Inverse regulation of SOS1 and HKT1 protein localization and stability by SOS3/CBL4 in Arabidopsis thaliana.

To control net sodium (Na+) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na+/H+ antiporter SOS1 to achieve Na+ efflux at the root and Na+ loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na+ unloading off the xylem. Together, these opposing transport systems govern the partition of Na+ within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na+ fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na+ export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na+ transport systems operating in vascular plants controlling plant tolerance to salinity.

Assessing the Nutritional Value of Root and Tuber Crops from Bolivia and Peru.

: All over the world, there are species which may be considered as neglected or underutilized despite their nutritious properties. At present, such crops contribute to food security in isolated areas by providing energy and nutrients in a diversified diet. Such genetic heritage-improved by ancient cultures-is under threat of losing biodiversity as well as the traditional knowledge associated with their cultivation and usage. Among these species, the Andean root and tuber crops (ARTCs) constitute a valuable resource which should be preserved and popularized because of their food and functional properties. We studied three ARTC species (mashua, arracacha, and yacon) to provide data on their composition, essential for increasing their use globally. We compared their nutritional values with the values of more widely used crops. Important differences in nutrient composition among ARTC landraces were found. Mineral nutrients showed significant differences among species. Considerable variations in the contents of prebiotics like fructooligosaccharides or functional elements (antioxidants and glucosinolates) were found among species and intraspecific samples. Certainly, these species are important assets to complement human nutrition and to secure supply of functional elements for healthy diets.

Regulation of K+ Nutrition in Plants.

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K+) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K+ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K+ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K+ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K+ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K+ starvation.

A Critical Role of Sodium Flux via the Plasma Membrane Na+/H+ Exchanger SOS1 in the Salt Tolerance of Rice.

Rice (Oryza sativa) stands among the world's most important crop species. Rice is salt sensitive, and the undue accumulation of sodium ions (Na+) in shoots has the strongest negative correlation with rice productivity under long-term salinity. The plasma membrane Na+/H+ exchanger protein Salt Overly Sensitive 1 (SOS1) is the sole Na+ efflux transporter that has been genetically characterized to date. Here, the importance of SOS1-facilitated Na+ flux in the salt tolerance of rice was analyzed in a reverse-genetics approach. A sos1 loss-of-function mutant displayed exceptional salt sensitivity that was correlated with excessive Na+ intake and impaired Na+ loading into the xylem, thus indicating that SOS1 controls net root Na+ uptake and long-distance Na+ transport to shoots. The acute Na+ sensitivity of sos1 plants at low NaCl concentrations allowed analysis of the transcriptional response to sodicity stress without effects of the osmotic stress intrinsic to high-salinity treatments. In contrast with that in the wild type, sos1 mutant roots displayed preferential down-regulation of stress-related genes in response to salt treatment, despite the greater intensity of stress experienced by the mutant. These results suggest there is impaired stress detection or an inability to mount a comprehensive response to salinity in sos1 In summary, the plasma membrane Na+/H+ exchanger SOS1 plays a major role in the salt tolerance of rice by controlling Na+ homeostasis and possibly contributing to the sensing of sodicity stress.

Active proton efflux, nutrient retention and boron-bridging of pectin are related to greater tolerance of proton toxicity in the roots of two Erica species.

BACKGROUND AND AIMS: Tolerance to soil acidity was studied in two species of Ericaceae that grow in mine-contaminated soils (S Portugal, SW Spain) to find out if there are interspecific variations in H+ tolerance which might be related to their particular location. METHODS: Tolerance to H+ toxicity was tested in nutrient solutions using seeds collected in SW Spain. Plant growth and nutrient contents in leaves, stems and roots were determined. Viability tests and proton exchange were studied in roots exposed, short-term, to acidic conditions. Membrane ATPase activity and the cell-wall pectic polysaccharide domain rhamnogalacturonan-II (RG-II) were analysed to find out interspecific differences. RESULTS: Variation in survival, growth and mineral composition was found between species. The H+-tolerant species (Erica andevalensis) showed greater concentration of nutrients than E. australis. Very low pH (pH 2) produced a significant loss of root nutrients (K, P, Mg) in the sensitive species. Root ATPase activity was slightly higher in the tolerant species with a correspondingly greater H+ efflux capacity. In both species, the great majority of the RG-II domains were in their boron-bridged dimeric form. However, shifting to a medium of pH 2 caused some of the boron bridges to break in the sensitive species. CONCLUSIONS: Variation in elements linked to the cell wall-membrane complex and the stability of their components (RG-II, H+-ATPases) are crucial for acid stress tolerance. Thus, by maintaining root cell structure, active proton efflux avoided toxic H+ build-up in the cytoplasm and supported greater nutrient acquisition in H+-tolerant species.

