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.

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.

Contents Summary 49 I. Introduction 49 II. Physiological and structural characteristics of plant Ca -permeable ion channels 50 III. Ca 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 Ca are enabled by its orchestrated transport across cell membranes, mediated by Ca -permeable ion channels, Ca -ATPases and Ca /H exchangers. Bioinformatics analysis has not determined any Ca -selective filters in plant ion channels, but electrophysiological tests do reveal Ca conductances in plant membranes. The biophysical characteristics of plant Ca conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca 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 Ca -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 in TPC1, which is passable for hydrated divalent cations. The hypothesis of a ROS-Ca hub is discussed, linking Ca transport to ROS generation. CNGC inactivation by cytosolic Ca , leading to the termination of Ca signals, is now mechanistically explained. The structure-function relationships of Ca -ATPases and Ca /H exchangers, and their regulation and physiological roles are analysed.

). † These authors contributed equally to this work. SUMMARYSoil 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.

During the symbiotic interaction between Medicago truncatula and the arbuscular mycorrhizal (AM) fungus Glomus intraradices, an endogenous increase in jasmonic acid (JA) occurs. Two full-length cDNAs coding for the JA-biosynthetic enzyme allene oxide cyclase (AOC) from M. truncatula, designated as MtAOC1 and MtAOC2, were cloned and characterized. The AOC protein was localized in plastids and found to occur constitutively in all vascular tissues of M. truncatula. In leaves and roots, MtAOCs are expressed upon JA application. Enhanced expression was also observed during mycorrhization with G. intraradices. A partial suppression of MtAOC expression was achieved in roots following transformation with Agrobacterium rhizogenes harboring the MtAOC1 cDNA in the antisense direction under control of the cauliflower mosaic virus 35S promoter. In comparison to samples transformed with 35STuidA, roots with suppressed MtAOC1 expression exhibited lower JA levels and a remarkable delay in the process of colonization with G. intraradices. Both the mycorrhization rate, quantified by fungal rRNA, and the arbuscule formation, analyzed by the expression level of the AM-specific gene MtPT4, were affected. Staining of fungal material in roots with suppressed MtAOC1 revealed a decreased number of arbuscules, but these did not exhibit an altered structure. Our results indicate a crucial role for JA in the establishment of AM symbiosis.