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PLANT PHYSIOLOGY , Vol 101, Issue 2 573-578, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Characterization of Glucose-6-Phosphate Incorporation into Starch by Isolated Intact Cauliflower-Bud Plastids
H. E. Neuhaus, G. Henrichs and R. Scheibe
Pflanzenphysiologie, Fachbereich Biologie/Chemie, Universitat Osnabruck, Barbarastrasse 11, D-4500 Osnabruck, Federal Republic of Germany
Intact plastids from cauliflower (Brassica oleracea var Prince de Bretagne)
buds were isolated according to the method described by Journet and Douce
(E.P. Journet and R. Douce [1985] Plant Physiol 79: 458-467). Incubation of
these plastids with various 14C-labeled compounds revealed that
glucose-6-phosphate can act as a precursor for starch synthesis. However,
significant rates (incorporation of 120 nmol glucose mg-1 protein h-1)
could only be observed when both 3-phosphoglyceric acid and ATP were
present as well. Starch synthesis in isolated plastids was strongly
dependent upon the intactness of the organelle. The presence of a
high-affinity ATP/ADP translocator with a Km for ATP of 12 [mu]M was
demonstrated by uptake experiments with [14C]ATP. ADP inhibited both ATP
uptake and effector-stimulated starch synthesis. Effector-stimulated
glucose-6-phosphate-dependent starch synthesis was not significantly
influenced by fructose-6-phosphate or 2-deoxyglucose-6-phosphate but was
strongly inhibited by triose phosphate and inorganic phosphate. Starch
synthesis was also inhibited by
4,4[prime]-diisothio-cyanostilbene-2,2[prime]-disulfonate, which is known
to be a potent inhibitor of the chloroplast phosphate translocator. The
data presented here support the view that starch biosynthesis in
heterotrophic tissues is powered by increasing levels of cytosolic
3-phosphoglyceric acid and ATP when glucose-6-phosphate is available.
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