Mechanisms of salinity tolerance

Co-expression of xerophyte Zygophyllum xanthoxylum ZxNHX and ZxVP1-1 enhances salt and drought tolerance in transgenic Lotus corniculatus by increasing cations accumulation

Lotus corniculatus L. is an important legume for forage, but is sensitive to salinity and drought. To develop salt- and drought-resistant L. corniculatus, ZxNHX and ZxVP1-1 genes encoding tonoplast Na+/H+ antiporter and H+-pyrophosphatase (H+-PPase) from a succulent xerophyte Zygophyllum xanthoxylum L., which is well adapted to arid environments through accumulating Na+ in its leaves, were transferred into this forage. We obtained the transgenic lines co-expressing ZxNHX and ZxVP1-1 genes (VX) as well as expressing ZxVP1-1 gene alone (VP). Compared with wild-type, both VX and VP transgenic lines grew better at 200mM NaCl, and also exhibited higher tolerance and faster recovery from water-deficit stress: these performances were associated with more Na+, K+ and Ca2+ accumulation in their leaves and roots, which caused lower leaf solute potential and thus retained more water. Moreover, the transgenic lines maintained lower relative membrane permeability and higher net photosynthesis rate under salt or water-deficit stress. These results indicate that expression of tonoplast Na+/H+ antiporter and H+-PPase genes from xerophyte enhanced salt and drought tolerance of L. corniculatus. Furthermore, compared with VP, VX showed higher shoot biomass, more cations accumulation, higher water retention, lesser cell membrane damage and higher photosynthesis capacity under salt or water-deficit condition, suggesting that co-expression of ZxVP1-1 and ZxNHX confers even greater performance to transgenic L. corniculatus than expression of the single ZxVP1-1.

A wheat allene oxide cyclase gene enhances salinity tolerance via jasmonate signaling

One of the two branches of the α-linolenic acid metabolism pathway is catalyzed by 12-oxo-phytodienoic acid reductase I, and the other is involved in jasmonic acid (JA) synthesis. The former is known to be active in the response to salinity tolerance in wheat (Triticum aestivum), but the participation of the latter in this response has not been established as yet. Here, the salinity-responsive bread wheat gene TaAOC1, which encodes an allene oxide cyclase involved in the α-linolenic acid metabolism pathway, was constitutively expressed in both bread wheat and Arabidopsis (Arabidopsis thaliana). In both species, transgenic lines exhibited an enhanced level of tolerance to salinity. The transgenic plants accumulated a higher content of JA and developed shorter roots. Both the shortened roots and the salinity tolerance were abolished in a background lacking a functional AtMYC2, a key component of the JA and abscisic acid signaling pathway, but were still expressed in a background deficient with respect to abscisic acid synthesis. We provide the first evidence, to our knowledge, suggesting that JA is also involved in the plant salinity response and that the α-linolenic acid metabolism pathway has a regulatory role over this response.

Abiotic stress responses in plant roots: a proteomics perspective

Abiotic stress conditions adversely affect plant growth, resulting in significant decline in crop productivity. To mitigate and recover from the damaging effects of such adverse environmental conditions, plants have evolved various adaptive strategies at cellular and metabolic levels. Most of these strategies involve dynamic changes in protein abundance that can be best explored through proteomics. This review summarizes comparative proteomic studies conducted with roots of various plant species subjected to different abiotic stresses especially drought, salinity, flood, and cold. The main purpose of this article is to highlight and classify the protein level changes in abiotic stress response pathways specifically in plant roots. Shared as well as stressor-specific proteome signatures and adaptive mechanism(s) are simultaneously described. Such a comprehensive account will facilitate the design of genetic engineering strategies that enable the development of broad-spectrum abiotic stress-tolerant crops.

The contribution of SERF1 to root-to-shoot signaling during salinity stress in rice

Stress perception and communication play important roles in the adaptation of plants to changing environmental conditions. Plant roots are the first organs to detect changes in the soil water potential induced by salt stress. In the presence of salinity stress, root-to-shoot communication occurs to adjust the growth of the whole plant. So far, the phytohormone abscisic acid (ABA), hydraulic signals and reactive oxygen species (ROS) have been proposed to mediate this communication under salt stress. Recently, we identified the rice transcription factor SALT-RESPONSIVE ERF1 (SERF1), which regulates a ROS-dependent transcriptional cascade in roots required for salinity tolerance. Upon salt stress, SERF1 knockout mutant plants show an increased leaf temperature as compared with wild type. As this occurs within the first 20 min of salt stress, we here evaluated the involvement of SERF1 in the perception of salt stress in the shoot. By metabolic profiling and expression analysis we show that the action of SERF1 in signal communication to the shoot is independent from ABA, but does affect the accumulation of ROS-related metabolites and transcripts under short-term salt stress.

The role of plant cell wall proteins in response to salt stress

Contemporary agriculture is facing new challenges with the increasing population and demand for food on Earth and the decrease in crop productivity due to abiotic stresses such as water deficit, high salinity, and extreme fluctuations of temperatures. The knowledge of plant stress responses, though widely extended in recent years, is still unable to provide efficient strategies for improvement of agriculture. The focus of study has been shifted to the plant cell wall as a dynamic and crucial component of the plant cell that could immediately respond to changes in the environment. The investigation of plant cell wall proteins, especially in commercially important monocot crops revealed the high involvement of this compartment in plants stress responses, but there is still much more to be comprehended. The aim of this review is to summarize the available data on this issue and to point out the future areas of interest that should be studied in detail.

Alleviation of exogenous 6-benzyladenine on two genotypes of eggplant (Solanum melongena Mill.) growth under salt stress

Cytokinins were recently shown to control plant adaptation to environmental stresses. To characterize the roles of cytokinins in the tolerance of eggplant (Solanum melongena Mill.) to salt stress, the protective effects of 6-benzyladenine (6-BA) on the growth, photosynthesis, and antioxidant capacity in the leaves of two eggplant cultivars Huqie12 (salt-sensitive) and Huqie4 (salt-tolerant) were investigated. Under 90 mM NaCl stress, Huqie4 showed higher biomass accumulation and less oxidative damage compared to the Huqie12. Application of exogenous 10 μM 6-BA significantly alleviated the growth suppression caused by salt stress in two eggplant genotypes. In parallel with the growth, 6-BA application in salt-stressed plants resulted in enhanced chlorophyll contents, as well as photosynthetic parameters such as net CO2 assimilation rate (P n), stomatal conductance (g s), transpiration rate (E), and intercellular CO2 concentration (C i). Furthermore, exogenous 6-BA also significantly reduced the O2 (-) production rate and malondialdehyde content and markedly increased the antioxidant enzymes superoxide dismutase and peroxidase, the antioxidant metabolites ascorbate and reduced glutathione (GSH), and proline in both genotypes under salt stress. The results indicate that exogenous 6-BA is useful to improve the salt resistance of eggplant, which is most likely related to the increase in photosynthesis and antioxidant capacity.

