Carbohydrate Metabolism in the Developing Endosperm of Rice Grains

The metabolism of carbohydrates in developing rice endospermwas characterized by a comparison of levels of activities of33 major enzymes between the endosperm and green leaves of rice.Activities of ADPglucose pyrophosphorylase, starch synthaseand branching enzyme (Q-en-zyme), compared on the basis of solubleprotein content, were markedly higher in endosperm than in greenleaves. The high levels of Q-enzyme may be responsible for theefficient production of starch in the rice endosperm. The measurementof levels of metabolic intermediates and the localization ofkey enzymes in isolated amyloplasts from rice endosperm supportthe view that sucrose is metabolized in the cytoplasm via thepathway: sucroseUDPglucosehexose-PFBPtriose-P. Triose-P thenenters the amyloplast, where it is converted to G1P via FBPand, finally, G1P is converted to starch by the concerted reactionsof ADPglucose pyrophosphorylase, starch synthase and Q-enzyme. ¹Present address: Yamagata Prefectural Agricultural Experi mentStation, Minorigaoka, Yamagata, 990-02 Japan. ²Present address: Institute of Biological Sciences, The University of Tsukuba, Tsukuba Science City, Ibaraki, 305 Japan. (Received February 15, 1989; Accepted June 10, 1989)