Mechanisms of salinity tolerance

Elemental profiles reflect plant adaptations to the environment

Most mineral elements found in plant tissues come exclusively from the soil, necessitating that plants adapt to highly variable soil compositions to survive and thrive. Profiling element concentrations in genetically diverse plant populations is providing insights into the plant-environment interactions that control elemental accumulation, as well as identifying the underlying genes. The resulting molecular understanding of plant adaptation to the environment both demonstrates how soils can shape genetic diversity and provides solutions to important agricultural challenges.

Proteomic study participating the enhancement of growth and salt tolerance of bottle gourd rootstock-grafted watermelon seedlings

An insertion grafting technique to do research on salt tolerance was applied using watermelon (Citrullus lanatus [Thunb.] Mansf. cv. Xiuli) as a scion and bottle gourd (Lagenaria siceraria Standl. cv. Chaofeng Kangshengwang) as a rootstock. Rootstock-grafting significantly relieved the inhibition of growth and photosynthesis induced by salt stress in watermelon plants. Proteomic analysis revealed 40 different expressed proteins in response to rootstock-grafting and/or salt stress. These proteins were involved in Calvin cycle, amino acids biosynthesis, carbohydrate and energy metabolism, ROS defense, hormonal biosynthesis and signal transduction. Most of these proteins were up-regulated by rootstock-grafting and/or susceptible to salt stress. The enhancement of the metabolic activities of Calvin cycle, biosynthesis of amino acids, carotenoids and peroxisomes, glycolytic pathway and tricarboxylic acid cycle will probably contribute to intensify the biomass and photosynthetic capacity in rootstock-grafted seedlings under condition without salt. The accumulation of key enzymes included in these biological processes described above seems to play an important role in the enhancement of salt tolerance of rootstock-grafted seedlings. Furthermore, leucine-rich repeat transmembrane protein kinase and phospholipase may be involved in transmitting the internal and external stimuli induced by grafting and/or salt stress.

Effect of salt stress on genes encoding translation-associated proteins in Arabidopsis thaliana

Salinity negatively affects plant growth and disturbs chloroplast integrity. Here, we aimed at identifying salt-responsive translation-related genes in Arabidopsis thaliana with an emphasis on those encoding plastid-located proteins. We used quantitative real-time PCR to test the expression of 170 genes after short-term salt stress (up to 24 h) and identified several genes affected by the stress including: PRPL11, encoding plastid ribosomal protein L11, ATAB2, encoding a chloroplast-located RNA-binding protein presumably functioning as an activator of translation, and PDF1B, encoding a peptide deformylase involved in N-formyl group removal from nascent proteins synthesized in chloroplasts. These genes were previously shown to have important functions in chloroplast biology and may therefore represent new targets for biotechnological optimization of salinity tolerance.

HbCIPK2, a novel CBL-interacting protein kinase from halophyte Hordeum brevisubulatum, confers salt and osmotic stress tolerance

Protein kinases play an important role in regulating the response to abiotic stress in plant. CIPKs are plant-specific signal transducers, and some members have been identified. However, the precise functions of novel CIPKs still remain unknown. Here we report that HbCIPK2 is a positive regulator of salt and osmotic stress tolerance. HbCIPK2 was screened out of the differentially expressed fragments from halophyte Hordeum brevisubulatum by cDNA-AFLP technique, and was a single-copy gene without intron. Expression of HbCIPK2 was increased by salt, drought and ABA treatment. HbCIPK2 is mainly localized to the plasma membrane and nucleus. Ectopic expression of 35S:HbCIPK2 not only rescued the salt hypersensitivity in Arabidopsis mutant sos2-1, but also enhanced salt tolerance in Arabidopsis wild type, and exhibited tolerance to osmotic stress during germination. The HbCIPK2 contributed to the ability to prevent K(+) loss in root and to accumulate less Na(+) in shoot resulting in K(+) /Na(+) homeostasis and protection of root cell from death, which is consistent with the gene expression profile of HbCIPK2-overexpressing lines. These findings imply possible novel HbCIPK2-mediated salt signalling pathways or networks in H. brevisubulatum.

Sugars and acid invertase mediate the physiological response of Schenkia spicata root cultures to salt stress

A heterotrophic model system was established in our studies in order to differentiate the effect of high salt concentrations in external medium on growth and sugar metabolism in roots from the effect of reduced sugar availability resulting from decreased photosynthesis under salinity. Soluble sugar content and the activity of acid invertase in root cultures of salt-tolerant (ST) and salt-sensitive (SS) Schenkia spicata (L.) Mansion genotypes were investigated during exposure to different NaCl concentrations (0-200 mM). Their response to severe salinity was characterized by a metabolic adjustment that led to the accumulation of sucrose (Suc) in root tissues. There was clear evidence that cell wall invertase (CW-Inv) is the major contributor to the Suc/hexose ratio in roots during exposure to elevated salinity. The results of CW-Inv activity and immunodetection assays in our study suggest that the regulation of CW-Inv expression is most likely achieved in a salt stress dependent manner. Also, NaCl modulated soluble acid invertase (SA-Inv) expression differentially in SS and ST genotypes of S. spicata. Regardless of the salt treatment, genotype, or the amount of enzyme, SA-Inv activity was generally low, indicating regulation at the posttranslational level. The results suggest no direct role of SA-Inv in the regulation of the root tissue carbohydrate pool and therefore in the control of the availability of glucose and fructose for the primary metabolism and/or osmotic adjustment in the present heterotrophic model system.

Salt-induced chloroplast protrusion is the process of exclusion of ribulose-1,5-bisphosphate carboxylase/oxygenase from chloroplasts into cytoplasm in leaves of rice

Chloroplast protrusions (CPs) are often observed under environmental stresses, but their role has not been elucidated. The formation of CPs was observed in the leaf of rice plants treated with 75 mm NaCl for 14 d. Some CPs were almost separated from the main chloroplast body. In some CPs, inner membrane structures and crystalline inclusions were included. Similar structures surrounded by double membranes were observed in the cytoplasm and vacuole. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) was detected in CPs and the similar structures in the cytoplasm and vacuole. These results suggest that CP is one of the pathways of Rubisco exclusion from chloroplasts into the cytoplasm under salinity, and the exclusions could be transported to vacuole for their degradation.

