Adenosine diphosphate glucose pyrophosphatase: A plastidial phosphodiesterase that prevents starch biosynthesis

Glycogen metabolism of the anammox bacterium "Candidatus Brocadia sinica"

Presence of glycogen granules in anaerobic ammonium-oxidizing (anammox) bacteria has been reported so far. However, very little is known about their glycogen metabolism and the exact roles. Here, we studied the glycogen metabolism in "Ca. Brocadia sinica" growing in continuous retentostat cultures with bicarbonate as a carbon source. The effect of the culture growth phase was investigated. During the growing phase, intracellular glycogen content increased up to 32.6 mg-glucose (g-biomass dry wt)-1 while the specific growth rate and ATP/ADP ratio decreased. The accumulated glycogen begun to decrease at the onset of entering the near-zero growth phase and was consumed rapidly when substrates were depleted. This clearly indicates that glycogen was synthesized and utilized as an energy storage. The proteomic analysis revealed that "Ca. B. sinica" synthesized glycogen via three known glycogen biosynthesis pathways and simultaneously degraded during the progress of active anammox, implying that glycogen is being continuously recycled. When cells were starved, a part of stored glycogen was converted to trehalose, a potential stress protectant. This suggests that glycogen serves at least as a primary carbon source of trehalose synthesis for survival. This study provides the first physiological evidence of glycogen metabolism in anammox bacteria and its significance in survival under natural substrate-limited habitat.

Structure of a nucleotide pyrophosphatase/phosphodiesterase (NPP) from Euphorbia characias latex characterized by small-angle X-ray scattering: clues for the general organization of plant NPPs

Little information is available concerning the structural features of nucleotide pyrophosphatase/phosphodiesterases (NPPs) of plant origin and the crystal structures of these proteins have not yet been reported. The aim of this study was to obtain insight into these aspects by carrying out a comparative analysis of the sequences of two different fragments of an NPP from the latex of the Mediterranean shrub Euphorbia characias (ELNPP) and by studying the low-resolution structure of the purified protein in solution by means of small-angle X-ray scattering. This is the first structure of a plant NPP in solution that has been reported to date. It is shown that the ELNPP sequence is highly conserved in many other plant species. Of note, the catalytic domains of these plant NPPs have the same highly conserved PDE-domain organization as mammalian NPPs. Moreover, ELNPP is a dimer in solution and this oligomerization state is likely to be common to other plant enzymes.

Optimization of nucleotide sugar supply for polysaccharide formation via thermodynamic buffering

Plant polysaccharides (cellulose, hemicellulose, pectin, starch) are either direct (i.e. leaf starch) or indirect products of photosynthesis, and they belong to the most abundant organic compounds in nature. Although each of these polymers is made by a specific enzymatic machinery, frequently in different cell locations, details of their synthesis share certain common features. Thus, the production of these polysaccharides is preceded by the formation of nucleotide sugars catalyzed by fully reversible reactions of various enzymes, mostly pyrophosphorylases. These 'buffering' enzymes are, generally, quite active and operate close to equilibrium. The nucleotide sugars are then used as substrates for irreversible reactions of various polysaccharide-synthesizing glycosyltransferases ('engine' enzymes), e.g. plastidial starch synthases, or plasma membrane-bound cellulose synthase and callose synthase, or ER/Golgi-located variety of glycosyltransferases forming hemicellulose and pectin backbones. Alternatively, the irreversible step might also be provided by a carrier transporting a given immediate precursor across a membrane. Here, we argue that local equilibria, established within metabolic pathways and cycles resulting in polysaccharide production, bring stability to the system via the arrangement of a flexible supply of nucleotide sugars. This metabolic system is itself under control of adenylate kinase and nucleoside-diphosphate kinase, which determine the availability of nucleotides (adenylates, uridylates, guanylates and cytidylates) and Mg2+, the latter serving as a feedback signal from the nucleotide metabolome. Under these conditions, the supply of nucleotide sugars to engine enzymes is stable and constant, and the metabolic process becomes optimized in its load and consumption, making the system steady and self-regulated.

Lethality caused by ADP-glucose accumulation is suppressed by salt-induced carbon flux redirection in cyanobacteria

Cyanobacteria are widely distributed photosynthetic organisms. During the day they store carbon, mainly as glycogen, to provide the energy and carbon source they require for maintenance during the night. Here, we generate a mutant strain of the freshwater cyanobacterium Synechocystis sp. PCC 6803 lacking both glycogen synthases. This mutant has a lethal phenotype due to massive accumulation of ADP-glucose, the substrate of glycogen synthases. This accumulation leads to alterations in its photosynthetic capacity and a dramatic decrease in the adenylate energy charge of the cell to values as low as 0.1. Lack of ADP-glucose pyrophosphorylase, the enzyme responsible for ADP-glucose synthesis, or reintroduction of any of the glycogen synthases abolishes the lethal phenotype. Viability of the glycogen synthase mutant is also fully recovered in NaCl-supplemented medium, which redirects the surplus of ADP-glucose to synthesize the osmolite glucosylglycerol. This alternative metabolic sink also suppresses phenotypes associated with the defective response to nitrogen deprivation characteristic of glycogen-less mutants, restoring the capacity to degrade phycobiliproteins. Thus, our system is an excellent example of how inadequate management of the adenine nucleotide pools results in a lethal phenotype, and the influence of metabolic carbon flux in cell viability and fitness.

Transcriptome Analysis and Identification of Genes Associated with Starch Metabolism in Castanea henryi Seed (Fagaceae)

Starch is the most important form of carbohydrate storage and is the major energy reserve in some seeds, especially Castanea henryi. Seed germination is the beginning of the plant's life cycle, and starch metabolism is important for seed germination. As a complex metabolic pathway, the regulation of starch metabolism in C. henryi is still poorly understood. To explore the mechanism of starch metabolism during the germination of C. henryi, we conducted a comparative gene expression analysis at the transcriptional level using RNA-seq across four different germination stages, and analyzed the changes in the starch and soluble sugar contents. The results showed that the starch content increased in 0-10 days and decreased in 10-35 days, while the soluble sugar content continuously decreased in 0-30 days and increased in 30-35 days. We identified 49 candidate genes that may be associated with starch and sucrose metabolism. Three ADP-glucose pyrophosphorylase (AGPase) genes, two nucleotide pyrophosphatase/phosphodiesterases (NPPS) genes and three starch synthases (SS) genes may be related to starch accumulation. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to validate the expression levels of these genes. Our study combined transcriptome data with physiological and biochemical data, revealing potential candidate genes that affect starch metabolism during seed germination, and provides important data about starch metabolism and seed germination in seed plants.

Proteomic Analysis of the Resistance Mechanisms in Sugarcane during Sporisorium scitamineum Infection

Smut disease is caused by Sporisorium scitamineum, an important sugarcane fungal pathogen causing an extensive loss in yield and sugar quality. The available literature suggests that there are two types of smut resistance mechanisms: external resistance by physical or chemical barriers and intrinsic internal resistance mechanisms operating at host–pathogen interaction at cellular and molecular levels. The nature of smut resistance mechanisms, however, remains largely unknown. The present study investigated the changes in proteome occurring in two sugarcane varieties with contrasting susceptibility to smut—F134 and NCo310—at whip development stage after S. scitamineum infection. Total proteins from pathogen inoculated and uninoculated (control) leaves were separated by two-dimensional gel electrophoresis (2D-PAGE). Protein identification was performed using BLASTp and tBLASTn against NCBI nonredundant protein databases and EST databases, respectively. A total of thirty proteins spots representing differentially expressed proteins (DEPs), 16 from F134 and 14 from NCo310, were identified and analyzed by MALDI-TOF/TOF MS. In F134, 4 DEPs were upregulated and nine were downregulated, while, nine were upregulated and three were downregulated in NCo310. The DEPs were associated with DNA binding, metabolic processes, defense, stress response, photorespiration, protein refolding, chloroplast, nucleus and plasma membrane. Finally, the expression of CAT, SOD, and PAL with recognized roles in S. scitamineum infection in both sugarcane verities were analyzed by real-time quantitative PCR (RT-qPCR) technique. Identification of genes critical for smut resistance in sugarcane will increase our knowledge of S. scitamineum-sugarcane interaction and help to develop molecular and conventional breeding strategies for variety improvement.

