PLANT PHYSIOLOGY , Vol 106, Issue 3 1123-1129, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Alteration of the Amount of the Chloroplast Phosphate Translocator in Transgenic Tobacco Affects the Distribution of Assimilate between Starch and Sugar
S. A. Barnes, J. S. Knight and J. C. Gray
Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom
Tobacco plants (Nicotiana tabacum L.) transformed with sense and antisense
constructs of a cDNA encoding the tobacco phosphate-triose
phosphate-3-phosphoglycerate translocator (phosphate translocator) were
shown to contain altered amounts of phosphate translocator mRNA and
protein. Phosphate translocator activity in intact chloroplasts isolated
from transformed plants showed a 15-fold variation, from 20% of the
wild-type activity in antisense transformants to 300% of the wild-type
activity in sense transformants. However, the maximal rates of
photosynthesis and the rates of photosynthetic carbon assimilation in
ambient CO2 showed no consistent differences between transformants. Starch
content was decreased by 20% and total soluble sugars were increased by 20%
in leaves of antisense transformants compared to sense transformants. The
40% decrease in the ratio of starch to total soluble sugars in antisense
transformants relative to sense transformants indicates that distribution
of assimilate between starch and sugar had been altered. However, the
amount of sucrose in the leaves was unchanged. The changes in total soluble
sugars were accounted for completely by changes in glucose and fructose,
suggesting the existence of a homeostatic mechanism for maintaining sucrose
concentrations in the leaves at the expense of glucose and fructose.
