Many genes whose expression is increased in response to drought or salt stresses have been cloned and characterized (Plant et al. 1991; Torres-Schumann et al. 1992; Chen et al. 1993,1994; Fray et al. 1994; Yu et al. 1996; Trevino and O'Connell 1998; Harrak et al. 2001; Tirajoh et al. 2005; Yesbergenova et al. 2005). Transgenic expression of these genes has been used for two different objectives, to determine the function of the protein or gene product in the stress response; or to attempt to confer stress tolerance on the transgenic plant (e.g., Imai et al. 1995; Cortina and Culianez-Macia 2005; Deguchi et al. 2006). All cases of drought and salt tolerances in wild tomato species are inherited as quantitative traits and were never simplyinher-ited; transgenic plants engineered to express a single gene do not seem likely to result in a robust stress tolerant phenotype. A thorough review of this topic has been presented (Flowers 2004). Genetic engineering of expression of stress responsive genes is still an important approach to determining the biochemical and physiological function of these genes in plant responses to the environment. One example of this type of analysis was on a unique class of chromatin proteins, H1 histones.
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