Mechanisms of salinity tolerance

Role of ENA ATPase in Na(+) efflux at high pH in bryophytes

Potassium or Na(+) efflux ATPases, ENA ATPases, are present in all fungi and play a central role in Na(+) efflux and Na(+) tolerance. Flowering plants lack ENA ATPases but two ENA ATPases have been identified in the moss Physcomitrella patens, PpENA1 and PpENA2. PpENA1 mediates Na(+) efflux in Saccharomyces cerevisiae. To propose a general function of ENA ATPases in bryophytes it was necessary to demonstrate that these ATPases mediate Na(+) efflux in planta and that they exist in more bryophytes than P. patens. For these demonstrations (1) we cloned a third ATPase from P. patens, PpENA3, and studied the expression pattern of the three PpENA genes; (2) we constructed and studied the single and double Deltappena1 and Deltappena2 mutants; and (3) we cloned two ENA ATPases from the liverwort Marchantia polymorpha, MpENA1 and MpENA2, and expressed them in S. cerevisiae. The results from the first two approaches revealed that the expression of ENA ATPases was greatly enhanced at high pH and that Na(+) efflux at high pH depended on PpENA1. The ENA1 ATPase of M. polymorpha suppressed the defective growth of a S. cerevisiae mutant at high K(+) or Na(+) concentrations, especially at high K(+).

Dynamic aspects of ion accumulation by vesicle traffic under salt stress in Arabidopsis

The intracellular membrane dynamics of Arabidopsis cells under high salt treatment were investigated. When Arabidopsis was treated with high levels of NaCl in hydroponic culture, root tip cells showed rapid changes in the vacuolar volume, a decrease in the number of small acid compartments, active movement of vesicles and accumulation of Na(+) both in the central vacuole and in the vesicles around the main vacuole observed with the Na(+)-dependent fluorescence of Sodium Green. Detailed observation of Arabidopsis suspension-cultured cells under high salt treatment showed a similar pattern of response to that observed in root tip cells. Immunostaining of suspension-cultured cells with antibodies against AtNHX1 clearly showed the occurrence of dotted fluorescence in the cytoplasm only under salt treatment. We also confirmed the existence of AtNHX1 in the vacuolar membrane isolated from suspension-cultured cells with immunofluorescence. Knockout of the vacuolar Q(a)-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein VAM3/SYP22 caused an increase in salt tolerance. In mutant plants, the distribution of Na(+) between roots and shoots differed from that of wild-type plants, with Na(+) accumulating more in roots and less in the shoots of the mutant plants. The role of vesicle traffic under salt stress is discussed.

Upgrading root physiology for stress tolerance by ectomycorrhizas: insights from metabolite and transcriptional profiling into reprogramming for stress anticipation

Ectomycorrhizas (EMs) alleviate stress tolerance of host plants, but the underlying molecular mechanisms are unknown. To elucidate the basis of EM-induced physiological changes and their involvement in stress adaptation, we investigated metabolic and transcriptional profiles in EM and non-EM roots of gray poplar (Populus x canescens) in the presence and absence of osmotic stress imposed by excess salinity. Colonization with the ectomycorrhizal fungus Paxillus involutus increased root cell volumes, a response associated with carbohydrate accumulation. The stress-related hormones abscisic acid and salicylic acid were increased, whereas jasmonic acid and auxin were decreased in EM compared with non-EM roots. Auxin-responsive reporter plants showed that auxin decreased in the vascular system. The phytohormone changes in EMs are in contrast to those in arbuscular mycorrhizas, suggesting that EMs and arbuscular mycorrhizas recruit different signaling pathways to influence plant stress responses. Transcriptome analyses on a whole genome poplar microarray revealed activation of genes related to abiotic and biotic stress responses as well as of genes involved in vesicle trafficking and suppression of auxin-related pathways. Comparative transcriptome analysis indicated EM-related genes whose transcript abundances were independent of salt stress and a set of salt stress-related genes that were common to EM non-salt-stressed and non-EM salt-stressed plants. Salt-exposed EM roots showed stronger accumulation of myoinositol, abscisic acid, and salicylic acid and higher K(+)-to-Na(+) ratio than stressed non-EM roots. In conclusion, EMs activated stress-related genes and signaling pathways, apparently leading to priming of pathways conferring abiotic stress tolerance.

HKT transporter-mediated salinity resistance mechanisms in Arabidopsis and monocot crop plants

The salinization of irrigated lands is increasingly detrimental to plant biomass production and agricultural productivity, as most plant species are sensitive to high concentrations of sodium (Na(+)), which causes combined Na(+) toxicity and osmotic stress. Plants have multiple Na(+)-transport systems to circumvent Na(+) toxicity. Essential physiological functions of major Na(+) transporters and their mechanisms mediating salinity resistance have been identified in Arabidopsis , including the AtSOS1, AtNHX and AtHKT1;1 transporters. As we discuss here, recent studies have demonstrated that a class of xylem-parenchyma-expressed Na(+)-permeable plant HKT transporters represent a primary mechanism mediating salt tolerance and Na(+) exclusion from leaves in Arabidopsis, and that major salt-tolerance quantitative trait loci in monocot crop plants are also based on this HKT-mediated mechanism.

Salt-dependent regulation of a CNG channel subfamily in Arabidopsis

BACKGROUND

In Arabidopsis thaliana, the family of cyclic nucleotide-gated channels (CNGCs) is composed of 20 members. Previous studies indicate that plant CNGCs are involved in the control of growth processes and responses to abiotic and biotic stresses. According to their proposed function as cation entry pathways these channels contribute to cellular cation homeostasis, including calcium and sodium, as well as to stress-related signal transduction. Here, we studied the expression patterns and regulation of CNGC19 and CNGC20, which constitute one of the five CNGC subfamilies.

RESULTS

GUS, GFP and luciferase reporter assays were used to study the expression of CNGC19 and CNGC20 genes from Arabidopsis thaliana in response to developmental cues and salt stress. CNGC19 and CNGC20 were differentially expressed in roots and shoots. The CNGC19 gene was predominantly active in roots already at early growth stages. Major expression was observed in the phloem. CNGC20 showed highest promoter activity in mesophyll cells surrounding the veins. Its expression increased during development and was maximal in mature and senescent leaves. Both genes were upregulated in the shoot in response to elevated NaCl but not mannitol concentrations. While in the root, CNGC19 did not respond to changes in the salt concentration, in the shoot it was strongly upregulated in the observed time frame (6-72 hours). Salt-induction of CNGC20 was also observed in the shoot, starting already one hour after stress treatment. It occurred with similar kinetics, irrespective of whether NaCl was applied to roots of intact plants or to the petiole of detached leaves. No differences in K and Na contents of the shoots were measured in homozygous T-DNA insertion lines for CNGC19 and CNGC20, respectively, which developed a growth phenotype in the presence of up to 75 mM NaCl similar to that of the wild type.

CONCLUSION

Together, the results strongly suggest that both channels are involved in the salinity response of different cell types in the shoot. Upon salinity both genes are upregulated within hours. CNGC19 and CNGC20 could assist the plant to cope with toxic effects caused by salt stress, probably by contributing to a re-allocation of sodium within the plant.

