Salt tolerance evolves more frequently in C4 grass lineages

Salt tolerance has evolved many times in the grass family, and yet few cereal crops are salt tolerant. Why has it been so difficult to develop crops tolerant of saline soils when salt tolerance has evolved so frequently in nature? One possible explanation is that some grass lineages have traits that predispose them to developing salt tolerance and that without these background traits, salt tolerance is harder to achieve. One candidate background trait is photosynthetic pathway, which has also been remarkably labile in grasses. At least 22 independent origins of the C₄ photosynthetic pathway have been suggested to occur within the grass family. It is possible that the evolution of C₄ photosynthesis aids exploitation of saline environments, because it reduces transpiration, increases water‐use efficiency and limits the uptake of toxic ions. But the observed link between the evolution of C₄ photosynthesis and salt tolerance could simply be due to biases in phylogenetic distribution of halophytes or C₄ species. Here, we use a phylogenetic analysis to investigate the association between photosynthetic pathway and salt tolerance in the grass family Poaceae. We find that salt tolerance is significantly more likely to occur in lineages with C₄ photosynthesis than in C₃ lineages. We discuss the possible links between C₄ photosynthesis and salt tolerance and consider the limitations of inferring the direction of causality of this relationship.     

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

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

栽培大豆种质资源耐盐性的研究进展

评价、筛选并利用栽培大豆的耐盐种质资源,对开发利用盐渍土具有极其重要的意义。本文从耐盐性评价方法、耐盐的生理生化基础、耐盐的分子生物学基础及耐盐种质的筛选与创新等4方面,对栽培大豆的耐盐性研究进展进行了系统综述。同时对栽培大豆耐盐性研究现存的问题与今后的发展方向进行了讨论,以期为栽培大豆耐盐性研究提供参考。     

Mini-Review of Knowledge Gaps in Salt Tolerance of Plants Applied to Willows and Poplars

Salt tolerance of agricultural crops has been studied since the 1940, but knowledge regarding salt tolerance of woody crops is still in its initial phase. Salt tolerance of agricultural crops has been expressed as the yield decrease due to a certain salt concentration within the root zone as compared to a non-saline control. The most well-known plant response curve to salinity has been a piece-wise linear regression relating crop yield to root zone salinity. This method used the hypothesis that crops tolerate salt up to a threshold after which their yield decreases approximately linearly. Critique to this method included its lack of sensitivity to dynamic factors such as weather conditions. As a result, other classification indices have been developed, but none is as well accepted as the threshold-slope model. In addition to a mini-review of the key salt tolerance studies, our objective was to classify salt tolerance levels of poplars and willows. Initial classification showed that salt tolerance of these genera ranged from sensitive to moderately tolerant.     

Comparative roles of brassinosteroids and polyamines in salt stress tolerance

Salt stress is among the major abiotic stresses that adversely affect the global crop production and its adverse impacts are getting more serious in the regions where saline water is used for irrigation. It induces reactive oxygen species, alters the activity of antioxidant system and adversely affects the process of photosynthesis. Various strategies have been employed to mitigate the deleterious effects of salt stress. Presently, the recommended strategies to overcome the adverse effects of salt stress include the use of tolerant cultivars, ameliorative water management and diverse cultural practices. However, none of these approaches have been found to be fully effective under salt stress conditions. An alternative and technically simpler approach to induce salt stress tolerance is the exogenous application of plant growth regulators (PGRs). This technique has gained significant importance during the past decade. PGRs have been implicated to regulate a wide range of metabolic and physiological activities in plants, ranging from cell division and organogenesis to protection against biotic and abiotic stresses. One of the important factors for enhanced plant productivity by PGRs is their efficiency to overcome the salt-induced stress conditions. Recent findings on the effects of brassinosteroids and polyamines on the salt stress tolerance of crops open new avenues to address the salinity problems. This review enlightens the role of brassinosteroids and polyamines in different plant processes like their role in regulation of photosynthesis, antioxidant systems and other related aspects, thereby improving overall performance of plants.     

