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Bob B. Buchanan – One of the best experts on this subject based on the ideXlab platform.

  • proteome of Amyloplasts isolated from developing wheat endosperm presents evidence of broad metabolic capability
    Journal of Experimental Botany, 2006
    Co-Authors: Yves Balmer, Bob B. Buchanan, William H. Vensel, Frances M. Dupont, William J Hurkman
    Abstract:

    By contrast to chloroplasts, our knowledge of Amyloplasts—organelles that synthesize and store starch in heterotrophic plant tissues—is in a formative stage. While our understanding of what is considered their primary function, i.e. the biosynthesis and degradation of starch, has increased dramatically in recent years, relatively little is known about other biochemical processes taking place in these organelles. To help fill this gap, a proteomic analysis of Amyloplasts isolated from the starchy endosperm of wheat seeds (10 d postanthesis) has been conducted. The study has led to the identification of 289 proteins that function in a range of processes, including carbohydrate metametabolism, cytoskeleton/plastid division, energetics, nitrogen and sulphur metabolism, nucleic acid-related reactions, synthesis of various building blocks, proteinrelated reactions, transport, signalling, stress, and a variety of other activities grouped under ‘miscellaneous’. The function of 12% of the proteins was unknown. The results highlight the role of the amyloplast as a starch-storing organelle that fulfills a spectrum of biosynthetic needs of the parent tissue. When compared with a recent proteomic analysis of whole endosperm, the current study demonstrates the advantage of using isolated organelles in proteomic studies.

  • a complete ferredoxin thioredoxin system regulates fundamental processes in Amyloplasts
    Proceedings of the National Academy of Sciences of the United States of America, 2006
    Co-Authors: Yves Balmer, William H. Vensel, Wanda Manieri, Peter Schürmann, William J Hurkman, Nick Cai, Bob B. Buchanan
    Abstract:

    †‡ A growing number of processes throughout biology are regulated by redox via thiol– disulfide exchange. This mechanism is particularly widespread in plants, where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protprotein. The current study extends regulation by Trx to Amyloplasts, organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxinTrx system (ferredoxin, ferredoxin–Trx reductase, and Trx), originally described for chloroplasts, in Amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin–NADP reductase. However, once reduced, ferredoxin appears to act as established for chloroplasts, i.e., via ferredoxin– Trx reductase and a Trx (m-type). A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids, and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves. redox regulation target proteins ferredoxinthioredoxin reductase

  • A complete ferredoxin/thioredoxin system regulates fundamental processes in Amyloplasts
    Proceedings of the National Academy of Sciences of the United States of America, 2006
    Co-Authors: Yves Balmer, William H. Vensel, Wanda Manieri, Peter Schürmann, William J Hurkman, Bob B. Buchanan
    Abstract:

    †‡ A growing number of processes throughout biology are regulated by redox via thiol– disulfide exchange. This mechanism is particularly widespread in plants, where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protprotein. The current study extends regulation by Trx to Amyloplasts, organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxinTrx system (ferredoxin, ferredoxin–Trx reductase, and Trx), originally described for chloroplasts, in Amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin–NADP reductase. However, once reduced, ferredoxin appears to act as established for chloroplasts, i.e., via ferredoxin– Trx reductase and a Trx (m-type). A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids, and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves. redox regulation target proteins ferredoxinthioredoxin reductase

Michael J. Emes – One of the best experts on this subject based on the ideXlab platform.

  • Multimeric states of starch phosphorylase determine protein-protein interactions with starch biosynthetic enzymes in Amyloplasts.
    Plant physiology and biochemistry : PPB, 2014
    Co-Authors: Renuka M. Subasinghe, Michael J. Emes, Fushan Liu, Ursula C. Polack, Elizabeth A. Lee, Ian J. Tetlow
    Abstract:

    Abstract Proteinprotein 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. Proteinprotein 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, proteinprotein 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-32P]-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.