Proteomics Analysis Reveals Non-Controlled Activation of Photosynthesis and Protein Synthesis in a Rice npp1 Mutant under High Temperature and Elevated CO₂ Conditions

Rice nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) catalyzes the hydrolytic breakdown of the pyrophosphate and phosphodiester bonds of a number of nucleotides including ADP-glucose and ATP. Under high temperature and elevated CO₂ conditions (HT + ECO₂), the npp1 knockout rice mutant displayed rapid growth and high starch content phenotypes, indicating that NPP1 exerts a negative effect on starch accumulation and growth. To gain further insight into the mechanisms involved in the NPP1 downregulation induced starch overaccumulation, in this study we conducted photosynthesis, leaf proteomic, and chloroplast phosphoproteomic analyses of wild-type (WT) and npp1 plants cultured under HT + ECO₂. Photosynthesis in npp1 leaves was significantly higher than in WT. Additionally, npp1 leaves accumulated higher levels of sucrose than WT. The proteomic analyses revealed upregulation of proteins related to carbohydrate metabolism and the protein synthesis system in npp1 plants. Further, our data indicate the induction of 14-3-3 proteins in npp1 plants. Our finding demonstrates a higher level of protein phosphorylation in npp1 chloroplasts, which may play an important role in carbohydrate accumulation. Together, these results offer novel targets and provide additional insights into carbohydrate metabolism regulation under ambient and adverse conditions.

Metabolomics and proteomics reveal drought-stress responses of leaf tissues from spring-wheat

To reveal the integrative biochemical networks of wheat leaves in response to water deficient conditions, proteomics and metabolomics were applied to two spring-wheat cultivars (Bahar, drought-susceptible; Kavir, drought-tolerant). Drought stress induced detrimental effects on Bahar leaf proteome, resulting in a severe decrease of total protein content, with impairments mainly in photosynthetic proteins and in enzymes involved in sugar and nitrogen metabolism, as well as in the capacity of detoxifying harmful molecules. On the contrary, only minor perturbations were observed at the protein level in Kavir stressed leaves. Metabolome analysis indicated amino acids, organic acids, and sugars as the main metabolites changed in abundance upon water deficiency. In particular, Bahar cv showed increased levels in proline, methionine, arginine, lysine, aromatic and branched chain amino acids. Tryptophan accumulation via shikimate pathway seems to sustain auxin production (indoleacrylic acid), whereas glutamate reduction is reasonably linked to polyamine (spermine) synthesis. Kavir metabolome was affected by drought stress to a less extent with only two pathways significantly changed, one of them being purine metabolism. These results comprehensively provide a framework for better understanding the mechanisms that govern plant cell response to drought stress, with insights into molecules that can be used for crop improvement projects.

The genetic architecture of amylose biosynthesis in maize kernel

Starch is the most abundant storage carbohydrate in maize kernel. The content of amylose and amylopectin confers unique properties in food processing and industrial application. Thus, the resurgent interest has been switched to the study of individual amylose or amylopectin rather than total starch, whereas the enzymatic machinery for amylose synthesis remains elusive. We took advantage of the phenotype of amylose content and the genotype of 9,007,194 single nucleotide polymorphisms from 464 inbred maize lines. The genome-wide association study identified 27 associated loci involving 39 candidate genes that were linked to amylose content including transcription factors, glycosyltransferases, glycosidases, as well as hydrolases. Except the waxy gene that encodes the granule-bound starch synthase, the remaining candidate genes were located in the upstream pathway of amylose synthesis, while the downstream members were already known from prior studies. The linked candidate genes could be transferred to manipulate amylose content and thus add value to maize kernel in the breeding programme.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

The starch-rich endosperms of the Poaceae, which includes wild grasses and their domesticated descendents the cereals, have provided humankind and their livestock with the bulk of their daily calories since the dawn of civilization up to the present day. There are currently unprecedented pressures on global food supplies, largely resulting from population growth, loss of agricultural land that is linked to increased urbanization, and climate change. Since cereal yields essentially underpin world food and feed supply, it is critical that we understand the biological factors contributing to crop yields. In particular, it is important to understand the biochemical pathway that is involved in starch biosynthesis, since this pathway is the major yield determinant in the seeds of six out of the top seven crops grown worldwide. This review outlines the critical stages of growth and development of the endosperm tissue in the Poaceae, including discussion of carbon provision to the growing sink tissue. The main body of the review presents a current view of our understanding of storage starch biosynthesis, which occurs inside the amyloplasts of developing endosperms.

New insights into the impacts of elevated CO2, nitrogen, and temperature levels on the regulation of C and N metabolism in durum wheat using network analysis

The use of correlation networks and hierarchical cluster analysis provides a framework to organize and study the coordination of parameters such as genes, metabolites, proteins and physiological parameters. We have analyzed 142 traits from primary C and N metabolism, including biochemical and gene expression analyses, in a range of 32 different growth conditions (various [CO₂] levels, temperatures, N supplies, growth stages and experimental methods). To test the integration of primary metabolism, particularly under climate change, we investigated which C and N metabolic traits and transcript levels are correlated in durum wheat flag leaves using a correlation network and a hierarchical cluster analysis. There was a high amount of positive correlation between traits involved in a wide range of biological processes, suggesting a close and intricate coordination between C-N metabolisms at the biochemical and transcriptional levels. Transcript levels for genes related to N uptake and assimilation were especially coexpressed with genes belonging to the respiratory pathway, highlighting the coordination between the synthesis of organic N compounds and provision of energy and C skeletons. Also involved in this coordination were Rubisco and nitrate reductase activities, which play a key role in the regulation of plant metabolism. Carbohydrate accumulation was linked with a down-regulation of photosynthetic and N metabolism genes and nitrate reductase activity. Based on the degree of connectivity between nodes, network exploration facilitated the identification of some traits that may be biologically relevant during plant abiotic stress tolerance, as most of them are involved in limiting steps of plant metabolism.

Functional and structural characterization of plastidic starch phosphorylase during barley endosperm development

The production of starch is essential for human nutrition and represents a major metabolic flux in the biosphere. The biosynthesis of starch in storage organs like barley endosperm operates via two main pathways using different substrates: starch synthases use ADP-glucose to produce amylose and amylopectin, the two major components of starch, whereas starch phosphorylase (Pho1) uses glucose-1-phosphate (G1P), a precursor for ADP-glucose production, to produce α-1,4 glucans. The significance of the Pho1 pathway in starch biosynthesis has remained unclear. To elucidate the importance of barley Pho1 (HvPho1) for starch biosynthesis in barley endosperm, we analyzed HvPho1 protein production and enzyme activity levels throughout barley endosperm development and characterized structure-function relationships of HvPho1. The molecular mechanisms underlying the initiation of starch granule biosynthesis, that is, the enzymes and substrates involved in the initial transition from simple sugars to polysaccharides, remain unclear. We found that HvPho1 is present as an active protein at the onset of barley endosperm development. Notably, purified recombinant protein can catalyze the de novo production of α-1,4-glucans using HvPho1 from G1P as the sole substrate. The structural properties of HvPho1 provide insights into the low affinity of HvPho1 for large polysaccharides like starch or amylopectin. Our results suggest that HvPho1 may play a role during the initiation of starch biosynthesis in barley.