低酚棉种质资源耐盐性鉴定

为了解决粮棉争地矛盾,棉花向更加盐碱干旱区域发展是大势所趋。因此进行耐盐鉴定筛选抗性种质意义重大。本研究在培养室条件下,利用发芽盒和石英砂对179份来源不同的低酚棉种质资源的耐盐性进行了评价。结果表明,179份低酚棉资源材料中,没有高抗材料;抗盐材料4份,占所鉴定材料的2.2%;耐盐材料54份,占30.2%;盐敏感材料121份,占67.6%。筛选出8份耐盐性突出的材料,其耐盐性达到了耐盐或抗盐的水平,包括豫无424、衡无87-306、中无1651、中无3385、多毛101、中无374-G、中无1038和中无642,这些材料在盐胁迫下的相对成活苗率均在70%以上。国内材料耐盐性好于国外材料,国内材料中河北省、山东省、河南省的材料耐盐性明显好于其他省份。本研究筛选出了耐盐性好的低酚棉材料,为低酚棉育种提供了优异的种质资源。     

Halophyte crop cultivation: The case for Salicornia and Sarcocornia

Increasing soil salinization and the growing scarcity of fresh water dictate the need for a creative solution to attain sustainable crop production. To accomplish this aim, the domestication of inherently salt tolerant plant species with economic value is proposed as a straightforward methodology. Most studies investigating salt tolerance mechanisms are linked to small, experimental systems that cannot be generalized to the real agricultural context. The crops Salicornia and Sarcocornia, however, with their extreme salt tolerance and long history of consumption by humans, make the ideal model plants on which to base a halophyte growth strategy. New applied technologies were developed for leafy vegetable production using small-scale greenhouse and in-field studies. Several cultivation systems adapted to the irrigation water salinity and the available soil conditions are described. Daylength manipulation and a repetitive harvest regime partially elucidated the flowering patterns of Salicornia and Sarcocornia and showed that flowering should be prevented for maximal vegetable production. Additionally, the beneficial effect of saline irrigation on quality parameters via the enhancement of stress-induced secondary metabolites with antioxidant capacity should be considered during cultivation. This review summarizes the recent developments in growing halophytes for food production with saline irrigation, using Salicornia and Sarcocornia as a case study.     

Physiological adaptative characteristics of Imperata cylindrica for salinity tolerance

Two populations of cogongrass [Imperata cylindrica (L.) Raeuschel], one from the saline regions of the Salt Range and the other from the non-saline regions of Faisalabad were assessed for salinity tolerance on the basis of some key morphological and physiological attributes. It was hypothesized that the tolerant population from the Salt Range must have developed some specific structural modifications, which are responsible for its better survival under high salinities. These adaptive components can be effectively used in modern technologies for improving degree of tolerance of other sensitive crops. The population from the Salt Range markedly excelled the Faisalabad population in terms of growth and physiological attributes measured in this study. The Faisalabad population of I. cylindrica was unable to survive at the highest salt level (200 mM NaCl). The tolerance of the Salt Range population to salt stress was found to be related to high accumulation of organic osmotica, particularly total free amino acids and proline as well as Ca²⁺ in the shoot. The distinctive structural modifications in the Salt Range population were found to be enhanced succulence, well-developed bulliform cells in leaves and smaller stomatal area.     

Kinetics of xylem loading,membrane potential maintenance,and sensitivity of K+‐permeable channels to reactive oxygen species: physiological traits that differentiate salinity tolerance between pea and barley

Salt sensitive (pea) and salt tolerant (barley) species were used to understand the physiological basis of differential salinity tolerance in crops. Pea plants were much more efficient in restoring otherwise depolarized membrane potential thereby effectively decreasing K⁺ efflux through depolarization‐activated outward rectifying potassium channels. At the same time, pea root apex was 10‐fold more sensitive to physiologically relevant H₂O₂ concentration and accumulated larger amounts of H₂O₂ under saline conditions. This resulted in a rapid loss of cell viability in the pea root apex. Barley plants rapidly loaded Na⁺ into the xylem; this increase was only transient, and xylem and leaf Na⁺ concentration remained at a steady level for weeks. On the contrary, pea plants restricted xylem Na⁺ loading during the first few days of treatment but failed to prevent shoot Na⁺ elevation in the long term. It is concluded that superior salinity tolerance of barley plants compared with pea is conferred by at least three different mechanisms: (1) efficient control of xylem Na⁺ loading; (2) efficient control of H₂O₂ accumulation and reduced sensitivity of non‐selective cation channels to H₂O₂ in the root apex; and (3) higher energy saving efficiency, with less ATP spent to maintain membrane potential under saline conditions.     

