Impact of ancestral wheat sodium exclusion genes Nax1 and Nax2 on grain yield of durum wheat on saline soils

Smart reprograming of plants against salinity stress using modern biotechnological tools

Climate change gives rise to numerous environmental stresses, including soil salinity. Salinity/salt stress is the second biggest abiotic factor affecting agricultural productivity worldwide by damaging numerous physiological, biochemical, and molecular processes. In particular, salinity affects plant growth, development, and productivity. Salinity responses include modulation of ion homeostasis, antioxidant defense system induction, and biosynthesis of numerous phytohormones and osmoprotectants to protect plants from osmotic stress by decreasing ion toxicity and augmented reactive oxygen species scavenging. As most crop plants are sensitive to salinity, improving salt tolerance is crucial in sustaining global agricultural productivity. In response to salinity, plants trigger stress-related genes, proteins, and the accumulation of metabolites to cope with the adverse consequence of salinity. Therefore, this review presents an overview of salinity stress in crop plants. We highlight advances in modern biotechnological tools, such as omics (genomics, transcriptomics, proteomics, and metabolomics) approaches and different genome editing tools (ZFN, TALEN, and CRISPR/Cas system) for improving salinity tolerance in plants and accomplish the goal of "zero hunger," a worldwide sustainable development goal proposed by the FAO.

Investigating the response mechanisms of bread wheat mutants to salt stress

Mutation breeding is among the most critical approaches to promoting genetic diversity when genetic diversity is narrowed for a long time using traditional breeding methods. In the current study, 15 wheat mutants created by gamma radiation and three salt-tolerant wheat cultivars were studied under no salinity stress (Karaj) and salinity stress (Yazd) during three consecutive growing seasons from 2017 to 2020 (M05 to M07 generations mutants). Results showed that salinity induced lipid peroxidation and enhanced ion leakage in all genotypes however, M6 and M15 showed the least ion leakage increment. It was also observed that the activity of antioxidant enzymes including SOD, CAT, POX, APX and GR increased with salinity; the maximum increase in antioxidant activity was belonged to M15, M09, M06 and M05. All genotypes had higher protein content in salinity stress conditions; M07 and M12 showed the lowest (1.8%) and the highest (17.3%) protein increase, respectively. Zeleny sedimentation volume increased under salinity stress conditions in all genotypes except M06, C2, C3, and M07. The result indicated that salinity stress increased wet gluten in all genotypes. M10 and M08 showed the highest (47.8%) and the lowest (4%) wet gluten increment, respectively. M06 and M11 mutants showed the lowest (6.1%) and the highest (60.7%) decrement of grain yield due to salinity stress, respectively. Finally, M04, M05, M07, M13, and M14 were known as genotypes with high grain yield in both no salinity and salinity stress conditions. In other word, these genotypes have higher yield stability. The results of the current study revealed that gamma irradiation could effectively be used to induce salinity tolerance in wheat.

Wheat adaptation to environmental stresses under climate change: Molecular basis and genetic improvement

Wheat (Triticum aestivum) is a staple food for about 40% of the world's population. As the global population has grown and living standards improved, high yield and improved nutritional quality have become the main targets for wheat breeding. However, wheat production has been compromised by global warming through the more frequent occurrence of extreme temperature events, which have increased water scarcity, aggravated soil salinization, caused plants to be more vulnerable to diseases, and directly reduced plant fertility and suppressed yield. One promising option to address these challenges is the genetic improvement of wheat for enhanced resistance to environmental stress. Several decades of progress in genomics and genetic engineering has tremendously advanced our understanding of the molecular and genetic mechanisms underlying abiotic and biotic stress responses in wheat. These advances have heralded what might be considered a "golden age" of functional genomics for the genetic improvement of wheat. Here, we summarize the current knowledge on the molecular and genetic basis of wheat resistance to abiotic and biotic stresses, including the QTLs/genes involved, their functional and regulatory mechanisms, and strategies for genetic modification of wheat for improved stress resistance. In addition, we also provide perspectives on some key challenges that need to be addressed.

Potential abiotic stress targets for modern genetic manipulation

Research into crop yield and resilience has underpinned global food security, evident in yields tripling in the past 5 decades. The challenges that global agriculture now faces are not just to feed 10+ billion people within a generation, but to do so under a harsher, more variable, and less predictable climate, and in many cases with less water, more expensive inputs, and declining soil quality. The challenges of climate change are not simply to breed for a "hotter drier climate," but to enable resilience to floods and droughts and frosts and heat waves, possibly even within a single growing season. How well we prepare for the coming decades of climate variability will depend on our ability to modify current practices, innovate with novel breeding methods, and communicate and work with farming communities to ensure viability and profitability. Here we define how future climates will impact farming systems and growing seasons, thereby identifying the traits and practices needed and including exemplars being implemented and developed. Critically, this review will also consider societal perspectives and public engagement about emerging technologies for climate resilience, with participatory approaches presented as the best approach.

The TaGSK1, TaSRG, TaPTF1, and TaP5CS Gene Transcripts Confirm Salinity Tolerance by Increasing Proline Production in Wheat (Triticum aestivum L.)

Salinity is an abiotic stress factor that reduces yield and threatens food security in the world's arid and semi-arid regions. The development of salt-tolerant genotypes is critical for mitigating yield losses, and this journey begins with the identification of sensitive and tolerant plants. Numerous physiologic and molecular markers for detecting salt-tolerant wheat genotypes have been developed. One of them is proline, which has been used for a long time but has received little information about proline-related genes in wheat genotypes. In this study, proline content and the expression levels of proline-related genes (TaPTF1, TaDHN, TaSRG, TaSC, TaPIMP1, TaMIP, TaHKT1;4, TaGSK, TaP5CS, and TaMYB) were examined in sensitive, moderate, and tolerant genotypes under salt stress (0, 50, 150, and 250 mM NaCl) for 0, 12, and 24 h. Our results show that salt stress increased the proline content in all genotypes, but it was found higher in salt-tolerant genotypes than in moderate and sensitive genotypes. The salinity stress increased gene expression levels in salt-tolerant and moderate genotypes. While salt-stress exposure for 12 and 24 h had a substantial effect on gene expression in wheat, TaPTF1, TaPIMP1, TaMIP, TaHKT1;4, and TaMYB genes were considerably upregulated in 24 h. The salt-tolerant genotypes showed a higher positive interaction than a negative interaction. The TaPTF1, TaP5CS, TaGSK1, and TaSRG genes were found to be more selective than the other analyzed genes under salt-stress conditions. Despite each gene's specific function, increasing proline biosynthesis functioned as a common mechanism for separating salt tolerance from sensitivity.

