In the facultative halophyte Mesembryanthemum crystallinum (common ice plant), irrigation with solutions containing NaCI induces an altemate mode of carbon dioxide fixation, Crassulacean acid metabolism (CAM). The salt stress protocol which we have established facilitates the study of CAM induction and the correlation of changes in metabolism and gene expression. We have studied the time course of mRNA induction for phosphoenolpyruvate carboxylase (PEPCase) (gene: ppc) and several other enzymes of carbon metabolism during stress. While CAM is not fully established for at least 10 days after the start of stress, mRNA amounts for PEPCase and for other CAM enzymes, such as Pyruvate orthophosphate dikinase, increase between day 2 and 3 after stress induction. Increases continue for at least 5 days. Concomitant with the increase of CAM transcripts, fluctuations in the mRNA amounts for genes rbcS and cab were observed. Transcript levels for these proteins decreased severalfold during a 3 to 4 day period.Plants are subjected to a number of environmental stimuli, including stresses imposed either by lack of water or by increases in electrolyte concentration. Both drought and salt stress are probably perceived similarly. The effects of both stresses have been studied mainly at physiological levels and only in part biochemically. Genetic traits which contribute to salt tolerance are not yet sufficiently understood. As salt tolerance appears to be a multigenic trait only the simultaneous change of several or many characters would produce desired phenotypes. Up to now, however, it has been impossible to discern responses that are at the basis of the tolerance from the plethora of accompanying adaptations in metabolism. In our attempts to learn more about the basic regulatory mechanisms determining the phenotype 'salt tolerant,' we are studying characteristics which distinguish halophytic (i.e. salt tolerant) plants from glycophytic (salt sensitive) plants.
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