Sodium transport in plants: a critical review

Effects of non-uniform root zone salinity on growth, ion regulation, and antioxidant defense system in two alfalfa cultivars

A split-root system was established to investigate the effects of uniform (0/0, 50/50, and 200/200 mM salt [NaCl]) and non-uniform (0/200 and 50/200 mM NaCl) salt stress on growth, ion regulation, and the antioxidant defense system of alfalfa (Medicago sativa) by comparing a salt-tolerant (Zhongmu No.1) and salt-sensitive (Algonquin) cultivar. We found that non-uniform salinity was associated with greater plant growth rate and shoot dry weight, lower leaf Na⁺ concentration, higher leaf potassium cation (K⁺) concentration, lower lipid peroxidation, and greater superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), and peroxidase (EC 1.11.1.7) activities, compared to uniform salt stress in both alfalfa cultivars. Under non-uniform salinity, a significant increase in Na⁺ concentration and Na⁺ efflux and a decline in K⁺ efflux in the no-saline or low-saline part of the roots alleviated salt damage. Our results also demonstrated that proline and antioxidant enzymes accumulated in both the no- or low-saline and high-saline roots, revealing that osmotic adjustment and antioxidant defense had systemic rather than localized effects in alfalfa plants, and there was a functional equilibrium within the root system under non-uniform salt stress. The salt-tolerant cultivar Zhongmu No.1 exhibited greater levels of growth compared to Algonquin under both uniform and non-uniform salt stress, with Na⁺ tolerance and efflux abilities more effective and greater antioxidant defense capacity evident for cultivar Zhongmu No.1.

Production of Low-Potassium Content Melon Through Hydroponic Nutrient Management Using Perlite Substrate

Chronic kidney disease patients are restricted to foods with high potassium content but our daily diets including melon are rich in potassium. Therefore, we investigated the production of low-potassium melon through hydroponic nutrient management in soilless culture using perlite substrate during autumn season of 2012, 2014 and spring season of 2016. In the first study, melon plants were supplied with 50% standard 'Enshi' nutrient solution until first 2 weeks of culture. In 3rd and 4th week, amount of applied potassium was 50, 75, 100, and 125% of required potassium nitrate for each plant per week (based on our previous study). It was found that, melon plants grown with 50% of its required potassium nitrate produced fruits with about 53% low-potassium compared to control. In the following study, four cultivars viz. Panna, Miyabi shunjuukei, Miyabi akifuyu412, and Miyabi soushun banshun309 were evaluated for their relative suitability of low-potassium melon production. Results showed insignificant difference in fruit potassium content among the cultivars used. Source of potassium fertilizer as potassium nitrate and potassium sulfate and their restriction (from 1 or 2 weeks after anthesis) were also studied. There were no influences on fruit potassium content and yield due to sources of potassium fertilizer and restriction timings. In our previous studies, it was evident that potassium can be translocated from leaves to fruits at maturity when it was supplied nutrient without potassium. Thus, we also studied total number of leaves per plant (23, 24, 25, 26, and 27 leaves per plant). It was evident that fruit potassium, yield, and quality were not influenced significantly due to differences in number of leaves per plant. These studies showed that restriction of potassium nitrate in the culture solution from anthesis to harvest could produce melon fruits with low-potassium (>20%) content compared to potassium content of greenhouse grown melon (340 mg/100 g FW). Quality testing and clinical validation of low-potassium melon also showed positive responses compared to greenhouse grown melon.

The High-Affinity Potassium Transporter EpHKT1;2 From the Extremophile Eutrema parvula Mediates Salt Tolerance

To survive salt stress, plants must maintain a balance between sodium and potassium ions. High-affinity potassium transporters (HKTs) play a key role in reducing Na+ toxicity through K+ uptake. Eutrema parvula (formerly known as Thellungiella parvula), a halophyte closely related to Arabidopsis, has two HKT1 genes that encode EpHKT1;1 and EpHKT1;2. In response to high salinity, the EpHKT1;2 transcript level increased rapidly; by contrast, the EpHKT1;1 transcript increased more slowly in response to salt treatment. Yeast cells expressing EpHKT1;2 were able to tolerate high concentrations of NaCl, whereas EpHKT1;1-expressing yeast cells remained sensitive to NaCl. Amino acid sequence alignment with other plant HKTs showed that EpHKT1;1 contains an asparagine residue (Asn-213) in the second pore-loop domain, but EpHKT1;2 contains an aspartic acid residue (Asp-205) at the same position. Yeast cells expressing EpHKT1;1, in which Asn-213 was substituted with Asp, were able to tolerate high concentrations of NaCl. In contrast, substitution of Asp-205 by Asn in EpHKT1;2 did not enhance salt tolerance and rather resulted in a similar function to that of AtHKT1 (Na+ influx but no K+ influx), indicating that the presence of Asn or Asp determines the mode of cation selectivity of the HKT1-type transporters. Moreover, Arabidopsis plants (Col-gl) overexpressing EpHKT1;2 showed significantly higher tolerance to salt stress and accumulated less Na+ and more K+ compared to those overexpressing EpHKT1;1 or AtHKT1. Taken together, these results suggest that EpHKT1;2 mediates tolerance to Na+ ion toxicity in E. parvula and is a major contributor to its halophytic nature.

Excess iron stress reduces root tip zone growth through nitric oxide-mediated repression of potassium homeostasis in Arabidopsis

The root tip zone is regarded as the principal action site for iron (Fe) toxicity and is more sensitive than other root zones, but the mechanism underpinning this remains largely unknown. We explored the mechanism underpinning the higher sensitivity at the Arabidopsis root tip and elucidated the role of nitric oxide (NO) using NO-related mutants and pharmacological methods. Higher Fe sensitivity of the root tip is associated with reduced potassium (K⁺ ) retention. NO in root tips is increased significantly above levels elsewhere in the root and is involved in the arrest of primary root tip zone growth under excess Fe, at least in part related to NO-induced K⁺ loss via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4) and reduced root tip zone cell viability. Moreover, ethylene can antagonize excess Fe-inhibited root growth and K⁺ efflux, in part by the control of root tip NO levels. We conclude that excess Fe attenuates root growth by effecting an increase in root tip zone NO, and that this attenuation is related to NO-mediated alterations in K⁺ homeostasis, partly via SNO1/SOS4.

Abiotic Stress Signaling in Wheat - An Inclusive Overview of Hormonal Interactions During Abiotic Stress Responses in Wheat

Rapid global warming directly impacts agricultural productivity and poses a major challenge to the present-day agriculture. Recent climate change models predict severe losses in crop production worldwide due to the changing environment, and in wheat, this can be as large as 42 Mt/°C rise in temperature. Although wheat occupies the largest total harvested area (38.8%) among the cereals including rice and maize, its total productivity remains the lowest. The major production losses in wheat are caused more by abiotic stresses such as drought, salinity, and high temperature than by biotic insults. Thus, understanding the effects of these stresses becomes indispensable for wheat improvement programs which have depended mainly on the genetic variations present in the wheat genome through conventional breeding. Notably, recent biotechnological breakthroughs in the understanding of gene functions and access to whole genome sequences have opened new avenues for crop improvement. Despite the availability of such resources in wheat, progress is still limited to the understanding of the stress signaling mechanisms using model plants such as Arabidopsis, rice and Brachypodium and not directly using wheat as the model organism. This review presents an inclusive overview of the phenotypic and physiological changes in wheat due to various abiotic stresses followed by the current state of knowledge on the identified mechanisms of perception and signal transduction in wheat. Specifically, this review provides an in-depth analysis of different hormonal interactions and signaling observed during abiotic stress signaling in wheat.

Elucidating the role of osmotic, ionic and major salt responsive transcript components towards salinity tolerance in contrasting chickpea (Cicer arietinum L.) genotypes

The growth of chickpea (Cicer arietinum L.) is extremely hampered by salt stress. Understanding of physio-biochemical and molecular attributes along with morphological traits contributing to the salinity tolerance is important for developing salt tolerant chickpea varieties. To explore these facts, two genotypes CSG8962 and HC5 with contrasting salt tolerance were evaluated in the salinity stress (Control and 120 mM NaCl) conditions. CSG8962 maintained lower Na/K ratio in root and shoot, trammeled Na translocation to the shoots from roots compared to HC5 which ascribed to better exclusion of salt from its roots and compartmentation in the shoot. In chickpea, salt stress specifically induced genes/sequences involved at several levels in the salt stress signaling pathway. Higher induction of trehalose 6 phosphate synthase and protein kinase genes pertaining to the osmotic and signaling modules, respectively, were evident in CSG8962 compared to HC5. Further transcripts of late embryogenesis abundant, non-specific lipid transfer protein, HI and 219 genes/sequences were also highly induced in CSG8962 compared to HC5 which emphasizes the better protection of cellular membranous network and membrane-bound macromolecules under salt stress. This further suppressed the stress enhanced electrolyte leakage, loss of turgidity, promoted the higher compatible solute accumulation and maintained better cellular ion homoeostasis in CSG8962 compared to HC5. Our study further adds to the importance of these genes in salt tolerance by comparing their behavior in contrasting chickpea genotypes.