The F130S point mutation in the Arabidopsis high-affinity K(+) transporter AtHAK5 increases K(+) over Na(+) and Cs(+) selectivity and confers Na(+) and Cs(+) tolerance to yeast under heterologous expression

Potassium (K(+)) is an essential macronutrient required for plant growth, development and high yield production of crops. Members of group I of the KT/HAK/KUP family of transporters, such as HAK5, are key components for K(+) acquisition by plant roots at low external K(+) concentrations. Certain abiotic stress conditions such as salinity or Cs(+)-polluted soils may jeopardize plant K(+) nutrition because HAK5-mediated K(+) transport is inhibited by Na(+) and Cs(+). Here, by screening in yeast a randomly-mutated collection of AtHAK5 transporters, a new mutation in AtHAK5 sequence is identified that greatly increases Na(+) tolerance. The single point mutation F130S, affecting an amino acid residue conserved in HAK5 transporters from several species, confers high salt tolerance, as well as Cs(+) tolerance. This mutation increases more than 100-fold the affinity of AtHAK5 for K(+) and reduces the K i values for Na(+) and Cs(+), suggesting that the F130 residue may contribute to the structure of the pore region involved in K(+) binding. In addition, this mutation increases the V max for K(+). All this changes occur without increasing the amount of the AtHAK5 protein in yeast and support the idea that this residue is contributing to shape the selectivity filter of the AtHAK5 transporter.

Neotyphodium coenophialum-infected tall fescue and its potential application in the phytoremediation of saline soils

The growth response of endophyte-infected (EI) and endophyte-free (EF) tall fescue to salt stress was investigated under two growing systems (hydroponic and soil in pots). The hydroponic experiment showed that endophyte infection significantly increased tiller and leaf number, which led to an increase in the total biomass of the host grass. Endophyte infection enhanced Na accumulation in the host grass and improved Na transport from the roots to the shoots. With a 15 g l(-1) NaCl treatment, the phytoextraction efficiency of EI tall fescue was 2.34-fold higher than EF plants. When the plants were grown in saline soils, endophyte infection also significantly increased tiller number, shoot height and the total biomass of the host grass. Although EI tall fescue cannot accumulate Na to a level high enough for it to be termed a halophyte, the increased biomass production and stress tolerance suggested that endophyte/plant associations had the potential to be a model for endophyte-assisted phytoextraction in saline soils.

A wheat SIMILAR TO RCD-ONE gene enhances seedling growth and abiotic stress resistance by modulating redox homeostasis and maintaining genomic integrity

Plant growth inhibition is a common response to salinity. Under saline conditions, Shanrong No. 3 (SR3), a bread wheat (Triticum aestivum) introgression line, performs better than its parent wheat variety Jinan 177 (JN177) with respect to both seedling growth and abiotic stress tolerance. Furthermore, the endogenous reactive oxygen species (ROS) was also elevated in SR3 relative to JN177. The SR3 allele of sro1, a gene encoding a poly(ADP ribose) polymerase (PARP) domain protein, was identified to be crucial for both aspects of its superior performance. Unlike RADICAL-INDUCED CELL DEATH1 and other Arabidopsis thaliana SIMILAR TO RCD-ONE (SRO) proteins, sro1 has PARP activity. Both the overexpression of Ta-sro1 in wheat and its heterologous expression in Arabidopsis promote the accumulation of ROS, mainly by enhancing the activity of NADPH oxidase and the expression of NAD(P)H dehydrogenase, in conjunction with the suppression of alternative oxidase expression. Moreover, it promotes the activity of ascorbate-GSH cycle enzymes and GSH peroxidase cycle enzymes, which regulate ROS content and cellular redox homeostasis. sro1 is also found to be involved in the maintenance of genomic integrity. We show here that the wheat SRO has PARP activity; such activity could be manipulated to improve the growth of seedlings exposed to salinity stress by modulating redox homeostasis and maintaining genomic stability.

Glycine betaine-mediated potentiation of HSP gene expression involves calcium signaling pathways in tobacco exposed to NaCl stress

Glycine betaine (GB) can enhance heat tolerance and the accumulation of heat-shock protein (HSP) in plants, but the effects of GB on HSP accumulation during salt stress were not previously known. To investigate the mechanism of how GB influences the expression of HSP, wild-type tobacco (Nicotiana tabacum) seedlings pretreated with exogenous GB and BADH-transgenic tobacco plants that accumulated GB in vivo were studied during NaCl stress. A transient Ca(2+) efflux was observed in the epidermal cells of the elongation zone of tobacco roots after NaCl treatment for 1-2 min. After 24 h of NaCl treatment, an influx of Ca(2+) was observed; a low concentration of GB significantly increased NaCl-induced Ca(2+) influx. GB increased the intracellular free calcium ion concentration and enhanced the expression of the calmodulin (CaM) and heat-shock transcription factor (HSF) genes resulting in potentiated levels of HSPs. Pharmacological experiments confirmed that Ca(2+) and CaM increased HSFs and HSPs gene expression, which coincided with increased the levels of HSP70 accumulation. These results suggest a mechanism by which GB acted as a cofactor in the NaCl induction of a Ca(2+) -permeable current. A possible regulatory model of Ca(2+) -CaM in the signal transduction pathway for induction of transcription and translation of the active HSPs is described.

Assessment of natural variation in the first pore domain of the tomato HKT1;2 transporter and characterization of mutated versions of SlHKT1;2 expressed in Xenopus laevis oocytes and via complementation of the salt sensitive athkt1;1 mutant

Single Nucleotide Polymorphisms (SNPs) within the coding sequence of HKT transporters are important for the functioning of these transporters in several plant species. To unravel the functioning of HKT transporters analysis of natural variation and multiple site-directed mutations studies are crucial. Also the in vivo functioning of HKT proteins, via complementation studies performed with athkt1;1 plants, could provide essential information about these transporters. In this work, we analyzed the natural variation present in the first pore domain of the HKT1;2 coding sequence of 93 different tomato accessions, which revealed that this region was conserved among all accessions analyzed. Analysis of mutations introduced in the first pore domain of the SlHKT1;2 gene showed, when heterologous expressed in Xenopus laevis oocytes, that the replacement of S70 by a G allowed SlHKT2;1 to transport K(+), but also caused a large reduction in both Na(+) and K(+) mediated currents. The study of the transport characteristics of SlHKT1;2 revealed that Na(+)-transport by the tomato SlHKT1;2 protein was inhibited by the presence of K(+) at the outside of the membrane. GUS expression under the AtHKT1;1 promoter gave blue staining in the vascular system of transgenic Arabidopsis. athkt1;1 mutant plants transformed with AtHKT1;1, SlHKT1;2, AtHKT1;1S68G, and SlHKT1;2S70G indicated that both AtHKT1;1 and SlHKT1;2 were able to restore the accumulation of K(+) in the shoot, although the low accumulation of Na(+) as shown by WT plants was only partially restored. The inhibition of Na(+) transport by K(+), shown by the SlHKT1;2 transporter in oocytes (and not by AtHKT1;1), was not reflected in Na(+) accumulation in the plants transformed with SlHKT1;2. Both AtHKT1;1-S68G and SlHKT1;2-S70G were not able to restore the phenotype of athkt1;1 mutant plants.

