Sodium (Na ⁺ ) at high millimolar concentrations in soils is toxic to most higher plants and severely reduces agricultural production worldwide. However, the molecular mechanisms for plant Na ⁺ uptake remain unknown. Here, the wheat root high-affinity potassium (K ⁺ ) uptake transporter HKT1 was shown to function as a high-affinity K ⁺ -Na ⁺ cotransporter. High-affinity K ⁺ uptake was activated by micromolar Na ⁺ concentrations; moreover, high-affinity Na ⁺ uptake was activated by K ⁺ (half-activation constant, 2.8 μM K ⁺ ). However, at physiologically detrimental concentrations of Na ⁺ , K ⁺ accumulation mediated by HKT1 was blocked and low-affinity Na ⁺ uptake occurred (Michaelis constant, ∼16 mM Na ⁺ ), which correlated to Na ⁺ toxicity in plants. Point mutations in the sixth putative transmembrane domain of HKT1 that increase Na ⁺ tolerance were isolated with the use of yeast as a screening system. Na ⁺ uptake and Na ⁺ inhibition of K ⁺ accumulation indicate a possible role for HKT1 in physiological Na ⁺ toxicity in plants.

The Na⁺-K⁺ co-transporter HKT1, first isolated from wheat, mediates high-affinity K⁺ uptake. The function of HKT1 in plants, however, remains to be elucidated, and the isolation of HKT1 homologs from Arabidopsis would further studies of the roles of HKT1 genes in plants. We report here the isolation of a cDNA homologous to HKT1 from Arabidopsis (AtHKT1) and the characterization of its mode of ion transport in heterologous systems. The deduced amino acid sequence of AtHKT1 is 41% identical to that of HKT1, and the hydropathy profiles are very similar.AtHKT1 is expressed in roots and, to a lesser extent, in other tissues. Interestingly, we found that the ion transport properties of AtHKT1 are significantly different from the wheat counterpart. As detected by electrophysiological measurements, AtHKT1 functioned as a selective Na⁺ uptake transporter inXenopus laevis oocytes, and the presence of external K⁺ did not affect the AtHKT1-mediated ion conductance (unlike that of HKT1). When expressed in Saccharomyces cerevisiae, AtHKT1 inhibited growth of the yeast in a medium containing high levels of Na⁺, which correlates to the large inward Na⁺ currents found in the oocytes. Furthermore, in contrast to HKT1, AtHKT1 did not complement the growth of yeast cells deficient in K⁺ uptake when cultured in K⁺-limiting medium. However, expression ofAtHKT1 did rescue Escherichia colimutants carrying deletions in K⁺ transporters. The rescue was associated with a less than 2-fold stimulation of K⁺uptake into K⁺-depleted cells. These data demonstrate that AtHKT1 differs in its transport properties from the wheat HKT1, and that AtHKT1 can mediate Na⁺ and, to a small degree, K⁺ transport in heterologous expression systems.

Many studies have described the response mechanisms of plants to salinity and heat applied individually; however, under field conditions some abiotic stresses often occur simultaneously. Recent studies revealed that the response of plants to a combination of two different stresses is specific and cannot be deduced from the stresses applied individually. Here, we report on the response of tomato plants to a combination of heat and salt stress. Interestingly, and in contrast to the expected negative effect of the stress combination on plant growth, our results show that the combination of heat and salinity provides a significant level of protection to tomato plants from the effects of salinity. We observed a specific response of plants to the stress combination that included accumulation of glycine betaine and trehalose. The accumulation of these compounds under the stress combination was linked to the maintenance of a high K⁺ concentration and thus a lower Na⁺/K⁺ ratio, with a better performance of the cell water status and photosynthesis as compared with salinity alone. Our findings unravel new and unexpected aspects of the response of plants to stress combination and provide a proposed list of enzymatic targets for improving crop tolerance to the abiotic field environment.

Abiotic stresses such as drought, heat or salinity are major causes of yield loss worldwide. Recent studies have revealed that the acclimation of plants to a combination of different environmental stresses is unique and therefore cannot be directly deduced from studying the response of plants to each of the different stresses applied individually. The efficient detoxification of reactive oxygen species (ROS) is thought to play a key role in enhancing the tolerance of plants to abiotic stresses. Here, we report on the role of melatonin in the protection of the photosynthetic apparatus through the increase in ROS detoxification in tomato plants grown under the combination of salinity and heat, two of the most common abiotic stresses known to act jointly. Plants treated with exogenous melatonin showed a different modulation in the expression on some antioxidant-related genes and their related enzymes. More specifically, ascorbate peroxidase, glutathione reductase, glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase (APX, GR, GPX and Ph-GPX, resepctively) showed an antagonistic regulation as compared to plants that did not receive melatonin. This translated into a better antioxidant capacity and to a lesser ROS accumulation under stress combination. The performance of the photosynthesis parameters and the photosystems was also increased in plants treated with exogenous melatonin under the combination of salinity and heat. In accordance with these findings, tomato plants treated with melatonin were found to grow better under stress combination that the non-treated ones. Our study highlights the important role that exogenous melatonin plays in the acclimation of plants to a combination of two different abiotic stresses, and how this compound can specifically regulate oxidative stress-related genes and enzymes to increase plant tolerance.

All living cells have an absolute requirement for K⁺, which must be taken up from the external medium. In contrast to marine organisms, which live in a medium with an inexhaustible supply of K⁺, terrestrial life evolved in oligotrophic environments where the low supply of K⁺ limited the growth of colonizing plants. In these limiting conditions Na⁺ could substitute for K⁺ in some cellular functions, but in others it is toxic. In the vacuole, Na⁺ is not toxic and can undertake osmotic functions, reducing the total K⁺ requirements and improving growth when the lack of K⁺ is a limiting factor. Because of these physiological requirements, the terrestrial life of plants depends on high-affinity K⁺ uptake systems and benefits from high-affinity Na⁺ uptake systems. In plants, both systems have received extensive attention during recent years and a clear insight of their functions is emerging. Some plant HAK transporters mediate high-affinity K⁺ uptake in yeast, mimicking K⁺ uptake in roots, while other members of the same family may be K⁺ transporters in the tonoplast. In parallel with the HAK transporters, some HKT transporters mediate high-affinity Na⁺ uptake without cotransporting K⁺. HKT transporters have two functions: (i) to take up Na⁺ from the soil solution to reduce K⁺ requirements when K⁺ is a limiting factor, and (ii) to reduce Na⁺ accumulation in leaves by both removing Na⁺ from the xylem sap and loading Na⁺ into the phloem sap.

The high-affinity K+ uptake system of plants plays a crucial role in nutrition and has been the subject of extensive kinetic studies. However, major components of this system remain to be identified. We isolated a cDNA from barley roots, HvHAK1, whose translated sequence shows homology to the Escherichia coli Kup and Schwanniomyces occidentalis HAK1 K+ transporters. HvHAK1 conferred high-affinity K+ uptake to a K(+)-uptake-deficient yeast mutant exhibiting the hallmark characteristics of the high-affinity K+ uptake described for barley roots. HvHAK1 also mediated low-affinity Na+ uptake. Another cDNA (HvHAK2) encoding a polypeptide 42% identical to HvHAK1 was also isolated. Analysis of several genomes of Triticeae indicates that HvHAK1 belongs to a multigene family. Translated sequences from bacterial DNAs and Arabidopsis, rice, and possibly human cDNAs show homology to the Kup-HAK1-HvHAK1 family of K+ transporters.