Coevolutionary Patterns in SOS1 and NHX1: Insights into Plant Ion Homeostasis Proteins

The SOS (salt overly sensitive) hypersensitivity pathway is a key mechanism for maintaining ion homeostasis at the cellular level and conferring plant resistance and tolerance to salt stress. Its components interact directly and indirectly with various proteins and regulatory mechanisms. We conducted the first coevolutionary analysis of two key proteins of the SOS interaction network (SOS1 and NHX1) across a broad taxonomic range of plant species, including halophytes and glycophytes. Due to sequence availability, our analyses primarily support intramolecular coevolutionary patterns, with preliminary indications of possible intermolecular associations. We assessed the functional and topological relevance of coevolving sites. Six coevolving amino acid pairs were identified in SOS1, and two in NHX1. Except for two residues in SOS1, all sites were associated with functionally and topologically conserved positions. In SOS1, the most relevant coevolving pairs were located in the cytoplasmic domain, which controls the activity of the Na⁺/H⁺ antiporter and plays a critical role in maintaining cellular Na⁺ homeostasis, whereas in NHX1, they were in the transmembrane domain. Our findings reveal previously unexplored molecular relationships in these critical ion homeostasis proteins. Understanding these interactions, which have significant implications for biotechnology and sustainable agriculture, can aid crop improvement and enhance agricultural sustainability under saline conditions.