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 increased phosphoenolpyruvate carboxylase (PEPC) activity on the CO2 compensation point, respiration in the light or dark as well as the partitioning of carbon into starch, soluble sugars, organic acids, and amino acids was investigated using transgenic potato plants. Engineered PEPC activity ranged from 0.5-fold wild-type level in antisense plants to 5-fold wild-type levels in lines overexpressing the cppc gene of Corynebacterium glutamicum encoding for a PEPC not modulated by protein phosphorylation. The CO2 compensation point determined according to Brooks and Farquhar (1985) was lower in PEPC overexpressors (32 µl l−1 CO2) compared to control potato lines (38 µl l−1 CO2), but was increased in antisense PEPC plants (42 µl l−1 CO2). 3-fold overexpression of PEPC gave a minimum CO2 compensation point of 32 µl l−1 CO2. Increased PEPC activity resulted in enhanced respiration in the light and dark. Altered PEPC activity had no effect on the pattern of 14CO2 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 discs from PEPC overexpressors. Metabolite levels were determined in 21 PEPC overexpressing lines after 8 h in the light. A 5-fold increase in PEPC over the wild-type increased malate (61%), starch (75%) and significantly increased sucrose contents (150%). Total amino acid contents were only marginally increased. From gas exchange characteristics and labelling experiments it was concluded that PEP carboxylation, followed by an increased rate of respiratory CO2 release, might work as a HCO−3/CO2 pump. This might result in elevated CO2/O2 ratios in the mesophyll, concomitant with a more favoured carboxylation/oxygenation ratio of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).