The capacity of plastids from developing pea cotyledons to synthesise acetyl CoA

Light induces gene expression to enhance the synthesis of storage reserves in Brassica napus L. embryos

In this manuscript, we disclosed the influence of light on the accumulation of storage reserves in B. napus embryos.1.Light induced the gene expression in the developing embryos of B. napus.2.Light promoted the starch synthesis in chloroplasts of B. napus embryos.3.Light enhanced the metabolic activity of storage reserve synthesis in B. napus embryos. Light influences the accumulation of storage reserves in embryos, but the molecular mechanism was not fully understood. Here, we monitored the effects of light on reserve biosynthesis in Brassica napus by comparing embryos from siliques grown in normal light conditions to those that were shaded or masked (i.e., darkened completely). Masked embryos developed more slowly, weighed less, and contained fewer proteins and lipids than control embryos. They also had fewer and smaller oil bodies than control embryos and lacked chloroplasts, where starch grains are usually synthesized. The levels of most amino acids, carbohydrates, and fatty acids were higher in masked embryos than in control or shaded embryos, whereas the levels of these metabolites in the masked endosperms were lower than those in control and shaded endosperm. Transcriptome analysis indicated that genes involved in photosynthesis (42 genes), amino acid biosynthesis (51 genes), lipid metabolism (61 genes), and sugar transport (13 genes) were significantly repressed in masked embryos. Our results suggest that light contributes to reserve accumulation in embryos by inducing the expression of metabolic genes, thereby enhancing the biosynthesis of storage reserves.

Starch granule initiation and morphogenesis-progress in Arabidopsis and cereals

Starch, the major storage carbohydrate in plants, is synthesized in plastids as semi-crystalline, insoluble granules. Many organs and cell types accumulate starch at some point during their development and maturation. The biosynthesis of the starch polymers, amylopectin and amylose, is relatively well understood and mostly conserved between organs and species. However, we are only beginning to understand the mechanism by which starch granules are initiated, and the factors that control the number of granules per plastid and the size/shape of granules. Here, we review recent progress in understanding starch granule initiation and morphogenesis. In Arabidopsis, granule initiation requires several newly discovered proteins with specific locations within the chloroplast, and also on the availability of maltooligosaccharides which act as primers for initiation. We also describe progress in understanding granule biogenesis in the endosperm of cereal grains-within which there is large interspecies variation in granule initiation patterns and morphology. Investigating whether this diversity results from differences between species in the functions of known proteins, and/or from the presence of novel, unidentified proteins, is a promising area of future research. Expanding our knowledge in these areas will lead to new strategies for improving the quality of cereal crops by modifying starch granule size and shape in vivo.

Analogous reserve distribution and tissue characteristics in quinoa and grass seeds suggest convergent evolution

Quinoa seeds are highly nutritious due to the quality of their proteins and lipids and the wide range of minerals and vitamins they store. Three compartments can be distinguished within the mature seed: embryo, endosperm, and perisperm. The distribution of main storage reserves is clearly different in those areas: the embryo and endosperm store proteins, lipids, and minerals, and the perisperm stores starch. Tissues equivalent (but not homologous) to those found in grasses can be identified in quinoa, suggesting the effectiveness of this seed reserve distribution strategy; as in cells of grass starchy endosperm, the cells of the quinoa perisperm endoreduplicate, increase in size, synthesize starch, and die during development. In addition, both systems present an extra-embryonic tissue that stores proteins, lipids and minerals: in gramineae, the aleurone layer(s) of the endosperm; in quinoa, the micropylar endosperm; in both cases, the tissues are living. Moreover, the quinoa micropylar endosperm and the coleorhiza in grasses play similar roles, protecting the root in the quiescent seed and controlling dormancy during germination. This investigation is just the beginning of a broader and comparative study of the development of quinoa and grass seeds. Several questions arise from this study, such as: how are synthesis and activation of seed proteins and enzymes regulated during development and germination, what are the genes involved in these processes, and lastly, what is the genetic foundation justifying the analogy to grasses.