The rice monovalent cation transporter OsHKT2;4: revisited ionic selectivity

The family of plant membrane transporters named HKT (for high-affinity K(+) transporters) can be subdivided into subfamilies 1 and 2, which, respectively, comprise Na(+)-selective transporters and transporters able to function as Na(+)-K(+) symporters, at least when expressed in yeast (Saccharomyces cerevisiae) or Xenopus oocytes. Surprisingly, a subfamily 2 member from rice (Oryza sativa), OsHKT2;4, has been proposed to form cation/K(+) channels or transporters permeable to Ca(2+) when expressed in Xenopus oocytes. Here, OsHKT2;4 functional properties were reassessed in Xenopus oocytes. A Ca(2+) permeability through OsHKT2;4 was not detected, even at very low external K(+) concentration, as shown by highly negative OsHKT2;4 zero-current potential in high Ca(2+) conditions and lack of sensitivity of OsHKT2;4 zero-current potential and conductance to external Ca(2+). The Ca(2+) permeability previously attributed to OsHKT2;4 probably resulted from activation of an endogenous oocyte conductance. OsHKT2;4 displayed a high permeability to K(+) compared with that to Na(+) (permeability sequence: K(+) > Rb(+) ≈ Cs(+) > Na(+) ≈ Li(+) ≈ NH(4)(+)). Examination of OsHKT2;4 current sensitivity to external pH suggested that H(+) is not significantly permeant through OsHKT2;4 in most physiological ionic conditions. Further analyses in media containing both Na(+) and K(+) indicated that OsHKT2;4 functions as K(+)-selective transporter at low external Na(+), but transports also Na(+) at high (>10 mm) Na(+) concentrations. These data identify OsHKT2;4 as a new functional type in the K(+) and Na(+)-permeable HKT transporter subfamily. Furthermore, the high permeability to K(+) in OsHKT2;4 supports the hypothesis that this system is dedicated to K(+) transport in the plant.

Tolerance responses of Brassica juncea to salinity, alkalinity and alkaline salinity

Soil salinity and alkalinity are common constraints to crop productivity in low rainfall regions of the world. These two stresses have been extensively studied but not the combined stress of alkaline salinity. To examine the effects of mild salinity (50mM NaCl) combined with alkalinity (5mM NaHCO3) on growth of Brassica juncea (L.) Czern., 30 genotypes were grown in hydroponics. Growth of all genotypes was substantially reduced by alkaline salinity after 4 weeks of stress. Based on large genotypic differences, NDR 8501 and Vaibhav were selected as tolerant and Xinyou 5 as highly sensitive for further detailed physiological study. Shoot and root biomass and leaf area of the selected genotypes showed greater reduction under alkaline salinity than salinity or alkalinity alone. Alkalinity alone imposed larger negative effect on growth than salinity. K+ and P concentrations in both shoot and root were significantly reduced by alkaline salinity but small difference existed among the selected genotypes. Leaf Fe concentration in Xinyou 5 decreased under alkaline salinity below a critical level of 50mgkg-1, which explained why more chlorosis and a larger growth reduction occurred than in NDR 8501 and Vaibhav. Relatively large shoot and root Na+ concentration also had additional adverse effect on growth under alkaline salinity. Low tissue K+, P and Fe concentrations by alkalinity were the major factors that reduced growth in the selected genotypes. Growth reduction by salinity was mainly caused by Na+ toxicity. Shoot Na+ concentration of NDR 8501 and Vaibhav was almost half those in Xinyou 5, suggesting NDR 8501 and Vaibhav excluded more Na+. However, Na+ exclusion was reduced by more than 50% under alkaline salinity than salinity in the selected genotypes. In conclusion, our results demonstrated that alkaline salinity reduced uptake of essential nutrients and Na+ exclusion that resulted in more negative consequences on growth than salinity alone.

Comparative transcriptomic analysis of salt adaptation in roots of contrasting Medicago truncatula genotypes

Evolutionary diversity can be driven by the interaction of plants with different environments. Molecular bases involved in ecological adaptations to abiotic constraints can be explored using genomic tools. Legumes are major crops worldwide and soil salinity is a main stress affecting yield in these plants. We analyzed in the Medicago truncatula legume the root transcriptome of two genotypes having contrasting responses to salt stress: TN1.11, sampled in a salty Tunisian soil, and the reference Jemalong A17 genotype. TN1.11 plants show increased root growth under salt stress as well as a differential accumulation of sodium ions when compared to A17. Transcriptomic analysis revealed specific gene clusters preferentially regulated by salt in root apices of TN1.11, notably those related to the auxin pathway and to changes in histone variant isoforms. Many genes encoding transcription factors (TFs) were also differentially regulated between the two genotypes in response to salt. Among those selected for functional studies, overexpression in roots of the A17 genotype of the bHLH-type TF most differentially regulated between genotypes improved significantly root growth under salt stress. Despite the global complexity of the differential transcriptional responses, we propose that an increase in this bHLH TF expression may be linked to the adaptation of M. truncatula to saline soil environments.

Differential expression of ion transporters and aquaporins in leaves may contribute to different salt tolerance in Malus species

Maintaining ion and water homeostasis in plants is an important defense strategy against salinity stress. Divergence in ion homeostasis between the salt-tolerant Malus hupehensis Rehd. and salt-sensitive Malus prunifolia 'yingyehaitang' was studied to understand their mechanisms for tolerance. Compared with the control on Day 15, plants of those two genotypes under high-salinity treatment had less K(+) in the leaves, stems, and roots. Contents were higher in the roots but lower in the leaves of M. hupehensis while levels in the stems were similar to those from M. prunifolia. For both genotypes, the sodium content increased after salinity treatment in all tissue types. However, the leaves from M. hupehensis had less Na(+) and maintained a lower Na(+)/K(+) ratio. To understand the basis for these differences, we studied the ion transporters and regulation of aquaporin transcripts in the leaves. Transcript levels for both MdHKT1 and MdSOS1 were higher in M. hupehensis, implying that this species had better capacity to exclude sodium so that less Na(+) occurred in the leaves but more in the stems. M. hupehensis also had a greater amount of MdNHX1 transcripts, which could have assisted in sequestering excess Na(+) into the vacuoles and sustaining a better cellular environment. A relatively higher level of aquaporin transcript was also found in M. hupehensis, suggesting that those plants were more capable of maintaining a better leaf water status and diluting excess ions effectively under high-salinity conditions. Therefore, these tested transporters may play important roles in determining how salinity tolerance is conferred in Malus species.

Comparative transcriptome analysis of two olive cultivars in response to NaCl-stress

BACKGROUND

Olive (Olea europaea L.) cultivation is rapidly expanding and low quality saline water is often used for irrigation. The molecular basis of salt tolerance in olive, though, has not yet been investigated at a system level. In this study a comparative transcriptomics approach was used as a tool to unravel gene regulatory networks underlying salinity response in olive trees by simulating as much as possible olive growing conditions in the field. Specifically, we investigated the genotype-dependent differences in the transcriptome response of two olive cultivars, a salt-tolerant and a salt-sensitive one.

