Starch degradation metabolism towards sucrose synthesis in germinating Araucaria araucana seeds

Abstract

As starch is the main seed reserve material in both species ofAraucaria of South America, A. araucana and A. angustifolia, it is important to understand starch breakdown in both embryo and megagametophyte tissues of Araucaria seeds. Sugar analysis by thin layer chromatography indicates that sucrose is the main sugar produced in both tissues. Enzyme reactions coupled to benzidine oxidation indicate that sucrose is the main sugar moved from the megagametophyte to the growing regions of the embryo via the cotyledons. Phosphorylase was detected in both embryo and megagmetophyte tissues by the formation of [32Pjglucose-I-P and by formation of [14q] amylopectin from [14Cjglucose-I-P. The enzyme activity increases 5-fold in both embryo and gametophyte to a peak 18 hours after the start of imbibition. Debranching enzyme, α-glucosidase, and hexokinase are also present in both embryonic and megapmetophytic tissues. Branched glucan oligosaccharides accumulate during this time, reaching a maximum 40 hours after imbibition starts, and decline after germination occurs. The pattern of activity of the enzymes studied in this work suggests that starch degradation is initiated by a-amylase and phosphorylase in the embryo and by phosphorylase mainly in the megagametophyte. Sucrose-P synthase seems to be the enzyme responsible for sucrose synthesis in both tissues.