Some important physiological selection criteria for salt tolerance in plants

Undoubtedly, plant breeders have made a significant achievement in the past few years, improving salinity tolerance in a number of potential crops using artificial selection and conventional breeding approaches, although molecular biology approaches are currently being intensively pursued for achieving this goal. However, most of the selection procedures used so far, were based merely on differences in agronomic characters. Agronomic characters represent the combined genetic and environmental effects on plant growth, and include the integration of the physiological phenomena conferring salinity tolerance. In fact, physiological criteria are able to supply more reliable information than agronomic characters. Although there are large numbers of reports in the literature mainly dealing with water relations, photosynthesis, and accumulation of various inorganic ions and organic metabolites in individual crops, there is little information available on the use of these attributes as selection criteria for improving salt tolerance through selection and breeding programs. In this review, the major adaptive components of salt tolerance such as osmotic adjustment, photosynthesis, water relations and ion relations are reviewed. In view of the complexity of salt tolerance and its great variation at intra-specific and inter-specific levels, it is difficult to identify single criteria, which could be used as effective selection targets. Rather it is most meaningful if physiological and biochemical indicators for individual species are determined rather than generic indicators.     

Genes and salt tolerance: bringing them together

Salinity tolerance comes from genes that limit the rate of salt uptake from the soil and the transport of salt throughout the plant, adjust the ionic and osmotic balance of cells in roots and shoots, and regulate leaf development and the onset of senescence. This review lists some candidate genes for salinity tolerance, and draws together hypotheses about the functions of these genes and the specific tissues in which they might operate. Little has been revealed by gene expression studies so far, perhaps because the studies are not tissue-specific, and because the treatments are often traumatic and unnatural. Suggestions are made to increase the value of molecular studies in identifying genes that are important for salinity tolerance.     

In vitro selection for salt tolerance in crop plants: Theoretical and practical considerations

Summary In recent years attempts have been made to supplement traditional breeding for the production of salt-tolerant plants with variability existing in cell culture. The potential causes suggested as an explanation for the limited success of the in vitro approach include: a) lack, or loss during selection, of regeneration capability; b) the development of epigenetically adapted cells; c) lack of correlation between the mechanisms of tolerance operating in cultured cells and mechanisms that operate in cells in the intact plant; and d) multigenicity of salt tolerance. The recent successful production of healthy, fertile, and genetically stable salt-tolerant regenerants from cells obtained from highly morphogenic explants which are selected early in culture (using one-step or short-term strategies) for salt tolerance, together with the demonstration that salt-sensitive plants can become tolerant by mutations in one or few genes, suggest that some of the potential limitations can be overcome and that some of them may not exist at all.     

Transport and accumulation of ions in two spring wheat lines differing in salt tolerance

Fifty-two-day old plants of a salt tolerant line, S24 and a salt sensitive, Yecora Rojo were subjected for 15 days to 125 mol·m⁻³ NaCl in Hoagland’s nutrient solution under glass-house conditions. The dry matter of shoots and roots of the salt tolerant line was significantly greater over all time intervals in saline substrate than the salt sensitive line, Yecora Rojo. In the leaves of salt-treated former line concentration of Na⁺ and Cl⁻ was lower as compared to the latter line. The lower Na⁺ and Cl⁻ concentrations in the leaves of S24 were found to be associated with lower transport of these ions to the shoots whereas the reverse was true for Yecora Rojo. The lines did not differ in accumulation of either ion in roots. It is concluded that salt tolerance in these two genotypes of spring wheat is associated with restricted accumulation of toxic Na⁺ and Cl⁻ ions to the shoots or with restricted transport.     

