Analysis of the physiological mechanism of salt-tolerant transgenic rice carrying a vacuolar Na+/H + antiporter gene from Suaeda salsa

Halophytes as new model plant species for salt tolerance strategies

Soil salinity is becoming a growing issue nowadays, severely affecting the world's most productive agricultural landscapes. With intersecting and competitive challenges of shrinking agricultural lands and increasing demand for food, there is an emerging need to build resilience for adaptation to anticipated climate change and land degradation. This necessitates the deep decoding of a gene pool of crop plant wild relatives which can be accomplished through salt-tolerant species, such as halophytes, in order to reveal the underlying regulatory mechanisms. Halophytes are generally defined as plants able to survive and complete their life cycle in highly saline environments of at least 200-500 mM of salt solution. The primary criterion for identifying salt-tolerant grasses (STGs) includes the presence of salt glands on the leaf surface and the Na⁺ exclusion mechanism since the interaction and replacement of Na⁺ and K⁺ greatly determines the survivability of STGs in saline environments. During the last decades or so, various salt-tolerant grasses/halophytes have been explored for the mining of salt-tolerant genes and testing their efficacy to improve the limit of salt tolerance in crop plants. Still, the utility of halophytes is limited due to the non-availability of any model halophytic plant system as well as the lack of complete genomic information. To date, although Arabidopsis (Arabidopsis thaliana) and salt cress (Thellungiella halophila) are being used as model plants in most salt tolerance studies, these plants are short-lived and can tolerate salinity for a shorter duration only. Thus, identifying the unique genes for salt tolerance pathways in halophytes and their introgression in a related cereal genome for better tolerance to salinity is the need of the hour. Modern technologies including RNA sequencing and genome-wide mapping along with advanced bioinformatics programs have advanced the decoding of the whole genetic information of plants and the development of probable algorithms to correlate stress tolerance limit and yield potential. Hence, this article has been compiled to explore the naturally occurring halophytes as potential model plant species for abiotic stress tolerance and to further breed crop plants to enhance salt tolerance through genomic and molecular tools.

Research Advances on Molecular Mechanism of Salt Tolerance in Suaeda

Plant growth and development are inevitably affected by various environmental factors. High salinity is the main factor leading to the reduction of cultivated land area, which seriously affects the growth and yield of plants. The genus Suaeda is a kind of euhalophyte herb, with seedlings that grow rapidly in moderately saline environments and can even survive in conditions of extreme salinity. Its fresh branches can be used as vegetables and the seed oil is rich in unsaturated fatty acids, which has important economic value and usually grows in a saline environment. This paper reviews the progress of research in recent years into the salt tolerance of several Suaeda species (for example, S. salsa, S. japonica, S. glauca, S. corniculata), focusing on ion regulation and compartmentation, osmotic regulation of organic solutes, antioxidant regulation, plant hormones, photosynthetic systems, and omics (transcriptomics, proteomics, and metabolomics). It helps us to understand the salt tolerance mechanism of the genus Suaeda, and provides a theoretical foundation for effectively improving crop resistance to salt stress environments.

Rewilding staple crops for the lost halophytism: Toward sustainability and profitability of agricultural production systems

Abiotic stress tolerance has been weakened during the domestication of all major staple crops. Soil salinity is a major environmental constraint that impacts over half of the world population; however, given the increasing reliance on irrigation and the lack of available freshwater, agriculture in the 21st century will increasingly become saline. Therefore, global food security is critically dependent on the ability of plant breeders to create high-yielding staple crop varieties that will incorporate salinity tolerance traits and account for future climate scenarios. Previously, we have argued that the current agricultural practices and reliance on crops that exclude salt from uptake is counterproductive and environmentally unsustainable, and thus called for a need for a major shift in a breeding paradigm to incorporate some halophytic traits that were present in wild relatives but were lost in modern crops during domestication. In this review, we provide a comprehensive physiological and molecular analysis of the key traits conferring crop halophytism, such as vacuolar Na⁺ sequestration, ROS desensitization, succulence, metabolic photosynthetic switch, and salt deposition in trichomes, and discuss the strategies for incorporating them into elite germplasm, to address a pressing issue of boosting plant salinity tolerance.

