Changes in Gene Expression Elicited by Salt Stress in Mesembryanthemum crystallinum

Abstract

Plants are subjected to a variety of environmental stresses such as extreme temperatures, changes in water potential, anaerobiosis, water pollutants or heavy-metal ions. With regard to water or salt stress, a great deal of physiological data has been reported which describes specific responses of plants to stresses induced by drought or salinity. Many plants accumulate solutes such as inorganic ions, soluble carbohydrates, amino acids, or quaternary ammonium compounds. So far, it has been difficult to develop a model which describes the activation and integration of different mechanisms utilized by higher plants during water stress. In fact, such a general response to this type of stress might not exist. Part of the problem in identifying and testing specific stress responses for adaptive value has been the lack of a well characterized model system. We describe here results from our work with the desert plant Mesembryanthemum crystallinum (iceplant). In this plant, water or salt stress induces a shift from C3 photosynthesis to Crassulacean acid metabolism (CAM). The plants survive rapid and extreme changes in soil salinity, apparently by utilizing both immediate and delayed responses to salt stress. While proline begins to accumulate within hours following irrigation with 500 mM NaCl, the CAM pathway is induced over a period of 7 to 10 days. Although the switch in carbon assimilation pathways is clearly a response to salt stress, CAM induction is probably not the primary mechanism confering salt tolerance in this species. cDNA clones coding for CAM enzymes should, however, provide excellent probes for investigating the molecular basis of stress regulated gene expression.