Salt Tolerance of Glycinebetaine-Deficient and -Containing Maize Lines

Unveiling the Synergistic Effects of Phosphorus Fertilization and Organic Amendments on Red Pepper Growth, Productivity and Physio-Biochemical Response under Saline Water Irrigation and Climate-Arid Stresses

In regions facing water scarcity and soil salinity, mitigating these abiotic stresses is paramount for sustaining crop production. This study aimed to unravel the synergistic effects of organic matter and phosphorus management in reducing the adverse effect of saline water for irrigation on red pepper (Capsicum annuum L.) production, fruit quality, plant physiology, and stress tolerance indicators. The study was carried out in the arid Tadla region of Morocco and involved two key experiments: (i) a field experiment during the 2019 growing season, where red pepper plants were subjected to varying phosphorus fertilizer rates (120, 140, and 170 kg of P2O5.ha-1) and saline water irrigation levels (0.7; 1.5; 3; and 5 dS.m-1); and (ii) a controlled pot experiment in 2021 for examining the interaction of saline water irrigation levels (EC values of 0.7, 2, 5, and 9 dS.m-1), phosphorus rates (30, 36, and 42 kg of P2O5.ha-1), and the amount of organic matter (4, 8, 12, and 16 t.ha-1). The field study highlighted that saline irrigation significantly affected red pepper yields and fruit size, although phosphorus fertilization helped enhance productivity. Additionally, biochemical markers of stress tolerance, such as proline and glycine betaine, along with stomatal conductance, were impacted by increasing salinity levels. The pot experiment showed that combining organic amendments and phosphorus improved soil properties and stimulated red pepper growth and root weight across all salinity levels. The integration of phosphorus fertilization and organic amendments proved instrumental for counteracting salinity-induced constraints on red pepper growth and yield. Nonetheless, caution is necessary as high salinity can still negatively impact red pepper productivity, necessitating the establishment of an irrigation water salinity threshold, set at 5 dS.m-1.

Designing salt stress-resilient crops: Current progress and future challenges

Excess soil salinity affects large regions of land and is a major hindrance to crop production worldwide. Therefore, understanding the molecular mechanisms of plant salt tolerance has scientific importance and practical significance. In recent decades, studies have characterized hundreds of genes associated with plant responses to salt stress in different plant species. These studies have substantially advanced our molecular and genetic understanding of salt tolerance in plants and have introduced an era of molecular design breeding of salt-tolerant crops. This review summarizes our current knowledge of plant salt tolerance, emphasizing advances in elucidating the molecular mechanisms of osmotic stress tolerance, salt-ion transport and compartmentalization, oxidative stress tolerance, alkaline stress tolerance, and the trade-off between growth and salt tolerance. We also examine recent advances in understanding natural variation in the salt tolerance of crops and discuss possible strategies and challenges for designing salt stress-resilient crops. We focus on the model plant Arabidopsis (Arabidopsis thaliana) and the four most-studied crops: rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), and soybean (Glycine max).

A Three-Year Plant Study of Salt-Tolerant Transgenic Maize Showed No Effects on Soil Enzyme Activity and Nematode Community

The environmental effects of genetically modified crops are now a global concern. It is important to monitor the potential environmental impact of transgenic corn after commercial release. In rhizosphere soil, plant roots interact with soil enzymes and microfauna, which can be affected by the transgenes of genetically modified crops. To determine the long-term impact of transgenic plant cultivation, we conducted a field study for 3 consecutive years (2018–2020) and observed the enzyme activities and nematode populations in plots planted with transgenic maize BQ-2, non-transgenic wild-type maize (Qi319), and inbred line B73. We took soil samples from three cornfields at four different growth stages (V3, V9, R1, and R6 stages); determined soil dehydrogenase, urease, and sucrase activities; and collected and identified soil nematodes to the genus level. The results demonstrated seasonal variations in dehydrogenase, urease, and sucrase activities. However, there was a consistent trend of change. The generic composition and diversity indices of the soil nematodes did not significantly differ, although significant seasonal variation was found in the individual densities of the principal trophic groups and the diversity indices of the nematodes in all three cornfields. The results of the study suggest that a 3-year cultivation of transgenic corn had no significant effects on soil enzyme activity and the soil nematode community. This study provides a theoretical basis for the environmental impact monitoring of transgenic corn.

Glycine betaine increases salt tolerance in maize (Zea mays L.) by regulating Na+ homeostasis

Improving crop salt tolerance is an adaptive measure to climate change for meeting future food demands. Previous studies have reported that glycine betaine (GB) plays critical roles as an osmolyte in enhancing plant salt resistance. However, the mechanism underlying the GB regulating plant Na⁺ homeostasis during response to salinity is poorly understood. In this study, hydroponically cultured maize with 125 mM NaCl for inducing salinity stress was treated with 100 μM GB. We found that treatment with GB improved the growth of maize plants under non-stressed (NS) and salinity-stressed (SS) conditions. Treatment with GB significantly maintained the properties of chlorophyll fluorescence, including Fv/Fm, ΦPSII, and ΦNPQ, and increased the activity of the antioxidant enzymes for mitigating salt-induced growth inhibition. Moreover, GB decreased the Na⁺/K⁺ ratio primarily by reducing the accumulation of Na⁺ in plants. The results of NMT tests further confirmed that GB increased Na⁺ efflux from roots under SS condition, and fluorescence imaging of cellular Na⁺ suggested that GB reduced the cellular allocation of Na⁺. GB additionally increased Na⁺ efflux in leaf protoplasts under SS condition, and treatment with sodium orthovanadate, a plasma membrane (PM) H⁺-ATPase inhibitor, significantly alleviated the positive effects of GB on Na⁺ efflux under salt stress. GB significantly improved the vacuolar activity of NHX but had no significant effects on the activity of V type H⁺-ATPases. In addition, GB significantly upregulated the expression of the PM H⁺-ATPase genes, ZmMHA2 and ZmMHA4, and the Na⁺/H⁺ antiporter gene, ZmNHX1. While, the V type H⁺-ATPases gene, ZmVP1, was not significantly regulated by GB. Altogether these results indicate that GB regulates cellular Na⁺ homeostasis by enhancing PM H+-ATPases gene transcription and protein activities to improve maize salt tolerance. This study provided an extended understanding of the functions of GB in plant responses to salinity, which can help the development of supportive measures using GB for obtaining high maize yield in saline conditions.

