Transgenic Potatoes (So/anum tuberosum) Results in a Highly Branched Starch

A chimeric gene containing the patatin promoter and the transitpeptide region of the small-subunit carboxylase gene was utilized to direct expression of Escherichia coli glycogen synthase (glgA) to potato (Solanum tuberosum) tuber amyloplasts. Expression of the glgA gene produd in tuber amyloplasts was between 0.007 and 0.028% of total protein in independent potato lines as determined by immunoblot analysis. Tubers from four transgenic potato lines were found to have a lowered specific gravity, a 30 to 50% reduction in the percentage of starch, and a decreased amylose/ amylopedin ratio. Total soluble sugar content in these seleded lines was increased by approximately 80%. Analysis of the starch from these potato lines also indicated a reduced phosphorous content. A very high degree of branching of the amylopectin fradion was detected by comparison of high and low molecular weight carbohydrate chains after debranching with isoamylase and corresponding high-performance liquid chromatography analysis of the products. Brabender viscoamylograph analysis and differential scanning calorimetry of the starches obtained from these transgenic potato lines also indicate a composition and structure much different from typical potato starch. Brabender analysis yielded very low stable paste viscosity values (about 30% of control values), whereas differential scanning calorimetry values indicated reduced enthalpy and gelatinization properties. The above parameters indicate a nove1 potato starch based on expression of the glgA E. coli gene produd in transgenic potato.