  • analysis of protein complexes in wheat Amyloplasts reveals functional interactions among starch biosynthetic enzymes
    Plant Physiology, 2008
    Co-Authors: Ian J. Tetlow, Fushan Liu, Robin Wait, Matthew K. Morell, Kim Gabriele Beisel, S Cameron, Amina Makhmoudova, Nicole S Bresolin, Michael J. Emes
    Abstract:

    Proteinprotein interactions among enzymes of amylopectin biosynthesis were investigated in developing wheat (Triticum aestivum) endosperm. Physical interactions between starch branching enzymes (SBEs) and starch synthases (SSs) were identified from endosperm Amyloplasts during the active phase of starch deposition in the developing grain using immunoprecipitation and cross-linking strategies. Coimmunoprecipitation experiments using peptide-specific antibodies indicate that at least two distinct complexes exist containing SSI, SSIIa, and either of SBEIIa or SBEIIb. Chemical cross linking was used to identify protein complexes containing SBEs and SSs from amyloplast extracts. Separation of extracts by gel filtfiltration chromatography demonstrated the presence of SBE and SS forms in protein complexes of around 260 kD and that SBEII forms may also exist as homodimers. Analysis of cross-linked 260-kD aggregation products from amyloplast lysates by mass spectrometry confirmed SSI, SSIIa, and SBEII forms as components of one or more protein complexes in Amyloplasts. In vitro phosphorylation experiments with γ-32P-ATP indicated that SSII and both forms of SBEII are phosphorylated. Treatment of the partially purified 260-kD SS-SBE complexes with alkaline phosphatase caused dissociation of the assembly into the respective monomeric proteins, indicating that formation of SS-SBE complexes is phosphorylation dependent. The 260-kD SS-SBEII protein complexes are formed around 10 to 15 d after pollination and were shown to be catalytically active with respect to both SS and SBE activities. Prior to this developmental stage, SSI, SSII, and SBEII forms were detectable only in monomeric form. High molecular weight forms of SBEII demonstrated a higher affinity for in vitro glucan substrates than monomers. These results provide direct evidence for the existence of protein complexes involved in amylopectin biosynthesis.

  • Characterization of ADP-glucose transport across the cereal endosperm amyloplast envelope
    Journal of experimental botany, 2007
    Co-Authors: Caroline G. Bowsher, Michael J. Emes, Edward F. A. L. Scrase-field, Sergio Esposito, Ian J. Tetlow
    Abstract:

    Most of the carbon used for starch biosynthesis in cereal endosperms is derived from ADP-glucose (ADP-Glc) synthesized by extra-plastidial AGPase activity, and imported directly across the amyloplast envelope. The properties of the wheat endosperm amyloplast ADP-Glc transporter were analysed with respect to substrate kinetics and specificities using reconstituted amyloplast envelope proteins in a proteoliposome-based assay system, as well as with isolated intact organelles. Experiments with liposomes showed that ADP-Glc transport was dependent on counter-exchange with other adenylates. Rates of ADP-Glc transport were highest with ADP and AMP as counter-exchange substrates, and kinetic analysis revealed that the transport system has a similar affinity for ADP and AMP. Measurement of ADP and AMP efflux from intact Amyloplasts showed that, under conditions of ADP-Glc-dependent starch biosynthesis, ADP is exported from the plastid at a rate equal to that of ADP-Glc utilization by starch synthases. Photo-affinity labelling of amyloplast membranes with the substrate analogue 8-azido-[alpha-32P]ADP-Glc showed that the polypeptide involved in substrate binding is an integral membrane protein of 38 kDa. This study shows that the ADP-Glc transporter in cereal endosperm Amyloplasts imports ADP-Glc in exchange for ADP which is produced as a by-product of the starch synthase reaction inside the plastid.

Ian J. Tetlow – One of the best experts on this subject based on the ideXlab platform.

