Induction of Protective Proteins by Salt Stress
The previous sections have made it clear that salt stress triggers numerous cellular reactions which are all interrelated and complement each other synergistically.
These reactions also include the synthesis of more or less specific protective proteins (Fig. 1.6.15). These may be, on the one hand, osmo-lyte-producing enzymes or, on the other, proteins from the LEA group (see Chap. 220.127.116.11) which ameliorate the dehydration stress triggered by high salt impact. In addition to these rather specifically acting proteins, another group of polypeptides is formed and accumulated during salinity stress whose function in relation to the stressor salt is not evident. This group includes both proteins which are also often formed during pathogen attacks and proteins of the cell wall and extracellular matrix.
The great variety of proteins which are synthesised and in some cases also strongly accumulated due to the effect of salt makes it difficult to assign the proteins to particular stress responses. The fact that LEA proteins are typical dehydration protection proteins (see Chap. 18.104.22.168) of course does not mean that they specifically ameliorate only the dehydration stress component of salinity.
One explanation for the occurrence of proteins which are not specific for salt stress in organisms subjected to salinity stems from the signal compounds leading to the strain resulting from salinity (see Chap. 1.6.2,1). Salt stress leads to an increase in the levels of ABA and jasmo-nate in the cell. ABA mediates the linkage of the responses of the cell to salt stress and drought. This stress response is carried over into the re sponses to stress caused by wounding or pathogen attack via the jasmonates (see Chaps. 1.10.1 and 1.10.2). However, a NaCl-dependent gene expression has been observed which can be assigned to neither ABA, nor jasmonate, nor the ethylene which is occasionally detected in connection with salt stress. This corresponds to the circumstances during drought stress which were described earlier (see Chap. 22.214.171.124).
Osmotins are proteins which accumulate in cells under salt stress and may constitute, e.g. in tobacco cell cultures, up to 12% of the total protein content. To the extent to which their expression is induced via ABA, they can also be synthesised and accumulated under drought conditions (Singh et al. 1989). If their expression is regulated via the jasmonate signal, they are also found upon pathogen attack. Although they were discovered during investigation of the cellular reaction to salt stress and were accordingly termed osmotins, these proteins are not specific for salt stress. Nowadays, they are rather classified as pathogen-related proteins (see Chap. 1.10.2) and referred to as osmotin-like proteins (OLPs), which are products of a multigene fami-
Osmotins are relatively small proteins (Mr between 24 and 50 kDa) which are conspicuous in terms of their alkaline isoelectric point (Table 1.6.7). They have a net positive charge at the pH values of the cell, i.e. they are cations. As such, they are probably able to interact efficiently with most anionic membrane proteins (Kononowicz et al. 1993). They possess a large number of disulfide bridges, but no free SH groups (Zhu et al. 1995).
Several osmotins have been found in each of the plants that have been subjected to detailed study; they differ not so much in their molecular mass as in their isoelectric points and other protein characteristics. For tobacco, the best-studied species, a pre-protein has been discovered in addition to the mature protein, from which it could be concluded that mature osmo-
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