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Abstract
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 studiedin vitroandin vivo. GBSSI, like other starch synthases, catalyses the transfer of a glucose residue from ADP-Glc (donor substrate) to the growing a-1,4 linked glucan (acceptor substrate). This reaction can be studiedin vitro(Chapter 2 and 3) 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 fromChlamydomonas wild typestrain 137C and mutant strain 17 (defective for the large subunit of AGPase) (Chapter 2). Starch isolated from 17 contained very little to no amylose, despite the presence of wild type GBSSI.