Proteome profiling of flax (Linum usitatissimum) seed: characterization of functional metabolic pathways operating during seed development

Flax (Linum usitatissimum L.) seeds are an important source of food and feed due to the presence of various health promoting compounds, making it a nutritionally and economically important plant. An in-depth analysis of the proteome of developing flax seed is expected to provide significant information with respect to the regulation and accumulation of such storage compounds. Therefore, a proteomic analysis of seven seed developmental stages (4, 8, 12, 16, 22, 30, and 48 days after anthesis) in a flax variety, NL-97 was carried out using a combination of 1D-SDS-PAGE and LC-MSE methods. A total 1716 proteins were identified and their functional annotation revealed that a majority of them were involved in primary metabolism, protein destination, storage and energy. Three carbon assimilatory pathways appeared to operate in flax seeds. Reverse transcription quantitative PCR of selected 19 genes was carried out to understand their roles during seed development. Besides storage proteins, methionine synthase, RuBisCO and S-adenosylmethionine synthetase were highly expressed transcripts, highlighting their importance in flax seed development. Further, the identified proteins were mapped onto developmental seed specific expressed sequence tag (EST) libraries of flax to obtain transcriptional evidence and 81% of them had detectable expression at the mRNA level. This study provides new insights into the complex seed developmental processes operating in flax.

Isolation and partial characterization of mutants with elevated lipid content in Chlorella sorokiniana and Scenedesmus obliquus

This paper describes the isolation and partial biomass characterization of high triacylglycerol (TAG) mutants of Chlorella sorokiniana and Scenedesmus obliquus, two algal species considered as potential source of biodiesel. Following UV mutagenesis, 2000 Chlorella and 2800 Scenedesmus colonies were screened with a method based on Nile Red fluorescence. Several mutants with high Nile Red fluorescence were selected by this high-throughput method in both species. Growth and biomass parameters of the strongest mutants were analyzed in detail. All of the four Chlorella mutants showed no significant changes in growth rate, cell weight, cell size, protein and chlorophyll contents on a per cell basis. Whereas all contained elevated total lipid and TAG content per unit of dry weight, two of them were also affected for starch metabolism, suggesting a change in biomass/storage carbohydrate composition. Two Scenedesmus mutants showed a 1.5 and 2-fold increased cell weight and larger cells compared to the wild type, which led to a general increase of biomass including total lipid and TAG content on a per cell basis. Such mutants could subsequently be used as commercial oleaginous algae and serve as an alternative to conventional petrol.

Protein association and dissociation regulated by extension peptide: a mode for iron control by phytoferritin in seeds

Most of the iron in legume seeds is stored in ferritin located in the amyloplast, which is used during seed germination. However, there is a lack of information on the regulation of iron by phytoferritin. In this study, soluble and insoluble forms of pea (Pisum sativum) seed ferritin (PSF) isolated from dried seeds were found to be identical 24-mer ferritins comprising H-1 and H-2 subunits. The insoluble form is favored at low pH, whereas the two forms reversibly interconvert in the pH range of 6.0 to 7.8, with an apparent pK(a) of 6.7. This phenomenon was not observed in animal ferritins, indicating that PSF is unique. The pH of the amyloplast was found to be approximately 6.0, thus facilitating PSF association, which is consistent with the role of PSF in long-term iron storage. Similar to previous studies, the results of this work showed that protein degradation occurs in purified PSF during storage, thus proving that phytoferritin also undergoes degradation during seedling germination. In contrast, no degradation was observed in animal ferritins, suggesting that this degradation of phytoferritin may be due to the extension peptide (EP), a specific domain found only in phytoferritin. Indeed, removal of EP from PSF significantly increased protein stability and prevented degradation under identical conditions while promoting protein dissociation. Correlated with such dissociation was a considerable increase in the rate of ascorbate-induced iron release from PSF at pH 6.0. Thus, phytoferritin may have facilitated the evolution of EP to enable it to regulate iron for storage or complement in seeds.

A novel EP-involved pathway for iron release from soya bean seed ferritin

Iron in phytoferritin from legume seeds is required for seedling germination and early growth. However, the mechanism by which phytoferritin regulates its iron complement to these physiological processes remains unknown. In the present study, protein degradation is found to occur in purified SSF (soya bean seed ferritin) (consisting of H-1 and H-2 subunits) during storage, consistent with previous results that such degradation also occurs during seedling germination. In contrast, no degradation is observed with animal ferritin under identical conditions, suggesting that SSF autodegradation might be due to the EP (extension peptide) on the exterior surface of the protein, a specific domain found only in phytoferritin. Indeed, EP-deleted SSF becomes stable, confirming the above hypothesis. Further support comes from a protease activity assay showing that EP-1 (corresponding to the EP of the H-1 subunit) exhibits significant serine protease-like activity, whereas the activity of EP-2 (corresponding to the EP of the H-2 subunit) is much weaker. Consistent with the observation above, rH-1 (recombinant H-1 ferritin) is prone to degradation, whereas its analogue, rH-2, becomes very stable under identical conditions. This demonstrates that SSF degradation mainly originates from the serine protease-like activity of EP-1. Associated with EP degradation is a considerable increase in the rate of iron release from SSF induced by ascorbate in the amyloplast (pH range, 5.8-6.1). Thus phytoferritin may have facilitated the evolution of the specific domain to control its iron complement in response to cell iron need in the seedling stage.

