Is There an Alternative Pathway for Starch Synthesis?

Okita, Thomas W.

doi:10.1104/pp.100.2.560

Cited by 133 publications

(78 citation statements)

References 19 publications

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“…Starch is considered to be the end product of a unidirectional and vectorial pathway in which the chloroplastic ADPG pyrophosphorylase (AGP) (IUBMB accession no. EC 2.7.7.27) is the only enzyme catalyzing the synthesis of ADPG, and it acts as the major limiting step of the gluconeogenic process (8)(9)(10). This popular view, consistent with the idea that the chloroplast is a complete photosynthetic unit that can produce starch from the atmospheric CO 2 (11,12), appears to be supported by a wide variety of mutant and transgenic lines in which enzymes and transport machineries have been modulated (13)(14)(15)(16)(17).…”

supporting

confidence: 57%

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

Baroja‐Fernández

Muñoz

Zandueta-Criado

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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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 ADP⅐glucose (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.

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supporting

confidence: 57%

“…Although its physiological relevance was questioned initially (9), this finding carried the inherent supposition that a sizable pool of the ADPG linked to starch biosynthesis in leaves occurs in the cytosol. In this respect, and based on the capacity of cytosolic sucrose synthase (SuSy) (IUBMB accession no.…”

mentioning

confidence: 99%

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

Baroja‐Fernández

Muñoz

Zandueta-Criado

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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“…This result was criticised for several reasons. (1) No enzyme reaction was known that allowed cytosolic synthesis of ADPGlc (for a review see [15]). (2) In some types of amyloplasts, exogenously supplied ADPGlc induced only low rates of starch biosynthesis [12].…”

Section: Introductionmentioning

confidence: 99%

ADP-glucose drives starch synthesis in isolated maize endosperm amyloplasts: characterization of starch synthesis and transport properties across the amyloplast envelope

Möhlmann

Tjaden

Henrichs

et al. 1997

Biochemical Journal

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We recently developed a method of purifying amyloplasts from developing maize (Zea mays L.) endosperm tissue [Neuhaus, Thom, Batz and Scheibe (1993) Biochem. J. 296, [395][396][397][398][399][400][401]. In the present paper we analyse how glucose 6-phosphate (Glc6P) and other phosphorylated compounds enter the plastid compartment. Using a proteoliposome system in which the plastid envelope membrane proteins are functionally reconstituted, we demonstrate that this type of plastid is able to transport ["%C]Glc6P or [$#P]P i in counter exchange with P i , Glc6P, dihydroxyacetone phosphate and phosphoenolpyruvate. Glucose 1-phosphate, fructose 6-phosphate and ribose 5-phosphate do not act as substrates for counter exchange. Besides hexose phosphates, ADP-glucose (ADPGlc) also acts as a substrate for starch synthesis in isolated maize endosperm amyloplasts. This process exhibits saturation kinetics with increasing concentrations of exogenously supplied ["%C]ADPGlc, reaching a maximum at

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“…AGPase is heterotetrameric in higher plants, consisting of two large (AGP-L) and two small (AGP-S) subunits encoded by Shrunken-2 and Brittle-2 genes, respectively [138,139]. AGPase subunits are encoded by multiple genes.…”

Section: Adp-glucose Pyrophosphorylasementioning

confidence: 99%

Causal Relations Among Starch Biosynthesis, Structure, and Properties

Wang

Henry

Gilbert

2014

Springer Science Reviews

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Starch, an important carbohydrate with widespread applications in human foods, animal feeds, and many industrial products, is synthesized by plants by the action of a complex system involving many enzymes. The differing activities of these enzymes contribute to variations in starch structure among different plant species, botanical organs, and genetic backgrounds, and thus affect the physicochemical properties and end-use functions of starch. The demand for starches with particular functional properties is increasing, but the ability to produce novel starches is still limited. Starches with specific properties can potentially be produced by biotechnical modification of the starch biosynthetic pathway; however, this requires further understanding of the starch biosynthesis-structureproperties relationships. This review summarizes the state of the art in the understanding of these causal relationships: the roles of the main starch-synthesizing enzymes on starch structure, hierarchical structure of starch, advanced molecular structure characterization methods, and impact of starch structure on some functional properties. A better understanding of these relationships among starch biosynthesis, structure, and properties provides direction for genetic modification and targeted breeding programs to produce starch with desired characteristics.

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Is There an Alternative Pathway for Starch Synthesis?

Cited by 133 publications

References 19 publications

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

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

ADP-glucose drives starch synthesis in isolated maize endosperm amyloplasts: characterization of starch synthesis and transport properties across the amyloplast envelope

Causal Relations Among Starch Biosynthesis, Structure, and Properties

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