Carbohydrate Metabolism in the Developing Endosperm of Rice Grains

The metabolism of carbohydrates in developing rice endosperm was characterized by a comparison of levels of activities of 33 major enzymes between the endosperm and green leaves of rice. Activities of ADPglucose pyrophosphorylase, starch synthase and branching enzyme (Q-en-zyme), compared on the basis of soluble protein content, were markedly higher in endosperm than in green leaves. The high levels of Q-enzyme may be responsible for the efficient production of starch in the rice endosperm. The measurement of levels of metabolic intermediates and the localization of key enzymes in isolated amyloplasts from rice endosperm support the view that sucrose is metabolized in the cytoplasm via the pathway: sucrose→UDPglucose→hexose-P→FBP→triose-P. Triose-P then enters the amyloplast, where it is converted to G1P via FBP and, finally, G1P is converted to starch by the concerted reactions of ADPglucose pyrophosphorylase, starch synthase and Q-enzyme.