Physiological and Biochemical Parameters of Salinity Resistance of Three Durum Wheat Genotypes

The area of farming lands affected by increasing soil salinity is growing significantly worldwide. For this reason, breeding works are conducted to improve the salinity tolerance of important crop species. The goal of the present study was to indicate physiological or biochemical parameters characterizing three durum wheat accessions with various tolerance to salinity. The study was carried out on germinating seeds and mature plants of a Polish SMH87 line, an Australian cultivar ‘Tamaroi’ (salt-sensitive), and the BC₅Nax₂ line (salt-tolerant) exposed to 0–150 mM NaCl. Germination parameters, electrolyte leakage (EL), and salt susceptibility index were determined in the germinating caryopses, whereas photosynthetic parameters, carbohydrate and phenolic content, antioxidant activity as well as yield were measured in fully developed plants. The parameters that most differentiated the examined accessions in the germination phase were the percentage of germinating seeds (PGS) and germination vigor (Vi). In the fully developed plants, parameters included whether the plants had the maximum efficiency of the water-splitting reaction on the donor side of photosystem II (PSII)–F_v/F₀, energy dissipation from PSII–DI_o/CS_m, and the content of photosynthetic pigments and hydrogen peroxide, which differentiated studied genotypes in terms of salinity tolerance degree. Salinity has a negative impact on grain yield by reducing the number of seeds per spike and the mass of one thousand seeds (MTS), which can be used as the most suitable parameter for determining tolerance to salinity stress. The most salt-tolerant BC₅Nax₂ line was characterized by the highest PGS, and Vi for NaCl concentration of 100–150 mM, content of chlorophyll a, b, carotenoids, and also MTS at all applied salt concentrations as compared with the other accessions. The most salt-sensitive cv. ‘Tamaroi’ demonstrated higher H₂O₂ concentration which proves considerable oxidative damage caused by salinity stress. Mentioned parameters can be helpful for breeders in the selection of genotypes the most resistant to this stress.

Molecular Cloning and Characterization of SaCLCd, SaCLCf, and SaCLCg, Novel Proteins of the Chloride Channel Family (CLC) from the Halophyte Suaeda altissima (L.) Pall

Coding sequences of the CLC family genes SaCLCd, SaCLCf, and SaCLCg, the putative orthologs of Arabidopsis thaliana AtCLCd, AtCLCf, and AtCLCg genes, were cloned from the euhalophyte Suaeda altissima (L.) Pall. The key conserved motifs and glutamates inherent in proteins of the CLC family were identified in SaCLCd, SaCLCf, and SaCLCg amino acid sequences. SaCLCd and SaCLCg were characterized by higher homology to eukaryotic (human) CLCs, while SaCLCf was closer to prokaryotic CLCs. Ion specificities of the SaCLC proteins were studied in complementation assays by heterologous expression of the SaCLC genes in the Saccharomyces cerevisiae GEF1 disrupted strain Δgef1. GEF1 encoded the only CLC family protein, the Cl^- transporter Gef1p, in undisrupted strains of this organism. Expression of SaCLCd in Δgef1 cells restored their ability to grow on selective media. The complementation test and the presence of both the "gating" and "proton" conservative glutamates in SaCLCd amino acid sequence and serine specific for Cl^- in its selectivity filter suggest that this protein operates as a Cl^-/H⁺ antiporter. By contrast, expression of SaCLCf and SaCLCg did not complement the growth defect phenotype of Δgef1 cells. The selectivity filters of SaCLCf and SaCLCg also contained serine. However, SaCLCf included only the "gating" glutamate, while SaCLCg contained the "proton" glutamate, suggesting that SaCLCf and SaCLCg proteins act as Cl^- channels. The SaCLCd, SaCLCf, and SaCLCg genes were shown to be expressed in the roots and leaves of S. altissima. In response to addition of NaCl to the growth medium, the relative transcript abundances of all three genes of S. altissima increased in the leaves but did not change significantly in the roots. The increase in expression of SaCLCd, SaCLCf, and SaCLCg in the leaves in response to increasing salinity was in line with Cl^- accumulation in the leaf cells, indicating the possible participation of SaCLCd, SaCLCf, and SaCLCg proteins in Cl^- sequestration in cell organelles. Generally, these results suggest the involvement of SaCLC proteins in the response of S. altissima plants to increasing salinity and possible participation in mechanisms underlying salt tolerance.

Identifying the genetic control of salinity tolerance in the bread wheat landrace Mocho de Espiga Branca

Salinity tolerance in bread wheat is frequently reported to be associated with low leaf sodium (Na+) concentrations. However, the Portuguese landrace, Mocho de Espiga Branca, accumulates significantly higher leaf Na+ but has comparable salinity tolerance to commercial bread wheat cultivars. To determine the genetic loci associated with the salinity tolerance of this landrace, an F2 mapping population was developed by crossing Mocho de Espiga Branca with the Australian cultivar Gladius. The population was phenotyped for 19 salinity tolerance subtraits using both non-destructive and destructive techniques. Genotyping was performed using genotyping-by-sequencing (GBS). Genomic regions associated with salinity tolerance were detected on chromosomes 1A, 1D, 4B and 5A for the subtraits of relative and absolute growth rate (RGR, AGR respectively), and on chromosome 2A, 2B, 4D and 5D for Na+, potassium (K+) and chloride (Cl-) accumulation. Candidate genes that encode proteins associated with salinity tolerance were identified within the loci including Na+/H+ antiporters, K+ channels, H+-ATPase, calcineurin B-like proteins (CBLs), CBL-interacting protein kinases (CIPKs), calcium dependent protein kinases (CDPKs) and calcium-transporting ATPase. This study provides a new insight into the genetic control of salinity tolerance in a Na+ accumulating bread wheat to assist with the future development of salt tolerant cultivars.

Development of new high-salt tolerant bread wheat (Triticum aestivum L.) genotypes and insight into the tolerance mechanisms

The loss of cropland soils, climate change, and population growth are directly affecting the food supply. Given the higher incidence of salinity and extreme events, the cereal performance and yield are substantially hampered. Wheat is forecast to decline over the coming years due to the salinization widespread as one of the oldest and most environmental severe constraints facing global cereal production. To increase salinity tolerance of wheat, in this study, we developed two new salt-tolerant bread wheats, named 'Maycan' and 'Yıldız'. The salinity tolerance of these lines, their parents, and a salt-sensitive cultivar has been tested from measurements of physiological, biochemical, and genes associated with osmotic adjustment/plant tolerance in cultures containing 0 and 150 mM NaCl at the seedling stage. Differential growth reductions to increased salinity were observed in the salt-sensitive cultivar, with those newly developed exhibiting significantly greater root length, growth of shoot and water content as salinity tolerances overall than their parents. 'Maycan' and 'Yıldız' had higher osmoregulator proline content and antioxidants enzyme activities under salinity than the other bread wheat tested. Notably, an important upregulation in the expression of genes related to cellular ion balance, osmolytes accumulation, and abscisic acid was observed in both new wheat germplasms, which may improve salt tolerance. These finding revealed that 'Maycan' and 'Yıldız' exhibit high-salt tolerance at the seedling stage and differing in their tolerance mechanisms to the other tested cultivars, thereby providing an opportunity for their exploitation as modern bread wheats.

