheat seed extracts as well as by recombinant Ca²⁺-dependent plant protein kinases. Further analysis showed that the preferential phosphorylation takes place on the L subunit. Results suggest that the ADP-Glc PPase is a phosphorylation target in seeds from grasses but not from oleaginous plants. Accompanying seed maturation and starch accumulation, a combined regulation of ADP-Glc PPase by metabolites and phosphorylation may provide an enzyme with stable levels of activity. Such concerted modulation would drive carbon skeletons to the synthesis of starch for its long-term storage, which later support seed germination.

NTRC and Thioredoxin f Overexpression Differentially Induces Starch Accumulation in Tobacco Leaves

Thioredoxin (Trx) f and NADPH-dependent Trx reductase C (NTRC) have both been proposed as major redox regulators of starch metabolism in chloroplasts. However, little is known regarding the specific role of each protein in this complex mechanism. To shed light on this point, tobacco plants that were genetically engineered to overexpress the NTRC protein from the chloroplast genome were obtained and compared to previously generated Trx f-overexpressing transplastomic plants. Likewise, we investigated the impact of NTRC and Trx f deficiency on starch metabolism by generating Nicotiana benthamiana plants that were silenced for each gene. Our results demonstrated that NTRC overexpression induced enhanced starch accumulation in tobacco leaves, as occurred with Trx f. However, only Trx f silencing leads to a significant decrease in the leaf starch content. Quantitative analysis of enzyme activities related to starch synthesis and degradation were determined in all of the genotypes. Zymographic analyses were additionally performed to compare the amylolytic enzyme profiles of both transplastomic tobacco plants. Our findings indicated that NTRC overexpression promotes the accumulation of transitory leaf starch as a consequence of a diminished starch turnover during the dark period, which seems to be related to a significant reductive activation of ADP-glucose pyrophosphorylase and/or a deactivation of a putative debranching enzyme. On the other hand, increased starch content in Trx f-overexpressing plants was connected to an increase in the capacity of soluble starch synthases during the light period. Taken together, these results suggest that NTRC and the ferredoxin/Trx system play distinct roles in starch turnover.

Target of rapamycin-signaling modulates starch accumulation via glycogenin phosphorylation status in the unicellular red alga Cyanidioschyzon merolae

The target of rapamycin (TOR) signaling pathway is involved in starch accumulation in various eukaryotic organisms; however, the molecular mechanism behind this phenomenon in eukaryotes has not been elucidated. We report a regulatory mechanism of starch accumulation by TOR in the unicellular red alga, Cyanidioschyzon merolae. The starch content in C. merolae after TOR-inactivation by rapamycin, a TOR-specific inhibitor, was increased by approximately 10-fold in comparison with its drug vehicle, dimethyl sulfoxide. However, our previous transcriptome analysis showed that the expression level of genes related to carbohydrate metabolism was unaffected by rapamycin, indicating that starch accumulation is regulated at post-transcriptional levels. In this study, we performed a phosphoproteome analysis using liquid chromatography-tandem mass spectrometry to investigate potential post-transcriptional modifications, and identified 52 proteins as candidate TOR substrates. Among the possible substrates, we focused on the function of CmGLG1, because its phosphorylation at the Ser613 residue was decreased after rapamycin treatment, and overexpression of CmGLG1 resulted in a 4.7-fold higher starch content. CmGLG1 is similar to the priming protein, glycogenin, which is required for the initiation of starch/glycogen synthesis, and a budding yeast complementation assay demonstrated that CmGLG1 can functionally substitute for glycogenin. We found an approximately 60% reduction in the starch content in a phospho-mimicking CmGLG1 overexpression strain, in which Ser613 was substituted with aspartic acid, in comparison with the wild-type CmGLG1 overexpression cells. Our results indicate that TOR modulates starch accumulation by changing the phosphorylation status of the CmGLG1 Ser613 residue in C. merolae.

Starch Trek: The Search for Yield

Starch is a plant storage polyglucan that accumulates in plastids. It is composed of two polymers, amylose and amylopectin, with different structures and plays several roles in helping to determine plant yield. In leaves, it acts as a buffer for night time carbon starvation. Genetically altered plants that cannot synthesize or degrade starch efficiently often grow poorly. There have been a number of successful approaches to manipulate leaf starch metabolism that has resulted in increased growth and yield. Its degradation is also a source of sugars that can help alleviate abiotic stress. In edible parts of plants, starch often makes up the majority of the dry weight constituting much of the calorific value of food and feed. Increasing starch in these organs can increase this as well as increasing yield. Enzymes involved in starch metabolism are well known, and there has been much research analyzing their functions in starch synthesis and degradation, as well as genetic and posttranslational regulatory mechanisms affecting them. In this mini review, we examine work on this topic and discuss future directions that could be used to manipulate this metabolite for improved yield.

Mechanism Underlying Heat Stability of the Rice Endosperm Cytosolic ADP-Glucose Pyrophosphorylase