Plastidial glycolysis in developing Arabidopsis embryos

During oilseed embryo development, carbon from sucrose is utilized for fatty acid synthesis in the plastid. The role of plastidial glycolysis in Arabidopsis embryo oil accumulation was investigated. Genes encoding enolases (ENO) and phosphoglyceromutases (PGlyM) were identified, and activities and subcellular locations were established by expression of recombinant and green fluorescent protein (GFP)-fusion proteins. Mutant Arabidopsis plants lacking putative plastidial isoforms were characterized with respect to isoform composition and embryo oil content. In the developing embryo, ENO1 and ENO2 account for most or all of the plastidial and cytosolic ENO activity, respectively, and PGLYM1 accounts for most or all of the plastidial PGlyM activity. The eno1 and pglym1 mutants, in which plastidic ENO and PGlyM activities were undetectable, had wild-type amounts of seed oil at maturity. It is concluded that although plastids of developing Arabidopsis embryos have the capacity to carry out the lower part of the glycolytic pathway, the cytosolic glycolytic pathway alone is sufficient to support the flux from 3-phosphoglycerate to phosphoenolpyruvate required for oil production. The results highlight the importance for oil production of translocators that facilitate interchange of glycolytic intermediates between the cytosol and the plastid stroma.

The evolution of the starch biosynthetic pathway in cereals and other grasses

In most species, the precursor for starch synthesis, ADPglucose, is made exclusively in the plastids by the enzyme ADPglucose pyrophosphorylase (AGPase). However, in the endosperm of grasses, including the economically important cereals, ADPglucose is also made in the cytosol via a cytosolic form of AGPase. Cytosolic ADPglucose is imported into plastids for starch synthesis via an ADPglucose/ADP antiporter (ADPglucose transporter) in the plastid envelope. The genes encoding the two subunits of cytosolic AGPase and the ADPglucose transporter are unique to grasses. In this review, the evolutionary origins of this unique endosperm pathway of ADPglucose synthesis and its functional significance are discussed. It is proposed that the genes encoding the pathway originated from a whole-genome-duplication event in an early ancestor of the grasses.

Isolation of amyloplasts

Two different methods for the preparation of starch-rich plastids are described together with protocols for the determination of plastid yield, purity, and intactness. The preparation of amyloplasts from maize endosperm and oilseed rape embryos are given as examples, but the protocols could be adapted for the isolation of starch-rich plastids from other plant organs. A method for the determination of the quantitative distribution of an enzyme between the plastids and cytosol is given. Typical results and references for marker enzymes for a range of subcellular compartments are listed.

Antisense inhibition of the plastidial glucose-6-phosphate/phosphate translocator in Vicia seeds shifts cellular differentiation and promotes protein storage

The glucose-6-phosphate/phosphate translocator (GPT) acts as an importer of carbon into the plastid. Despite the potential importance of GPT for storage in crop seeds, its regulatory role in biosynthetic pathways that are active during seed development is poorly understood. We have isolated GPT1 from Vicia narbonensis and studied its role in seed development using a transgenic approach based on the seed-specific legumin promoter LeB4. GPT1 is highly expressed in vegetative sink tissues, flowers and young seeds. In the embryo, localized upregulation of GPT1 at the onset of storage coincides with the onset of starch accumulation. Embryos of transgenic plants expressing antisense GPT1 showed a significant reduction (up to 55%) in the specific transport rate of glucose-6-phosphate as determined using proteoliposomes prepared from embryos. Furthermore, amyloplasts developed later and were smaller in size, while the expression of genes encoding plastid-specific translocators and proteins involved in starch biosynthesis was decreased. Metabolite analysis and stable isotope labelling demonstrated that starch biosynthesis was also reduced, although storage protein biosynthesis increased. This metabolic shift was characterized by upregulation of genes related to nitrogen uptake and protein storage, morphological variation of the protein-storing vacuoles, and a crude protein content of mature seeds of transgenics that was up to 30% higher than in wild-type. These findings provide evidence that (1) the prevailing level of GPT1 abundance/activity is rate-limiting for the synthesis of starch in developing seeds, (2) GPT1 exerts a controlling function on assimilate partitioning into storage protein, and (3) GPT1 is essential for the differentiation of embryonic plastids and seed maturation.

