Expression of different glucansucrases in potato tubers: implications for starch biosynthesis

This thesis describes the production of dextran (polymers with -(1→6)-linked glucosyl residues), mutan (polymers with -(1→3)-linked glucosyl residues) and alternan (polymers with alternating -(1→3)/ -(1→6)-linked glucosyl residues) in potato tubers. Modifications of the physicochemical properties of potato starch were investigated as well as the possibilities to introduce novel linkage types in starch. The most pronounced alterations at the tuber and starch levels were obtained in the mutan-accumulating potato plants. The tuber phenotype exhibited brownish regions, granule morphology was dramatically affected and the expression level of the sucrose-regulated AGPase and GBSSI genes was down-regulated. Furthermore, the starches showed more pronounced retrogradation after gelatinization, and had a 1.7-fold higher viscosity after cooling than untransformed starches. In general, introduction of novel linkage types in starch was not evidenced. More research is needed to investigate the possibilities of engineering a second generation of glucansucrases with improved capacities to introduce novel linkage types in starch