Salt Overly Sensitive (SOS) pathway (SOS3/CBL4 - SOS2/CIPK24 - SOS1 Na+/H+ antiporter)

The Salt Overly Sensitive (SOS) pathway is a calcium-decoding kinase relay that drives Na+ extrusion from plant cells under salinity. A salt-induced cytosolic Ca2+ signal is sensed by N-myristoylated calcineurin B-like (CBL) EF-hand calcium sensors: SOS3/CBL4 predominantly in roots and SCaBP8/CBL10 in shoots. The Ca2+-bound CBL binds the C-terminal NAF/FISL motif of the CBL-interacting protein kinase SOS2/CIPK24, relieving its autoinhibition and recruiting it to the plasma membrane. The CBL-CIPK24 complex phosphorylates the C-terminal autoinhibitory domain of SOS1/NHX7, an electroneutral plasma-membrane Na+/H+ antiporter, relieving its autoinhibition so that SOS1 uses the proton-motive force to export Na+ to the apoplast; SOS1 also controls long-distance Na+ partitioning at the xylem/symplast boundary. The module ends at SOS1-mediated Na+ efflux. The salt-sensing upstream step (GIPC-mediated Ca2+ influx), SOS2 regulation by ABI2, 14-3-3 proteins and GI, the vacuolar CBL10-CIPK24 branch and NHX-type vacuolar antiporters, and HKT1-mediated xylem Na+ retrieval are outside this module's boundary.

Arabidopsis thaliana proteins are used as representative members, grounded with reviewed UniProtKB accessions: SOS1/NHX7 Q9LKW9 and SOS2/CIPK24 Q9LDI3 from the gene review folders; SOS3/CBL4 O81223 and SCaBP8/CBL10 Q7FRS8 from UniProt (no gene reviews yet). Annoton molecular functions match the gene reviews' core functions: CIPK24 protein serine/threonine kinase activity (GO:0004674) and SOS1 sodium:proton antiporter activity (GO:0015385). The CBL sensor annoton uses protein kinase activator activity (GO:0030295), supported by SOS3 binding and activating SOS2 (PMID:10725350); calcium ion sensor activity (GO:0061891) would be an alternative framing and is described in the role text. CBL4 (roots) and CBL10 (shoots) are treated as tissue-preferential members of one Ca2+-sensor role rather than as a formal variant set, because both act on the same SOS2-SOS1 relay at the plasma membrane; CBL10 also targets CIPK24 to the tonoplast, a branch not modeled here. Boundary decision on HKT1 (Q84TI7, reviewed): HKT1 is a plasma-membrane Na+ uniporter (GO:0015081) that retrieves Na+ from the xylem sap and recirculates it to roots. It acts in parallel with SOS1 on whole-plant Na+ partitioning but is not a substrate of the CBL-CIPK24 relay, so including it would turn this signaling module into a broader Na+ homeostasis program; it is excluded and would belong in a future whole-plant Na+ partitioning module together with SOS1's long-distance role. No PANTHER family ids are asserted. No production GO-CAM in gocams/index.tsv contains these Arabidopsis proteins (checked 2026-10-14 by UniProt accession), so no gocam_associations are given.