Starch biosynthesis from triose-phosphate in transgenic potato tubers expressing plastidic fructose-1,6-bisphosphatase.
A full length cDNA clone encoding plastidic fructose-1,6-bisphosphatase (cp-FBPase), together with a transit peptide, was isolated from a potato (Solanum tuberosum L.) leaf cDNA library. Potato plants were transformed with the isolated cp-FBPase sequence behind a patatin class I promoter to ensure tuber-specific expression of the enzyme. Plant lines were selected which expressed up to 250 mU (g FW)-1 in the developing tubers, which is 10- to 20-fold the activity found in wild-type tubers. Intact amyloplasts were isolated from in vitro-grown minitubers developed in darkness. Comparison with marker enzymes showed that cp-FBPase activity in transgenic tubers, as well as the low FBPase activity in the wild-type tubers, was localised inside the amyloplasts. The intact amyloplasts isolated from both wild-type and transgenic tubers synthesised starch from [U-14C] glucose-6-phosphate. Conversely, only the transgenic tubers expressing cp-FBPase showed appreciable synthesis of starch from [U-14C] dihydroxyacetone phosphate, and this synthesis rate was correlated to the activity of cp-FBPase. Thus, the expression of cp-FBPase in tubers allows for a new route of starch biosynthesis from triose-phosphates imported from the cytosol. The transgenic tubers did not differ from wild-type tubers with respect to starch content, or the levels of neutral sugars and phosphorylated hexoses.
Thorbjørnsen
Tine
T
Plant Biochemistry Laboratory, Department of Plant Biology, Royal Veterinary and Agricultural University, Frederiksberg C, Denmark.
Asp
Torben
T
Jørgensen
Kirsten
K
Nielsen
Tom Hamborg
TH
D002241:Carbohydrates / Q000096:biosynthesis; D002244:Carbon / Q000378:metabolism; D002250:Carbon Radioisotopes; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics / Q000302:isolation & purification; D004099:Dihydroxyacetone Phosphate / Q000378:metabolism*; D006597:Fructose-Bisphosphatase / Q000235:genetics / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D019298:Glucose-6-Phosphate / Q000378:metabolism; D018515:Plant Leaves / Q000235:genetics / Q000378:metabolism; D018547:Plant Stems / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D018087:Plastids / Q000201:enzymology; D011198:Solanum tuberosum / Q000235:genetics / Q000378:metabolism*; D013213:Starch / Q000096:biosynthesis*
D002241:Carbohydrates; D002250:Carbon Radioisotopes; D018076:DNA, Complementary; D019298:Glucose-6-Phosphate; D004099:Dihydroxyacetone Phosphate; D002244:Carbon; D013213:Starch; D006597:Fructose-Bisphosphatase