Variation in tissue Na(+) content and the activity of SOS1 genes among two species and two related genera of Chrysanthemum

Genome-wide identification, characterization, and functional analysis of the CHX, SOS, and RLK genes in Solanum lycopersicum under salt stress

The cation/proton exchanger (CHX), salt overly sensitive (SOS), and receptor-like kinase (RLK) genes play significant roles in the response to salt stress in plants. This study is the first to identify the SOS gene in Solanum lycopersicum (tomato) through genome-wide analysis under salt stress conditions. Quantitative reverse transcription PCR (qRT-PCR) results indicated that the expression levels of CHX, SOS, and RLK genes were upregulated, with fold changes of 1.83, 1.49, and 1.55, respectively, after 12 h of exposure to salt stress. Genome-wide analysis revealed 21 CHX, 5 SOS, and 86 RLK genes in S. lycopersicum. CHX genes were found on chromosomes 2, 3, 4, 5, 6, 7, 8, 9, 11, and 12 of S. lycopersicum. SOS genes were found on chromosomes 1, 4, 6, and 10. RLK genes were found on all chromosomes of S. lycopersicum. The Ka/Ks ratios indicate that the CHX, SOS, and RLK genes have been primarily influenced by purifying selection. This suggests that these genes have faced strong environmental pressures throughout their evolution. Purifying selection typically results in a decrease in genetic diversity. The estimated duplication time for CHX paralogous gene pairs ranged from approximately 26.965 to 245.413 million years ago (Mya), while the duplication time for SOS paralogous gene pairs ranged from around 116.682 to 275.631 Mya. For RLK paralogous gene pairs, the duplication time varied from approximately 27.689 to 239.376 Mya. Synteny analysis of the CHX, SOS, and RLK genes demonstrated collinear relationships with orthologous genes in Arabidopsis thaliana, but no collinearity orthologous relationships in Oryza sativa (rice). Furthermore, the analysis revealed that there were 6 orthologous SlCHX genes, 2 orthologous SlSOS genes, and 44 orthologous SlRLK genes paired with those in A. thaliana. The results of the present study may help to elucidate the role of the CHX, SOS, and RLK genes in salt stress in S. lycopersicum.

A putative Na+/H+ antiporter BpSOS1 contributes to salt tolerance in birch

White birch (Betula platyphylla Suk.) is an important pioneer tree which plays a critical role in maintaining ecosystem stability and forest regeneration. The growth of birch is dramatically inhibited by salt stress, especially the root inhibition. Salt Overly Sensitive 1 (SOS1) is the only extensively characterized Na+ efflux transporter in multiple plant species. The salt-hypersensitive mutant, sos1, display significant inhibition of root growth by NaCl. However, the role of SOS1 in birch responses to salt stress remains unclear. Here, we characterized a putative Na+/H+ antiporter BpSOS1 in birch and generated the loss-of-function mutants of the birch BpSOS1 by CRISPR/Cas9 approach. The bpsos1 mutant exhibit exceptional increased salt sensitivity which links to excessive Na+ accumulation in root, stem and old leaves. We observed a dramatic reduction of K+ contents in leaves of the bpsos1 mutant plants under salt stress. Furthermore, the Na+/K+ ratio of roots and leaves is significant higher in the bpsos1 mutants than the wild-type plants under salt stress. The ability of Na+ efflux in the root meristem zone is found to be impaired which might result the imbalance of Na+ and K+ in the bpsos1 mutants. Our findings indicate that the Na+/H+ exchanger BpSOS1 plays a critical role in birch salt tolerance by maintaining Na+ homeostasis and provide evidence for molecular breeding to improve salt tolerance in birch and other trees.

SOS1 gene family in mangrove (Kandelia obovata): Genome-wide identification, characterization, and expression analyses under salt and copper stress

BACKGROUND

Salt Overly Sensitive 1 (SOS1), a plasma membrane Na+/H+ exchanger, is essential for plant salt tolerance. Salt damage is a significant abiotic stress that impacts plant species globally. All living organisms require copper (Cu), a necessary micronutrient and a protein cofactor for many biological and physiological processes. High Cu concentrations, however, may result in pollution that inhibits the growth and development of plants. The function and production of mangrove ecosystems are significantly impacted by rising salinity and copper contamination.

RESULTS

A genome-wide analysis and bioinformatics techniques were used in this study to identify 20 SOS1 genes in the genome of Kandelia obovata. Most of the SOS1 genes were found on the plasma membrane and dispersed over 11 of the 18 chromosomes. Based on phylogenetic analysis, KoSOS1s can be categorized into four groups, similar to Solanum tuberosum. Kandelia obovata's SOS1 gene family expanded due to tandem and segmental duplication. These SOS1 homologs shared similar protein structures, according to the results of the conserved motif analysis. The coding regions of 20 KoSOS1 genes consist of amino acids ranging from 466 to 1221, while the exons include amino acids ranging from 3 to 23. In addition, we found that the 2.0 kb upstream promoter region of the KoSOS1s gene contains several cis-elements associated with phytohormones and stress responses. According to the expression experiments, seven randomly chosen genes experienced up- and down-regulation of their expression levels in response to copper (CuCl2) and salt stressors.