Rice grains accumulate starch as their major storage reserve whose biosynthesis is sensitive to heat. ADP-glucose pyrophosphorylase (AGPase) is among the starch biosynthetic enzymes severely affected by heat stress during seed maturation. To increase the heat tolerance of the rice enzyme, we engineered two dominant AGPase subunits expressed in developing endosperm, the large (L2) and small (S2b) subunits of the cytosol-specific AGPase. Bacterial expression of the rice S2b with the rice L2, potato tuber LS (pLS), or with the mosaic rice-potato large subunits, L2-pLS and pLS-L2, produced heat-sensitive recombinant enzymes, which retained less than 10% of their enzyme activities after 5 min incubation at 55°C. However, assembly of the rice L2 with the potato tuber SS (pSS) showed significantly increased heat stability comparable to the heat-stable potato pLS/pSS. The S2b assembled with the mosaic L2-pLS subunit showed 3-fold higher sensitivity to 3-PGA than L2/S2b, whereas the counterpart mosaic pLS-L2/S2b showed 225-fold lower sensitivity. Introduction of a QTC motif into S2b created an N-terminal disulfide linkage that was cleaved by dithiothreitol reduction. The QTC enzyme showed moderate heat stability but was not as stable as the potato AGPase. While the QTC AGPase exhibited approximately fourfold increase in 3-PGA sensitivity, its substrate affinities were largely unchanged. Random mutagenesis of S2b^QTC produced six mutant lines with elevated production of glycogen in bacteria. All six lines contained a L379F substitution, which conferred enhanced glycogen production in bacteria and increased heat stability. Modeled structure of this mutant enzyme revealed that this highly conserved leucine residue is located in the enzyme's regulatory pocket that provides interaction sites for activators and inhibitors. Our molecular dynamic simulation analysis suggests that introduction of the QTC motif and the L379F mutation improves enzyme heat stability by stabilizing their backbone structures possibly due to the increased number of H-bonds between the small subunits and increased intermolecular interactions between the two SSs and two LSs at elevated temperature.

Redox Regulation of Starch Metabolism

Metabolism of starch is a major biological integrator of plant growth supporting nocturnal energy dynamics by transitory starch degradation as well as periods of dormancy, re-growth, and reproduction by utilization of storage starch. Especially, the extraordinarily well-tuned and coordinated rate of transient starch biosynthesis and degradation suggests the presence of very sophisticated regulatory mechanisms. Together with the circadian clock, land plants (being autotrophic and sessile organisms) need to monitor, sense, and recognize the photosynthetic rate, soil mineral availability as well as various abiotic and biotic stress factors. Currently it is widely accepted that post-translational modifications are the main way by which the diel periodic activity of enzymes of transient starch metabolism are regulated. Among these mechanisms, thiol-based redox regulation is suggested to be of fundamental importance and in chloroplasts, thioredoxins (Trx) are tightly linked up to photosynthesis and mediate light/dark regulation of metabolism. Also, light independent NADP-thioredoxin reductase C (NTRC) plays a major role in reactive oxygen species scavenging. Moreover, Trx and NTRC systems are interconnected at several levels and strongly influence each other. Most enzymes involved in starch metabolism are demonstrated to be redox-sensitive in vitro. However, to what extent their redox sensitivity is physiologically relevant in synchronizing starch metabolism with photosynthesis, heterotrophic energy demands, and oxidative protection is still unclear. For example, many hydrolases are activated under reducing (light) conditions and the strict separation between light and dark metabolic pathways is now challenged by data suggesting degradation of starch during the light period.

On the Roles of Wheat Endosperm ADP-Glucose Pyrophosphorylase Subunits

The ADP-glucose pyrophosphorylase from wheat endosperm controls starch synthesis in seeds and has unique regulatory properties compared to others from this family. It comprises two types of subunits, but despite its importance little is known about their roles. Here, we synthesized de novo the wheat endosperm ADP-glucose pyrophosphorylase small (S) and large (L) subunit genes, heterologously expressed them in Escherichia coli, and kinetically characterized the recombinant proteins. To understand their distinct roles, we co-expressed them with well characterized subunits from the potato tuber enzyme to obtain hybrids with one S subunit from one source and an L subunit from the other. After kinetic analyses of these hybrids, we concluded that the unusual insensitivity to activation of the wheat endosperm enzyme is caused by a pre-activation of the L subunit. In addition, the heat stability and sensitivity to phosphate are given by the S subunit.

The Deep Thioredoxome in Chlamydomonas reinhardtii: New Insights into Redox Regulation

Thiol-based redox post-translational modifications have emerged as important mechanisms of signaling and regulation in all organisms, and thioredoxin plays a key role by controlling the thiol-disulfide status of target proteins. Recent redox proteomic studies revealed hundreds of proteins regulated by glutathionylation and nitrosylation in the unicellular green alga Chlamydomonas reinhardtii, while much less is known about the thioredoxin interactome in this organism. By combining qualitative and quantitative proteomic analyses, we have comprehensively investigated the Chlamydomonas thioredoxome and 1188 targets have been identified. They participate in a wide range of metabolic pathways and cellular processes. This study broadens not only the redox regulation to new enzymes involved in well-known thioredoxin-regulated metabolic pathways but also sheds light on cellular processes for which data supporting redox regulation are scarce (aromatic amino acid biosynthesis, nuclear transport, etc). Moreover, we characterized 1052 thioredoxin-dependent regulatory sites and showed that these data constitute a valuable resource for future functional studies in Chlamydomonas. By comparing this thioredoxome with proteomic data for glutathionylation and nitrosylation at the protein and cysteine levels, this work confirms the existence of a complex redox regulation network in Chlamydomonas and provides evidence of a tremendous selectivity of redox post-translational modifications for specific cysteine residues.

