Halophytes--an emerging trend in phytoremediation

Halophytic plants are of special interest because these plants are naturally present in environments characterized by an excess of toxic ions, mainly sodium and chloride. Several studies have revealed that these plants may also tolerate other stresses including heavy metals based on the findings that tolerance to salt and to heavy metals may, at least partly, rely on common physiological mechanisms. In addition, it has been shown that salt-tolerant plants may also be able to accumulate metals. Therefore, halophytes have been suggested to be naturally better adapted to cope with environmental stresses, including heavy metals compared to salt-sensitive crop plants commonly chosen for phytoextraction purposes. Thus, potentially halophytes are ideal candidates for phytoextraction orphytostabilization of heavy metal polluted soils and moreover of heavy metal polluted soils affected by salinity. Some halophytes use excretion processes in order to remove the excess of salt ions from their sensitive tissues and in some cases these glandular structures are not always specific to Na+ and Cl- and other toxic elements such as cadmium, zinc, lead, or copper are accumulated and excreted by salt glands or trichomes on the surface of the leaves--a novel phytoremediation process called "phytoexcretion". Finally, the use of halophytes has also been proposed for soil desalination through salt accumulation in the plant tissue or dissolution of soil calcite in the rhizosphere to provide Ca2+ that can be exchanged with Na+ at cation exchange sites.     

Physiological and molecular mechanisms of plant salt tolerance

Salt tolerance is an important economic trait for crops growing in both irrigated fields and marginal lands. The plant kingdom contains plant species that possess highly distinctive capacities for salt tolerance as a result of evolutionary adaptation to their environments. Yet, the cellular mechanisms contributing to salt tolerance seem to be conserved to some extent in plants although some highly salt-tolerant plants have unique structures that can actively excrete salts. In this review, we begin by summarizing the research in Arabidopsis with a focus on the findings of three membrane transporters that are important for salt tolerance: SOS1, AtHKT1, and AtNHX1. We then review the recent studies in salt tolerance in crops and halophytes. Molecular and physiological mechanisms of salt tolerance in plants revealed by the studies in the model plant, crops, and halophytes are emphasized. Utilization of the Na⁺ transporters to improve salt tolerance in plants is also summarized. Perspectives are provided at the end of this review.     

Ecotypic differentiation within Festuca rubra L. occurring in a heterogeneous coastal environment

In order to study the genetic differentiation between Festuca rubra L. individuals growing in a heterogeneous environment, indices of salt tolerance, mean relative growth rates and the numbers of tillers formed by plants grown in a Hoagland solution, were determined. It was found that plants from salt marsh sites have a high index of salt tolerance, a high mean relative growth rate and numerous tillers; plants from coastal sand dunes are less tolerant, grow slowly and form few tillers; plants from the inland polder sites are rather salt sensitive, fast growing and form a high number of tillers. The heritability of the mean relative growth rate and the tiller number appeared to differ from zero. Apparently, these characters have been under recent selection and thus give a picture of the adaptations of individual plants to the different environments encountered. An indication of gene flow has been found, although the effect of gene flow seems to be small in the face of the force of selection. It was concluded that the distinction of three ecotypes within the species F. rubra is insufficient to describe the differentiation found. Considering the differences observed, it seems more reasonable to speak of ecotypic variation.     

Principles and strategies in breeding for higher salt tolerance

Summary Salinity is an environmental component that usually reduces yield. Recent advances in the understanding of salt effects on plants have not revealed a reliable physiological or biochemical marker that can be used to rapidly screen for salt tolerance. The necessity of measuring salt tolerance based upon growth in saline relative to non-saline environments makes salt tolerance measurements and selection for tolerance difficult. Additionally, high variability in soil salinity and environmental interactions makes it questionable whether breeding should be conducted for tolerance or for high yield. Genetic techniques can be used to identify the components of variation attributable to genotype and environment, and the extent of genetic variation in saline and nonsaline environments can be used to estimate the potential for improving salt tolerance. Absolute salt tolerance can be improved best by increasing both absolute yield and relative salt tolerance.     