Physiological and Biochemical Response to Fusarium culmorum Infection in Three Durum Wheat Genotypes at Seedling and Full Anthesis Stage

Fusarium culmorum is a worldwide, soil-borne plant pathogen. It causes diseases of cereals, reduces their yield, and fills the grain with toxins. The main direction of modern breeding is to select wheat genotypes the most resistant to Fusarium diseases. This study uses seedlings and plants at the anthesis stage to analyze total soluble carbohydrates, total and cell-wall bound phenolics, chlorophyll content, antioxidant activity, hydrogen peroxide content, mycotoxin accumulation, visual symptoms of the disease, and Fusarium head blight index (FHBi). These results determine the resistance of three durum wheat accessions. We identify physiological or biochemical markers of durum wheat resistance to F. culmorum. Our results confirm correlations between FHBi and mycotoxin accumulation in the grain, which results in grain yield decrease. The degree of spike infection (FHBi) may indicate accumulation mainly of deoxynivalenol and nivalenol in the grain. High catalase activity in the infected leaves could be considered a biochemical marker of durum sensitivity to this fungus. These findings allowed us to formulate a strategy for rapid evaluation of the disease severity and the selection of plants with higher level, or resistance to F. culmorum infection.

Up-regulated 2-alkenal reductase expression improves low-nitrogen tolerance in maize by alleviating oxidative stress

In plants, cellular lipid peroxidation is enhanced under low nitrogen (LN) stress; this increases the lipid-derived reactive carbonyl species (RCS) levels. The cellular toxicity of RCS can be reduced by various RCS-scavenging enzymes. However, the roles of these enzymes in alleviating oxidative stress and improving nutrient use efficiency (NUE) under nutrient stress remain unknown. Here, we overexpressed maize endogenous NADPH-dependent 2-alkenal reductase (ZmAER) in maize; it significantly increased the tolerance of transgenic plants (OX-AER) to LN stress. Under LN condition, the biomass, nitrogen accumulation, NUE, and leaf photosynthesis of the OX-AER plants were significantly higher than those of the wild-type (WT) plants. The leaf and root malondialdehyde and H₂ O₂ levels in the transgenic plants were significantly lower than those in WT. The expression of antioxidant enzyme-related genes ZmCAT3, ZmPOD5 and ZmPOD13 was significantly higher in the transgenic lines than in WT. Under LN stress, the nitrate reductase activity in the OX-AER leaves was significantly increased compared with that in the WT leaves. Furthermore, under LN stress, ZmNRT1.1 and ZmNRT2.5 expression was upregulated in the OX-AER plants compared with that in WT. Overall, up-regulated ZmAER expression could enhance maize's tolerance to LN stress by alleviating oxidative stress and improve NUE.

Agro-Physiologic Responses and Stress-Related Gene Expression of Four Doubled Haploid Wheat Lines under Salinity Stress Conditions

Simple Summary

Productivity of wheat can be enhanced using salt-tolerant genotypes. However, the assessment of salt tolerance potential in wheat through agro-physiological traits and stress-related gene expression analysis could potentially minimize the cost of breeding programs and be a powerful way for the selection of the most salt-tolerant genotype. The study evaluated the salt tolerance potential of four doubled haploid lines of wheat and compared them with the check cultivar Sakha-93 using an extensive set of agro-physiologic parameters and salt-stress-related gene expressions. The results indicated that the five genotypes tested displayed reduction in all traits evaluated except the canopy temperature and electrical conductivity, which had the greatest decline occurring in the check cultivar and the least decline in DHL2. The genotypes DHL21 and DHL5 exhibited increased expression rate of salt-stress-related genes under salt stress conditions. The multiple linear regression model and path coefficient analysis showed a coefficient of determination of 0.93. Concluding, the number of spikelets, and/or number of kernels were identified to be unbiased traits for assessing wheat DHLs under salinity conditions, given their contribution and direct impact on the grain yield. Moreover, the two most salt-tolerant genotypes DHL2 and DHL21 can be useful as genetic resources for future breeding programs.

Abstract

Salinity majorly hinders horizontal and vertical expansion in worldwide wheat production. Productivity can be enhanced using salt-tolerant wheat genotypes. However, the assessment of salt tolerance potential in bread wheat doubled haploid lines (DHL) through agro-physiological traits and stress-related gene expression analysis could potentially minimize the cost of breeding programs and be a powerful way for the selection of the most salt-tolerant genotype. We used an extensive set of agro-physiologic parameters and salt-stress-related gene expressions. Multivariate analysis was used to detect phenotypic and genetic variations of wheat genotypes more closely under salinity stress, and we analyzed how these strategies effectively balance each other. Four doubled haploid lines (DHLs) and the check cultivar (Sakha93) were evaluated in two salinity levels (without and 150 mM NaCl) until harvest. The five genotypes showed reduced growth under 150 mM NaCl; however, the check cultivar (Sakha93) died at the beginning of the flowering stage. Salt stress induced reduction traits, except the canopy temperature and initial electrical conductivity, which was found in each of the five genotypes, with the greatest decline occurring in the check cultivar (Sakha-93) and the least decline in DHL2. The genotypes DHL21 and DHL5 exhibited increased expression rate of salt-stress-related genes (TaNHX1, TaHKT1, and TaCAT1) compared with DHL2 and Sakha93 under salt stress conditions. Principle component analysis detection of the first two components explains 70.78% of the overall variation of all traits (28 out of 32 traits). A multiple linear regression model and path coefficient analysis showed a coefficient of determination (R²) of 0.93. The models identified two interpretive variables, number of spikelets, and/or number of kernels, which can be unbiased traits for assessing wheat DHLs under salinity stress conditions, given their contribution and direct impact on the grain yield.

Genome-wide association study of yield and related traits in common wheat under salt-stress conditions

BACKGROUND

Soil salinization is a major threat to wheat production. It is essential to understand the genetic basis of salt tolerance for breeding and selecting new salt-tolerant cultivars that have the potential to increase wheat yield.

RESULT

In this study, a panel of 191 wheat accessions was subjected to genome wide association study (GWAS) to identify SNP markers linked with adult-stage characters. The population was genotyped by Wheat660K SNP array and eight phenotype traits were investigated under low and high salinity environments for three consecutive years. A total of 389 SNPs representing 11 QTLs were significantly associated with plant height, spike number, spike length, grain number, thousand kernels weight, yield and biological mass under different salt treatments, with the phenotypic explanation rate (R²) ranging from 9.14 to 50.45%. Of these, repetitive and pleiotropic loci on chromosomes 4A, 5A, 5B and 7A were significantly linked to yield and yield related traits such as thousand kernels weight, spike number, spike length, grain number and so on under low salinity conditions. Spike length-related loci were mainly located on chromosomes 1B, 3B, 5B and 7A under different salt treatments. Two loci on chromosome 4D and 5A were related with plant height in low and high salinity environment, respectively. Three salt-tolerant related loci were confirmed to be important in two bi-parental populations. Distribution of favorable haplotypes indicated that superior haplotypes of pleiotropic loci on group-5 chromosomes were strongly selected and had potential for increasing wheat salt tolerance. A total of 14 KASP markers were developed for nine loci associating with yield and related traits to improve the selection efficiency of wheat salt-tolerance breeding.