Plant salt tolerance: adaptations in halophytes

Background Most of the water on Earth is seawater, each kilogram of which contains about 35 g of salts, and yet most plants cannot grow in this solution; less than 0·2 % of species can develop and reproduce with repeated exposure to seawater. These ‘extremophiles’ are called halophytes.Scope Improved knowledge of halophytes is of importance to understanding our natural world and to enable the use of some of these fascinating plants in land re-vegetation, as forages for livestock, and to develop salt-tolerant crops. In this Preface to a Special Issue on halophytes and saline adaptations, the evolution of salt tolerance in halophytes, their life-history traits and progress in understanding the molecular, biochemical and physiological mechanisms contributing to salt tolerance are summarized. In particular, cellular processes that underpin the ability of halophytes to tolerate high tissue concentrations of Na⁺ and Cl⁻, including regulation of membrane transport, their ability to synthesize compatible solutes and to deal with reactive oxygen species, are highlighted. Interacting stress factors in addition to salinity, such as heavy metals and flooding, are also topics gaining increased attention in the search to understand the biology of halophytes.Conclusions Halophytes will play increasingly important roles as models for understanding plant salt tolerance, as genetic resources contributing towards the goal of improvement of salt tolerance in some crops, for re-vegetation of saline lands, and as ‘niche crops’ in their own right for landscapes with saline soils.     

Salt tolerance in Lycopersicon species. IV. Efficiency of marker-assisted selection for salt tolerance improvement

The usefulness of marker-assisted selection (MAS) to develop salt-tolerant breeding lines from a F₂ derived from L. esculentum x L. pimpinellifolium has been studied. Interval mapping methodology of quantitative trait locus (QTL) analysis was used to locate more precisely previously detected salt tolerance QTLs. A new QTL for total fruit weight under salinity (TW) near TG24 was detected. Most of the detected QTLs [3 for TW, 5 for fruit number, (FN) and 4 for fruit weight (FW)] had low R ² values, except the FW QTL in the TG180-TG48 interval, which explains 36.6% of the total variance. Dominant and overdominant effects were detected at the QTLs for TW, whereas gene effects at the QTLs for FJV and FW ranged from additive to partial dominance. Phenotypic selection of F₂ familes and marker-assisted selection of F₃ families were carried out. Yield under salinity decreased in the F₂ generation. F₃ means were similar to those of the F₁ as a consequence of phentoypic selection. The most important selection response for every trait was obtained from the F₃ to F₄ where MAS was applied. While F₃ variation was mainly due to the within-family component, in the F₄ the FN and FW between-family component was larger than the within-family one, indicating an efficient compartmentalization and fixation of QTLs into the F₄ families. Comparison of the yield of these families under control versus saline conditions showed that fruit weight is a key trait to success in tomato salt-tolerance improvement using wild Lycopersicon germplasm. The QTLs we have detected under salinity seem to be also working under control conditions, although the interaction family x treatment was significant for TW, thereby explaining the fact that the selected families responded differently to salinity.     

生物炭调控盐胁迫下水稻幼苗耐盐性能

土壤盐渍化降低土壤生产力.探索生物炭对盐胁迫下水稻幼苗耐盐性能的影响,对调控盐渍区水稻生产潜力具有重要意义.本研究通过生物炭介入盐胁迫稻田土壤的盆栽试验,调查了生物炭对盐胁迫下土壤环境和水稻幼苗耐盐性能的影响.盐胁迫设置4个水平,分别为0 g NaCl·kg-1土(S0),1 g NaCl·kg-1土(S1),2 g ...     

Improving Glycyrrhiza uralensis salt tolerance with N+ ion irradiation

Low energy (25 keV) N⁺ ions were implanted into liquorice (Glycyrrhiza uralensis) seeds at a fluency of either zero (control) or 900 × (2.6 × 10¹³) ions/cm². After irradiation, all the seeds were planted in the plastic pots for a growth period of one month. Thereafter, the seedlings in the pot were subjected to saline stress at 600 mM for about 3 days. The morphological and physiological characteristics such as total chlorophyll content, proline level, activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ATPase, and triphenyl tetrazolium chloride (TTC) reduction in seedling roots were investigated. Our results indicated that ion irradiation significantly increased the shoot height, leaflet number, taproot lenght, lateral root number, and shoot and root weights of liquorice seedlings with or without saline stress. Furthermore, the total chlorophyll content, proline level, SOD, POD, CAT, ATPase activities, and root TTC reduction vigor of seedlings were all found to be significantly increased under saline stress by ion irradiation compared with their corresponding controls. These results indicated that ion irradiation can strengthen the resistance of liquorice seedlings to saline stress and may have a potential application for the improvement of plants in sand areas.     

