Summary: Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺‐ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl⁻ concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.

Caesium (Cs⁺) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs⁺ is chemically similar to potassium (K⁺), and much is known about K⁺ transport mechanisms, it is not clear through which K⁺ transport mechanisms Cs⁺ is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K⁺ and Cs⁺ uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K⁺ deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K⁺ deprivation. Under K⁺-deficient conditions in the presence of Cs⁺, Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs⁺ accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K⁺in planta, AtHAK5 also transports Cs⁺. Further experiments showed that AtHAK5 mediated Cs⁺ uptake into yeast cells and that, although the K⁺ deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH₄⁺in planta, Cs⁺ uptake by yeast was stimulated by low concentrations of NH₄⁺. Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs⁺ than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K⁺ and Cs⁺ under conditions of low K⁺ availability.

Salinity is one of the most decisive environmental factors threatening the productivity of crop plants. Understanding the mechanisms of plant salt tolerance is critical to be able to maintain or improve crop yield under these adverse environmental conditions. Plant membranes act as biological barriers, protecting the contents of cells and organelles from biotic and abiotic stress, including salt stress. Alterations in membrane lipids in response to salinity have been observed in a number of plant species including both halophytes and glycophytes. Changes in membrane lipids can directly affect the properties of membrane proteins and activity of signaling molecules, adjusting the fluidity and permeability of membranes, and activating signal transduction pathways. In this review, we compile evidence on the salt stress responses of the major membrane lipids from different plant tissues, varieties, and species. The role of membrane lipids as signaling molecules in response to salinity is also discussed. Advances in mass spectrometry (MS)-based techniques have largely expanded our knowledge of salt-induced changes in lipids, however only a handful studies have investigated the underlying mechanisms of membrane lipidome regulation. This review provides a comprehensive overview of the recent works that have been carried out on lipid remodeling of plant membranes under salt treatment. Challenges and future perspectives in understanding the mechanisms of salt-induced changes to lipid metabolisms are proposed.

Read the full review for this F1000Prime recommended article: The Arabidopsis cax1 mutant exhibits impaired ion homeostasis, development, and hormonal responses and reveals interplay among vacuolar transporters.

Aquaporin protein regulation and redistribution in response to osmotic stress was investigated. Ice plant (Mesembryanthemum crystallinum) McTIP1;2 (McMIPF) mediated water flux when expressed in Xenopus leavis oocytes. Mannitol-induced water imbalance resulted in increased protein amounts in tonoplast fractions and a shift in protein distribution to other membrane fractions, suggesting aquaporin relocalization. Indirect immunofluorescence labeling also supports a change in membrane distribution for McTIP1;2 and the appearance of a unique compartment where McTIP1;2 is expressed. Mannitol-induced redistribution of McTIP1;2 was arrested by pretreatment with brefeldin A, wortmannin, and cytochalasin D, inhibitors of vesicle trafficking-related processes. Evidence suggests a role for glycosylation and involvement of a cAMP-dependent signaling pathway in McTIP1;2 redistribution. McTIP1;2 redistribution to endosomal compartments may be part of a homeostatic process to restore and maintain cellular osmolarity under osmotic-stress conditions.

In plants, Na⁺/H⁺ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na⁺ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na⁺/H⁺ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na⁺/H⁺ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mm NaCl and increased with prolonged salt exposure up to 8 d. H⁺-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na⁺/H⁺ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent K_m) of the transporter for Na⁺ is 22.8 mm. Data from two experimental approaches supports electroneutral exchange (one Na⁺ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na⁺/H⁺ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na⁺/H⁺ exchanger and its relative contribution to the maintenance of cellular Na⁺ homeostasis during plant growth in salt.