The Arabidopsis gibberellin methyl transferase 1 suppresses gibberellin activity, reduces whole-plant transpiration and promotes drought tolerance in transgenic tomato

Previous studies have shown that reduced gibberellin (GA) level or signal promotes plant tolerance to environmental stresses, including drought, but the underlying mechanism is not yet clear. Here we studied the effects of reduced levels of active GAs on tomato (Solanum lycopersicum) plant tolerance to drought as well as the mechanism responsible for these effects. To reduce the levels of active GAs, we generated transgenic tomato overexpressing the Arabidopsis thaliana GA METHYL TRANSFERASE 1 (AtGAMT1) gene. AtGAMT1 encodes an enzyme that catalyses the methylation of active GAs to generate inactive GA methyl esters. Tomato plants overexpressing AtGAMT1 exhibited typical GA-deficiency phenotypes and increased tolerance to drought stress. GA application to the transgenic plants restored normal growth and sensitivity to drought. The transgenic plants maintained high leaf water status under drought conditions, because of reduced whole-plant transpiration. The reduced transpiration can be attributed to reduced stomatal conductance. GAMT1 overexpression inhibited the expansion of leaf-epidermal cells, leading to the formation of smaller stomata with reduced stomatal pores. It is possible that under drought conditions, plants with reduced GA activity and therefore, reduced transpiration, will suffer less from leaf desiccation, thereby maintaining higher capabilities and recovery rates.

Using Arabidopsis-related model species (ARMS): growth, genetic transformation, and comparative genomics

The Arabidopsis-related model species (ARMS) Thellungiella salsuginea and Thellungiella parvula have generated broad interest in salt stress research. While general growth characteristics of these species are similar to Arabidopsis, some aspects of their life cycle require particular attention in order to obtain healthy plants, with a large production of seeds in a relatively short time. This chapter describes basic procedures for growth, maintenance, and Agrobacterium-mediated transformation of ARMS. Where appropriate, differences in requirements between Thellungiella spp. and Arabidopsis are highlighted, along with basic growth requirements of other less studied candidate model species. Current techniques for comparative genomics analysis between Arabidopsis and ARMS are also described in detail.

Dissipation of excess photosynthetic energy contributes to salinity tolerance: a comparative study of salt-tolerant Ricinus communis and salt-sensitive Jatropha curcas

The relationships between salt tolerance and photosynthetic mechanisms of excess energy dissipation were assessed using two species that exhibit contrasting responses to salinity, Ricinus communis (tolerant) and Jatropha curcas (sensitive). The salt tolerance of R. communis was indicated by unchanged electrolyte leakage (cellular integrity) and dry weight in leaves, whereas these parameters were greatly affected in J. curcas. The leaf Na+ content was similar in both species. Photosynthesis was intensely decreased in both species, but the reduction was more pronounced in J. curcas. In this species biochemical limitations in photosynthesis were more prominent, as indicated by increased C(i) values and decreased Rubisco activity. Salinity decreased both the V(cmax) (in vivo Rubisco activity) and J(max) (maximum electron transport rate) more significantly in J. curcas. The higher tolerance in R. communis was positively associated with higher photorespiratory activity, nitrate assimilation and higher cyclic electron flow. The high activity of these alternative electron sinks in R. communis was closely associated with a more efficient photoprotection mechanism. In conclusion, salt tolerance in R. communis, compared with J. curcas, is related to higher electron partitioning from the photosynthetic electron transport chain to alternative sinks.

Ultrastructural and physiological responses of potato (Solanum tuberosum L.) plantlets to gradient saline stress

Salinity is one of the major abiotic stresses that impacts plant growth and reduces the productivity of field crops. Compared to field plants, test tube plantlets offer a direct and fast approach to investigate the mechanism of salt tolerance. Here we examined the ultrastructural and physiological responses of potato (Solanum tuberosum L. c.v. "Longshu No. 3") plantlets to gradient saline stress (0, 25, 50, 100, and 200 mM NaCl) with two consequent observations (2 and 6 weeks, respectively). The results showed that, with the increase of external NaCl concentration and the duration of treatments, (1) the number of chloroplasts and cell intercellular spaces markedly decreased, (2) cell walls were thickened and even ruptured, (3) mesophyll cells and chloroplasts were gradually damaged to a complete disorganization containing more starch, (4) leaf Na and Cl contents increased while leaf K content decreased, (5) leaf proline content and the activities of catalase (CAT) and superoxide dismutase (SOD) increased significantly, and (6) leaf malondialdehyde (MDA) content increased significantly and stomatal area and chlorophyll content decline were also detected. Severe salt stress (200 mM NaCl) inhibited plantlet growth. These results indicated that potato plantlets adapt to salt stress to some extent through accumulating osmoprotectants, such as proline, increasing the activities of antioxidant enzymes, such as CAT and SOD. The outcomes of this study provide ultrastructural and physiological insights into characterizing potential damages induced by salt stress for selecting salt-tolerant potato cultivars.

Effect of salinity on zinc uptake by Brassica juncea

Salinity is a major worldwide problem that affects agricultural soils and limits the reclamation of contaminated sites. Despite the large number of research papers published about salt tolerance in Brassica juncea L., there are very few accounts concerning the influence of salinity on the uptake of trace metals. In this study, B. juncea plants divided through soil sets comprising 0, 900 and 1800 mg Zn kg(-1), were treated with solutions containing 0, 60 and 120 mmol L(-1) of NaCl, with the purpose of observing the effect of salt on Zn uptake, and some physiological responses throughout the 90 days experiment. Increasing concentrations of NaCl and Zn produced a decline in the ecophysiological and biochemical properties of the plants, with observable synergistic effects on parameters like shoot dry weight, leaf area, or photochemical efficiency. Nevertheless, plants treated with 60 mmol L(-1) of NaCl accumulated striking harvestable amounts of Zn per plant that largely exceed those reported for Thlaspi caerulescens. It was concluded that salinity could play an important role on the uptake of Zn by B. juncea. The potential mechanisms behind these results are discussed, as well as the implications for phytoremediation of Zn on saline and non-saline soils.

A gateway with a guard: how the endodermis regulates growth through hormone signaling

The endodermis is a defining feature of plant roots and is most widely studied as a differentially permeable barrier limiting solute uptake from the soil into the vascular stream. Recent work has revealed that this inner cell layer is also an important signaling center for hormone-mediated control of growth. Auxin, gibberellic acid, abscisic acid and strigalactones all appear to depend on the endodermis to regulate root biology and point to this cell type as having important inter-cell layer regulatory activity, as well. In this review I discuss recent work detailing the importance of the endodermis in growth control and how this function is affected during responses to the environment.