Understanding the roles of osmolytes for acclimatizing plants to changing environment: a review of potential mechanism

Abiotic stresses are significant environmental issues that restrict plant growth, productivity, and survival while also posing a threat to global food production and security. Plants produce compatible solutes known as osmolytes to adapt themselves in such changing environment. Osmolytes contribute to homeostasis maintenance, provide the driving gradient for water uptake, maintain cell turgor by osmotic adjustment, and redox metabolism to remove excess level of reactive oxygen species (ROS) and reestablish the cellular redox balance as well as protect cellular machinery from osmotic stress and oxidative damage. Perceiving the mechanisms how plants interpret environmental signals and transmit them to cellular machinery to activate adaptive responses is important for crop improvement programs to get stress-tolerant varieties. A large number of studies conducted in the last few decades have shown that osmolytes accumulate in plants and have strong associations with abiotic stress tolerance. Production of abundant osmolytes is needed for tolerance in many plant species. In addition, transgenic plants overexpressing genes for different osmolytes showed enhanced tolerance to various abiotic stresses. Many important aspects of their mechanisms of action are yet to be largely identified, especially regarding the relevance and relative contribution of specific osmolytes to the stress tolerance of a given species. Therefore, more efforts and resources should be invested in the study of the abiotic stress responses of plants in their natural habitats. The present review focuses on the possible roles and mechanisms of osmolytes and their association toward abiotic stress tolerance in plants. This review would help the readers in learning more about osmolytes and how they behave in changing environments as well as getting an idea of how this knowledge could be applied to develop stress tolerance in plants.

Metabolomics-driven gene mining and genetic improvement of tolerance to salt-induced osmotic stress in maize

The farmland of the world's main corn-producing area is increasingly affected by salt stress. Therefore, the breeding of salt-tolerant cultivars is necessary for the long-term sustainability of global corn production. Previous studies have shown that natural maize varieties display a large diversity of salt tolerance, yet the genetic variants underlying such diversity remain poorly discovered and applied, especially those mediating the tolerance to salt-induced osmotic stress (SIOS). Here we report a metabolomics-driven understanding and genetic improvement of maize SIOS tolerance. Using a LC-MS-based untargeted metabolomics approach, we profiled the metabolomes of 266 maize inbred lines under control and salt conditions, and then identified 37 metabolite biomarkers of SIOS tolerance (METO1-37). Follow-up metabolic GWAS (mGWAS) and genotype-to-phenotype modeling identified 10 candidate genes significantly associating with the SIOS tolerance and METO abundances. Furthermore, we validated that a citrate synthase, a glucosyltransferase and a cytochrome P450 underlie the genotype-METO-SIOS tolerance associations, and showed that their favorable alleles additively improve the SIOS tolerance of elite maize inbred lines. Our study provides a novel insight into the natural variation of maize SIOS tolerance, which boosts the genetic improvement of maize salt tolerance, and demonstrates a metabolomics-based approach for mining crop genes associated with this complex agronomic trait.

Metabolic engineering of osmoprotectants to elucidate the mechanism(s) of salt stress tolerance in crop plants

Previous studies on engineering osmoprotectant metabolic pathway genes focused on the performance of transgenic plants under salt stress conditions rather than elucidating the underlying mechanism(s), and hence, the mechanism(s) remain(s) unclear. Salt stress negatively impacts agricultural crop yields. Hence, to meet future food demands, it is essential to generate salt stress-resistant varieties. Although traditional breeding has improved salt tolerance in several crops, this approach remains inadequate due to the low genetic diversity of certain important crop cultivars. Genetic engineering is used to introduce preferred gene(s) from any genetic reserve or to modify the expression of the existing gene(s) responsible for salt stress response or tolerance, thereby leading to improved salt tolerance in plants. Although plants naturally produce osmoprotectants as an adaptive mechanism for salt stress tolerance, they offer only partial protection. Recently, progress has been made in the identification and characterization of genes involved in the biosynthetic pathways of osmoprotectants. Exogenous application of these osmoprotectants, and genetic engineering of enzymes in their biosynthetic pathways, have been reported to enhance salt tolerance in different plants. However, no clear mechanistic model exists to explain how osmoprotectant accumulation in transgenic plants confers salt tolerance. This review critically examines the results obtained thus far for elucidating the underlying mechanisms of osmoprotectants for improved salt tolerance, and thus, crop yield stability under salt stress conditions, through the genetic engineering of trehalose, glycinebetaine, and proline metabolic pathway genes.