METHODOLOGY/PRINCIPAL FINDINGS

A 135-day long salinity experiment was conducted using one-year old trees exposed to NaCl stress for 90 days followed by 45 days of post-stress period during the summer. A cDNA library made of olive seedling mRNAs was sequenced and an olive microarray was constructed. Total RNA was extracted from root samples after 15, 45 and 90 days of NaCl-treatment as well as after 15 and 45 days of post-treatment period and used for microarray hybridizations. SAM analysis between the NaCl-stress and the post-stress time course resulted in the identification of 209 and 36 differentially expressed transcripts in the salt-tolerant and salt-sensitive cultivar, respectively. Hierarchical clustering revealed two major, distinct clusters for each cultivar. Despite the limited number of probe sets, transcriptional regulatory networks were constructed for both cultivars while several hierarchically-clustered interacting transcription factor regulators such as JERF and bZIP homologues were identified.

CONCLUSIONS/SIGNIFICANCE

A systems biology approach was used and differentially expressed transcripts as well as regulatory interactions were identified. The comparison of the interactions among transcription factors in olive with those reported for Arabidopsis might indicate similarities in the response of a tree species with Arabidopsis at the transcriptional level under salinity stress.

A broader model for C₄ photosynthesis evolution in plants inferred from the goosefoot family (Chenopodiaceae s.s.)

C(4) photosynthesis is a fascinating example of parallel evolution of a complex trait involving multiple genetic, biochemical and anatomical changes. It is seen as an adaptation to deleteriously high levels of photorespiration. The current scenario for C(4) evolution inferred from grasses is that it originated subsequent to the Oligocene decline in CO(2) levels, is promoted in open habitats, acts as a pre-adaptation to drought resistance, and, once gained, is not subsequently lost. We test the generality of these hypotheses using a dated phylogeny of Amaranthaceae s.l. (including Chenopodiaceae), which includes the largest number of C(4) lineages in eudicots. The oldest chenopod C(4) lineage dates back to the Eocene/Oligocene boundary, representing one of the first origins of C(4) in plants, but still corresponding with the Oligocene decline of atmospheric CO(2). In contrast to grasses, the rate of transitions from C(3) to C(4) is highest in ancestrally drought resistant (salt-tolerant and succulent) lineages, implying that adaptation to dry or saline habitats promoted the evolution of C(4); and possible reversions from C(4) to C(3) are apparent. We conclude that the paradigm established in grasses must be regarded as just one aspect of a more complex system of C(4) evolution in plants in general.

A single locus is responsible for salinity tolerance in a Chinese landrace barley (Hordeum vulgare L.)

Introduction

Salinity and waterlogging are two major abiotic stresses severely limiting barley production. The lack of a reliable screening method makes it very hard to improve the tolerance through breeding programs.

Methods

This work used 188 DH lines from a cross between a Chinese landrace variety, TX9425 (waterlogging and salinity tolerant), and a Japanese malting barley, Naso Nijo (waterlogging and salinity sensitive), to identify QTLs associated with the tolerance.

Results

Four QTLs were found for waterlogging tolerance. The salinity tolerance was evaluated with both a hydroponic system and in potting mixture. In the trial with potting mixture, only one major QTL was identified to associate with salinity tolerance. This QTL explained nearly 50% of the phenotypic variation, which makes it possible for further fine mapping and cloning of the gene. This QTL was also identified in the hydroponic experiment for different salt-related traits. The position of this QTL was located at a similar position to one of the major QTLs for waterlogging tolerance, indicating the possibility of similar mechanisms controlling both waterlogging and salinity tolerance.

Conclusion

The markers associated with the QTL provided a unique opportunity in breeding programs for selection of salinity and waterlogging tolerance.

Studying plant salt tolerance with the voltage clamp technique

Voltage clamp is one of the key techniques for the dissection, identification, and monitoring of ion transporters in plant cells. Voltage clamp-based research work on salinity stress in plants enables the characterization of many plant ATP-dependent pumps, ion channels, and ion-coupled carriers through heterologous expression in Xenopus laevis oocytes and in vivo measurements in salt-tolerant and salt-sensitive giant green algae such as Chara and many plant species. We have modified and developed a reliable set of procedures for voltage clamp analysis in intact guard cells and root epidermal cells from Arabidopsis thaliana with potentially broad applications in the salinity response of plants. These procedures greatly extend the duration of measurements and scope for analysis of the predominant K(+) and anion channels.

Comparative transcriptome profiling of chilling stress responsiveness in two contrasting rice genotypes

Rice is sensitive to chilling stress, especially at the seedling stage. To elucidate the molecular genetic mechanisms of chilling tolerance in rice, comprehensive gene expressions of two rice genotypes (chilling-tolerant LTH and chilling-sensitive IR29) with contrasting responses to chilling stress were comparatively analyzed. Results revealed a differential constitutive gene expression prior to stress and distinct global transcription reprogramming between the two rice genotypes under time-series chilling stress and subsequent recovery conditions. A set of genes with higher basal expression were identified in chilling-tolerant LTH compared with chilling-sensitive IR29, indicating their possible role in intrinsic tolerance to chilling stress. Under chilling stress, the major effect on gene expression was up-regulation in the chilling- tolerant genotype and strong repression in chilling-sensitive genotype. Early responses to chilling stress in both genotypes featured commonly up-regulated genes related to transcription regulation and signal transduction, while functional categories for late phase chilling regulated genes were diverse with a wide range of functional adaptations to continuous stress. Following the cessation of chilling treatments, there was quick and efficient reversion of gene expression in the chilling-tolerant genotype, while the chilling-sensitive genotype displayed considerably slower recovering capacity at the transcriptional level. In addition, the detection of differentially-regulated TF genes and enriched cis-elements demonstrated that multiple regulatory pathways, including CBF and MYBS3 regulons, were involved in chilling stress tolerance. A number of the chilling-regulated genes identified in this study were co-localized onto previously fine-mapped cold-tolerance-related QTLs, providing candidates for gene cloning and elucidation of molecular mechanisms responsible for chilling tolerance in rice.