Exogenous trehalose largely alleviates ionic unbalance, ROS burst, and PCD occurrence induced by high salinity in Arabidopsis seedlings

Trehalose (Tre) has been reported to play a critical role in plant response to salinity and the involved mechanisms remain to be investigated in detail. Here, the putative roles of Tre in regulation of ionic balance, cellular redox state, cell death were studied in Arabidopsis under high salt condition. Our results found that the salt-induced restrictions on both vegetative and reproductive growth in salt-stressed plants were largely alleviated by exogenous supply with Tre. The microprobe analysis of ionic dynamics in the leaf and stem of florescence highlighted the Tre ability to retain K and K/Na ratio in plant tissues to improve salt tolerance. The flow cytometry assay of cellular levels of reactive oxygen species and programmed cell death displayed that Tre was able to antagonized salt-induced damages in redox state and cell death and sucrose did not play the same role with Tre. By comparing ionic distribution in leaf and inflorescence stem (IS), we found that Tre was able to restrict Na transportation to IS from leaves since that the ratio of Na accumulation in leaves relative to IS was largely improved due to Tre. The marked decrease of Na ion and improved sucrose level in IS might account for the promoted floral growth when Tre was included in the saline solution. At the same time, endogenous soluble sugars and antioxidant enzyme activities in the salt-stressed plants were also elevated by Tre to counteract high salt stress. We concluded that Tre could improve Arabidopsis salt resistance with respect to biomass accumulation and floral transition in the means of regulating plant redox state, cell death, and ionic distribution.

Functional characterization in Xenopus oocytes of Na+ transport systems from durum wheat reveals diversity among two HKT1;4 transporters

Plant tolerance to salinity constraint involves complex and integrated functions including control of Na(+) uptake, translocation, and compartmentalization. Several members of the high-affinity K(+) transporter (HKT) family, which comprises plasma-membrane transporters permeable to K(+) and Na(+) or to Na(+) only, have been shown to play major roles in plant Na(+) and K(+) homeostasis. Among them, HKT1;4 has been identified as corresponding to a quantitative trait locus (QTL) of salt tolerance in wheat but was not functionally characterized. Here, we isolated two HKT1;4-type cDNAs from a salt-tolerant durum wheat (Triticum turgidum L. subsp. durum) cultivar, Om Rabia3, and investigated the functional properties of the encoded transporters using a two-electrode voltage-clamp technique, after expression in Xenopus oocytes. Both transporters displayed high selectivity for Na(+), their permeability to other monovalent cations (K(+), Li(+), Cs(+), and Rb(+)) being ten times lower than that to Na(+). Both TdHKT1;4-1 and TdHKT1;4-2 transported Na(+) with low affinity, although the half-saturation of the conductance was observed at a Na(+) concentration four times lower in TdHKT1;4-1 than in TdHKT1;4-2. External K(+) did not inhibit Na(+) transport through these transporters. Quinine slightly inhibited TdHKT1;4-2 but not TdHKT1;4-1. Overall, these data identified TdHKT1;4 transporters as new Na(+)-selective transporters within the HKT family, displaying their own functional features. Furthermore, they showed that important differences in affinity exist among durum wheat HKT1;4 transporters. This suggests that the salt tolerance QTL involving HKT1;4 may be at least in part explained by functional variability among wheat HKT1;4-type transporters.

Alleviation of salt stress in citrus seedlings inoculated with arbuscular mycorrhizal fungi depends on the rootstock salt tolerance

Seedlings of Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and Alemow (Citrus macrophylla Wester) were inoculated with a mixture of AM fungi (Rhizophagus irregularis and Funneliformis mosseae) (+AM), or left non-inoculated (-AM). From forty-five days after fungal inoculation onwards, half of +AM or -AM plants were irrigated with nutrient solution containing 50 mM NaCl. Three months later, AM significantly increased plant growth in both Cleopatra mandarin and Alemow rootstocks. Plant growth was higher in salinized +AM plants than in non-salinized -AM plants, demonstrating that AM compensates the growth limitations imposed by salinity. Whereas AM-inoculated Cleopatra mandarin seedlings had a very good response under saline treatment, inoculation in Alemow did not alleviate the negative effect of salinity. The beneficial effect of mycorrhization is unrelated with protection against the uptake of Na or Cl and the effect of AM on these ions did not explain the different response of rootstocks. This response was related with the nutritional status since our findings confirm that AM fungi can alter host responses to salinity stress, improving more the P, K, Fe and Cu plant nutrition in Cleopatra mandarin than in Alemow plants. AM inoculation under saline treatments also increased root Mg concentration but it was higher in Cleopatra mandarin than in Alemow. This could explain why AM fungus did not completely recovered chlorophyll concentrations in Alemow and consequently it had lower photosynthesis rate than control plants. AM fungi play an essential role in citrus rootstock growth and biomass production although the intensity of this response depends on the rootstock salinity tolerance.

Physiology and proteome responses of two contrasting rice mutants and their wild type parent under salt stress conditions at the vegetative stage

Salinity is one of the major environmental limiting factors that affects growth and productivity of rice (Oryza sativa L.) worldwide. Rice is among the most sensitive crops to salinity, especially at early vegetative stages. In order to get a better understanding of molecular pathways affected in rice mutants showing contrasting responses to salinity, we exploited the power of 2-DE based proteomics to explore the proteome changes associated with salt stress response. Our physiological observations showed that standard evaluation system (SES) scores, Na+ and K+ concentrations in shoots and Na+/K+ ratio were significantly different in contrasting mutants under salt stress condition. Proteomics analysis showed that, out of 854 protein spots which were reproducibly detected, 67 protein spots showed significant responses to salt stress. The tandem mass spectrometry analysis of these significantly differentially accumulated proteins resulted in identification of 34 unique proteins. These proteins are involved in various molecular processes including defense to oxidative stresses, metabolisms, photosynthesis, protein synthesis and processing, signal transduction. Several of the identified proteins were emerged as key participants in salt stress tolerance. The possible implication of salt responsive proteins in plant adaptation to salt stress is discussed.

(22)Na influx is significantly lower in salt tolerant groundnut (Arachis hypogaea) varieties

Distinct varieties differing in salt tolerance were initially identified from two separate green house experiments using two systems; solution as well as soil culture. The first screening involved a diverse group of 27 cultivars. Several physiological traits; Chlorophyll Stability Index (CSI), Salt Tolerance Index (STI) and ion content were determined to screen the cultivars for differences in salt tolerance using solution culture in the first experiment. A set of six varieties (three tolerant and three susceptible) were selected from this experiment and then subjected again to salt stress adopting a natural soil system in the second experiment which involved a screening approach essentially similar to that of the first experiment. In the third experiment using two distinct cultivars differing in salt tolerance selected from experiment II, (22)Na influx rate was determined in the root and shoot at the end of a 24 h salt imposition in Hoagland's nutrient system containing 180 KBq of (22)Na. The results suggested that there were distinct differences in (22)Na influx rate into root and concurrently in the shoot. The salt tolerant Spanish improved and one of the moderately tolerant Trombay variety TAG 24, showed good regulation of (22)Na influx resulting in low (22)Na concentration. The salt susceptible variety JSP39 had nearly 7-8 fold higher root (22)Na content as compared to the tolerant and moderately tolerant cultivars. The results have highlighted the importance of Na exclusion as an important determinant of salt tolerance in groundnut.