A different role for hydrogen peroxide and the antioxidative system under short and long salt stress in Brassica oleracea roots

Salinity affects normal growth and development of plants depending on their capacity to overcome the induced stress. The present study was focused on the response and regulation of the antioxidant defence system in Brassica oleracea roots under short and long salt treatments. The function and the implications of hydrogen peroxide as a stressor or as a signalling molecule were also studied. Two different zones were analysed--the elongation and differentiation zone and the fully differentiated root zone--in order to broaden the knowledge of the different effects of salt stress in root. In general, an accumulation of hydrogen peroxide was observed in both zones at the highest (80 mM NaCl) concentration. A higher accumulation of hydrogen peroxide was observed in the stele of salt-treated roots. At the subcellular level, mitochondria accumulated hydrogen peroxide in salt-treated roots. The results confirm a drastic decrease in the antioxidant enzymes catalase, ascorbate peroxidase, and peroxidases under short salt treatments. However, catalase and peroxidase activities were recovered under long salt stress treatments. The two antioxidant molecules analysed, ascorbate and glutathione, showed a different trend during salt treatments. Ascorbate was progressively accumulated and its redox state maintained, but glutathione was highly accumulated at 24 h of salt treatment, but then its concentration and redox state progressively decreased. Concomitantly, the antioxidant enzymes involved in ascorbate and glutathione regeneration were modified under salt stress treatments. In conclusion, the increase in ascorbate levels and the maintenance of the redox state seem to be critical for root growth and development under salt stress.

Development of wheat-Lophopyrum elongatum recombinant lines for enhanced sodium 'exclusion' during salinity stress

Lophopyrum elongatum (tall wheatgrass), a wild relative of wheat, can be used as a source of novel genes for improving salt tolerance of bread wheat. Sodium 'exclusion' is a major physiological mechanism for salt tolerance in a wheat-tall wheatgrass amphiploid, and a large proportion ( approximately 50%) for reduced Na(+) accumulation in the Xag leaf, as compared to wheat, was earlier shown to be contributed by genetic effects from substitution of chromosome 3E from tall wheatgrass for wheat chromosomes 3A and 3D. Homoeologous recombination between 3E and wheat chromosomes 3A and 3D was induced using the ph1b mutant, and putative recombinants were identified as having SSR markers specific for tall wheatgrass loci. As many as 14 recombinants with smaller segments of tall wheatgrass chromatin were identified and low-resolution breakpoint analysis was achieved using wheat SSR loci. Seven recombinants were identified to have leaf Na+ concentrations similar to those in 3E(3A) or 3E(3D) substitution lines, when grown in 200 mM NaCl in nutrient solution. Phenotypic analysis identified recombinants with introgressions at the distal end on the long arm of homoeologous group 3 chromosomes being responsible for Na(+) 'exclusion'. A total of 55 wheat SSR markers mapped to the long arm of homoeologous group 3 markers by genetic and deletion bin mapping were used for high resolution of wheat-tall wheatgrass chromosomal breakpoints in selected recombinants. Molecular marker analysis and genomic in situ hybridisation confirmed the 524-568 recombinant line as containing the smallest introgression of tall wheatgrass chromatin on the distal end of the long arm of wheat chromosome 3A and identified this line as suitable for developing wheat germplasm with Na(+) 'exclusion'.

Isolated Thellungiella shoots do not require roots to survive NaCl and Na2SO4 salt stresses

Shoots of Thellungiella derived by micropropagation were used to estimate the plants' salt tolerance and ability to regulate Na+ uptake. Two species with differing salt tolerances were studied: Thellungiella salsuginea (halophilla), which is less tolerant, and Thellungiella botschantzevii, which is more tolerant. Although the shoots of neither ecotype survived at 700 mM NaCl or 200 mM Na2SO4, micropropagated shoots of T. botschantzevii were more tolerant to Na2SO4 (10-100 mM) and NaCl (100-300 mM). In the absence of roots, Na2SO4 salinity reduced shoot growth more dramatically than NaCl salinity. Plantlets of both species were able to adapt to salt stress even when they did not form roots. First, there was no significant correlation between Na+ accumulation in shoots and Na+ concentration in the growth media. Second, K+ concentrations in the shoots exposed to different salt concentrations were maintained at equivalent levels to control plants grown in medium without NaCl or Na2SO4. These results suggest that isolated shoots of Thellungiella possess their own mechanisms for enabling salt tolerance, which contribute to salt tolerance in intact plants.

A new screening method for osmotic component of salinity tolerance in cereals using infrared thermography

A high-throughput, automated image analysis protocol for the capture, identification and analysis of thermal images acquired with a long-wave infrared (IR) camera was developed to quantify the osmotic stress response of wheat and barley to salinity. There was a strong curvilinear relationship between direct measurements of stomatal conductance and leaf temperature of barley grown in a range of salt concentrations. This indicated that thermography accurately reflected the physiological status of salt-stressed barley seedlings. Leaf temperature differences between barley grown at 200 mM NaCl and 0 mM NaCl reached 1.6°C - the sensitivity of the IR signal increasing at higher salt concentrations. Seventeen durum wheat genotypes and one barley genotype, known to vary for osmotic stress tolerance, were grown in control (no salt) and 150 mM NaCl treatments to validate the newly-developed automated thermal imaging protocol. The ranking of the 18 genotypes based on both a growth study and the IR measurements was consistent with previous reports in the literature for these genotypes. This study shows the potential of IR thermal imaging for the screening of large numbers of genotypes varying for stomatal traits, specifically those related to salt tolerance.

Assessment of variation in antioxidative defense system in salt-treated pea (Pisum sativum) cultivars and its putative use as salinity tolerance markers

The present work describes whether the changes in the activities of antioxidant enzymes and the levels of some non-enzymatic antioxidants could be used as markers of salt tolerance in nine genetically diverse pea (Pisum sativum) cultivars. All cultivars were exposed to four levels of NaCl i.e., 0, 40, 80 and 120mM in sand culture. Plant fresh biomass, total phenolics, total soluble proteins, hydrogen peroxide (H(2)O(2)), malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in leaves while different forms of tocopherols (alpha-, gamma- and Delta-tocopherol) in fresh seed of salt-stressed and non-stressed plants were analyzed. On the basis of percent inhibition in shoot biomass at the highest salt level (120mM) cultivars 2001-35, 2001-55 and Climax were ranked as tolerant (percent inhibition less than 60%), 2001-20, 9800-5 and 9800-10 moderately tolerant (percent inhibition 60-70%), and 2001-40, 9200 and Tere 2 salt sensitive (percent inhibition more than 70%). Salt stress markedly enhanced the activities of SOD and POD, levels of total phenolics and gamma- and Delta-tocopherols, and decreased the total soluble proteins and CAT activity, while the internal levels of H(2)O(2) remained unaffected in all pea cultivars. Although salt-induced oxidative stress occurred in all pea cultivars, the response of salt-tolerant and salt-sensitive cultivars with respect to the generation of enzymatic and non-enzymatic metabolites measured in the present study was not consistent. Of different antioxidant enzymes and metabolites analyzed, only CAT activity was found to be a reliable marker of salt tolerance in the set of pea cultivars examined.

