The architecture of salt tolerance: A multi-scale view of sodium transport in plants

Soil salinity compromises agricultural productivity by disrupting plant ionic and osmotic equilibrium. While sodium (Na⁺) is the principal cytotoxic ion in saline soils, it can also function as a beneficial element at low concentrations, contributing to cellular osmotic balance and partially offsetting potassium (K⁺) requirements for turgor. Resolving how plants manage this concentration-dependent duality is critical for developing salt-tolerant crops. Achieving this objective requires a comprehensive understanding of how plants regulate Na⁺ via mechanisms such as ion sensing, membrane transport, subcellular sequestration, and tissue-level distribution. Such insights are critical for developing crop varieties with enhanced salt tolerance without compromising yield. To address this fundamental topic, this review integrates data from classical approaches and single-cell RNA sequencing (scRNA-seq) analyses to provide a multi-scale view of Na⁺ homeostasis. We leverage publicly available scRNA-seq datasets to generate cell-type-specific expression profiles of the primary Na⁺ transporter SOS1 and its Ca²⁺-dependent regulators, as well as high-affinity K⁺ transporters (HKTs) that contribute to Na⁺/K⁺ balance. This approach reveals key differences in how dicotyledons (Arabidopsis thaliana) and monocotyledons (Oryza sativa) regulate ion levels. This review also highlights the significance of subcellular localization and endomembrane trafficking of ion transporters, which determine transporter density and stability at target membranes. By connecting cellular-level mechanisms to tissue-level organization, our synthesis tackles a pressing question in agriculture and biology: how do plants coordinate ion movement across different spatial domains to survive in saline environments? This integrated perspective offers mechanistic insights into plant salinity tolerance and supports the development of salt-resilient crops.