Enzymic Mechanism of Starch Breakdown in Germinating Rice Seeds II. Scutellum as the Site of Sucrose Synthesis

Abstract

A bstract. Time-sequence analyses of carbohydrate breakdown in germinating rice seeds shows that a rapid breakdown of starch reserve in endosperm starts after about 4 days of germination. Although the major soluble carbohydrate in the dry seed is sucrose, a marked increase in the production of glucose and maltooligosaccharides accompanies the breakdown of starch. Maltotriose was found to constitute the greatest portion of the oligosaccharides throughout the germination stage. a-Amylase activities were found to parallel the pattern of starch breakdown. Assays for phosphorylase activity showed that this enzyme may account for much smaller amounts of starch breakdown per grain, as compared to the amounts hydrolyzed by ay-amylase. There was a transient decline in the content of sucrose in the initiall 4 days of seed germination, followed by the gradual increase in later germination stages. During the entire germination stage, sucrose synthetase activity was not detected in the endosperm, although appreciable enzyme activity was present in the growing shoot tissues as well as in the frozen rice seeds harvested at the mid-milky stage. We propose the predominant formation of glucose from starch reserves in the endosperm by the taction of a-amylase and aocompanying hydrolytic enzyme(s) and that this sugar is eventually mobilized to the growing tissues, shoots or roots. Recent biochemical studies have elucidated the