Purification of Maize Nucleotide Pyrophosphatase/Phosphodiesterase Casts Doubt on the Existence of Zeatin Cis-Trans Isomerase in Plants

Almost 25 years ago, an enzyme named zeatin cis-trans isomerase from common bean has been described by Bassil et al. (1993). The partially purified enzyme required an external addition of FAD and dithiothreitol for the conversion of cis-zeatin to its trans- isomer that occurred only under light. Although an existence of this important enzyme involved in the metabolism of plant hormones cytokinins was generally accepted by plant biologists, the corresponding protein and encoding gene have not been identified to date. Based on the original paper, we purified and identified an enzyme from maize, which shows the described zeatin cis-trans isomerase activity. The enzyme belongs to nucleotide pyrophosphatase/phosphodiesterase family, which is well characterized in mammals, but less known in plants. Further experiments with the recombinant maize enzyme obtained from yeast expression system showed that rather than the catalytic activity of the enzyme itself, a non-enzymatic flavin induced photoisomerization is responsible for the observed zeatin cis-trans interconversion in vitro. An overexpression of the maize nucleotide pyrophosphatase/phosphodiesterase gene led to decreased FAD and increased FMN and riboflavin contents in transgenic Arabidopsis plants. However, neither contents nor the ratio of zeatin isomers was altered suggesting that the enzyme is unlikely to catalyze the interconversion of zeatin isomers in vivo. Using enhanced expression of a homologous gene, functional nucleotide pyrophosphatase/phosphodiesterase was also identified in rice.

N-Glycomic and Microscopic Subcellular Localization Analyses of NPP1, 2 and 6 Strongly Indicate that trans-Golgi Compartments Participate in the Golgi to Plastid Traffic of Nucleotide Pyrophosphatase/Phosphodiesterases in Rice

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) are widely distributed N-glycosylated enzymes that catalyze the hydrolytic breakdown of numerous nucleotides and nucleotide sugars. In many plant species, NPPs are encoded by a small multigene family, which in rice are referred to NPP1-NPP6 Although recent investigations showed that N-glycosylated NPP1 is transported from the endoplasmic reticulum (ER)-Golgi system to the chloroplast through the secretory pathway in rice cells, information on N-glycan composition and subcellular localization of other NPPs is still lacking. Computer-assisted analyses of the amino acid sequences deduced from different Oryza sativa NPP-encoding cDNAs predicted all NPPs to be secretory glycoproteins. Confocal fluorescence microscopy observation of cells expressing NPP2 and NPP6 fused with green fluorescent protein (GFP) revealed that NPP2 and NPP6 are plastidial proteins. Plastid targeting of NPP2-GFP and NPP6-GFP was prevented by brefeldin A and by the expression of ARF1(Q71L), a dominant negative mutant of ADP-ribosylation factor 1 that arrests the ER to Golgi traffic, indicating that NPP2 and NPP6 are transported from the ER-Golgi to the plastidial compartment. Confocal laser scanning microscopy and high-pressure frozen/freeze-substituted electron microscopy analyses of transgenic rice cells ectopically expressing the trans-Golgi marker sialyltransferase fused with GFP showed the occurrence of contact of Golgi-derived membrane vesicles with cargo and subsequent absorption into plastids. Sensitive and high-throughput glycoblotting/mass spectrometric analyses showed that complex-type and paucimannosidic-type glycans with fucose and xylose residues occupy approximately 80% of total glycans of NPP1, NPP2 and NPP6. The overall data strongly indicate that the trans-Golgi compartments participate in the Golgi to plastid trafficking and targeting mechanism of NPPs.

Transcriptome and selected metabolite analyses reveal points of sugar metabolism in jackfruit (Artocarpus heterophyllus Lam.)

Artocarpus heterophyllus Lam., commonly known as jackfruit, produces the largest tree-borne fruit known thus far. The edible part of the fruit develops from the perianths, and contains many sugar-derived compounds. However, its sugar metabolism is poorly understood. A fruit perianth transcriptome was sequenced on an Illumina HiSeq 2500 platform, producing 32,459 unigenes with an average length of 1345nt. Sugar metabolism was characterized by comparing expression patterns of genes related to sugar metabolism and evaluating correlations with enzyme activity and sugar accumulation during fruit perianth development. During early development, high expression levels of acid invertases and corresponding enzyme activities were responsible for the rapid utilization of imported sucrose for fruit growth. The differential expression of starch metabolism-related genes and corresponding enzyme activities were responsible for starch accumulated before fruit ripening but decreased during ripening. Sucrose accumulated during ripening, when the expression levels of genes for sucrose synthesis were elevated and high enzyme activity was observed. The comprehensive transcriptome analysis presents fundamental information on sugar metabolism and will be a useful reference for further research on fruit perianth development in jackfruit.

Human cytomegalovirus RL13 protein interacts with host NUDT14 protein affecting viral DNA replication

The interaction between the host and human cytomegalovirus (HCMV) is important in determining the outcome of a viral infection. The HCMV RL13 gene product exerts independent, inhibitory effects on viral growth in fibroblasts and epithelial cells. At present, there are few reports on the interactions between the HCMV RL13 protein and human host proteins. The present study provided direct evidence for the specific interaction between HCMV RL13 and host nucleoside diphosphate linked moiety X (nudix)‑type motif 14 (NUDT14), a UDP‑glucose pyrophosphatase, using two‑hybrid screening, an in vitro glutathione S‑transferase pull‑down assay, and co‑immunoprecipitation in human embryonic kidney HEK293 cells. Additionally, the RL13 protein was shown to co‑localize with the NUDT14 protein in the HEK293 cell membrane and cytoplasm, demonstrated using fluorescence confocal microscopy. Decreasing the expression level of NUDT14 via NUDT14‑specific small interfering RNAs increased the number of viral DNA copies in the HCMV‑infected cells. However, the overexpression of NUDT14 in a stably expressing cell line did not affect viral DNA levels significantly in the HCMV infected cells. Based on the known functions of NUDT14, the results of the present study suggested that the interaction between the RL13 protein and NUDT14 protein may be involved in HCMV DNA replication, and that NUDT14 may offer potential in the modulation of viral infection.

Multimeric states of starch phosphorylase determine protein-protein interactions with starch biosynthetic enzymes in amyloplasts

Protein-protein interactions between starch phosphorylase (SP) and other starch biosynthetic enzymes were investigated using isolated maize endosperm amyloplasts and a recombinant maize enzyme. Plastidial SP is a stromal enzyme existing as a multimeric protein in amyloplasts. Biochemical analysis of the recombinant maize SP indicated that the tetrameric form was catalytically active in both glucan-synthetic and phosphorolytic directions. Protein-protein interaction experiments employing the recombinant SP as an affinity ligand with amyloplast extracts showed that the multimeric state of SP determined interactions with other enzymes of the starch biosynthetic pathway. The monomeric form of SP interacts with starch branching enzyme I (SBEI) and SBEIIb, whereas only SBEI interacts with the tetrameric form of SP. In all cases, protein-protein interactions were broken when amyloplast lysates were dephosphorylated in vitro, and enhanced following pre-treatment with ATP, suggesting a mechanism of protein complex formation regulated by protein phosphorylation. In vitro protein phosphorylation experiments with [γ-(32)P]-ATP show that SP is phosphorylated by a plastidial protein kinase. Evidence is presented which suggests SBEIIb modulates the catalytic activity of SP through the formation of a heteromeric protein complex.

