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

Abstract

Researchwww.newphytologist.org 775 Summary• 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 C 4 -like photosynthesis system in the stems of this C 3 tree species.• The activity, optimal pH and L-malate sensitivity of PEPC, and the MichaelisMenten constant (K m ) 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 C 3 PEPCs, with an optimal pH of c. 7.8, a K m for PEP of c. 0.3 mM and a IC 50 for L-malate (the L-malate concentration that inhibits 50% of PEPC activity at the K m 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 C 4 -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 CO 2 and fixation via the anaplerotic pathway).New Phytologist (2007) 176: [775][776][777][778][779][780][781]

Effect of soil nitrogen supply on carbon assimilation by tree stems

“…A likely stem N sink candidate is PEP carboxylase. As shown in previous studies (Berveiller and Damesin, 2008;Berveiller et al, 2007a;2007b), PEP carboxylase activity was higher in stems than in leaves, suggesting that this enzyme is involved in stem photosynthesis (Fig. 5b).…”

“…The relationship between light-saturated photosynthetic rate and N concentration is widely documented for leaves (Evans, 1989;Field and Mooney, 1986). This relationship exists between stems of various tree species (Berveiller et al, 2007a;2007b) and this study shows that it is valid at an intraspecific level. This relationship is generally linear, but in our study, the mass-based and area-based relationships were found to be hyperbolic.…”

“…According to Berveiller et al (2007b), PEP carboxylase activity was measured at pH 7.8 in a buffer containing 100 mM HEPES, 10 mM MgCl 2 , 5 mM NaHCO 3 , 0.2 mM NADH, and 3 U mL −1 malate dehydrogenase. This assay was initiated by the addition of 5 mM phosphoenolpyruvate (Tietz and Wild, 1991;Uedan and Sugiyama, 1976).…”

“…67 (2010) do not play a major role in stem carbon exchange (Wittmann et al, 2006). Although of a lower magnitude than in leaves, some ecophysiological characteristics of stem photosynthesis are similar to those observed in leaves, such as the positive response of gas exchange to light and temperature (Wittmann and Pfanz, 2007;Wittmann et al, 2001), the positive response of electron transport rate to light (Manetas, 2004), and the presence of Rubisco (Berveiller and Damesin, 2008;Berveiller et al, 2007b;Schaedle and Brayman, 1986). Over a whole season, current-year stems of European beech (Fagus sylvatica) are able to assimilate the equivalent of 40% of the C lost by respiration (Damesin, 2003).…”

“…Phosphoenolpyruvate (PEP) carboxylase is an important cross point between the two metabolisms as it is involved in supplying oxaloacetate to the Krebs cycle, thus ensuring a continuous replenishment of C 4 -dicarboxylic acid, which is necessary for nitrogen assimilation and amino acid biosynthesis (Huppe and Turpin, 1994). We have recently shown a very high activity of PEP carboxylase in young stems in comparison to leaves of adult European beeches (Berveiller and Damesin, 2008;Berveiller et al, 2007b). Stem photosynthesis is positively correlated to nitrogen concentration in an interspecific study (Berveiller et al, 2007a).…”