Distinct osmo‐sensing protein kinase pathways are involved in signalling moderate and severe hyper‐osmotic stress

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Abstract
Summary: Plant growth is severely affected by hyper‐osmotic salt conditions. Although a number of salt‐induced genes have been isolated, the sensing and signal transduction of salt stress is little understood. We provide evidence that alfalfa cells have two osmo‐sensing protein kinase pathways that are able to distinguish between moderate and extreme hyper‐osmotic conditions. A 46 kDa protein kinase was found to be activated by elevated salt concentrations (above 125 mm NaCl). In contrast, at high salt concentrations (above 750 mm NaCl), a 38 kDa protein kinase, but not the 46 kDa kinase, became activated. By biochemical and immunological analysis, the 46 kDa kinase was identified as SIMK, a member of the family of MAPKs (mitogen‐activated protein kinases). SIMK is not only activated by NaCl, but also by KCl and sorbitol, indicating that the SIMK pathway is involved in mediating general hyper‐osmotic conditions. Salt stress induces rapid but transient activation of SIMK, showing maximal activity between 8 and 16 min before slow inactivation. When inactive, most mammalian and yeast MAPKs are cytoplasmic but undergo nuclear transloca‐ tion upon activation. By contrast, SIMK was found to be a constitutively nuclear protein and the activity of the kinase was not correlated with changes in its intra‐cellular compartmentation, suggesting an intra‐nuclear mechanism for the regulation of SIMK activity.