CONCLUSION

Utilizing a Wheat660K SNPs chip, QTLs for yield and its related traits were detected under salt treatment in a natural wheat population. Important salt-tolerant related loci were validated in RIL and DH populations. This study provided reliable molecular markers that could be crucial for marker-assisted selection in wheat salt tolerance breeding programs.

Novel Tools for Adjusting Spatial Variability in the Early Sugarcane Breeding Stage

The detection of spatial variability in field trials has great potential for accelerating plant breeding progress due to the possibility of better controlling non-genetic variation. Therefore, we aimed to evaluate a digital soil mapping approach and a high-density soil sampling procedure for identifying and adjusting spatial dependence in the early sugarcane breeding stage. Two experiments were conducted in regions with different soil classifications. High-density sampling of soil physical and chemical properties was performed in a regular grid to investigate the structure of spatial variability. Soil apparent electrical conductivity (ECa) was measured in both experimental areas with an EM38-MK2^® sensor. In addition, principal component analysis (PCA) was employed to reduce the dimensionality of the physical and chemical soil data sets. After conducting the PCA and obtaining different thematic maps, we determined each experimental plot's exact position within the field. Tons of cane per hectare (TCH) data for each experiment were obtained and analyzed using mixed linear models. When environmental covariates were considered, a previous forward model selection step was applied to incorporate the variables. The PCA based on high-density soil sampling data captured part of the total variability in the data for Experimental Area 1 and was suggested to be an efficient index to be incorporated as a covariate in the statistical model, reducing the experimental error (residual variation coefficient, CVe). When incorporated into the different statistical models, the ECa information increased the selection accuracy of the experimental genotypes. Therefore, we demonstrate that the genetic parameter increased when both approaches (spatial analysis and environmental covariates) were employed.

Natural Genetic Resources from Diverse Plants to Improve Abiotic Stress Tolerance in Plants

The current agricultural system is biased for the yield increase at the cost of biodiversity. However, due to the loss of precious genetic diversity during domestication and artificial selection, modern cultivars have lost the adaptability to cope with unfavorable environments. There are many reports on variations such as single nucleotide polymorphisms (SNPs) and indels in the stress-tolerant gene alleles that are associated with higher stress tolerance in wild progenitors, natural accessions, and extremophiles in comparison with domesticated crops or model plants. Therefore, to gain a better understanding of stress-tolerant traits in naturally stress-resistant plants, more comparative studies between the modern crops/model plants and crop progenitors/natural accessions/extremophiles are required. In this review, we discussed and summarized recent progress on natural variations associated with enhanced abiotic stress tolerance in various plants. By applying the recent biotechniques such as the CRISPR/Cas9 gene editing tool, natural genetic resources (i.e., stress-tolerant gene alleles) from diverse plants could be introduced to the modern crop in a non-genetically modified way to improve stress-tolerant traits.

High affinity Na+ transport by wheat HKT1;5 is blocked by K. Plant Direct 2020;4:e00275. [PMID: 33103046 PMCID: PMC7576878 DOI: 10.1002/pld3.275]

The wheat sodium transporters TmHKT1;5-A and TaHKT1;5-D are encoded by genes underlying the major shoot Na+ exclusion loci Nax2 and Kna1 from Triticum monococcum (Tm) and Triticum aestivum (Ta), respectively. In contrast to HKT2 transporters that have been shown to exhibit high affinity K+-dependent Na+ transport, HKT1 proteins have, with one exception, only been shown to catalyze low affinity Na+ transport and no K+ transport. Here, using heterologous expression in Xenopus laevis oocytes we uncover a novel property of HKT1 proteins, that both TmHKT1;5-A and TaHKT1;5-D encode dual (high and low) affinity Na+-transporters with the high-affinity component being abolished when external K+ is in excess of external Na+. Three-dimensional structural modeling suggested that, compared to Na+, K+ is bound more tightly in the selectivity filter region by means of additional van der Waals forces, which is likely to explain the K+ block at the molecular level. The low-affinity component for Na+ transport of TmHKT1;5-A had a lower K m than that of TaHKT1;5-D and was less sensitive to external K+. We propose that these properties contribute towards the improvements in shoot Na+-exclusion and crop plant salt tolerance following the introgression of TmHKT1;5-A into diverse wheat backgrounds.

Antioxidant activity as a response to cadmium pollution in three durum wheat genotypes differing in salt-tolerance

Durum wheat is commonly used in various food industry industries and cultivated worldwide. A serious problem with the species cultivation is its capability to accumulate cadmium (Cd) in the grains. The aim of this study is to investigate whether antioxidant activity may be used as a marker of Cd tolerance in durum wheat. The experiment involved three durum wheat genotypes/lines differing in salt tolerance. The plant response to Cd was appraised based on the activity of ascorbate–glutathione (AsA–GSH) cycle enzymes, ascorbate-to-dehydroascorbate ratio, reduced-to-oxidized glutathione ratio (GSH:GSSG), as well as Cd content in the seeds. The highest activity of dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione reductase was noted in control plants of salt-sensitive cultivar “Tamaroi.” In the presence of Cd, activity of these enzymes was considerably reduced. “Tamaroi” plants demonstrated also the highest Cd content in the grain. In conclusion, we identified the cultivar “Tamaroi” as most susceptible to cadmium, and the level of durum wheat sensitivity to the element can be evaluated based on a significant decrease in the activity of AsA–GSH cycle enzymes and GSH:GSSG ratio.

Transcriptome profiling at osmotic and ionic phases of salt stress response in bread wheat uncovers trait-specific candidate genes

BACKGROUND

Bread wheat is one of the most important crops for the human diet, but the increasing soil salinization is causing yield reductions worldwide. Improving salt stress tolerance in wheat requires the elucidation of the mechanistic basis of plant response to this abiotic stress factor. Although several studies have been performed to analyze wheat adaptation to salt stress, there are still some gaps to fully understand the molecular mechanisms from initial signal perception to the onset of responsive tolerance pathways. The main objective of this study is to exploit the dynamic salt stress transcriptome in underlying QTL regions to uncover candidate genes controlling salt stress tolerance in bread wheat. The massive analysis of 3'-ends sequencing protocol was used to analyze leave samples at osmotic and ionic phases. Afterward, stress-responsive genes overlapping QTL for salt stress-related traits in two mapping populations were identified.

RESULTS

Among the over-represented salt-responsive gene categories, the early up-regulation of calcium-binding and cell wall synthesis genes found in the tolerant genotype are presumably strategies to cope with the salt-related osmotic stress. On the other hand, the down-regulation of photosynthesis-related and calcium-binding genes, and the increased oxidative stress response in the susceptible genotype are linked with the greater photosynthesis inhibition at the osmotic phase. The specific up-regulation of some ABC transporters and Na+/Ca2+ exchangers in the tolerant genotype at the ionic stage indicates their involvement in mechanisms of sodium exclusion and homeostasis. Moreover, genes related to protein synthesis and breakdown were identified at both stress phases. Based on the linkage disequilibrium blocks, salt-responsive genes within QTL intervals were identified as potential components operating in pathways leading to salt stress tolerance. Furthermore, this study conferred evidence of novel regions with transcription in bread wheat.