Overexpression of KcNHX1 gene confers tolerance to multiple abiotic stresses in Arabidopsis thaliana

Abiotic stresses such as drought, salinity, and heat affect plant growth and development. Karelinia caspica is a unique perennial herb that grows in desert area for a long time and has strong tolerance to environmental stresses. In order to explore the functions of the Na⁺/H⁺ antiporter gene from eremophyte K. caspica (KcNHX1) in the abiotic stress response of K. caspica and the underlying regulatory mechanisms, we constructed a vector overexpressing KcNHX1 and transformed it into Arabidopsis thaliana. The physiological results showed that the overexpression of KcNHX1 in A. thaliana not only enhanced the plant's tolerance to salt stress, but also enhanced its tolerance to drought and heat stress at the seedling stage. In addition, KcNHX1-overexpressing plants exhibited enhanced reproductive growth under high temperature, which was mediated by increased auxin accumulation. Taken together, our results indicate that KcNHX1 from an eremophyte can be used as a candidate gene to improve multiple stress tolerance in other plants.

Reassessing the role of ion homeostasis for improving salinity tolerance in crop plants

Soil salinity is a constraint for major agricultural crops leading to severe yield loss, which may increase with the changing climatic conditions. Disruption in the cellular ionic homeostasis is one of the primary responses induced by elevated sodium ions (Na⁺ ). Therefore, unraveling the mechanism of Na⁺ uptake and transport in plants along with the characterization of the candidate genes facilitating ion homeostasis is obligatory for enhancing salinity tolerance in crops. This review summarizes the current advances in understanding the ion homeostasis mechanism in crop plants, emphasizing the role of transporters involved in the regulation of cytosolic Na⁺ level along with the conservation of K⁺ /Na⁺ ratio. Furthermore, expression profiles of the candidate genes for ion homeostasis were also explored under various developmental stages and tissues of Oryza sativa based on the publicly available microarray data. The review also gives an up-to-date summary on the efforts to increase salinity tolerance in crops by manipulating selected stress-associated genes. Overall, this review gives a combined view on both the ionomic and molecular background of salt stress tolerance in plants.

A Na+/H+ antiporter, K2-NhaD, improves salt and drought tolerance in cotton (Gossypium hirsutum L.)

Overexpression of K2-NhaD in transgenic cotton resulted in phenotypes with strong salinity and drought tolerance in greenhouse and field experiments, increased expression of stress-related genes, and improved regulation of metabolic pathways, such as the SOS pathway. Drought and salinity are major abiotic stressors which negatively impact cotton yield under field conditions. Here, a plasma membrane Na⁺/H⁺ antiporter gene, K2-NhaD, was introduced into upland cotton R15 using an Agrobacterium tumefaciens-mediated transformation system. Homozygous transgenic lines K9, K17, and K22 were identified by PCR and glyphosate-resistance. TAIL-PCR confirmed that T-DNA carrying the K2-NhaD gene in transgenic lines K9, K17 and K22 was inserted into chromosome 3, 19 and 12 of the cotton genome, respectively. Overexpression of K2-NhaD in transgenic cotton plants grown in greenhouse conditions and subjected to drought and salinity stress resulted in significantly higher relative water content, chlorophyll, soluble sugar, proline levels, and SOD, CAT, and POD activity, relative to non-transgenic plants. The expression of stress-related genes was significantly upregulated, and this resulted in improved regulation of metabolic pathways, such as the salt overly sensitive pathway. K2-NhaD transgenic plants growing under field conditions displayed strong salinity and drought tolerance, especially at high levels of soil salinity and drought. Seed cotton yields in transgenic line were significantly higher than in wild-type plants. In conclusion, the data indicate that K2-NhaD transgenic lines have great potential for the production of stress-tolerant cotton under field conditions.