Engineering the expression level of cytosolic nucleoside diphosphate kinase in transgenic Solanum tuberosum roots alters growth, respiration and carbon metabolism

Nucleoside diphosphate kinase (NDPK) is a ubiquitous enzyme that catalyzes the transfer of the γ-phosphate from a donor nucleoside triphosphate to an acceptor nucleoside diphosphate. In this study we used a targeted metabolomic approach and measurement of physiological parameters to report the effects of the genetic manipulation of cytosolic NDPK (NDPK1) expression on physiology and carbon metabolism in potato (Solanum tuberosum) roots. Sense and antisense NDPK1 constructs were introduced in potato using Agrobacterium rhizogenes to generate a population of root clones displaying a 40-fold difference in NDPK activity. Root growth, O₂ uptake, flux of carbon between sucrose and CO₂ , levels of reactive oxygen species and some tricarboxylic acid cycle intermediates were positively correlated with levels of NDPK1 expression. In addition, NDPK1 levels positively affected UDP-glucose and cellulose contents. The activation state of ADP-glucose pyrophosphorylase, a key enzyme in starch synthesis, was higher in antisense roots than in roots overexpressing NDPK1. Further analyses demonstrated that ADP-glucose pyrophosphorylase was more oxidized, and therefore less active, in sense clones than antisense clones. Consequently, antisense NDPK1 roots accumulated more starch and the starch to cellulose ratio was negatively affected by the level of NDPK1. These data support the idea that modulation of NDPK1 affects the distribution of carbon between starch and cellulose biosynthetic pathways.

The Unprecedented Versatility of the Plant‎ Thioredoxin System

Thioredoxins are ubiquitous enzymes catalyzing reversible disulfide-bond formation to regulate structure and function of many proteins in diverse organisms. In recent years, reverse genetics and biochemical approaches were used to resolve the functions, specificities, and interactions of the different thioredoxin isoforms and reduction systems in planta and revealed the most versatile thioredoxin system of all organisms. Here we review the emerging roles of the thioredoxin system, namely the integration of thylakoid energy transduction, metabolism, gene expression, growth, and development under fluctuating environmental conditions. We argue that these new developments help us to understand why plants organize such a divergent composition of thiol redox networks and provide insights into the regulatory hierarchy that operates between them.

Dithiol disulphide exchange in redox regulation of chloroplast enzymes in response to evolutionary and structural constraints

Redox regulation of chloroplast enzymes via disulphide reduction is believed to control the rates of CO₂ fixation. The study of the thioredoxin reduction pathways and of various target enzymes lead to the following guidelines.

Molecular cloning, subcellular localization and characterization of two adenylate kinases from cassava, Manihot esculenta Crantz cv. KU50

Adenylate kinase (ADK) is a phosphotransferase that plays an important role in cellular energy homeostasis. Many isozymes located in different subcellular compartments have been reported. In this study, we focus on the characterization of cassava (Manihot esculenta) ADKs. We found 15 ADKs that are publicly available in the African cassava genome database. We cloned two ADKs, namely MeADK1 and MeADK2, which are phylogenetically grouped together with the plastidial ADK in potato. Both MeADK1 and MeADK2 showed 66% identity in the amino acid sequences with plastidial ADK in potato. However, we demonstrated that they are localized to mitochondria using GFP fusions of MeADK1 and MeADK2. The Escherichia coli-produced recombinant MeADK1 and MeADK2 preferred forward reactions that produce ATP. They exhibited similar specific activities. The semi-quantitative RT-PCR analysis showed that MeADK1 and MeADK2 in 2-month-old leaves have similar expression patterns under a diurnal light-dark cycle. However, MeADK2 transcripts were expressed at much higher levels than MeADK1 in 5-month-old leaves and roots. Thus, we conclude that MeADK2 might play a vital role in energy homeostasis in cassava mitochondria.

The Path to Thioredoxin and Redox Regulation in Chloroplasts

After a brief discussion of my graduate work at Duke University, I describe a series of investigations on redox proteins at the University of California, Berkeley. Starting with ferredoxin from fermentative bacteria, the Berkeley research fostered experiments that uncovered a pathway for fixing CO2 in bacterial photosynthesis. The carbon work, in turn, opened new vistas, including the discovery that thioredoxin functions universally in regulating the Calvin-Benson cycle in oxygenic photosynthesis. These experiments, which took place over a 50-year period, led to the formulation of a set of biological principles and set the stage for research demonstrating a role for redox in the regulation of previously unrecognized processes extending far beyond photosynthesis.

ACHT4-driven oxidation of APS1 attenuates starch synthesis under low light intensity in Arabidopsis plants

The regulatory mechanisms that use signals of low levels of reactive oxygen species (ROS) could be obscured by ROS produced under stress and thus are better investigated under homeostatic conditions. Previous studies showed that the chloroplastic atypical thioredoxin ACHT1 is oxidized by 2-Cys peroxiredoxin (2-Cys Prx) in Arabidopsis plants illuminated with growth light and in turn transmits a disulfide-based signal via yet unknown target proteins in a feedback regulation of photosynthesis. Here, we studied the role of a second chloroplastic paralog, ACHT4, in plants subjected to low light conditions. Likewise, ACHT4 reacted in planta with 2-Cys Prx, indicating that it is oxidized by a similar disulfide exchange reaction. ACHT4 further reacted uniquely with the small subunit (APS1) of ADP-glucose pyrophosphorylase (AGPase), the first committed enzyme of the starch synthesis pathway, suggesting that it transfers the disulfides it receives from 2-Cys Prx to APS1 and turns off AGPase. In accordance, ACHT4 participated in an oxidative signal that quenched AGPase activity during the diurnal transition from day to night, and also in an attenuating oxidative signal of AGPase in a dynamic response to small fluctuations in light intensity during the day. Increasing the level of expressed ACHT4 or of ACHT4ΔC, a C terminus-deleted form that does not react with APS1, correspondingly decreased or increased the level of reduced APS1 and decreased or increased transitory starch content. These findings imply that oxidative control mechanisms act in concert with reductive signals to fine tune starch synthesis during daily homeostatic conditions.