Vetiver (Vetiveria zizanioides) responses to fertilization and salinity under irrigation conditions

Vetiver (Vetiveria zizanioides) has not been widely introduced in arid and semi-arid regions where irrigation, fertilization, and salinity are important factors in plant growth. The main objective of this study was to determine the response of vetiver to fertilization (fertigation) and salinity and their interactions under irrigated conditions. The experiment was conducted in a greenhouse in 10-L pots. Combined effects of three nutrients concentrations and three salinity levels of electrical conductivity (EC) 1, 3 and 6 dS/m in the irrigation water on growth and transpiration of vetiver plants and the content of different elements in their foliage were studied. Similar contents of approximately 3.7 g/kg Na, approximately 5.77 g/kg Ca and approximately 2.55 g/kg Mg were found in the foliage of all the plants irrigated with the different fertilizer and salinity levels. Concentrations of 59 mg/L N and 36.1mg/L K in the irrigation water were sufficient for vetiver plants needs at the different salinity levels tested. The salinity threshold (the maximum EC in the soil solution that does not cause a significant yield reduction) for vetiver was between 3 and 6 dS/m. A concentration of 15.2mg/L P in the irrigation water was the optimum value for vetiver growth in the three salinity levels, resulting in an average content of 5.95 g/kg P in plant foliage. It is suggested that vetiver is sensitive to excess P (>8.66 g/kg). Increasing EC in the irrigation water to 6 dS/m decreased plant foliage biomass mainly due to an increase in the osmotic potential of the irrigation water and high Cl(-) concentration in the foliage.

A proteomic study of the response to salinity and drought stress in an introgression strain of bread wheat

The effect of drought and salinity stress on the seedlings of the somatic hybrid wheat cv. Shanrong No. 3 (SR3) and its parent bread wheat cv. Jinan 177 (JN177) was investigated using two-dimensional gel electrophoresis and mass spectrometry. Of a set of 93 (root) and 65 (leaf) differentially expressed proteins (DEPs), 34 (root) and six (leaf) DEPs were cultivar-specific. The remaining DEPs were salinity/drought stress-responsive but not cultivar-specific. Many of the DEPs were expressed under both drought and salinity stresses. The amounts of stress-responsive DEPs between SR3 and JN177 were almost equivalent, whereas only some of these DEPs were shared by the two cultivars. Overall, the number of salinity-responsive DEPs was greater than the number of drought-responsive DEPs. And most of the drought-responsive DEPs also responded to salinity. There are both similarities and differences in the responses of wheat to salinity and drought. A parallel transcriptomics analysis showed that the correlation between transcriptional and translational patterns of DEPs was poor. The enhanced drought/salinity tolerance of SR3 appears to be governed by a superior capacity for osmotic and ionic homeostasis, a more efficient removal of toxic by-products, and ultimately a better potential for growth recovery.

Calcium mediates root K+/Na+ homeostasis in poplar species differing in salt tolerance

Using the non-invasively ion-selective microelectrode technique, flux profiles of K(+), Na(+) and H(+) in mature roots and apical regions, and the effects of Ca(2+) on ion fluxes were investigated in salt-tolerant poplar species, Populus euphratica Oliver and salt-sensitive Populus simonii x (P. pyramidalis + Salix matsudana) (Populus popularis 35-44, P. popularis). Compared to P. popularis, P. euphratica roots exhibited a greater capacity to retain K(+) after exposure to a salt shock (SS, 100 mM NaCl) and a long-term (LT) salinity (50 mM NaCl, 3 weeks). Salt shock-induced K(+) efflux in the two species was markedly restricted by K(+) channel blocker, tetraethylammonium chloride, but enhanced by sodium orthovanadate, the inhibitor of plasma membrane (PM) H(+)-ATPase, suggesting that the K(+) efflux is mediated by depolarization-activated (DA) channels, e.g., KORCs (outward rectifying K(+) channels) and NSCCs (non-selective cation channels). Populus euphratica roots were more effective to exclude Na(+) than P. popularis in an LT experiment, resulting from the Na(+)/H(+) antiport across the PM. Moreover, pharmacological evidence implies that the greater ability to control K(+)/Na(+) homeostasis in salinized P. euphratica roots is associated with the higher H(+)-pumping activity, which provides an electrochemical H(+) gradient for Na(+)/H(+) exchange and simultaneously decreases the NaCl-induced depolarization of PM, thus reducing Na(+) influx via NSCCs and K(+) efflux through DA-KORCs and DA-NSCCs. Ca(2+) application markedly limited salt-induced K(+) efflux but enhanced the apparent Na(+) efflux, thus enabling the two species, especially the salt-sensitive poplar, to retain K(+)/Na(+) homeostasis in roots exposed to prolonged NaCl treatment.

Metabolic responses to salt stress of barley (Hordeum vulgare L.) cultivars, Sahara and Clipper, which differ in salinity tolerance

Plants show varied cellular responses to salinity that are partly associated with maintaining low cytosolic Na(+) levels and a high K(+)/Na(+) ratio. Plant metabolites change with elevated Na(+), some changes are likely to help restore osmotic balance while others protect Na(+)-sensitive proteins. Metabolic responses to salt stress are described for two barley (Hordeum vulgare L.) cultivars, Sahara and Clipper, which differed in salinity tolerance under the experimental conditions used. After 3 weeks of salt treatment, Clipper ceased growing whereas Sahara resumed growth similar to the control plants. Compared with Clipper, Sahara had significantly higher leaf Na(+) levels and less leaf necrosis, suggesting they are more tolerant to accumulated Na(+). Metabolite changes in response to the salt treatment also differed between the two cultivars. Clipper plants had elevated levels of amino acids, including proline and GABA, and the polyamine putrescine, consistent with earlier suggestions that such accumulation may be correlated with slower growth and/or leaf necrosis rather than being an adaptive response to salinity. It is suggested that these metabolites may be an indicator of general cellular damage in plants. By contrast, in the more tolerant Sahara plants, the levels of the hexose phosphates, TCA cycle intermediates, and metabolites involved in cellular protection increased in response to salt. These solutes remain unchanged in the more sensitive Clipper plants. It is proposed that these responses in the more tolerant Sahara are involved in cellular protection in the leaves and are involved in the tolerance of Sahara leaves to high Na(+).

Genetic technologies for the identification of plant genes controlling environmental stress responses

Abiotic conditions such as light, temperature, water availability and soil parameters determine plant growth and development. The adaptation of plants to extreme environments or to sudden changes in their growth conditions is controlled by a well balanced, genetically determined signalling system, which is still far from being understood. The identification and characterisation of plant genes which control responses to environmental stresses is an essential step to elucidate the complex regulatory network, which determines stress tolerance. Here, we review the genetic approaches, which have been used with success to identify plant genes which control responses to different abiotic stress factors. We describe strategies and concepts for forward and reverse genetic screens, conventional and insertion mutagenesis, TILLING, gene tagging, promoter trapping, activation mutagenesis and cDNA library transfer. The utility of the various genetic approaches in plant stress research we review is illustrated by several published examples.

Leaf expansion in grasses under salt stress

Restriction of leaf growth is among the earliest visible effects of many stress conditions, including salinity. Because leaves determine radiation interception and are the main photosynthetic organs, salinity effects on leaf expansion and function are directly related to yield constraints under saline conditions. The expanding zone of leaf blades spans from the meristem to the region in which cells reach their final length. Kinematic methods are used to describe cell division and cell expansion activities. Analyses of this type have indicated that the reduction in leaf expansion by salinity may be exerted through effects on both cell division and expansion. In turn, the components of vacuole-driven cell expansion may be differentially affected by salinity, and examination of salinity effects on osmotic and mechanical constraints to cell expansion have gradually led to the identification of the gene products involved in such control. The study of how reactive oxygen species affect cell expansion is an emerging topic in the study of salinity's regulation of leaf growth.