Spatial and Temporal Profile of Glycine Betaine Accumulation in Plants Under Abiotic Stresses

Several halophytes and a few crop plants, including Poaceae, synthesize and accumulate glycine betaine (GB) in response to environmental constraints. GB plays an important role in osmoregulation, in fact, it is one of the main nitrogen-containing compatible osmolytes found in Poaceae. It can interplay with molecules and structures, preserving the activity of macromolecules, maintaining the integrity of membranes against stresses and scavenging ROS. Exogenous GB applications have been proven to induce the expression of genes involved in oxidative stress responses, with a restriction of ROS accumulation and lipid peroxidation in cultured tobacco cells under drought and salinity, and even stabilizing photosynthetic structures under stress. In the plant kingdom, GB is synthesized from choline by a two-step oxidation reaction. The first oxidation is catalyzed by choline monooxygenase (CMO) and the second oxidation is catalyzed by NAD+-dependent betaine aldehyde dehydrogenase. Moreover, in plants, the cytosolic enzyme, named N-methyltransferase, catalyzes the conversion of phosphoethanolamine to phosphocholine. However, changes in CMO expression genes under abiotic stresses have been observed. GB accumulation is ontogenetically controlled since it happens in young tissues during prolonged stress, while its degradation is generally not significant in plants. This ability of plants to accumulate high levels of GB in young tissues under abiotic stress, is independent of nitrogen (N) availability and supports the view that plant N allocation is dictated primarily to supply and protect the growing tissues, even under N limitation. Indeed, the contribution of GB to osmotic adjustment and ionic and oxidative stress defense in young tissues, is much higher than that in older ones. In this review, the biosynthesis and accumulation of GB in plants, under several abiotic stresses, were analyzed focusing on all possible roles this metabolite can play, particularly in young tissues.

The Promise of Molecular and Genomic Techniques for Biodiversity Research and DNA Barcoding of the Arabian Peninsula Flora

The Arabian Peninsula is known to have a comprehensive and rich endowment of unique and genetically diverse plant genetic resources. Analysis and conservation of biological diversity is a crucial issue to the whole Arabian Peninsula. The rapid and accurate delimitation and identification of a species is crucial to genetic diversity analysis and the first critical step in the assessment of distribution, population abundance and threats related to a particular target species. During the last two decades, classical strategies of evaluating genetic variability, such as morphology and physiology, have been greatly complemented by phylogenetic, taxonomic, genetic diversity and breeding research molecular studies. At present, initiatives are taking place around the world to generate DNA barcode libraries for vascular plant flora and to make these data available in order to better understand, conserve and utilize biodiversity. The number of herbarium collection-based plant evolutionary genetics and genomics studies being conducted has been increasing worldwide. The herbaria provide a rich resource of already preserved and identified material, and these as well as freshly collected samples from the wild can be used for creating a reference DNA barcode library for the vascular plant flora of a region. This review discusses the main molecular and genomic techniques used in plant identification and biodiversity analysis. Hence, we highlight studies emphasizing various molecular techniques undertaken during the last 10 years to study the plant biodiversity of the Arabian Peninsula. Special emphasis on the role of DNA barcoding as a powerful tool for plant biodiversity analysis is provided, along with the crucial role of herbaria in creating a DNA barcode library.

Metabolite Profiles of Maize Leaves in Drought, Heat, and Combined Stress Field Trials Reveal the Relationship between Metabolism and Grain Yield

The development of abiotic stress-resistant cultivars is of premium importance for the agriculture of developing countries. Further progress in maize (Zea mays) performance under stresses is expected by combining marker-assisted breeding with metabolite markers. In order to dissect metabolic responses and to identify promising metabolite marker candidates, metabolite profiles of maize leaves were analyzed and compared with grain yield in field trials. Plants were grown under well-watered conditions (control) or exposed to drought, heat, and both stresses simultaneously. Trials were conducted in 2010 and 2011 using 10 tropical hybrids selected to exhibit diverse abiotic stress tolerance. Drought stress evoked the accumulation of many amino acids, including isoleucine, valine, threonine, and 4-aminobutanoate, which has been commonly reported in both field and greenhouse experiments in many plant species. Two photorespiratory amino acids, glycine and serine, and myoinositol also accumulated under drought. The combination of drought and heat evoked relatively few specific responses, and most of the metabolic changes were predictable from the sum of the responses to individual stresses. Statistical analysis revealed significant correlation between levels of glycine and myoinositol and grain yield under drought. Levels of myoinositol in control conditions were also related to grain yield under drought. Furthermore, multiple linear regression models very well explained the variation of grain yield via the combination of several metabolites. These results indicate the importance of photorespiration and raffinose family oligosaccharide metabolism in grain yield under drought and suggest single or multiple metabolites as potential metabolic markers for the breeding of abiotic stress-tolerant maize.

Stress-related hormones and glycinebetaine interplay in protection of photosynthesis under abiotic stress conditions

Plants subjected to abiotic stresses such as extreme high and low temperatures, drought or salinity, often exhibit decreased vegetative growth and reduced reproductive capabilities. This is often associated with decreased photosynthesis via an increase in photoinhibition, and accompanied by rapid changes in endogenous levels of stress-related hormones such as abscisic acid (ABA), salicylic acid (SA) and ethylene. However, certain plant species and/or genotypes exhibit greater tolerance to abiotic stress because they are capable of accumulating endogenous levels of the zwitterionic osmolyte-glycinebetaine (GB). The accumulation of GB via natural production, exogenous application or genetic engineering, enhances plant osmoregulation and thus increases abiotic stress tolerance. The final steps of GB biosynthesis occur in chloroplasts where GB has been shown to play a key role in increasing the protection of soluble stromal and lumenal enzymes, lipids and proteins, of the photosynthetic apparatus. In addition, we suggest that the stress-induced GB biosynthesis pathway may well serve as an additional or alternative biochemical sink, one which consumes excess photosynthesis-generated electrons, thus protecting photosynthetic apparatus from overreduction. Glycinebetaine biosynthesis in chloroplasts is up-regulated by increases in endogenous ABA or SA levels. In this review, we propose and discuss a model describing the close interaction and synergistic physiological effects of GB and ABA in the process of cold acclimation of higher plants.