The Role of Cysteine Residues in Redox Regulation and Protein Stability of Arabidopsis thaliana Starch Synthase 1

Starch biosynthesis in Arabidopsis thaliana is strictly regulated. In leaf extracts, starch synthase 1 (AtSS1) responds to the redox potential within a physiologically relevant range. This study presents data testing two main hypotheses: 1) that specific thiol-disulfide exchange in AtSS1 influences its catalytic function 2) that each conserved Cys residue has an impact on AtSS1 catalysis. Recombinant AtSS1 versions carrying combinations of cysteine-to-serine substitutions were generated and characterized in vitro. The results demonstrate that AtSS1 is activated and deactivated by the physiological redox transmitters thioredoxin f1 (Trxf1), thioredoxin m4 (Trxm4) and the bifunctional NADPH-dependent thioredoxin reductase C (NTRC). AtSS1 displayed an activity change within the physiologically relevant redox range, with a midpoint potential equal to -306 mV, suggesting that AtSS1 is in the reduced and active form during the day with active photosynthesis. Cys164 and Cys545 were the key cysteine residues involved in regulatory disulfide formation upon oxidation. A C164S_C545S double mutant had considerably decreased redox sensitivity as compared to wild type AtSS1 (30% vs 77%). Michaelis-Menten kinetics and molecular modeling suggest that both cysteines play important roles in enzyme catalysis, namely, Cys545 is involved in ADP-glucose binding and Cys164 is involved in acceptor binding. All the other single mutants had essentially complete redox sensitivity (98–99%). In addition of being part of a redox directed activity “light switch”, reactivation tests and low heterologous expression levels indicate that specific cysteine residues might play additional roles. Specifically, Cys265 in combination with Cys164 can be involved in proper protein folding or/and stabilization of translated protein prior to its transport into the plastid. Cys442 can play an important role in enzyme stability upon oxidation. The physiological and phylogenetic relevance of these findings is discussed.

Substrate binding properties of potato tuber ADP-glucose pyrophosphorylase as determined by isothermal titration calorimetry

Substrate binding properties of the large (LS) and small (SS) subunits of potato tuber ADP-glucose pyrophosphorylase were investigated by using isothermal titration calorimetry. Our results clearly show that the wild type heterotetramer (S(WT)L(WT)) possesses two distinct types of ATP binding sites, whereas the homotetrameric LS and SS variant forms only exhibited properties of one of the two binding sites. The wild type enzyme also exhibited significantly increased affinity to this substrate compared to the homotetrameric enzyme forms. No stable binding was evident for the second substrate, glucose-1-phosphate, in the presence or absence of ATPγS suggesting that interaction of glucose-1-phosphate is dependent on hydrolysis of ATP and supports the Theorell-Chance bi bi reaction mechanism.

The role of the large subunit in redox regulation of the rice endosperm ADP-glucose pyrophosphorylase

The starch regulatory enzyme ADP-glucose pyrophosphorylase is activated by 3-phosphoglyceric acid (3-PGA) and inhibited by inorganic phosphate (Pi ). The activity of the plastid-localized enzyme is also subject to fine regulation by redox control in response to changing light and sugar levels. The less active oxidized form of the enzyme contains an inter-subunit disulfide bond formed between the pair of small subunit's Cys12 residues of the heterotetrameric enzyme. Although this cysteine residue is not conserved in the small subunits of cereal endosperm cytosolic AGPases, biochemical studies of the major rice endosperm enzyme indicate that the cytosolic isoform, like the plastidial enzymes, is subject to redox control. Kinetic analysis revealed that the reduced forms of the partially purified native and purified recombinant AGPases have 6- and 3.4-fold, respectively, more affinity to 3-PGA, rendering the enzymes more active at lower 3-PGA concentration than the non-reduced enzyme. Truncation of the large subunit by removal of N-terminal peptide resulted in a decrease in 3-PGA affinity and loss of redox response of the enzyme. Site-directed mutagenesis of the conserved cysteine residues at the N-terminal of the large subunit showed that C47 and C58, but not C12, are essential for proper redox response of the enzyme. Overall, our results show that the major rice endosperm AGPase activity is controlled by a combination of allosteric regulation and redox control, the latter through modification of the large subunit instead of the small subunit as evident in the plastid-localized enzyme.

Metabolic control of redox and redox control of metabolism in plants

SIGNIFICANCE

Reduction-oxidation (Redox) status operates as a major integrator of subcellular and extracellular metabolism and is simultaneously itself regulated by metabolic processes. Redox status not only dominates cellular metabolism due to the prominence of NAD(H) and NADP(H) couples in myriad metabolic reactions but also acts as an effective signal that informs the cell of the prevailing environmental conditions. After relay of this information, the cell is able to appropriately respond via a range of mechanisms, including directly affecting cellular functioning and reprogramming nuclear gene expression.

RECENT ADVANCES

The facile accession of Arabidopsis knockout mutants alongside the adoption of broad-scale post-genomic approaches, which are able to provide transcriptomic-, proteomic-, and metabolomic-level information alongside traditional biochemical and emerging cell biological techniques, has dramatically advanced our understanding of redox status control. This review summarizes redox status control of metabolism and the metabolic control of redox status at both cellular and subcellular levels.

CRITICAL ISSUES

It is becoming apparent that plastid, mitochondria, and peroxisome functions influence a wide range of processes outside of the organelles themselves. While knowledge of the network of metabolic pathways and their intraorganellar redox status regulation has increased in the last years, little is known about the interorganellar redox signals coordinating these networks. A current challenge is, therefore, synthesizing our knowledge and planning experiments that tackle redox status regulation at both inter- and intracellular levels.

FUTURE DIRECTIONS

Emerging tools are enabling ever-increasing spatiotemporal resolution of metabolism and imaging of redox status components. Broader application of these tools will likely greatly enhance our understanding of the interplay of redox status and metabolism as well as elucidating and characterizing signaling features thereof. We propose that such information will enable us to dissect the regulatory hierarchies that mediate the strict coupling of metabolism and redox status which, ultimately, determine plant growth and development.

