Cellular aspects of Na ⁺ homeostasis in plants: Quantitative approaches

Under saline conditions, plants consistently maintain cytosolic Na⁺ concentrations between 10 and 30 mM, sequestering excess Na⁺ to the vacuole. We demonstrate that this cytosolic Na⁺ homeostasis is regulated by inward Na⁺-permeable channels and outward Na⁺:H⁺ antiporters at both the plasma membrane and tonoplast. Sodium's interplay with K⁺ transport adds complexity and selective transport is crucial to avoid conflicting ion fluxes. Our models predict that Na⁺:H⁺ antiport regulation at the plasma membrane significantly impacts cytosolic Na⁺ levels, while channel and antiport regulation are equally important at the tonoplast. The energetic implications of these transport mechanisms are discussed. In contrast to the cytosol, chloroplast Na⁺ concentrations vary significantly between species and increase with soil salinity, raising questions as to how C₄ and CAM plants acquire pyruvate under saline conditions. However, modelling transport activity at the chloroplast membrane requires far more knowledge of the associated transport systems and the chloroplastic Na⁺ content.