CONCLUSIONS

For the first time, this work systematically identified SOS1 genes in Kandelia obovata. Our investigations also encompassed physicochemical properties, evolution, and expression patterns, thereby furnishing a theoretical framework for subsequent research endeavours aimed at functionally characterizing the Kandelia obovata SOS1 genes throughout the life cycle of plants.

Overexpression of a plasmalemma Na+/H+ antiporter from the halophyte Nitraria sibirica enhances the salt tolerance of transgenic poplar

The plasmalemma Na+/H+ antiporter Salt Overly Sensitive 1 (SOS1) is responsible for the efflux of Na+ from the cytoplasm, an important determinant of salt resistance in plants. In this study, an ortholog of SOS1, referred to as NsSOS1, was cloned from Nitraria sibirica, a typical halophyte that grows in deserts and saline-alkaline land, and its expression and function in regulating the salt tolerance of forest trees were evaluated. The expression level of NsSOS1 was higher in leaves than in roots and stems of N. sibirica, and its expression was upregulated under salt stress. Histochemical staining showed that β-glucuronidase (GUS) driven by the NsSOS1 promoter was strongly induced by abiotic stresses and phytohormones including salt, drought, low temperature, gibberellin, and methyl jasmonate, suggesting that NsSOS1 is involved in the regulation of multiple signaling pathways. Transgenic 84 K poplar (Populus alba × P. glandulosa) overexpressing NsSOS1 showed improvements in survival rate, root biomass, plant height, relative water levels, chlorophyll and proline levels, and antioxidant enzyme activities versus non-transgenic poplar (NT) under salt stress. Transgenic poplars accumulated less Na+ and more K+ in roots, stems, and leaves, which had a lower Na+/K+ ratio compared to NT under salt stress. These results indicate that NsSOS1-mediated Na+ efflux confers salt tolerance to transgenic poplars, which show more efficient photosynthesis, better scavenging of reactive oxygen species, and improved osmotic adjustment under salt stress. Transcriptome analysis of transgenic poplars confirmed that NsSOS1 not only mediates Na+ efflux but is also involved in the regulation of multiple metabolic pathways. The results provide insight into the regulatory mechanisms of NsSOS1 and suggest that it could be used to improve the salt tolerance of forest trees.

Molecular cloning and characterization of a salt overly sensitive3 (SOS3) gene from the halophyte Pongamia

A high concentration of sodium (Na+) is the primary stressor for plants in high salinity environments. The Salt Overly Sensitive (SOS) pathway is one of the best-studied signal transduction pathways, which confers plants the ability to export too much Na+ out of the cells or translocate the cytoplasmic Na+ into the vacuole. In this study, the Salt Overly Sensitive3 (MpSOS3) gene from Pongamia (Millettia pinnata Syn. Pongamia pinnata), a semi-mangrove, was isolated and characterized. The MpSOS3 protein has canonical EF-hand motifs conserved in other calcium-binding proteins and an N-myristoylation signature sequence. The MpSOS3 gene was significantly induced by salt stress, especially in Pongamia roots. Expression of the wild-type MpSOS3 but not the mutated nonmyristoylated MpSOS3-G2A could rescue the salt-hypersensitive phenotype of the Arabidopsis sos3-1 mutant, which suggested the N-myristoylation signature sequence of MpSOS3 was required for MpSOS3 function in plant salt tolerance. Heterologous expression of MpSOS3 in Arabidopsis accumulated less H2O2, superoxide anion radical (O2-), and malondialdehyde (MDA) than wild-type plants, which enhanced the salt tolerance of transgenic Arabidopsis plants. Under salt stress, MpSOS3 transgenic plants accumulated a lower content of Na+ and a higher content of K+ than wild-type plants, which maintained a better K+/Na+ ratio in transgenic plants. Moreover, no development and growth discrepancies were observed in the MpSOS3 heterologous overexpression plants compared to wild-type plants. Our results demonstrated that the MpSOS3 pathway confers a conservative salt-tolerant role and provided a foundation for further study of the SOS pathway in Pongamia.