Analysis of short-term metabolic alterations in Arabidopsis following changes in the prevailing environmental conditions

Although a considerable increase in our knowledge concerning the importance of metabolic adjustments to unfavorable growth conditions has been recently provided, relatively little is known about the adjustments which occur in response to fluctuation in environmental factors. Evaluating the metabolic adjustments occurring under changing environmental conditions thus offers a good opportunity to increase our current understanding of the crosstalk between the major pathways which are affected by such conditions. To this end, plants growing under normal conditions were transferred to different light and temperature conditions which were anticipated to affect (amongst other processes) the rates of photosynthesis and photorespiration and characterized at the physiological, molecular, and metabolic levels following this transition. Our results revealed similar behavior in response to both treatments and imply a tight connectivity of photorespiration with the major pathways of plant metabolism. They further highlight that the majority of the regulation of these pathways is not mediated at the level of transcription but that leaf metabolism is rather pre-poised to adapt to changes in these input parameters.

Detergent-like stressor and nutrient in metabolism of Penicillium chrysogenum

The influence of detergents on the metabolism of Penicillium chrysogenum from two aspects, as a stress factor and potential nutrient, was studied. The fungus was isolated from the river bed Lepenica, Kragujevac, at a place where sewage domestic wastewater discharged into the river. The fungus was grown in a liquid nutrient medium according to Czapek with and without addition of commercial detergent (MERIX, Henkel, Serbia) at a concentration of 0.3% and 0.5%. The biochemical changes of pH, redox potential, free and total organic acids, total dry weight biomass, activity of alkaline and acid invertase and alkaline phosphatase were evaluated from day 3 to day 16 of the fungus growth. At the same time, detergent disappearance in terms of methylene blue active substances in the medium was measured. The detergent at a concentration of 0.5% showed a fungicide effect. In the medium with 0.3% of detergent, there was increased pH and concentration of organic acids, but decreased redox potential and total dry weight biomass. The detergent also showed an inhibitory effect on invertase and phosphatase activity. P. chrysogenum decomposed 50.2% of the total detergent concentration for an experimental period of 16 days.

Ongoing applicative studies of plant thioredoxins

Studies triggered by the discovery of the function of thioredoxin (Trx) in photosynthesis have revealed its role throughout biology. Parallel biochemical and proteomic analyses have led to the identification of its numerous putative targets. Recently, to verify the biological significance of these targets, in vivo studies using transformants in which Trx is overexpressed or suppressed are in progress, and the transformants themselves that are being used in such studies show their potential applicative values. Moreover, Trx's mitigation of allergenicity for some proteins offers promising prospects in the food industry. Practical studies based on redox regulation, once only on the horizon, are now achieving new dimensions. This short review focuses on the industrial applications of Trx studies, the current situation, and future perspectives. The putative targets obtained by the proteomics approach in comparison with in vivo observations of the transformants are also examined. Applicative studies of glutathione, a counterpart of Trx, are also discussed briefly.

Redox regulation of the Calvin-Benson cycle: something old, something new

Reversible redox post-translational modifications such as oxido-reduction of disulfide bonds, S-nitrosylation, and S-glutathionylation, play a prominent role in the regulation of cell metabolism and signaling in all organisms. These modifications are mainly controlled by members of the thioredoxin and glutaredoxin families. Early studies in photosynthetic organisms have identified the Calvin-Benson cycle, the photosynthetic pathway responsible for carbon assimilation, as a redox regulated process. Indeed, 4 out of 11 enzymes of the cycle were shown to have a low activity in the dark and to be activated in the light through thioredoxin-dependent reduction of regulatory disulfide bonds. The underlying molecular mechanisms were extensively studied at the biochemical and structural level. Unexpectedly, recent biochemical and proteomic studies have suggested that all enzymes of the cycle and several associated regulatory proteins may undergo redox regulation through multiple redox post-translational modifications including glutathionylation and nitrosylation. The aim of this review is to detail the well-established mechanisms of redox regulation of Calvin-Benson cycle enzymes as well as the most recent reports indicating that this pathway is tightly controlled by multiple interconnected redox post-translational modifications. This redox control is likely allowing fine tuning of the Calvin-Benson cycle required for adaptation to varying environmental conditions, especially during responses to biotic and abiotic stresses.

Transcriptome profiling of fruit development and maturation in Chinese white pear (Pyrus bretschneideri Rehd)

BACKGROUND

Pear (Pyrus spp) is an important fruit species worldwide; however, its genetics and genomic information is limited. Combining the Solexa/Illumina RNA-seq high-throughput sequencing approach (RNA-seq) with Digital Gene Expression (DGE) analysis would be a powerful tool for transcriptomic study. This paper reports the transcriptome profiling analysis of Chinese white pear (P. bretschneideri) using RNA-seq and DGE to better understand the molecular mechanisms in fruit development and maturation of Chinese white pear.

RESULTS

De novo transcriptome assembly and gene expression analysis of Chinese white pear were performed in an unprecedented depth (5.47 gigabase pairs) using high-throughput Illumina RNA-seq combined with a tag-based Digital Gene Expression (DGE) system. Approximately, 60.77 million reads were sequenced, trimmed, and assembled into 90,227 unigenes. These unigenes comprised 17,619 contigs and 72,608 singletons with an average length of 508 bp and had an N50 of 635 bp. Sequence similarity analyses against six public databases (Uniprot, NR, and COGs at NCBI, Pfam, InterPro, and KEGG) found that 61,636 unigenes can be annotated with gene descriptions, conserved protein domains, or gene ontology terms. By BLASTing all 61,636 unigenes in KEGG, a total of 31,215 unigenes were annotated into 121 known metabolic or signaling pathways in which a few primary, intermediate, and secondary metabolic pathways are directly related to pear fruit quality. DGE libraries were constructed for each of the five fruit developmental stages. Variations in gene expression among all developmental stages of pear fruit were significantly different in a large amount of unigenes.

