INVESTIGATION OF PHOSPHOENOLPYRUVATE CARBOXYLASE (PEPCASE) IN MESEMBRYANTHEMUM-CRYSTALLINUM L IN C3 AND CAM PHOTOSYNTHETIC STATES

When exposed to osmotic stress, Mesembryanthemum crystallinum plants switch from C3 to CAM photosynthesis. Phosphoenolpyruvate carboxylase (PEPCase) is a key enzyme in CAM plants, being responsible for the initial fixation of CO2. In C3 plants the enzyme has been shown to be involved in the replenishing of TCA cycle intermediates and in the operation of stomatal guard cells. Multiple PEPCase isoforms were observed in C3-performing leaves with four isoelectric points of 5.2,5.5, 5.6 and 5.9 and four apparent subunit molecular masses of 105, 108, 113 and 116 kDa. In some instances, subunits of different size possessed exactly the same pI. The induction of CAM led to the predominance of a new isoform of pI 6.5 with subunit molecular mass of 108 kDa, but in addition, changes were observed in some of the isoforms present in the C3 plant. PEPCase subunits were purified from the C3 and CAM forms of M. crystallinum and subjected to peptide mapping. Two distinct though similar sets of maps were obtained, one from the CAM isoform (pI 6.5) and C3-associated subunits of pI 5-9 and another for C3 subunits of pI 5.2 and 5.5. It was inferred from these data that the C3 isoforms expressed in the leaf were derived from at least two genes. The C3 isoform (pI 5.9) showing greatest similarity to the CAM isoform in terms of peptide mapping also increased in response to salt stress. It is speculated that the CAM isoform may have evolved from this enzyme.