栽培大豆和野生大豆耐盐性及离子效应的比较

以国际上常用的耐盐大豆（Glycine max L.)品种Lee68为对照,在发芽期和苗期两个阶段,利用发芽指数、指害指数和耐盐系数等指标对一年生具盐腺野生大豆（Glycine soja L.)和部分栽培大豆（Glycine max L.)及某些野生大豆品系或品种的耐盐性进行了比较,讨论了耐盐指标的可行性。从离子效应方面比较了Na^ 和Cl^-对大豆发芽率的影响,并对具盐腺野生大豆的耐盐机理进行了初步分析。结果表明,大豆品种的耐盐性在发芽期和苗期无一致相关性。轻度等渗胁迫下,Na^ 对种子发芽率的抑制作用大于Cl^-,而重度等渗胁迫下则相反。通过减少由根系吸收的Na^ 、Cl^-向叶片的运输,维持叶片中较高含量的K^ ,减轻盐离子毒害,可能是具盐腺野生大事耐盐的主要生理机制之一。     

Effects of stabilizing solutes on salt activation of phosphoenolpyruvate carboxylase from various plant sources

Phosphoenolpyruvate carboxylase (PEP carboxylase EC 4.1.1.31) was extracted from various halophytic, semi-halophytic and glycophytic plant species. When the enzyme of those extracts was substrate protected, and in the presence of 1.6 m M PEP in the reaction mixture, the activity of PEP carboxylase was increased by 100 m M NaCl, and the activity range in the presence of NaCl was expanded. No correlation could be established between the response of the enzyme to ions and various plant characteristics, such as taxonomic status, salt tolerance or carbon fixation pathways. Salt activation of PEP carboxylase was substrate (PEP) dependent, but the minimal substrate concentration varied in different species.
Effects of the stabilizing solutes PEP, betaine, proline and glycerol on the kinetic properties of PEP carboxylase from Zea mays (L.) cv. Hazera were analyzed. In the absence of NaCl the slope of the Hill plot (n_It) tended to rise in the presence of these solutes. Stabilization of the enzyme with betaine or glycerol caused a decrease in K'. while K' and VTO increased in the presence of PEP. NaCl (100 mM) caused an increase in both K' and V^max in the protected as well as in the unprotected enzyme, except for PEP protection, where K' decreased somewhat. In the presence of the protectants, glycerol and PEP, the effect of NaCl on V^max, was 2–4 times higher than its effect on the non-protected enzyme.     

The use of Box-Behnken design of experiments to study in vitro salt tolerance by Pisolithus tinctorius

The ectomycorrhizal fungus Pisolithus tinctorius was grown in vitro with different concentrations and combinations of three different sodium salts viz., sodium chloride, sodium sulphate and trisodium citrate (NaCl, Na₂SO4 and Na₃C₆H₅O₇) for 30 days. At the end of the experiment the mycelial biomass and protein content of the fungus was evaluated. Based on the salts tested the combination of NaCl and Na₂SO₄ in optimum concentrations actually promoted the growth of P. tinctorius. Box-Behnken design with three variables like NaCl, Na₂SO₄ and Na₃C₆H₅O₇ was used to identify a significant correlation between the effect of these variables on mycelial biomass production. The experimental values are in good agreement with predicted values, the correlation coefficient being 0.9880.     