  • Multimeric states of starch phosphorylase determine protein-protein interactions with starch biosynthetic enzymes in Amyloplasts.
    Plant physiology and biochemistry : PPB, 2014
    Co-Authors: Renuka M. Subasinghe, Michael J. Emes, Fushan Liu, Ursula C. Polack, Elizabeth A. Lee, Ian J. Tetlow
    Abstract:

    Abstract 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-32P]-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.

  • analysis of protein complexes in wheat Amyloplasts reveals functional interactions among starch biosynthetic enzymes
    Plant Physiology, 2008
    Co-Authors: Ian J. Tetlow, Fushan Liu, Robin Wait, Matthew K. Morell, Kim Gabriele Beisel, S Cameron, Amina Makhmoudova, Nicole S Bresolin, Michael J. Emes
    Abstract:

    Protein-protein interactions among enzymes of amylopectin biosynthesis were investigated in developing wheat (Triticum aestivum) endosperm. Physical interactions between starch branching enzymes (SBEs) and starch synthases (SSs) were identified from endosperm Amyloplasts during the active phase of starch deposition in the developing grain using immunoprecipitation and cross-linking strategies. Coimmunoprecipitation experiments using peptide-specific antibodies indicate that at least two distinct complexes exist containing SSI, SSIIa, and either of SBEIIa or SBEIIb. Chemical cross linking was used to identify protein complexes containing SBEs and SSs from amyloplast extracts. Separation of extracts by gel filtration chromatography demonstrated the presence of SBE and SS forms in protein complexes of around 260 kD and that SBEII forms may also exist as homodimers. Analysis of cross-linked 260-kD aggregation products from amyloplast lysates by mass spectrometry confirmed SSI, SSIIa, and SBEII forms as components of one or more protein complexes in Amyloplasts. In vitro phosphorylation experiments with γ-32P-ATP indicated that SSII and both forms of SBEII are phosphorylated. Treatment of the partially purified 260-kD SS-SBE complexes with alkaline phosphatase caused dissociation of the assembly into the respective monomeric proteins, indicating that formation of SS-SBE complexes is phosphorylation dependent. The 260-kD SS-SBEII protein complexes are formed around 10 to 15 d after pollination and were shown to be catalytically active with respect to both SS and SBE activities. Prior to this developmental stage, SSI, SSII, and SBEII forms were detectable only in monomeric form. High molecular weight forms of SBEII demonstrated a higher affinity for in vitro glucan substrates than monomers. These results provide direct evidence for the existence of protein complexes involved in amylopectin biosynthesis.

  • Characterization of ADP-glucose transport across the cereal endosperm amyloplast envelope
    Journal of experimental botany, 2007
    Co-Authors: Caroline G. Bowsher, Michael J. Emes, Edward F. A. L. Scrase-field, Sergio Esposito, Ian J. Tetlow
    Abstract:

    Most of the carbon used for starch biosynthesis in cereal endosperms is derived from ADP-glucose (ADP-Glc) synthesized by extra-plastidial AGPase activity, and imported directly across the amyloplast envelope. The properties of the wheat endosperm amyloplast ADP-Glc transporter were analysed with respect to substrate kinetics and specificities using reconstituted amyloplast envelope proteins in a proteoliposome-based assay system, as well as with isolated intact organelles. Experiments with liposomes showed that ADP-Glc transport was dependent on counter-exchange with other adenylates. Rates of ADP-Glc transport were highest with ADP and AMP as counter-exchange substrates, and kinetic analysis revealed that the transport system has a similar affinity for ADP and AMP. Measurement of ADP and AMP efflux from intact Amyloplasts showed that, under conditions of ADP-Glc-dependent starch biosynthesis, ADP is exported from the plastid at a rate equal to that of ADP-Glc utilization by starch synthases. Photo-affinity labelling of amyloplast membranes with the substrate analogue 8-azido-[alpha-32P]ADP-Glc showed that the polypeptide involved in substrate binding is an integral membrane protein of 38 kDa. This study shows that the ADP-Glc transporter in cereal endosperm Amyloplasts imports ADP-Glc in exchange for ADP which is produced as a by-product of the starch synthase reaction inside the plastid.