Rootstock-mediated changes in xylem ionic and hormonal status are correlated with delayed leaf senescence, and increased leaf area and crop productivity in salinized tomato

Tomato crop productivity under salinity can be improved by grafting cultivars onto salt-tolerant wild relatives, thus mediating the supply of root-derived ionic and hormonal factors that regulate leaf area and senescence. A tomato cultivar was grafted onto rootstocks from a population of recombinant inbred lines (RILs) derived from a Solanum lycopersicum x Solanum cheesmaniae cross and cultivated under moderate salinity (75 mM NaCl). Concentrations of Na(+), K(+) and several phytohormones [abscisic acid (ABA); the cytokinins (CKs) zeatin, Z; zeatin riboside, ZR; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)] were analysed in leaf xylem sap in graft combinations of contrasting vigour. Scion leaf area correlated with photosystem II (PSII) efficiency (F(v)/F(m)) and determined fruit productivity. Xylem K(+) (but not Na(+)), K(+)/Na(+), the active CK Z, the ratio with its storage form Z/ZR and especially the ratio between CKs and ACC (Z/ACC and Z + ZR/ACC) were positively loaded into the first principal component (PC) determining both leaf growth and PSII efficiency. In contrast, the ratio ACC/ABA was negatively correlated with leaf biomass. Although the underlying physiological mechanisms by which rootstocks mediate leaf area or chlorophyll fluorescence (and thus influence tomato salt tolerance) seem complex, a putative potassium-CK interaction involved in regulating both processes merits further attention.

Shoot Na+ exclusion and increased salinity tolerance engineered by cell type-specific alteration of Na+ transport in Arabidopsis

Soil salinity affects large areas of cultivated land, causing significant reductions in crop yield globally. The Na+ toxicity of many crop plants is correlated with overaccumulation of Na+ in the shoot. We have previously suggested that the engineering of Na+ exclusion from the shoot could be achieved through an alteration of plasma membrane Na+ transport processes in the root, if these alterations were cell type specific. Here, it is shown that expression of the Na+ transporter HKT1;1 in the mature root stele of Arabidopsis thaliana decreases Na+ accumulation in the shoot by 37 to 64%. The expression of HKT1;1 specifically in the mature root stele is achieved using an enhancer trap expression system for specific and strong overexpression. The effect in the shoot is caused by the increased influx, mediated by HKT1;1, of Na+ into stelar root cells, which is demonstrated in planta and leads to a reduction of root-to-shoot transfer of Na+. Plants with reduced shoot Na+ also have increased salinity tolerance. By contrast, plants constitutively expressing HKT1;1 driven by the cauliflower mosaic virus 35S promoter accumulated high shoot Na+ and grew poorly. Our results demonstrate that the modification of a specific Na+ transport process in specific cell types can reduce shoot Na+ accumulation, an important component of salinity tolerance of many higher plants.

Cardiolipin and the osmotic stress responses of bacteria

Cells control their own hydration by accumulating solutes when they are exposed to high osmolality media and releasing solutes in response to osmotic down-shocks. Osmosensory transporters mediate solute accumulation and mechanosensitive channels mediate solute release. Escherichia coli serves as a paradigm for studies of cellular osmoregulation. Growth in media of high salinity alters the phospholipid headgroup and fatty acid compositions of bacterial cytoplasmic membranes, in many cases increasing the ratio of anionic to zwitterionic lipid. In E. coli, the proportion of cardiolipin (CL) increases as the proportion of phosphatidylethanolamine (PE) decreases when osmotic stress is imposed with an electrolyte or a non-electrolyte. Osmotic induction of the gene encoding CL synthase (cls) contributes to these changes. The proportion of phosphatidylglycerol (PG) increases at the expense of PE in cls(-) bacteria and, in Bacillus subtilis, the genes encoding CL and PG synthases (clsA and pgsA) are both osmotically regulated. CL is concentrated at the poles of diverse bacterial cells. A FlAsH-tagged variant of osmosensory transporter ProP is also concentrated at E. coli cell poles. Polar concentration of ProP is CL-dependent whereas polar concentration of its paralogue LacY, a H(+)-lactose symporter, is not. The proportion of anionic lipids (CL and PG) modulates the function of ProP in vivo and in vitro. These effects suggest that the osmotic induction of CL synthesis and co-localization of ProP with CL at the cell poles adjust the osmolality range over which ProP activity is controlled by placing it in a CL-rich membrane environment. In contrast, a GFP-tagged variant of mechanosensitive channel MscL is not concentrated at the cell poles but anionic lipids bind to a specific site on each subunit of MscL and influence its function in vitro. The sub-cellular locations and lipid dependencies of other osmosensory systems are not known. Varying CL content is a key element of osmotic adaptation by bacteria but much remains to be learned about its roles in the localization and function of osmoregulatory proteins.

The role of root apoplastic transport barriers in salt tolerance of rice (Oryza sativa L.)

Increasing soil salinity reduces crop yields worldwide, with rice being particularly affected. We have examined the correlation between apoplastic barrier formation in roots, Na+ uptake into shoots and plant survival for three rice (Oryza sativa L.) cultivars of varying salt sensitivity: the salt-tolerant Pokkali, moderately tolerant Jaya and sensitive IR20. Rice plants grown hydroponically or in soil for 1 month were subjected to both severe and moderate salinity stress. Apoplastic barriers in roots were visualized using fluorescence microscopy and their chemical composition determined by gas chromatography and mass spectrometry. Na+ content was estimated by flame photometry. Suberization of apoplastic barriers in roots of Pokkali was the most extensive of the three cultivars, while Na+ accumulation in the shoots was the least. Saline stress induced the strengthening of these barriers in both sensitive and tolerant cultivars, with increase in mRNAs encoding suberin biosynthetic enzymes being detectable within 30 min of stress. Enhanced barriers were detected after several days of moderate stress. Overall, more extensive apoplastic barriers in roots correlated with reduced Na+ uptake and enhanced survival when challenged with high salinity.

Potassium nutrition, sodium toxicity, and calcium signaling: connections through the CBL-CIPK network

Plant roots take up numerous minerals from the soil. Some minerals (e.g., K(+)) are essential nutrients and others (e.g., Na(+)) are toxic for plant growth and development. In addition to the absolute level, the balance among the minerals is critical for their physiological functions. For instance, [K(+)]/[Na(+)] ratio and homeostasis often determine plant growth rate. Either low-K or high-Na in the soil represents a stress condition that severely affects plant life and agricultural production. Earlier observations indicated that higher soil Ca2(+) improve plants growth under low-K or high-Na condition, implying functional interaction among the three cations. Recent studies have begun to delineate the signaling mechanisms underlying such interactions. Either low-K(+) or high-Na(+) can trigger cellular Ca2(+) changes that lead to activation of complex signaling networks. One such network consists of Ca2(+) sensor proteins (e.g., CBLs) interacting with their target kinases (CIPKs). The CBL-CIPK signaling modules interact with and regulate the activity of a number of transporting proteins involved in the uptake and translocation of K(+) and Na(+), maintaining the "balance" of these cations in plants under stress conditions.