CONCLUSION

Extensive transcriptome and DGE profiling data at five fruit developmental stages of Chinese white pear have been obtained from a deep sequencing, which provides comprehensive gene expression information at the transcriptional level. This could facilitate understanding of the molecular mechanisms in fruit development and maturation. Such a database can also be used as a public information platform for research on molecular biology and functional genomics in pear and other related species.

Plastid thioredoxins: a "one-for-all" redox-signaling system in plants

The sessile nature of plants forces them to face an ever-changing environment instead of escape from hostile conditions as animals do. In order to overcome this survival challenge, a fine monitoring and controlling of the status of the photosynthetic electron transport chain and the general metabolism is vital for these organisms. Frequently, evolutionary plant adaptation has consisted in the appearance of multigenic families, comprising an array of enzymes, structural components, or sensing, and signaling elements, in numerous occasions with highly conserved primary sequences that sometimes make it difficult to discern between redundancy and specificity among the members of a same family. However, all this gene diversity is aimed to sort environment-derived plant signals to efficiently channel the external incoming information inducing a right physiological answer. Oxygenic photosynthesis is a powerful source of reactive oxygen species (ROS), molecules with a dual oxidative/signaling nature. In response to ROS, one of the most frequent post-translational modifications occurring in redox signaling proteins is the formation of disulfide bridges (from Cys oxidation). This review is focused on the role of plastid thioredoxins (pTRXs), proteins containing two Cys in their active site and largely known as part of the plant redox-signaling network. Several pTRXs types have been described so far, namely, TRX f, m, x, y, and z. In recent years, improvements in proteomic techniques and the study of loss-of-function mutants have enabled us to grasp the importance of TRXs for the plastid physiology. We will analyze the specific signaling function of each TRX type and discuss about the emerging role in non-photosynthetic plastids of these redox switchers.

Feedback inhibition of starch degradation in Arabidopsis leaves mediated by trehalose 6-phosphate

Many plants accumulate substantial starch reserves in their leaves during the day and remobilize them at night to provide carbon and energy for maintenance and growth. In this paper, we explore the role of a sugar-signaling metabolite, trehalose-6-phosphate (Tre6P), in regulating the accumulation and turnover of transitory starch in Arabidopsis (Arabidopsis thaliana) leaves. Ethanol-induced overexpression of trehalose-phosphate synthase during the day increased Tre6P levels up to 11-fold. There was a transient increase in the rate of starch accumulation in the middle of the day, but this was not linked to reductive activation of ADP-glucose pyrophosphorylase. A 2- to 3-fold increase in Tre6P during the night led to significant inhibition of starch degradation. Maltose and maltotriose did not accumulate, suggesting that Tre6P affects an early step in the pathway of starch degradation in the chloroplasts. Starch granules isolated from induced plants had a higher orthophosphate content than granules from noninduced control plants, consistent either with disruption of the phosphorylation-dephosphorylation cycle that is essential for efficient starch breakdown or with inhibition of starch hydrolysis by β-amylase. Nonaqueous fractionation of leaves showed that Tre6P is predominantly located in the cytosol, with estimated in vivo Tre6P concentrations of 4 to 7 µm in the cytosol, 0.2 to 0.5 µm in the chloroplasts, and 0.05 µm in the vacuole. It is proposed that Tre6P is a component in a signaling pathway that mediates the feedback regulation of starch breakdown by sucrose, potentially linking starch turnover to demand for sucrose by growing sink organs at night.

Improving starch yield in cereals by over-expression of ADPglucose pyrophosphorylase: expectations and unanticipated outcomes

Significant improvements in crop productivity are required to meet the nutritional requirements of a growing world population. This challenge is magnified by an increased demand for bioenergy as a means to mitigate carbon inputs into the environment. Starch is a major component of the harvestable organs of many crop plants, and various endeavors have been taken to improve the yields of starchy organs through the manipulation of starch synthesis. Substantial efforts have centered on the starch regulatory enzyme ADPglucose pyrophosphorylase (AGPase) due to its pivotal role in starch biosynthesis. These efforts include over-expression of this enzyme in cereal plants such as maize, rice and wheat as well as potato and cassava, as they supply the bulk of the staple food worldwide. In this perspective, we describe efforts to increase starch yields in cereal grains by first providing an introduction about the importance of source-sink relationship and the motives behind the efforts to alter starch biosynthesis and turnover in leaves. We then discuss the catalytic and regulatory properties of AGPase and the molecular approaches used to enhance starch synthesis by manipulation of this process during grain filling using seed-specific promoters. Several studies have demonstrated increases in starch content per seed using endosperm-specific promoters, but other studies have demonstrated an increase in seed number with only marginal impact on seed weight. Potential mechanisms that may be responsible for this paradoxical increase in seed number will also be discussed. Finally, we describe current efforts and future prospects to improve starch yield in cereals. These efforts include further enhancement of starch yield in rice by augmenting the process of ADPglucose transport into amyloplast as well as other enzymes involved in photoassimilate partitioning in seeds.