Ammonium enhances resistance to salinity stress in citrus plants

In this work, we demonstrate that NH₄⁺ nutrition in citrange Carrizo plants acts as an inducer of resistance against salinity conditions. We investigated its mode of action and provide evidence that NH₄⁺ confers resistance by priming abscisic acid and polyamines, and enhances H₂O₂ and proline basal content. Moreover, we observed reduced Cl⁻ uptake as well as enhanced PHGPx expression after salt stress. Control and N-NH₄⁺ plants showed optimal growth. However, N-NH₄⁺ plants displayed greater dry weight and total lateral roots than control plants, but these differences were not observed for primary root length. Our results revealed that N-NH₄⁺ treatment induces a similar phenotypical response to the recent stress-induced morphogenetic response (SIMRs). The hypothesis is that N-NH₄⁺ treatment triggers mild chronic stress in citrange Carrizo plants, which might explain the SIMR observed. Moreover, we observed modulators of stress signaling, such as H₂O₂ in N-NH₄⁺ plants, which could acts as an intermediary between stress and the development of the SIMR phenotype. This observation suggests that NH₄⁺ treatments induce a mild stress condition that primes the citrange Carrizo defense response by stress imprinting and confers protection against subsequent salt stress.

Effects of exogenous proline and glycinebetaine on the salt tolerance of rice cultivars

Salinity significantly increased trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS) uptake and decreased the K(+)/Na(+) ratio in salt-sensitive rice (Nipponbare) but did not markedly in salt-tolerant rice (Pokkali). Proline and glycinebetaine (betaine) suppressed the increase in PTS uptake and the decrease in the K(+)/Na(+) ratio in Nipponbare, but did not affect PTS uptake or the K(+)/Na(+) ratio in Pokkali.

Overexpression of SlSOS2 (SlCIPK24) confers salt tolerance to transgenic tomato

The Ca(2+)-dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na(+) and K(+) under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na(+)/H(+) (SlSOS1) and endosomal-vacuolar K(+), Na(+)/H(+) (LeNHX2 and LeNHX4) antiporters, responsible for Na(+) extrusion out of the root, active loading of Na(+) into the xylem, and Na(+) and K(+) compartmentalization.

Paxillus involutus strains MAJ and NAU mediate K(+)/Na(+) homeostasis in ectomycorrhizal Populus x canescens under sodium chloride stress

Salt-induced fluxes of H(+), Na(+), K(+), and Ca(2+) were investigated in ectomycorrhizal (EM) associations formed by Paxillus involutus (strains MAJ and NAU) with the salt-sensitive poplar hybrid Populus × canescens. A scanning ion-selective electrode technique was used to measure flux profiles in non-EM roots and axenically grown EM cultures of the two P. involutus isolates to identify whether the major alterations detected in EM roots were promoted by the fungal partner. EM plants exhibited a more pronounced ability to maintain K(+)/Na(+) homeostasis under salt stress. The influx of Na(+) was reduced after short-term (50 mm NaCl, 24 h) and long-term (50 mm NaCl, 7 d) exposure to salt stress in mycorrhizal roots, especially in NAU associations. Flux data for P. involutus and susceptibility to Na(+)-transport inhibitors indicated that fungal colonization contributed to active Na(+) extrusion and H(+) uptake in the salinized roots of P. × canescens. Moreover, EM plants retained the ability to reduce the salt-induced K(+) efflux, especially under long-term salinity. Our study suggests that P. involutus assists in maintaining K(+) homeostasis by delivering this nutrient to host plants and slowing the loss of K(+) under salt stress. EM P. × canescens plants exhibited an enhanced Ca(2+) uptake ability, whereas short-term and long-term treatments caused a marked Ca(2+) efflux from mycorrhizal roots, especially from NAU-colonized roots. We suggest that the release of additional Ca(2+) mediated K(+)/Na(+) homeostasis in EM plants under salt stress.

Hydrogeochemistry and water quality assessment of the Kor-Sivand Basin, Fars province, Iran

In order to assess the quality and suitability of waters in the Kor-Sivand river basin, 60 water samples from the Kor river and 90 water samples from wells in the basin were studied. Assessments were based on Piper's and Gibbs' diagrams for water quality, Food and Agricultural Organization's (FAO) guidelines, and US Salinity Laboratory diagram for water suitability. The results showed that the river water is of Ca-HCO(3) type, while well water is of Ca-Cl and Na-Cl type. Based on Gibbs' diagram, the source of soluble ions in the river water samples is the weathering of stones over which water flows, while evaporation was found to be the dominant process in the ion concentration of the well samples. According to the FAO Guidelines, the salinity of surface water for irrigation did not cause great restrictions; however, many of these waters could create potential permeability problems. In the groundwater samples, a high salt concentration is more important than the infiltration problem. Mg hazard values at some sites limit its use for agricultural purposes. One third of the river water samples and two thirds of well waters had more than 50% magnesium. Saturation indices showed that 94% of the analyzed water samples are supersaturated with calcite, aragonite, and dolomite. Based on the US Salinity Laboratory diagram, river water samples were classified as C(2)S(1) and C(3)S(1), while C(4)S(3), C(4)S(4), C(2)S(1), and C(3)S(1) were the most dominant classes in well samples. Some management practices necessary for sustainable development of water resources in the study area were discussed briefly, including appropriate selection of crops, adequate drainage, leaching, blending and cyclic use of saline water, proper irrigation method, and addition of soil amendment.

Organ-dependent oxylipin signature in leaves and roots of salinized tomato plants (Solanum lycopersicum)

Oxylipins have been extensively studied in plant defense mechanisms or as signal molecules. Depending on the stress origin (e.g. wounding, insect, pathogen), and also on the plant species or organ, a specific oxylipin signature can be generated. Salt stress is frequently associated with secondary stress such as oxidative damage. Little is known about the damage caused to lipids under salt stress conditions, especially with respect to oxylipins. In order to determine if an organ-specific oxylipin signature could be observed during salt stress, tomato (Solanum lycopersicum cv. Money Maker) plants were submitted to salt stress (100 mM of NaCl) for a 30-d period. A complete oxylipin profiling and LOX related-gene expression measurement were achieved in leaves and roots. As expected, salt stress provoked premature senescence in leaves, as revealed by a decrease in photosystem II efficiency (F(v)/F(m) ratio) and sodium accumulation in leaves. In roots, a significant decrease in several oxylipins (9- and 13-hydro(pero)xy linole(n)ic acids, keto and divinyl ether derivatives) was initiated at day 5 and intensified at day 21 after salt treatment, whereas jasmonic acid content increased. In leaves, the main changes in oxylipins were observed later (at day 30), with an increase in some 9- and 13-hydro(pero)xy linole(n)ic acids and a decrease in some keto-derivatives and in jasmonic acid. Oxylipin enantiomeric characterization revealed that almost all compounds were formed enzymatically, and therefore a massive auto-oxidation of lipids that can be encountered in abscission processes can be excluded here.