Membrane fluxes, bypass flows, and sodium stress in rice: the influence of silicon

Provision of silicon (Si) to roots of rice (Oryza sativa L.) can alleviate salt stress by blocking apoplastic, transpirational bypass flow of Na+ from root to shoot. However, little is known about how Si affects Na+ fluxes across cell membranes. Here, we measured radiotracer fluxes of 24Na+, plasma membrane depolarization, tissue ion accumulation, and transpirational bypass flow, to examine the influence of Si on Na+ transport patterns in hydroponically grown, salt-sensitive (cv. IR29) and salt-tolerant (cv. Pokkali) rice. Si increased growth and lowered [Na+] in shoots of both cultivars, with minor effects in roots; neither root nor shoot [K+] were affected. In IR29, Si lowered shoot [Na+] via a large reduction in bypass flow, while in Pokkali, where bypass flow was small and not affected by Si, this was achieved mainly via a growth dilution of shoot Na+. Si had no effect on unidirectional 24Na+ fluxes (influx and efflux), or on Na+-stimulated plasma-membrane depolarization, in either IR29 or Pokkali. We conclude that, while Si can reduce Na+ translocation via bypass flow in some (but not all) rice cultivars, it does not affect unidirectional Na+ transport or Na+ cycling in roots, either across root cell membranes or within the bulk root apoplast.

Variation in the Abundance of OsHAK1 Transcript Underlies the Differential Salinity Tolerance of an indica and a japonica Rice Cultivar

Salinity imposes a major constraint over the productivity of rice. A set of chromosome segment substitution lines (CSSLs), derived from a cross between the japonica type cultivar (cv.) Nipponbare (salinity sensitive) and the indica type cv. 9311 (moderately tolerant), was scored using a hydroponics system for their salinity tolerance at the seedling stage. Two of the CSSLs, which share a ∼1.2 Mbp stretch of chromosome 4 derived from cv. Nipponbare, were as sensitive to the stress as cv. Nipponbare itself. Fine mapping based on an F2 population bred from a backcross between one of these CSSLs and cv. 9311 narrowed this region to 95 Kbp, within which only one gene (OsHAK1) exhibited a differential (lower) transcript abundance in cv. Nipponbare and the two CSSLs compared to in cv. 9311. The gene was up-regulated by exposure to salinity stress both in the root and the shoot, while a knockout mutant proved to be more salinity sensitive than its wild type with respect to its growth at both the vegetative and reproductive stages. Seedlings over-expressing OsHAK1 were more tolerant than wild type, displaying a superior photosynthetic rate, a higher leaf chlorophyll content, an enhanced accumulation of proline and a reduced level of lipid peroxidation. At the transcriptome level, the over-expression of OsHAK1 stimulated a number of stress-responsive genes as well as four genes known to be involved in Na+ homeostasis and the salinity response (OsHKT1;5, OsSOS1, OsLti6a and OsLti6b). When the stress was applied at booting through to maturity, the OsHAK1 over-expressors out-yielded wild type by 25%, and no negative pleiotropic effects were expressed in plants gown under non-saline conditions. The level of expression of OsHAK1 was correlated with Na+/K+ homeostasis, which implies that the gene should be explored a target for molecular approaches to the improvement of salinity tolerance in rice.

Transcriptome sequencing and comparative analysis of differentially-expressed isoforms in the roots of Halogeton glomeratus under salt stress

Although Halogeton glomeratus (H. glomeratus) has been confirmed to have a unique mechanism to regulate Na⁺ efflux from the cytoplasm and compartmentalize Na⁺ into leaf vacuoles, little is known about the salt tolerance mechanisms of roots under salinity stress. In the present study, transcripts were sequenced using the BGISEQ-500 sequencing platform (BGI, Wuhan, China). After quality control, approximately 24.08 million clean reads were obtained and the average mapping ratio to the reference gene was 70.00%. When comparing salt-treated samples with the control, a total of 550, 590, 1411 and 2063 DEIs were identified at 2, 6, 24 and 72h, respectively. Numerous differentially-expressed isoforms that play important roles in response and adaptation to salt condition are related to metabolic processes, cellular processes, single-organism processes, localization, biological regulation, responses to stimulus, binding, catalytic activity and transporter activity. Fifty-eight salt-induced isoforms were common to different stages of salt stress; most of these DEIs were related to signal transduction and transporters, which maybe the core isoforms regulating Na⁺ uptake and transport in the roots of H. glomeratus. The expression patterns of 18 DEIs that were detected by quantitative real-time polymerase chain reaction were consistent with their respective changes in transcript abundance as identified by RNA-Seq technology. The present study thoroughly explored potential isoforms involved in salt tolerance on H. glomeratus roots at five time points. Our results may serve as an important resource for the H. glomeratus research community, improving our understanding of salt tolerance in halophyte survival under high salinity stress.

Mitigation of salt stress in white clover (Trifolium repens) by Azospirillum brasilense and its inoculation effect

BACKGROUND

Salinity is one of the increasingly serious environmental problems worldwide for cultivating agricultural crops. The present study was aimed to ascertain the potential of beneficial soil bacterium Azospirillum brasilense to alleviate saline stress in Trifolium repens. Experimental plants (white clover) were grown from seeds and inoculated with or without A. brasilense bacterial strain supplemented with 0, 40, 80, or 120 mM NaCl into soil.

RESULTS

The growth attributes including, shoot heights, root lengths, fresh and dry weights, leaf area and chlorophyll content were significantly enhanced in T. repens plants grown in A. brasilense inoculated soil than un-inoculated controls, particularly under elevated salinity conditions (40, 80 and 120 mM NaCl). Malondialdehyde content of leaf was recorded to be declined under saline conditions. Moreover, the K⁺/Na⁺ ratio was also improved in bacterium-inoculated plants, since A. brasilense significantly reduced the root and shoot Na⁺ level under high salty environment.

CONCLUSIONS

Results revealed that soil inoculation with A. brasilense could significantly promote T. repens growth under both non-saline and saline environments, and this study might be extended to other vegetables and crops for the germination and growth enhancement.

Plant ionic relation and whole-plant physiological responses to waterlogging, salinity and their combination in barley

Waterlogging and salinity stresses significantly affect crop growth and global food production, and these stresses are often interrelated because waterlogging can lead to land salinisation by transporting salts to the surface. Although the physiological and molecular mechanisms of plant responses to each of these environmental constraints have been studied in detail, fewer studies have dealt with potential mechanisms underlying plant tolerance to the combined stress. This gap in knowledge is jeopardising the success of breeding programs. In the present work we studied the physiological and agronomical responses of 12 barley varieties contrasting in salinity stress tolerance to waterlogging (WL), salinity (NaCl) and combined (WL/NaCl) stresses. Stress damage symptoms were much greater in plants under combined WL/NaCl stress than those under separate stresses. The shoot biomass, chlorophyll content, maximum photochemical efficiency of PSII and shoot K+ concentration were significantly reduced under WL/NaCl conditions, whereas shoot Na+ concentration increased. Plants exposed to salinity stress showed lower damage indexes compared with WL. Chlorophyll fluorescence Fv/Fm value showed the highest correlation with the stress damage index under WL/NaCl conditions (r=-0.751) compared with other measured physiological traits, so was nominated as a good parameter to rank the tolerance of varieties. Average FW was reduced to 73±2, 52±1 and 23±2 percent of the control under NaCl, WL and combined WL/NaCl treatments respectively. Generally, the adverse effect of WL/NaCl stress was much greater in salt-sensitive varieties than in more tolerant varieties. Na+ concentrations of the shoot under control conditions were 97±10µmolg-1 DW, and increased to 1519±123, 179±11 and 2733±248µmolg-1 under NaCl, WL and combined WL/NaCl stresses respectively. K+ concentrations were 1378±66, 1260±74, 1270±79 and 411±92µmolg-1 DW under control, NaCl, WL and combined WL/NaCl stresses respectively. No significant correlation was found between the overall salinity stress tolerance and amount of Na+ accumulated in plant shoots after 15 days of exposure to 250mM NaCl stress. However, plants exposed to combined salinity and WL stress showed a negative correlation between shoot Na+ accumulation and extent of salinity damage. Overall, the reported results indicate that K+ reduction in the plants under combined WL/NaCl stress, but not stress-induced Na+ accumulation in the shoot, was the most critical feature in determining the overall plant performance under combined stress conditions.

