Plant Physiology 100:560-564 (1992)
© 1992 American Society of Plant Biologists
Update on Metabolism
Is There an Alternative Pathway for Starch Synthesis? 1
Thomas W. Okita
Institute of Biological Chemistry, Washington State University, Pullman, Washington
In leaf tissue, carbon enters starch via the gluconeogenesis pathway where D-glycerate 3-phosphate formed from CO2 fixation is converted into hexose monophosphates within the chloroplast stroma. In starch-containing sink organs, evidence has been obtained indicating that the flow of carbon into starch follows a different pathway whereby hexose monophosphates formed from sucrose are transported into the amyloplast, a plastid specialized in starch accumulation. In both chloroplasts and amyloplasts, the formation of ADPglucose, the substrate for starch synthase, is controlled by the activity of ADPglucose pyrophosphorylase, a key regulatory enzyme of starch synthesis localized in the plastid. Recently, an alternative pathway of starch synthesis has been proposed in which ADPglucose is synthesized from sucrose and transported directly into the plastid compartment, where it is used for starch synthesis. On the basis of the biochemical phenotypes exhibited by various plant mutants with defined genetic lesions, it is concluded that ADPglucose pyrophosphorylase is essential for starch synthesis, whereas the alternative pathway has only a minor role in this process.
1 Supported in part by Department of Energy grant DE-FG06-87ER13699, The Rockefeller Foundation, and Project 0590, College of Agriculture and Home Economics, Washington State University, Pullman, WA 99164.
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