Growing floricultural crops with brackish water

In the current review we focus on the opportunity to use brackish water in the cultivation of floricultural plants, plants for which, due to their high economic value, growers have traditionally used good quality water for irrigation. Now, even for these crops the use of alternative water sources for irrigating nursery plants is needed because of the limited supplies of fresh water in many countries; understanding how saline water can be used will also enhance sustainable development in floriculture. While salt stress usually reduces plant growth, any such reduction might not be negative for ornamentals, where shoot vigour is sometime undesirable, although on flower crops salt stress can delay flowering or decrease flower quality characteristics. However, a decrease in growth rate is not enough to characterize the salt tolerance of ornamental plants, but traits like tip and marginal leaf burn, as consequence of sodium and chlorine accumulation, have to be considered for their effects on aesthetical value. With this in mind, some halophytes should be considered for floriculture because of their ability to cope with saline environments; their potential to tolerate salt is an important factor in reducing production costs. Consequently, the identification of ornamental halophytes is important for producing a commercially acceptable crop when irrigated with brackish waters. Many aspects of a plant's reaction to salt are genetically determined, so selection of suitable genotypes or breeding for salt tolerance in ornamentals are interesting options. Developing salt-tolerant floricultural crops, together with typical management practices that avoid excessive salinity stress in the root media, will provide the grower with economically and environmentally sound wastewater reuse options.     

Characterization of salt tolerant alfalfa (Medicago sativa L.) plants regenerated from salt tolerant cell lines

Salt tolerant cell lines have been selected from Medicago sativa, by a single step selection process on tissue culture medium containing 1% NaCl. Plants regenerated from these lines show improved salt tolerance compared to parent plants. The regenerated plants are vigorous, have flowered and are self fertile. The cellular salt tolerance characteristic can be passaged through the regenerated plants, since callus cultures initiated from immature ovaries of the salt tolerant regenerated plants are salt tolerant without additional selection on 1% NaCl. Several of these second generation callus cultures have been regenerated to produce vigorous plants which maintain the salt tolerance characteristic. The tolerance phenotype appears dominant in seeds obtained from self fertilization of the tolerant plants. The regenerated salt tolerant plants are therefore a valuable source as genotypes in plant breeding for salt tolerance and isolation, identification and manipulation of genes which confer salt tolerance in alfalfa.Abbreviations SH Schenk and Hildebrandt medium - 2,4-D 2,4-dichlorophenoxyacetic acid     

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.     

Improving salinity tolerance in crop plants: a biotechnological view

Salinity limits the production capabilities of agricultural soils in large areas of the world. Both breeding and screening germplasm for salt tolerance encounter the following limitations: (a) different phenotypic responses of plants at different growth stages, (b) different physiological mechanisms, (c) complicated genotype × environment interactions, and (d) variability of the salt-affected field in its chemical and physical soil composition. Plant molecular and physiological traits provide the bases for efficient germplasm screening procedures through traditional breeding, molecular breeding, and transgenic approaches. However, the quantitative nature of salinity stress tolerance and the problems associated with developing appropriate and replicable testing environments make it difficult to distinguish salt-tolerant lines from sensitive lines. In order to develop more efficient screening procedures for germplasm evaluation and improvement of salt tolerance, implementation of a rapid and reliable screening procedure is essential. Field selection for salinity tolerance is a laborious task; therefore, plant breeders are seeking reliable ways to assess the salt tolerance of plant germplasm. Salt tolerance in several plant species may operate at the cellular level, and glycophytes are believed to have special cellular mechanisms for salt tolerance. Ion exclusion, ion sequestration, osmotic adjustment, macromolecule protection, and membrane transport system adaptation to saline environments are important strategies that may confer salt tolerance to plants. Cell and tissue culture techniques have been used to obtain salt tolerant plants employing two in vitro culture approaches. The first approach is selection of mutant cell lines from cultured cells and plant regeneration from such cells (somaclones). In vitro screening of plant germplasm for salt tolerance is the second approach, and a successful employment of this method in durum wheat is presented here. Doubled haploid lines derived from pollen culture of F₁ hybrids of salt-tolerant parents are promising tools to further improve salt tolerance of plant cultivars. Enhancement of resistance against both hyper-osmotic stress and ion toxicity may also be achieved via molecular breeding of salt-tolerant plants using either molecular markers or genetic engineering.     

