EFFECTS OF ALTERED PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITIES ON TRANSGENIC C3 PLANT SOLANUM-TUBEROSUM

Phosphoenolpyruvate carboxylase (PEPC) genes from Corynebacterium glut amicum (eppc), Escherichia coli (eppc) or Flaveria trinervia (fppc) we re transferred to Solanum tuberosum. Plant regenerants producing forei gn PEPC were identified by Western blot analysis. Maximum PEPC activit ies measured in eppc and fppc plants grown in the greenhouse were doub led compared to control plants. For cppc a transgenic plant line could be selected which exhibited a fourfold increase in PEPC activity. In the presence of acetyl-CoA, a known activator of the procaryotic PEPC, a sixfold higher activity level was observed. In cppc plants grown in axenic culture PEPC activities were even higher. There was a 6-fold o r 12-fold increase in the PEPC activities compared to the controls mea sured in the absence or presence of acetyl-CoA, respectively. Comparab le results were obtained by transient expression in Nicotiana tabacum protoplasts. PEPC of C. glutamicum (PEPC C.g.) in S. tuberosum leaf ex tracts displays its characteristic K-m (PEP) value. Plant growth was e xamined with plants showing high expression of PEPC and, moreover, wit h a plant cell line expressing an antisense S. tuberosum (anti-sppc) g ene. In axenic culture the growth rate of a cppc plant cell line was a ppreciably diminished, whereas growth rates of an anti-sppc line were similar or slightly higher than in controls. Malate levels were increa sed in cppc plants and decreased in antisense plants. There were no si gnificant differences in photosynthetic electron transport or steady s tate CO2 assimilation between control plants and transformants overexp ressing PEPC C.g. or anti-sppc plants. However, a prolonged dark treat ment resulted in a delayed induction of photosynthetic electron transp ort in plants with less PEPC. Rates of CO2 release in the dark determi ned after a 45 min illumination period at a high proton flux density w ere considerably enhanced in cppc plants and slightly diminished in an ti-sppc plants. When CO2 assimilation rates were corrected for estimat ed rates of mitochondrial respiration in the light, the electron requi rement for CO2 assimilation determined in low CO2 was slightly lower i n transformants with higher PEPC, whereas transformants with decreased PEPC exhibited an appreciably elevated electron requirement. The CO2 compensation point remained unchanged in plants (cppc) with high PEPC activity, but might be increased in an antisense plant cell line. Stom atal opening was delayed in antisense plants, but was accelerated in p lants overexpressing PEPC C.g. compared to the controls.