STARCH SYNTHESIS AND CARBOHYDRATE OXIDATION IN AMYLOPLASTS FROM DEVELOPING WHEAT ENDOSPERM

The rates of incorporation of various metabolites into starch by isola ted amyloplasts from developing endosperm of spring wheat (Triticum ae stivum L. cv. Axona) were examined. Of the metabolites tested that wer e likely to be present in the cytosol at concentrations sufficient to sustain starch synthesis, only glucose 1-phosphate (Glc1P) supported p hysiologically relevant rates of starch synthesis. Incorporation of Gl c1P into starch was both dependent on the presence of ATP and intact o rganelles. The rate of incorporation of hexose into starch became satu rated at a Glc1P concentration of less than 1 mol m(-3) in the presenc e of 1 mol m(-3) ATP. Starch synthesis from 5 mol m(-3) ADP-glucose su pplied to the organelles occurred at rates 15-fold higher than from si milar concentrations of Glc1P, but it is argued that this is probably of little physiological relevance. The net incorporation of hexose uni ts into starch from Glc1P was inhibited 50% by 100 mmol.m(-3) carboxya tractyloside. Carbohydrate oxidation in the amyloplast was stimulated by the addition of 2-oxoglutarate and glutamine, and in such circumsta nces incorporation of C-14-labelled metabolites into starch was reduce d. Glucose 6-phosphate proved to be a better substrate for oxidative p athways than Glc1P. Our results suggest that Glc1P is the primary subs trate for starch synthesis in developing wheat endosperm, and that ATP required for starch synthesis is imported via an adenylate translocat or.