A two-staged model of Na+ exclusion in rice explained by 3D modeling of HKT transporters and alternative splicing

The HKT family of Na(+) and Na(+)/K(+) transporters is implicated in plant salinity tolerance. Amongst these transporters, the cereal HKT1;4 and HKT1;5 are responsible for Na(+) exclusion from photosynthetic tissues, a key mechanism for plant salinity tolerance. It has been suggested that Na(+) is retrieved from the xylem transpiration stream either in the root or the leaf sheath, protecting the leaf blades from excessive Na(+) accumulation. However, direct evidence for this scenario is scarce. Comparative modeling and evaluation of rice (Oryza sativa) HKT-transporters based on the recent crystal structure of the bacterial TrkH K(+) transporter allowed to reconcile transcriptomic and physiological data. For OsHKT1;5, both transcript abundance and protein structural features within the selectivity filter could control shoot Na(+) accumulation in a range of rice varieties. For OsHKT1;4, alternative splicing of transcript and the anatomical complexity of the sheath needed to be taken into account. Thus, Na(+) accumulation in a specific leaf blade seems to be regulated by abundance of a correctly spliced OsHKT1;4 transcript in a corresponding sheath. Overall, allelic variation of leaf blade Na(+) accumulation can be explained by a complex interplay of gene transcription, alternative splicing and protein structure.

Novel durum wheat genes up-regulated in response to a combination of heat and drought stress

We report the effect of heat, drought and combined stress on the expression of a group of genes that are up-regulated under these conditions in durum wheat (Triticum turgidum subsp. durum) plants. Modulation of gene expression was studied by cDNA-AFLP performed on RNAs extracted from flag leaves. By this approach, we identified several novel durum wheat genes whose expression is modulated under different stress conditions. We focused on a group of hitherto undescribed up-regulated genes in durum wheat, among these, 7 are up-regulated by heat, 8 by drought stress, 15 by combined heat and drought stress, 4 are up-regulated by both heat and combined stress, and 3 by both drought and combined stress. The functional characterization of these genes will provide new data that could help the developing of strategies aimed at improving durum wheat tolerance to field stress.

The ethylene response factor AtERF98 enhances tolerance to salt through the transcriptional activation of ascorbic acid synthesis in Arabidopsis

Ascorbic acid (AsA) is an important antioxidant in plants, and its biosynthesis is finely regulated through developmental and environmental cues; however, the regulatory mechanism remains unclear. In this report, the knockout and knockdown mutants of Arabidopsis AtERF98 decreased the AsA level, whereas the overexpression of AtERF98 increased it, which suggests that AtERF98 plays an important role in regulating AsA biosynthesis. AtERF98-overexpressing plants showed enhanced expression of AsA synthesis genes in the d-mannose/l-galactose (d-Man/l-Gal) pathway and the myo-inositol pathway gene MIOX4, as well as of AsA turnover genes. In contrast, AtERF98 mutants showed decreased expression of AsA synthesis genes in the d-Man/l-Gal pathway but not of the myo-inositol pathway gene or AsA turnover genes. In addition, the role of AtERF98 in regulating AsA production was significantly impaired in the d-Man/l-Gal pathway mutant vtc1-1, but the expression of the myo-inositol pathway gene or AsA turnover genes was not affected, which indicates that the regulation of AtERF98 in AsA synthesis is primarily mediated by the d-Man/l-Gal pathway. Transient expression and chromatin immunoprecipitation assays further showed that AtERF98 binds to the promoter of VTC1, which indicates that AtERF98 modulates AsA biosynthesis by directly regulating the expression of the AsA synthesis genes. Moreover, the knockout mutant aterf98-1 displayed decreased salt-induced AsA synthesis and reduced tolerance to salt. The supplementation of exogenous AsA increased the salt tolerance of aterf98-1; coincidently, the enhanced salt tolerance of AtERF98-overexpressing plants was impaired in vtc1-1. Thus, our data provide evidence that the regulation of AtERF98 in AsA biosynthesis contributes to enhanced salt tolerance in Arabidopsis.

Identification of salt treated proteins in sorghum using gene ontology linkage

Sorghum bicolor (L.) is an important crop of arid and semi arid zones with most of its varieties tolerant to drought, heat and salt stress. Functional identification of many salt tolerant proteins has been reported in Arabidopsis, rice and other plants, however only little functional information has been predicted in sorghum till date. A 2-D gel electrophoresis based proteomic approach with MALDI-TOF mass spectrometer was utilized to analyze the salt stress response of sorghum. Major changes in protein complement were observed at 200 mM NaCl in hydroponic culture after 96 h of salt-stress. Highly expressed five proteins were excised for functional identification. We developed shortest path (SP) analysis based method on Gene Ontology (GO) hierarchy using sum of GO-term's semantic similarities. In this study, we observed that majority of expressed proteins belonged to the functional category of energy production and conversion, signal transduction mechanisms and ribosome maturation. These identified functions suggest a distinct mechanism of salt-stress adaptation in sorghum plant. The proposed method in this paper potentially has great importance to further understanding of newly identified proteins that can help in plant development.

Molecular cloning and characterization of plasma membrane- and vacuolar-type Na⁺/H⁺ antiporters of an alkaline-salt-tolerant monocot, Puccinellia tenuiflora

A better understanding of salt tolerance in plants might lead to the genetic engineering of crops that can grow in saline soils. Here we cloned and characterized plasma membrane and vacuolar Na⁺/H⁺ antiporters of a monocotyledonous alkaline-tolerant halophyte, Puccinellia tenuiflora. The predicted amino acid sequence of the transporters were very similar to those of orthologs in monocotyledonous crops. Expression analysis showed that (1) NHA was more strongly induced by NaCl in the roots of P. tenuiflora while in rice it was rather induced in the shoots, suggesting that the role of NHA in salt excretion from the roots partly accounts for the difference in the tolerance of the two species, and that (2) NHXs were specifically induced by NaHCO₃ but not by NaCl in the roots of both species, suggesting that vacuolar-type Na⁺/H⁺ antiporters play roles in pH regulation under alkaline salt conditions. Overexpression of the antiporters resulted in increased tolerance of shoots to NaCl and roots to NaHCO₃. Overexpression lines exhibited a lower Na⁺ content and a higher K⁺ content in shoots under NaCl treatments, leading to a higher Na⁺/H⁺ ratio.