HPLC-MS/MS analyses show that the near-Starchless aps1 and pgm leaves accumulate wild type levels of ADPglucose: further evidence for the occurrence of important ADPglucose biosynthetic pathway(s) alternative to the pPGI-pPGM-AGP pathway

In leaves, it is widely assumed that starch is the end-product of a metabolic pathway exclusively taking place in the chloroplast that (a) involves plastidic phosphoglucomutase (pPGM), ADPglucose (ADPG) pyrophosphorylase (AGP) and starch synthase (SS), and (b) is linked to the Calvin-Benson cycle by means of the plastidic phosphoglucose isomerase (pPGI). This view also implies that AGP is the sole enzyme producing the starch precursor molecule, ADPG. However, mounting evidence has been compiled pointing to the occurrence of important sources, other than the pPGI-pPGM-AGP pathway, of ADPG. To further explore this possibility, in this work two independent laboratories have carried out HPLC-MS/MS analyses of ADPG content in leaves of the near-starchless pgm and aps1 mutants impaired in pPGM and AGP, respectively, and in leaves of double aps1/pgm mutants grown under two different culture conditions. We also measured the ADPG content in wild type (WT) and aps1 leaves expressing in the plastid two different ADPG cleaving enzymes, and in aps1 leaves expressing in the plastid GlgC, a bacterial AGP. Furthermore, we measured the ADPG content in ss3/ss4/aps1 mutants impaired in starch granule initiation and chloroplastic ADPG synthesis. We found that, irrespective of their starch contents, pgm and aps1 leaves, WT and aps1 leaves expressing in the plastid ADPG cleaving enzymes, and aps1 leaves expressing in the plastid GlgC accumulate WT ADPG content. In clear contrast, ss3/ss4/aps1 leaves accumulated ca. 300 fold-more ADPG than WT leaves. The overall data showed that, in Arabidopsis leaves, (a) there are important ADPG biosynthetic pathways, other than the pPGI-pPGM-AGP pathway, (b) pPGM and AGP are not major determinants of intracellular ADPG content, and (c) the contribution of the chloroplastic ADPG pool to the total ADPG pool is low.

Nucleotide pyrophosphatase/phosphodiesterase 1 exerts a negative effect on starch accumulation and growth in rice seedlings under high temperature and CO2 concentration conditions

Nucleotide pyrophosphatase/phosphodiesterase (NPP) is a widely distributed enzymatic activity occurring in both plants and mammals that catalyzes the hydrolytic breakdown of the pyrophosphate and phosphodiester bonds of a number of nucleotides. Unlike mammalian NPPs, the physiological function of plant NPPs remains largely unknown. Using a complete rice NPP1-encoding cDNA as a probe, in this work we have screened a rice shoot cDNA library and obtained complete cDNAs corresponding to six NPP genes (NPP1-NPP6). As a first step to clarify the role of NPPs, recombinant NPP1, NPP2 and NPP6 were purified from transgenic rice cells constitutively expressing NPP1, NPP2 and NPP6, respectively, and their enzymatic properties were characterized. NPP1 and NPP6 exhibited hydrolytic activities toward ATP, UDP-glucose and the starch precursor molecule, ADP-glucose, whereas NPP2 did not recognize nucleotide sugars as substrates, but hydrolyzed UDP, ADP and adenosine 5'-phosphosulfate. To gain insight into the physiological function of rice NPP1, an npp1 knockout mutant was characterized. The ADP-glucose hydrolytic activities in shoots of npp1 rice seedlings were 8% of those of the wild type (WT), thus indicating that NPP1 is a major determinant of ADP-glucose hydrolytic activity in rice shoots. Importantly, when seedlings were cultured at 160 Pa CO2 under a 28°C/23°C (12 h light/12 h dark) regime, npp1 shoots and roots were larger than those of wild-type (WT) seedlings. Furthermore, the starch content in the npp1 shoots was higher than that of WT shoots. Growth and starch accumulation were also enhanced under an atmospheric CO2 concentration (40 Pa) when plants were cultured under a 33°C/28°C regime. The overall data strongly indicate that NPP1 exerts a negative effect on plant growth and starch accumulation in shoots, especially under high CO2 concentration and high temperature conditions.

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.

Primary and secondary metabolism in the sun-exposed peel and the shaded peel of apple fruit

The metabolism of carbohydrates, organic acids, amino acids and phenolics was compared between the sun-exposed peel and the shaded peel of apple fruit. Contents of sorbitol and glucose were higher in the sun-exposed peel, whereas those of sucrose and fructose were almost the same in the two peel types. This was related to lower sorbitol dehydrogenase activity and higher activities of sorbitol oxidase, neutral invertase and acid invertase in the sun-exposed peel. The lower starch content in the sun-exposed peel was related to lower sucrose synthase activity early in fruit development. Dark respiratory metabolism in the sun-exposed peel was enhanced by the high peel temperature due to high light exposure. Activities of most enzymes in respiratory metabolism were higher in the sun-exposed peel, but the concentrations of most organic acids were relatively stable, except pyruvate and oxaloacetate. Due to the different availability of carbon skeletons from dark respiration in the two peel types, amino acids with higher C/N ratios are accumulated in the sun-exposed peel whereas those with lower C/N ratios are accumulated in the shaded peel. Contents of anthocyanins and flavonols and activities of phenylalanine ammonia-lyase, UDP-galactose:flavonoid 3-O-glucosyltransferase and several other enzymes were higher in the sun-exposed peel than in the shaded peel, indicating the entire phenylpropanoid pathway is upregulated in the sun-exposed peel. Comprehensive analyses of the metabolites and activities of enzymes involved in primary metabolism and secondary metabolism have allowed us to gain a full picture of the metabolic network in the two peel types under natural light exposure.

Proteomic characterization of the Rph15 barley resistance gene-mediated defence responses to leaf rust

Background

Leaf rust, caused by the biotrophic fungal pathogen Puccinia hordei, is one of the most important foliar disease of barley (Hordeum vulgare) and represents a serious threat in many production regions of the world. The leaf rust resistance gene Rph15 is of outstanding interest for resistance breeding because it confers resistance to over 350 Puccinia hordei isolates collected from around the world. Molecular and biochemical mechanisms responsible for the Rph15 effectiveness are currently not investigated. The aim of the present work was to study the Rph15-based defence responses using a proteomic approach.

Results

Protein pattern changes in response to the leaf rust pathogen infection were investigated in two barley near isogenic lines (NILs), Bowman (leaf rust susceptible) and Bowman-Rph15 (leaf rust resistant), differing for the introgression of the leaf rust resistance gene Rph15. Two infection time points, 24 hours and four days post inoculation (dpi), were analysed. No statistically significant differences were identified at the early time point, while at 4 dpi eighteen protein spots were significantly up or down regulated with a fold-change equal or higher than two in response to pathogen infection. Almost all the pathogen-responsive proteins were identified in the Bowman-Rph15 resistant NIL. Protein spots were characterized by LC-MS/MS analysis and found to be involved in photosynthesis and energy metabolism, carbohydrate metabolism, protein degradation and defence. Proteomic data were complemented by transcriptional analysis of the respective genes. The identified proteins can be related to modulation of the photosynthetic apparatus components, re-direction of the metabolism to sustain defence responses and deployment of defence proteins.

Conclusions

The identification of leaf rust infection-modulated defence responses restricted to the resistant NIL support the hypothesis that basal defence responses of Bowman, but not the Rph15 resistance gene-based ones, are suppressed or delayed by pathogen effectors to levels below the detection power of the adopted proteomic approach. Additionally, Rph15-mediated resistance processes identified mainly resides on a modulation of primary metabolism, affecting photosyntesis and carbohydrate pool.

Does ear C sink strength contribute to overcoming photosynthetic acclimation of wheat plants exposed to elevated CO2?