Iron deficiency tolerance traits in wild (Hordeum maritimum) and cultivated barley (Hordeum vulgare)

Phytosiderophores (PS) are Fe(III)-solubilizing compounds released by Poaceae roots under iron deficiency conditions. Several studies focused on the capacity of these plants to secrete PS as a center of their iron deficiency tolerance, and little information is available on other traits such as root/shoot biomass ratios, iron use efficiency, photosynthetic activity, and iron mobilization capacity that might also contribute to iron deficiency tolerance. In this study, we evaluated some traits other than PS release capacity that could be responsible for differences in iron deficiency tolerance in two barley species, Hordeum maritimum and Hordeum vulgare. Results showed that under iron starvation, biomass production was affected in both species, but H. maritimum kept higher root/shoot ratios due to the distribution efficiency of carbohydrates within the plant and the growth flexibility of its organs. Both species responded to iron starvation by an early release of PS, but they differed in their secretion capacity. In cultivated barley, the PS release rate was 1.5-2-fold higher than that of wild barley. This behavior was also concomitant with no modification in shoot iron concentration of the latter, which may lead to a low stimulation of its PS release as compared to the former. The amount of Fe(3+) mobilized by root exudates was determined at different pH values (between 5.6 and 8.6). Results showed a decrease in the mobilization capacity with the increasing pH, mainly in H. vulgare. At 8.6, it was reduced by 50% in H. vulgare and 30% in H. maritimum. These data suggest that differences in Poaceae tolerance to iron deficiency is attributed not only to PS secretion capacity, but also to carbohydrate distribution within the plant, Fe use efficiency, and root exudates capacity to mobilize Fe(III).

Maintenance of stress related transcripts in tolerant cultivar at a level higher than sensitive one appears to be a conserved salinity response among plants

Response of plants towards salinity is multigenic in nature with its various components playing diverse roles in stress perception, relay or response. For the purpose of dissecting the genetic determinants of salinity response in crops, the family Brassicaceae presents an excellent model since significant inter-and intra-specific variations have been reported for salinity tolerance. Using these intraspecific variations of Brassica, we show that one of the possible mechanism by which a genotype is able to exhibit tolerance better than another is by keeping the basal levels of stress responsive transcripts higher than the sensitive genotype. This is quite reflected when we analyze members of a specific pathway such as SOS pathway or even when we extend the analysis to a range of molecules including those playing important role in stress perception, signal transduction or stress response. However, these investigations need to be extended to genome level transcript analysis to further validate the hypothesis of "well preparedness" in tolerant genotypes and we propose the suitability of Brassica genotypes for this endevours.

High external phosphate (Pi) increases sodium ion uptake and reduces salt tolerance of 'Pi-tolerant' soybean

Previous studies on the interaction between environmental inorganic phosphate (Pi) and salinity stress using soybean cultivars sensitive to high external Pi had two limitations: (1) the phenotype was dominated by overaccumulation of phosphorus (P); and (2) no detailed analysis was performed for sodium ion uptake. In this study, we focused on the effects of high external Pi on the sodium ion uptake in 'Pi-tolerant' soybean cultivars. The P accumulation in Pi-tolerant soybean Union was much lower [9.0 mg g(-)(1) dry weight (DW); contrasting to 38-76 mg g(-)(1) DW in the 'Pi-sensitive' soybean cultivars]. At in planta level, high level of external Pi significantly (P < 0.001) increased net sodium ion uptake and aggravated salinity stress symptoms. The effects of high external Pi diminished when de-rooted plants were used, suggesting that root is the primary organ interacting with Pi in the growth medium. Two-cell models, including soybean suspension cells and the tobacco Bright Yellow-2 cell line, were also employed to study the effects of high external Pi at the cellular level. Consistent to in planta results, high external Pi uplifted cellular sodium ion uptake and reduced cell viability under salinity stress. Gene expression analyses further showed that HPi (2 mM Pi supplements; excessive level of Pi) could reduce the fold of induction of GmSOS1 and GmCNGC under salinity stress, suggesting that they may be possible molecular targets involved in the interaction between high external Pi and Na(+) uptake.

Ion flux profiles and plant ion homeostasis control under salt stress

The ability of a plant to maintain an ionic homeostasis is crucial in plant salt tolerance. Direct evidence based on data from the non-invasive measurement of ion fluxes would not only offer new insight about the function of the transporter but also provide a whole plant approach for dissecting salt adaptation mechanisms. Here, we review some reports using the ion-selective microelectrodes to characterize the net ion fluxes of tissues or cells.

A novel protein phosphorylation pathway involved in osmotic-stress response in tobacco plants

Osmotic stress is one of the severest environmental pressures for plants, commonly occurring under natural growing condition due to drought, salinity, cold and wounding. Plants sensitively respond to these stresses by activating a set of genes, which encode proteins necessary to overcome the crises. We screened such genes from tobacco plants, and identified a particular clone, which encoded a 45 kDa protein kinase belonging to the plant receptor-like cytoplasmic protein kinase class-VII, NAK (novel Arabidopsis protein kinase) group. The clone was consequently designated as NtNAK (Nicotiana tabacum NAK, accession number: DQ447159). GFP-NtNAK fusion protein was localized in both cytoplasm and nucleus, and bacterially expressed NtNAK exhibited in vitro kinase activity. Its transcripts were clearly induced upon treatments of leaves with salt, mannitol, low temperature and also with abscisic and jasmonic acids and ethylene. These properties indicated NtNAK to be a typical osmo-stress-responsive protein kinase. Its target protein(s) were then screened by the yeast two-hybrid system, and one clone encoding a 32 kDa protein was identified. The protein resembled a potato stress-responsive protein CK251806, and designated as NtCK25 (accession number: DQ448851). Bacterially expressed NtCK25 was phosphorylated by NtNAK, and NtCK25-GFP fusion protein was exclusively localized in nucleus. The structure of NtCK25 was found to be similar to a human nuclear body protein, SP110, which is involved in DNA/protein binding regulation. This suggested that, perceiving osmo-stress signal, NtNAK phosphorylates and activates NtCK25, which might function in regulation of nucleus function. The present study thus suggests that NtNAK/NtCK25 constitutes a novel phosphorylation pathway for osmotic-stress response in plants.

Plant NHX cation/proton antiporters

Although physiological and biochemical data since long suggested that Na(+)/H(+) and K(+)/H(+) antiporters are involved in intracellular ion and pH regulation in plants, it has taken a long time to identify genes encoding antiporters that could fulfil these roles. Genome sequencing projects have now shown that plants contain a very large number of putative Cation/Proton antiporters, the function of which is only beginning to be studied. The intracellular NHX transporters constitute the first Cation/Proton exchanger family studied in plants. The founding member, AtNHX1, was identified as an important salt tolerance determinant and suggested to catalyze Na(+) accumulation in vacuoles. It is, however, becoming increasingly clear, that this gene and other members of the family also play crucial roles in pH regulation and K(+) homeostasis, regulating processes from vesicle trafficking and cell expansion to plant development.