Beneficial soil bacterium Bacillus subtilis (GB03) augments salt tolerance of white clover

Soil salinity is an increasingly serious problem worldwide that reduces agricultural output potential. Selected beneficial soil bacteria can promote plant growth and augment tolerance to biotic and abiotic stresses. Bacillus subtilis strain GB03 has been shown to confer growth promotion and abiotic stress tolerance in the model plant Arabidopsis thaliana. Here we examined the effect of this beneficial soil bacterium on salt tolerance in the legume forage crop, white clover. Plants of white clover (Trifolium repens L. cultivar Huia) were grown from seeds with or without soil inoculation of the beneficial soil bacterium Bacillus subtilis GB03 supplemented with 0, 50, 100, or 150 mM NaCl water into soil. Growth parameters, chlorophyll content, malondialdehyde (MDA) content and osmotic potential were monitored during the growth cycle. Endogenous Na(+) and K(+) contents were determined at the time of harvest. White clover plants grown in GB03-inoculated soil were significantly larger than non-inoculated controls with respect to shoot height, root length, plant biomass, leaf area and chlorophyll content; leaf MDA content under saline condition and leaf osmotic potential under severe salinity condition (150 mM NaCl) were significantly decreased. Furthermore, GB03 significantly decreased shoot and root Na(+) accumulation and thereby improved K(+)/Na(+) ratio when GB03-inoculated plants were grown under elevated salt conditions. The results indicate that soil inoculation with GB03 promotes white clover growth under both non-saline and saline conditions by directly or indirectly regulating plant chlorophyll content, leaf osmotic potential, cell membrane integrity and ion accumulation.

PHIV-RootCell: a supervised image analysis tool for rice root anatomical parameter quantification

We developed the PHIV-RootCell software to quantify anatomical traits of rice roots transverse section images. Combined with an efficient root sample processing method for image acquisition, this program permits supervised measurements of areas (those of whole root section, stele, cortex, and central metaxylem vessels), number of cell layers and number of cells per cell layer. The PHIV-RootCell toolset runs under ImageJ, an independent operating system that has a license-free status. To demonstrate the usefulness of PHIV-RootCell, we conducted a genetic diversity study and an analysis of salt stress responses of root anatomical parameters in rice (Oryza sativa L.). Using 16 cultivars, we showed that we could discriminate between some of the varieties even at the 6 day-olds stage, and that tropical japonica varieties had larger root sections due to an increase in cell number. We observed, as described previously, that root sections become enlarged under salt stress. However, our results show an increase in cell number in ground tissues (endodermis and cortex) but a decrease in external (peripheral) tissues (sclerenchyma, exodermis, and epidermis). Thus, the PHIV-RootCell program is a user-friendly tool that will be helpful for future genetic and physiological studies that investigate root anatomical trait variations.

Management of Atriplex nummularia Lindl. in a salt affected soil in a semi arid region of Brazil

This study aims to investigate the behavior of Atriplex nummularia under field conditions, including its growth, periodic cuttings, salt extraction, and soil chemical properties monitored for 16 months. Three treatments were evaluated: soil cultivated with Atriplex pruned at 6 and 12 months after transplanting (MAT); soil cultivated with plants that were harvested only at the end of the experiment (16 MAT); and a control (uncultivated soil) with four replications. Soil samplings were taken at 0, 6, 12, and 16 MAT. The samples were taken at depths of 0-20, 20-40, 40-60, and 60-80 cm. Biometric variables for growth were monitored monthly. The shoot was divided into leaves, thin stems (< or = 3 mm diameter), and thick stems (> 3 mm diameter) to determine its content of Ca, Mg, Na, K, and Cl. We concluded that pruning regime for Atriplex was efficient mainly because it stimulated regrowth of less lignified material (leaves and stems < or = 3 mm). We found that elements extracted by plant tissue can be quantified accurately, making them valuable indicators of the efficiency of the recovery process. The use of the Atriplex is recommended because the the possibility of revegetating areas inhospitable to most species used in conventional farming.

Phosphatidic acid, a versatile water-stress signal in roots

Adequate water supply is of utmost importance for growth and reproduction of plants. In order to cope with water deprivation, plants have to adapt their development and metabolism to ensure survival. To maximize water use efficiency, plants use a large array of signaling mediators such as hormones, protein kinases, and phosphatases, Ca(2) (+), reactive oxygen species, and low abundant phospholipids that together form complex signaling cascades. Phosphatidic acid (PA) is a signaling lipid that rapidly accumulates in response to a wide array of abiotic stress stimuli. PA formation provides the cell with spatial and transient information about the external environment by acting as a protein-docking site in cellular membranes. PA reportedly binds to a number of proteins that play a role during water limiting conditions, such as drought and salinity and has been shown to play an important role in maintaining root system architecture. Members of two osmotic stress-activated protein kinase families, sucrose non-fermenting 1-related protein kinase 2 and mitogen activated protein kinases were recently shown bind PA and are also involved in the maintenance of root system architecture and salinity stress tolerance. In addition, PA regulates several proteins involved in abscisic acid-signaling. PA-dependent recruitment of glyceraldehyde-3-phosphate dehydrogenase under water limiting conditions indicates a role in regulating metabolic processes. Finally, a recent study also shows the PA recruits the clathrin heavy chain and a potassium channel subunit, hinting toward additional roles in cellular trafficking and potassium homeostasis. Taken together, the rapidly increasing number of proteins reported to interact with PA implies a broad role for this versatile signaling phospholipid in mediating salt and water stress responses.

Physiological mechanisms for high salt tolerance in wild soybean (Glycine soja) from Yellow River Delta, China: photosynthesis, osmotic regulation, ion flux and antioxidant capacity

Glycine soja (BB52) is a wild soybean cultivar grown in coastal saline land in Yellow River Delta, China. In order to reveal the physiological mechanisms adapting to salinity, we examined photosynthesis, ion flux, antioxidant system and water status in Glycine soja under NaCl treatments, taking a cultivated soybean, ZH13, as control. Upon NaCl exposure, higher relative water content and water potential were maintained in the leaf of BB52 than ZH13, which might depend on the more accumulation of osmotic substances such as glycinebetaine and proline. Compared with ZH13, activities of antioxidant enzymes including superoxide dismutase, catalase, ascorbate peroxidase and contents of ascorbate, glutathione and phenolics were enhanced to a higher level in BB52 leaf under NaCl stress, which could mitigate the salt-induced oxidative damage in BB52. Consistently, lipid peroxidation indicated by malondialdehyde content was lower in BB52 leaf. Photosynthetic rate (Pn) was decreased by NaCl stress in BB52 and ZH13, and the decrease was greater in ZH13. The decreased Pn in BB52 was mainly due to stomatal limitation. The inhibited activation of rubisco enzyme in ZH13 due to the decrease of rubisco activase content became an important limiting factor of Pn, when NaCl concentration increased to 200 mM. Rubisco activase in BB52 was not affected by NaCl stress. Less negative impact in BB52 derived from lower contents of Na(+) and Cl(-) in the tissues, and non-invasive micro-test technique revealed that BB52 roots had higher ability to extrude Na(+) and Cl(-). Wild soybean is a valuable genetic resource, and our study may provide a reference for molecular biologist to improve the salt tolerance of cultivated soybean in face of farmland salinity.