Deletion of chloroplast NADPH-dependent thioredoxin reductase results in inability to regulate starch synthesis and causes stunted growth under short-day photoperiods

Plastid-localized NADPH-dependent thioredoxin reductase C (NTRC) is a unique NTR enzyme containing both reductase and thioredoxin domains in a single polypeptide. Arabidopsis thaliana NTRC knockout lines (ntrc) show retarded growth, especially under short-day (SD) photoperiods. This study identified chloroplast processes that accounted for growth reduction in SD-acclimated ntrc. The strongest reduction in ntrc growth occurred under photoperiods with nights longer than 14 h, whereas knockout of the NTRC gene did not alter the circadian-clock-controlled growth of Arabidopsis. Lack of NTRC modulated chloroplast reactive oxygen species (ROS) metabolism, but oxidative stress was not the primary cause of retarded growth of SD-acclimated ntrc. Scarcity of starch accumulation made ntrc leaves particularly vulnerable to photoperiods with long nights. Direct interaction of NTRC and ADP-glucose pyrophosphorylase, a key enzyme in starch synthesis, was confirmed by yeast two-hybrid analysis. The ntrc line was not able to maximize starch synthesis during the light period, which was particularly detrimental under SD conditions. Acclimation of Arabidopsis to SD conditions also involved an inductive rise of ROS production in illuminated chloroplasts that was not counterbalanced by the activation of plastidial anti-oxidative systems. It is proposed that knockout of NTRC challenges redox regulation of starch synthesis, resulting in stunted growth of the mutant lines acclimated to the SD photoperiod.

The potato tuber, maize endosperm and a chimeric maize-potato ADP-glucose pyrophosphorylase exhibit fundamental differences in Pi inhibition

ADP-glucose pyrophosphorylase (AGPase) is highly regulated by allosteric effectors acting both positively and negatively. Enzymes from various sources differ, however, in the mechanism of allosteric regulation. Here, we determined how the effector, inorganic phosphate (Pi), functions in the presence and absence of saturating amounts of the activator, 3-phosphoglyceric acid (3-PGA). This regulation was examined in the maize endosperm enzyme, the oxidized and reduced forms of the potato tuber enzyme as well as a small subunit chimeric AGPase (MP), which contains both maize endosperm and potato tuber sequences paired with a wild-type maize large subunit. These data, combined with our previous kinetic studies of these enzymes led to a model of Pi inhibition for the various enzymes. The Pi inhibition data suggest that while the maize enzyme contains a single effector site that binds both 3-PGA and Pi, the other enzymes exhibit more complex behavior and most likely have at least two separate interacting binding sites for Pi. The possible physiological implications of the differences in Pi inhibition distinguishing the maize endosperm and potato tuber AGPases are discussed.

Starch biosynthesis, its regulation and biotechnological approaches to improve crop yields

Structurally composed of the glucose homopolymers amylose and amylopectin, starch is the main storage carbohydrate in vascular plants, and is synthesized in the plastids of both photosynthetic and non-photosynthetic cells. Its abundance as a naturally occurring organic compound is surpassed only by cellulose, and represents both a cornerstone for human and animal nutrition and a feedstock for many non-food industrial applications including production of adhesives, biodegradable materials, and first-generation bioethanol. This review provides an update on the different proposed pathways of starch biosynthesis occurring in both autotrophic and heterotrophic organs, and provides emerging information about the networks regulating them and their interactions with the environment. Special emphasis is given to recent findings showing that volatile compounds emitted by microorganisms promote both growth and the accumulation of exceptionally high levels of starch in mono- and dicotyledonous plants. We also review how plant biotechnologists have attempted to use basic knowledge on starch metabolism for the rational design of genetic engineering traits aimed at increasing starch in annual crop species. Finally we present some potential biotechnological strategies for enhancing starch content.

Overexpression of plastidial thioredoxin f leads to enhanced starch accumulation in tobacco leaves

Starch, the most abundant storage carbohydrate in plants, has been a major feedstock for first-generation biofuels. Growing fuel demands require, however, that the starch yields of energy crops be improved. Leaf starch is synthesised during the day and degraded at night to power nonphotosynthetic metabolism. Redox regulation has been associated with the coordination of the enzymes involved in starch metabolism, but neither the signals nor mechanisms that regulate this metabolism are entirely clear. In this work, the thioredoxin (Trx) f and m genes, which code for key enzymes in plastid redox regulation, were overexpressed from the plastid genome. Tobacco plants overexpressing Trx f, but not Trx m, showed an increase of up to 700% in leaf starch accumulation, accompanied by an increase in leaf sugars, specific leaf weight (SLW), and leaf biomass yield. To test the potential of these plants as a nonfood energy crop, tobacco leaves overexpressing Trx f were subjected to enzymatic hydrolysis, and around a 500% increase in the release of fermentable sugars was recorded. The results show that Trx f is a more effective regulator of photosynthetic carbon metabolism in planta than Trx m. The overexpression of Trx f might therefore provide a means of increasing the carbohydrate content of plants destined for use in biofuel production. It might also provide a means of improving the nutritional properties of staple food crops.

Inactivation of thioredoxin f1 leads to decreased light activation of ADP-glucose pyrophosphorylase and altered diurnal starch turnover in leaves of Arabidopsis plants

Chloroplast thioredoxin f (Trx f) is an important regulator of primary metabolic enzymes. However, genetic evidence for its physiological importance is largely lacking. To test the functional significance of Trx f in vivo, Arabidopsis mutants with insertions in the trx f1 gene were studied, showing a drastic decrease in Trx f leaf content. Knockout of Trx f1 led to strong attenuation in reductive light activation of ADP-glucose pyrophosphorylase (AGPase), the key enzyme of starch synthesis, in leaves during the day and in isolated chloroplasts, while sucrose-dependent redox activation of AGPase in darkened leaves was not affected. The decrease in light-activation of AGPase in leaves was accompanied by a decrease in starch accumulation, an increase in sucrose levels and a decrease in starch-to-sucrose ratio. Analysis of metabolite levels at the end of day shows that inhibition of starch synthesis was unlikely due to shortage of substrates or changes in allosteric effectors. Metabolite profiling by gas chromatography-mass spectrometry pinpoints only a small number of metabolites affected, including sugars, organic acids and ethanolamine. Interestingly, metabolite data indicate carbon shortage in trx f1 mutant leaves at the end of night. Overall, results provide in planta evidence for the role played by Trx f in the light activation of AGPase and photosynthetic carbon partitioning in plants.