A low-starch barley mutant, risø 16, lacking the cytosolic small subunit of ADP-glucose pyrophosphorylase, reveals the importance of the cytosolic isoform and the identity of the plastidial small subunit

To provide information on the roles of the different forms of ADP-glucose pyrophosphorylase (AGPase) in barley (Hordeum vulgare) endosperm and the nature of the genes encoding their subunits, a mutant of barley, Risø 16, lacking cytosolic AGPase activity in the endosperm was identified. The mutation specifically abolishes the small subunit of the cytosolic AGPase and is attributable to a large deletion within the coding region of a previously characterized small subunit gene that we have called Hv.AGP.S.1. The plastidial AGPase activity in the mutant is unaffected. This shows that the cytosolic and plastidial small subunits of AGPase are encoded by separate genes. We purified the plastidial AGPase protein and, using amino acid sequence information, we identified the novel small subunit gene that encodes this protein. Studies of the Risø 16 mutant revealed the following. First, the reduced starch content of the mutant showed that a cytosolic AGPase is required to achieve the normal rate of starch synthesis. Second, the mutant makes both A- and B-type starch granules, showing that the cytosolic AGPase is not necessary for the synthesis of these two granule types. Third, analysis of the phylogenetic relationships between the various small subunit proteins both within and between species, suggest that the cytosolic AGPase single small subunit gene probably evolved from a leaf single small subunit gene.

Characterization of the genes encoding the cytosolic and plastidial forms of ADP-glucose pyrophosphorylase in wheat endosperm

In most species, the synthesis of ADP-glucose (Glc) by the enzyme ADP-Glc pyrophosphorylase (AGPase) occurs entirely within the plastids in all tissues so far examined. However, in the endosperm of many, if not all grasses, a second form of AGPase synthesizes ADP-Glc outside the plastid, presumably in the cytosol. In this paper, we show that in the endosperm of wheat (Triticum aestivum), the cytosolic form accounts for most of the AGPase activity. Using a combination of molecular and biochemical approaches to identify the cytosolic and plastidial protein components of wheat endosperm AGPase we show that the large and small subunits of the cytosolic enzyme are encoded by genes previously thought to encode plastidial subunits, and that a gene, Ta.AGP.S.1, which encodes the small subunit of the cytosolic form of AGPase, also gives rise to a second transcript by the use of an alternate first exon. This second transcript encodes an AGPase small subunit with a transit peptide. However, we could not find a plastidial small subunit protein corresponding to this transcript. The protein sequence of the purified plastidial small subunit does not match precisely to that encoded by Ta.AGP.S.1 or to the predicted sequences of any other known gene from wheat or barley (Hordeum vulgare). Instead, the protein sequence is most similar to those of the plastidial small subunits from chickpea (Cicer arietinum) and maize (Zea mays) and rice (Oryza sativa) seeds. These data suggest that the gene encoding the major plastidial small subunit of AGPase in wheat endosperm has yet to be identified.

Carbon flux and fatty acid synthesis in plants

The de novo synthesis of fatty acids in plants occurs in the plastids through the activity of fatty acid synthetase. The synthesis of the malonyl-coenzyme A that is required for acyl-chain elongation requires the import of metabolites from the cytosol and their subsequent metabolism. Early studies had implicated acetate as the carbon source for plastidial fatty acid synthesis but more recent experiments have provided data that argue against this. A range of cytosolic metabolites including glucose 6-phosphate, malate, phosphoenolpyruvate and pyruvate support high rates of fatty acid synthesis by isolated plastids, the relative utilisation of which depends upon the plant species and the organ from which the plastids are isolated. The import of these metabolites occurs via specific transporters on the plastid envelope and recent advances in the understanding of the role of these transporters are discussed. Chloroplasts are able to generate the reducing power and ATP required for fatty acid synthesis by capture of light energy in the reactions of photosynthetic electron transport. Regulation of chloroplast fatty acid synthesis is mediated by the response of acetyl-CoA carboxylase to the redox state of the plastid, which ensures that the carbon metabolism is linked to the energy status. The regulation of fatty acid synthesis in plastids of heterotrophic cells is much less well understood and is of particular interest in the tissues that accumulate large amounts of the storage oil, triacylglycerol. In these heterotrophic cells the plastids import ATP and oxidise imported carbon sources to produce the required reducing power. The sequencing of the genome of Arabidopsis thaliana has now enabled a number of aspects of plant fatty acid synthesis to be re-addressed, particularly those areas in which in vitro biochemical analysis had provided equivocal answers. Examples of such aspects and future opportunities for our understanding of plant fatty acid synthesis are presented and discussed.