Optimal Extraction of Gallic Acid fromSuaeda glaucaBge. Leaves and Enhanced Efficiency by Ionic Liquids

The ultrasound-assisted extraction (UAE) was initially applied to extract gallic acid fromSuaeda glaucaBge. using 70% ethanol as extraction solvent. Temperature, liquid-solid ratio, and extraction time were optimized by response surface methodology (RSM), obtaining maximum levels of gallic acid (6.30 mg·g−1) at 51°C, 19.52 mL·g−1, and 42.68 min, respectively. The obtained model was statistically significant (p<0.0001). The verification experiments at the optimum conditions yielded gallic acid for 6.21 mg·g−1. Subsequently, under optimal conditions, four ionic liquids were used to extract gallic acid fromSuaeda glaucaBge. The results indicated that the presence of 1-hexyl-3-methylimidazolium chloride allowed increasing the EE of gallic acid up to 8.90 mg·g−1. This might be interpreted in terms of the molecular interaction between ionic liquid and gallic acid. The use of ionic liquids involves a stronger gallic acid extraction capacity than conventional organic volatile solvents. A promising alternative process is proposed for the extraction of gallic acid ofSuaeda glaucaBge.

Characterization of the superoxide dismutase genes of the halophyte Suaeda maritima in Japan and Egypt

Suaeda maritima varieties native to Japan and Egypt were cultured under aseptic conditions. The varieties differed in genetic distance but exhibited similar expression profiles of superoxide dismutase isozyme genes. The expression characteristics of superoxide dismutase (SOD; EC 1.15.1.1) isozyme genes from halophytic Suaeda marit ima plants native to Japan and Egypt were analyzed using young plants grown under aseptic conditions. A phylogenetic tree based on internal transcribed spacer sequences suggested that Egyptian S. maritima is related to European and India S. maritima, while Japanese S. maritima belongs to a separate clade. An in-gel SOD activity staining assay revealed that leaves from both the Egyptian and Japanese varieties showed high levels of CuZn-SOD and Fe-SOD activity, but no Mn-SOD activity; conversely, stems from both varieties showed Mn-SOD activity as well as other SOD isozyme activities. In Japanese S. maritima leaves, SOD activity was increased by incubation in growth medium containing 400 mM NaCl, while Egyptian S. maritima leaves showed elevated SOD activity in the absence of high salt. Genes encoding Mn-SOD and Fe-SOD were isolated from both plant types. RT-PCR analysis revealed that all SOD isozyme-encoding genes were expressed at the same levels in leaves from both plant types grown in normal or high-salt medium. In contrast, the expression of genes encoding choline monooxygenase and betaine aldehyde dehydrogenase, which are involved in betacyanin biosynthesis, was increased in high-salt medium. In leaves of Japanese S. maritima plants, Fe deficiency without high salt exposure preferentially decreased Fe-SOD activity. On the other hand, Fe deficiency with high salt exposure decreased not only Fe-SOD activity but also CuZn-SOD activity, suggesting that Fe availability is involved in the up-regulation of SOD isozymes mediating salt tolerance.

Understanding salinity responses and adopting 'omics-based' approaches to generate salinity tolerant cultivars of rice

Soil salinity is one of the main constraints affecting production of rice worldwide, by reducing growth, pollen viability as well as yield of the plant. Therefore, detailed understanding of the response of rice towards soil salinity at the physiological and molecular level is a prerequisite for its effective management. Various approaches have been adopted by molecular biologists or breeders to understand the mechanism for salinity tolerance in plants and to develop salt tolerant rice cultivars. Genome wide analysis using 'omics-based' tools followed by identification and functional validation of individual genes is becoming one of the popular approaches to tackle this task. On the other hand, mutation breeding and insertional mutagenesis has also been exploited to obtain salinity tolerant crop plants. This review looks into various responses at cellular and whole plant level generated in rice plants toward salinity stress thus, evaluating the suitability of intervention of functional genomics to raise stress tolerant plants. We have tried to highlight the usefulness of the contemporary 'omics-based' approaches such as genomics, proteomics, transcriptomics and phenomics towards dissecting out the salinity tolerance trait in rice. In addition, we have highlighted the importance of integration of various 'omics' approaches to develop an understanding of the machinery involved in salinity response in rice and to move forward to develop salt tolerant cultivars of rice.

