Abstract
BSA, bovine serum albumin CAM, Crassulacean acid metabolism DTT, dithiothreitol EDTA, ethylenediaminetetraacetic acid FPLCfast protein liquid chromatography HEPES, N‐(2‐hydroxyethyl)piperazine‐Ń‐(ethanesulphonic acid) ME, β‐mercaptoethanol NBT, nitro blue tetrazolium PAGE, polyacrylamide gel electrophoresis SDS, sodium dodecyl sulphate SDS‐PAGE, sodium dodecyl sulphate polyacrylamide gel electrophoresis Rubisco, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (EC 4.1.1.39) SOD, superoxide dismutase (EC 1.15.1.1) TEMED, N,N,Ń,Ń‐tetramethylethylenediamine Tris, Tris (hydroxymethyl) aminomethane Tricine, N‐Tris(hydroxymethyl)methylglycine Treatment of Mesembryanthemum crystallinum for several days with 0·4 kmol m–3 NaCl in the root medium, in parallel to an increase of the cell sap osmolarity enhances activity of important antioxidative enzymes, such as superoxide dismutases (SODs). M. crystallinum is equipped with three SOD isoforms. These isoforms were identified as Mn‐, Fe‐, and Cu/Zn‐SODs, respectively. Mn‐SOD was found in the mitochondrial fraction, Fe‐SOD in the chloroplast fraction, and Cu/Zn‐SOD is probably localized in the cytosol. The Fe‐SOD found in M. crystallinum is the first iron‐containing SOD enzyme to be characterized in the plant family Aizoaceae. Salt treatment increases the activity of this isoform earlier than the other SODs. Molecular masses of SOD isoforms were determined as 82, 48 and 34 kDa for Mn‐, Fe‐, Cu/Zn‐SODs, respectively. Native Mn‐SOD seems to be a tetramer, while Fe‐SOD and Cu/Zn‐SOD are dimers. All SOD isoforms show high thermal stability. Mn‐SOD is active even after short heating at 90 °C and Fe‐SOD at 70 °C. Moreover, high concentrations of β‐mercaptoethanol used as a reducing agent did not destroy the function of all isoforms. With the salinity treatment in M. crystallinum, Crassulacean acid metabolism (CAM) is induced. Build‐up of large stationary O2 concentrations in the leaf air spaces is associated with the photosynthetic CO2 reduction behind closed stomata in phase III of CAM. This illustrates why M. crystallinum may require higher antioxidative activities under NaCl stress and also explains earlier findings that CAM plants are more resistant than C3 plants to environmental stress as imposed by, for example, SO2 and O3.
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