Overexpression of phosphoenolpyruvate carboxylase from Corynebacterium glutamicum lowers the CO2 compensation point (*) and enhances dark and light respiration in transgenic potato

The effect of decreased or increasedphosphoenolpyruvate carboxylase (PEPC) activity on theCO2 compensation point, respiration in the light or dark as well as thepartitioning of carbon into starch, soluble sugars, organic acids, andamino acids was investigated using transgenic potato plants. EngineeredPEPC activity ranged form 0.5-fold wild-type level in antisense plants to5-fold wild-type levels in lines overexpressing theccpc gene of Corynebacteriumglutamicum encoding for a PEPC not modulated by proteinphosphorylation. The CO2 compensation point determined according to Brooksand Farquhar (1985) was lower in PEPC overexpressors (32 l l^-1 CO2) compared to controlpotato lines (38 l l^-1CO2), but was increased in antisense PEPC plants (42 l^-1 CO2). 3-fold overexpression of PEPC gave aminimum CO2 compensation point of 32 l l^-1CO2. Increased PEPC activity resulted in enhanced respiration in the lightand dark. Altered PEPC activity had no effect on the pattern of¹⁴CO2 incorporation into leaf discs in the light.¹⁴C pulse-chase experiments in the dark,demonstrated that substantially more total label was lost in the leaf discsfrom PEPC overexpressors. Metabolite levels were determined in 21 PEPCoverexpressing lines after 8 h in the light. A 5-fold increase in PEPC overthe wild-type increased malate (61%), starch (75%) and significantlyincreased sucrose contents 9150%). Total amino acid contents were onlymarginally increased. From gas exchange characteristics and labelingexperiments it was concluded that PEP carboxylation, followed by anincreased rate of respiratory CO2 release, might work as aHCO3^-/CO2 pump. This might result in elevated CO2/O2ratios in the mesophyll, concomitant with a more favouredcarboxylation/oxygenation ratio of ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco).