Amylose biosynthesis in potato : interaction between substrate availability and GBSSI activity, regulated at the allelic level = [Amylose biosynthese in aardappel : interactie tussen beschikbaar substraat en KGZ activiteit, gereguleerd op allel niveau]

Besides the role of starch as the primary source of calories in both human and animal diet, it is used as raw material for industrial application such as the paper industry, textile industry, chemical industry, and pharmaceutical industry. Starch isolated for industrial applications, is mainly derived from corn. However in Europe a significant proportion of starch is isolated from potato tubers. Starch consists of two glucose polymers, amylose, which is essentially linear and amylopectin, which is highly branched. Industrially important properties, which are responsible for the functional quality of starch, are highly affected by the ratio of amylose to amylopectin. Therefore, different strategies have been applied to obtain starch with altered amylose content. Mutations leading to the selective loss of amylose have been described in many species, including potato. From the synthesis of amylose-free starch in mutants lacking granule-bound starch synthase I (GBSSI) it is clear that GBSSI is responsible for amylose biosynthesis.The goal of this thesis research was to unravel the amylose biosynthesis in potato and to determine the role and regulation of GBSSI in this process. Therefore amylose biosynthesis was studied <em>in vitro</em> and <em>in vivo</em> . GBSSI, like other starch synthases, catalyses the transfer of a glucose residue from ADP-Glc (donor substrate) to the growingα-1,4 linked glucan (acceptor substrate). This reaction can be studied <em>in vitro</em> by the incubation of purified starch granules, containing the active GBSSI, with radiolabelled ADP-Glc. In this thesis we show a new mechanism for amylose biosynthesis, using amylopectin as acceptor substrate. Transfer of chains from amylopectin to amylose was evidenced from pulse-chase experiments performed with starch isolated from <em>Chlamydomonas</em> and higher plants.To alter the amylose content within the plant, different strategies can be applied. Subtle changes in amylose content, can be obtained by classical breeding. These relatively small changes may significantly improve quality as was shown by the effect of GBSSI on amylose content and noodle quality in wheat. The genetics behind amylose content in potato is described in this thesis. Amylose-free potato starch can be obtained by suppression of GBSSI using antisense technology. The effect of allelic composition on antisense inhibition was shown after analysing a large number of transgenics per variety.The allelic composition of a variety can therefore be used as a selection criterion in breeding programs with the aim to obtain cultivars with altered starch composition.