Yves Balmer – One of the best experts on this subject based on the ideXlab platform.

  • proteome of Amyloplasts isolated from developing wheat endosperm presents evidence of broad metabolic capability
    Journal of Experimental Botany, 2006
    Co-Authors: Yves Balmer, Bob B. Buchanan, William H. Vensel, Frances M. Dupont, William J Hurkman
    Abstract:

    By contrast to chloroplasts, our knowledge of Amyloplasts—organelles that synthesize and store starch in heterotrophic plant tissues—is in a formative stage. While our understanding of what is considered their primary function, i.e. the biosynthesis and degradation of starch, has increased dramatically in recent years, relatively little is known about other biochemical processes taking place in these organelles. To help fill this gap, a proteomic analysis of Amyloplasts isolated from the starchy endosperm of wheat seeds (10 d postanthesis) has been conducted. The study has led to the identification of 289 proteins that function in a range of processes, including carbohydrate metabolism, cytoskeleton/plastid division, energetics, nitrogen and sulphur metabolism, nucleic acid-related reactions, synthesis of various building blocks, proteinrelated reactions, transport, signalling, stress, and a variety of other activities grouped under ‘miscellaneous’. The function of 12% of the proteins was unknown. The results highlight the role of the amyloplast as a starch-storing organelle that fulfills a spectrum of biosynthetic needs of the parent tissue. When compared with a recent proteomic analysis of whole endosperm, the current study demonstrates the advantage of using isolated organelles in proteomic studies.

  • a complete ferredoxin thioredoxin system regulates fundamental processes in Amyloplasts
    Proceedings of the National Academy of Sciences of the United States of America, 2006
    Co-Authors: Yves Balmer, William H. Vensel, Wanda Manieri, Peter Schürmann, William J Hurkman, Nick Cai, Bob B. Buchanan
    Abstract:

    †‡ A growing number of processes throughout biology are regulated by redox via thiol– disulfide exchange. This mechanism is particularly widespread in plants, where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protein. The current study extends regulation by Trx to Amyloplasts, organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxinTrx system (ferredoxin, ferredoxin–Trx reductase, and Trx), originally described for chloroplasts, in Amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin–NADP reductase. However, once reduced, ferredoxin appears to act as established for chloroplasts, i.e., via ferredoxin– Trx reductase and a Trx (m-type). A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids, and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves. redox regulation target proteins ferredoxin–thioredoxin reductase

  • A complete ferredoxin/thioredoxin system regulates fundamental processes in Amyloplasts
    Proceedings of the National Academy of Sciences of the United States of America, 2006
    Co-Authors: Yves Balmer, William H. Vensel, Wanda Manieri, Peter Schürmann, William J Hurkman, Bob B. Buchanan
    Abstract:

    †‡ A growing number of processes throughout biology are regulated by redox via thiol– disulfide exchange. This mechanism is particularly widespread in plants, where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protein. The current study extends regulation by Trx to Amyloplasts, organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxinTrx system (ferredoxin, ferredoxin–Trx reductase, and Trx), originally described for chloroplasts, in Amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin–NADP reductase. However, once reduced, ferredoxin appears to act as established for chloroplasts, i.e., via ferredoxin– Trx reductase and a Trx (m-type). A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids, and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves. redox regulation target proteins ferredoxin–thioredoxin reductase

Takashi Akazawa – One of the best experts on this subject based on the ideXlab platform.