Salinity Adaptation and the Contribution of Parental Environmental Effects in Medicago truncatula

High soil salinity negatively influences plant growth and yield. Some taxa have evolved mechanisms for avoiding or tolerating elevated soil salinity, which can be modulated by the environment experienced by parents or offspring. We tested the contribution of the parental and offspring environments on salinity adaptation and their potential underlying mechanisms. In a two-generation greenhouse experiment, we factorially manipulated salinity concentrations for genotypes of Medicago truncatula that were originally collected from natural populations that differed in soil salinity. To compare population level adaptation to soil salinity and to test the potential mechanisms involved we measured two aspects of plant performance, reproduction and vegetative biomass, and phenological and physiological traits associated with salinity avoidance and tolerance. Saline-origin populations had greater biomass and reproduction under saline conditions than non-saline populations, consistent with local adaptation to saline soils. Additionally, parental environmental exposure to salt increased this difference in performance. In terms of environmental effects on mechanisms of salinity adaptation, parental exposure to salt spurred phenological differences that facilitated salt avoidance, while offspring exposure to salt resulted in traits associated with greater salt tolerance. Non-saline origin populations expressed traits associated with greater growth in the absence of salt while, for saline adapted populations, the ability to maintain greater performance in saline environments was also associated with lower growth potential in the absence of salt. Plastic responses induced by parental and offspring environments in phenology, leaf traits, and gas exchange contribute to salinity adaptation in M. truncatula. The ability of plants to tolerate environmental stress, such as high soil salinity, is likely modulated by a combination of parental effects and within-generation phenotypic plasticity, which are likely to vary in populations from contrasting environments.     

Epidermal bladder cells confer salinity stress tolerance in the halophyte quinoa and Atriplex species

Epidermal bladder cells (EBCs) have been postulated to assist halophytes in coping with saline environments. However, little direct supporting evidence is available. Here, Chenopodium quinoa plants were grown under saline conditions for 5 weeks. One day prior to salinity treatment, EBCs from all leaves and petioles were gently removed by using a soft cosmetic brush and physiological, ionic and metabolic changes in brushed and non‐brushed leaves were compared. Gentle removal of EBC neither initiated wound metabolism nor affected the physiology and biochemistry of control‐grown plants but did have a pronounced effect on salt‐grown plants, resulting in a salt‐sensitive phenotype. Of 91 detected metabolites, more than half were significantly affected by salinity. Removal of EBC dramatically modified these metabolic changes, with the biggest differences reported for gamma‐aminobutyric acid (GABA), proline, sucrose and inositol, affecting ion transport across cellular membranes (as shown in electrophysiological experiments). This work provides the first direct evidence for a role of EBC in salt tolerance in halophytes and attributes this to (1) a key role of EBC as a salt dump for external sequestration of sodium; (2) improved K⁺ retention in leaf mesophyll and (3) EBC as a storage space for several metabolites known to modulate plant ionic relations.     

碱蓬属植物耐盐机理研究进展

碱蓬属(Suaeda)植物是一类典型的真盐生植物,属于重要的盐生植物资源,全球广泛分布.人们已经对20种碱蓬属植物进行了观察和盐胁迫实验,研究了不同器官或组织的生理生化特征及其对盐胁迫的反应,并基于这些研究分析了盐胁迫的应答机制.叶片肉质化、细胞内离子区域化、渗透调节物质增加和抗氧化系统能力增强是碱蓬属植物响应和适应盐胁迫的重要方式和途径.但迄今为止的研究工作尚有一定的局限性,主要包括:研究工作主要集中在植物地上部分,而对植物地下部分的研究较少;多是少数生物学指标或生理学现象的单独观察,而缺乏对生理代谢过程的整体和综合分析;针对某种碱蓬的独立分析较多,而与近缘种的比较研究较少;植物对中性盐胁迫的反应研究较多,而对碱性盐的研究较少.为进一步系统阐明碱蓬属植物的耐盐机制,今后的工作应注重碱蓬属植物响应和适应盐胁迫的信号网络和调控机制研究,基于系统生物学研究思路,采用现代组学技术探索该属植物响应盐胁迫的由复杂信号网络调控的特殊生理特征和特异代谢途径.     