Chloroplasts of salt-grown Arabidopsis seedlings are impaired in structure, genome copy number and transcript levels

The chloroplast is the most prominent and metabolically active plastid in photosynthetic plants. Chloroplasts differentiate from proplastids in the plant meristem. Plant plastids contain multiple copies of a small circular genome. The numbers of chloroplasts per mesophyll cell and of plastid genome copies are affected by developmental stage and environmental signals. We compared chloroplast structure, gene expression and genome copy number in Arabidopsis seedlings germinated and grown under optimal conditions to those in seedlings germinated and grown in the presence of NaCl. Chloroplasts of the NaCl-grown seedlings were impaired, with less developed thylakoid and granum membranes than control seedlings. In addition, chloroplasts of salt-grown Arabidopsis seedlings accumulated more starch grains than those in the respective control plants. Steady-state transcript levels of chloroplast-encoded genes and of nuclear genes encoding chloroplast proteins were reduced in salt-grown seedlings. This reduction did not result from a global decrease in gene expression, since the expression of other nuclear genes was induced or not affected. Average cellular chloroplast genome copy number was reduced in salt-grown seedlings, suggesting that the reduction in steady-state transcript levels of chloroplast-encoded genes might result from a decrease in template DNA.

Ionomic responses and correlations between elements and metabolites under salt stress in wild and cultivated barley

A thorough understanding of ionic detoxification and homeostasis is imperative for improvement of salt tolerance in crops. However, the homeostasis of elements and their relationship to metabolites under salt stress have not been fully elucidated in plants. In this study, Tibetan wild barley accessions, XZ16 and XZ169, differing in salt tolerance, and a salt-tolerant cultivar CM72 were used to investigate ionomic profile changes in tissues in response to 150 and 300 mM NaCl at the germination and seedling stages. At the germination stage, the contents of Ca and Fe significantly decreased in roots, while K and S contents increased, and Ca and Mg contents decreased in shoots, after 10 d of treatment. At the seedling stage, the contents of K, Mg, P and Mn in roots and of K, Ca, Mg and S in shoots decreased significantly after 21 d of treatment. Moreover, Na had a significant negative correlation with metabolites involved in glycolysis, α-ketoglutaric acid, maleic acid and alanine in roots, and metabolites associated with the tricarboxylic acid (TCA) cycle, sucrose, polyols and aspartate in leaves. The salt-tolerant genotypes XZ16 and CM72 showed a lower Na content in tissues, and less reduction in Zn and Cu in roots, of Ca, Mg and S in leaves, and shoot DW than the sensitive genotype XZ169, when exposed to a higher salt level. The results indicated that restriction of Na accumulation and rearrangement of nutrient elements and metabolites in barley tissues are possibly attributable to development of salt tolerance.

Ability of leaf mesophyll to retain potassium correlates with salinity tolerance in wheat and barley

This work investigated the importance of the ability of leaf mesophyll cells to control K(+) flux across the plasma membrane as a trait conferring tissue tolerance mechanism in plants grown under saline conditions. Four wheat (Triticum aestivum and Triticum turgidum) and four barley (Hordeum vulgare) genotypes contrasting in their salinity tolerance were grown under glasshouse conditions. Seven to 10-day-old leaves were excised, and net K(+) and H(+) fluxes were measured from either epidermal or mesophyll cells upon acute 100 mM treatment (mimicking plant failure to restrict Na(+) delivery to the shoot) using non-invasive microelectrode ion flux estimation (the MIFE) system. To enable net ion flux measurements from leaf epidermal cells, removal of epicuticular waxes was trialed with organic solvents. A series of methodological experiments was conducted to test the efficiency of different methods of wax removal, and the impact of experimental procedures on cell viability, in order to optimize the method. A strong positive correlation was found between plants' ability to retain K(+) in salt-treated leaves and their salinity tolerance, in both wheat and especially barley. The observed effects were related to the ionic but not osmotic component of salt stress. Pharmacological experiments have suggested that voltage-gated K(+) -permeable channels mediate K(+) retention in leaf mesophyll upon elevated NaCl levels in the apoplast. It is concluded that MIFE measurements of NaCl-induced K(+) fluxes from leaf mesophyll may be used as an efficient screening tool for breeding in cereals for salinity tissue tolerance.

The K+/H+ antiporter LeNHX2 increases salt tolerance by improving K+ homeostasis in transgenic tomato

The endosomal LeNHX2 ion transporter exchanges H(+) with K(+) and, to lesser extent, Na(+) . Here, we investigated the response to NaCl supply and K(+) deprivation in transgenic tomato (Solanum lycopersicum L.) overexpressing LeNHX2 and show that transformed tomato plants grew better in saline conditions than untransformed controls, whereas in the absence of K(+) the opposite was found. Analysis of mineral composition showed a higher K(+) content in roots, shoots and xylem sap of transgenic plants and no differences in Na(+) content between transgenic and untransformed plants grown either in the presence or the absence of 120 mm NaCl. Transgenic plants showed higher Na(+)/H(+) and, above all, K(+)/H(+) transport activity in root intracellular membrane vesicles. Under K(+) limiting conditions, transgenic plants enhanced root expression of the high-affinity K(+) uptake system HAK5 compared to untransformed controls. Furthermore, tomato overexpressing LeNHX2 showed twofold higher K(+) depletion rates and half cytosolic K(+) activity than untransformed controls. Under NaCl stress, transgenic plants showed higher uptake velocity for K(+) and lower cytosolic K(+) activity than untransformed plants. These results indicate the fundamental role of K(+) homeostasis in the better performance of LeNHX2 overexpressing tomato under NaCl stress.

Transcriptional variation in response to salt stress in commonly used Arabidopsis thaliana accessions

Transcriptional variation is increasingly recognized as a component of genetic diversity and environmental adaptation. It can also provide insights into stress responsive determinants and underlying adaptive mechanisms. Prior studies showed phenotypic differences in response to salinity stress for two widely used Arabidopsis thaliana accessions, Wassilewskija-2 (Ws) and Columbia-0 (Col). This study examined changes in global gene expression in relation to differences in response to salt stress among Ws, Col, and the glabrous mutant of Col [Col(gl)]. Transcripts most highly affected by accession and salt stress were related to abiotic or biotic stress responses. Approximately 60% of salt-induced changes in Ws overlapped with changes in Col, suggesting common salt stress responses. However, a markedly greater number of genes was altered in the highly salt sensitive Col, likely reflecting both adaptive responses and salt injury. The Col(gl) transcriptome was least affected by salt. Many salt-responsive transcripts observed in Col were altered in Col(gl) prior to salt stress, indicating that even without salt, the gl1-1 mutation induced a suite of stress responsive genes. Regardless of salt stress, there were greater transcriptomic differences between Col and Col(gl) than between Col and Ws. The transcript expression differences between [Ws vs. Col] and [Col(gl) vs. Col] formed largely non-overlapping sets. Thus, although Ws, Col and Col(gl) are commonly and sometimes interchangeably used, here they displayed distinct responses. Collectively, their observed expression differences likely reflect a combination of adaptive traits, response to injury, or phenotypic buffering of mutational effects.