Functional and expression analyses of two kinds of betaine aldehyde dehydrogenases in a glycinebetaine-hyperaccumulating graminaceous halophyte, Leymus chinensis

Glycinebetaine (GB) is an important compatible solute for salinity tolerance in many plants. In this study, we analyzed the enzymatic activity and the expression level of betaine aldehyde dehydrogenase (BADH), an important enzyme that catalyzes the last step in the GB synthesis in Leymus chinensis, a GB-hyperaccumulating graminaceous halophyte, and compared with those of barley, a graminaceous glycophyte. We have isolated cDNAs for two BADH genes, LcBADH1 and LcBADH2. LcBADH1 has a putative peroxisomal signal peptide (PTS1) at its C-terminus, while LcBADH2 does not have any typical signal peptide. Using immunofluorescent labeling, we showed that BADH proteins were localized to the cytosol and dot-shaped organelles in the mesophyll and bundle sheath cells of L.chinensis leaves. The affinity of recombinant LcBADH2 for betaine aldehyde was comparable to other plant BADHs, whereas recombinant LcBADH1 showed extremely low affinity for betaine aldehyde, indicating that LcBADH2 plays a major role in GB synthesis in L. chinensis. In addition, the recombinant LcBADH2 protein was tolerant to NaCl whereas LcBADH1 wasn't. The kinetics, subcellular and tissue localization of BADH proteins were comparable between L. chinensis and barley. The activity and expression level of BADH proteins were higher in L. chinensis compared with barley under both normal and salinized conditions, which may be related to the significant difference in the amount of GB accumulation between two plants.

Transformation of tomato with a bacterial codA gene enhances tolerance to salt and water stresses

Genetically engineered tomato (Lycopersicon esculentum) with the ability to synthesize glycinebetaine was generated by introducing the codA gene encoding choline oxidase from Arthrobacter globiformis. Integration of the codA gene in transgenic tomato plants was verified by PCR analysis and DNA blot hybridization. Transgenic expression of gene was verified by RT-PCR analysis and RNA blot hybridization. The codA-transgenic plants showed higher tolerance to salt stress during seed germination, and subsequent growth of young seedlings than wild-type plants. The codA transgene enhanced the salt tolerance of whole plants and leaves. Mature leaves of codA-transgenic plants revealed higher levels of relative water content, chlorophyll content, and proline content than those of wild-type plants under salt and water stresses. Results from the current study suggest that the expression of the codA gene in transgenic tomato plants induces the synthesis of glycinebetaine and improves the tolerance of plants to salt and water stresses.

Drought, metabolites, and Arabidopsis natural variation: a promising combination for understanding adaptation to water-limited environments

Drought elicits substantial changes in plant metabolism and it remains a challenge to determine which of these changes represent adaptive responses and which of them are merely neutral effects or even symptoms of damage. Arabidopsis primarily uses low water potential/dehydration avoidance strategies to respond to water limitation. The large variation in evolved stress responses among accessions can be a powerful tool to identify ecologically important and adaptive traits; however, collection of relevant phenotype data under controlled water stress is often a limiting factor. Quantitative genetics of Arabidopsis has great potential to find the genes underlying variation in drought-affected metabolic traits, for example proline metabolism, as well as overall adaptation.

Transgenic Brassica chinensis plants expressing a bacterial codA gene exhibit enhanced tolerance to extreme temperature and high salinity

Transgenic Brassica compestris L. spp. chinensis plants expressing a choline oxidase (codA) gene from Arthrobacter globiformis were obtained through Agrobacterium tumefaciens-mediated transformation. In the transgenic plants, codA gene expression and its product transportation to chloroplasts were detected by the enzyme-linked immunosorbent assay (ELISA) examination, immunogold localization, and (1)H-nuclear magnetic resonance ((1)H-NMR). Stress tolerance was evaluated in the T(3) plants under extreme temperature and salinity conditions. The plants of transgenic line 1 (L1) showed significantly higher net photosynthetic rate (P(n)) and P(n) recovery rate under high (45 °C, 4 h) and low temperature (1 °C, 48 h) treatments, and higher photosynthetic rate under high salinity conditions (100, 200, and 300 mmol/L NaCl, respectively) than the wild-type plants. The enhanced tolerance to high temperature and high salinity stresses in transgenic plants is associated with the accumulation of betaine, which is not found in the wild-type plants. Our results indicate that the introduction of codA gene from Arthrobacter globiformis into Brassica compestris L. spp. chinensis could be a potential strategy for improving the plant tolerance to multiple stresses.

Salt-induced accumulation of glycine betaine is inhibited by high light in durum wheat

In this study, we determined the effects of both salinity and high light on the metabolism of durum wheat (Triticum durum Desf. cv. Ofanto) seedlings, with a special emphasis on the potential role of glycine betaine in their protection. Unexpectedly, it appears that high light treatment inhibits the synthesis of glycine betaine, even in the presence of salt stress. Additional solutes such as sugars and especially amino acids could partially compensate for the decrease in its synthesis upon exposure to high light levels. In particular, tyrosine content was strongly increased by high light, this effect being enhanced by salt treatment. Interestingly, a large range of well-known detoxifying molecules were also not induced by salt treatment in high light conditions. Taken together, our results question the role of glycine betaine in salinity tolerance under light conditions close to those encountered by durum wheat seedlings in their natural environment and suggest the importance of other mechanisms, such as the accumulation of minor amino acids.

Glycinebetaine-induced water-stress tolerance in codA-expressing transgenic indica rice is associated with up-regulation of several stress responsive genes

Rice (Oryza sativa L.), a non-accumulator of glycinebetaine (GB), is highly susceptible to abiotic stress. Transgenic rice with chloroplast-targeted choline oxidase encoded by the codA gene from Arthrobacter globiformis has been evaluated for inheritance of transgene up to R5 generation and water-stress tolerance. During seedling, vegetative and reproductive stages, transgenic plants could maintain higher activity of photosystem II and they show better physiological performance, for example, enhanced detoxification of reactive oxygen species compared to wild-type plants under water-stress. Survival rate and agronomic performance of transgenic plants is also better than wild-type following prolonged water-stress. Choline oxidase converts choline into GB and H2O2 in a single step. It is possible that H2O2/GB might activate stress response pathways and prepare transgenic plants to mitigate stress. To check this possibility, microarray-based transcriptome analysis of transgenic rice has been done. It unravelled altered expression of many genes involved in stress responses, signal transduction, gene regulation, hormone signalling and cellular metabolism. Overall, 165 genes show more than two-fold up-regulation at P-value < 0.01 in transgenic rice. Out of these, at least 50 genes are known to be involved in plant stress response. Exogenous application of H2O2 or GB to wild-type plants also induces such genes. Our data show that metabolic engineering for GB is a promising strategy for introducing stress tolerance in crop plants and which could be imparted, in part, by H2O2- and/or GB-induced stress response genes.