The transgene pyramiding tobacco with betaine synthesis and heterologous expression of AtNHX1 is more tolerant to salt stress than either of the tobacco lines with betaine synthesis or AtNHX1

Previous studies have shown that the overexpression of betA (encoding choline dehydrogenase from Escherichia coli) or AtNHX1 (a vacuolar Na(+)/H(+) antiport from Arabidopsis thaliana) gene can improve the salt tolerance of transgenic plants. However, little is known about the effects of the transgene pyramiding of betA and AtNHX1. Here, betA + AtNHX1 transgene pyramiding tobacco was produced by sexual crossing, and the salt tolerance was evaluated at the cellular and plant levels. In NaCl stress, the Na(+) concentration in vacuoles and vacuolar membrane potential of transgene pyramiding cells were similar to those of AtNHX1-transgenics, and much higher than those of betA-transgenics when detected using fluorescent dye staining; transgene pyramiding cells showed a higher protoplast viability and comparable mitochondrial activity as compared with single transgenics; and transgene pyramiding plants showed comparable Na(+) content in leaves as compared with AtNHX1-transgenics and remarkably higher than betA-transgenics; and transgene pyramiding lines exhibited higher percentage of seed germination, better seedling growth and higher fresh weight than lines that had betA or AtNHX1 alone. Based on the integrative analysis of salt tolerance, the consistency between the cellular level and the whole plant level was confirmed and the transgene pyramiding plants exhibited improved salt tolerance, but compared with the plants with betA or AtNHX1 alone, the differences were relatively small. Other mechanisms involved in salt tolerance should be considered to further enhance transgene pyramiding plants salt tolerance.

Quantifying the three main components of salinity tolerance in cereals

Salinity stress is a major factor inhibiting cereal yield throughout the world. Tolerance to salinity stress can be considered to contain three main components: Na(+) exclusion, tolerance to Na(+) in the tissues and osmotic tolerance. To date, most experimental work on salinity tolerance in cereals has focused on Na(+) exclusion due in part to its ease of measurement. It has become apparent, however, that Na(+) exclusion is not the sole mechanism for salinity tolerance in cereals, and research needs to expand to study osmotic tolerance and tissue tolerance. Here, we develop assays for high throughput quantification of Na(+) exclusion, Na(+) tissue tolerance and osmotic tolerance in 12 Triticum monococcum accessions, mainly using commercially available image capture and analysis equipment. We show that different lines use different combinations of the three tolerance mechanisms to increase their total salinity tolerance, with a positive correlation observed between a plant's total salinity tolerance and the sum of its proficiency in Na(+) exclusion, osmotic tolerance and tissue tolerance. The assays developed in this study can be easily adapted for other cereals and used in high throughput, forward genetic experiments to elucidate the molecular basis of these components of salinity tolerance.

Membrane transporters and carbon metabolism implicated in chloride homeostasis differentiate salt stress responses in tolerant and sensitive Citrus rootstocks

Salinity tolerance in Citrus is strongly related to leaf chloride accumulation. Both chloride homeostasis and specific genetic responses to Cl(-) toxicity are issues scarcely investigated in plants. To discriminate the transcriptomic network related to Cl(-) toxicity and salinity tolerance, we have used two Cl(-) salt treatments (NaCl and KCl) to perform a comparative microarray approach on two Citrus genotypes, the salt-sensitive Carrizo citrange, a poor Cl(-) excluder, and the tolerant Cleopatra mandarin, an efficient Cl(-) excluder. The data indicated that Cl(-) toxicity, rather than Na(+) toxicity and/or the concomitant osmotic perturbation, is the primary factor involved in the molecular responses of citrus plant leaves to salinity. A number of uncharacterized membrane transporter genes, like NRT1-2, were differentially regulated in the tolerant and the sensitive genotypes, suggesting its potential implication in Cl(-) homeostasis. Analyses of enriched functional categories showed that the tolerant rootstock induced wider stress responses in gene expression while repressing central metabolic processes such as photosynthesis and carbon utilization. These features were in agreement with phenotypic changes in the patterns of photosynthesis, transpiration, and stomatal conductance and support the concept that regulation of transpiration and its associated metabolic adjustments configure an adaptive response to salinity that reduces Cl(-) accumulation in the tolerant genotype.

NaCl-induced alternations of cellular and tissue ion fluxes in roots of salt-resistant and salt-sensitive poplar species

Using the scanning ion-selective electrode technique, fluxes of H+, Na+, and Cl- were investigated in roots and derived protoplasts of salt-tolerant Populus euphratica and salt-sensitive Populus popularis 35-44 (P. popularis). Compared to P. popularis, P. euphratica roots exhibited a higher capacity to extrude Na+ after a short-term exposure to 50 mM NaCl (24 h) and a long term in a saline environment of 100 mM NaCl (15 d). Root protoplasts, isolated from the long-term-stressed P. euphratica roots, had an enhanced Na+ efflux and a correspondingly increased H+ influx, especially at an acidic pH of 5.5. However, the NaCl-induced Na+/H+ exchange in root tissues and cells was inhibited by amiloride (a Na+/H+ antiporter inhibitor) or sodium orthovanadate (a plasma membrane H+-ATPase inhibitor). These results indicate that the Na+ extrusion in stressed P. euphratica roots is the result of an active Na+/H+ antiport across the plasma membrane. In comparison, the Na+/H+ antiport system in salt-stressed P. popularis roots was insufficient to exclude Na+ at both the tissue and cellular levels. Moreover, salt-treated P. euphratica roots retained a higher capacity for Cl- exclusion than P. popularis, especially during a long term in high salinity. The pattern of NaCl-induced fluxes of H+, Na+, and Cl- differs from that caused by isomotic mannitol in P. euphratica roots, suggesting that NaCl-induced alternations of root ion fluxes are mainly the result of ion-specific effects.

A novel plant leucine-rich repeat receptor kinase regulates the response of Medicago truncatula roots to salt stress

In plants, a diverse group of cell surface receptor-like protein kinases (RLKs) plays a fundamental role in sensing external signals to regulate gene expression. Roots explore the soil environment to optimize their growth via complex signaling cascades, mainly analyzed in Arabidopsis thaliana. However, legume roots have significant physiological differences, notably their capacity to establish symbiotic interactions. These major agricultural crops are affected by environmental stresses such as salinity. Here, we report the identification of a leucine-rich repeat RLK gene, Srlk, from the legume Medicago truncatula. Srlk is rapidly induced by salt stress in roots, and RNA interference (RNAi) assays specifically targeting Srlk yielded transgenic roots whose growth was less inhibited by the presence of salt in the medium. Promoter-beta-glucuronidase fusions indicate that this gene is expressed in epidermal root tissues in response to salt stress. Two Srlk-TILLING mutants also failed to limit root growth in response to salt stress and accumulated fewer sodium ions than controls. Furthermore, early salt-regulated genes are downregulated in Srlk-RNAi roots and in the TILLING mutant lines when submitted to salt stress. We propose a role for Srlk in the regulation of the adaptation of M. truncatula roots to salt stress.

Learning from evolution: Thellungiella generates new knowledge on essential and critical components of abiotic stress tolerance in plants

Thellungiella salsuginea (halophila) is a close relative of Arabidopsis thaliana but, unlike A. thaliana, it grows well in extreme conditions of cold, salt, and drought as well as nitrogen limitation. Over the last decade, many laboratories have started to use Thellungiella to investigate the physiological, metabolic, and molecular mechanisms of abiotic stress tolerance in plants, and new knowledge has been gained in particular with respect to ion transport and gene expression. The advantage of Thellungiella over other extremophile model plants is that it can be directly compared with Arabidopsis, and therefore generate information on both essential and critical components of stress tolerance. Thellungiella research is supported by a growing body of technical resources comprising physiological and molecular protocols, ecotype collections, expressed sequence tags, cDNA-libraries, microarrays, and a pending genome sequence. This review summarizes the current state of knowledge on Thellungiella and re-evaluates its usefulness as a model for research into plant stress tolerance.