Wheat plants (Triticum durum Desf., cv. Regallo) were grown in the field to study the effects of contrasting [CO(2)] conditions (700 versus 370 μmol mol(-1)) on growth, photosynthetic performance, and C management during the post-anthesis period. The aim was to test whether a restricted capacity of sink organs to utilize photosynthates drives a loss of photosynthetic capacity in elevated CO(2). The ambient (13)C/(12)C isotopic composition (δ(13)C) of air CO(2) was changed from -10.2‰ in ambient [CO(2)] to -23.6‰ under elevated [CO(2)] between the 7th and the 14th days after anthesis in order to study C assimilation and partitioning between leaves and ears. Elevated [CO(2)] had no significant effect on biomass production and grain filling, and caused an accumulation of C compounds in leaves. This was accompanied by up-regulation of phosphoglycerate mutase and ATP synthase protein content, together with down-regulation of adenosine diphosphate glucose pyrophosphatase protein. Growth in elevated [CO(2)] negatively affected Rubisco and Rubisco activase protein content and induced photosynthetic down-regulation. CO(2) enrichment caused a specific decrease in Rubisco content, together with decreases in the amino acid and total N content of leaves. The C labelling revealed that in flag leaves, part of the C fixed during grain filling was stored as starch and structural C compounds whereas the rest of the labelled C (mainly in the form of soluble sugars) was completely respired 48 h after the end of labelling. Although labelled C was not detected in the δ(13)C of ear total organic matter and respired CO(2), soluble sugar δ(13)C revealed that a small amount of labelled C reached the ear. The (12)CO(2) labelling suggests that during the beginning of post-anthesis the ear did not contribute towards overcoming flag leaf carbohydrate accumulation, and this had a consequent effect on protein expression and photosynthetic acclimation.

Influence of high temperature during grain filling on the accumulation of storage proteins and grain quality in rice (Oryza sativa L.)

The present study was performed to understand the effects of high temperature (HT) during filling on the expression of storage proteins and the quality of rice grains. HT (35/30 °C day/night) reduced the weight, amylose content, and flour gel consistency of grains. It increased the accumulation of all classes of storage proteins at early filling stage but decreased the accumulation of prolamins at maturation. For albumins, the expressions of cyclophilin 2, peroxiredoxin, and HSP16.9 were differentially enhanced by HT. For globulins, HT decreased the accumulation of globulin but increased that of glyoxalase I and peroxiredoxin. HT enhanced the transcription of genes for glutelins, prolamins, globulins, and protein disulfide isomerase at early filling stage but decreased the expression of these genes at a later stage. Low amounts of prolamins and globulins, as well as low pH value, were found in sound, immature, and dead kernels grown under HT. The relationships among HT, storage proteins, and grain quality are discussed.

Differential proteomic response to heat stress in thermal Agrostis scabra and heat-sensitive Agrostis stolonifera

Knowledge of heat-responsive proteins is critical for further understanding of the molecular mechanisms of heat tolerance. The objective of this study was to compare proteins differentially expressed in two C(3) grass species contrasting in heat tolerance, heat-tolerant thermal Agrostis scabra and heat-sensitive Agrostis stolonifera L., and to identify heat-responsive proteins for short- and long-term responses. Plants were exposed to 20/15 degrees C (day/night, control) or 40/35 degrees C (day/night, heat stress) in growth chambers. Leaves were harvested at 2 and 10 days after temperature treatment. Proteins were extracted and separated by fluorescence difference gel electrophoresis (DIGE). Thermal A. scabra had superior heat tolerance than A. stolonifera, as indicated by the maintenance of higher chlorophyll content and photochemical efficiency under heat stress. The two-dimensional difference electrophoresis detected 68 heat-responsive proteins in the two species. Thermal A. scabra had more protein spots either down- or up-regulated at 2 days of heat stress, but fewer protein spots were altered at 10 days of heat stress compared with A. stolonifera. Many protein spots exhibited transient down-regulation in thermal A. scabra (only at 2 days of heat treatment), whereas down-regulation of many proteins was also found at 10 days of heat treatment in A. stolonifera, which suggested that protein metabolism in thermal A. scabra might acclimate to heat stress more rapidly than those in A. stolonifera. The sequences of 56 differentially expressed protein spots were identified using mass spectrometry. The results suggest that the maintenance or less severe down-regulation of proteins during long-term (10 days) heat stress may contribute to the superior heat tolerance in thermal A. scabra, including those involved in photosynthesis [RuBisCo, RuBisCo activase, chloroplastic glyceraldehydes-3-phosphate dehydrogenase (GAPDH), chloroplastic aldolase, oxygen-evolving complex, photosystem I subunits], dark respiration (cytosolic GAPDH, cytoplasmic aldolase, malate dehydrogenase, hydroxypyruvate reductase, sedoheptulose-1,7-bisphosphatase), photorespiration [(hydroxypyruvate reductase, alanine aminotransferase (AlaAT), hydroxymethyltransferase (SHMT), glycine decarboxylase (GDC)], as well as heat and oxidative stress protection [heat shock cognate (HSC) 70 and FtsH-like protein].

Characterization of mouse UDP-glucose pyrophosphatase, a Nudix hydrolase encoded by the Nudt14 gene

Recombinant mouse UDP-glucose pyrophosphatase (UGPPase), encoded by the Nudt14 gene, was produced in Escherichia coli and purified close to homogeneity. The enzyme catalyzed the conversion of [beta-(32)P]UDP-glucose to [(32)P]glucose-1-P and UMP, confirming that it hydrolyzed the pyrophosphate of the nucleoside diphosphate sugar to generate glucose-1-P and UMP. The enzyme was also active toward ADP-ribose. Activity is dependent on the presence of Mg(2+) and was greatest at alkaline pH above 8. Kinetic analysis indicated a K(m) of approximately 4mM for UDP-glucose and approximately 0.3mM for ADP-ribose. Based on V(max)/K(m) values, the enzyme was approximately 20-fold more active toward ADP-ribose. UGPPase behaves as a dimer in solution and can be cross-linked to generate a species of M(r) 54,000 from a monomer of 30,000 as judged by SDS-PAGE. The dimerization was not affected by the presence of glucose-1-P or UDP-glucose. Using antibodies raised against the recombinant protein, Western analysis indicated that UGPPase was widely expressed in mouse tissues, including skeletal muscle, liver, kidney, heart, lung, fat, heart and pancreas with a lower level in brain. It was generally present as a doublet when analyzed by SDS-PAGE, suggesting the occurrence of some form of post-translational modification. Efforts to interconvert the species by adding or inhibiting phosphatase activity were unsuccessful, leaving the nature of the modification unknown. Sequence alignments and database searches revealed related proteins in species as distant as Drosophila melanogaster and Caenorhabditis elegans.

Diphosphonucleotide phosphatase/phosphodiesterase (PPD1) from yellow lupin (Lupinus luteus L.) contains an iron-manganese center

Yellow lupin diphosphonucleotide phosphatase/phosphodiesterase (PPD1) represents a novel group of enzymes. Here we report that it possesses one iron atom and one manganese atom (1:1 molar ratio) per subunit. The enzyme exhibits visible absorption maximum at approximately 530 nm. Prolonged oxidation of PPD1 leads to loss of the charge-transfer band and catalytic activity, whereas after reduction PPD1 remains active. Replacement of conserved amino-acid residues coordinating metals results in the loss of enzymatic activity. Despite low amino-acid sequence homology of PPD1 to well-characterized approximately 55-kDa purple acid phosphatases, their overall fold, topology of active center and metal content are highly similar.

