Development of Transgenic Potatoes to Attain Novel Starch Qualities

Sammanfattning: Starch is a plant-derived polysaccharide with many uses in different food and non-food applications. In this thesis, the production of two novel potato starch qualities, amylopectin and high-amylose, and the modifications and the characteristics of the produced plants and starches are described. The novel potato starch qualities will find uses in e.g. the paper, adhesive, textile and packing industries. Amylopectin potato lines were produced by antisense inhibition of a single gene, coding for a granule-bound starch synthase (GBSS). One amylopectin line, EH92-527-1, is currently in an approval phase in the form of a European market notification. EH92-527-1 has been thoroughly characterised and molecular and chemical data for the line are presented. During the characterisation it was found that the antisense gene had been rearranged upon insertion, creating a truncated asymmetric inverted repeat. The inverted repeat is probably what is yielding the very efficient GBSS inhibition. The line was also shown to have an increased vitamin C level compared with the parental variety, probably due to an alteration in the sugar metabolism in the line. One issue often debated concerning genetically modified plants is the use of a selection gene transformed together with the trait gene/genes to be able to select for transgenic lines. A neomycin phosphotransferase gene (nptII), coding for a protein giving the plant resistance to the antibiotic kanamycin, is almost exclusively used as selection system during potato transformation. A non-antibiotic selection system based on a mutated acetohydroxyacid synthase (AHAS) and herbicides of the imidazolinone family was developed, yielding a higher transformation rate than when nptII is used. In contrast to amylopectin potatoes, the expression of two genes, coding for starch branching enzymes (SBE), had to be down-regulated to produce the required high-amylose potato starch. The starch had up to 7 times lower branching degree and a major increase in the amount of bound phosphate compared with the parental line. High-amylose lines were first produced by antisense inhibition of the two sbe genes but the frequency of lines with high-amylose starch quality was very low. To improve this, an RNA interference (RNAi) method was developed, which yielded a 10-fold higher frequency of inhibited lines. Another major advantage found with the RNAi method was a reduction in the number of T-DNA inserts compared with the previously used antisense technique. One disadvantage of the high-amylose lines is a reduction in starch yield. To increase the starch content in the high-amylose lines two novel genes, coding for Solanum tuberosum glycogenin homologue proteins (StGH) with a possible starch initiation function, were over-expressed in potato.