Salt stress-induced alterations in the root proteome of barley genotypes with contrasting response towards salinity

In addition to drought and extreme temperatures, soil salinity represents a growing threat to crop productivity. Among the cereal crops, barley is considered as notably salt tolerant, and cultivars show considerable variation for tolerance towards salinity stress. In order to unravel the molecular mechanisms underlying salt stress tolerance and to utilize the natural genetic variation of barley accessions, a series of hydroponics-based salinity stress experiments was conducted using two genetic mapping parents, cvs Steptoe and Morex, which display contrasting levels of salinity tolerance. The proteome of roots from both genotypes was investigated as displayed by two-dimensional gel electrophoresis, and comparisons were made between plants grown under non-saline and saline conditions. Multivariate analysis of the resulting protein patterns revealed cultivar-specific and salt stress-responsive protein expression. Mass spectrometry-based identification was successful for 26 out of 39 selected protein spots. Hierarchical clustering was applied to detect similar protein expression patterns. Among those, two proteins involved in the glutathione-based detoxification of reactive oxygen species (ROS) were more abundant in the tolerant genotype, while proteins involved in iron uptake were expressed at a higher level in the sensitive one. This study emphasizes the role of proteins involved in ROS detoxification during salinity stress, and identified potential candidates for increasing salt tolerance in barley.

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 mM NaCl) on protective enzyme activities under ambient (350 micromol mol(-1)) and elevated (700 micromol mol(-1)) CO(2) concentrations were investigated in two barley cultivars (Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO(2), upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO(2) alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO(2) and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO(2) mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.

Activity of tonoplast proton pumps and Na+/H+ exchange in potato cell cultures is modulated by salt

The efficient exclusion of excess Na from the cytoplasm and vacuolar Na(+) accumulation are the main mechanisms for the adaptation of plants to salt stress. This is typically carried out by transmembrane transport proteins that exclude Na(+) from the cytosol in exchange for H(+), a secondary transport process which is energy-dependent and driven by the proton-motive force generated by plasma-membrane and tonoplast proton pumps. Tonoplast enriched-vesicles from control and 150 mM NaCl-tolerant calli lines were used as a model system to study the activity of V-H(+)-PPase and V-H(+)-ATPase and the involvement of Na(+) compartmentalization into the vacuole as a mechanism of salt tolerance in Solanum tuberosum. Both ATP- and pyrophosphate (PP(i))-dependent H(+)-transport were higher in tonoplast vesicles from the salt-tolerant line than in vesicles from control cells. Western blotting of tonoplast proteins confirmed that changes in V-H(+)-PPase activity are correlated with increased protein amount. Conversely, immunodetection of the A-subunit of V-H(+)-ATPase revealed that a mechanism of post-translational regulation is probably involved. Na(+)-dependent dissipation of a pre-established pH gradient was used to measure Na(+)/H(+) exchange in tonoplast vesicles. The initial rates of proton efflux followed Michaelis-Menten kinetics and the V(max) of proton dissipation was 2-fold higher in NaCl-tolerant calli when compared to the control. H(+)-coupled exchange was specific for Na(+) and Li(+) and not for K(+). The increase of both the pH gradient across the tonoplast and the Na(+)/H(+) antiport activity in response to salt strongly suggests that Na(+) sequestration into the vacuole contributes to salt tolerance in potato.

Cloning of a high-affinity K+ transporter gene PutHKT2;1 from Puccinellia tenuiflora and its functional comparison with OsHKT2;1 from rice in yeast and Arabidopsis

A high-affinity K+ transporter PutHKT2;1 cDNA was isolated from the salt-tolerant plant Puccinellia tenuiflora. Expression of PutHKT2;1 was induced by both 300 mM NaCl and K+-starvation stress in roots, but only slightly regulated by those stresses in shoots. PutHKT2;1 transcript levels in 300 mM NaCl were doubled by the depletion of potassium. Yeast transformed with PutHKT2;1, like those transformed with PhaHKT2;1 from salt-tolerant reed plants (Phragmites australis), (i) were able to take up K+ in low K+ concentration medium or in the presence of NaCl, and (ii) were permeable to Na+. This suggests that PutHKT2;1 has a high affinity K+-Na+ symport function in yeast. Arabidopsis over-expressing PutHKT2;1 showed increased sensitivities to Na+, K+, and Li+, while Arabidopsis over-expressing OsHKT2;1 from rice (Oryza sativa) showed increased sensitivity only to Na+. In contrast to OsHKT2;1, which functions in Na+-uptake at low external K+ concentrations, PutHKT2;1 functions in Na+-uptake at higher external K+ concentrations. These results show that the modes of action of PutHKT2;1 in transgenic yeast and Arabidopsis differ from the mode of action of the closely related OsHKT2;1 transporter.

Tolerance of Hordeum marinum accessions to O2 deficiency, salinity and these stresses combined

BACKGROUND AND AIMS

When root-zone O(2) deficiency occurs together with salinity, regulation of shoot ion concentrations is compromised even more than under salinity alone. Tolerance was evaluated amongst 34 accessions of Hordeum marinum, a wild species in the Triticeae, to combined salinity and root-zone O(2) deficiency. Interest in H. marinum arises from the potential to use it as a donor for abiotic stress tolerance into wheat.

METHODS

Two batches of 17 H. marinum accessions, from (1) the Nordic Gene Bank and (2) the wheat belt of Western Australia, were exposed to 0.2 or 200 mol m(-3) NaCl in aerated or stagnant nutrient solution for 28-29 d. Wheat (Triticum aestivum) was included as a sensitive check species. Growth, root porosity, root radial O(2) loss (ROL) and leaf ion (Na(+), K(+), Cl(-)) concentrations were determined.

KEY RESULTS

Owing to space constraints, this report is focused mainly on the accessions from the Nordic Gene Bank. The 17 accessions varied in tolerance; relative growth rate was reduced by 2-38 % in stagnant solution, by 8-42 % in saline solution (aerated) and by 39-71 % in stagnant plus saline treatment. When in stagnant solution, porosity of adventitious roots was 24-33 %; salinity decreased the root porosity in some accessions, but had no effect in others. Roots grown in stagnant solution formed a barrier to ROL, but variation existed amongst accessions in apparent barrier 'strength'. Leaf Na(+) concentration was 142-692 micromol g(-1) d. wt for plants in saline solution (aerated), and only increased to 247-748 micromol g(-1) d. wt in the stagnant plus saline treatment. Leaf Cl(-) also showed only small effects of stagnant plus saline treatment, compared with saline alone. In comparison with H. marinum, wheat was more adversely affected by each stress alone, and particularly when combined; growth reductions were greater, adventitious root porosity was 21 %, it lacked a barrier to ROL, leaf K(+) declined to lower levels, and leaf Na(+) and Cl(-) concentrations were 3.1-9-fold and 2.8-6-fold higher, respectively, in wheat.