Gene expression profiling of tolerant barley in response to Diuraphis noxia (Hemiptera: Aphididae) feeding

Aphids are, arguably, the single most damaging group of agricultural insect pests throughout the world. Plant tolerance, which is a plant response to an insect pest, is viewed as an excellent management strategy. Developing testable hypotheses based on genome-wide and more focused methods will help in understanding the molecular underpinnings of plant tolerance to aphid herbivory. As a first step in this process, we undertook transcript profiling with Affymetrix GeneChip Barley Genome arrays using RNA extracted from tissues of tolerant and susceptible genotypes collected at three hours, three days and six days after Diuraphis noxia introduction. Acquired data were compared to identify changes unique to the tolerant barley at each harvest date. Transcript abundance of 4086 genes was differentially changed over the three harvest dates in tolerant and susceptible barley in response to D. noxia feeding. Across the three harvest dates, the greatest number of genes was differentially expressed in both barleys at three days after aphid introduction. A total of 909 genes showed significant levels of change in the tolerant barley in response to D. noxia feeding as compared to susceptible plants infested with aphids. Many of these genes could be assigned to specific metabolic categories, including several associated with plant defense and scavenging of reactive oxygen species (ROS). Interestingly, two peroxidase genes, designated HvPRXA1 and HvPRXA2, were up-regulated to a greater degree in response to D. noxia feeding on tolerant barley plants, indicating that specific peroxidases could be important for the tolerance process. These findings suggest that the ability to elevate and sustain levels of ROS-scavenging enzymes could play an important role in the tolerant response.

Effects of beta-1,3-glucan from Septoria tritici on structural defence responses in wheat

The accumulation of the pathogenesis-related (PR) proteins beta-1,3-glucanase and chitinase and structural defence responses were studied in leaves of wheat either resistant or susceptible to the hemibiotrophic pathogen Septoria tritici. Resistance was associated with an early accumulation of beta-1,3-glucanase and chitinase transcripts followed by a subsequent reduction in level. Resistance was also associated with high activity of beta-1,3-glucanase, especially in the apoplastic fluid, in accordance with the biotrophic/endophytic lifestyle of the pathogen in the apoplastic spaces, thus showing the highly localized accumulation of defence proteins in the vicinity of the pathogen. Isoform analysis of beta-1,3-glucanase from the apoplastic fluid revealed that resistance was associated with the accumulation of an endo-beta-1,3-glucanase, previously implicated in defence against pathogens, and a protein with identity to ADPG pyrophosphatase (92%) and germin-like proteins (93%), which may be involved in cell wall reinforcement. In accordance with this, glycoproteins like extensin were released into the apoplast and callose accumulated to a greater extent in cell walls, whereas lignin and polyphenolics were not found to correlate with defence. Treatment of a susceptible wheat cultivar with purified beta-1,3-glucan fragments from cell walls of S. tritici gave complete protection against disease and this was accompanied by increased gene expression of beta-1,3-glucanase and the deposition of callose. Collectively, these data indicate that resistance is dependent on a fast, initial recognition of the pathogen, probably due to beta-1,3-glucan in the fungal cell walls, and this results in the accumulation of beta-1,3-glucanase and structural defence responses, which may directly inhibit the pathogen and protect the host against fungal enzymes and toxins.

Identification of leaf proteins differentially accumulated during cold acclimation between Festuca pratensis plants with distinct levels of frost tolerance

Festuca pratensis (meadow fescue) as the most frost-tolerant species within the Lolium-Festuca complex was used as a model for research aimed at identifying the cellular components involved in the cold acclimation (CA) of forage grasses. The work presented here also comprises the first comprehensive proteomic research on CA in a group of monocotyledonous species which are able to withstand winter conditions. Individual F. pratensis plants with contrasting levels of frost tolerance, high frost tolerant (HFT) and low frost tolerant (LFT) plants, were selected for comparative proteomic research. The work focused on the analysis of leaf protein accumulation before and after 2, 8, and 26 h, and 3, 5, 7, 14, and 21 d of CA, using high-throughput two-dimensional electrophoresis, and on the identification of proteins which were accumulated differentially between the selected plants by the application of mass spectrometry. The analyses of approximately 800 protein profiles revealed a total of 41 (5.1%) proteins that showed a minimum of a 1.5-fold difference in abundance, at a minimum of one time point of CA for HFT and LFT genotypes. It was shown that significant differences in profiles of protein accumulation between the analysed plants appeared relatively early during cold acclimation, most often after 26 h (on the 2nd day) of CA and one-half of the differentially accumulated proteins were all parts of the photosynthetic apparatus. Several proteins identified here have been reported to be differentially accumulated during cold conditions for the first time in this paper. The functions of the selected proteins in plant cells and their probable influence on the level of frost tolerance in F. pratensis, are discussed.

Plastidial localization of a potato 'Nudix' hydrolase of ADP-glucose linked to starch biosynthesis

Escherichia coli and potato (Solanum tuberosum) ADP-sugar pyrophosphatases (EcASPP and StASPP, respectively) are 'Nudix' hydrolases of the bacterial glycogen and starch precursor molecule, ADP-glucose (ADPG). We have previously shown that potato leaves expressing EcASPP either in the cytosol or in the chloroplast exhibited large reductions in the levels of starch, suggesting the occurrence of cytosolic and plastidial pools of ADPG linked to starch biosynthesis. In this work, we produced and characterized potato and Arabidopsis plants expressing EcASPP and StASPP fused with green fluorescent protein (GFP). Confocal fluorescence microscopy analyses of these plants confirmed that EcASPP-GFP has a cytosolic localization, whereas StASPP-GFP occurs in the plastid stroma. Both source leaves and potato tubers from EcASPP-GFP-expressing plants showed a large reduction of the levels of both ADPG and starch. In contrast, StASPP-GFP-expressing leaves and tubers exhibited reduced starch and normal ADPG contents when compared with control plants. With the exception of starch synthase in StASPP-GFP-expressing plants, no pleiotropic changes in maximum catalytic activities of enzymes closely linked to starch metabolism could be detected in EcASPP-GFP- and StASPP-GFP-expressing plants. The overall data (i) show that potato plants possess a plastidial ASPP that has access to ADPG linked to starch biosynthesis and (ii) are consistent with the occurrence of plastidic and cytosolic pools of ADPG linked to starch biosynthesis.

Molecular characterization of organelle-type Nudix hydrolases in Arabidopsis

Nudix (for nucleoside diphosphates linked to some moiety X) hydrolases act to hydrolyze ribonucleoside and deoxyribonucleoside triphosphates, nucleotide sugars, coenzymes, or dinucleoside polyphosphates. Arabidopsis (Arabidopsis thaliana) contains 27 genes encoding Nudix hydrolase homologues (AtNUDX1 to -27) with a predicted distribution in the cytosol, mitochondria, and chloroplasts. Previously, cytosolic Nudix hydrolases (AtNUDX1 to -11 and -25) were characterized. Here, we conducted a characterization of organelle-type AtNUDX proteins (AtNUDX12 to -24, -26, and -27). AtNUDX14 showed pyrophosphohydrolase activity toward both ADP-ribose and ADP-glucose, although its K(m) value was approximately 100-fold lower for ADP-ribose (13.0+/-0.7 microm) than for ADP-glucose (1,235+/-65 microm). AtNUDX15 hydrolyzed not only reduced coenzyme A (118.7+/-3.4 microm) but also a wide range of its derivatives. AtNUDX19 showed pyrophosphohydrolase activity toward both NADH (335.3+/-5.4 microm) and NADPH (36.9+/-3.5 microm). AtNUDX23 had flavin adenine dinucleotide pyrophosphohydrolase activity (9.1+/-0.9 microm). Both AtNUDX26 and AtNUDX27 hydrolyzed diadenosine polyphosphates (n=4-5). A confocal microscopic analysis using a green fluorescent protein fusion protein showed that AtNUDX15 is distributed in mitochondria and AtNUDX14 -19, -23, -26, and -27 are distributed in chloroplasts. These AtNUDX mRNAs were detected ubiquitously in various Arabidopsis tissues. The T-DNA insertion mutants of AtNUDX13, -14, -15, -19, -20, -21, -25, -26, and -27 did not exhibit any phenotypical differences under normal growth conditions. These results suggest that Nudix hydrolases in Arabidopsis control a variety of metabolites and are pertinent to a wide range of physiological processes.

