Figure 2.

Schematic overview of early components involved in salt sensing. High external Na⁺ concentration leads to elevation of intracellular Ca²⁺, phosphatidic acid (PA), and cGMP. Cld PA can activate NHA (SOS1) in an independent manner. The main target of Ca²⁺ is CIBL4 (SOS3). The CIBL4 is capable to form the complex with CBL-interacting serine/threonine-protein kinase 24 (CIPK24, SOS2). The CIBL4-CIPK24 complex activates NHA (SOS1) and inhibits Na⁺ uptake by HKT2. CIPK24 together with SCaBP (SOS3 like protein) is involved in activation of the V-ATPase. CIPK24 participates in the activation of vacuolar transporters such as CAX and NHX. The rise in cytosolic Ca²⁺ concentration could trigger interaction of RSA1-RITF1. RSA1-RITF1 complex activates promoter of SOS1 gene. PA is involved in activation of mitogen-activated protein kinase 6 (MPK6). MPK6 can directly phosphorylate SOS1. The Ca²⁺-dependent kinase (CDPK3) and cytosolic Ca²⁺ lead to activation of vacuolar two-pore K⁺ channels (TPKs) and subsequent K⁺ release from vacuole. Due to the plasma membrane localization, SOS5 protein is considered to be potential candidate for extracellular Na⁺ sensing and helps maintain of cell wall integrity and architecture. Annexin1 (ANN1) is capable to sense the high concentrations of extracellular Na⁺ by mediating ROS-activated Ca²⁺ influx through the plasma membrane of plant cells. The rise of cGMP leads to inhibition of Na⁺ uptake, possibly via cyclic nucleotide–gated ion channels (CNGSs) and glutamate receptor (GLRs). PIP2;1, CNGCs, and GLRs could be blocked by exogenous Ca²⁺. The ROS production leads to K⁺ leak via activation of outward K⁺ channels – KOR (guard cells outward K⁺ channel, GORK) and NSCC. The intracellular accumulation of ROS at high levels can trigger programmed cell death (PCD).