Dynamics of tonoplast proton pumps and other tonoplast proteins of Mesembryanthemum crystallinum L. during the induction of Crassulacean acid metabolism

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Abstract
In plants of Mesembryanthemum crystallinum the activities of the two proton pumps on the tonoplast, i.e. the ATPase and the pyrophosphatase, and the gelelectrophoretic pattern of the total tonoplast proteins were analyzed during the transition of the metabolic state from C3 photosynthesis to Crassulacean acid metabolism (CAM). In one series, CAM was induced by watering the plants with NaCl. In another series, the change of the metabolic state to CAM was a consequence of the aging of the plants. No significant differences in the specific activities of ATP hydrolysis were found in plants performing C3 photosynthesis and CAM, respectively. However, with both series the protein content of tonoplast preparations and, in parallel, the total ATP hydrolytic activity of the tonoplast ATPase were higher after the change to CAM. In contrast, the specific activity of pyrophosphate hydrolysis was maximum in the preparations of young plants and diminished after the induction of CAM in both series. Therefore the tonoplast ATPase seems to be the main enzyme responsible for the energization of malate accumulation in CAM. The tonoplast pyrophosphatase is important in the early stages of plant growth and plays a minor role in CAM. With M. crystallinum the change from C3 photosynthesis to CAM is accompanied by de-novo synthesis of tonoplast proteins. Several polypeptides with relative molecular masses (Mrs) of 55, 41, and 36 kDa were clearly more pronounced in the gel-electrophoretic pattern of the total tonoplast protein after CAM induction. These changes were independent of the CAM-inducing salt treatment or aging. Moreover, two subunits of the tonoplast ATPase with Mrs of about 27 and 31 kDa showed particularly high intensities only in the CAM state. It is assumed that the subunit composition of the tonoplast ATPase differs in the two metabolic states and that the two subunits induced modify the regulation of the ATPase in CAM. In addition, the reaction of the plants to the NaCl treatment per se was the induction at the tonoplast of a polypeptide with an Mr of 24 kDa.