Starch Biosynthesis in Rice Endosperm

Abstract

Starches are the most important form of carbohydrates for most organisms on earth. However, the starch structure and biosynthesis mechanisms have not been completely resolved. At least four classes of enzymes catalyze the reactions of starch biosynthesis in plants: starch synthase (SS) elongates α-glucan chains of starch, ADP-glucose pyrophosphorylase (AGPase) supplies the substrate for SS, branching enzyme (BE) forms the α-1,6glycosidic bonds of amylopectin, and debranching enzyme (DBE) trims improper branches generated by BE. Many isozymes of these enzymes encoding different genes exist in green plants. To understand the starch biosynthesis mechanisms, the author tried to isolate rice mutant lines and transgenic rice lines of the genes that account for starch biosynthetic enzymes. Through the biochemical and physiological analyses of these materials during the last 15 years, the function of the isozymes expressed in the endosperm of rice has been better understood. We built the model of amylopectin biosynthesis based on the function of each isozyme. The unique starches that accumulate in the endosperm of mutant lines are quite different from those of the wild type. In the near future, the author hopes that unique starches that accumulate in the mutant lines will be useful for industrial applications.