We have earlier discussed the sos pathway under the aspect of controlling sodiumhomeostasis, but detection of the pathway provides an example of another kind because sos genes were first found in a salt-sensitive, glycophytic species. Indeed, it has become clear that the cells of virtually all plants possess the capability to sense and respond to a saline environment. Tolerance is, to a species specific and widely varying degree, genetically possible (Serrano and Rodriguez-Navarro, 2002). Although the salt tolerance of many halophytes is constitutive, the tolerance of others is induced by the salinity level of the environment (facultative halophytes) but functions that determine halophytism are ubiquitous. From many physiological and biochemical studies, it has also become clear that the plant adaptive response to salinity involves four basic co-coordinated adjustments, outlined in Fig. 12.3: ion homeostasis, osmotic compensation (water homeostasis), injury repair or avoidance, and growth reduction (Zhu, 2001). Yet, the mechanisms and precise genetic components involved in adaptation to salinity have remained a mystery for long. Our understanding of how plants perceive the salinity of their environment and adjust appropriately has improved tremendously with the introduction and use of Arabidopsis as a model system. Although Arabidopsis is not salt tolerant in the sense of halophytic tolerance, screening for mutants with lower tolerance than the wild type in Arabidopsis in the mid 1990s has been successful and much work has confirmed that Arabidopsis does have genetic components that control the ability to survive and grow in a salinized environment (Ishitani et al., 1997; Warren et al., 1996; Werner and Finkelstein, 1995; Xiong et al., 2002b). Our present understanding of plant signal transduction for adaptation to salinity has become possible based on common ancestry of land plants. The approach has been successful because the advantages of well-developed genetic and molecular tools developed for Arabidopsis could be exploited.
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