Catalase plays a key role in salt stress acclimation induced by hydrogen peroxide pretreatment in maize

Pretreatment in plants is recognized as a valuable strategy to stimulate plant defenses, leading to better plant development. This study evaluated the effects of H₂O₂ leaf spraying pretreatment on plant growth and investigated the antioxidative mechanisms involved in the response of maize plants to salt stress. It was found that salinity reduced maize seedling growth when compared to control conditions, and H₂O₂ foliar spraying was effective in minimizing this effect. Analysis of the antioxidative enzymes catalase (EC 1.11.1.6), guaiacol peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.1) and superoxide dismutase (EC 1.15.1.1) revealed that H₂O₂ spraying increased antioxidant enzyme activities. Catalase (CAT) was the most responsive of these enzymes to H₂O₂, with higher activity early (48 h) in the treatment, while guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) were responsive only at later stages (240 h) of treatment. Increased CAT activity appears linked to gene expression regulation. Lower malondialdehyde levels were detected in plants with higher CAT activity, which may result from the protective function of this enzyme. Overall, we can conclude that pretreatment with H₂O₂ leaf spraying was able to reduce the deleterious effects of salinity on seedling growth and lipid peroxidation. These responses could be attributed to the ability of H₂O₂ to induce antioxidant defenses, especially CAT activity.

Biochemical and molecular analyses of copper-zinc superoxide dismutase from a C4 plant Pennisetum glaucum reveals an adaptive role in response to oxidative stress

Superoxide dismutases (SODs) form the foremost line of defense against ROS in aerobes. Pennisetum glaucum cDNA library is constructed to isolate superoxide dismutase cDNA clone (PgCuZnSOD) of 798 bp comprising 5'UTR (111 bp), an ORF (459 bp) and 3'UTR (228 bp). Deduced protein of 152 amino acids (16.7 kDa) with an estimated isoelectric point of 5.76 shared highest homology to cytoplasmic CuZnSODs from monocots i.e., maize, rice. Predicted 3D model reveals a conserved eight-stranded ß-barrel with active site held between barrel and two surface loops. Purified recombinant protein is relatively thermo-stable with maximal activity at pH 7.6 and shows inhibition with H(2)O(2) (4.3 mM) but not with azide (10 mM). In Pennisetum seedlings, abiotic stress induced PgCuZnSOD transcript up-regulation directly correlates to high protein and activity induction. Overexpression of PgCuZnSOD confers comparatively enhanced tolerance to methyl viologen (MV) induced oxidative stress in bacteria. Results imply that PgCuZnSOD plays a functional role in conferring oxidative stress tolerance to prokaryotic system and may hold significant potential to impart oxidative stress tolerance in higher plants through transgenic approach.

Dissecting the effects of nitrate, sucrose and osmotic potential on Arabidopsis root and shoot system growth in laboratory assays

Studying the specific effects of water and nutrients on plant development is difficult because changes in a single component can often trigger multiple response pathways. Such confounding issues are prevalent in commonly used laboratory assays. For example, increasing the nitrate concentration in growth media alters both nitrate availability and osmotic potential. In addition, it was recently shown that a change in the osmotic potential of media alters the plant's ability to take up other nutrients such as sucrose. It can also be difficult to identify the initial target tissue of a particular environmental cue because there are correlated changes in development of many organs. These growth changes may be coordinately regulated, or changes in development of one organ may trigger changes in development of another organ as a secondary effect. All these complexities make analyses of plant responses to environmental factors difficult to interpret. Here, we review the literature on the effects of nitrate, sucrose and water availability on root system growth and discuss the mechanisms underlying these effects. We then present experiments that examine the impact of nitrate, sucrose and water on root and shoot system growth in culture using an approach that holds all variables constant except the one under analysis. We found that while all three factors also alter root system size, changes in sucrose and osmotic potential also altered shoot system size. In contrast, we found that, when osmotic effects are controlled, nitrate specifically inhibits root system growth while having no effect on shoot system growth. This effectively decreases the root : shoot ratio. Alterations in root : shoot ratio have been widely observed in response to nitrogen starvation, where root growth is selectively increased, but the present results suggest that alterations in this ratio can be triggered across a wide spectrum of nitrate concentrations.

Ion Transporters and Abiotic Stress Tolerance in Plants

Adaptation of plants to salt stress requires cellular ion homeostasis involving net intracellular Na⁺ and Cl⁻ uptake and subsequent vacuolar compartmentalization without toxic ion accumulation in the cytosol. Sodium ions can enter the cell through several low- and high-affinity K⁺ carriers. Some members of the HKT family function as sodium transporter and contribute to Na⁺ removal from the ascending xylem sap and recirculation from the leaves to the roots via the phloem vasculature. Na⁺ sequestration into the vacuole depends on expression and activity of Na⁺/H⁺ antiporter that is driven by electrochemical gradient of protons generated by the vacuolar H⁺-ATPase and the H⁺-pyrophosphatase. Sodium extrusion at the root-soil interface is presumed to be of critical importance for the salt tolerance. Thus, a very rapid efflux of Na⁺ from roots must occur to control net rates of influx. The Na⁺/H⁺ antiporter SOS1 localized to the plasma membrane is the only Na⁺ efflux protein from plants characterized so far. In this paper, we analyze available data related to ion transporters and plant abiotic stress responses in order to enhance our understanding about how salinity and other abiotic stresses affect the most fundamental processes of cellular function which have a substantial impact on plant growth development.

A comparison of hydroponic and soil-based screening methods to identify salt tolerance in the field in barley

Success in breeding crops for yield and other quantitative traits depends on the use of methods to evaluate genotypes accurately under field conditions. Although many screening criteria have been suggested to distinguish between genotypes for their salt tolerance under controlled environmental conditions, there is a need to test these criteria in the field. In this study, the salt tolerance, ion concentrations, and accumulation of compatible solutes of genotypes of barley with a range of putative salt tolerance were investigated using three growing conditions (hydroponics, soil in pots, and natural saline field). Initially, 60 genotypes of barley were screened for their salt tolerance and uptake of Na(+), Cl(-), and K(+) at 150 mM NaCl and, based on this, a subset of 15 genotypes was selected for testing in pots and in the field. Expression of salt tolerance in saline solution culture was not a reliable indicator of the differences in salt tolerance between barley plants that were evident in saline soil-based comparisons. Significant correlations were observed in the rankings of genotypes on the basis of their grain yield production at a moderately saline field site and their relative shoot growth in pots at EC(e) 7.2 [Spearman's rank correlation (rs)=0.79] and EC(e) 15.3 (rs=0.82) and the crucial parameter of leaf Na(+) (rs=0.72) and Cl(-) (rs=0.82) concentrations at EC(e) 7.2 dS m(-1). This work has established screening procedures that correlated well with grain yield at sites with moderate levels of soil salinity. This study also showed that both salt exclusion and osmotic tolerance are involved in salt tolerance and that the relative importance of these traits may differ with the severity of the salt stress. In soil, ion exclusion tended to be more important at low to moderate levels of stress but osmotic stress became more important at higher stress levels. Salt exclusion coupled with a synthesis of organic solutes were shown to be important components of salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop-improvement programmes.

