EXPRESSION OF ESCHERICHIA-COLI GLYCOGEN-SYNTHASE IN THE TUBERS OF TRANSGENIC POTATOES (SOLANUM-TUBEROSUM) RESULTS IN A HIGHLY BRANCHED STARCH

A chimeric gene containing the patatin promoter and the transit-peptid e region of the small-subunit carboxylase gene was utilized to direct expression of Escherichia coli glycogen synthase (glgA) to potato (Sol anum tuberosum) tuber amyloplasts. Expression of the glgA gene product 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 specif ic gravity, a 30 to 50% reduction in the percentage of starch, and a d ecreased amylose/amylopectin ratio. Total soluble sugar content in the se selected lines was increased by approximately 80%. Analysis of the starch from these potato lines also indicated a reduced phosphorous co ntent. A very high degree of branching of the amylopectin fraction was detected by comparison of high and low molecular weight carbohydrate chains after debranching with isoamylase and corresponding high-perfor mance liquid chromatography analysis of the products. Brabender viscoa mylograph analysis and differential scanning calorimetry of the starch es obtained from these transgenic potato lines also indicate a composi tion and structure much different from typical potato starch. Brabende r analysis yielded very low stable paste viscosity values (about 30% o f control values), whereas differential scanning calorimetry values in dicated reduced enthalpy and gelatinization properties. The above para meters indicate a novel potato starch based on expression of the glgA E. coli gene product in transgenic potato.