*5.5.3 SOS pathway*

The SOS signaling path includes three significant enzymes, SOS1, SOS2, and SOS3. SOS1 protein codes for Na+ /H+ anti-porter plasma membrane. This protein is essential for cell-level regulation of Na+ efflux. Na+ long-distance transportation from root to

shoot is also facilitated. This protein's overexpression is related to plant salt tolerance [81]. Salt stress-stimulated signals from Ca2+ activate the SOS2 gene encoding serine/ threonine kinase. This protein includes a correctly established catalytic N-terminal domain and a regulatory C-terminal domain. The SOS3 protein is the third protein involved in the SOS stress signaling pathway. It is a myristylated Ca+ binding protein and contains an N-terminus myristylation site. In salt tolerance, this site shows an important role. In the C-terminal regulatory domain of the SOS2 protein, FISL motif is present (also known as NAF domain) that is approximately 21 lengthy sequence of amino acids, and helps to interact with the Ca2+ binding SOS3 protein [82]. Kinase activation is the consequence of the SOS2–SOS3 protein interaction. The kinase caused then phosphorylated SOS1 protein thus enhancing its initially identified yeast transportation activity. SOS1 protein is defined by a long cytosolic C-terminal tail composed of a putative nucleotide binding motif and an auto inhibitory domain, which is roughly 700 amino acids long [83]. The target site for SOS2 phosphorylation is this auto inhibitory domain. In relation to salinity tolerance, it regulates trafficking of membrane vesicle, pH homeostasis and functions of vacuole. There is thus a significant increase in intracellular Ca2+ level with the increase in Na+ concentration, which in turn encourages its binding with SOS3 protein. Ca2+ controls homeostasis of intracellular Na+ along with SOS proteins. Then the SOS3 protein interacts and activation of the SOS2 protein occurs by releasing its self-inhibition. The complex SOS2-SOS3 goes down to plasma membrane where SOS1 reacts. The result of phosphorylated SOS1 is improved Na+ efflux, reducing Na+ toxicity [84].
