Review article. Molecular biology of salt tolerance in the context of whole-plant physiology

The halobacteria are the only organisms that are tolerant of salinity atthe molecular level. All other bacteria, all fungi, all plants, and allanimals avoid the need for salt tolerance for most of their macromoleculesby maintaining defined and conserved conditions in the cytoplasm. Theseconditions favour potassium over sodium, the limitation of total inorganicion activity, and the supplementation of this where necessary with organicsolutes which are metabolically neutral osmolytes that may also beosmoprotectant. The salt tolerance of an organism depends upon the range ofexternal salinity over which it is able to sustain these conditions in thecytoplasm. There is substantial and increasing knowledge of the molecularbiology and molecular genetics of the processes of ion and organic solutetransport, solute synthesis, and compartmentation that underpin cell-basedtolerance. Much of recent research focuses on the identification of genesand gene products that affect cell-based tolerance, commonly derived fromsingle-cell models. There is commonly the implicit or explicit assumptionthat incorporation of these genes will benefit the salt tolerance of foodcrop species. While this essential experimental approach is giving enormousinsight there should not be rash or premature expectations. The unique andoverriding consideration for the salinity tolerance of terrestrial plantsis the net flux of water due to transpiration and so resides at a higherlevel of organization. Processes that are advantageous to a single cell inan aqueous medium may be lethal to a cell in a leaf in the air. The likelyimpact of single structural-gene changes in ion and solute transport uponco-ordinated plant response is probably over-estimated, and recent viewsconsider regulatory processes and multiple gene transfers. While thetechnical ability for plant transformation increases daily, thepracticality of using transgenic plants in complex breeding programmesseems rarely to be given enough thought. If intervention at the molecularlevel is to lead to salt-tolerant crop plants than it will be essential toview this in the contexts of whole plants and of plant breeding. Recentindications that a relatively small number of quantitative trait loci (QTL)may govern complex physiological characters offer the most hope for thefuture.