Expression of P-Enolpyruvate Carboxylase and other Aspects of CAM during the Development of Peperomia camptotricha Leaves

During the development of Peperomia camptotricha leaves, metabolism changes from C₃-photosynthesis to Crassulacean acid metabolism (CAM). The youngest leaves showed no diurnal fluctuation of organic acids or P-enolpyruvate carboxylase (PEPc) activity. There was little evidence for PEPc protein using PEPc antibodies prepared from the CAM form of PEPc, nor was there evidence for PEPc mRNA when tested using a cDNA probe made from CAM P. scandens. As leaves matured, there was a parallel increase in titratable acidity, PEPc activity, PEPc protein, and PEPc mRNA. In leaf whorls 1 through 6, there was a significant linear correlation between the diurnal fluctuation of organic acids and PEPc activity indicating a functional relationship. The specific activity of PEPc increased as leaves matured and the K_m (PEP) decreased indicating that the enzyme was becoming more active. The ratio of PEPc protein to PEPc mRNA decreased as leaves matured. During the expression of CAM, the spongy mesophyll where most of the CAM activity occurs increased in thickness and per cent air space, whereas the palisade mesophyll where most of the C₃ activity occurs did not increase in size dramatically. The diurnal fluctuation of organic acids and the expression of PEPc activity, protein, and mRNA increased as the thickness of the spongy mesophyll increased. During the expression of CAM in Peperomia camptotricha, there appears to be coordinated expression of PEPc mRNA, protein, and activity, the commencement of diurnal organic acid fluctuation, and the development of the CAM-like spongy mesophyll. Thus the evidence suggests that CAM in this species is expressed during normal development and not in response to environmental signals.