Rising from the sea: correlations between sulfated polysaccharides and salinity in plants

High salinity soils inhibit crop production worldwide and represent a serious agricultural problem. To meet our ever-increasing demand for food, it is essential to understand and engineer salt-resistant crops. In this study, we evaluated the occurrence and function of sulfated polysaccharides in plants. Although ubiquitously present in marine algae, the presence of sulfated polysaccharides among the species tested was restricted to halophytes, suggesting a possible correlation with salt stress or resistance. To test this hypothesis, sulfated polysaccharides from plants artificially and naturally exposed to different salinities were analyzed. Our results revealed that the sulfated polysaccharide concentration, as well as the degree to which these compounds were sulfated in halophytic species, were positively correlated with salinity. We found that sulfated polysaccharides produced by Ruppia maritima Loisel disappeared when the plant was cultivated in the absence of salt. However, subjecting the glycophyte Oryza sativa Linnaeus to salt stress did not induce the biosynthesis of sulfated polysaccharides but increased the concentration of the carboxylated polysaccharides; this finding suggests that negatively charged cell wall polysaccharides might play a role in coping with salt stress. These data suggest that the presence of sulfated polysaccharides in plants is an adaptation to high salt environments, which may have been conserved during plant evolution from marine green algae. Our results address a practical biological concept; additionally, we suggest future strategies that may be beneficial when engineering salt-resistant crops.     

Plasticity in salt tolerance traits allows for invasion of novel habitat by Japanese knotweed s. l. (Fallopia japonica and F.xbohemica, Polygonaceae)

Japanese knotweeds are among the most invasive organisms in the world. Their recent expansion into salt marsh habitat provides a unique opportunity to investigate how invasives establish in new environments. We used morphology, cytology, and AFLP genotyping to identify taxa and clonal diversity in roadside and salt marsh populations. We conducted a greenhouse study to determine the ability to tolerate salt and whether salt marsh populations are more salt tolerant than roadside populations as measured by the efficiency of PSII, leaf area, succulence, height, root-to-shoot ratio, and total biomass. Clonal diversity was extremely low with one F. japonica clone and five F. ×bohemica genotypes. The two taxa were significantly different in several traits, but did not vary in biomass or plasticity of any trait. All traits were highly plastic in response to salinity, but differed significantly among genets. Despite this variation, plants from the salt marsh habitats did not perform better in the salt treatment, suggesting that they are not better adapted to tolerate salt. Instead, our data support the hypothesis that plasticity in salt tolerance traits may allow these taxa to live in saline habitats without specific adaptation to tolerate salt.     

The importance of ecotype diversity on duckweed growth with and without salt stress

生态型多样性对存在盐分胁迫和不存在盐分胁迫下浮萍生长的重要性 淡水生态系统受到的污染正在威胁着全世界淡水植物物种的多样性。浮萍(Lemna minor)等淡水植物对新出现的逆境条件具有潜在的敏感性。为了测试生态型的多样性是否可以增强这类植物对逆境的抗性，本研究使用了7个浮萍种群，并沿着一个生态型多样性梯度对存在和不存在中度盐分胁迫时的种群生长速率进行了测量。这些浮萍种群在92个实验围隔中生长了5个月，其中有生态型单一栽培，也有在5或3个同种生态型(23种独特组合)中混合栽培。在无扰条件下生长一段时间(阶段1)后，这些浮萍栽培物将被置于中等盐分胁迫(50 mmol/L NaCl)条件下数周时间(阶段2)。实验进行时存在着与不同生态型伴生的天然表观微生物群落(epimicrobial community)。在阶段2中，这些藻类的一部分受到了随机二次胁迫。这些生态型表现出了不同的生长速率，其中最快的生长速率是其它生态型的两倍。多样性的环境进一步影响了生态型的生长速率，生态型混合栽培的植株在实验结束时具有更高的丰度，因此，随着时间的推移，环境逐渐恶化，生态型的多样性也将变得更为重要。上述研究结果表明，种内生长速率的差异体现了生态型多样性对种群丰度的正向影响。在中等盐度水平下的暴露并未显著影响浮萍的生长速率，尽管这种效应可能被盐性环境中更小的藻类压力所掩盖。     

