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 ch anges from C3-photosynthesis to Crassulacean acid metabolism (CAM). Th e youngest leaves showed no diurnal fluctuation of organic acids or P- enolpyruvate carboxylase (PEPc) activity. There was little evidence fo r PEPc protein using PEPc antibodies prepared from the CAM form of PEP c, 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 in crease in titratable acidity, PEPc activity, PEPc protein, and PEPc mR NA. In leaf whorls 1 through 6, there was a significant linear correla tion between the diurnal fluctuation of organic acids and PEPc activit y indicating a functional relationship. The specific activity of PEPc increased as leaves matured and the K(m) (PEP) decreased indicating th at the enzyme was becoming more active. The ratio of PEPc protein to P EPc mRNA decreased as leaves matured. During the expression of CAM, th e 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 a ctivity, protein, and mRNA increased as the thickness of the spongy me sophyll increased. During the expression of CAM in Peperomia camptotri cha, there appears to be coordinated expression of PEPc mRNA, protein, and activity, the commencement of diurnal organic acid fluctuation, a nd the development of the CAM-like spongy mesophyll. Thus the evidence suggests that CAM in this species is expressed during normal developm ent and not in response to environmental signals.