CONCLUSION

The dynamic transcriptome analysis allowed the comparison of osmotic and ionic phases of the salt stress response and gave insights into key molecular mechanisms involved in the salt stress adaptation of contrasting bread wheat genotypes. The leveraging of the highly contiguous chromosome-level reference genome sequence assembly facilitated the QTL dissection by targeting novel candidate genes for salt tolerance.

Transgenic Breeding Approaches for Improving Abiotic Stress Tolerance: Recent Progress and Future Perspectives

The recent rapid climate changes and increasing global population have led to an increased incidence of abiotic stress and decreased crop productivity. Environmental stresses, such as temperature, drought, nutrient deficiency, salinity, and heavy metal stresses, are major challenges for agriculture, and they lead to a significant reduction in crop growth and productivity. Abiotic stress is a very complex phenomenon, involving a variety of physiological and biochemical changes in plant cells. Plants exposed to abiotic stress exhibit enhanced levels of ROS (reactive oxygen species), which are highly reactive and toxic and affect the biosynthesis of chlorophyll, photosynthetic capacity, and carbohydrate, protein, lipid, and antioxidant enzyme activities. Transgenic breeding offers a suitable alternative to conventional breeding to achieve plant genetic improvements. Over the last two decades, genetic engineering/transgenic breeding techniques demonstrated remarkable developments in manipulations of the genes for the induction of desired characteristics into transgenic plants. Transgenic approaches provide us with access to identify the candidate genes, miRNAs, and transcription factors (TFs) that are involved in specific plant processes, thus enabling an integrated knowledge of the molecular and physiological mechanisms influencing the plant tolerance and productivity. The accuracy and precision of this phenomenon assures great success in the future of plant improvements. Hence, transgenic breeding has proven to be a promising tool for abiotic stress improvement in crops. This review focuses on the potential and successful applications, recent progress, and future perspectives of transgenic breeding for improving abiotic stress tolerance and productivity in plants.

Development and characterization of an EMS-mutagenized wheat population and identification of salt-tolerant wheat lines

BACKGROUND

Triticum aestivum (wheat) is one of the world's oldest crops and has been used for >8000 years as a food crop in North Africa, West Asia and Europe. Today, wheat is one of the most important sources of grain for humans, and is cultivated on greater areas of land than any other crop. As the human population increases and soil salinity becomes more prevalent, there is increased pressure on wheat breeders to develop salt-tolerant varieties in order to meet growing demands for yield and grain quality. Here we developed a mutant wheat population using the moderately salt-tolerant Bangladeshi variety BARI Gom-25, with the primary goal of further increasing salt tolerance.

RESULTS

After titrating the optimal ethyl methanesulfonate (EMS) concentration, ca 30,000 seeds were treated with 1% EMS, and 1676 lines, all originating from single seeds, survived through the first four generations. Most mutagenized lines showed a similar phenotype to BARI Gom-25, although visual differences such as dwarfing, giant plants, early and late flowering and altered leaf morphology were seen in some lines. By developing an assay for salt tolerance, and by screening the mutagenized population, we identified 70 lines exhibiting increased salt tolerance. The selected lines typically showed a 70% germination rate on filter paper soaked in 200 mM NaCl, compared to 0-30% for BARI Gom-25. From two of the salt-tolerant OlsAro lines (OA42 and OA70), genomic DNA was sequenced to 15x times coverage. A comparative analysis against the BARI Gom-25 genomic sequence identified a total of 683,201 (OA42), and 768,954 (OA70) SNPs distributed throughout the three sub-genomes (A, B and D). The mutation frequency was determined to be approximately one per 20,000 bp. All the 70 selected salt-tolerant lines were tested for root growth in the laboratory, and under saline field conditions in Bangladesh. The results showed that all the lines selected for tolerance showed a better salt tolerance phenotype than both BARI Gom-25 and other local wheat varieties tested.

CONCLUSION

The mutant wheat population developed here will be a valuable resource in the development of novel salt-tolerant varieties for the benefit of saline farming.

Bread Wheat With High Salinity and Sodicity Tolerance

Soil salinity and sodicity are major constraints to global cereal production, but breeding for tolerance has been slow. Narrow gene pools, over-emphasis on the sodium (Na+) exclusion mechanism, little attention to osmotic stress/tissue tolerance mechanism(s) in which accumulation of inorganic ions such as Na+ is implicated, and lack of a suitable screening method have impaired progress. The aims of this study were to discover novel genes for Na+ accumulation using genome-wide association studies, compare growth responses to salinity and sodicity in low-Na+ bread Westonia with Nax1 and Nax2 genes and high-Na+ bread wheat Baart-46, and evaluate growth responses to salinity and sodicity in bread wheats with varying leaf Na+ concentrations. The novel high-Na+ bread wheat germplasm, MW#293, had higher grain yield under salinity and sodicity, in absolute and relative terms, than the other bread wheat entries tested. Genes associated with high Na+ accumulation in bread wheat were identified, which may be involved in tissue tolerance/osmotic adjustment. As most modern bread wheats are efficient at excluding Na+, further reduction in plant Na+ is unlikely to provide agronomic benefit. The salinity and sodicity tolerant germplasm MW#293 provides an opportunity for the development of future salinity/sodicity tolerant bread wheat.

Tackling Salinity in Sustainable Agriculture—What Developing Countries May Learn from Approaches of the Developed World

Soil salinity is a common problem of the developing world as well as the developed world. However, the pace to reduce salinity is much slower in the developing world. The application of short-term approaches with an unsustainable supply of funds are the major reasons of low success. In contrast, the developed world has focused on long-term and sustainable techniques, and considerable funds per unit area have been allocated to reduce soil salinity. Here, we review the existing approaches in both worlds. Approaches like engineering and nutrient use were proven to be unsustainable, while limited breeding and biosaline approaches had little success in the developing countries. In contrast, advanced breeding and genetics tools were implemented in the developed countries to improve the salinity tolerance of different crops with more success. Resultantly, developed countries not only reduced the area for soil salinity at a higher rate, but more sustainable and cheaper ways to resolve the issue were implemented at the farmers’ field. Similarly, plant microbial approaches and the application of fertigation through drip irrigation have great potential for both worlds, and farmer participatory approaches are required to obtain fruitful outcomes. In this regard, a challenging issue is the transition of sustainable approaches from developed countries to developing ones, and possible methods for this are discussed.

