MICROBIAL MODELS AND SALT STRESS TOLERANCE IN PLANTS

Improving salt tolerance in crop plants remains an urgent issue in plant molecular biology. The adaptation of plants to NaCl involves metabolic reactions (synthesis of organic solutes) and transport phenomena (ion extrusion at the plasma membrane and vacuolar compartmentation). In addition, a plethora of salt-induced genes with a bewildering variety of suggested functions have been described. The uncertainties about the physiological roles and/or molecular bases of many of these phenomena make it difficult to select genes that could improve salt tolerance (halotolerance) in transgenic plants. We suggest that the field of salt tolerance can benefit by complementing the present phenomenological or descriptive approaches with a functional strategy directed toward isolating genes that, by overexpression of the corresponding protein, could improve salt tolerance. These halotolerance genes not only could illuminate the critical steps for salt tolerance, but also could provide the tools for improvement. Microbial genetics facilitates the implementation of this genetic approach. Studies using the prokaryotic organism Escherichia coli suggest that the synthesis of organic solutes may be the crucial step for salt tolerance because the first described bacterial halotolerance gene (proB-74) determines the overaccumulation of proline. In the eukaryotic microorganism Saccharomyces cerevisiae, however, potassium homeostasis seems to be the most critical response to salt stress. The first halotolerance gene isolated from this organism (HAL1) seems to modulate potassium transport, increasing the intracellular level of this cation in NaCl-containing media. The existence of plant homologues to HAL1 indicates that yeast may be a useful model for the genetics of salt tolerance in plants.