Induction of a Crassulacean acid like metabolism in the C4 succulent plant, Portulaca oleracea L.:: physiological and morphological changes are accompanied by specific modifications in phosphoenolpyruvate carboxylase

The induction of a Crassulacean acid like metabolism ( CAM) was evidenced after 21-23 days of drought stress in the C-4 succulent plant Portulaca oleracea L. by changes in the CO2 exchange pattern, in malic acid content and in titratable acidity during the day - night cycle. Light microscopy studies also revealed differences in the leaf structure after the drought treatment. Following the induction of the CAM-like metabolism, the regulatory properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), the enzyme responsible for the diurnal fixation of CO2 in C-4 plants but nocturnal in CAM plants, were studied. The enzyme from stressed plants showed different kinetic properties with respect to controls, notably its lack of cooperativity, higher sensitivity to L-malate inhibition, higher PEP affinity and lower enzyme content on a protein basis. In both conditions, PEPC's subunit mass was 110 kDa, although changes in the isoelectric point and electrophoretic mobility of the native enzyme were observed. In vivo phosphorylation and native isoelectrofocusing studies indicated variations in the phosphorylation status of the enzyme of samples collected during the night and day, which was clearly different for the control and stressed groups of plants. The results presented suggest that PEPC activity and regulation are modified upon drought stress treatment in a way that allows P. oleracea to perform a CAM-like metabolism.