Crab herbivory regulates re‐colonization of disturbed patches in a southwestern Atlantic salt marsh

Recent work exploring the effects of physical stress and herbivory on secondary succession in estuarine plant communities agrees with basic stress models and reveal that herbivory is an important force in brackish and oligohaline marshes but negligible in physically stressful salt marshes. In these systems, herbivores are terrestrial, and thus negatively affected by the same stressful factors that affect marsh plants (i.e. frequent flooding or high salinities). We evaluated the effects of a marine herbivore (i.e. the crab Neohelice granulata) on plant secondary succession in a southwestern Atlantic salt marsh. Field surveys revealed that disturbance‐generated bare patches have harsh physical conditions and that their edges suffer higher herbivore pressure compared to the marsh matrix. A factorial experiment demonstrated that asexual expansion of the surrounding plants is the only possible mechanism to re‐colonize disturbed patches and that crab exclusion can increase this colonization rate by more than 30 times. Our results show that even in highly stressful environments, herbivores strongly impact marsh structure by regulating patch recovery. The synergism of physical stress and herbivory may make plant succession an extremely slow process and lead to the prevalence of bare areas.     

红树植物水分关系研究进展

红树林生长于受潮汐影响的海滨特殊生境,具有重要的生态功能及应用价值。红树植物的水分利用特点一直是研究热点。由于受环境盐分影响,红树植物水势普遍较低;水分运输系统抗气穴化能力极强;水分利用保守,用水量处于同径级的热带陆生树木用水量的低值范围;表型可塑性大,可通过耐旱、耐盐和多样的水分管理策略适应潮间带环境;水分运输效率不低,能在环境条件适宜时进行高效的光合合成。本文通过大量的文献分析,综述了红树植物的水分关系特点、水分和盐分管理策略,对未来从多角度、结合新研究技术的红树植物水分关系研究进行了展望。     

耐盐转基因植物研究进展

高盐是限制作物生长、发育和产量的最严重的非生物胁迫之一。长期以来,改善作物的耐盐性一直是一个伟大的目标。然而,由于耐盐反应是一个极为复杂的过程,过去,通过传统的育种和遗传工程取得的成功有限。近十年来,由于分子生物学的发展,发现了一些与耐盐相关的新基因,对于这些基因的表达方式及其在耐盐反应中的作用已逐步得到了解,这为转基因工程提供了新的材料。通过控制耐盐相关基因在植物体内的表达,已获得了一些提高耐盐性的转基因植物,展示了诱人的前景,但该领域研究仍然存在许多困难和问题,文章重点讨论耐盐转基因植物的进展。     

Reversed animal-plant interactions: the evolution of insectivorous and ant-fed plants

Insectivorous plants and ant-fed plants represent the two ways in which plants have evolved to utilize directly nutrients derived from animals. This paper addresses the limitations under which selection acts to favour the evolution of one or the other of these nutrient-gathering tactics. Both tactics have evolved independently at least six times under similar ecological conditions, indicating that the evolutionary solutions to ecological problems are limited by the historical make-up of communities and are, to some extent, predictable. Both insectivorous and ant-fed plants evolve in environments with very low levels of availability of nutrients in the substrate; the primary use of the animal-food is probably nitrogen; the vast majority of species are perennial, and most species are tropical or subtropical, although some insectivorous genera are primarily temperate.
Although these two nutrient-gathering tactics evolve in response to similar ecological problems, whether plants evolve an insectivorous habit or the ant-fed habit depends on the growth forms of the plants and the habitats in which they grow. Most insectivorous plants evolve as herbs in wet, sterile soils or in sterile aquatic habitats; ant-fed plants evolve as epiphytes on trees in open-canopied habitats. These kinds of animal-plant interactions are relatively rare because the environments in which they are favoured by selection are uncommon.