Gene expression responses in Suaeda salsa after cadmium exposure

Coastal line is now polluted by many kinds of sewage including heavy metals discharged by intensive human activities. Cadmium is a nonessential heavy metal for organisms and can cause many kinds of adverse effect on the organisms. Suaeda salsa, a pioneer halophyte in intertidal zone of the Bohai coast, was proved to have cadmium-tolerant capacity. Given that, S. salsa was suggested as a potential coastal bio-indicator plant for cadmium contamination in the intertidal zone. Therefore, it is essential to investigate the responsive mechanism of S. salsa to cadmium since few studies focus on this subject till now. In the present study, six genes were selected to investigate the variation profiles of mRNA expression by fluorescent real-time quantitative PCR, including those involved in myo-inositol synthesis, redox reaction, salt-tolerant reaction. Results showed that cadmium exposure significantly modulate the mRNA expressions of MIPS, Nhx1, CAT2, GST, Prx Q genes. It suggested that cadmium exposure exerted an oxidative stress on S. salsa, disturbed Na⁺ homeostasis across membranes and interfered with the metabolism of inositol. In addition, CAT2 gene could be used as a gene marker in S. salsa to indicate cadmium pollution.

Phytoremediation potential of Arabidopsis thaliana, expressing ectopically a vacuolar proton pump, for the industrial waste phosphogypsum

Phosphogypsum (PG) is a by-product of the phosphorus-fertiliser industry and represents an environmental concern since it contains pollutants such as cadmium (Cd). We have recently shown that the overexpression of a proton pump gene (TaVP1) in transgenic tobacco (Nicotiana tabacum) led to an enhanced Cd tolerance and accumulation. The aim of this study was to evaluate the potential of transgenic Arabidopsis thaliana plants harbouring the TaVP1 gene to phytoremediate phosphogypsum. A pot experiment was carried out under greenhouse conditions. Transgenic A. thaliana plants harbouring the TaVP1 gene were grown on various substrates containing phosphogypsum (0, 25, 50 and 100 %) for 40 days. At the end of the growth period, we examined the growth (germination, root length, fresh weight) and physiological parameters (chlorophyll and protein contents, catalase activity and proteolysis) as well as the cadmium, Mg, Ca, and P contents of the A. thaliana plants. In order to evaluate Cd tolerance of the A. thaliana lines harbouring the TaVP1 gene, an in vitro experiment was also carried out. One week-old seedlings were transferred to Murashige and Skoog agar plates containing various concentrations of cadmium; the germination, total leaf area and root length were determined. The growth and physiological parameters of all A. thaliana plants were significantly altered by PG. The germination capacity, root growth and biomass production of wild-type (WT) plants were more severely inhibited by PG compared with the TaVP1 transgenic A. thaliana lines. In addition, TaVP1 transgenic A. thaliana plants maintained a higher antioxidant capacity than the WT. Interestingly, elemental analysis of leaf material derived from plants grown on PG revealed that the transgenic A. thaliana line accumulated up to ten times more Cd than WT. Despite its higher Cd content, the transgenic A. thaliana line performed better than the WT counterpart. In vitro evaluation of Cd tolerance showed that TaVP1 transgenic A. thaliana lines were more Cd-tolerant than the WT plants. These results suggested that ectopic expression of a vacuolar proton pump in A. thaliana plants can lead to various biotechnological applications including the phytoremediation of industrial wastes.

Simple yet stringent screening methodologies for evaluation of putative transformants for abiotic stress tolerance: salt and cadmium stress as a paradigm

Rigorous and stringent screening methodologies to select transformants at both seedling and plant level under cadmium or NaCl stress were developed. At seedling level, two screening strategies were standardized. One involved germination on filter paper/agar in the presence of either CdCl2 (125 μM) or NaCl (350-450 mM) for 9 days and selection of tolerant putative transformants. The other involved germination of the seedlings on soilrite by irrigation of 450 mM NaCl. Further, at plant level, in vitro evaluation for stress tolerance involved a simple leaf senescence bioassay. Combination of the seedling and plant level screening strategies would result in the initial identification of promising transformants for further analysis.

