The last century has seen enormous gains in plant productivity and in resistance to a variety of pests and diseases through much innovative plant breeding and more recently molecular engineering to prevent plant damage by insects. In contrast, improvements to salt and drought tolerance in crop and ornamental plants has been elusive, partially because they are quantitative traits and part of the multigenic responses detectable under salt/drought stress conditions. However, the rapidly expanding base of information on molecular strategies in plant adaptation to stress is likely to improve experimental strategies to achieve improved tolerance. Recently studies of salinity tolerance in crop plants have ranged from genetic mapping to molecular characterization of salt/drought induced gene products. With our increasing understanding of biochemical pathways and mechanisms that participate in plant stress responses it has also become apparent that many of these responses are common protective mechanisms that can be activated by salt, drought and cold, albeit sometimes through different signalling pathways. This review focuses on recent progress in molecular engineering to improve salt tolerance in plants in context of our current knowledge of metabolic changes elicited by salt/drought stress and the known plant characteristics useful for salt tolerance. While it is instructive to draw parallels between molecular mechanisms responsive to salt-stress with accumulating evidence from studies of related abiotic stress-responses, more data are needed to delineate those mechanisms specific for salt tolerance. Also discussed is the alternative genetic strategy that combines single-step selection of salt tolerant cells in culture, followed by regeneration of salt tolerant plants and identification of genes important in the acquired salt tolerance. Currently, transgenic plants have been used to test the effect af overexpression of specific prokaryotic or plant genes, known to be up-regulated by salt/drought stress. The incremental success of these experiments indicates a potentially useful role for these stress-induced genes in achieving long term tolerance. In addition, it is possible that enhanced expression of gene products that function in physiological systems especially sensitive to disruption by salt, could incrementally improve salt tolerance. Current knowledge points towards a need to reconcile our findings that many genes are induced by stress with the practical limitations of overexpressing all of them in a plant in a tissue specific manner that would maintain developmental control as needed. New approaches are being developed towards being able to manipulate expression of functionally related classes of genes by characterization of signalling pathways in salt/drought stress and characterization and cloning of transcription factors that regulate the expression of many genes that could contribute to salt/drought tolerance. Transcription factors that regulate functionally related genes could be particularly attractive targets for such investigations, since they may also function in regulating quantitative traits. Transgenic manipulation of such transcription factors should help us understand more about multigene regulation and its relationship to tolerance. (C) 1998 Annals of Botany Company.