Mapping of novel salt tolerance QTL in an Excalibur × Kukri doubled haploid wheat population

KEY MESSAGE

Novel QTL for salinity tolerance traits have been detected using non-destructive and destructive phenotyping in bread wheat and were shown to be linked to improvements in yield in saline fields. Soil salinity is a major limitation to cereal production. Breeding new salt-tolerant cultivars has the potential to improve cereal crop yields. In this study, a doubled haploid bread wheat mapping population, derived from the bi-parental cross of Excalibur × Kukri, was grown in a glasshouse under control and salinity treatments and evaluated using high-throughput non-destructive imaging technology. Quantitative trait locus (QTL) analysis of this population detected multiple QTL under salt and control treatments. Of these, six QTL were detected in the salt treatment including one for maintenance of shoot growth under salinity (QG(1-5).asl-7A), one for leaf Na+ exclusion (QNa.asl-7A) and four for leaf K+ accumulation (QK.asl-2B.1, QK.asl-2B.2, QK.asl-5A and QK:Na.asl-6A). The beneficial allele for QG(1-5).asl-7A (the maintenance of shoot growth under salinity) was present in six out of 44 mainly Australian bread and durum wheat cultivars. The effect of each QTL allele on grain yield was tested in a range of salinity concentrations at three field sites across 2 years. In six out of nine field trials with different levels of salinity stress, lines with alleles for Na+ exclusion and/or K+ maintenance at three QTL (QNa.asl-7A, QK.asl-2B.2 and QK:Na.asl-6A) excluded more Na+ or accumulated more K+ compared to lines without these alleles. Importantly, the QK.asl-2B.2 allele for higher K+ accumulation was found to be associated with higher grain yield at all field sites. Several alleles at other QTL were associated with higher grain yields at selected field sites.

Response to in vitro salt stress in sugarcane is conditioned by concentration and condition of exposure to NaCl

La salinidad es uno de los principales factores de estrés ambiental, además de interferir en el crecimiento de las plantas perjudica directamente la producción agrícola. En ese contexto, se destaca la importancia de investigaciones direccionadas a la respuesta de las plantas sometidas al estrés salino, con el fin de evaluar el comportamiento fisiológico y bioquímico con el objetivo de seleccionar genotipos tolerantes a dicha condición. Una de las técnicas más utilizadas para uniformizar la respuesta de las plantas a una condición en particular es el cultivo de tejidos in vitro. Por lo tanto, el objetivo de este estudio fue evaluar la respuesta de dos variedades comerciales de caña de azúcar (RB931011 e RB872552) expuestas a estrés salino con NaCl (56 mM e 112 mM) en diferentes condiciones, gradual y abrupta. Las respuestas del sistema antioxidante enzimático (catalasa, peroxidasa y ascorbato peroxidasa) y prolina libre, asi como las concentraciones de Na+ e K+ fueron evaluadas 30 días después del inicio de los tratamientos. Fueron observadas diferencias en la respuesta de las variedades en función del modo de inducción del estrés salino, graduado o abrupto, y no solo en función de las concentraciones de NaCl en el medio de cultivo. La respuesta al estrés es condicionada no solo por la concentración de sal sino también por la forma de exposición al medio salino.

Variation in shoot tolerance mechanisms not related to ion toxicity in barley

Soil salinity can severely reduce crop growth and yield. Many studies have investigated salinity tolerance mechanisms in cereals using phenotypes that are relatively easy to measure. The majority of these studies measured the accumulation of shoot Na+ and the effect this has on plant growth. However, plant growth is reduced immediately after exposure to NaCl before Na+ accumulates to toxic concentrations in the shoot. In this study, nondestructive and destructive measurements are used to evaluate the responses of 24 predominately Australian barley (Hordeum vulgare L.) lines at 0, 150 and 250mM NaCl. Considerable variation for shoot tolerance mechanisms not related to ion toxicity (shoot ion-independent tolerance) was found, with some lines being able to maintain substantial growth rates under salt stress, whereas others stopped growing. Hordeum vulgare spp. spontaneum accessions and barley landraces predominantly had the best shoot ion independent tolerance, although two commercial cultivars, Fathom and Skiff, also had high tolerance. The tolerance of cv. Fathom may be caused by a recent introgression from H. vulgare L. spp. spontaneum. This study shows that the most salt-tolerant barley lines are those that contain both shoot ion-independent tolerance and the ability to exclude Na+ from the shoot (and thus maintain high K+:Na+ ratios).

Comparative studies on tolerance of rice genotypes differing in their tolerance to moderate salt stress

BACKGROUND

Moderate salt stress, which often occurs in most saline agriculture land, suppresses crop growth and reduces crop yield. Rice, as an important food crop, is sensitive to salt stress and rice genotypes differ in their tolerance to salt stress. Despite extensive studies on salt tolerance of rice, a few studies have specifically investigated the mechanism by which rice plants respond and tolerate to moderate salt stress. Two rice genotypes differing in their tolerance to saline-alkaline stress, Dongdao-4 and Jigeng-88, were used to explore physiological and molecular mechanisms underlying tolerance to moderate salt stress.

RESULTS

Dongdao-4 plants displayed higher biomass, chlorophyll contents, and photosynthetic rates than Jigeng-88 under conditions of salt stress. No differences in K+ concentrations, Na+ concentrations and Na+/K+ ratio in shoots between Dongdao-4 and Jigeng-88 plants were detected when challenged by salt stress, suggesting that Na+ toxicity may not underpin the greater tolerance of Dongdao-4 to salt stress than that of Jigeng-88. We further demonstrated that Dongdao-4 plants had greater capacity to accumulate soluble sugars and proline (Pro) than Jigeng-88, thus conferring greater tolerance of Dongdao-4 to osmotic stress than Jigeng-88. Moreover, Dongdao-4 suffered from less oxidative stress than Jigeng-88 under salt stress due to higher activities of catalase (CAT) in Dongdao-4 seedlings. Finally, RNA-seq revealed that Dongdao-4 and Jigeng-88 differed in their gene expression in response to salt stress, such that salt stress changed expression of 456 and 740 genes in Dongdao-4 and Jigeng-88, respectively.

CONCLUSION

Our results revealed that Dongdao-4 plants were capable of tolerating to salt stress by enhanced accumulation of Pro and soluble sugars to tolerate osmotic stress, increasing the activities of CAT to minimize oxidative stress, while Na+ toxicity is not involved in the greater tolerance of Dongdao-4 to moderate salt stress.

A walk on the wild side: Oryza species as source for rice abiotic stress tolerance

Oryza sativa, the common cultivated rice, is one of the most important crops for human consumption, but production is increasingly threatened by abiotic stresses. Although many efforts have resulted in breeding rice cultivars that are relatively tolerant to their local environments, climate changes and population increase are expected to soon call for new, fast generation of stress tolerant rice germplasm, and current within-species rice diversity might not be enough to overcome such needs. The Oryza genus contains other 23 wild species, with only Oryza glaberrima being also domesticated. Rice domestication was performed with a narrow genetic diversity, and the other Oryza species are a virtually untapped genetic resource for rice stress tolerance improvement. Here we review the origin of domesticated Oryza sativa from wild progenitors, the ecological and genomic diversity of the Oryza genus, and the stress tolerance variation observed for wild Oryza species, including the genetic basis underlying the tolerance mechanisms found. The summary provided here is important to indicate how we should move forward to unlock the full potential of these germplasms for rice improvement.