Transcriptional profiling analysis in Populus yunnanensis provides insights into molecular mechanisms of sexual differences in salinity tolerance

Physiological responses to abiotic stress in plants exhibit sexual differences. Females usually experience greater negative effects than males; however, little is known about the molecular mechanisms of sexual differences in abiotic stress responses. In the present study, transcriptional responses to salinity treatments were compared between male and female individuals of the poplar Populus yunnanensis. It was found that several functional groups of genes involved in important pathways were differentially expressed, including photosynthesis-related genes, which were mainly up-regulated in males but down-regulated in females. This gene expression pattern is consistent with physiological observations showing that salinity inhibited photosynthetic capacity more in females than in males. Furthermore, genes located in autosomes rather than in the female-specific region of the W chromosome are the major contributors to the sexual differences in the salinity tolerance of poplars. In conclusion, this study provided molecular evidence of sexual differences in the salinity tolerance of poplars. The identified sex-related genes in salinity tolerance and their functional groups will enhance our understanding of sexual differences in salinity stress at the transcription level.

Comparison of ion balance and nitrogen metabolism in old and young leaves of alkali-stressed rice plants

Background

Alkali stress is an important agricultural contaminant and has complex effects on plant metabolism. The aim of this study was to investigate whether the alkali stress has different effects on the growth, ion balance, and nitrogen metabolism in old and young leaves of rice plants, and to compare functions of both organs in alkali tolerance.

Methodology/Principal Findings

The results showed that alkali stress only produced a small effect on the growth of young leaves, whereas strongly damaged old leaves. Rice protected young leaves from ion harm via the large accumulation of Na⁺ and Cl⁻ in old leaves. The up-regulation of OsHKT1;1, OsAKT1, OsHAK1, OsHAK7, OsHAK10 and OsHAK16 may contribute to the larger accumulation of Na⁺ in old leaves under alkali stress. Alkali stress mightily reduced the NO₃⁻ contents in both organs. As old leaf cells have larger vacuole, under alkali stress these scarce NO₃⁻ was principally stored in old leaves. Accordingly, the expression of OsNRT1;1 and OsNRT1;2 in old leaves was up-regulated by alkali stress, revealing that the two genes might contribute to the accumulation of NO₃⁻ in old leaves. NO₃⁻ deficiency in young leaves under alkali stress might induce the reduction in OsNR1 expression and the subsequent lacking of NH₄⁺, which might be main reason for the larger down-regulation of OsFd-GOGAT and OsGS2 in young leaves.

Conclusions/Significance

Our results strongly indicated that, during adaptation of rice to alkali stress, young and old leaves have distinct mechanisms of ion balance and nitrogen metabolism regulation. We propose that the comparative studies of young and old tissues may be important for abiotic stress tolerance research.

Physiological and growth responses of six turfgrass species relative to salinity tolerance

The demand for salinity-tolerant turfgrasses is increasing due to augmented use of effluent or low-quality water (sea water) for turf irrigation and the growing turfgrass industry in coastal areas. Experimental plants, grown in plastic pots filled with a mixture of river sand and KOSAS^R peat (9 : 1), were irrigated with sea water at different dilutions imparting salinity levels of 0, 8, 16, 24, 32, 40, or 48 dS m⁻¹. Salinity tolerance was evaluated on the basis of leaf firing, shoot and root growth reduction, proline content, and relative water content. Paspalum vaginatum was found to be most salt tolerant followed by Zoysia japonica and Zoysia matrella, while Digitaria didactyla, Cynodon dactylon “Tifdwarf,” and Cynodon dactylon “Satiri” were moderately tolerant. The results indicate the importance of turfgrass varietal selection for saline environments.

NADP-dependent isocitrate dehydrogenase from Arabidopsis roots contributes in the mechanism of defence against the nitro-oxidative stress induced by salinity

NADPH regeneration appears to be essential in the mechanism of plant defence against oxidative stress. Plants contain several NADPH-generating dehydrogenases including isocitrate dehydrogenase (NADP-ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME). In Arabidopsis seedlings grown under salinity conditions (100 mM NaCl) the analysis of physiological parameters, antioxidant enzymes (catalase and superoxide dismutase) and content of superoxide radical (O₂^∙−), nitric oxide (NO), and peroxynitrite (ONOO⁻) indicates a process of nitro-oxidative stress induced by NaCl. Among the analysed NADPH-generating dehydrogenases under salinity conditions, the NADP-ICDH showed the maximum activity mainly attributable to the root NADP-ICDH. Thus, these data provide new insights on the relevance of the NADP-ICDH which could be considered as a second barrier in the mechanism of response against the nitro-oxidative stress generated by salinity.

Microbial amelioration of crop salinity stress

The use of soil and irrigation water with a high content of soluble salts is a major limiting factor for crop productivity in the semi-arid areas of the world. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity, as well as new approaches to discover new mechanisms involved in stress tolerance. Although these mechanisms are not always well understood, beneficial physiological effects include improved nutrient and water uptake, growth promotion, and alteration of plant hormonal status and metabolism. This review aims to evaluate the beneficial effects of soil biota on the plant response to saline stress, with special reference to phytohormonal signalling mechanisms that interact with key physiological processes to improve plant tolerance to the osmotic and toxic components of salinity. Improved plant nutrition is a quite general beneficial effect and may contribute to the maintenance of homeostasis of toxic ions under saline stress. Furthermore, alteration of crop hormonal status to decrease evolution of the growth-retarding and senescence-inducing hormone ethylene (or its precursor 1-aminocyclopropane-1-carboxylic acid), or to maintain source-sink relations, photosynthesis, and biomass production and allocation (by altering indole-3-acetic acid and cytokinin biosynthesis) seem to be promising target processes for soil biota-improved crop salt tolerance.

