Manipulating the starch composition of potato

Starch can be fractionated into two types of glucose polymers: amylose and amylopectin. Amylose consists of essentially linear chains of alpha-(1,4)-linked glucose residues, whereas amylopectin is built up from alpha-(1,4)-linked chains with alpha-(1,6)-linked branches. The composition and fine structure of starch are responsible for many of the physicochemical properties and thus determines its industrial uses. Variation in starch structure and composition can be found between and within crops. In the latter case it can be found in mutants, often resulting from the loss of function of one or more of the genes involved in starch biosynthesis. In maize, the most extensively studied crop, mutant genotypes are known for nearly every gene identified as being involved in starch biosynthesis. Differences in starch compositon can also be achieved by genetic modifications such as antisense inhibition of genes or overexpression of (heterologous) genes. Most examples of genetic modification of starch composition are in potato, which can easily be transformed. Antisense inhibition of enzymes in the biosynthetic pathway, such as ADP glucose phosphorylase (AGP), (granule-bound) starch synthase or branching enzyme, lead to an altered starch content and/or composition. In addition, the introduction and expression of bacterial genes, such as genes of the Escherichia coli glycogen synthesis pathway, in potato leads to starches with altered content, composition, structure and physicochemical properties. Studying the physicochemical properties of these altered starches will, together with the information obtained by research on starches of mutants, help to clarify the precise relationship between structural and functional features of starch.