ADP-glucose pyrophosphorylase is located in the plastid in developing tomato fruit

The subcellular location of activity and protein of ADP-glucose pyrophosphorylase (AGPase) in developing tomato (Lycopersicon esculentum) fruit was determined following a report that the enzyme might be present inside and outside the plastids in this organ. Plastids prepared from crude homogenates of columella and pericarp, the starch-accumulating tissues of developing fruit, contained 8% to 18% of the total activity of enzymes known to be confined to plastids, and 0.2% to 0.5% of the total activity of enzymes known to be confined to the cytosol. The proportion of the total activity of AGPase in the plastids was the same as that of the enzymes known to be confined to the plastid. When samples of plastid and total homogenate fractions were subjected to immunoblotting with an antiserum raised to AGPase, most or all of the protein detected was plastidial. Taken as a whole, these data provide strong evidence that AGPase is confined to the plastids in developing tomato fruit.

A cytosolic ADP-glucose pyrophosphorylase is a feature of graminaceous endosperms, but not of other starch-storing organs

The occurrence of an extra-plastidial isoform of ADP-glucose (Glc) pyrophosphorylase (AGPase) among starch-storing organs was investigated in two ways. First, the possibility that an extra-plastidial isoform arose during the domestication of cereals was studied by comparing the intracellular distribution of enzyme activity and protein in developing endosperm of noncultivated Hordeum species with that previously reported for cultivated barley (Hordeum vulgare). As in cultivated barley, the AGPase of H. vulgare subsp. spontaneum and Hordeum murinum endosperm is accounted for by a major extra-plastidial and a minor plastidial isoform. Second, the ratio of ADP-Glc to UDP-Glc was used as an indication of the intracellular location of the AGPase activity in a wide range of starch-synthesizing organs. The ratio is expected to be high in organs in which UDP-Glc and ADP-Glc are synthesized primarily in the cytosol, because the reactions catalyzed by AGPase and UDP-Glc pyrophosphorylase will be coupled and close to equilibrium. This study revealed that ADP-Glc contents and the ratio of ADP-Glc to UDP-Glc were higher in developing graminaceous endosperms than in any other starch-storing organs. Taken as a whole the results indicate that an extra-plastidial AGPase is important in ADP-Glc synthesis in graminaceous endosperms, but not in other starch-storing organs.

Promoter trapping of a novel medium-chain acyl-CoA oxidase, which is induced transcriptionally during Arabidopsis seed germination

The first step of peroxisomal fatty acid beta-oxidation is catalyzed by a family of acyl-CoA oxidase isozymes with distinct fatty acyl-CoA chain-length specificities. Here we identify a new acyl-CoA oxidase gene from Arabidopsis (AtACX3) following the isolation of a promoter-trapped mutant in which beta-glucuronidase expression was initially detected in the root meristem. In acx3 mutant seedlings medium-chain acyl-CoA oxidase activity was reduced by 95%, whereas long- and short-chain activities were unchanged. Despite this reduction in activity lipid catabolism and seedling development were not perturbed. AtACX3 was cloned and expressed in Escherichia coli. The recombinant enzyme displayed medium-chain acyl-CoA substrate specificity. Analysis of beta-glucuronidase activity in acx3 revealed that, in addition to constitutive expression in the root axis, AtACX3 is also up-regulated strongly in the hypocotyl and cotyledons of germinating seedlings. This suggests that beta-oxidation is regulated predominantly at the level of transcription in germinating oilseeds. After the discovery of AtACX3, the Arabidopsis acyl-CoA oxidase gene family now comprises four isozymes with substrate specificities that encompass the full range of acyl-CoA chain lengths that exist in vivo.

The role of pyruvate dehydrogenase and acetyl-coenzyme A synthetase in fatty acid synthesis in developing Arabidopsis seeds

Acetyl-coenzyme A (acetyl-CoA) formed within the plastid is the precursor for the biosynthesis of fatty acids and, through them, a range of important biomolecules. The source of acetyl-CoA in the plastid is not known, but two enzymes are thought to be involved: acetyl-CoA synthetase and plastidic pyruvate dehydrogenase. To determine the importance of these two enzymes in synthesizing acetyl-CoA during lipid accumulation in developing Arabidopsis seeds, we isolated cDNA clones for acetyl-CoA synthetase and for the ptE1alpha- and ptE1beta-subunits of plastidic pyruvate dehydrogenase. To our knowledge, this is the first reported acetyl-CoA synthetase sequence from a plant source. The Arabidopsis acetyl-CoA synthetase preprotein has a calculated mass of 76,678 D, an apparent plastid targeting sequence, and the mature protein is a monomer of 70 to 72 kD. During silique development, the spatial and temporal patterns of the ptE1beta mRNA level are very similar to those of the mRNAs for the plastidic heteromeric acetyl-CoA carboxylase subunits. The pattern of ptE1beta mRNA accumulation strongly correlates with the formation of lipid within the developing embryo. In contrast, the level of mRNA for acetyl-CoA synthetase does not correlate in time and space with lipid accumulation. The highest level of accumulation of the mRNA for acetyl-CoA synthetase during silique development is within the funiculus. These mRNA data suggest a predominant role for plastidic pyruvate dehydrogenase in acetyl-CoA formation during lipid synthesis in seeds.