Nitrate-dependent shoot sodium accumulation and osmotic functions of sodium in Arabidopsis under saline conditions

Improving crop plants to be productive in saline soils or under irrigation with saline water would be an important technological advance in overcoming the food and freshwater crises that threaten the world population. However, even if the transformation of a glycophyte into a plant that thrives under seawater irrigation was biologically feasible, current knowledge about Na⁺ effects would be insufficient to support this technical advance. Intriguingly, crucial details about Na⁺ uptake and its function in the plant have not yet been well established. We here propose that under saline conditions two nitrate-dependent transport systems in series that take up and load Na⁺ into the xylem constitute the major pathway for the accumulation of Na⁺ in Arabidopsis shoots; this pathway can also function with chloride at high concentrations. In nrt1.1 nitrate transport mutants, plant Na⁺ accumulation was partially defective, which suggests that NRT1.1 either partially mediates or modulates the nitrate-dependent Na⁺ transport. Arabidopsis plants exposed to an osmotic potential of -1.0 MPa (400 mOsm) for 24 h showed high water loss and wilting in sorbitol or Na/MES, where Na⁺ could not be accumulated. In contrast, in NaCl the plants that accumulated Na⁺ lost a low amount of water, and only suffered transitory wilting. We discuss that in Arabidopsis plants exposed to high NaCl concentrations, root Na⁺ uptake and tissue accumulation fulfil the primary function of osmotic adjustment, even if these processes lead to long-term toxicity.

The salt glands of Tamarix usneoides E. Mey. ex Bunge (South African Salt Cedar)

Tamarix usneoides is a halophyte tree endemic to south-western Africa. This species is known to excrete a range of ions from specialized glandular structures on its leaves. To understand the mechanisms involved in the transport, sequestration and excretion of ions by the glands, a study was performed on salt gland distribution and ultrastructure. The glands are vesiculated trichomes, comprised of eight cells viz. two basal collecting cells and six excretory cells, partially bounded by a secondary cell wall that could serve as an impermeable barrier, forcing excess ions to move from the apoplast of the surrounding tissue into the cytoplasm of the basal excretory cells. It was hypothesized that the ions are moved across the excretory cells in endocytotic vesicles that fuse with the plasmalemma or form junctional complexes, allowing ion movement from one excretory cell to the next. In the apical cell, the vesicles fuse with the plasmalemma, releasing the ions into the network of cell wall ingrowths which channel the ions to the outside surface of the cell. This study shows that there are distinct structural adaptations for the processing of ions for excretion, although the mechanism by which ions enter the cells still needs to be determined.

Halophytes: Potential Resources for Salt Stress Tolerance Genes and Promoters

Halophytes have demonstrated their capability to thrive under extremely saline conditions and thus considered as one of the best germplasm for saline agriculture. Salinity is a worldwide problem, and the salt-affected areas are increasing day-by-day because of scanty rainfall, poor irrigation system, salt ingression, water contamination, and other environmental factors. The salinity stress tolerance mechanism is a very complex phenomenon, and some pathways are coordinately linked for imparting salinity tolerance. Though a number of salt responsive genes have been reported from the halophytes, there is always a quest for promising stress-responsive genes that can modulate plant physiology according to the salt stress. Halophytes such as Aeluropus, Mesembryanthemum, Suaeda, Atriplex, Thellungiella, Cakile, and Salicornia serve as a potential candidate for the salt-responsive genes and promoters. Several known genes like antiporters (NHX, SOS, HKT, VTPase), ion channels (Cl-, Ca2+, aquaporins), antioxidant encoding genes (APX, CAT, GST, BADH, SOD) and some novel genes such as USP, SDR1, SRP etc. were isolated from halophytes and explored for developing stress tolerance in the crop plants (glycophytes). It is evidenced that stress triggers salt sensors that lead to the activation of stress tolerance mechanisms which involve multiple signaling proteins, up- or down-regulation of several genes, and finally the distinctive or collective effects of stress-responsive genes. In this review, halophytes are discussed as an excellent platform for salt responsive genes which can be utilized for developing salinity tolerance in crop plants through genetic engineering.

The sodium transporter encoded by the HKT1;2 gene modulates sodium/potassium homeostasis in tomato shoots under salinity

Excessive soil salinity diminishes crop yield and quality. In a previous study in tomato, we identified two closely linked genes encoding HKT1-like transporters, HKT1;1 and HKT1;2, as candidate genes for a major quantitative trait locus (kc7.1) related to shoot Na⁺ /K⁺ homeostasis - a major salt tolerance trait - using two populations of recombinant inbred lines (RILs). Here, we determine the effectiveness of these genes in conferring improved salt tolerance by using two near-isogenic lines (NILs) that were homozygous for either the Solanum lycopersicum allele (NIL17) or for the Solanum cheesmaniae allele (NIL14) at both HKT1 loci; transgenic lines derived from these NILs in which each HKT1;1 and HKT1;2 had been silenced by stable transformation were also used. Silencing of ScHKT1;2 and SlHKT1;2 altered the leaf Na⁺ /K⁺ ratio and caused hypersensitivity to salinity in plants cultivated under transpiring conditions, whereas silencing SlHKT1;1/ScHKT1;1 had a lesser effect. These results indicate that HKT1;2 has the more significant role in Na⁺ homeostasis and salinity tolerance in tomato.

Regulation of Na+ and K+ homeostasis in plants: towards improved salt stress tolerance in crop plants

Soil salinity is a major abiotic stress that results in considerable crop yield losses worldwide. However, some plant genotypes show a high tolerance to soil salinity, as they manage to maintain a high K⁺/Na⁺ ratio in the cytosol, in contrast to salt stress susceptible genotypes. Although, different plant genotypes show different salt tolerance mechanisms, they all rely on the regulation and function of K⁺ and Na⁺ transporters and H⁺ pumps, which generate the driving force for K⁺ and Na⁺ transport. In this review we will introduce salt stress responses in plants and summarize the current knowledge about the most important ion transporters that facilitate intra- and intercellular K⁺ and Na⁺ homeostasis in these organisms. We will describe and discuss the regulation and function of the H⁺-ATPases, H⁺-PPases, SOS1, HKTs, and NHXs, including the specific tissues where they work and their response to salt stress.

Genome-wide expression profiling in leaves and roots of date palm (Phoenix dactylifera L.) exposed to salinity

BACKGROUND

Date palm, as one of the most important fruit crops in North African and West Asian countries including Oman, is facing serious growth problems due to salinity, arising from persistent use of saline water for irrigation. Although date palm is a relatively salt-tolerant plant species, its adaptive mechanisms to salt stress are largely unknown.

RESULTS

In order to get an insight into molecular mechanisms of salt tolerance, RNA was profiled in leaves and roots of date palm seedlings subjected to NaCl for 10 days. Under salt stress, photosynthetic parameters were differentially affected; all gas exchange parameters were decreased but the quantum yield of PSII was unaffected while non-photochemical quenching was increased. Analyses of gene expression profiles revealed 2630 and 4687 genes were differentially expressed in leaves and roots, respectively, under salt stress. Of these, 194 genes were identified as commonly responding in both the tissue sources. Gene ontology (GO) analysis in leaves revealed enrichment of transcripts involved in metabolic pathways including photosynthesis, sucrose and starch metabolism, and oxidative phosphorylation, while in roots genes involved in membrane transport, phenylpropanoid biosynthesis, purine, thiamine, and tryptophan metabolism, and casparian strip development were enriched. Differentially expressed genes (DEGs) common to both tissues included the auxin responsive gene, GH3, a putative potassium transporter 8 and vacuolar membrane proton pump.

CONCLUSIONS

Leaf and root tissues respond differentially to salinity stress and this study has revealed genes and pathways that are associated with responses to elevated NaCl levels and thus may play important roles in salt tolerance providing a foundation for functional characterization of salt stress-responsive genes in the date palm.