  • distinct isoforms of adpglucose pyrophosphatase and adpglucose pyrophosphorylase occur in the suspension cultured cells of sycamore acer pseudoplatanus l
    FEBS Letters, 2000
    Co-Authors: Edurne Barojafernandez, Aitor Zanduetacriado, Milagros Rodriguezlopez, Takashi Akazawa, Javier Pozuetaromero
    Abstract:

    The intracellular localizations of ADPglucose pyrophosphatase (AGPPase) and ADPglucose pyrophosphorylase (AGPase) have been studied using protoplasts prepared from suspension-cultured cells of sycamore (Acer pseudoplatanus L.). Subcellular fractionation studies revealed that all the AGPPase present in the protoplasts is associated with Amyloplasts, whereas more than 60% of AGPase is in the extraplastidial compartment. Immunoblots of amyloplast- and extraplastid-enriched extracts further confirmed that AGPase is located mainly outside the amyloplast. Experiments carried out to identify possible different isoforms of AGPPase in the amyloplast revealed the presence of soluble and starch grangranule-bound isoforms. We thus propose that ADPglucose levels linked to starch biosynthesis in sycamore cells are controlled by enzymatic reactions catalyzing the synthesis and breakdown of ADPglucose, which take place both inside and outside the amyloplast.

  • characterization and intraorganellar distribution of protein kinases in Amyloplasts isolated from cultured cells of sycamore acer pseudoplatanus l
    Plant Physiology, 1991
    Co-Authors: Alejandro M. Viale, Jarunya Ngernprasirtsiri, Takashi Akazawa
    Abstract:

    Incubation of Amyloplasts isolated from cultured cells of sycamore (Acer pseudoplatanus L.) with [γ-32P]ATP resulted in the rapid phosphorylation (half-time of 40 seconds at 25 degrees Celcius) of organellar polypeptides. The preferred substrate for amyloplast protein kinases was Mg2+. ATP, and recovery of only [32P]serine after partial acid hydrolysis indicated the predominance of protein serine kinases in the organelle. These activities were located in the envelope and stromal fractions of the plastid, which showed different specificities toward exogenous protein substrates and distinct patterns of phosphorylation of endogenous polypeptides. A 66-kilodalton polypeptide, inaccessible to an exogenously added protease, was one of the major phosphorylated products found in intact Amyloplasts at low [γ-32P] adenosine triphosphate concentrations. This polypeptide represented the major phosphoprotein observed with the isolated envelope fraction. The patterns of polypeptide phosphorylation found in intact Amyloplasts and chloroplasts from cultured cell lines of sycamore were clearly distinguishable. The overall results indicate the presence of protein phosphorylation systems unique to this reserve plastid present in nonphotosynthetic tissues.

  • Direct transport of ADPglucose by an adenylate translocator is linked to starch biosynthesis in Amyloplasts.
    Proceedings of the National Academy of Sciences of the United States of America, 1991
    Co-Authors: Javier Pozueta-romero, Marco Frehner, Alejandro M. Viale, Takashi Akazawa
    Abstract:

    Abstract Starch biosynthesis has been studied by using Amyloplasts isolated from cultured cells of sycamore trees (Acer pseudoplatanus L.). Highly purified intact Amyloplasts, free from mitochondria and starch granules derived from broken Amyloplasts, were isolated from a Percoll step gradient. Subsequently, the double silicone oil layer centrifugation technique was used to study adenylate transport in the Amyloplasts. An adenylate-specific carrier was found to be active in the uptake of ATP, ADP, AMP, and most importantly, ADPglucose (ADP-Glc). Kinetic analyses showed that the uptake of these adenylates was mutually competitive with each other. In contrast to the mitochondrial adenylate carrier, in Amyloplasts only ATP and ADP-Glc uptake were inhibited by carboxyatractyloside. Evidence is presented that the ADP-Glc transported into the amyloplast stroma can be used in starch synthesis catalyzed by starch synthase (ADP-Glc:1,4-alpha-D-glucan 4-alpha-D-glucosyltransferase, EC 2.4.1.21). We propose that starch biosynthesis in Amyloplasts is tightly coupled with the direct transport of ADP-Glc synthesized in the cytosol by sucrose synthase (ADP-Glc:D-fructose 2-alpha-D-glucosyltransferase, EC 2.4.1.13)