Gene Expression and Signal Transduction in Water-Stress Response.

Land plants suffer from dehydration or water stress not only under drought and high-salt-concentration conditions but also under low-temperature conditions. They respond and adapt to water stress to survive these environmental stress conditions. Water stress induces various biochemical and physiological responses in plants. Under water-stress conditions plant cells lose water and decrease turgor pressure. The plant hormone ABA increases as a result of water stress, and ABA has important roles in the tolerance of plants to drought, high salinity, and cold. A number of genes that respond to drought, salt, and cold stress at the transcriptional level have recently been described (for review, see Ingram and Bartels, 1996; Shinozaki and Yamaguchi-Shinozaki, 1996; Bray, 1997). The mRNAs of water-stress-inducible genes decrease when the plants are released from stress conditions, which is consistent with evidence that shows that these genes respond to water stress or dehydration. The functions of some gene products have been predicted from sequence homology with known proteins and are thought to have a role in protecting the cells from water deficit (Ingram and Bartels, 1996; Bray, 1997). Expression patterns of dehydration-inducible genes are complex. Some genes respond to water stress very rapidly, whereas others are induced slowly after the accumulation of ABA. Most of the genes that respond to drought, salt, and cold stress are also induced by exogenous application of ABA (for review, see Shinozaki and YamaguchiShinozaki, 1996; Bray et al., 1997). It appears that dehydration triggers the production of ABA, which in turn induces various genes. Severa1 genes that are induced by water stress are not responsive to exogenous ABA treatment. These findings suggest the existence of both ABAindependent and ABA-dependent signal transduction cascades between the initial signal of drought or cold stress and the expression of specific genes (Shinozaki and Yamaguchi-Shinozaki, 1996; Bray et al., 1997). Promoter analysis of droughtand cold-inducible genes has identified severa1 cis-acting elements that are involved in ABAdependent and ABA-independent responses to conditions of water stress. Details of molecular mechanisms regulating responses of plant genes to water stress remain to be discovered, and there are many questions to be examined at the molecular level. These include the sensing mechanisms of water stress or osmotic stress, modulation of the stress signals to cellular signals, transduction of the cellular signals to the nucleus, transcriptional control of stress-inducible genes, and the function and cooperation of stress-inducible genes allowing water-stress tolerance. This Update focuses on recent progress toward understanding the signal transduction cascades leading to expression of water-stressinducible genes. Possible sensors of osmotic stress in plants are discussed based on our knowledge of yeast and bacteria1 sensors. A glossary of terms is included to facilitate the reading.