Xylem parenchyma rig. 1.6.3. Model of the regulation of ion homeostasis by Ca+ under salt stress. A Comparison of the response of yeast and plant cells to salt stress modulated by Ca+. Cam Calmodulin. B The SOS system in a plant. NHE Sodium-proton antiporter in the tonoplast; SOS protein of the overly sensitive salt cascade (see text); TF transcription factor. (A, after Bressan et al. 1998; B, after Hasegawa et al. 2000b)
tie is known about osmosensors, particularly as they apply to salt stress, the next step - the genesis of the signal - will now be considered. Genes and proteins have recently been identified which point to a biochemical connection between the osmotic adaptation of the plant and an increase in the cytosolic Ca2+ level. Yeast has served in this regard as a model, for which it has been known for some time that there is an interaction between Ca2+, the subunit B of the calcium-binding protein calcineurin and salt tolerance. In this organism the protein phosphatase calcineurin activates on the one hand transcription factors, which lead to increased expression of a plasma membrane Na+-ATPase and a tono-plastic Na+/H+ antiporter. On the other hand, active calcineurin modulates a not very selective K+/H+ symporter in such a way that it becomes both more effective and more strongly selective for K+ (Fig. 1.6.3 A). A gene family has been correspondingly discovered in Arabidopsis, a mutation in which makes plants particularly sensitive to Na+ and also to Li+ ("salt overly sensitive": SOS). The functions of three SOS proteins have now been elucidated: SOS1 is a Na+-H+ antiporter in the plasma membrane, SOS2 is a serine-threonine protein kinase which activates SOS1, and SOS3 is a calcium-binding protein with an EF-hand domain typical of such proteins, which - after being activated - itself activates SOS2 (Fig. 1.6.3 B; Hasegawa et al. 2000). In addition to the direct activation of SOS1, the SOS cascade also positively influences the activation of transcription factors and leads to increased expression of the antiporter.
Corresponding to the ways in which plant cells respond to drought, there are several parallel calcium-dependent regulatory systems which apply to NaCl stress as well. These are only understood in part, however, e.g. a protein kinase (calcium-dependent protein kinase, CDPK) involved in the expression of a LEA protein or non-stress-specific MAP kinases. It is known that certain phytohormones also act as signals.
A strongly elevated abscisic acid (ABA) level has often been determined in salt-stressed plants (Fig. 1.6.4). An enhanced formation of ethylene has also been observed, whereas the cytokinin content usually decreases.
Since salinity has an ionic as well as a dehydration component (due to the binding of water to ions) and ABA plays an important role as a phytohormonal signal during drought (see Fig, 1.5.8), it cannot be excluded that the in-
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