The composite water and solute transport of barley (Hordeum vulgare) roots: effect of suberized barriers

BACKGROUND AND AIMS

Roots have complex anatomical structures, and certain localized cell layers develop suberized apoplastic barriers. The size and tightness of these barriers depend on the growth conditions and on the age of the root. Such complex anatomical structures result in a composite water and solute transport in roots.

METHODS

Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography-mass spectometry (GC-MS). The hydraulic conductivity of roots ( Lp r ) and of cortical cells ( Lp c ) was measured using root and cell pressure probes.

KEY RESULTS

When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones. However, they developed an endodermis. Endodermal Casparian bands first appeared as 'dots' as early as at 20 mm from the apex, whereas a patchy suberin lamellae appeared at 60 mm. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lp r . Comparison of Lp r and Lp c suggested that cell to cell pathways dominate for water transport in roots. However, the calculation of Lp r from Lp c showed that at least 26 % of water transport occurs through the apoplast. Roots had different solute permeabilities ( P sr ) and reflection coefficients ( σ sr ) for the solutes used. The σ sr was below unity for the solutes, which have virtually zero permeability for semi-permeable membranes.

CONCLUSIONS

Suberized endodermis significantly reduces Lp r of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series (cortex, endodermis) in addition to the currently included components arranged in parallel (apoplastic, cell to cell pathways).

Integrative Control Between Proton Pumps and SOS1 Antiporters in Roots is Crucial for Maintaining Low Na+ Accumulation and Salt Tolerance in Ammonium-Supplied Sorghum bicolor

An effective strategy for re-establishing K+ and Na+ homeostasis is a challenge for the improvement of plant performance in saline soil. Specifically, attempts to understand the mechanisms of Na+ extrusion from plant cells, the control of Na+ loading in the xylem and the partitioning of the accumulated Na+ between different plant organs are ongoing. Our goal was to provide insight into how an external nitrogen source affects Na+ accumulation in Sorghum bicolor under saline conditions. The NH4+ supply improved the salt tolerance of the plant by restricting Na+ accumulation and improving the K+/Na+ homeostasis in shoots, which was consistent with the high activity and expression of Na+/H+ antiporters and proton pumps in the plasma membrane and vacuoles in the roots, resulting in low Na+ loading in the xylem. Conversely, although NO3--grown plants had exclusion and sequestration mechanisms for Na+, these responses were not sufficient to reduce Na+ accumulation. In conclusion, NH4+ acts as an efficient signal to activate co-ordinately responses involved in the regulation of Na+ homeostasis in sorghum plants under salt stress, which leads to salt tolerance.

Role of LOTR1 in Nutrient Transport through Organization of Spatial Distribution of Root Endodermal Barriers

The formation of Casparian strips and suberin lamellae at the endodermis limits the free diffusion of nutrients and harmful substances via the apoplastic space between the soil solution and the stele in roots [1-3]. Casparian strips are ring-like lignin polymers deposited in the middle of anticlinal cell walls between endodermal cells and fill the gap between them [4-6]. Suberin lamellae are glycerolipid polymers covering the endodermal cells and likely function as a barrier to limit transmembrane movement of apoplastic solutes into the endodermal cells [7, 8]. However, the current knowledge on the formation of these two distinct endodermal barriers and their regulatory role in nutrient transport is still limited. Here, we identify an uncharacterized gene, LOTR1, essential for Casparian strip formation in Arabidopsis thaliana. The lotr1 mutants display altered localization of CASP1, an essential protein for Casparian strip formation [9], disrupted Casparian strips, ectopic suberization of endodermal cells, and low accumulation of shoot calcium (Ca). Degradation by expression of a suberin-degrading enzyme in the mutants revealed that the ectopic suberization at the endodermal cells limits Ca transport through the transmembrane pathway, thereby causing reduced Ca delivery to the shoot. Moreover, analysis of the mutants showed that suberin lamellae function as an apoplastic diffusion barrier to the stele at sites of lateral root emergence where Casparian strips are disrupted. Our findings suggest that the transmembrane pathway through unsuberized endodermal cells, rather than the sites of lateral root emergence, mediates the transport of apoplastic substances such as Ca into the xylem.

Overexpression of a Plasma Membrane Bound Na+/H+ Antiporter-Like Protein (SbNHXLP) Confers Salt Tolerance and Improves Fruit Yield in Tomato by Maintaining Ion Homeostasis

A Na+/H+ antiporter-like protein (NHXLP) was isolated from Sorghum bicolor L. (SbNHXLP) and validated by overexpressing in tomato for salt tolerance. Homozygous T2 transgenic lines when evaluated for salt tolerance, accumulated low Na+ and displayed enhanced salt tolerance compared to wild-type plants (WT). This is consistent with the amiloride binding assay of the protein. Transgenics exhibited higher accumulation of proline, K+, Ca2+, improved cambial conductivity, higher PSII, and antioxidative enzyme activities than WT. Fluorescence imaging results revealed lower Na+ and higher Ca2+ levels in transgenic roots. Co-immunoprecipitation experiments demonstrate that SbNHXLP interacts with a Solanum lycopersicum cation proton antiporter protein2 (SlCHX2). qRT-PCR results showed upregulation of SbNHXLP and SlCHX2 upon treatment with 200 mM NaCl and 100 mM potassium nitrate. SlCHX2 is known to be involved in K+ acquisition, and the interaction between these two proteins might help to accumulate more K+ ions, and thus maintain ion homeostasis. These results strongly suggest that plasma membrane bound SbNHXLP involves in Na+ exclusion, maintains ion homeostasis in transgenics in comparison with WT and alleviates NaCl stress.

A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. I. Clarifying the Roles of Endodermal Barriers in the Salt Stress Response

In this paper, we present a detailed and comprehensive mathematical model of active and passive ion and water transport in plant roots. Two key features are the explicit consideration of the separate, but interconnected, apoplastic, and symplastic transport pathways for ions and water, and the inclusion of both active and passive ion transport mechanisms. The model is used to investigate the respective roles of the endodermal Casparian strip and suberin lamellae in the salt stress response of plant roots. While it is thought that these barriers influence different transport pathways, it has proven difficult to distinguish their separate functions experimentally. In particular, the specific role of the suberin lamellae has been unclear. A key finding based on our simulations was that the Casparian strip is essential in preventing excessive uptake of Na⁺ into the plant via apoplastic bypass, with a barrier efficiency that is reflected by a sharp gradient in the steady-state radial distribution of apoplastic Na⁺ across the barrier. Even more significantly, this function cannot be replaced by the action of membrane transporters. The simulations also demonstrated that the positive effect of the Casparian strip of controlling Na⁺ uptake, was somewhat offset by its contribution to the osmotic stress component: a more effective barrier increased the detrimental osmotic stress effect. In contrast, the suberin lamellae were found to play a relatively minor, even non-essential, role in the overall response to salt stress, with the presence of the suberin lamellae resulting in only a slight reduction in Na⁺ uptake. However, perhaps more significantly, the simulations identified a possible role of suberin lamellae in reducing plant energy requirements by acting as a physical barrier to preventing the passive leakage of Na⁺ into endodermal cells. The model results suggest that more and particular experimental attention should be paid to the properties of the Casparian strip when assessing the salt tolerance of different plant varieties and species. Indeed, the Casparian strip appears to be a more promising target for plant breeding and plant genetic engineering efforts than the suberin lamellae for the goal of improving salt tolerance.

Tissue tolerance: an essential but elusive trait for salt-tolerant crops

For a plant to persist in saline soil, osmotic adjustment of all plant cells is essential. The more salt-tolerant species accumulate Na+ and Cl- to concentrations in leaves and roots that are similar to the external solution, thus allowing energy-efficient osmotic adjustment. Adverse effects of Na+ and Cl- on metabolism must be avoided, resulting in a situation known as 'tissue tolerance'. The strategy of sequestering Na+ and Cl- in vacuoles and keeping concentrations low in the cytoplasm is an important contributor to tissue tolerance. Although there are clear differences between species in the ability to accommodate these ions in their leaves, it remains unknown whether there is genetic variation in this ability within a species. This viewpoint considers the concept of tissue tolerance, and how to measure it. Four conclusions are drawn: (1) osmotic adjustment is inseparable from the trait of tissue tolerance; (2) energy-efficient osmotic adjustment should involve ions and only minimal organic solutes; (3) screening methods should focus on measuring tolerance, not injury; and (4) high-throughput protocols that avoid the need for control plants and multiple Na+ or Cl- measurements should be developed. We present guidelines to identify useful genetic variation in tissue tolerance that can be harnessed for plant breeding of salt tolerance.

