Tree stem phosphoenolpyruvate carboxylase (PEPC): lack of biochemical and localization evidence for a C4-like photosynthesis system

Here, the kinetic properties and immunolocalization of phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in young stems of Fagus sylvatica were investigated. The aim of the study was to test the hypothesis that there is a C4-like photosynthesis system in the stems of this C3 tree species. The activity, optimal pH and L-malate sensitivity of PEPC, and the Michaelis-Menten constant (Km) for phosphoenolpyruvate (PEP), were measured in protein extracts from current-year stems and leaves. A gel blot experiment and immunolocalization studies were performed to examine the isozyme complexity of PEPC and the tissue distribution of PEPC and Rubisco in stems. Leaf and stem PEPCs exhibited similar, classical values characteristic of C3 PEPCs, with an optimal pH of c. 7.8, a Km for PEP of c. 0.3 mM and a IC50 for L-malate (the L-malate concentration that inhibits 50% of PEPC activity at the Km for PEP) of c. 0.1 mM. Western blot analysis showed the presence of two PEPC subunits (molecular mass c. 110 kDa) both in leaves and in stems. Immunogold labelling did not reveal any differential localization of PEPC and Rubisco, neither between nor inside cells. This study suggests that C4-type photosynthesis does not occur in stems of F. sylvatica and underlines the importance of PEPC in nonphotosynthetic carbon fixation by most stem tissues (fixation of respired CO2 and fixation via the anaplerotic pathway).