Nitrogen fixation persists under conditions of salt stress in transgenic Medicago truncatula plants expressing a cyanobacterial flavodoxin

Several recent studies have demonstrated that the expression of a cyanobacterial flavodoxin in plants can provide tolerance to a wide range of environmental stresses. Indeed, this strategy has been proposed as a potentially powerful biotechnological tool to generate multiple‐tolerant crops. To determine whether flavodoxin expression specifically increased tolerance to salt stress and whether it might also preserve legume nitrogen fixation under saline conditions, the flavodoxin gene was introduced into the model legume Medicago truncatula. Expression of flavodoxin did not confer saline tolerance to the whole plant, although the sensitive nitrogen‐fixing activity was maintained under salt stress in flavodoxin‐expressing plants. Our results indicate that flavodoxin induced small but significant changes in the enzymatic activities involved in the nodule redox balance that might be responsible for the positive effect on nitrogen fixation. Expression of flavodoxin can be regarded as a potential tool to improve legume symbiotic performance under salt stress, and possibly other environmental stresses.     

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.     

Glycinebetaine in saline conditions: an assessment of the current state of knowledge

Salt stress is one of the environmental threats that have devastating impacts on plant distribution, growth and production. Different plants are believed to have salt tolerance mechanisms that occur at the cellular level. One facet of the cellular mechanisms of adaptation to salinity stress is to accumulate either inorganic and/or organic solutes. Glycinebetaine (GB), as well as other organic solutes, has been referred to as compatible solutes, for the reason that they are innocent with essential biochemical reactions even at high concentrations. GB has been assumed to be involved in osmotic adjustment and/or osmoprotection of cellular functional macromolecules and, hence, can improve tolerance to saline conditions. However, the exact mechanism and direct evidences for such correlative data are still lacking despite many attempts to improve growth under saline conditions by exogenous application as well as genetic engineering of metabolic pathways involved in metabolism of GB. Despite the enormous amount of information accumulated in this regard, the exact function of GB in the adaptation to saline environments is not fully clear to this point, and even GB functions have been argued. Because of that, inconsistencies exist in the published data regarding GB accumulation and functions under salt stress. In this review, we provide an update on evidence supporting each of these arguments to reassess how GB affects plant growth and physiological traits under salt imposition, and whether its effects correlate with salt tolerance.     

Salinity and crop yield

Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop‐specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na⁺, also Cl^?, Mg²⁺, SO₄^2‐ or HCO₃^‐ contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.     

Physiological and biochemical adaptations of Cynodon dactylon (L.) Pers. from the Salt Range (Pakistan) to salinity stress