New Insights on Plant Salt Tolerance Mechanisms and Their Potential Use for Breeding

Soil salinization is a major threat to agriculture in arid and semi-arid regions, where water scarcity and inadequate drainage of irrigated lands severely reduce crop yield. Salt accumulation inhibits plant growth and reduces the ability to uptake water and nutrients, leading to osmotic or water-deficit stress. Salt is also causing injury of the young photosynthetic leaves and acceleration of their senescence, as the Na+ cation is toxic when accumulating in cell cytosol resulting in ionic imbalance and toxicity of transpiring leaves. To cope with salt stress, plants have evolved mainly two types of tolerance mechanisms based on either limiting the entry of salt by the roots, or controlling its concentration and distribution. Understanding the overall control of Na+ accumulation and functional studies of genes involved in transport processes, will provide a new opportunity to improve the salinity tolerance of plants relevant to food security in arid regions. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress. Moreover, associations of cultures with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi could serve as an alternative and sustainable strategy to increase crop yields in salt-affected fields.

Salinity effects on chloroplast PSII performance in glycophytes and halophytes

The effects of NaCl stress and K+ nutrition on photosynthetic parameters of isolated chloroplasts were investigated using PAM fluorescence. Intact mesophyll cells were able to maintain optimal photosynthetic performance when exposed to salinity for more than 24h whereas isolated chloroplasts showed declines in both the relative electron transport rate (rETR) and the maximal photochemical efficiency of PSII (Fv/Fm) within the first hour of treatment. The rETR was much more sensitive to salt stress compared with Fv/Fm, with 40% inhibition of rETR observed at apoplastic NaCl concentration as low as 20mM. In isolated chloroplasts, absolute K+ concentrations were more essential for the maintenance of the optimal photochemical performance (Fv/Fm values) rather than sodium concentrations per se. Chloroplasts from halophyte species of quinoa (Chenopodium quinoa Willd.) and pigface (Carpobrotus rosii (Haw.) Schwantes) showed less than 18% decline in Fv/Fm under salinity, whereas the Fv/Fm decline in chloroplasts from glycophyte pea (Pisum sativum L.) and bean (Vicia faba L.) species was much stronger (31 and 47% respectively). Vanadate (a P-type ATPase inhibitor) significantly reduced Fv/Fm in both control and salinity treated chloroplasts (by 7 and 25% respectively), whereas no significant effects of gadolinium (blocker of non-selective cation channels) were observed in salt-treated chloroplasts. Tetraethyl ammonium (TEA) (K+ channel inhibitor) and amiloride (inhibitor of the Na+/H+ antiporter) increased the Fv/Fm of salinity treated chloroplasts by 16 and 17% respectively. These results suggest that chloroplasts' ability to regulate ion transport across the envelope and thylakoid membranes play a critical role in leaf photosynthetic performance under salinity.

Assessment of Stress Tolerance, Productivity, and Forage Quality in T1 Transgenic Alfalfa Co-overexpressing ZxNHX and ZxVP1-1 from Zygophyllum xanthoxylum

Salinization, desertification, and soil nutrient deprivation are threatening the production of alfalfa (Medicago sativa L.) in northern China. We have previously generated T0 transgenic alfalfa co-overexpressing Zygophyllum xanthoxylum ZxNHX and ZxVP1-1 genes with enhanced salt and drought tolerance. To further develop this excellent breeding material into the new forage cultivar, stress tolerance, productivity, and forage quality of T1 transgenic alfalfa (GM) were assessed in this study. The GM inherited the traits of salt and drought tolerance from T0 generation. Most importantly, co-overexpression of ZxNHX and ZxVP1-1 enhanced the tolerance to Pi deficiency in GM, which was associated with more Pi accumulation in plants. Meanwhile, T1 transgenic alfalfa developed a larger root system with increased root size, root dry weight and root/shoot ratio, which may be one important reason for the improvement of phosphorus nutrition and high biomass accumulation in GM under various conditions. GM also accumulated more crude protein, crude fiber, crude fat, and crude ash than wild-type (WT) plants, especially under stress conditions and in the field. More interestingly, the crude fat contents sharply dropped in WT (by 66-74%), whereas showed no change or decreased less in GM, when subjected to salinity, drought or low-Pi. Our results indicate that T1 transgenic alfalfa co-overexpressing ZxNHX and ZxVP1-1 shows stronger stress tolerance, higher productivity and better forage quality. This study provides a solid foundation for creating the alfalfa cultivars with high yield, good quality and wide adaptability on saline, dry, and nutrient-deprived marginal lands of northern China.

Characterization of Two HKT1;4 Transporters from Triticum monococcum to Elucidate the Determinants of the Wheat Salt Tolerance Nax1 QTL

TmHKT1;4-A1 and TmHKT1;4-A2 are two Na⁺ transporter genes that have been identified as associated with the salt tolerance Nax1 locus found in a durum wheat (Triticum turgidum L. subsp. durum) line issued from a cross with T. monococcum. In the present study, we were interested in getting clues on the molecular mechanisms underpinning this salt tolerance quantitative trait locus (QTL). By analyzing the phylogenetic relationships between wheat and T. monococcum HKT1;4-type genes, we found that durum and bread wheat genomes possess a close homolog of TmHKT1;4-A1, but no functional close homolog of TmHKT1;4-A2. Furthermore, performing real-time reverse transcription-PCR experiments, we showed that TmHKT1;4-A1 and TmHKT1;4-A2 are similarly expressed in the leaves but that TmHKT1;4-A2 is more strongly expressed in the roots, which would enable it to contribute more to the prevention of Na⁺ transfer to the shoots upon salt stress. We also functionally characterized the TmHKT1;4-A1 and TmHKT1;4-A2 transporters by expressing them in Xenopus oocytes. The two transporters displayed close functional properties (high Na⁺/K⁺ selectivity, low affinity for Na⁺, stimulation by external K⁺ of Na⁺ transport), but differed in some quantitative parameters: Na⁺ affinity was 3-fold lower and the maximal inward conductance was 3-fold higher in TmHKT1;4-A2 than in TmHKT1;4-A1. The conductance of TmHKT1;4-A2 at high Na⁺ concentration (>10 mM) was also shown to be higher than that of the two durum wheat HKT1;4-type transporters so far characterized. Altogether, these data support the hypothesis that TmHKT1;4-A2 is responsible for the Nax1 trait and provide new insight into the understanding of this QTL.

Essential and Beneficial Trace Elements in Plants, and Their Transport in Roots: a Review

The essentiality of 14 mineral elements so far have been reported in plant nutrition. Eight of these elements were known as micronutrients due to their lower concentrations in plants (usually ≤100 mg/kg/dw). However, it is still challenging to mention an exact number of plant micronutrients since some elements have not been strictly proposed yet either as essential or beneficial. Micronutrients participate in very diverse metabolic processes, including from the primary and secondary metabolism to the cell defense, and from the signal transduction to the gene regulation, energy metabolism, and hormone perception. Thus, the attempt to understand the molecular mechanism(s) behind their transport has great importance in terms of basic and applied plant sciences. Moreover, their deficiency or toxicity also caused serious disease symptoms in plants, even plant destruction if not treated, and many people around the world suffer from the plant-based dietary deficiencies or metal toxicities. In this sense, shedding some light on this issue, the 13 mineral elements (Fe, B, Cu, Mn, Mo, Si, Zn, Ni, Cl, Se, Na, Al, and Co), required by plants at trace amounts, has been reviewed with the primary focus on the transport proteins (transporters/channels) in plant roots. So, providing the compiled but extensive information about the structural and functional roles of micronutrient transport genes/proteins in plant roots.

Moderate salt treatment alleviates ultraviolet-B radiation caused impairment in poplar plants

The effects of moderate salinity on the responses of woody plants to UV-B radiation were investigated using two Populus species (Populus alba and Populus russkii). Under UV-B radiation, moderate salinity reduced the oxidation pressure in both species, as indicated by lower levels of cellular H2O2 and membrane peroxidation, and weakened the inhibition of photochemical efficiency expressed by O-J-I-P changes. UV-B-induced DNA lesions in chloroplast and nucleus were alleviated by salinity, which could be explained by the higher expression levels of DNA repair system genes under UV-B&salt condition, such as the PHR, DDB2, and MutSα genes. The salt-induced increase in organic osmolytes proline and glycine betaine, afforded more efficient protection against UV-B radiation. Therefore moderate salinity induced cross-tolerance to UV-B stress in poplar plants. It is thus suggested that woody plants growing in moderate salted condition would be less affected by enhanced UV-B radiation than plants growing in the absence of salt. Our results also showed that UV-B signal genes in poplar plants PaCOP1, PaSTO and PaSTH2 were quickly responding to UV-B radiation, but not to salt. The transcripts of PaHY5 and its downstream pathway genes (PaCHS1, PaCHS4, PaFLS1 and PaFLS2) were differently up-regulated by these treatments, but the flavonoid compounds were not involved in the cross-tolerance since their concentration increased to the same extent in both UV-B and combined stresses.

