Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) is a CO2 acquisition, CO2 transient storage and CO2 concentrating mechanism based on organic acid synthesis. CO2 acquisition occurs in the dark. CO2 is fixed and stored in the form of organic acids. Remobilization of CO2 from the organic acids occurs in the light and leads to massive CO2 concentration inside photosynthesizing plant organs (green leaves or stems, in some cases also aerial roots of epiphytes). No particular biochemical reactions or enzymes participate in CAM. Although special isoforms of enzymesmay be involved, basically all reactions have wellknown housekeeping functions in all green plants. It is the special linkage of metabolic elements that makes up the CAM network (Figure 1; Cushman and Bohnert, 1997; Lüttge, 1998). The key enzyme of CO2 dark fixation is phosphoenolpyruvate carboxylase (PEPC, (1) in Figure 1) in the cytosol. It forms oxaloacetate, which is then reduced to malate. This is the only, or at least the dominating, form of nocturnalCO2 storage inmostCAMplants. In someCAM plants, however, citrate may also play a considerable role additionally or alternatively to malate. It can be formed in the mitochondria. Protons are generated together with the organic acid anions. To maintain cytosolic pH homeostasis, these are pumped into the cell sap vacuole by tonoplast H pumps, mainly the vacuolar H ATPase (V-ATPase, (2) in Figure 1). Proton pumping energizes vacuolar organic acid accumulation as it establishes an H electrochemical gradient (Dmd) driving organic acid anions across the tonoplast, the vacuolar membrane. Malate must be removed from the cytosol because it effects a feedback inhibition of its own synthesis via PEPC. The central aqueous cell sap vacuole is a suitable storage compartment occupying often 98% or even more of the volume of cells in succulent CAM tissues (leaf or stem succulence). Nocturnally accumulated organic acids typically amount to 100–300mmol L 1 malate but may be as high as 1.4mol L 1 titratable proton equivalents in species ofClusia, which accumulate bothmalate and citrate with a stoichiometry of 2 and 3 titratable protons, respectively, per malate and citrate (Lüttge, 1998).