Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na⁺ and Cl⁻ uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na⁺ and Cl⁻ uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K⁺ in plant salinity stress are discussed.

Contents Summary 49 I. Introduction 49 II. Physiological and structural characteristics of plant Ca²⁺‐permeable ion channels 50 III. Ca²⁺extrusion systems 61 IV. Concluding remarks 64 Acknowledgements 64 References 64 Summary: Calcium is an essential structural, metabolic and signalling element. The physiological functions of Ca²⁺are enabled by its orchestrated transport across cell membranes, mediated by Ca²⁺‐permeable ion channels, Ca²⁺‐ATPases and Ca²⁺/H⁺exchangers. Bioinformatics analysis has not determined any Ca²⁺‐selective filters in plant ion channels, but electrophysiological tests do reveal Ca²⁺conductances in plant membranes. The biophysical characteristics of plant Ca²⁺conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca²⁺conductances are mediated by several families of ion channels, including cyclic nucleotide‐gated channels (CNGCs), ionotropic glutamate receptors, two‐pore channel 1 (TPC1), annexins and several types of mechanosensitive channels. Key Ca²⁺‐mediated reactions (e.g. sensing of temperature, gravity, touch and hormones, and cell elongation and guard cell closure) have now been associated with the activities of specific subunits from these families. Structural studies have demonstrated a unique selectivity filter inTPC1, which is passable for hydrated divalent cations. The hypothesis of aROS‐Ca²⁺hub is discussed, linking Ca²⁺transport toROSgeneration.CNGCinactivation by cytosolic Ca²⁺, leading to the termination of Ca²⁺signals, is now mechanistically explained. The structure–function relationships of Ca²⁺‐ATPases and Ca²⁺/H⁺exchangers, and their regulation and physiological roles are analysed.

Summary: Soil salinity is an increasing menace that affects agriculture across the globe. Plant adaptation to high salt concentrations involves integrated functions, including control of Na⁺ uptake, translocation and compartmentalization. Na⁺ transporters belonging to the HKT family have been shown to be involved in tolerance to mild salt stress in glycophytes such as Arabidopsis, wheat and rice by contributing to Na⁺ exclusion from aerial tissues. Here, we have analysed the role of the HKT transporter HKT2;1, which is permeable to K⁺ and Na⁺, in barley, a relatively salt‐tolerant crop that displays a salt‐including behaviour. In Xenopus oocytes, HvHKT2;1 co‐transports Na⁺ and K⁺ over a large range of concentrations, displaying low affinity for Na⁺, variable affinity for K⁺ depending on external Na⁺ concentration, and inhibition by K⁺ (K_i approximately 5 mm). HvHKT2;1 is predominantly expressed in the root cortex. Transcript levels are up‐regulated in both roots and shoots by low K⁺ growth conditions, and in shoots by high Na⁺ growth conditions. Over‐expression of HvHKT2;1 led to enhanced Na⁺ uptake, higher Na⁺ concentrations in the xylem sap, and enhanced translocation of Na⁺ to leaves when plants were grown in the presence of 50 or 100 mm NaCl. Interestingly, these responses were correlated with increased barley salt tolerance. This suggests that one of the factors that limits barley salt tolerance is the capacity to translocate Na⁺ to the shoot rather than accumulation or compartmentalization of this cation in leaf tissues. Thus, over‐expression of HvHKT2;1 leads to increased salt tolerance by reinforcing the salt‐including behaviour of barley.

The Arabidopsis thaliana genome contains five genes that encode two pore K ⁺ (TPK) channels. The most abundantly expressed isoform of this family, TPK1, is expressed at the tonoplast where it mediates K ⁺ -selective currents between cytoplasmic and vacuolar compartments. TPK1 open probability depends on both cytoplasmic Ca ²⁺ and cytoplasmic pH but not on the tonoplast membrane voltage. The channel shows intrinsic rectification and can be blocked by Ba ²⁺ , tetraethylammonium, and quinine. TPK1 current was found in all shoot cell types and shows all of the hallmarks of the previously described vacuolar K (VK) tonoplast channel characterized in guard cells. Characterization of TPK1 loss-of-function mutants and TPK1-overexpressing plants shows that TPK1 has a role in intracellular K ⁺ homeostasis affecting seedling growth at high and low ambient K ⁺ levels. In stomata, TPK1 function is consistent with vacuolar K ⁺ release, and removal of this channel leads to slower stomatal closure kinetics. During germination, TPK1 contributes to the radicle development through vacuolar K ⁺ deposition to provide expansion growth or in the redistribution of essential minerals.