Identification of gamma ray irradiation-induced mutations in membrane transport genes in a rice population by TILLING

A high-salt environment represents environmental stress for most plants. Those that can grow and thrive in such an environment must have membrane transport systems that can respond effectively. Plant roots absorb Na⁺ from the soil, and the plant must maintain Na⁺ homeostasis to survive salt stress. A major mechanism by which salt-tolerant plants adapt to salt stress is through modulation of ion transport genes. We have subjected a population of rice plants to mutagenesis, and identified lines with both single-nucleotide polymorphisms (SNPs) in membrane transport genes and altered responses to salt stress. Primers labeled with FAM or HEX fluorescent dyes were designed for nine target genes encoding membrane transport proteins that are believed to regulate salt stress tolerance. A TILLING (Targeting Induced Local Lesions IN Genome) assay was performed on 2,961 M₂ rice mutant lines using electrophoresis. After the TILLING assay, a total of 41 mutant lines containing SNPs in the target genes were identified and screened. The average number of mutations per gene was 1/492 kb in lines having SNPs, and the percentage of mutation sites per total sequence was 0.67. Among the 41 lines, nine had altered sequences in the exon region of the genes. Of these nine lines, seven were tolerant to salt stress after exposure to 170 mM NaCl for three weeks, while the other two lines were not more salt-tolerant than the control lines. Furthermore, five mutant lines containing SNPs in the coding region of OsAKT1, OsHKT6, OsNSCC2, OsHAK11 and OsSOS1 showed changed expression levels for each gene. We conclude that variation in membrane transport genes, such as expression levels and protein structures, may affect the rice plant's tolerance to salt stress. These mutations represent traits that may be selected for large rice mutant populations, permitting efficient acquisition of salt-tolerant lines.

Salt sensitivity in chickpea is determined by sodium toxicity

Salt sensitivity in chickpea is determined by Na(+) toxicity, whereas relatively high leaf tissue concentrations of Cl(-) were tolerated, and the osmotic component of 60-mM NaCl was not detrimental. Chickpea (Cicer arietinum L.) is sensitive to salinity. This study dissected the responses of chickpea to osmotic and ionic components (Na(+) and/or Cl(-)) of salt stress. Two genotypes with contrasting salt tolerances were exposed to osmotic treatments (-0.16 and -0.29 MPa), Na(+)-salts, Cl(-)-salts, or NaCl at 0, 30, or 60 mM for 42 days and growth, tissue ion concentrations and leaf gas-exchange were assessed. The osmotic treatments and Cl(-)-salts did not affect growth, whereas Na(+)-salts and NaCl treatments equally impaired growth in either genotype. Shoot Na(+) and Cl(-) concentrations had markedly increased, whereas shoot K(+) had declined in the NaCl treatments, but both genotypes had similar shoot concentrations of each of these individual ions after 14 and 28 days of treatments. Genesis836 achieved higher net photosynthetic rate (64-84 % of control) compared with Rupali (35-56 % of control) at equivalent leaf Na(+) concentrations. We conclude that (1) salt sensitivity in chickpea is determined by Na(+) toxicity, and (2) the two contrasting genotypes appear to differ in 'tissue tolerance' of high Na(+). This study provides a basis for focus on Na(+) tolerance traits for future varietal improvement programs for salinity tolerance in chickpea.

Chloride and sulfate salinity differently affect biomass, mineral nutrient composition and expression of sulfate transport and assimilation genes in Brassica rapa

BACKGROUND AND AIMS

It remains uncertain whether a higher toxicity of either NaCl or Na2SO4 in plants is due to an altered toxicity of sodium or a different toxicity of the anions. The aim of this study was to determine the contributions of sodium and the two anions to the different toxicities of chloride and sulfate salinity. The effects of the different salts on physiological parameters, mineral nutrient composition and expression of genes of sulfate transport and assimilation were studied.

METHODS

Seedlings of Brassica rapa L. have been exposed to NaCl, Na2SO4, KCl and K2SO4 to assess the potential synergistic effect of the anions with the toxic cation sodium, as well as their separate toxicities if accompanied by the non-toxic cation potassium. Biomass production, stomatal resistance and Fv/fm were measured to determine differences in ionic and osmotic stress caused by the salts. Anion content (HPLC), mineral nutrient composition (ICP-AES) and gene expression of sulfate transporters and sulfur assimilatory enzymes (real-time qPCR) were analyzed.

RESULTS

Na2SO4 impeded growth to a higher extent than NaCl and was the only salt to decrease Fv/fm. K2SO4 reduced plant growth more than NaCl. Analysis of mineral nutrient contents of plant tissue revealed that differences in sodium accumulation could not explain the increased toxicity of sulfate over chloride salts. Shoot contents of calcium, manganese and phosphorus were decreased more strongly by exposure to Na2SO4 than by NaCl. The expression levels of genes encoding proteins for sulfate transport and assimilation were differently affected by the different salts. While gene expression of primary sulfate uptake at roots was down-regulated upon exposure to sulfate salts, presumably to prevent an excessive uptake, genes encoding for the vacuolar sulfate transporter Sultr4;1 were upregulated. Gene expression of ATP sulfurylase was hardly affected by salinity in shoot and roots, the transcript level of 5'-adenylylsulfate reductase (APR) was decreased upon exposure to sulfate salts in roots. Sulfite reductase was decreased in the shoot by all salts similarly and remained unaffected in roots.

CONCLUSIONS

The higher toxicity of Na2SO4 over NaCl in B. rapa seemed to be due to an increased toxicity of sulfate over chloride, as indicated by the higher toxicity of K2SO4 over KCl. Thus, toxicity of sodium was not promoted by sulfate. The observed stronger negative effect on the tissue contents of calcium, manganese and phosphorus could contribute to the increased toxicity of sulfate over chloride. The upregulation of Sultr4;1 and 4;2 under sulfate salinity might lead to a detrimental efflux of stored sulfate from the vacuole into the cytosol and the chloroplasts. It remains unclear why expression of Sultr4;1 and 4;2 was upregulated. A possible explanation is a control of the gene expression of these transporters by the sulfate gradient across the tonoplast.

Effects of Salt Stress on Three Ecologically Distinct Plantago Species

Comparative studies on the responses to salt stress of taxonomically related taxa should help to elucidate relevant mechanisms of stress tolerance in plants. We have applied this strategy to three Plantago species adapted to different natural habitats, P. crassifolia and P. coronopus-both halophytes-and P. major, considered as salt-sensitive since it is never found in natural saline habitats. Growth inhibition measurements in controlled salt treatments indicated, however, that P. major is quite resistant to salt stress, although less than its halophytic congeners. The contents of monovalent ions and specific osmolytes were determined in plant leaves after four-week salt treatments. Salt-treated plants of the three taxa accumulated Na+ and Cl- in response to increasing external NaCl concentrations, to a lesser extent in P. major than in the halophytes; the latter species also showed higher ion contents in the non-stressed plants. In the halophytes, K+ concentration decreased at moderate salinity levels, to increase again under high salt conditions, whereas in P. major K+ contents were reduced only above 400 mM NaCl. Sorbitol contents augmented in all plants, roughly in parallel with increasing salinity, but the relative increments and the absolute values reached did not differ much in the three taxa. On the contrary, a strong (relative) accumulation of proline in response to high salt concentrations (600-800 mM NaCl) was observed in the halophytes, but not in P. major. These results indicate that the responses to salt stress triggered specifically in the halophytes, and therefore the most relevant for tolerance in the genus Plantago are: a higher efficiency in the transport of toxic ions to the leaves, the capacity to use inorganic ions as osmotica, even under low salinity conditions, and the activation, in response to very high salt concentrations, of proline accumulation and K+ transport to the leaves of the plants.