Mutagenesis of cysteine 81 prevents dimerization of the APS1 subunit of ADP-glucose pyrophosphorylase and alters diurnal starch turnover in Arabidopsis thaliana leaves

Many plants, including Arabidopsis thaliana, retain a substantial portion of their photosynthate in leaves in the form of starch, which is remobilized to support metabolism and growth at night. ADP-glucose pyrophosphorylase (AGPase) catalyses the first committed step in the pathway of starch synthesis, the production of ADP-glucose. The enzyme is redox-activated in the light and in response to sucrose accumulation, via reversible breakage of an intermolecular cysteine bridge between the two small (APS1) subunits. The biological function of this regulatory mechanism was investigated by complementing an aps1 null mutant (adg1) with a series of constructs containing a full-length APS1 gene encoding either the wild-type APS1 protein or mutated forms in which one of the five cysteine residues was replaced by serine. Substitution of Cys81 by serine prevented APS1 dimerization, whereas mutation of the other cysteines had no effect. Thus, Cys81 is both necessary and sufficient for dimerization of APS1. Compared to control plants, the adg1/APS1(C81S) lines had higher levels of ADP-glucose and maltose, and either increased rates of starch synthesis or a starch-excess phenotype, depending on the daylength. APS1 protein levels were five- to tenfold lower in adg1/APS1(C81S) lines than in control plants. These results show that redox modulation of AGPase contributes to the diurnal regulation of starch turnover, with inappropriate regulation of the enzyme having an unexpected impact on starch breakdown, and that Cys81 may play an important role in the regulation of AGPase turnover.

Post-translational redox modification of ADP-glucose pyrophosphorylase in response to light is not a major determinant of fine regulation of transitory starch accumulation in Arabidopsis leaves

ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme comprising two small and two large subunits that catalyze the production of ADP-glucose linked to starch biosynthesis. The current paradigm on leaf starch metabolism assumes that post-translational redox modification of AGP in response to light is a major determinant of fine regulation of transitory starch accumulation. According to this view, under oxidizing conditions occurring during the night the two AGP small subunits (APS1) are covalently linked via an intermolecular disulfide bridge that inactivates the protein, whereas under reducing conditions occurring during the day NADP-thioredoxin reductase C (NTRC)-dependent reductive monomerization of APS1 activates the enzyme. In this work we have analyzed changes in the redox status of APS1 during dark-light transition in leaves of plants cultured under different light intensities. Furthermore, we have carried out time-course analyses of starch content in ntrc mutants, and in aps1 mutants expressing the Escherichia coli redox-insensitive AGP (GlgC) in the chloroplast. We also characterized aps1 plants expressing a redox-insensitive, mutated APS1 (APS1mut) form in which the highly conserved Cys81 residue involved in the formation of the intermolecular disulfide bridge has been replaced by serine. We found that a very moderate, NTRC-dependent APS1 monomerization process in response to light occurred only when plants were cultured under photo-oxidative conditions. We also found that starch accumulation rates during the light in leaves of both ntrc mutants and GlgC-expressing aps1 mutants were similar to those of wild-type leaves. Furthermore, the pattern of starch accumulation during illumination in leaves of APS1mut-expressing aps1 mutants was similar to that of APS1-expressing aps1 mutants at any light intensity. The overall data demonstrate that post-translational redox modification of AGP in response to light is not a major determinant of fine regulation of transitory starch accumulation in Arabidopsis.

Use of TILLING and robotised enzyme assays to generate an allelic series of Arabidopsis thaliana mutants with altered ADP-glucose pyrophosphorylase activity

ADP-glucose pyrophosphorylase (AGPase) catalyses the synthesis of ADP-glucose, and is a highly regulated enzyme in the pathway of starch synthesis. In Arabidopsis thaliana, the enzyme is a heterotetramer, containing two small subunits encoded by the APS1 gene and two large subunits encoded by the APL1-4 genes. TILLING (Targeting Induced Local Lesions IN Genomes) of a chemically mutagenised population of A. thaliana plants identified 33 novel mutations in the APS1 gene, including 21 missense mutations in the protein coding region. High throughput measurements using a robotised cycling assay showed that maximal AGPase activity in the aps1 mutants varied from <15 to 117% of wild type (WT), and that the kinetic properties of the enzyme were altered in several lines, indicating a role for the substituted amino acid residues in catalysis or substrate binding. These results validate the concept of using such a platform for efficient high-throughput screening of very large populations of mutants, natural accessions or introgression lines. AGPase was estimated to have a flux control coefficient of 0.20, indicating that the enzyme exerted only modest control over the rate of starch synthesis in plants grown under short day conditions (8 h light/16 h dark) with an irradiance of 150 μmol quanta m(-2)s(-1). Redox activation of the enzyme, via reduction of the intermolecular disulphide bridge between the two small subunits, was increased in several lines. This was sometimes, but not always, associated with a decrease in the abundance of the APS1 protein. In conclusion, the TILLING technique was used to generate an allelic series of aps1 mutants in A. thaliana that revealed new insights into the multi-layered regulation of AGPase. These mutants offer some advantages over the available loss-of-function mutants, e.g. adg1, for investigating the effects of subtle changes in the enzyme's activity on the rate of starch synthesis.