Pleiotropic effects of enhancing vacuolar K/H exchange in tomato.

Cation antiporters of the NHX family are widely regarded as determinants of salt tolerance due to their capacity to drive sodium (Na) and sequester it into vacuoles. Recent work shows, however, that NHX transporters are primarily involved in vacuolar potassium (K) storage. Over-expression of the K/H antiporter AtNHX1 in tomato increases K accumulation into vacuoles and plant sensitivity to K deprivation. Here we show that the appearance of early leaf symptoms of K deficiency was associated with higher concentration of polyamines. Transgenic roots exhibited a greater sensitivity than shoots to K deprivation with changes in the composition of the free amino acids pool, total sugars and organic acids. Concentrations of amides (glutamine), amino acids (arginine) and sugars significantly increased in root, together with a reduction in malate and succinate concentrations. The concentration of pyruvate and the activity of pyruvate kinase were greater in the transgenic roots before K withdrawal although both parameters were depressed by K deprivation and approached wild-type levels. In the longer term, the over-expression of the NHX1 antiporter affected root growth and biomass partitioning (shoot/root ratio). Greater ethylene release produced longer stem internodes and leaf curling in the transgenic line. Our data show that enhanced sequestration of K by the NHX antiporter in the vacuoles altered cellular K homeostasis and had deeper physiological consequences than expected. Early metabolic changes lead later on to profound morphological and physiological adjustments resulting eventually in the loss of nutrient use efficiency.

Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake.

Stomatal movements rely on alterations in guard cell turgor. This requires massive K(+) bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K(+) into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K(+)/H(+) exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K(+) and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K(+) accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K(+) is a requirement for stomatal opening and a critical component in the overall K(+) homeostasis essential for stomatal closure, and suggest that vacuolar K(+) fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

A constitutively active form of a durum wheat Na⁺/H⁺ antiporter SOS1 confers high salt tolerance to transgenic Arabidopsis.

The SOS signaling pathway has emerged as a key mechanism in preserving the homeostasis of Na⁺ and K⁺ under saline conditions. We have recently identified and functionally characterized, by complementation studies in yeast, the gene encoding the durum wheat plasma membrane Na⁺/H⁺ antiporter (TdSOS1). To extend these functional studies to the whole plant level, we complemented Arabidopsis sos1-1 mutant with wild-type TdSOS1 or with the hyperactive form TdSOS1∆972 and compared them to the Arabidopsis AtSOS1 protein. The Arabidopsis sos1-1 mutant is hypersensitive to both Na⁺ and Li⁺ ions. Compared with sos1-1 mutant transformed with the empty binary vector, seeds from TdSOS1 or TdSOS1∆972 transgenic plants had better germination under salt stress and more robust seedling growth in agar plates as well as in nutritive solution containing Na⁺ or Li⁺ salts. The root elongation of TdSOS1∆972 transgenic lines was higher than that of Arabidopsis sos1-1 mutant transformed with TdSOS1 or with the endogenous AtSOS1 gene. Under salt stress, TdSOS1∆972 transgenic lines showed greater water retention capacity and retained low Na⁺ and high K⁺ in their shoots and roots. Our data showed that the hyperactive form TdSOS1∆972 conferred a significant ionic stress tolerance to Arabidopsis plants and suggest that selection of hyperactive alleles of the SOS1 transport protein may pave the way for obtaining salt-tolerant crops.

Ion exchangers NHX1 and NHX2 mediate active potassium uptake into vacuoles to regulate cell turgor and stomatal function in Arabidopsis.

Intracellular NHX proteins are Na(+),K(+)/H(+) antiporters involved in K(+) homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na(+),K(+)/H(+) antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K(+). By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na(+) sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K(+) pool, which in turn provoked greater K(+) retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K(+) uptake at the tonoplast, for turgor regulation, and for stomatal function.

Expression of wheat Na(+)/H(+) antiporter TNHXS1 and H(+)- pyrophosphatase TVP1 genes in tobacco from a bicistronic transcriptional unit improves salt tolerance.