The Role of Silicon in Higher Plants under Salinity and Drought Stress

Although deemed a "non-essential" mineral nutrient, silicon (Si) is clearly beneficial to plant growth and development, particularly under stress conditions, including salinity and drought. Here, we review recent research on the physiological, biochemical, and molecular mechanisms underlying Si-induced alleviation of osmotic and ionic stresses associated with salinity and drought. We distinguish between changes observed in the apoplast (i.e., suberization, lignification, and silicification of the extracellular matrix; transpirational bypass flow of solutes and water), and those of the symplast (i.e., transmembrane transport of solutes and water; gene expression; oxidative stress; metabolism), and discuss these features in the context of Si biogeochemistry and bioavailability in agricultural soils, evaluating the prospect of using Si fertilization to increase crop yield and stress tolerance under salinity and drought conditions.

A New Insight of Salt Stress Signaling in Plant

Many studies have been conducted to understand plant stress responses to salinity because irrigation-dependent salt accumulation compromises crop productivity and also to understand the mechanism through which some plants thrive under saline conditions. As mechanistic understanding has increased during the last decades, discovery-oriented approaches have begun to identify genetic determinants of salt tolerance. In addition to osmolytes, osmoprotectants, radical detoxification, ion transport systems, and changes in hormone levels and hormone-guided communications, the Salt Overly Sensitive (SOS) pathway has emerged to be a major defense mechanism. However, the mechanism by which the components of the SOS pathway are integrated to ultimately orchestrate plant-wide tolerance to salinity stress remains unclear. A higher-level control mechanism has recently emerged as a result of recognizing the involvement of GIGANTEA (GI), a protein involved in maintaining the plant circadian clock and control switch in flowering. The loss of GI function confers high tolerance to salt stress via its interaction with the components of the SOS pathway. The mechanism underlying this observation indicates the association between GI and the SOS pathway and thus, given the key influence of the circadian clock and the pathway on photoperiodic flowering, the association between GI and SOS can regulate growth and stress tolerance. In this review, we will analyze the components of the SOS pathways, with emphasis on the integration of components recognized as hallmarks of a halophytic lifestyle.

Roles and Transport of Sodium and Potassium in Plants

The two alkali cations Na(+) and K(+) have similar relative abundances in the earth crust but display very different distributions in the biosphere. In all living organisms, K(+) is the major inorganic cation in the cytoplasm, where its concentration (ca. 0.1 M) is usually several times higher than that of Na(+). Accumulation of Na(+) at high concentrations in the cytoplasm results in deleterious effects on cell metabolism, e.g., on photosynthetic activity in plants. Thus, Na(+) is compartmentalized outside the cytoplasm. In plants, it can be accumulated at high concentrations in vacuoles, where it is used as osmoticum. Na(+) is not an essential element in most plants, except in some halophytes. On the other hand, it can be a beneficial element, by replacing K(+) as vacuolar osmoticum for instance. In contrast, K(+) is an essential element. It is involved in electrical neutralization of inorganic and organic anions and macromolecules, pH homeostasis, control of membrane electrical potential, and the regulation of cell osmotic pressure. Through the latter function in plants, it plays a role in turgor-driven cell and organ movements. It is also involved in the activation of enzymes, protein synthesis, cell metabolism, and photosynthesis. Thus, plant growth requires large quantities of K(+) ions that are taken up by roots from the soil solution, and then distributed throughout the plant. The availability of K(+) ions in the soil solution, slowly released by soil particles and clays, is often limiting for optimal growth in most natural ecosystems. In contrast, due to natural salinity or irrigation with poor quality water, detrimental Na(+) concentrations, toxic for all crop species, are present in many soils, representing 6 % to 10 % of the earth's land area. Three families of ion channels (Shaker, TPK/KCO, and TPC) and 3 families of transporters (HAK, HKT, and CPA) have been identified so far as contributing to K(+) and Na(+) transport across the plasmalemma and internal membranes, with high or low ionic selectivity. In the model plant Arabidopsis thaliana, these families gather at least 70 members. Coordination of the activities of these systems, at the cell and whole plant levels, ensures plant K(+) nutrition, use of Na(+) as a beneficial element, and adaptation to saline conditions.

Analysis of the sodium chloride-dependent respiratory kinetics of wheat mitochondria reveals differential effects on phosphorylating and non-phosphorylating electron transport pathways

A number of previous studies have documented the gross response of mitochondrial respiration to salinity treatment, but it is unclear how NaCl directly affects the kinetics of plant phosphorylating and non-phosphorylating electron transport pathways. This study investigates the direct effects of NaCl upon different respiratory pathways in wheat, by measuring rates of isolated mitochondrial oxygen consumption across different substrate oxidation pathways in saline media. We also profile the abundance of respiratory proteins by using targeted selected reaction monitoring (SRM) mass spectrometry of mitochondria isolated from control and salt-treated wheat plants. We show that all pathways of electron transport were inhibited by NaCl concentrations above 400 mM; however electron transfer chains showed divergent responses to NaCl concentrations between 0 and 200 mM. Stimulation of oxygen consumption was measured in response to NaCl in scenarios where exogenous NADH was provided as substrate and electron flow was coupled to the generation of a proton gradient across the inner membrane. Protein abundance measurements show that several enzymes with activities less affected by NaCl are induced by salinity, whereas enzymes with activities inhibited by NaCl are depleted. These data deepen our understanding of how plant respiration responds to NaCl, offering new mechanistic explanations for the divergent salinity responses of whole-plant respiratory rate in the literature.

Measurement of Differential Na(+) Efflux from Apical and Bulk Root Zones of Intact Barley and Arabidopsis Plants

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na(+) toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na(+) from root cells is believed to be mediated by Salt Overly-Sensitive-1, although expression of this Na(+)/H(+) antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of (24)Na(+) efflux in these two regions of root. In barley, steady-state release of (24)Na(+) was about four times higher from the distal root than from the bulk roots. In the distal root, (24)Na(+) release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na(+) efflux in the distal root, but not in the bulk root.

Co-expression of tonoplast Cation/H(+) antiporter and H(+)-pyrophosphatase from xerophyte Zygophyllum xanthoxylum improves alfalfa plant growth under salinity, drought and field conditions

Salinity and drought are major environmental factors limiting the growth and productivity of alfalfa worldwide as this economically important legume forage is sensitive to these kinds of abiotic stress. In this study, transgenic alfalfa lines expressing both tonoplast NXH and H(+)-PPase genes, ZxNHX and ZxVP1-1 from the xerophyte Zygophyllum xanthoxylum L., were produced via Agrobacterium tumefaciens-mediated transformation. Compared with wild-type (WT) plants, transgenic alfalfa plants co-expressing ZxNHX and ZxVP1-1 grew better with greater plant height and dry mass under normal or stress conditions (NaCl or water-deficit) in the greenhouse. The growth performance of transgenic alfalfa plants was associated with more Na(+), K(+) and Ca(2+) accumulation in leaves and roots, as a result of co-expression of ZxNHX and ZxVP1-1. Cation accumulation contributed to maintaining intracellular ions homoeostasis and osmoregulation of plants and thus conferred higher leaf relative water content and greater photosynthesis capacity in transgenic plants compared to WT when subjected to NaCl or water-deficit stress. Furthermore, the transgenic alfalfa co-expressing ZxNHX and ZxVP1-1 also grew faster than WT plants under field conditions, and most importantly, exhibited enhanced photosynthesis capacity by maintaining higher net photosynthetic rate, stomatal conductance, and water-use efficiency than WT plants. Our results indicate that co-expression of tonoplast NHX and H(+)-PPase genes from a xerophyte significantly improved the growth of alfalfa, and enhanced its tolerance to high salinity and drought. This study laid a solid basis for reclaiming and restoring saline and arid marginal lands as well as improving forage yield in northern China.

MAPK-mediated enhanced expression of vacuolar H(+)-ATPase confers the improved adaption to NaCl stress in a halotolerate peppermint (Mentha piperita L.)