NMR metabolic profiling of transgenic maize with the Cry1Ab gene

The metabolic profiles of seeds from the transgenic maize variety 33P67 and of the corresponding traditional variety were investigated using one- and two-dimensional NMR techniques. The transgenic variety carries a functional Cry1A(b) gene, which confers to the plant the ability to produce Bt insect toxin. About 40 water-soluble metabolites in the maize seed extracts were identified, providing a more complete (1)H and (13)C NMR assignment with respect to the assignment reported in the literature. In particular ethanol, lactic acid, citric acid, lysine, arginine, glycine-betaine, raffinose, trehalose, alpha-galactose, and adenine were identified for the first time in the (1)H NMR spectrum of maize seeds extracts. The (1)H spectra of transgenic and nontransgenic seed maize samples turned out to be conservative, showing the same signals and therefore the same metabolites. However, a higher concentration of ethanol, citric acid, glycine-betaine, trehalose, as well as of another compound not yet completely identified, was observed in the transgenic extracts than in nontransgenic samples. So, it was possible to discriminate between transgenic and nontransgenic metabolic profilings through the use of an appropriate statistical analysis.

Barium toxicity effects in soybean plants

Barium (Ba)-induced phytotoxicity at 100, 1000, or 5000 microM Ba in soybean plants (Glycine max) was investigated under hydroponic culture conditions. Soybean growth and leaf photosynthetic activity were significantly inhibited by all three levels of Ba treatments. In the case of photosynthetic activity, 5000 microM Ba treatment shutdown stomatal opening and perturbed carbon fixation metabolism and translocation. However, 100 and 1000 microM Ba treatments shut down stomatal opening and inhibited carbon fixation, but without perturbation of leaf carbon fixation-related metabolism. Potassium (K) absorption by soybean roots was also reduced in all three Ba treatments. This decreased K absorption reduced K localization at guard cells. Barium accumulation in guard cells also inhibited K transport from epidermal cells to guard cells. This lack of K in guard cells resulted in stomatal closure. As a result of inhibition of K transport into guard cells and stomatal shutdown, photosynthetic activity and plant productivity were inhibited. Our experiment indicates that Ba has phytotoxic effects on soybean plants by inhibiting photosynthesis.

Improved salt tolerance in tobacco plants by co-transformation of a betaine synthesis gene BADH and a vacuolar Na+/H+ antiporter gene SeNHX1

Three types of transgenic tobacco plants were acquired by separate transformation or co-transformation of a vacuolar Na(+)/H(+) antiporter gene, SeNHX1, and a betaine synthesis gene, BADH. When exposed to 200 mM NaCl, the dual gene-transformed plants displayed greater accumulation of betaine and Na(+) than their wild-type counterparts. Photosynthetic rate and photosystem II activity in the transgenic plants were less affected by salt stress than wild-type plants. Transgenic plants exhibited a greater increase in osmotic pressure than wild-type plants when exposed to NaCl. More importantly, the dual gene transformed plants accumulated higher biomass than either of the single transgenic plants under salt stress. Taken together, these findings indicate that simultaneous transformation of BADH and SeNHX1 genes into tobacco plants can enable plants to accumulate betaine and Na(+), thus conferring them more tolerance to salinity than either of the single gene transformed plants or wild-type tobacco plants.

Mechanisms of salinity tolerance

The physiological and molecular mechanisms of tolerance to osmotic and ionic components of salinity stress are reviewed at the cellular, organ, and whole-plant level. Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na(+) or Cl() exclusion, and the tolerance of tissue to accumulated Na(+) or Cl(). Our understanding of the role of the HKT gene family in Na(+) exclusion from leaves is increasing, as is the understanding of the molecular bases for many other transport processes at the cellular level. However, we have a limited molecular understanding of the overall control of Na(+) accumulation and of osmotic stress tolerance at the whole-plant level. Molecular genetics and functional genomics provide a new opportunity to synthesize molecular and physiological knowledge to improve the salinity tolerance of plants relevant to food production and environmental sustainability.

Genetic engineering of the biosynthesis of glycinebetaine leads to increased tolerance of photosynthesis to salt stress in transgenic tobacco plants

Genetically engineered tobacco (Nicotiana tabacum L.) with the ability to synthesis glycinebetaine (GB) in chloroplasts was established by introducing the BADH gene for betaine aldehyde dehydrogenase from spinach (Spinacia oleracea L.). The genetic engineering resulted in enhanced tolerance of growth of young seedlings to salt stress. This increased tolerance was not due to improved water status, since there were no significant differences in accumulation of sodium and chloride, leaf water potential, and relative water content between wild type and transgenic plants under salt stress. Salt stress resulted in a decrease in CO2 assimilation and such a decrease was much greater in wild type plants than in transgenic plants. Though salt stress showed no damage to PSII, there were a decrease in the maximal PSII electron transport rate in vivo and an increase in non-photochemical quenching (NPQ) and these changes were greater in wild type plants than in transgenic plants. In addition, salt stress inhibited the activities of ribulose 1,5-bisphosphate carboxylase/oxygenase, chloroplastic fructose-1,6-bisphosphatase, fructose-1,6-bisphosphate aldolase, and phosphoribulokinase and such a decrease was also greater in wild type plants than in transgenic plants, suggesting that GB protects these enzymes against salt stress. However, there were no significant changes in the activities of phosphoglycerate kinase, triose phosphate isomerase, ribulose-5-phosphate isomerase, transketolase, and sedoheptulose-1,7-bisphosphatase in both wild type and transgenic plants. The results in this study suggest that enhanced tolerance of CO2 assimilation to salt stress may be one of physiological bases for increased tolerance of growth of transgenic plants to salt stress.

