Yeast Halotolerance Genes: Crucial Ion Transport and Metabolic Reactions in Salt Tolerance

Abstract

The improvement of salt tolerance in crop plants is an urgent challenge for plant biotechnology. The technology for transfering genes to plants is readily available (Potrykus, 1991), but the major problem is the isolation of genes with the capability to improve salt tolerance, which I have called halotolerance genes (Serrano and Gaxiola, 1993). These genes could correspond to either stress — sensitive systems or to tolerance responses involving either ion transport or osmolyte synthesis (figure 1). Salt stress is a very complicated phenomenon, involving both ionic toxicity and osmotic stress. The cytoplasmic loading of Cl^- and Na⁺ may be toxic to many cellular systems. On the other hand, water loss from the cells induced by external salt reduces turgo and may also affect cellular systems sensitive to water activity. Any of these forms of the stress trigger a signal transduction pathway which results in some kind of a daptation or tolerance. Both ion transport phenomena and synthesis of organic osmolytes are involv and regulation can occur by either changes in enzyme activy or gene regulation.