The role of nitrogen availability for the salt-tolerance of two different varieties of durum wheat

Salt stress tolerance of durum wheat was assessed in control and 200 and 300 mM NaCl-exposed seed of two cultivars (BidiAP4 and Azizi). These salt treatments were accompanied by different levels of nitrate (Ca(NO3)2) added to the media (0.1, 3, 10 mM). The data showed that NaCl stress increased Na(+) and Cl(-) contents and lowered K(+) and NO3 (-) levels in seeds of BidiAP4 cultivar. In Azizi seeds exposed to NaCl, Na(+) and K(+) were highly accumulated while low levels of NO3 (-) and Cl(-) were detected. Those findings highlight the difference in the salt stress tolerance of these two durum wheat cultivars also depending on nitrogen (N) availability, Azizi cultivar being less sensitive to NaCl treatment than BidiAP4. These data also suggested a relationship between salt tolerance capacity and enhancement or maintenance of nitrogen and carbon metabolisms enzyme activity.

AtALMT9 is a malate-activated vacuolar chloride channel required for stomatal opening in Arabidopsis

Water deficit strongly affects crop productivity. Plants control water loss and CO₂ uptake by regulating the aperture of the stomatal pores within the leaf epidermis. Stomata aperture is regulated by the two guard cells forming the pore and changing their size in response to ion uptake and release. While our knowledge about potassium and chloride fluxes across the plasma membrane of guard cells is advanced, little is known about fluxes across the vacuolar membrane. Here we present the molecular identification of the long-sought-after vacuolar chloride channel. AtALMT9 is a chloride channel activated by physiological concentrations of cytosolic malate. Single-channel measurements demonstrate that this activation is due to a malate-dependent increase in the channel open probability. Arabidopsis thaliana atalmt9 knockout mutants exhibited impaired stomatal opening and wilt more slowly than the wild type. Our findings show that AtALMT9 is a vacuolar chloride channel having a major role in controlling stomata aperture.

Aluminium-activated malate transporters are exclusive to plants, regulating the transport of ions across the membranes on which they are expressed. De Angeli and colleagues show that AtALMT9 acts as a vacuolar chloride channel that is activated by cytosolic malate, and that this regulates stomata aperture.

Differentially delayed root proteome responses to salt stress in sugar cane varieties

Soil salinity is a limiting factor to sugar cane crop development, although in general plants present variable mechanisms of tolerance to salinity stress. The molecular basis underlying these mechanisms can be inferred by using proteomic analysis. Thus, the objective of this work was to identify differentially expressed proteins in sugar cane plants submitted to salinity stress. For that, a greenhouse experiment was established with four sugar cane varieties and two salt conditions, 0 mM (control) and 200 mM NaCl. Physiological and proteomics analyses were performed after 2 and 72 h of stress induction by salt. Distinct physiological responses to salinity stress were observed in the varieties and linked to tolerance mechanisms. In proteomic analysis, the roots soluble protein fraction was extracted, quantified, and analyzed through bidimensional electrophoresis. Gel images analyses were done computationally, where in each contrast only one variable was considered (salinity condition or variety). Differential spots were excised, digested by trypsin, and identified via mass spectrometry. The tolerant variety RB867515 showed the highest accumulation of proteins involved in growth, development, carbohydrate and energy metabolism, reactive oxygen species metabolization, protein protection, and membrane stabilization after 2 h of stress. On the other hand, the presence of these proteins in the sensitive variety was verified only in stress treatment after 72 h. These data indicate that these stress responses pathways play a role in the tolerance to salinity in sugar cane, and their effectiveness for phenotypical tolerance depends on early stress detection and activation of the coding genes expression.

Biochar mitigates negative effects of salt additions on two herbaceous plant species

Addition of pyrolyzed biomass ("biochar") to soils has commonly been shown to increase crop yields and alleviate plant stresses associated with drought and exposure to toxic materials. Here we investigate the ability of biochar (at two dosages: 5 and 50 t ha(-1)) to mitigate salt-induced stress, simulating road salt additions in a factorial glasshouse experiment involving the broadleaved herbaceous plants Abutilon theophrasti and Prunella vulgaris. Salt additions of 30 g m(-2) NaCl to unamended soils resulted in high mortality rates for both species. Biochar (Fagus grandifolia sawdust pyrolyzed at 378 °C), when applied at 50 t ha(-1) as a top dressing, completely alleviated salt-induced mortality in A. theophrasti and prolonged survival of P. vulgaris. Surviving A. theophrasti plants that received both 50 t ha(-1) biochar and salt addition treatments showed growth rates and physiological performance similar to plants without salt addition. Biochar treatments alone also substantially increased biomass of P. vulgaris, with a ∼50% increase relative to untreated controls at both biochar dosages. Biochar did not significantly affect photosynthetic carbon gain (Amax), water use efficiency, or chlorophyll fluorescence (Fv/Fm) in either species. Our results indicate that biochar can ameliorate salt stress effects on plants through salt sorption, suggesting novel applications of biochar to mitigate effects of salinization in agricultural, urban, and contaminated soils.

Phenotypic effects of salt and heat stress over three generations in Arabidopsis thaliana

Current and predicted environmental change will force many organisms to adapt to novel conditions, especially sessile organisms such as plants. It is therefore important to better understand how plants react to environmental stress and to what extent genotypes differ in such responses. It has been proposed that adaptation to novel conditions could be facilitated by heritable epigenetic changes induced by environmental stress, independent of genetic variation. Here we assessed phenotypic effects of heat and salt stress within and across three generations using four highly inbred Arabidopsis thaliana genotypes (Col, Cvi, Ler and Sha). Salt stress generally decreased fitness, but genotypes were differently affected, suggesting that susceptibility of A. thaliana to salt stress varies among genotypes. Heat stress at an early rosette stage had less detrimental effects but accelerated flowering in three out of four accessions. Additionally, we found three different modes of transgenerational effects on phenotypes, all harboring the potential of being adaptive: heat stress in previous generations induced faster rosette growth in Sha, both under heat and control conditions, resembling a tracking response, while in Cvi, the phenotypic variance of several traits increased, resembling diversified bet-hedging. Salt stress experienced in earlier generations altered plant architecture of Sha under salt but not control conditions, similar to transgenerational phenotypic plasticity. However, transgenerational phenotypic effects depended on the type of stress as well as on genotype, suggesting that such effects may not be a general response leading to adaptation to novel environmental conditions in A. thaliana.

Transcriptome analysis of Salicornia europaea under saline conditions revealed the adaptive primary metabolic pathways as early events to facilitate salt adaptation

BACKGROUND

Halophytes such as Salicornia europaea have evolved to exhibit unique mechanisms controlled by complex networks and regulated by numerous genes and interactions to adapt to habitats with high salinity. However, these mechanisms remain unknown.