Coordinate changes in carbon partitioning and plastidial metabolism during the development of oilseed rape embryos

Measurements of metabolic fluxes in whole embryos and isolated plastids have revealed major changes in the pathways of carbon utilization during cotyledon filling by oilseed rape (Brassica napus L.) embryos. In the early cotyledon stage (stage A), embryos used sucrose (Suc) predominantly for starch synthesis. Plastids isolated from these embryos imported glucose-6-phosphate (Glc-6-P) and partitioned it to starch and fatty acids synthesis and to the oxidative pentose phosphate pathway in the ratio of 2:1:1 on a hexose basis. Of the substrates tested, Glc-6-P gave the highest rates of fatty acid synthesis by the plastids and pyruvate was used weakly. By the mid- to late-cotyledon stage (stage C), oil accumulation by the embryos was rapid, as was their utilization of Suc for oil synthesis in vitro. Plastids from C-stage embryos differed markedly from those of stage-A embryos: (a) pyruvate uptake and utilization for fatty acid synthesis increased by respectively 18- and 25-fold; (b) Glc-6-P partitioning was predominantly to the oxidative pentose phosphate pathway (respective ratios of 1:1:3); and (c) the rate of plastidial fatty acid synthesis more than doubled. This increased rate of fatty synthesis was dependent upon the increase in pyruvate uptake and was mediated through the induction of a saturable transporter activity.

Characterization of starch-debranching enzymes in pea embryos

Two distinct types of debranching enzymes have been identified in developing pea (Pisum sativum L.) embryos using native gel analysis and tests of substrate preference on purified or partially purified activities. An isoamylase-like activity capable of hydrolyzing amylopectin and glycogen but not pullulan is present throughout development and is largely or entirely confined to the plastid. Activities capable of hydrolyzing pullulan are present both inside and outside of the plastid, and extraplastidial activity increases relative to the plastidial activity during development. Both types of debranching enzyme are also present in germinating embryos. We argue that debranching enzymes are likely to have a role in starch metabolism in the plastid of the developing embryo and in starch degradation during germination.

Cloning and characterization of a dihydrolipoamide acetyltransferase (E2) subunit of the pyruvate dehydrogenase complex from Arabidopsis thaliana

A cDNA encoding a dihydrolipoamide acetyltransferase (E2) subunit of the pyruvate dehydrogenase complex has been isolated from Arabidopsis thaliana. A cell culture cDNA expression library was screened with a monoclonal antibody (JIM 63) raised against nuclear matrix proteins, and four clones were isolated. One of these was 2175 base pairs in length, and it contained an open reading frame with an amino acid sequence and domain structure with strong similarity to the E2s of other eukaryotic and prokaryotic organisms. The organization and number of functional domains within the Arabidopsis protein are identical to those of the human E2, although the amino acid sequences within these domains are equally similar to those of the yeast and human proteins. The predicted amino acid sequence reveals the presence of a putative amino-terminal leader sequence with characteristics similar to those of other proteins, which are targeted to the plant mitochondrial matrix. The cross-reactivities of plant mitochondrial matrix proteins with JIM 63 and antibodies raised against the E2 and protein X components of eukaryotic pyruvate dehydrogenase complexes are consistent with the clone encoding a mitochondrial form of E2 and not the smaller protein X. The E2 mRNA of 2.2 kilobases was expressed in a range of Arabidopsis and Brassica napus tissues.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.