Characterization of progeny of Coleus blumei following an in vitro selection for salt tolerance

An in vitro selection method was developed for Coleus blumei to enhance salt tolerance of this amenity species. Leaf disc explants were incubated on a Murashige & Skoog medium containing benzylaminopurine, 2 mg l^-1, and napthalene acetic acid, 1 mg l^-1, which initiated both callus and plantlets from the explants. A large number of explants were incubated on this differentiating medium containing 90 mM NaCl, which inhibited over 90% of plantlet formation. Surviving plantlets. were grown to maturity, when apical cuttings were taken and propagated. Plants were also allowed to flower and set seed. Cuttings from the selected regenerated plants showed consistently better growth in the presence of NaCl than unselected cuttings. Seed progeny of selected plants also showed more vigorous growth in the presence and absence of NaCl than progeny from unselected plants. The in vitro selection was compared with the results of an earlier in vivo selection to assess the contribution from tissue culture derived somaclonal variation. Progeny from the in vitro selection showed a higher level of tolerance than progeny from the in vivo selection.     

植物耐盐的分子生物学基础

植物分子水平的耐盐研究是近年的研究热点,通过综述与耐盐有关的几种重要分子的性质和作用,总结了几种与植物耐盐有关的基因以及它们在盐分胁迫下的表达和调控。     

Microsatellite signature of ecological selection for salt tolerance in a wild sunflower hybrid species, Helianthus paradoxus

The hybrid sunflower species Helianthus paradoxus inhabits sporadic salt marshes in New Mexico and southwest Texas, USA, whereas its parental species, Helianthus annuus and Helianthus petiolaris, are salt sensitive. Previous studies identified three genomic regions - survivorship quantitative trait loci (QTLs) - that were under strong selection in experimental hybrids transplanted into the natural habitat of H. paradoxus. Here we ask whether these same genomic regions experienced significant selection during the origin and evolution of the natural hybrid, H. paradoxus. This was accomplished by comparing the variability of microsatellites linked to the three survivorship QTLs with those from genomic regions that were neutral in the experimental hybrids. As predicted if one or more selective sweeps had occurred in these regions, microsatellites linked to the survivorship QTLs exhibited a significant reduction in diversity in populations of the natural hybrid species. In contrast, no difference in diversity levels was observed between the two microsatellite classes in parental populations.     

Inducing salt tolerance in maize (Zea mays L.) through seed priming with chloride salts: Growth and ion transport at early growth stages

The study was conducted to determine whether salt tolerance could be induced in maize at germination stage by soaking of seeds for 8 h in distilled water or in 200 meq·L⁻¹ of NaCl, KCl, CaCl₂·2H₂O. Both primed and un-primed seeds were subjected for 14 days to 0, 100 or 200 mol·m⁻³ NaCl under controlled conditions. Although all priming agents were effective in alleviating adverse effects of salt stress on maize at germination stage, CaCl₂·2H₂O proved to be more effective since the seeds primed with this salt had significantly higher final germination, rate of germination and fresh and dry weights of plumules and radicles than those treated with other salts or distilled water. Concentration of Na⁺, K⁺ and Ca²⁺ increased significantly in all parts of germinating seeds of maize seeds primed with NaCl, KCl, or CaCl₂·2H₂O, respectively. In addition, seeds primed with CaCl₂·2H₂O were the highest in Cl⁻ accumulation in all parts of the germinating seeds, followed by seeds treated with NaCl and KCl. Most of the Ca²⁺ was retained in seeds and mesocotyl, because of which, transport of this ion to plumules and radicles was low.     

