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PLANT PHYSIOLOGY , Vol 108, Issue 4 1569-1577, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Influence of Elevated Fructose-2,6-Bisphosphate Levels on Starch Mobilization in Transgenic Tobacco Leaves in the Dark
P. Scott and N. J. Kruger
Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
The aim of this work was to study the effect of elevated
fructose-2,6-bisphosphate (Fru-2,6-bisP) levels on carbohydrate metabolism
in leaves in the dark. In transgenic tobacco (Nicotiana tabacum L.) lines
containing mammalian 6-phosphofructo-2-kinase activity there is an inverse
relationship between the level of Fru-2,6-bisP in leaves and the rate of
starch breakdown in the dark. Estimates of the flux response coefficient
for the rate of net starch degradation with respect to changes in
Fru-2,6-bisP level are -0.57 for whole leaves and -0.69 to -0.89 for
excised leaf discs. We suggest that this decrease in the net rate of starch
breakdown is caused, at least in part, by stimulation of unidirectional
starch synthesis. Measurements of the levels of metabolic intermediates and
the metabolism of [U-14C]glucose indicate that the stimulation of starch
synthesis in the dark is a result of high Fru-2,6-bisP levels, increasing
the 3-phosphoglycerate:inorganic phosphate ratio in leaves. We argue that
the observed response to changes in the level of Fru-2,6-bisP are effected
through activation of pyrophosphate:fructose-6-phosphate
1-phosphotransferase. However, the extent to which changes in Fru-2,6-bisP
influence starch metabolism in wild-type plants is not known.
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