METHODS

To investigate the mechanism by which halophytes tolerate salt based on changes in the whole transcriptome, we performed transcriptome sequencing and functional annotation by database search. Using the unigene database, we conducted digital gene expression analysis of S. europaea at various time points after these materials were treated with NaCl. We also quantified ion uptakes. Gene functional enrichment analysis was performed to determine the important pathways involved in this process.

RESULTS

A total of 57,151 unigenes with lengths of >300 bp were assembled, in which 57.5% of these unigenes were functionally annotated. Differentially expressed genes indicated that cell wall metabolism and lignin biosynthetic pathways were significantly enriched in S. europaea to promote the development of the xylem under saline conditions. This result is consistent with the increase in sodium uptake as ions pass through the xylem. Given that PSII efficiency remained unaltered, salt treatment activated the expression of electron transfer-related genes encoded by the chloroplast chromosome. Chlorophyll biosynthesis was also inhibited, indicating the energy-efficient state of the electron transfer system of S. europaea.

CONCLUSIONS

The key function of adjusting important primary metabolic pathways in salt adaption was identified by analyzing the changes in the transcriptome of S. europaea. These pathways could involve unique salt tolerance mechanisms in halophytes. This study also provided information as the basis of future investigations on salt response genes in S. europaea. Ample gene resources were also provided to improve the genes responsible for the salt tolerance ability of crops.

Responses of African Grasses in the Genus Sporobolus to Defoliation and Sodium Stress: Tradeoffs, Cross-Tolerance, or Independent Responses?

In the Serengeti ecosystem of East Africa, grazing ungulates prefer areas with elevated grass Na, suggesting that some grasses tolerate both high soil Na and defoliation. We performed a factorial Na-by-defoliation greenhouse study with five abundant Sporobolus congeners to explore whether Serengeti grasses possess traits which: (i) confer tolerance to both Na and defoliation (cross-tolerance); (ii) display a tradeoff; or (iii) act independently in their tolerances. Our expectation was that related grasses would exhibit cross-tolerance when simultaneously subjected to Na and defoliation. Instead, we found that physiological tolerances and growth responses to Na and defoliation did not correlate but instead acted independently: species characterized by intense grazing in the field showed no growth or photosynthetic compensation for combined Na and defoliation. Additionally, in all but the highest Na dosage, mortality was higher when species were exposed to both Na and defoliation together. Across species, mortality rates were greater in short-statured species which occur on sodic soils in heavily grazed areas. Mortality among species was positively correlated with specific leaf area, specific root length, and relative growth rate, suggesting that rapidly growing species which invest in low cost tissues have higher rates of mortality when exposed to multiple stressors. We speculate that the prevalence of these species in areas of high Na and disturbance is explained by alternative strategies, such as high fecundity, a wide range of germination conditions, or further dispersal, to compensate for the lack of additional tolerance mechanisms.

Effect of long-term salinity on cellular antioxidants, compatible solute and fatty acid profile of Sweet Annie (Artemisia annua L.)

Impact of long-term salinity and subsequent oxidative stress was studied on cellular antioxidants, proline accumulation and lipid profile of Artemisia annua L. (Sweet Annie or Qinghao) which yields artemisinin (Qinghaosu), effective against cerebral malaria-causing strains of Plasmodium falciparum. Under salinity (0.0-160 mM NaCl), in A. annua, proline accumulation, contents of ascorbate and glutathione and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT) increased, but the contents of reduced forms of glutathione (GSH) and ascorbate declined. The fatty-acid profiling revealed a major salinity-induced shift towards long-chain and mono-saturated fatty acids. Myristic acid (14:0), palmitoleic acid (16:1), linoleic acid (18:2) and erucic acid (22:1) increased by 141%, 186%, 34% and 908%, respectively, in comparison with the control. Contents of oleic acid (18:1), linolenic acid (18:3), arachidonic acid (22:0) and lignoceric acid (24:0) decreased by 50%, 17%, 44% and 78%, respectively. Thus, in A. annua, salinity declines ascorbate and GSH contents. However, increased levels of proline and total glutathione (GSH+GSSG), and activities of antioxidant enzymes might provide a certain level of tolerance. Modification in fatty-acid composition might be a membrane adaptation to long-term salinity and oxidative stress.

Learning from halophytes: physiological basis and strategies to improve abiotic stress tolerance in crops

BACKGROUND

Global annual losses in agricultural production from salt-affected land are in excess of US$12 billion and rising. At the same time, a significant amount of arable land is becoming lost to urban sprawl, forcing agricultural production into marginal areas. Consequently, there is a need for a major breakthrough in crop breeding for salinity tolerance. Given the limited range of genetic diversity in this trait within traditional crops, stress tolerance genes and mechanisms must be identified in extremophiles and then introduced into traditional crops.

SCOPE AND CONCLUSIONS

This review argues that learning from halophytes may be a promising way of achieving this goal. The paper is focused around two central questions: what are the key physiological mechanisms conferring salinity tolerance in halophytes that can be introduced into non-halophyte crop species to improve their performance under saline conditions and what specific genes need to be targeted to achieve this goal? The specific traits that are discussed and advocated include: manipulation of trichome shape, size and density to enable their use for external Na(+) sequestration; increasing the efficiency of internal Na(+) sequestration in vacuoles by the orchestrated regulation of tonoplast NHX exchangers and slow and fast vacuolar channels, combined with greater cytosolic K(+) retention; controlling stomata aperture and optimizing water use efficiency by reducing stomatal density; and efficient control of xylem ion loading, enabling rapid shoot osmotic adjustment while preventing prolonged Na(+) transport to the shoot.

Populus euphratica XTH overexpression enhances salinity tolerance by the development of leaf succulence in transgenic tobacco plants

Populus euphratica is a salt-tolerant tree species that develops leaf succulence after a prolonged period of salinity stress. In the present study, a putative xyloglucan endotransglucosylase/hydrolase gene (PeXTH) from P. euphratica was isolated and transferred to tobacco plants. PeXTH localized exclusively to the endoplasmic reticulum and cell wall. Plants overexpressing PeXTH were more salt tolerant than wild-type tobacco with respect to root and leaf growth, and survival. The increased capacity for salt tolerance was due mainly to the anatomical and physiological alterations caused by PeXTH overexpression. Compared with the wild type, PeXTH-transgenic plants contained 36% higher water content per unit area and 39% higher ratio of fresh weight to dry weight, a hallmark of leaf succulence. However, the increased water storage in the leaves in PeXTH-transgenic plants was not accompanied by greater leaf thickness but was due to highly packed palisade parenchyma cells and fewer intercellular air spaces between mesophyll cells. In addition to the salt dilution effect in response to NaCl, these anatomical changes increased leaf water-retaining capacity, which lowered the increase of salt concentration in the succulent tissues and mesophyll cells. Moreover, the increased number of mesophyll cells reduced the intercellular air space, which improved carbon economy and resulted in a 47-78% greater net photosynthesis under control and salt treatments (100-150 mM NaCl). Taken together, the results indicate that PeXTH overexpression enhanced salt tolerance by the development of succulent leaves in tobacco plants without swelling.