Higher-plant cofactor-independent phosphoglyceromutase: purification, molecular characterization and expression

Cofactor-independent phosphoglyceromutase (PGM) was purified to homogeneity from developing castor seed endosperm. Immunological characterization using monospecific antisera raised against this protein indicates that the enzyme is located in the cytosol and that there is no immunologically related polypeptide in the leucoplast from this tissue. Isolation and sequence determination of full-length cDNA clones for castor and tobacco PGM demonstrate that the protein is highly conserved in these plants and is closely related to the maize enzyme. A comparison of the amino acid sequence of peptides derived from Neurospora crassa PGM with the cofactor-independent enzyme from higher plants demonstrated that they are related and may have diverged from a common ancestral gene. The previously proposed relationship between higher-plant PGM and alkaline phosphatases is not supported by sequence analysis of the castor and tobacco enzymes. Expression of the single castor cytosolic PGM gene correlates well with other cytosolic glycolytic genes in developing and germinating castor seeds, and with the appearance of enzyme activity and PGM polypeptides in these tissues.

Starch synthesis in developing pea embryos

The pea embryo stores about half of its carbon as starch and has proved to be an excellent system on which to study the nature and regulation of the pathway of starch synthesis. The developing embryo receives its carbon as sucrose, which is metabolized via glycolysis in the cytosol of cotyledonary cells. Glucose 6-phosphate enters the amyloplast - probably via a phosphate-exchange translocator - where it is converted to ADPglucose via phosphoglucomutase and ADPglucose pyrophosphorylase. ADPglucose pyrophosphorylase is the site of action of a mutation at the rb locus, which reduces activity by more than 90 % and the rate of starch synthesis by about 50 %. Study of mutant and wildtype embryos reveals that one of four putative subunits of the enzyme is eliminated by the mutation. Three distinct isoforms of starch synthase catalyze the incorporation of the glucosyl moiety of ADPglucose into starch. Two of these are probably active in the soluble phase of the amyloplast and become incorporated into the granule as it grows, while the third is almost exclusively granule-bound. Analysis of cDNA clones for starch synthases shows that the exclusively granule-bound form is very similar to the 'waxy' gene product believed to be responsible for amylose synthesis in cereal endosperms. The soluble starch synthases show some similarities to the 'waxy' proteins, but clearly belong to a different and previously undescribed class of starch synthases. The pea embryo contains two forms of starch branching enzyme, which are encoded by different genes, are maximally expressed at different times in development, and have different kinetic properties. It is likely that they play different roles in the synthesis of the granule. A mutation at the r locus, which reduces the rate of starch synthesis by about 50% and increases the amylose content of the starch from 30% to 70%, consists of a transposon-like insertion in the gene encoding starch-branching enzyme I. Activity of this isoform is abolished by the mutation. CONTENTS Summary 21 I. Introduction 21 II. The supply of sucrose to the embryo 22 III. The timing and location of starch synthesis 23 IV. The supply of carbon to the amyloplast 23 V. Mutations affecting the committed pathway of starch synthesis 26 VI. The committed pathway 28 Acknowledgements 31 References 31.

Characteristics of plastids responsible for starch synthesis in developing pea embryos

The nature of the starch-synthesising plastids in developing pea (Pisum sativum L.) embryos has been investigated. Chlorophyll and starch were distributed throughout the cotyledon during development. Chlorophyll content increased initially, then showed little change up to the point of drying out of the embryo. Starch content per embryo increased dramatically throughout development. The chlorophyll content per unit volume was highest on the outer edge of the cotyledon, while the starch content was highest on inner face. Nycodenz gradients, which fractionated mechanically-prepared plastids according to their starch content, failed to achieve any significant separation of plastids rich in starch and ADP-glucose pyrophosphorylase from those rich in chlorophyll and a Calvin-cycle marker enzyme, NADP-glyceraldehyde-3-phosphate dehydrogenase. However, material that was not sufficiently dense to enter the gradients was enriched in activity of the Calvin-cycle marker enzyme relative to that of ADP-glucose pyrophosphorylase. Nomarski and epi-fluorescence microscopy showed that intact, isolated plastids, including those with very large starch grains, invariably contained chlorophyll in stromal structures peripheral to the starch grain. We suggest that the starch-storing plastids of developing pea embryos are derived directly from chloroplasts, and retain chloroplast-like characteristics throughout their development. Developing pea embryos also contain chloroplasts which store little or no starch. These are probably located primarily on the outer edge of the cotyledons where there is sufficient light for photosynthesis at some stages of development.

