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PLANT PHYSIOLOGY , Vol 105, Issue 1 111-117, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Starch Phosphorylation in Potato Tubers Proceeds Concurrently with de Novo Biosynthesis of Starch
T. H. Nielsen, B. Wischmann, K. Enevoldsen and B. L. Moller
Department of Plant Biology, Royal Agricultural and Veterinary University, 40 Thorvaldsensvej, DK-1871 Frederiksberg C, Copenhagen, Denmark (T.H.N., B.W., B.L.M.)
The in vivo phosphorylation of starch was studied in Solanum tuberosum cv
Dianella and Posmo. Small starch granules contain 25% more ester-bound
phosphate per glucose residue than large starch granules. The degree of
phosphorylation was found to be almost constant during tuber development.
Isolated tuber discs synthesize starch from externally supplied glucose at
a significant rate. Tuber discs supplied with glucose and
[32P]orthophosphate incorporate radiolabeled phosphorus into the starch.
The level of 32P incorporation is proportional to the amount of starch
synthesized. The incorporation of 32P from orthophosphate is correlated to
de novo synthesis of starch, since the incorporation of 32P is diminished
upon inhibition of starch synthesis by fluoride. Based on the amount of
[14C]glucose phosphate isolated after hydrolysis of purified starch from
tuber discs incubated in the presence of [U-14C]glucose, approximately 0.5%
of the glucose residues of the de novo-synthesized starch are
phosphorylated. This value is in general agreement with the observed levels
of phosphorus in starch accumulated during tuber development. Thus, the
enzyme system responsible for starch phosphorylation is fully active in the
isolated tuber discs, and the starch phosphorylation proceeds as an
integrated part of de novo starch synthesis.
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