Back into the wild-Apply untapped genetic diversity of wild relatives for crop improvement

Deleterious effects of climate change and human activities, as well as diverse environmental stresses, present critical challenges to food production and the maintenance of natural diversity. These challenges may be met by the development of novel crop varieties with increased biotic or abiotic resistance that enables them to thrive in marginal lands. However, considering the diverse interactions between crops and environmental factors, it is surprising that evolutionary principles have been underexploited in addressing these food and environmental challenges. Compared with domesticated cultivars, crop wild relatives (CWRs) have been challenged in natural environments for thousands of years and maintain a much higher level of genetic diversity. In this review, we highlight the significance of CWRs for crop improvement by providing examples of CWRs that have been used to increase biotic and abiotic stress resistance/tolerance and overall yield in various crop species. We also discuss the surge of advanced biotechnologies, such as next-generation sequencing technologies and omics, with particular emphasis on how they have facilitated gene discovery in CWRs. We end the review by discussing the available resources and conservation of CWRs, including the urgent need for CWR prioritization and collection to ensure continuous crop improvement for food sustainability.

Tissue tolerance: an essential but elusive trait for salt-tolerant crops

For a plant to persist in saline soil, osmotic adjustment of all plant cells is essential. The more salt-tolerant species accumulate Na+ and Cl- to concentrations in leaves and roots that are similar to the external solution, thus allowing energy-efficient osmotic adjustment. Adverse effects of Na+ and Cl- on metabolism must be avoided, resulting in a situation known as 'tissue tolerance'. The strategy of sequestering Na+ and Cl- in vacuoles and keeping concentrations low in the cytoplasm is an important contributor to tissue tolerance. Although there are clear differences between species in the ability to accommodate these ions in their leaves, it remains unknown whether there is genetic variation in this ability within a species. This viewpoint considers the concept of tissue tolerance, and how to measure it. Four conclusions are drawn: (1) osmotic adjustment is inseparable from the trait of tissue tolerance; (2) energy-efficient osmotic adjustment should involve ions and only minimal organic solutes; (3) screening methods should focus on measuring tolerance, not injury; and (4) high-throughput protocols that avoid the need for control plants and multiple Na+ or Cl- measurements should be developed. We present guidelines to identify useful genetic variation in tissue tolerance that can be harnessed for plant breeding of salt tolerance.

Characterization of Two HKT1;4 Transporters from Triticum monococcum to Elucidate the Determinants of the Wheat Salt Tolerance Nax1 QTL

TmHKT1;4-A1 and TmHKT1;4-A2 are two Na⁺ transporter genes that have been identified as associated with the salt tolerance Nax1 locus found in a durum wheat (Triticum turgidum L. subsp. durum) line issued from a cross with T. monococcum. In the present study, we were interested in getting clues on the molecular mechanisms underpinning this salt tolerance quantitative trait locus (QTL). By analyzing the phylogenetic relationships between wheat and T. monococcum HKT1;4-type genes, we found that durum and bread wheat genomes possess a close homolog of TmHKT1;4-A1, but no functional close homolog of TmHKT1;4-A2. Furthermore, performing real-time reverse transcription-PCR experiments, we showed that TmHKT1;4-A1 and TmHKT1;4-A2 are similarly expressed in the leaves but that TmHKT1;4-A2 is more strongly expressed in the roots, which would enable it to contribute more to the prevention of Na⁺ transfer to the shoots upon salt stress. We also functionally characterized the TmHKT1;4-A1 and TmHKT1;4-A2 transporters by expressing them in Xenopus oocytes. The two transporters displayed close functional properties (high Na⁺/K⁺ selectivity, low affinity for Na⁺, stimulation by external K⁺ of Na⁺ transport), but differed in some quantitative parameters: Na⁺ affinity was 3-fold lower and the maximal inward conductance was 3-fold higher in TmHKT1;4-A2 than in TmHKT1;4-A1. The conductance of TmHKT1;4-A2 at high Na⁺ concentration (>10 mM) was also shown to be higher than that of the two durum wheat HKT1;4-type transporters so far characterized. Altogether, these data support the hypothesis that TmHKT1;4-A2 is responsible for the Nax1 trait and provide new insight into the understanding of this QTL.

Uncoupling of sodium and chloride to assist breeding for salinity tolerance in crops

The separation of toxic effects of sodium (Na(+)) and chloride (Cl(-)) by the current methods of mixed salts and subsequent determination of their relevance to breeding has been problematic. We report a novel method (Na(+) humate) to study the ionic effects of Na(+) toxicity without interference from Cl(-), and ionic and osmotic effects when combined with salinity (NaCl). Three cereal species (Hordeum vulgare, Triticum aestivum and Triticum turgidum ssp. durum with and without the Na(+) exclusion gene Nax2) differing in Na(+) exclusion were grown in a potting mix under sodicity (Na(+) humate) and salinity (NaCl), and water use, leaf nutrient profiles and yield were determined. Under sodicity, Na(+)-excluding bread wheat and durum wheat with the Nax2 gene had higher yield than Na(+)-accumulating barley and durum wheat without the Nax2 gene. However, under salinity, despite a 100-fold difference in leaf Na(+), all species yielded similarly, indicating that osmotic stress negated the benefits of Na(+) exclusion. In conclusion, Na(+) exclusion can be an effective mechanism for sodicity tolerance, while osmoregulation and tissue tolerance to Na(+) and/or Cl(-) should be the main foci for further improvement of salinity tolerance in cereals. This represents a paradigm shift for breeding cereals with salinity tolerance.

Introgression of genes from bread wheat enhances the aluminium tolerance of durum wheat

The aluminium tolerance of durum wheat was markedly enhanced by introgression of TaALMT1 and TaMATE1B from bread wheat. In contrast to bread wheat, TaMATE1B conferred greater aluminium tolerance than TaALMT1. Durum wheat (tetraploid AABB, Triticum turgidum) is a species that grows poorly on acid soils due to its sensitivity of Al(3+). By contrast, bread wheat (hexaploid AABBDD, T. aestivum) shows a large variation in Al(3+) tolerance which can be attributed to a major gene (TaALMT1) located on chromosome 4D as well as to other genes of minor effect such as TaMATE1B. Genotypic variation for Al(3+) tolerance in durum germplasm is small and the introgression of genes from bread wheat is one option for enhancing the ability of durum wheat to grow on acid soils. Introgression of a large fragment of the 4D chromosome previously increased the Al(3+) tolerance of durum wheat demonstrating the viability of transferring the TaALMT1 gene to durum wheat to increase its Al(3+) tolerance. Here, we used a ph1 (pairing homoeologous) mutant of durum wheat to introgress a small fragment of the 4D chromosome harboring the TaALMT1 gene. The size of the 4D chromosomal fragment introgressed into durum wheat was estimated by markers, fluorescence in situ hybridisation and real-time quantitative PCR. In a parallel strategy, we introgressed TaMATE1B from bread wheat into durum wheat using conventional crosses. Both genes separately increased the Al(3+) tolerance of durum wheat in both hydroponics and soil cultures. In contrast to bread wheat, the TaMATE1B gene was more effective than TaALMT1 in increasing the Al(3+) tolerance of durum wheat grown on acid soil.

