Starch Biosynthesis in Rice Endosperm

© 2014 TERRAPUB, Tokyo. All rights reserved. doi:10.5047/agbm.2014.00401.0001 water (Fig. 1) or treating it with alkaline, urea, or dimethyl sulfoxide through the cutting of hydrogen bonds between the starch molecules. It is thought that digestion of hydrogen bonds results in the unwinding of the double helices of parallel α-1,4 glucan chains in the crystal lamellae of amylopectin molecules. The reactions involved in the formation of gelatinized starch are not reversible (Fig. 1). Retrogradation of starch is thought to be the incomplete winding of neighboring chains of gelatinized starch. Retrograded starch can be gelatinized again by heating, indicating that the reactions between starch retrogradation and gelatinization are reversible (Fig. 1). Both gelatinization and retrogradation of starch play essential roles for food chemistry and industrial applications. Starches from plants with different genetic backgrounds have different physico-chemical properties and can be used for different purposes. Corn starches have widespread use for many applications because of their low cost and high performance. The advent of genetic engineering expanded the possibilities for production of tailor-made starches with specific properties that can be synthesized in the storage tissues of plants. People must use recyclable carbohydrate starches whose properties are 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. Starch Biosynthesis in Rice Endosperm