SOS Independent Pathways and Protein Kinase Systems

The most highly developed model for NaCl (osmotic) signaling and response systems is the common baker’s yeast, S. cerevisiae. Apart from the two-component Hik receptor SLN1, an Src-homology 3 (SH3) domain protein is also involved in osmosensing (Hohmann, 2002). These sensors control both the ion homeostasis and osmotic adjustment responses to NaCl stress in yeast, through the activation of MAPK signal cascades. These are also commonly involved in environmental signal responses of animals’ systems (Chang and Karin, 2001). Several MAPKs of plants have been shown to be activated by osmotic or NaCl stress (Jonak et al., 2002). In addition, the transcript levels of MAPK genes have been found to be elevated by osmotic stresses (Ichimura et al., 2000). Although there is considerable likelihood that some of these MAPK-encoding genes are involved in controlling adaptive responses of plants to NaCl/osmotic stress, very few genetic studies have been conducted to determine the effects of loss of function or overexpression of such genes on stress tolerance phenotypes. Their definitive roles in stress adaptation therefore, remain elusive. Several other protein kinases besides MAPKs have also been implicated in control of stress response in plants. Especially interesting is a 42 kDa nonspecific protein kinase belonging to the sucrose nonfermenting (SNF1) kinase group that is activated by NaCl (Mikolajczyk et al., 2000). This group of kinases is apparently independent of the sos pathway, and being unrelated to the MAPK family, suggests that plants may have a novel signal system to respond to NaCl stress.