Rice plastidial N-glycosylated nucleotide pyrophosphatase/phosphodiesterase is transported from the ER-golgi to the chloroplast through the secretory pathway

A nucleotide pyrophosphatase/phosphodiesterase (NPP) activity that catalyzes the hydrolytic breakdown of ADP-glucose (ADPG) has been shown to occur in the plastidial compartment of both mono- and dicotyledonous plants. To learn more about this enzyme, we purified two NPPs from rice (Oryza sativa) and barley (Hordeum vulgare) seedlings. Both enzymes are glycosylated, since they bind to concanavalin A, stain with periodic acid-Schiff reagent, and are digested by Endo-H. A complete rice NPP cDNA, designated as NPP1, was isolated, characterized, and overexpressed in transgenic plants displaying high ADPG hydrolytic activity. Databank searches revealed that NPP1 belongs to a functionally divergent group of plant nucleotide hydrolases. NPP1 contains numerous N-glycosylation sites and a cleavable hydrophobic signal sequence that does not match with the N-terminal part of the mature protein. Both immunocytochemical analyses and confocal fluorescence microscopy of rice cells expressing NPP1 fused with green fluorescent protein (GFP) revealed that NPP1-GFP occurs in the plastidial compartment. Brefeldin A treatment of NPP1-GFP-expressing cells prevented NPP1-GFP accumulation in the chloroplasts. Endo-H digestibility studies revealed that both NPP1 and NPP1-GFP in the chloroplast are glycosylated. Collectively, these data demonstrate the trafficking of glycosylated proteins from the endoplasmic reticulum-Golgi system to the chloroplast in higher plants.

Cloning, expression and characterization of a Nudix hydrolase that catalyzes the hydrolytic breakdown of ADP-glucose linked to starch biosynthesis in Arabidopsis thaliana

'Nudix' hydrolases are widely distributed nucleotide pyrophosphatases that possess a conserved GX5EX7REUXEEXGU motif where U is usually isoleucine, leucine or valine. Among them, Escherichia coli ADP-sugar pyrophosphatase (ASPP) has been shown to catalyze the hydrolytic breakdown of ADP-glucose linked to bacterial glycogen biosynthesis. Comparisons of the 31 different Nudix-encoding sequences of the Arabidopsis genome with those coding for known bacterial and mammalian ASPPs identified one sequence possessing important divergences in the Nudix motif that, once expressed in E. coli, produced a protein with ASPP activity. This protein, designated as AtASPP, shares strong homology with hypothetical rice and potato proteins, indicating that ASPPs are widely distributed in both mono- and dicotyledonous plants. As a first step to test the possible involvement of plant ASPPs in regulating the intracellular levels of ADP-glucose linked to starch biosynthesis, we produced and characterized AtASPP-overexpressing Arabidopsis plants. Source leaves from these plants exhibited a large reduction in the levels of both ADP-glucose and starch, indicating that plant ASPPs catalyze the hydrolytic breakdown of a sizable pool of ADP-glucose linked to starch biosynthesis. No pleiotropic changes in maximum catalytic activities of enzymes closely linked to starch metabolism could be detected in AtASPP-overexpressing leaves. The overall information provides the first evidence for the existence of plant Nudix hydrolases that have access to an intracellular pool of ADP-glucose linked to starch biosynthesis.

Structure and mechanism of an ADP-glucose phosphorylase from Arabidopsis thaliana

The X-ray crystal structure of the At5g18200.1 protein has been determined to a nominal resolution of 2.30 A. The structure has a histidine triad (HIT)-like fold containing two distinct HIT-like motifs. The sequence of At5g18200.1 indicates a distant family relationship to the Escherichia coli galactose-1-P uridylyltransferase (GalT): the determined structure of the At5g18200.1 protein confirms this relationship. The At5g18200.1 protein does not demonstrate GalT activity but instead catalyzes adenylyl transfer in the reaction of ADP-glucose with various phosphates. The best acceptor among those evaluated is phosphate itself; thus, the At5g18200.1 enzyme appears to be an ADP-glucose phosphorylase. The enzyme catalyzes the exchange of (14)C between ADP-[(14)C]glucose and glucose-1-P in the absence of phosphate. The steady state kinetics of exchange follows the ping-pong bi-bi kinetic mechanism, with a k(cat) of 4.1 s(-)(1) and K(m) values of 1.4 and 83 microM for ADP-[(14)C]glucose and glucose-1-P, respectively, at pH 8.5 and 25 degrees C. The overall reaction of ADP-glucose with phosphate to produce ADP and glucose-1-P follows ping-pong bi-bi steady state kinetics, with a k(cat) of 2.7 s(-)(1) and K(m) values of 6.9 and 90 microM for ADP-glucose and phosphate, respectively, at pH 8.5 and 25 degrees C. The kinetics are consistent with a double-displacement mechanism that involves a covalent adenylyl-enzyme intermediate. The X-ray crystal structure of this intermediate was determined to 1.83 A resolution and shows the AMP group bonded to His(186). The value of K(eq) in the direction of ADP and glucose-1-P formation is 5.0 at pH 7.0 and 25 degrees C in the absence of a divalent metal ion, and it is 40 in the presence of 1 mM MgCl(2).

Cloning and expression analysis of Zmglp1, a new germin-like protein gene in maize

The cDNA and genomic DNA of a green tissue-specific gene were cloned from maize (Zea mays L.) using cDNA-amplified fragment length polymorphism (cDNA-AFLP) and library screening. The deduced protein was highly similar to Hordeum vulgare germin-like protein 1 (HvGLP1), and the maize gene was therefore designated Zmglp1. Northern blot specifically detected the mRNA of Zmglp1 in young whorl leaves at the early-whorl stage. However, at the late-whorl, tassel, and silk stages, Zmglp1 transcripts were highly abundant in young whorl leaves; less abundant in mature leaves, young tassels, and cobs; and not detectable in roots, immature kernels, and stalks. RNA in situ hybridization revealed that Zmglp1 expressed only in mesophyllous, phloem, and guard cells in the young whorl leaves. Deletion analysis of the promoter in transgenic Arabidopsis resulted in the identification of several regions containing important regulatory cis-elements controlling the expression levels and circadian rhythm-oscillated patterns of Zmglp1.

Development and mapping of EST-derived simple sequence repeat markers for hexaploid wheat

Expressed sequence tags (ESTs) are a valuable source of molecular markers. To enhance the resolution of an existing linkage map and to identify putative functional polymorphic gene loci in hexaploid wheat (Triticum aestivum L.), over 260,000 ESTs from 5 different grass species were analyzed and 5418 SSR-containing sequences were identified. Using sequence similarity analysis, 156 cross-species superclusters and 138 singletons were used to develop primer pairs, which were then tested on the genomic DNA of barley (Hordeum vulgare), maize (Zea mays), rice (Oryza sativa), and wheat. Three-hundred sixty-eight primer pairs produced PCR amplicons from at least one species and 227 primer pairs amplified DNA from two or more species. EST-SSR sequences containing dinucleotide motifs were significantly more polymorphic (74%) than those containing trinucleotides (56%), and polymorphism was similar for markers in both coding and 5' untranslated (UTR) regions. Out of 112 EST-SSR markers, 90 identified 149 loci that were integrated into a reference wheat genetic map. These loci were distributed on 19 of the 21 wheat chromosomes and were clustered in the distal chromosomal regions. Multiple-loci were detected by 39% of the primer pairs. Of the 90 mapped ESTs, putative functions for 22 were identified using BLASTX queries. In addition, 80 EST-SSR markers (104 loci) were located to chromosomes using nullisomic-tetrasomic lines. The enhanced map from this study provides a basis for comparative mapping using orthologous and PCR-based markers and for identification of expressed genes possibly affecting important traits in wheat.

