PLANT PHYSIOLOGY , Vol 104, Issue 4 1159-1166, Copyright © 1994 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Expression of Escherichia coli Glycogen Synthase in the Tubers of Transgenic Potatoes (Solanum tuberosum) Results in a Highly Branched Starch
C. K. Shewmaker, C. D. Boyer, D. P. Wiesenborn, D. B. Thompson, M. R. Boersig, J. V. Oakes and D. M. Stalker
Calgene, Inc., 1920 Fifth Street, Davis, California 95616
A chimeric gene containing the patatin promoter and the transit-peptide
region of the small-subunit carboxylase gene was utilized to direct
expression of Escherichia coli glycogen sythase (glgA) to potato (Solanum
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 specific gravity, a 30
to 50% reduction in the percentage of starch, and a decreased
amylose/amylopectin ratio. Total soluble sugar content in these selected
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 fraction 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 novel potato
starch based on expression of the glgA E. coli gene product in transgenic
potato.