Salinity tolerance of Arabidopsis: a good model for cereals?

Arabidopsis is a glycophyte species that is sensitive to moderate levels of NaCl. Arabidopsis offers unique benefits to genetic and molecular research and has provided much information about both Na(+) transport processes and Na(+) tolerance. A compilation of data available on Na(+) accumulation and Na(+) tolerance in Arabidopsis is presented, and comparisons are made with several crop plant species. The relationship between Na(+) tolerance and Na(+) accumulation is different in Arabidopsis and cereals, with an inverse relationship often found within cereal species that is not as evident in Arabidopsis ecotypes. Results on salinity tolerance obtained in Arabidopsis should therefore be extrapolated to cereals with caution. Arabidopsis remains a useful model to study and discover plant Na(+) transport processes.

Salt resistance is determined by osmotic adjustment and abscisic acid in newly developed maize hybrids in the first phase of salt stress

This study investigated the mechanisms of salt resistance of four maize (Zea mays L.) hybrids [cultivar (cv.) Pioneer 3906 and newly developed hybrids SR03, SR12 and SR13] during the first phase of salt stress. Plants were grown in aerated nutrient solutions at 1 mM Na+ (control) and 100 mM Na+ (salt stress). Stress was imposed in 25 mM steps and plants were harvested after 2 days at 100 mM Na+. At 100 mM Na+ the area of the fourth leaf, which developed under salt stress, did not change significantly in SR03 and SR12 whereas significant reductions were observed in cv. Pioneer 3906 and SR13. Concentrations of assimilates (i.e. glucose, fructose and sucrose) in the shoot sap were significantly greater under salt stress in SR03 and SR12. However, the greater assimilate supply was not responsible for their salt resistance as there were no significant reductions in assimilate concentrations even in the other two genotypes. Shoot turgor and growth were maintained in SR03 and SR12 at 100 mM Na+ through significant increases in osmolality of the shoot sap. Concentrations of free ABA and ABA-glucose esters (ABA-GE) in the growing region of the fourth leaf increased significantly under salt stress in all genotypes. Leaf area at 100 mM Na(+), expressed as a percentage of that at 1 mM, showed significant positive relationships with free ABA (R(2) = 0.62) and the sum of free ABA and ABA-GE (R(2) = 0.65). Results of this study indicate clearly that a combination of partial osmotic adjustment, a possible reduction of the sensitivity of leaf growth under salt stress to increased ABA concentrations and a growth-promoting function regulated by ABA is responsible for salt resistance in the first phase of salt stress. Genotypic variation in these mechanisms can be utilized to breed salt-resistant genotypes in maize.

Impact of genetics and environment on nutritional and metabolite components of maize grain

The Organization for Economic Co-operation and Development (OECD) recommends the measurement of specific plant components for compositional assessments of new biotechnology-derived crops. These components include proximates, nutrients, antinutrients, and certain crop-specific secondary metabolites. A considerable literature on the natural variability of these components in conventional and biotechnology-derived crops now exists. Yet the OECD consensus also suggests measurements of any metabolites that may be directly associated with a newly introduced trait. Therefore, steps have been initiated to assess natural variation in metabolites not typically included in the OECD consensus but which might reasonably be expected to be affected by new traits addressing, for example, nutritional enhancement or improved stress tolerance. The compositional study reported here extended across a diverse genetic range of maize hybrids derived from 48 inbreds crossed against two different testers. These were grown at three different, but geographically similar, locations in the United States. In addition to OECD analytes such as proximates, total amino acids and free fatty acids, the levels of free amino acids, sugars, organic acids, and selected stress metabolites in harvested grain were assessed. The major free amino acids identified were asparagine, aspartate, glutamate, and proline. The major sugars were sucrose, glucose, and fructose. The most predominant organic acid was citric acid, with only minor amounts of other organic acids detected. The impact of genetic background and location was assessed for all components. Overall, natural variation in free amino acids, sugars, and organic acids appeared to be markedly higher than that observed for the OECD analytes.

Impact of genetics and environment on nutritional and metabolite components of maize grain

The Organization for Economic Co-operation and Development (OECD) recommends the measurement of specific plant components for compositional assessments of new biotechnology-derived crops. These components include proximates, nutrients, antinutrients, and certain crop-specific secondary metabolites. A considerable literature on the natural variability of these components in conventional and biotechnology-derived crops now exists. Yet the OECD consensus also suggests measurements of any metabolites that may be directly associated with a newly introduced trait. Therefore, steps have been initiated to assess natural variation in metabolites not typically included in the OECD consensus but which might reasonably be expected to be affected by new traits addressing, for example, nutritional enhancement or improved stress tolerance. The compositional study reported here extended across a diverse genetic range of maize hybrids derived from 48 inbreds crossed against two different testers. These were grown at three different, but geographically similar, locations in the United States. In addition to OECD analytes such as proximates, total amino acids and free fatty acids, the levels of free amino acids, sugars, organic acids, and selected stress metabolites in harvested grain were assessed. The major free amino acids identified were asparagine, aspartate, glutamate, and proline. The major sugars were sucrose, glucose, and fructose. The most predominant organic acid was citric acid, with only minor amounts of other organic acids detected. The impact of genetic background and location was assessed for all components. Overall, natural variation in free amino acids, sugars, and organic acids appeared to be markedly higher than that observed for the OECD analytes.