CONCLUSIONS

Stagnant treatment plus salinity reduced growth more than salinity alone, or stagnant alone, but some accessions of H. marinum were still relatively tolerant of these combined stresses, maintaining Na(+) and Cl(-) 'exclusion' even in an O(2)-deficient, saline rooting medium.

Cell-specific localization of Na+ in roots of durum wheat and possible control points for salt exclusion

Sodium exclusion from leaves is an important mechanism for salt tolerance in durum wheat. To characterize possible control points for Na(+) exclusion, quantitative cryo-analytical scanning electron microscopy was used to determine cell-specific ion profiles across roots of two durum wheat genotypes with contrasting rates of Na(+) transport from root to shoot grown in 50 mm NaCl. The Na(+) concentration in Line 149 (low transport genotype) declined across the cortex, being highest in the epidermal and sub-epidermal cells (48 mm) and lowest in the inner cortical cells (22 mm). Na(+) was high in the pericycle (85 mm) and low in the xylem parenchyma (34 mm). The Na(+) profile in Tamaroi (high transport genotype) had a similar trend but with a high concentration (130 mm) in the xylem parenchyma. The K(+) profiles were generally inverse to those of Na(+). Chloride was only detected in the epidermis. These data suggest that the epidermal and cortical cells removed most of the Na(+) and Cl(-) from the transpiration stream before it reached the endodermis, and that the endodermis is not the control point for salt uptake by the plant. The pericycle as well as the xylem parenchyma may be important in the control of net Na(+) loading of the xylem.

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 micromol g (root FW)(-1) h(-1)], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only approximately 50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional 'leak-pump' model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes.

NaCl-induced changes in cytosolic free Ca2+ in Arabidopsis thaliana are heterogeneous and modified by external ionic composition

Increases in cytosolic free Ca(2+) ([Ca(2+)](cyt)) are common to many stress-activated signalling pathways, including the response to saline environments. We have investigated the nature of NaCl-induced [Ca(2+)](cyt) signals in whole Arabidopsis thaliana seedlings using aequorin. We found that NaCl-induced increases in [Ca(2+)](cyt) are heterogeneous and mainly restricted to the root. Both the concentration of NaCl and the composition of the solution bathing the root have profound effects on the magnitude and dynamics of NaCl-induced increases in [Ca(2+)](cyt). Alteration of external K(+) concentration caused changes in the temporal and spatial pattern of [Ca(2+)](cyt) increase, providing evidence for Na(+)-induced Ca(2+) influx across the plasma membrane. The effects of various pharmacological agents on NaCl-induced increases in [Ca(2+)](cyt) indicate that NaCl may induce influx of Ca(2+) through both plasma membrane and intracellular Ca(2+)-permeable channels. Analysis of spatiotemporal [Ca(2+)](cyt) dynamics using photon-counting imaging revealed additional levels of complexity in the [Ca(2+)](cyt) signal that may reflect the oscillatory nature of NaCl-induced changes in single cells.

Changes in physiological indicators associated with salt tolerance in two contrasting cashew rootstocks

In order to identify salt tolerance indicators, several physiological variables were evaluated in two contrasting cashew (Anacardium occidentale L.) rootstocks in response to salt stress. The tolerant CCP 09 genotype showed better growth performance after two weeks under a large range of NaCl salinity (50, 100, 150 and 200 mM). The NaCl treatments induced a significant drop in transpiration as a consequence of an increased stomatal resistance in both genotypes. No significant differences in Na+, Cl, and K+ concentrations were found in both roots and leaves regardless of rootstocks. The tolerant genotype exhibited lower relative water content and less negative leaf osmotic potential as compared with the sensitive genotype and, therefore, these variables could not be related to salt tolerance. Salt stress caused more significant changes in protein and amino acid concentrations in roots than in leaves. Among the physiological indicators, leaf membrane damage was closely associated with the differences in salt tolerance between the two cashew genotypes. Furthermore, under NaCl salinity the tolerant rootstock showed greater ability to accumulate compatible organic solutes (amino acids, proline and soluble sugars) in leaves in addition to maintaining the soluble sugar concentration in roots as compared with the sensitive rootstock.

Salinity tolerance in halophytes

Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mm NaCl or more, constitute about 1% of the world's flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na+, K+ and Cl- and the synthesis of organic 'compatible' solutes, even where salt glands are operative. Although there is evidence that different species may utilize different transporters in their accumulation of Na+, in general little is known of the proteins and regulatory networks involved. Consequently, it is not yet possible to assign molecular mechanisms to apparent differences in rates of Na+ and Cl- uptake, in root-to-shoot transport (xylem loading and retrieval), or in net selectivity for K+ over Na+. At the cellular level, H+-ATPases in the plasma membrane and tonoplast, as well as the tonoplast H+-PPiase, provide the trans-membrane proton motive force used by various secondary transporters. The widespread occurrence, taxonomically, of halophytes and the general paucity of information on the molecular regulation of tolerance mechanisms persuade us that research should be concentrated on a number of 'model' species that are representative of the various mechanisms that might be involved in tolerance.

Mechanisms of salinity tolerance

The physiological and molecular mechanisms of tolerance to osmotic and ionic components of salinity stress are reviewed at the cellular, organ, and whole-plant level. Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na(+) or Cl() exclusion, and the tolerance of tissue to accumulated Na(+) or Cl(). Our understanding of the role of the HKT gene family in Na(+) exclusion from leaves is increasing, as is the understanding of the molecular bases for many other transport processes at the cellular level. However, we have a limited molecular understanding of the overall control of Na(+) accumulation and of osmotic stress tolerance at the whole-plant level. Molecular genetics and functional genomics provide a new opportunity to synthesize molecular and physiological knowledge to improve the salinity tolerance of plants relevant to food production and environmental sustainability.

Hormonal changes in relation to biomass partitioning and shoot growth impairment in salinized tomato (Solanum lycopersicum L.) plants

Following exposure to salinity, the root/shoot ratio is increased (an important adaptive response) due to the rapid inhibition of shoot growth (which limits plant productivity) while root growth is maintained. Both processes may be regulated by changes in plant hormone concentrations. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated hydroponically for 3 weeks under high salinity (100 mM NaCl) and five major plant hormones (abscisic acid, ABA; the cytokinins zeatin, Z, and zeatin-riboside, ZR; the auxin indole-3-acetic acid, IAA; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, ACC) were determined weekly in roots, xylem sap, and leaves. Salinity reduced shoot biomass by 50-60% and photosynthetic area by 20-25% both by decreasing leaf expansion and delaying leaf appearance, while root growth was less affected, thus increasing the root/shoot ratio. ABA and ACC concentrations strongly increased in roots, xylem sap, and leaves after 1 d (ABA) and 15 d (ACC) of salinization. By contrast, cytokinins and IAA were differentially affected in roots and shoots. Salinity dramatically decreased the Z+ZR content of the plant, and induced the conversion of ZR into Z, especially in the roots, which accounted for the relative increase of cytokinins in the roots compared to the leaf. IAA concentration was also strongly decreased in the leaves while it accumulated in the roots. Decreased cytokinin content and its transport from the root to the shoot were probably induced by the basipetal transport of auxin from the shoot to the root. The auxin/cytokinin ratio in the leaves and roots may explain both the salinity-induced decrease in shoot vigour (leaf growth and leaf number) and the shift in biomass allocation to the roots, in agreement with changes in the activity of the sink-related enzyme cell wall invertase.