Vacuolar H(+)-ATPase (V-H(+)-ATPase) has been proved to be of importance in maintenance of ion homeostasis inside plant cells under NaCl stress. In this study, the expression levels and salt-tolerate function of V-H(+)-ATPase genes were investigated in the roots and leaves of a halotolerate peppermint (Mentha × piperita L.) Keyuan-1 treated with different concentrations of NaCl. Results showed that the expressions of V-H(+)-ATPase in the transcriptional, protein and activity levels were significantly enhanced in the halotolerate peppermint Keyuan-1 compared to the wild-type (WT) peppermint under 50, 100, and 150 mM NaCl treatment. Moreover, inhibition experiments exhibited that V-H(+)-ATPase activity played vital roles in the salt tolerance of peppermint Keyuan-1 to 150 mM NaCl stress through increasing the vacuolar H(+) pumping activity and Na(+) compartmentalization capacity. Furthermore, results of Western blots showed that the activity of a mitogen-activated protein kinase (MAPK) was significantly increased under different concentrations of NaCl in the halotolerate peppermint Keyuan-1, which was much higher than that of WT peppermint. Further experiments with inhibitors suggested that this MAPK protein was responsible for the enhanced expression of V-H(+)-ATPase in the halotolerate peppermint Keyuan-1. In response to NaCl stress, increase of cytoplasmic calcium concentration ([Ca(2+)]cyt) occurred upstream of MAPK activation in the halotolerate peppermint Keyuan-1. In all, these findings demonstrated that increased V-H(+)-ATPase activity was positively correlated with the enhanced salt tolerance in the halotolerate peppermint Keyuan-1, providing the theoretic basis for the further development and utilization of peppermint in saline areas.

How high do ion fluxes go? A re-evaluation of the two-mechanism model of K(+) transport in plant roots

Potassium (K(+)) acquisition in roots is generally described by a two-mechanism model, consisting of a saturable, high-affinity transport system (HATS) operating via H(+)/K(+) symport at low (<1mM) external [K(+)] ([K(+)]ext), and a linear, low-affinity system (LATS) operating via ion channels at high (>1mM) [K(+)]ext. Radiotracer measurements in the LATS range indicate that the linear rise in influx continues well beyond nutritionally relevant concentrations (>10mM), suggesting K(+) transport may be pushed to extraordinary, and seemingly limitless, capacity. Here, we assess this rise, asking whether LATS measurements faithfully report transmembrane fluxes. Using (42)K(+)-isotope and electrophysiological methods in barley, we show that this flux is part of a K(+)-transport cycle through the apoplast, and masks a genuine plasma-membrane influx that displays Michaelis-Menten kinetics. Rapid apoplastic cycling of K(+) is corroborated by an absence of transmembrane (42)K(+) efflux above 1mM, and by the efflux kinetics of PTS, an apoplastic tracer. A linear apoplastic influx, masking a saturating transmembrane influx, was also found in Arabidopsis mutants lacking the K(+) transporters AtHAK5 and AtAKT1. Our work significantly revises the model of K(+) transport by demonstrating a surprisingly modest upper limit for plasma-membrane influx, and offers insight into sodium transport under salt stress.

The Water to Solute Permeability Ratio Governs the Osmotic Volume Dynamics in Beetroot Vacuoles

Plant cell vacuoles occupy up to 90% of the cell volume and, beyond their physiological function, are constantly subjected to water and solute exchange. The osmotic flow and vacuole volume dynamics relies on the vacuole membrane -the tonoplast- and its capacity to regulate its permeability to both water and solutes. The osmotic permeability coefficient (P_f ) is the parameter that better characterizes the water transport when submitted to an osmotic gradient. Usually, P_f determinations are made in vitro from the initial rate of volume change, when a fast (almost instantaneous) osmolality change occurs. When aquaporins are present, it is accepted that initial volume changes are only due to water movements. However, in living cells osmotic changes are not necessarily abrupt but gradually imposed. Under these conditions, water flux might not be the only relevant driving force shaping the vacuole volume response. In this study, we quantitatively investigated volume dynamics of isolated Beta vulgaris root vacuoles under progressively applied osmotic gradients at different pH, a condition that modifies the tonoplast P_f . We followed the vacuole volume changes while simultaneously determining the external osmolality time-courses and analyzing these data with mathematical modeling. Our findings indicate that vacuole volume changes, under progressively applied osmotic gradients, would not depend on the membrane elastic properties, nor on the non-osmotic volume of the vacuole, but on water and solute fluxes across the tonoplast. We found that the volume of the vacuole at the steady state is determined by the ratio of water to solute permeabilites (P_f /P_s ), which in turn is ruled by pH. The dependence of the permeability ratio on pH can be interpreted in terms of the degree of aquaporin inhibition and the consequently solute transport modulation. This is relevant in many plant organs such as root, leaves, cotyledons, or stems that perform extensive rhythmic growth movements, which very likely involve considerable cell volume changes within seconds to hours.

Comparison between Arabidopsis and Rice for Main Pathways of K(+) and Na(+) Uptake by Roots

K(+) is an essential macronutrient for plants. It is acquired by specific uptake systems located in roots. Although the concentrations of K(+) in the soil solution are widely variable, K(+) nutrition is secured by uptake systems that exhibit different affinities for K(+). Two main systems have been described for root K(+) uptake in several species: the high-affinity HAK5-like transporter and the inward-rectifier AKT1-like channel. Other unidentified systems may be also involved in root K(+) uptake, although they only seem to operate when K(+) is not limiting. The use of knock-out lines has allowed demonstrating their role in root K(+) uptake in Arabidopsis and rice. Plant adaptation to the different K(+) supplies relies on the finely tuned regulation of these systems. Low K(+)-induced transcriptional up-regulation of the genes encoding HAK5-like transporters occurs through a signal cascade that includes changes in the membrane potential of root cells and increases in ethylene and reactive oxygen species concentrations. Activation of AKT1 channels occurs through phosphorylation by the CIPK23/CBL1 complex. Recently, activation of the Arabidopsis HAK5 by the same complex has been reported, pointing to CIPK23/CBL as a central regulator of the plant's adaptation to low K(+). Na(+) is not an essential plant nutrient but it may be beneficial for some plants. At low concentrations, Na(+) improves growth, especially under K(+) deficiency. Thus, high-affinity Na(+) uptake systems have been described that belong to the HKT and HAK families of transporters. At high concentrations, typical of saline environments, Na(+) accumulates in plant tissues at high concentrations, producing alterations that include toxicity, water deficit and K(+) deficiency. Data concerning pathways for Na(+) uptake into roots under saline conditions are still scarce, although several possibilities have been proposed. The apoplast is a significant pathway for Na(+) uptake in rice grown under salinity conditions, but in other plant species different mechanisms involving non-selective cation channels or transporters are under discussion.

Male poplars have a stronger ability to balance growth and carbohydrate accumulation than do females in response to a short-term potassium deficiency

Potassium (K) deficiency influences plant performance, such as ion uptake and carbohydrate transport. However, little is known about differences between males and females in response to K deficiency. In this study, dry matter accumulation, photosynthetic capacity, allocation patterns of K(+) , Na(+) and carbohydrates, and ultrastructural changes in males and females of Populus cathayana exposed to K deficiency were investigated. The results indicated that males maintained a significantly higher K(+) content and K(+) /Na(+) ratio in leaves and stems than did females under K deficiency. Moreover, K deficiency significantly increased the sucrose content of females, whereas no significant effect on males was detected. In addition, a comparative analysis showed that males allocated more resources to roots, while females allocated more to leaves, which resulted in sexually different root/shoot (R/S) ratios. Transmission electron microscopic (TEM) observations showed that males suffered fewer injuries than did females. These results suggested that males have a better ability to cope with K deficiency. In addition, the combined effects of salinity and K deficiency on poplars were studied. The results indicated that salt stress aggravates the negative effects caused by K deficiency. Taken together, our study provided evidence for gender-specific strategies in ion and carbohydrate allocation in poplars exposed to a short-term K deficiency. In leaves and stems, the lower K(+) accumulation inhibited sucrose translocation and resulted in a decreased R/S ratio, which may contribute to males having a stronger ability to balance growth and carbohydrate accumulation when compared with females.

Breeding and Domesticating Crops Adapted to Drought and Salinity: A New Paradigm for Increasing Food Production

World population is expected to reach 9.2 × 10(9) people by 2050. Feeding them will require a boost in crop productivity using innovative approaches. Current agricultural production is very dependent on large amounts of inputs and water availability is a major limiting factor. In addition, the loss of genetic diversity and the threat of climate change make a change of paradigm in plant breeding and agricultural practices necessary. Average yields in all major crops are only a small fraction of record yields, and drought and soil salinity are the main factors responsible for yield reduction. Therefore there is the need to enhance crop productivity by improving crop adaptation. Here we review the present situation and propose the development of crops tolerant to drought and salt stress for addressing the challenge of dramatically increasing food production in the near future. The success in the development of crops adapted to drought and salt depends on the efficient and combined use of genetic engineering and traditional breeding tools. Moreover, we propose the domestication of new halophilic crops to create a 'saline agriculture' which will not compete in terms of resources with conventional agriculture.