The evolution of halophytes, glycophytes and crops, and its implications for food security under saline conditions

The effective development of salt tolerant crops requires an understanding that the evolution of halophytes, glycophytes and our major grain crops has involved significantly different processes. Halophytes (and other edaphic endemics) generally arose through colonization of habitats in severe disequilibrium by pre-adapted individuals, rather than by gradual adaptation from populations of 'glycophytes'. Glycophytes, by contrast, occur in low sodium ecosystems, where sodium was and is the major limiting nutrient in herbivore diets, suggesting that their evolution reflects the fact that low sodium individuals experienced lower herbivory and had higher fitness. For domestication/evolution of crop plants, the selective pressure was human imposed and involved humans co-opting functions of defense and reproductive security. Unintended consequences of this included loss of tolerance to various stresses and loss of the genetic variability needed to correct that. Understanding, combining and manipulating all three modes of evolution are now critical to the development of salt tolerant crops, particularly those that will offer food security in countries with few economic resources and limited infrastructure. Such efforts will require exploiting the genetic structures of recently evolved halophytes, the genetic variability of model plants, and endemic halophytes and 'minor' crops that already exist.

Genetic mechanisms of abiotic stress tolerance that translate to crop yield stability

Crop yield reduction as a consequence of increasingly severe climatic events threatens global food security. Genetic loci that ensure productivity in challenging environments exist within the germplasm of crops, their wild relatives and species that are adapted to extreme environments. Selective breeding for the combination of beneficial loci in germplasm has improved yields in diverse environments throughout the history of agriculture. An effective new paradigm is the targeted identification of specific genetic determinants of stress adaptation that have evolved in nature and their precise introgression into elite varieties. These loci are often associated with distinct regulation or function, duplication and/or neofunctionalization of genes that maintain plant homeostasis.

Macroevolutionary patterns of salt tolerance in angiosperms

BACKGROUND

Halophytes are rare, with only 0·25% of angiosperm species able to complete their life cycle in saline conditions. This could be interpreted as evidence that salt tolerance is difficult to evolve. However, consideration of the phylogenetic distribution of halophytes paints a different picture: salt tolerance has evolved independently in many different lineages, and halophytes are widely distributed across angiosperm families. In this Viewpoint, I will consider what phylogenetic analysis of halophytes can tell us about the macroevolution of salt tolerance.

HYPOTHESIS

Phylogenetic analyses of salt tolerance have shown contrasting patterns in different families. In some families, such as chenopods, salt tolerance evolved early in the lineage and has been retained in many lineages. But in other families, including grasses, there have been a surprisingly large number of independent origins of salt tolerance, most of which are relatively recent and result in only one or a few salt-tolerant species. This pattern of many recent origins implies either a high transition rate (salt tolerance is gained and lost often) or a high extinction rate (salt-tolerant lineages do not tend to persist over macroevolutionary timescales). While salt tolerance can evolve in a wide range of genetic backgrounds, some lineages are more likely to produce halophytes than others. This may be due to enabling traits that act as stepping stones to developing salt tolerance. The ability to tolerate environmental salt may increase tolerance of other stresses or vice versa.

CONCLUSIONS

Phylogenetic analyses suggest that enabling traits and cross-tolerances may make some lineages more likely to adapt to increasing salinization, a finding that may prove useful in assessing the probable impact of rapid environmental change on vegetation communities, and in selecting taxa to develop for use in landscape rehabilitation and agriculture.

The plasma membrane transport systems and adaptation to salinity

Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

The Na(+) transporter, TaHKT1;5-D, limits shoot Na(+) accumulation in bread wheat

Bread wheat (Triticum aestivum L.) has a major salt tolerance locus, Kna1, responsible for the maintenance of a high cytosolic K(+) /Na(+) ratio in the leaves of salt stressed plants. The Kna1 locus encompasses a large DNA fragment, the distal 14% of chromosome 4DL. Limited recombination has been observed at this locus making it difficult to map genetically and identify the causal gene. Here, we decipher the function of TaHKT1;5-D, a candidate gene underlying the Kna1 locus. Transport studies using the heterologous expression systems Saccharomyces cerevisiae and Xenopus laevis oocytes indicated that TaHKT1;5-D is a Na(+) -selective transporter. Transient expression in Arabidopsis thaliana mesophyll protoplasts and in situ polymerase chain reaction indicated that TaHKT1;5-D is localised on the plasma membrane in the wheat root stele. RNA interference-induced silencing decreased the expression of TaHKT1;5-D in transgenic bread wheat lines which led to an increase in the Na(+) concentration in the leaves. This indicates that TaHKT1;5-D retrieves Na(+) from the xylem vessels in the root and has an important role in restricting the transport of Na(+) from the root to the leaves in bread wheat. Thus, TaHKT1;5-D confers the essential salinity tolerance mechanism in bread wheat associated with the Kna1 locus via shoot Na(+) exclusion and is critical in maintaining a high K(+) /Na(+) ratio in the leaves. These findings show there is potential to increase the salinity tolerance of bread wheat by manipulation of HKT1;5 genes.

Functional characterization in Xenopus oocytes of Na+ transport systems from durum wheat reveals diversity among two HKT1;4 transporters

Plant tolerance to salinity constraint involves complex and integrated functions including control of Na(+) uptake, translocation, and compartmentalization. Several members of the high-affinity K(+) transporter (HKT) family, which comprises plasma-membrane transporters permeable to K(+) and Na(+) or to Na(+) only, have been shown to play major roles in plant Na(+) and K(+) homeostasis. Among them, HKT1;4 has been identified as corresponding to a quantitative trait locus (QTL) of salt tolerance in wheat but was not functionally characterized. Here, we isolated two HKT1;4-type cDNAs from a salt-tolerant durum wheat (Triticum turgidum L. subsp. durum) cultivar, Om Rabia3, and investigated the functional properties of the encoded transporters using a two-electrode voltage-clamp technique, after expression in Xenopus oocytes. Both transporters displayed high selectivity for Na(+), their permeability to other monovalent cations (K(+), Li(+), Cs(+), and Rb(+)) being ten times lower than that to Na(+). Both TdHKT1;4-1 and TdHKT1;4-2 transported Na(+) with low affinity, although the half-saturation of the conductance was observed at a Na(+) concentration four times lower in TdHKT1;4-1 than in TdHKT1;4-2. External K(+) did not inhibit Na(+) transport through these transporters. Quinine slightly inhibited TdHKT1;4-2 but not TdHKT1;4-1. Overall, these data identified TdHKT1;4 transporters as new Na(+)-selective transporters within the HKT family, displaying their own functional features. Furthermore, they showed that important differences in affinity exist among durum wheat HKT1;4 transporters. This suggests that the salt tolerance QTL involving HKT1;4 may be at least in part explained by functional variability among wheat HKT1;4-type transporters.