Identification and characterization of plant glycerophosphodiester phosphodiesterase

GPX-PDE (glycerophosphodiester phosphodiesterase; EC 3.1.4.46) is a relatively poorly characterized enzyme that catalyses the hydrolysis of various glycerophosphodiesters (glycerophosphocholine, glycerophosphoethanolamine, glycerophosphoglycerol, glycerophosphoserine and bis-glycerophosphoglycerol), releasing sn-glycerol 3-phosphate and the corresponding alcohol. In a previous study, we demonstrated the existence of a novel GPX-PDE in the cell walls and vacuoles of plant cells. Since no GPX-PDE had been identified in any plant organism, the purification of GPX-PDE from carrot cell walls was attempted. After extraction of cell wall proteins from carrot cell suspension cultures with CaCl2, GPX-PDE was purified up to 2700-fold using, successively, ammonium sulphate precipitation, gel filtration and concanavalin A-Sepharose. Internal sequence analysis of a 55 kDa protein identified in the extract following 2700-fold purification revealed strong similarity to the primary sequence of GLPQ, a bacterial GPX-PDE. To confirm the identity of plant GPX-PDE, an Arabidopsis thaliana cDNA similar to that encoding the bacterial GPX-PDE was cloned and overexpressed in a bacterial expression system, and was used to raise antibodies against the putative Arabidopsis thaliana GPX-PDE. Immunochemical assays performed on carrot cell wall proteins extracted by CaCl2 treatment showed a strong correlation between GPX-PDE activity and detection of the 55 kDa protein, validating the identity of the plant GPX-PDE. Finally, various properties of the purified enzyme were investigated. GPX-PDE is a multimeric enzyme, specific for glycerophosphodiesters, exhibiting a K(m) of 36 microM for glycerophosphocholine and active within a wide pH range (from 4 to 10). Since these properties are similar to those of GLPQ, the bacterial GPX-PDE, the similarities between plant and bacterial enzymes are also discussed.

Most of ADP x glucose linked to starch biosynthesis occurs outside the chloroplast in source leaves

Sucrose and starch are end products of two segregated gluconeogenic pathways, and their production takes place in the cytosol and chloroplast of green leaves, respectively. According to this view, the plastidial ADP.glucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule ADPG. However, a growing body of evidences indicates that starch formation involves the import of cytosolic ADPG to the chloroplast. This evidence is consistent with the idea that synthesis of the ADPG linked to starch biosynthesis takes place in the cytosol by means of sucrose synthase, whereas AGP channels the glucose units derived from the starch breakdown. To test this hypothesis, we first investigated the subcellular localization of ADPG. Toward this end, we constructed transgenic potato plants that expressed the ADPG-cleaving adenosine diphosphate sugar pyrophosphatase (ASPP) from Escherichia coli either in the chloroplast or in the cytosol. Source leaves from plants expressing ASPP in the chloroplast exhibited reduced starch and normal ADPG content as compared with control plants. Most importantly however, leaves from plants expressing ASPP in the cytosol showed a large reduction of the levels of both ADPG and starch, whereas hexose phosphates increased as compared with control plants. No pleiotropic changes in photosynthetic parameters and maximum catalytic activities of enzymes closely linked to starch and sucrose metabolism could be detected in the leaves expressing ASPP in the cytosol. The overall results show that, essentially similar to cereal endosperms, most of the ADPG linked to starch biosynthesis in source leaves occurs in the cytosol.

Impact of heterologous expression of Escherichia coli UDP-glucose pyrophosphorylase on trehalose and glycogen synthesis in Corynebacterium glutamicum

Trehalose is a disaccharide with a wide range of applications in the food industry. We recently proposed a strategy for trehalose production based on improved strains of the gram-positive bacterium Corynebacterium glutamicum. This microorganism synthesizes trehalose through two major pathways, OtsBA and TreYZ, by using UDP-glucose and ADP-glucose, respectively, as the glucosyl donors. In this paper we describe improvement of the UDP-glucose supply through heterologous expression in C. glutamicum of the UDP-glucose pyrophosphorylase gene from Escherichia coli, either expressed alone or coexpressed with the E. coli ots genes (galU otsBA synthetic operon). The impact of such expression on trehalose accumulation and excretion, glycogen accumulation, and the growth pattern of new recombinant strains is described. Expression of the galU otsBA synthetic operon resulted in a sixfold increase in the accumulated and excreted trehalose relative to that in a wild-type strain. Surprisingly, single expression of galU also resulted in an increase in the accumulated trehalose. This increase in trehalose synthesis was abolished upon deletion of the TreYZ pathway. These results proved that UDP-glucose has an important role not only in the OtsBA pathway but also in the TreYZ pathway.

Sucrose synthase catalyzes the de novo production of ADPglucose linked to starch biosynthesis in heterotrophic tissues of plants

By using barley seeds, developmental changes of ADPglucose (ADPG)-producing sucrose synthase (SS) and ADPG pyrophosphorylase (AGPase) have been compared with those of UDPglucose (UDPG), ADPG, sucrose (Suc) and starch contents. Both ADPG-synthesizing SS and AGPase activity patterns were found to correlate well with those of ADPG and starch contents. Remarkably, however, maximal activities of ADPG-synthesizing SS were found to be several fold higher than those of AGPase throughout seed development, the highest rate of starch accumulation being well accounted for by SS. Kinetic analyses of SS from barley endosperms and potato tubers in the Suc cleavage direction showed similar K(m) values for ADP and UDP, whereas apparent affinity for Suc was shown to be higher in the presence of UDP than with ADP. Moreover, measurements of transglucosylation activities in starch granules incubated with purified SS, ADP and [U-(14)C]Suc revealed a low inhibitory effect of UDP. The ADPG and UDPG contents in the transgenic S-112 SS and starch deficient potato mutant [Zrenner et al. (1995) Plant J. 7: 97] were found to be 35% and 30% of those measured in wild-type plants, whereas both glucose-1-phosphate and glucose-6-phosphate contents were found to be normal as compared with those of wild-type plants. The overall results thus strongly support a novel gluconeogenic mechanism reported previously [Pozueta-Romero et al. (1999) CRIT: Rev. Plant Sci. 18: 489] wherein SS catalyses directly the de novo production of ADPG linked to starch biosynthesis in heterotrophic tissues of plants.

Cloning, expression and characterization of a mammalian Nudix hydrolase-like enzyme that cleaves the pyrophosphate bond of UDP-glucose

A distinct UDP-glucose (UDPG) pyrophosphatase (UGPPase, EC 3.6.1.45) has been characterized using pig kidney ( Sus scrofa ). This enzyme hydrolyses UDPG, the precursor molecule of numerous glycosylation reactions in animals, to produce glucose 1-phosphate (G1P) and UMP. Sequence analyses of the purified enzyme revealed that, similar to the case of a nucleotide-sugar hydrolase controlling the intracellular levels of ADP-glucose linked to glycogen biosynthesis in Escherichia coli [Moreno-Bruna, Baroja-Fernández, Muñoz, Bastarrica-Berasategui, Zandueta-Criado, Rodri;guez-López, Lasa, Akazawa and Pozueta-Romero (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 8128-8132], UGPPase appears to be a member of the ubiquitously distributed group of nucleotide pyrophosphatases designated Nudix hydrolases. A complete cDNA of the UGPPase-encoding gene, designated UGPP, was isolated from a human thyroid cDNA library and expressed in E. coli. The resulting cells accumulated a protein that showed kinetic properties identical to those of pig UGPPase.

Starch synthesis and carbon partitioning in developing endosperm

The biosynthesis of starch is the major determinant of yield in cereal grains. In this short review, attention is focused on the synthesis of the soluble substrate for starch synthesis, ADPglucose (ADPG). Consideration is given to the pathway of ADPG production, its subcellular compartmentation, and the role of metabolite transporters in mediating its delivery to the site of starch synthesis. As ADPG is an activated sugar, the dependence of its production on respiration, changes which occur during development, and the constraints which ATP production may place on carbon partitioning into different end-products are discussed.