Starch synthesis and carbohydrate oxidation in amyloplasts from developing wheat endosperm

Abstract

The rates of incorporation of various metabolites into starch by isolated amyloplasts from developing endosperm of spring wheat (Triticum aestivum L. cv. Axona) were examined. Of the metabolites tested that were likely to be present in the cytosol at concentrations sufficient to sustain starch synthesis, only glucose 1-phosphate (Glc1P) supported physiologically relevant rates of starch synthesis. Incorporation of Glc1P into starch was both dependent on the presence of ATP and intact organelles. The rate of incorporation of hexose into starch became saturated at a Glc1P concentration of less than 1 mol·m-3 in the presence of 1 mol·m-3 ATP. Starch synthesis from 5 mol · m-3 ADP-glucose supplied to the organelles occurred at rates 15-fold higher than from similar concentrations of Glc1P, but it is argued that this is probably of little physiological relevance. The net incorporation of hexose units into starch from GlclP was inhibited 50% by 100 mmol.m-3 carboxyatractyloside. Carbohydrate oxidation in the amyloplast was stimulated by the addition of 2-oxoglutarate and glutamine, and in such circumstances incorporation of14C-labelled metabolites into starch was reduced. Glucose 6-phosphate proved to be a better substrate for oxidative pathways than Glc1P. Our results suggest that Glc1P is the primary substrate for starch synthesis in developing wheat endosperm, and that ATP required for starch synthesis is imported via an adenylate translocator.