Naturally adapted salt tolerant populations provide a valuable material for exploring the adaptive components of salt tolerance. Under this aspect, two populations of Cynodon dactylon (L.) Pers. were subjected to salt stress in hydroponics. One was collected from a heavily salt-affected soil in the vicinity of a natural salt lake, Uchhali Lake, in the Salt Range of the Punjab province of Pakistan, and the other from a normal non-saline habitat from the Faisalabad region. The NaCl treatments in Hoagland's nutrient solution were: Control (no salt), 50, 100, 150 and 200 mM of NaCl. After 8 weeks of growth in hydroponics produced biomass, ion relations, and photosynthetic capacity were measured in the differently adapted ecotypes. In the ecotype of C. dactylon from the Salt Range, shoot dry weight was only slightly affected by varying levels of salt. However, in contrast, its root weight was markedly increased. On the other hand, the ecotype from Faisalabad (non-saline habitat) showed a marked decrease in shoot and root dry weights under saline regimes. The ecotype from the Salt Range accumulated relatively less amount of Na⁺ in the shoot than did that from Faisalabad, particularly at higher salt levels. Shoot or root K⁺ and Ca²⁺ contents varied inconsistently in both ecotypes under salt stress. All the photosynthetic parameters, leaf water potential and osmotic potential, and chlorophyll content in both ecotypes were adversely affected by salt stress, but all these physiological attributes except turgor potential and soluble sugars were less affected at high salinities in the salt tolerant ecotype from Salt Range. This ecotype accumulated significantly higher organic osmotica (total free amino acids, proline, total soluble proteins, and total soluble sugars) under saline conditions than its intolerant counterpart. Overall, the salt tolerant ecotype of C. dactylon from the Salt Range showed high salt tolerance due to its restricted uptake of Na⁺ accompanied by an increased uptake of K⁺ and Ca²⁺ in the roots as well as shoot due to its higher photosynthetic capacity and accumulation of organic osmotica such as free amino acids and proline under saline conditions.     

Salt‐tolerance diversity in diploid and polyploid cotton (Gossypium) species

The development of salt‐tolerant genotypes is pivotal for the effective utilization of salinized land and to increase global crop productivity. Several cotton species comprise the most important source of textile fibers globally, and these are increasingly grown on marginal or increasingly saline agroecosystems. The allopolyploid cotton species also provide a model system for polyploid research, of relevance here because polyploidy was suggested to be associated with increased adaptation to stress. To evaluate the genetic variation of salt tolerance among cotton species, 17 diverse accessions of allopolyploid (AD‐genome) and diploid (A‐ and D‐genome) Gossypium were evaluated for a total of 29 morphological and physiological traits associated with salt tolerance. For most morphological and physiological traits, cotton accessions showed highly variable responses to 2 weeks of exposure to moderate (50 mm NaCl) and high (100 mm NaCl) hydroponic salinity treatments. Our results showed that the most salt‐tolerant species were the allopolyploid Gossypium mustelinum from north‐east Brazil, the D‐genome diploid Gossypium klotzschianum from the Galapagos Islands, followed by the A‐genome diploids of Africa and Asia. Generally, A‐genome accessions outperformed D‐genome cottons under salinity conditions. Allopolyploid accessions from either diploid genomic group did not show significant differences in salt tolerance, but they were more similar to one of the two progenitor lineages. Our findings demonstrate that allopolyploidy in itself need not be associated with increased salinity stress tolerance and provide information for using the secondary Gossypium gene pool to breed for improved salt tolerance.     

Biological features and regulatory mechanisms of salt tolerance in plants

Halophytes play a vital role in saline agriculture because these plants are necessary to increase the food supply to meet the demands of the growing world population. In addition, the transfer of salt-resistance genes from halophytes using genetic technologies has the potential to increase the salt tolerance of xerophytes. Characterization of some particularly promising halophyte model organisms has revealed the important new insights into the salt tolerance mechanisms used by plants. Numerous advances using these model systems have improved our understanding of salt tolerance regulation and salt tolerance-associated changes in gene expression, and these mechanisms have important implications for saline agriculture. Recent findings provide a basis for future studies of salt tolerance in plants, as well as the development of improved strategies for saline agriculture to increase yields of food, feed, and fuel crops.     

Salt-stress signaling

Salinity stress has a major impact on plant growth and development. Increasing concentrations of salt in farm soils means that researchers must develop tolerant crops if the global food supply is to be sustained. Salt adaptation involves a complex network of different mechanisms whose responses to high salinity are regulated in an integrated fashion. The salt-stress signaling cascade(s) that activates these mechanisms starts by perceiving the saline environment. However, little is known about the components involved in either the perception or signaling of this stress. The mechanisms that are activated under such conditions include those responsible for ion homeostasis and osmotic adjustment. Here, we review the current understanding of those molecular mechanisms used by plants to respond and adapt to salt stress. Particular attention is paid to the information yielded by genetic analyses of the yeast modelSaccharomyces cerevisiae and the higher-plant model system ofArabidopsis.