The tolerance of Jatropha curcas seedlings to NaCl: an ecophysiological analysis

Jatropha curcas L. is a biodiesel crop that is resistant to drought stress. However, the salt tolerance of this plant has not yet been studied. To address this question, J. curcas seedlings were grown in a fertilised substrate to evaluate the effects of salinity stress on growth, leaf water relation and organic solutes, leaf and root mineral concentrations, chlorophyll fluorescence parameters, and carbohydrate concentration. The experiment consisted of six treatments with different concentrations of NaCl in the irrigation water: 0 (control), 30, 60, 90, 120 and 150 mM. The total biomass exhibited a salt-induced decrease in the 60 mM or higher NaCl concentrations. The Cl(r) concentration was higher than the Na(+) concentration in all of the plant tissues. The water potential and relative water content of the leaves were not affected by any of the salt treatments. However, salinity induced a decline in the leaf K(+) concentration, together with a significant enhancement in the leaf P, S, Fe, Zn, Mn and Cu levels. The net assimilation of CO₂ also decreased with the salt treatment, due in part to non-stomatal limitation from the increase in C(a)/C(i) and a decrease in the maximum quantum efficiency (F(v)/F(m)) of photosystem II and soil plant analysis development (SPAD) units. This work suggests that J. curcas seedlings exhibit a moderate tolerance to salinity, as the plants were able to tolerate up to 4 dS m(-1) (EC water irrigation; 30 mM NaCl). The negative influences of salinity in this crop are mainly due to Cl(r) and/or Na(+) toxicity and to a nutritional imbalance caused by an increase in the Na(+)/K(+) ratio. The osmotic effect of salinity in this species is negligible, perhaps due to its strong control of leaf transpiration, which reduces water loss.

Genetic behaviour of physiological traits conferring cytosolic K+/Na+ homeostasis in wheat

A plant's ability to maintain an optimal cytosolic K(+)/Na(+) ratio has long been cited as a key feature of salinity tolerance. As traditional whole-leaf nutrient analysis does not account for tissue and organelle-specific ion sequestration, the predictive value of this index at the whole-plant level is not always satisfactory. Consequently, suitable in situ methods for functionally assessing the activity of the key membrane transporters contributing to this trait at the cellular level need to be developed. The aim of this work was to investigate the extent to which plasma membrane transporter-mediated Na(+) exclusion and KOR-mediated K(+) retention traits, measured with the microelectrode ion flux measuring (MIFE) technique, are inheritable in wheat, and whether the MIFE technique has the potential to be used in combination with molecular markers to determine QTLs for these transporter proteins. Experiments involved two bread (Triticum aestivum) and two durum (Triticum turgidum) wheat lines contrasting in their salinity tolerance. Net Na(+), K(+) and H(+) fluxes were measured from 6-day-old roots of parental lines and their F(1) hybrids upon addition and removal of NaCl. These results were complemented by assessment of whole-plant physiological and agronomic characteristics. We show evidence for a strong heritability of plasma membrane transporter-mediated Na(+) exclusion and K(+) retention traits in wheat at the cellular level. This opens the prospect of using the MIFE technique to map the position of these transporters on particular loci of wheat chromosomes. The next obvious step would be to pyramid these traits in one ideotype with superior salinity tolerance.

Enhanced salt stress tolerance of rice plants expressing a vacuolar H+ -ATPase subunit c1 (SaVHAc1) gene from the halophyte grass Spartina alterniflora Löisel

The physiological role of a vacuolar ATPase subunit c1 (SaVHAc1) from a halophyte grass Spartina alterniflora was studied through its expression in rice. The SaVHAc1-expressing plants showed enhanced tolerance to salt stress than the wild-type plants, mainly through adjustments in early stage and preparatory physiological responses. In addition to the increased accumulation of its own transcript, SaVHAc1 expression led to increased accumulation of messages of other native genes in rice, especially those involved in cation transport and ABA signalling. The SaVHAc1-expressing plants maintained higher relative water content under salt stress through early stage closure of the leaf stoma and reduced stomata density. The increased K(+) /Na(+) ratio and other cations established an ion homoeostasis in SaVHAc1-expressing plants to protect the cytosol from toxic Na(+) and thereby maintained higher chlorophyll retention than the WT plants under salt stress. Besides, the role of SaVHAc1 in cell wall expansion and maintenance of net photosynthesis was implicated by comparatively higher root and leaf growth and yield of rice expressing SaVHAc1 over WT under salt stress. The study indicated that the genes contributing toward natural variation in grass halophytes could be effectively manipulated for improving salt tolerance of field crops within related taxa.

Dissection of genetic overlap of drought and low-temperature tolerance QTLs at the germination stage using backcross introgression lines in soybean

Northeast of China is the main soybean production area, drought and low-temperature tolerance are both main factors involved in reducing soybean yield and limiting planting regions, the most effective way to solve this problem is to breed cultivars with drought and low-temperature tolerance. A set of the BC(2)F(3) lines was constructed with Hongfeng 11 as recurrent parent and Harosoy as donor parent, and screened in drought and low-temperature condition at the germination stage. Related QTLs were obtained by Chi-test and ANOVA analysis with genotypic and phenotypic data. Eighteen QTLs of drought tolerance and 23 QTLs of low-temperature tolerance were detected. Among them, 12 QTLs were correlated with both drought and low-temperature tolerance, which showed a partial genetic overlap between drought and low-temperature tolerance at the germination stage in soybean. Among the 12 genetic overlap QTLs, Satt253, Satt513, Satt693, Satt240, Satt323, and Satt255 were detected by at least one method for both drought and low-temperature tolerance. Satt557, Satt452, Sat_331, Satt338, Satt271, and Satt588 were detected by only one analysis method. The QTLs detected above were significant loci for drought or low-temperature tolerance in soybean. This will play an important role in MAS for development of both drought and low-temperature tolerance variety.

Identification of genes associated with adaptation to NaCl toxicity in perennial ryegrass (Lolium perenne L.)

Perennial ryegrass (Lolium perenne L.) is a popular turfgrass species. To understand the molecular mechanisms of salinity tolerance, a suppression subtractive cDNA library was constructed for a salinity-tolerant ryegrass accession, with NaCl-treated (255 mM) plants as the tester. Differentially expressed cDNA fragments were cloned and screened. BLAST search revealed that 268 clones exhibited significant homologies to known genes. These genes could be categorized into 11 different functional groups, including metabolism, energy transfer, detoxification, compatible solute, cellular transport, transcription, signal transduction, etc. The salinity-regulated expression of selected genes was confirmed by RT-PCR analysis. The results suggested that these putatively salinity up-regulated genes may play a vital role in the salinity tolerance of perennial ryegrass. They can be used as candidate genes for creating stress-tolerant grasses and for understanding molecular mechanisms of plant adaptation to salinity stress.

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.