Improving salt tolerance of wheat and barley: future prospects

Cropping on saline land is restricted by the low tolerance of crops to salinity and waterlogging. Prospects for improving salt tolerance in wheat and barley include the use of: (i) intra-specific variation, (ii) variation for salt tolerance in the progenitors of these cereals, (iii) wide-hybridisation with halophytic ‘wild’ relatives (an option for wheat, but not barley), and (iv) transgenic techniques. In this review, key traits contributing to salt tolerance, and sources of variation for these within the Triticeae, are identified and recommendations for use of these traits in screening for salt tolerance are summarised. The potential of the approaches to deliver substantial improvements in salt tolerance is discussed, and the importance of adverse interactions between waterlogging and salinity are emphasised. The potential to develop new crops from the diverse halophytic flora is also considered.

Engineering of enhanced glycine betaine synthesis improves drought tolerance in maize

Glycine betaine plays an important role in some plants, including maize, in conditions of abiotic stress, but different maize varieties vary in their capacity to accumulate glycine betaine. An elite maize inbred line DH4866 was transformed with the betA gene from Escherichia coli encoding choline dehydrogenase (EC 1.1.99.1), a key enzyme in the biosynthesis of glycine betaine from choline. The transgenic maize plants accumulated higher levels of glycine betaine and were more tolerant to drought stress than wild-type plants (non-transgenic) at germination and the young seedling stage. Most importantly, the grain yield of transgenic plants was significantly higher than that of wild-type plants after drought treatment. The enhanced glycine betaine accumulation in transgenic maize provides greater protection of the integrity of the cell membrane and greater activity of enzymes compared with wild-type plants in conditions of drought stress.

Abscisic acid is involved in the water stress-induced betaine accumulation in pear leaves

ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.

Abscisic acid is involved in the water stress-induced betaine accumulation in pear leaves

ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.

The effects of salt stress on growth, nitrate reduction and proline and glycinebetaine accumulation in Prosopis alba

Prosopis alba (algarrobo) is one of the most important salt-tolerant legumes used in the food and furniture industries. The effects of salinity on some growth and physiological parameters in algarrrobo seedlings were investigated. 17-Day-old seedlings were subjected to three salt treatments by adding NaCl to the growth medium in 50 mmol.L-1 increments every 24 h until the final concentrations of 0, 300 and 600 mmol.L-1 were reached. Only the highest NaCl concentration affected all of the considered parameters. Thus, 600 mmol.L-1 NaCl caused a significant reduction in root and shoot growth, but an increase in the root/shoot ratio. Leaf relative water content, nitrate content and nitrate reductase activity in leaves and roots were also decreased. At 300 and 600 mmol.L-1, the glycinebetaine content was significantly increased in both leaves and roots but this was not found for proline content. Total soluble carbohydrates increased only in roots. The results suggest that glycinebetaine enhancement may be important for osmotic adjustment in Prosopis alba under salinity stress.

Effects of salt stress on plant growth, stomatal response and solute accumulation of different maize genotypes

Seeds from eight different maize genotypes (BR3123, BR5004, BR5011, BR5026, BR5033, CMS50, D766 and ICI8447) were sown in vermiculite, and after germination they were transplanted into nutrient solution or nutrient solution containing 100 mmol.L-1 of NaCl and placed in a greenhouse. During the experimental period plant growth (dry matter, shoot to root dry mass ratio, leaf area, relative growth rate, and net assimilation rate), leaf temperature, stomatal conductance, transpiration, predawn water potential, sodium, potassium, soluble amino acids and soluble carbohydrate contents were determined in both control and salt stressed plants of all genotypes studied. Salt stress reduced plant growth of all genotypes but the genotypes BR5033 and BR5011 were characterized as the most salt-tolerant and salt-sensitive, respectively. Stomatal response of the salt-tolerant genotype was not affected by salinity. Among the studied parameters, shoot to root dry mass ratio, leaf sodium content and leaf soluble organic solute content showed no relation with salt tolerance, i.e., they could not be considered as good morpho-physiological markers for maize salt tolerance. In contrast, sodium and soluble organic solutes accumulation in the roots as a result of salt stress appeared to play an important role in the acclimation to salt stress of the maize genotypes studied, suggesting that they could be used as physiological markers during the screening for salt tolerance.

Temporal progression of gene expression responses to salt shock in maize roots

Using a cDNA microarray containing 7943 ESTs, the behavior of the maize root transcriptome has been monitored in a time course for 72 h after imposition of salinity stress (150 mM NaCI). Under these conditions, root sodium amounts increased faster than in leaves, and root potassium decreased significantly. Although the overall free amino acid concentration was not affected, amino acid composition was changed with proline and asparagine increasing. Microarray analysis identified 916 ESTs representing genes whose steady-state RNA levels were significantly altered at various time points, corresponding to 11% of the ESTs printed. The response of the transcriptome to sub-lethal salt stress was rapid and transient, leading to a burst of changes at the three-hour time point. The salt-regulated ESTs represented 472 tentatively unique genes (TUGs), which, based on functional category analysis, are involved in a broad range of cellular and biochemical activities, prominent amongst which were transport and signal transduction pathways. Clustering of regulated transcripts based on the timing and duration of changes suggests a structured succession of induction and repression for salt responsive genes in multiple signal and response cascades. Within this framework, 16 signaling molecules, including six protein kinases, two protein phosphatases and eight transcription factors, were regulated with distinct expression patterns by high salinity.

Na+ tolerance and Na+ transport in higher plants

Tolerance to high soil [Na(+)] involves processes in many different parts of the plant, and is manifested in a wide range of specializations at disparate levels of organization, such as gross morphology, membrane transport, biochemistry and gene transcription. Multiple adaptations to high [Na(+)] operate concurrently within a particular plant, and mechanisms of tolerance show large taxonomic variation. These mechanisms can occur in all cells within the plant, or can occur in specific cell types, reflecting adaptations at two major levels of organization: those that confer tolerance to individual cells, and those that contribute to tolerance not of cells per se, but of the whole plant. Salt-tolerant cells can contribute to salt tolerance of plants; but we suggest that equally important in a wide range of conditions are processes involving the management of Na(+) movements within the plant. These require specific cell types in specific locations within the plant catalysing transport in a coordinated manner. For further understanding of whole plant tolerance, we require more knowledge of cell-specific transport processes and the consequences of manipulation of transporters and signalling elements in specific cell types.