Studies on the salt tolerance of some flower crops grown under glass

Summary The salt sensitivity of carnations, gerberas, anthuriums, chrysanthemums and hippeastrums was studied in an investigation. The crops were grown in basins filled with soil and irrigated with the aid of low level sprinklers. Two different cultivars of each crop were included in the experiments. The crops were irrigated with water containing different levels of salts. The EC of the irrigation water (EC_w) ranged between 0.2 and 3.9 mS.cm^–1 at 25°C.The salt applications had a deleterious effect on the development of all the flower crops used in the investigation. Carnations and chrysanthemums proved to be the least sensitive. Gerberas and hippeastrums showed a medium sensitivity and anthuriums proved to be the most salt sensitive. The latter crop also showed a specific sensitivity to sodium chloride.The salinity threshold values were low for most crops,i.e. EC_w<0.6. Slightly higher values were found for carnations only. The salinity decrease values of EC_w showed very wide variations and ranged from 6 to 34%. A method is proposed for the conversion of EC_w values into EC values for the saturation extract (EC_e) with the aid of the soil analytical results.The results of tissue analyses showed that the sodium and chloride contents in particular were affected by the salt applications. The effects of the salt applications on the uptake of the major nutrient elements varied from crop to crop.     

Manipulation of monoubiquitin improves salt tolerance in transgenic tobacco

Ubiquitin (Ub) is regarded as a stress protein involved in many stress responses. In this paper, sense and antisense transgenic tobacco plants, as well as the wild type and vector control, were used to study the role of Ub in salt tolerance of plants. In sense Ta-Ub2 transgenic tobacco plants, there was higher expression of Ub protein conjugates than in the wild type and vector control, but the reverse trend was observed in antisense Nt-Ub1 transgenic plants. The germination rate of tobacco seed, growth status and photosynthesis of the tobacco plants suggested that over-expressing Ub promoted the growth of transgenic tobacco plants and enhanced their salt tolerance, but the opposite effect was seen in plants with repressed Ub expression. Changes in antioxidant capacity may be one of the mechanisms underlying Ub-regulated salt tolerance. Furthermore, improved tolerance to a combination of stresses was also observed in the sense transgenic tobacco plants. These findings imply that Ub is involved in the tolerance of plants to abiotic stress.     

Expression of a calcineurin gene improves salt stress tolerance in transgenic rice

Calcineurin is a Ca²⁺- and calmodulin-dependent serine/threonine phosphatase and has multiple functions in animal cells including regulating ionic homeostasis. We generated transgenic rice plants that not only expressed a truncated form of the catalytic subunit of mouse calcineurin, but also were able to grow and fertilize normally in the field. Notably, the expression of the mouse calcineurin gene in rice resulted in its higher salt stress tolerance than the non-transgenic rice. Physiological studies have indicated that the root growth of transgenic plants was less inhibited than the shoot growth, and that less Na⁺ was accumulated in the roots of transgenic plants after a prolonged period of salt stress. These findings imply that the heterologous calcineurin plays a significant role in maintaining ionic homeostasis and the integrity of plant roots when exposed to salt. In addition, the calcineurin gene expression in the stems of transgenic plants correlated with the increased expression of the Rab16A gene that encodes a group 2-type late-embryogenesis-abundant (LEA) protein. Altogether our findings provide the first genetic and physiological evidence that expression of the mouse calcineurin protein functionally improves the salt stress tolerance of rice partly by limiting Na⁺ accumulation in the roots.     

盐胁迫对4树种叶片中K^＋和Na^＋的影响及其耐盐能力的评价

通过盆栽试验,对我国南方银杏（Ginkgo biloba L.)、侧柏[Patycladus orentalis(L.)Franco]、火炬松（Pinus taeda L.)和剌槐(Robinia Pseudoacacia L.)4造林树种苗木叶片中K^ 、N^ 浓度浓度和N^ ／K^ 比进行测定,盐处理水平为0.0％（CK）、0.1%、0.3%和0.5％。随着盐浓度的提高,各树种叶片中Na^ 浓度和N^ ／K^ 有逐渐升高的趋势,但K^ 浓度变化较小;随着盐胁迫时间的推移,各树种叶片中Na^ 、K^ 浓度和Na^ /K^ 比都没有明显的变化规律。方差分析和多层比较表明,侧柏、火炬松和剌槐叶片中Na^ 、K^ 浓度和N^ ／K^ 比在4组处理间的差异均达显著或极显著水平。4树种中剌槐和侧柏的耐盐能力最强,银杏次之,火炬松最弱。