STARCH BIOSYNTHESIS AND MODIFICATION OF STARCH STRUCTURE IN TRANSGENIC PLANTS

Starch is synthesised through the ADP-glucose pathway, involving the t hree enzymes ADP-glucose pyrophosphorylase, starch synthase and starch branching enzyme. ADP-glucose pyrophosphorylase is the key enzyme of the pathway, determining the flux of carbon into starch. It generates ADP-glucose, which is the substrate for the starch synthases, from glu cose-1-phosphate and ATP releasing pyrophosphate. The enzyme is stimul ated by 3-phosphoglycerate and inhibited through inorganic phosphate. The starch synthases, which catalyse the transfer of glucose from ADP- glucose to the nonreducing end of a growing alpha-1,4-glucan, are divi ded into two classes, the granule-bound starch synthases (GBSS) and th e soluble starch synthases (SS). In both classes several isoforms have been described from many different plant species. The branching enzym e, which introduces branchpoints into the amylopectin, can also occur in different isoforms. Other enzymes present in plants, which also act on alpha-1,4-glucans, such as the starch phosphorylases, disproportio nating enzyme and different starch hydrolases, might also be important for dertermining the starch structure and, therefore, its processibil ity. Many aspects of starch synthesis are not fully understood to date . Starch metabolism can be manipulated through genetic engineering, ei ther by the ectopic expression of different heterologous genes, or thr ough the repression of the expression of endogenous genes using antise nse RNA technology. This not only allows the functional analysis of st arch biosynthetic proteins, but also the manipulation of starch struct ure in order to widen its industrial applications. In this way many di fferent potato lines have been generated, containing either different amounts of starch, or which synthesize a structurally modified starch. These structural changes relate to the amylose content, the phosphate content, or the gelatinisation and gelation characteristics of the st arch.