Molecular and immunological characterization of plastid and cytosolic pyruvate kinase isozymes from castor-oil-plant endosperm and leaf

1. Monospecific antiserum was raised in rabbits to homogeneous cytosolic pyruvate kinase isolated from 5-day-old germinating endosperm of the castor oil plant, Ricinus communis. An earlier study demonstrated that the purified enzyme is putatively heterotetrameric, composed of two subunits which migrate as 57-kDa and 56-kDa proteins upon sodium dodecyl sulfate/polyacrylamide gel electrophoresis [Plaxton, W. C. (1988) Plant Physiol. (Bethesda) 86, 1065-1069]. Both proteins were detected on Western blots of extracts prepared under denaturing conditions from 4-8-day-old, but not 0-3-day-old, germinating-endosperm tissue. This suggests that both subunits exist in vivo, and that the large increase in pyruvate kinase activity which occurs around the fourth day of germination is due to an increase in pyruvate kinase concentration. 2. The cytosolic and plastidic pyruvate kinase isozymes (termed PKc and PKp, respectively) from castor-oil-plant developing endosperm and expanding leaf tissue were separated by anion-exchange chromatography on Q-Sepharose. The antigenic reaction of the partially purified enzyme preparations to rabbit polyclonal antibodies raised against homogeneous germinating-castor-bean PKc was tested by immunoprecipitation and Western blotting. Although developing-endosperm and leaf PKc appeared to be antigenically very similar to germinating-endosperm PKc, they differed from the heterotetrameric germinating-endosperm enzyme by being composed of a single type of subunit with a molecular mass of about 56 kDa. No cross-reactivity of the PKc antibodies was observed with either developing-endosperm or leaf PKp, nor with rabbit muscle or Bacillus stearothermophilus pyruvate kinase. Conversely, none of the castor-oil-plant pyruvate kinase preparations showed significant cross-reactivity with antibodies raised against purified yeast or rabbit muscle pyruvate kinases. 3. To investigate the structural relationship between the two germinating-endosperm-PKc subunits, each polypeptide was characterized by amino acid composition analysis and peptide mapping by CNBr fragmentation. The amino acid compositions and CNBr cleavage patterns of the two subunits were similar, but not identical, suggesting that these polypeptides are related, but distinct, proteins. Mild tryptic attack of native enzyme led to an approximate 6-kDa reduction in the apparent molecular mass of both subunits, further indicating sequence similarity between the two polypeptides. 4. Native molecular masses of the various castor-oil-plant pyruvate kinases were estimated by Superose-6 gel-filtration chromatography.(ABSTRACT TRUNCATED AT 400 WORDS)

Pyruvate kinase isozymes from the green alga, Selenastrum minutum. I. Purification and physical and immunological characterization

Pyruvate kinase from the green alga Selenastrum minutum consists of two isoforms (PK1 and PK2) separable by Q-Sepharose chromatography. The two isoforms have been highly purified to respective final specific activities of 42 and 23 (mumol pyruvate produced/min)/mg protein. Purification steps included salt fractionation, anion-exchange, hydrophobic interaction, and gel filtration chromatography. The final enzyme preparations differ significantly in physical and immunological properties. PK1 is heat labile and is completely inactivated following reaction with N-ethylmaleimide. In contrast, PK2 is heat-stable and is only partially inactivated following N-ethylmaleimide treatment. PK1 appears to be homotetrameric with a native molecular mass of about 240 kDa, whereas PK2 appears to be homodecameric with a native molecular mass of approximately 590 kDa. The antigenic reaction of both final PK preparations to rabbit antiserum prepared against homogeneous germinating castor bean endosperm cytosolic pyruvate kinase was tested by immunoprecipitation and Western blotting. The two algal pyruvate kinases are immunologically unrelated as only PK2 cross-reacts with the cytosolic pyruvate kinase antibodies. These data indicate that the S. minutum pyruvate kinase isoforms, PK1 and PK2, are not interconvertible forms of the same protein, but probably represent chloroplastic and cytosolic isozymes, respectively.

Major differences in isoforms of starch-branching enzyme between developing embryos of round- and wrinkled-seeded peas (Pisum sativum L.)

In order to determine whether round-and wrinkled-seeded peas (Pisum sativum L.) differ in the activity and properties of starch-branching enzyme (1,4-α-D-glucan, 1,4-α-D-glucan-6-glycosyl transferase; EC 2.4.1.18) in their developing embryos, essentially isogenic lines of peas, differing only at the r (rugosus) locus that confers the round (RR, Rr) or wrinkled (rr) phenotype, were studied. Activity of the enzyme rises rapidly from an early stage of development in embryos of round peas, but only at later stages in embryos of wrinkled peas. The purified enzyme from mature embryos of round peas can be resolved into two isoforms that differ in molecular weight and in their ability to branch amylose. The purified enzyme from mature embryos of wrinkled peas is a single protein with the same molecular weight and branching properties as one of the isoforms from embryos of round peas. The difference in activity of starch-branching enzyme between embryos of round and wrinkled peas is likely to be due to the absence from embryos of wrinkled peas of one of the isoforms occurring in embryos of round peas.