Abiotic stress tolerance of plants is a very complex trait and involves multiple physiological and biochemical processes. Thus, the improvement of plant stress tolerance should involve pyramiding of multiple genes. In the present study, we report the construction and application of a bicistronic system, involving the internal ribosome entry site (IRES) sequence from the 5'UTR of the heat-shock protein of tobacco gene NtHSF-1, to the improvement of salt tolerance in transgenic tobacco plants. Two genes from wheat encoding two important vacuolar ion transporters, Na(+)/H(+) antiporter (TNHXS1) and H(+)-pyrophosphatase (TVP1), were linked via IRES to generate the bicistronic construct TNHXS1-IRES-TVP1. Molecular analysis of transgenic tobacco plants revealed the correct integration of the TNHXS1-IRES-TVP1construct into tobacco genome and the production of the full-length bicistronic mRNA from the 35S promoter. Ion transport analyses with tonoplast vesicles isolated from transgenic lines confirmed that single-transgenic lines TVP1cl19 and TNHXS1cl7 had greater H(+)-PPiase and Na(+)/H(+) antiport activity, respectively, than the WT. Interestingly, the co-expression of TVP1 and TNHXS1 increased both Na(+)/H(+) antiport and H(+)-PPiase activities and induced the H(+) pumping activity of the endogenous V-ATPase. Transgenic tobacco plants expressing TNHXS1-IRES-TVP1 showed a better performance than either of the single gene-transformed lines and the wild type plants when subjected to salt treatment. In addition, the TNHXS1-IRES-TVP1 transgenic plants accumulated less Na(+) and more K(+) in their leaf tissue than did the wild type and the single gene-transformed lines. These results demonstrate that IRES system, described herein, can co-ordinate the expression of two important abiotic stress-tolerance genes and that this expression system is a valuable tool for obtaining transgenic plants with improved salt tolerance.

Selective uptake of major and trace elements in Erica andevalensis, an endemic species to extreme habitats in the Iberian Pyrite Belt.

To assess the ecophysiological traits and the phytoremediation potential of the endemic heather Erica andevalensis, we determined the concentrations of major and trace elements in different plant parts and in rizosphere soils from Riotinto mining district (Huelva, Spain). The results showed that E. andevalensis may grow on substrates with very high As, Cu, Fe and Pb concentrations (up to 4114, 1050, 71900 and 15614 microg/g dry weight, respectively), very low availability of macro- and micronutrients and with pH values ranging from 3.3 to 4.9. In these harsh edaphic conditions E. andevalensis selectively absorbed and translocated essential nutrients and excludes potentially phytotoxic elements, which were accumulated in the root epidermis. The concentrations of major and trace elements in E. andevalensis aerial parts from the Riotinto mining district were in the normal range for plants; likewise other Erica species it accumulated Mn and only in a very polluted site we measured leaf concentrations of As and Pb within the excessive or toxic limits for plants. Differently from previous studies, which emphasized the soil pH and bioavailability of phytotoxic elements as the main stress factors, this study showed that in the Riotinto region, E. andevalensis can tolerate wide range of pH and toxic element concentrations; the harshest environments colonized by monospecific patches of this species were characterized above all by very low availability of nutrients. The extraordinary capability to adapt to these extreme habitats made E. andevalensis a priority species to promote the phytostabilization and the development of a self-sustaining vegetative cover on Riotinto mine tailings.

Effects of silicon on copper toxicity in Erica andevalensis Cabezudo and Rivera: a potential species to remediate contaminated soils.

The influence of silicon on responses to copper excess was studied in plants of Erica andevalensis. Plantlets were grown in nutrient solutions containing two Cu (1 and 500 µM) and three Si concentrations (0, 0.5 and 1 mM). Plant growth, water content, and mineral nutrient concentration were determined. Plants grown with 500 µM Cu showed differences in growth and shoot water content depending on Si supply. The addition of 1 mM Si in high-Cu nutrient solutions significantly improved plant growth and reduced water loss preventing plant death related to Cu-excess. Silicon supply reduced significantly leaf Cu concentration (up to 32%) and increased Cu concentration in roots. Phytoliths isolated from leaves were analysed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Such phytoliths consisted in silica deposits associated with Cu and other elements (K, Ca, P). Improvement by Si of Cu tolerance in E. andevalensis was clearly related to the inhibition of Cu upward transport. The leaf phytoliths formed in Si-treated plants might have some contribution to tolerance by Cu immobilisation and inactivation.

How do vacuolar NHX exchangers function in plant salt tolerance?