The role of glycine betaine in the protection of plants from stress: clues from transgenic plants

The acclimation of a plant to a constantly changing environment involves the accumulation of certain organic compounds of low molecular mass, known collectively as compatible solutes, in the cytoplasm. The evidence from numerous investigations of the physiology, genetics, biophysics and biochemistry of plants strongly suggests that glycine betaine (GB), an amphoteric quaternary amine, plays an important role as a compatible solute in plants under various types of environmental stress, such as high levels of salts and low temperature. Plant species vary in their capacity to synthesize GB and some plants, such as spinach and barley, accumulate relatively high levels of GB in their chloroplasts while others, such as Arabidopsis and tobacco, do not synthesize this compound. Genetic engineering has allowed the introduction into GB-deficient species of biosynthetic pathways to GB from both micro-organisms and higher plants; this approach has facilitated investigations of the importance of GB in stress protection. In this review, we summarize recent progress in the genetic manipulation of the synthesis of GB, with special emphasis on the relationship between the protective effects of GB in vivo and those documented in vitro.

Genetic engineering of glycinebetaine synthesis in plants: current status and implications for enhancement of stress tolerance

Metabolic acclimation via the accumulation of compatible solutes is regarded as a basic strategy for the protection and survival of plants in extreme environments. Certain plants accumulate significant amounts of glycinebetaine (betaine), a compatible quaternary amine, in response to high salinity, cold and drought. It is likely that betaine is involved in the protection of macrocomponents of plant cells, such as protein complexes and membranes, under stress conditions. Genetic engineering of the biosynthesis of betaine from choline has been the focus of considerable attention as a potential strategy for increasing stress tolerance in stress-sensitive plants that are incapable of synthesizing this compatible/protective solute. Three distinct pathways for the synthesis of betaine have been identified in spinach, Escherichia coli and Arthrobacter globiformis, and various genes and cDNAs for the proteins involved are available. Moreover, each of the pathways has been exploited to a greater or lesser extent in efforts to convert betaine-deficient plants to betaine accumulators. In this review, the potential of several recent examples of transgenic approaches to the enhancement of stress tolerance in plants is summarized and discussed.

Metabolic engineering of rice leading to biosynthesis of glycinebetaine and tolerance to salt and cold

Genetically engineered rice (Oryza sativa L.) with the ability to synthesize glycinebetaine was established by introducing the codA gene for choline oxidase from the soil bacterium Arthrobacter globiformis. Levels of glycinebetaine were as high as 1 and 5 micromol per gram fresh weight of leaves in two types of transgenic plant in which choline oxidase was targeted to the chloroplasts (ChlCOD plants) and to the cytosol (CytCOD plants), respectively. Although treatment with 0.15 M NaCl [corrected] inhibited the growth of both wild-type and transgenic plants, the transgenic plants began to grow again at the normal rate after a significantly less time than the wild-type plants after elimination of the salt stress. Inactivation of photosynthesis, used as a measure of cellular damage, indicated that ChlCOD plants were more tolerant than CytCOD plants to photoinhibition under salt stress and low-temperature stress. These results indicated that the subcellular compartmentalization of the biosynthesis of glycinebetaine was a critical element in the efficient enhancement of tolerance to stress in the engineered plants.

Molecular characterization of two genes encoding betaine aldehyde dehydrogenase from amaranth. Expression in leaves under short-term exposure to osmotic stress or abscisic acid

A genomic clone (ahybadh4) and a cDNA (ahybadh17) both encoding betaine aldehyde dehydrogenase (BADH; EC 1.2.1.8) were isolated from the plant Amaranthus hypochondriacus L. The ahybadh4 gene extends 9 kilobases (kb) containing 15 exons with an open reading frame (ORF) of 501 amino acids (aa), a 1.3kb 5' untranslated region (UTR) and a 3' UTR of 0.3kb. The ahybadh17 cDNA encodes a BADH isoform of 500aa which contains 10aa substitutions with respect to AHYBADH4. Both encoded proteins share 98% identity at the amino acid level. Comparison of amaranth BADHs with other reported sequences showed high similarity. Analysis of ahybadh17 expression in amaranth leaves showed that mRNA and BADH protein are present in non-treated amaranth leaves and both transiently increased under short-term exposure to abscisic acid (ABA) and osmotic stress treatments.

Expression of a Late Embryogenesis Abundant Protein Gene, HVA1, from Barley Confers Tolerance to Water Deficit and Salt Stress in Transgenic Rice

A late embryogenesis abundant (LEA) protein gene, HVA1, from barley (Hordeum vulgare L.) was introduced into rice suspension cells using the Biolistic-mediated transformation method, and a large number of independent transgenic rice (Oryza sativa L.) plants were generated. Expression of the barley HVA1 gene regulated by the rice actin 1 gene promoter led to high-level, constitutive accumulation of the HVA1 protein in both leaves and roots of transgenic rice plants. Second-generation transgenic rice plants showed significantly increased tolerance to water deficit and salinity. Transgenic rice plants maintained higher growth rates than nontransformed control plants under stress conditions. The increased tolerance was also reflected by delayed development of damage symptoms caused by stress and by improved recovery upon the removal of stress conditions. We also found that the extent of increased stress tolerance correlated with the level of the HVA1 protein accumulated in the transgenic rice plants. Using a transgenic approach, this study provides direct evidence supporting the hypothesis that LEA proteins play an important role in the protection of plants under water-or salt-stress conditions. Thus, LEA genes hold considerable potential for use as molecular tools for genetic crop improvement toward stress tolerance.