Changes in Properties of Phosphoenolpyruvate Carboxylase with Induction of Crassulacean Acid Metabolism (CAM) in the C<sub>4</sub> Plant Portulaca Oleracea

Abstract

Aiming at understanding the odd case of CAM expression by a C4 plant, some properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31, orthophosphate: oxaloacetate carboxylyase, phosphorylating) were comparatively studied in leaves of CAM-expressing and non-expressing Portulaca oleracea L. plants. CAM expression was induced by growing plants under an 8-h photoperiod and under water-stress. CAM induction in leaves of these plants (designated as CAM) is indicated by the nocturnal acidification and by the clear diurnal oscillation pattern and amplitude of acidity, malic acid, and PEPC activity characteristic of CAM plants. Treatment of the other plant group (designated as C4) by growth under a 16-h photoperiod and well-watered conditions did not induce expression of the tested criteria of CAM in plants. In these C4 plants, the mentioned CAM criteria were undetectable. PEPC from CAM and C4Portulaca responded differently to any of the studied assay conditions or effectors. For example, extent and timing of sensitivity of PEPC to pH change, inhibition by malate, activation by glucose-6-phosphate or inorganic phosphate, and the enzyme affinity to the substrate PEP were reversed with induction of CAM from the C4-P. oleracea. These contrasting responses indicate distinct kinetic and regulatory properties of PEPC of the two modes. Thus by shifting to CAM in the C4Portulaca a new PEPC isoform may be synthesised to meet CAM requirements. Simultaneous occurrence of both C4 and CAM is suggested in P. oleracea when challenged with growth under stress.