A working model for RSA1 and RITF1 function under salt stress.

<p>The calcium-binding protein RSA1 senses salt-induced changes in nuclear free calcium and interacts with a bHLH transcription factor, RITF1. RITF1 may be phosphorylated by nuclear-localized MAPKs. The RSA1-RITF1 complex controls expression of genes important for detoxification of salt-induced ROS and for Na⁺ homeostasis under salt stress. Some RITF1 target genes may play a role in salt tolerance with so far unknown mechanisms. The calcium-binding protein SOS3 senses salt-induced cytosolic calcium increases and interacts with SOS2, a protein kinase. The SOS3-SOS2 protein kinase complex then phosphorylates and thereby activates the plasma membrane-localized Na⁺/H⁺ antiporter SOS1.