Proline Accumulation in Halophytes
The phenomenon of accumulation of “compatible solutes” in plants has turned out to be an interesting subject in stress physiology. The synthesis of proline occurs in a great number of halophytes, but its regulation is unknown up to now. The influence of NaCl on pyrroline-5-carboxylate reductase, the enzyme synthesizing proline can be regarded as one possibility of regulation. The activity of p-5-c reductase in salt-treated or salt-shocked plants is considerably higher than in non-treated plants, which can explain the increased proline concentration after NaCl treatment. The different in vitro NaCl inhibition of p-5-c reductase from NaCl- treated and non-treated plants points to an alteration of the enzyme during salt adaptation.
Leaves of Mesembryanthemum nodiflorum submerged in solutions of different osmotic potential proved to be suitable plant material for the study of osmotic adaptation. After transfer into NaCl solutions these leaves lose water immediately. But corresponding with proline synthesis and the uptake of NaCl (correlation coefficient r = 0.92 - 0.98) the water content of the cells starts to increase within a short time. After 15 h (400 mM NaCl) the initial loss of water is compensated. During the next hours the water content and the turgor of the cells increase furthermore until they contain up to 60% more water than the original leaves. In sucrose-stressed leaves no restoration of initial water content occurs. This demonstrates that halophyte cells under salt stress only are able to adjust their osmotic potential to the surrounding solution by uptake of NaCl and accumulation of proline. Under sucrose stress no rapid rise in vacuole osmotic potential is possible, although proline (in the cytoplasm) is synthesized in a high degree.
Tissue suspension cultures of different halophytes were tested for the purpose of their applicability to salt-tolerance studies as a undifferentiated homogeneous plant material. First results show that isolated cells — enclosed by a cell wall — behave like normal cells of halophyte tissue: plasmolysis occurring after salt stress is overcome within a short time, and growth (measured as production of protein and of cell wall material) increases with advancing salt tolerance of the parent plants and with increasing salt concentrations of the growing solution. Proline synthesis is highest in the most salt-tolerant cells and increases with salt concentration. The highest proline/protein ratio occurs at 400 mM NaCl. Up to now there seems to exist no difference between whole plant cells and isolated cells of halophytes in suspension culture concerning their salt tolerance and proline synthesis.