Gene flow from genetically modified rice to its wild relatives: Assessing potential ecological consequences

Pollen-mediated gene flow is the major pathway for transgene escape from GM rice to its wild relatives. Transgene escape to wild Oryza species having AA-genome will occur if GM rice is released to environments with these wild Oryza species. Transgenes may persist to and spread in wild populations after gene flow, resulting unwanted ecological consequences. For assessing the potential consequences caused by transgene escape, it is important to understand the actual gene flow frequencies from GM rice to wild relatives, transgene expression and inheritance in the wild relatives, as well as fitness changes that brought to wild relatives by the transgenes. This article reviews studies on transgene escape from rice to its wild relatives via gene flow and its ecological consequences. A framework for assessing potential ecological consequences caused by transgene escape from GM rice to its wild relatives is discussed based on studies of gene flow and fitness changes.

Raising salinity tolerant rice: recent progress and future perspectives

With the rapid growth in population consuming rice as staple food and the deteriorating soil and water quality around the globe, there is an urgent need to understand the response of this important crop towards these environmental abuses. With the ultimate goal to raise rice plant with better suitability towards rapidly changing environmental inputs, intensive efforts are on worldwide employing physiological, biochemical and molecular tools to perform this task. In this regard, efforts of plant breeders need to be duly acknowledged as several salinity tolerant varieties have reached the farmers field. Parallel efforts from molecular biologists have yielded relevant knowledge related to perturbations in gene expression and proteins during stress. Employing transgenic technology, functional validation of various target genes involved in diverse processes such as signaling, transcription, ion homeostasis, antioxidant defense etc for enhanced salinity stress tolerance has been attempted in various model systems and some of them have been extended to crop plant rice too. However, the fact remains that these transgenic plants showing improved performance towards salinity stress are yet to move from 'lab to the land'. Pondering this, we propose that future efforts should be channelized more towards multigene engineering that may enable the taming of this multigene controlled trait. Recent technological achievements such as the whole genome sequencing of rice is leading to a shift from single gene based studies to genome wide analysis that may prove to be a boon in re-defining salt stress responsive targets.

Emerging trends in the functional genomics of the abiotic stress response in crop plants

Plants are exposed to different abiotic stresses, such as water deficit, high temperature, salinity, cold, heavy metals and mechanical wounding, under field conditions. It is estimated that such stress conditions can potentially reduce the yield of crop plants by more than 50%. Investigations of the physiological, biochemical and molecular aspects of stress tolerance have been conducted to unravel the intrinsic mechanisms developed during evolution to mitigate against stress by plants. Before the advent of the genomics era, researchers primarily used a gene-by-gene approach to decipher the function of the genes involved in the abiotic stress response. However, abiotic stress tolerance is a complex trait and, although large numbers of genes have been identified to be involved in the abiotic stress response, there remain large gaps in our understanding of the trait. The availability of the genome sequences of certain important plant species has enabled the use of strategies, such as genome-wide expression profiling, to identify the genes associated with the stress response, followed by the verification of gene function by the analysis of mutants and transgenics. Certain components of both abscisic acid-dependent and -independent cascades involved in the stress response have already been identified. Information originating from the genome-wide analysis of abiotic stress tolerance will help to provide an insight into the stress-responsive network(s), and may allow the modification of this network to reduce the loss caused by stress and to increase agricultural productivity.

Salt stress response in rice: genetics, molecular biology, and comparative genomics

Significant progress has been made in unraveling the molecular biology of rice in the past two decades. Today, rice stands as a forerunner amongst the cereals in terms of details known on its genetics. Evidence show that salt tolerance in plants is a quantitative trait. Several traditional cultivars, landraces, and wild types of rice like Pokkali, CSR types, and Porteresia coarctata appear as promising materials for donation of requisite salt tolerance genes. A large number of quantitative trait loci (QTL) have been identified for salt tolerance in rice through generation of recombinant inbred lines and are being mapped using different types of DNA markers. Salt-tolerant transgenic rice plants have been produced using a host of different genes and transcript profiling by micro- and macroarray-based methods has opened the gates for the discovery of novel salt stress mechanisms in rice, and comparative genomics is turning out to be a critical input in this respect. In this paper, we present a comprehensive review of the genetic, molecular biology, and comparative genomics effort towards the generation of salt-tolerant rice. From the data on comprehensive transcript expression profiling of clones representing salt-stress-associated genes of rice, it is shown that transcriptional and translational machineries are important determinants in controlling salt stress response, and gene expression response in tolerant and susceptible rice plants differs mainly in quantitative terms.