Potassium (K(+)) is a major osmoticum of plant cells, and the vacuolar accumulation of this element is a especially crucial feature for plants under high-salt conditions. Emerging evidence indicates that cation/proton transporters of the NHX family are instrumental in the H(+)-linked K(+) transport that mediate active K(+) uptake at the tonoplast for the unequal partitioning of K(+) between vacuole and cytosol. However, and in spite of tenuous supporting evidence, NHX proteins are widely regarded as key players in the sequestration of sodium (Na(+)) into vacuoles to avert ion toxicity in the cytosol of plants under salinity stress. Here, we propose an updated model positing that NHX proteins fulfill a protective function to minimize salt-related stress mainly through the vacuolar compartmentalization of K(+) and, in some cases, of Na(+) as well thereby preventing toxic Na(+)-K(+) ratios in the cytosol while accruing solutes for osmotic balance.

The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato.

NHX-type antiporters in the tonoplast have been reported to increase the salt tolerance of various plants species, and are thought to mediate the compartmentation of Na(+) in vacuoles. However, all isoforms characterized so far catalyze both Na(+)/H(+) and K(+)/H(+) exchange. Here, we show that AtNHX1 has a critical involvement in the subcellular partitioning of K(+), which in turn affects plant K(+) nutrition and Na(+) tolerance. Transgenic tomato plants overexpressing AtNHX1 had larger K(+) vacuolar pools in all growth conditions tested, but no consistent enhancement of Na(+) accumulation was observed under salt stress. Plants overexpressing AtNHX1 have a greater capacity to retain intracellular K(+) and to withstand salt-shock. Under K(+)-limiting conditions, greater K(+) compartmentation in the vacuole occurred at the expense of the cytosolic K(+) pool, which was lower in transgenic plants. This caused the early activation of the high-affinity K(+) uptake system, enhanced K(+) uptake by roots, and increased the K(+) content in plant tissues and the xylem sap of transformed plants. Our results strongly suggest that NHX proteins are likely candidates for the H(+)-linked K(+) transport that is thought to facilitate active K(+) uptake at the tonoplast, and the partitioning of K(+) between vacuole and cytosol.

Alkali cation exchangers: roles in cellular homeostasis and stress tolerance.

Uptake and translocation of cations play essential roles in plant nutrition, signal transduction, growth, and development. Among them, potassium (K+) and sodium (Na+) have been the focus of numerous physiological studies because K+ is an essential macronutrient and the most abundant inorganic cation in plant cells, whereas Na+ toxicity is a principal component of the deleterious effects associated with salinity stress. Although the homeostasis of these two ions was long surmised to be fine tuned and under complex regulation, the myriad of candidate membrane transporters mediating their uptake, intracellular distribution, and long-distance transport is nevertheless perplexing. Recent advances have shown that, in addition to their function in vacuolar accumulation of Na+, proteins of the NHX family are endosomal transporters that also play critical roles in K+ homeostasis, luminal pH control, and vesicle trafficking. The plasma membrane SOS1 protein from Arabidopsis thaliana, a highly specific Na+/H+ exchanger that catalyses Na+ efflux and that regulates its root/shoot distribution, has also revealed surprising interactions with K+ uptake mechanisms by roots. Finally, the function of individual members of the large CHX family remains largely unknown but two CHX isoforms, AtCHX17 and AtCH23, have been shown to affect K+ homeostasis and the control of chloroplast pH, respectively. Recent advances on the understanding of the physiological processes that are governed by these three families of cation exchangers are reviewed and discussed.

Phenotypic and genotypic characterization of rhizobia from diverse geographical origin that nodulate Pachyrhizus species.

Legumes from the genus Pachyrhizus, commonly known as yam bean, are cultivated in several countries from the American continent and constitute an alternative source for sustainable starch, oil and protein production. The endosymbionts of these legumes have been poorly studied although it is known that this legume is nodulated by fast and slow growing rhizobia. In this study we have analyzed a collection of strains isolated in several countries using different phenotypic and molecular methods. The results obtained by SDS-PAGE analysis, LPS profiling and TP-RAPD fingerprinting showed the high diversity of the strains analyzed, although all of them presented slow growth in yeast mannitol agar (YMA) medium. These results were confirmed using 16S-23S internal transcribed spacer (ITS) region and complete sequencing of the 16S rRNA gene, showing that most strains analyzed belong to different species of genus Bradyrhizobium. Three strains were closely related to B. elkanii and the rest of the strains were related to the phylogenetic group constituted by B. japonicum, B. liaoningense, B. yuanmingense and B. betae. These results support that the study of rhizobia nodulating unexplored legumes in different geographical locations will allow the discovery of new species able to establish legume symbioses.