33 Genetic Dissection of the Biosynthesis, Degradation, and Biological Functions of Starch

Abstract

Starch is a polymer composed of glucose residues linked through a backbone of α-(1,4)-glycosidic bonds with branch points in the backbone formed by α-(1,6)-glycosidic bonds. Starch can be categorized as amylose, which typically contains few if any branch points, and amylopectin, which consists of short linear chains (about 20 residues in length on average) linked together into a branched structure (for a review of starch structure, see Kainuma 1988). In addition to differences in degree of branching, starches from different sources differ in the organization of the branch points, the lengths of the backbone chains, and the presence of small amounts of covalently or noncovalently bound protein, lipid, and phosphate. Starch accumulates in insoluble, partially crystalline particles called starch grains, which range in size from less than 1 μm in green tissues to more than 100 μm in diameter in potato tubers. Starch is made only inside plastids (chloroplasts and amyloplasts), and due to its insoluble nature, remains inside the plastid except in specialized cases such as cereal endosperm, where cellular compartments degrade as the endosperm matures.

Starch is synthesized by all higher plants and accumulates in large amounts in most plants. Transitory starch plays a key role in photosynthetic carbon metabolism, where it is synthesized inside chloroplasts during photosynthesis and then degraded to supply energy and carbon skeletons for metabolism during the subsequent dark period. Reserve starch is synthesized in amyloplasts in storage tissues (seeds, tubers, etc.) where it accumulates during one developmental phase (e.g., seed development) and then...