The signalling pathways, calcineurin B-like protein 5 (CBL5)-CBL-interacting protein kinase 8 (CIPK8)/CIPK24-salt overly sensitive 1 (SOS1), transduce salt signals in seed germination in Arabidopsis

For plant growth under salt stress, sensing and transducing salt signals are central to cellular Na+ homoeostasis. The calcineurin B-like protein (CBL)-CBL-interacting protein kinase (CIPK) complexes play critical roles in transducing salt signals in plants. Here, we show that CBL5, an ortholog of CBL4 and CBL10 in Arabidopsis, interacts with and recruits CIPK8/CIPK24 to the plasma membrane. Yeast cells coexpressing CBL5, CIPK8/CIPK24 and SOS1 demonstrated lesser Na+ accumulation and a better growth phenotype than the untransformed or SOS1 transgenic yeast cells under salinity. Overexpression of CBL5 improved the growth of the cipk8 or cipk24 single mutant but not the cipk8 cipk24 double mutant under salt stress, suggesting that CIPK8 and CIPK24 were the downstream targets of CBL5. Interestingly, seed germination in cbl5 was severely inhibited by NaCl, which was recovered by the overexpression of CBL5. Furthermore, CBL5 was mainly expressed in the cotyledons and hypocotyls, which are essential to seed germination. Na+ efflux activity in the hypocotyls of cbl5 was reduced relative to the wild-type under salt stress, enhancing Na+ accumulation. These findings indicate that CBL5 functions in seed germination and protects seeds and germinating seedlings from salt stress through the CBL5-CIPK8/CIPK24-SOS1 pathways.

Comparative Physiological and Transcriptome Analysis of Crossostephium chinense Reveals Its Molecular Mechanisms of Salt Tolerance

Crossostephium chinense is a wild species with strong salt tolerance that has great potential to improve the salt tolerance of cultivated chrysanthemums. Conversely, the unique salt-tolerant molecular mechanisms of Cr. chinense are still unclear. This study performed a comparative physiological and transcriptome analysis of Cr. chinense, Chrysanthemum lavandulifolium, and three hybrids to investigate the salt-tolerant molecular mechanisms of Cr. chinense. The physiological results showed that Cr. chinense maintained higher superoxide dismutase (SOD) activity, alleviating oxidative damage to the membrane. KEGG enrichment analysis showed that plant hormone signaling transduction and the MAPK signaling pathway were mostly enriched in Cr. chinense and hybrids under salt stress. Further weighted gene co-expression network analysis (WGCNA) of DEGs suggested that abscisic acid (ABA) signaling transduction may play a significant role in the salt-tolerant mechanisms of Cr. chinense and hybrids. The tissue-specific expression patterns of the candidate genes related to ABA signaling transduction and the MAPK signaling pathway indicate that genes related to ABA signaling transduction demonstrated significant expression levels under salt stress. This study offers important insights into exploring the underlying salt-tolerant mechanisms of Cr. chinense mediated by ABA signaling transduction and broadens our understanding of the breeding strategies for developing salt-tolerant cultivars utilizing salt-tolerant chrysanthemum germplasms.

Overexpression of TCP9-like gene enhances salt tolerance in transgenic soybean

TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factors are a plant-specific family and play roles in plant growth, development, and responses to biotic and abiotic stresses. However, little is known about the functions of the TCP transcription factors in the soybean cultivars with tolerance to salt stress. In this study, TCP9-like, a TCP transcription factor, was identified in the soybean cultivars exposed to salt stress. The expression of TCP9-like gene in the roots of salt-tolerant soybean cultivars was higher than that in salt-sensitive cultivars treated with NaCl. The overexpression of TCP9-like enhanced the salt tolerance of the salt-sensitive soybean cultivar 'DN50'. In T2 generation, the plants with TCP9-like overexpression had significantly lower Na+ accumulation and higher K+ accumulation than the WT plants exposed to 200 or 250 mmol/L NaCl. The K+/Na+ ratio in the plants overexpressing TCP9-like was significantly higher than that in WT plants treated with 200 mmol/L NaCl. Meanwhile, the overexpression of TCP9-like up-regulated the expression levels of GmNHX1, GmNHX3, GmSOS1, GmSOS2-like, and GmHKT1, which were involved in the K+/Na+ homeostasis pathway. The findings indicated that TCP9-like mediated the regulation of both Na+ and K+ accumulation to improve the tolerance of soybean to salt stress.

Genome-wide characterization of SOS1 gene family in potato (Solanum tuberosum) and expression analyses under salt and hormone stress

Salt Overly Sensitive 1 (SOS1) is one of the members of the Salt Overly Sensitive (SOS) signaling pathway and plays critical salt tolerance determinant in plants, while the characterization of the SOS1 family in potato (Solanum tuberosum) is lacking. In this study, 37 StSOS1s were identified and found to be unevenly distributed across 10 chromosomes, with most of them located on the plasma membrane. Promoter analysis revealed that the majority of these StSOS1 genes contain abundant cis-elements involved in various abiotic stress responses. Tissue specific expression showed that 21 of the 37 StSOS1s were widely expressed in various tissues or organs of the potato. Molecular interaction network analysis suggests that 25 StSOS1s may interact with other proteins involved in potassium ion transmembrane transport, response to salt stress, and cellular processes. In addition, collinearity analysis showed that 17, 8, 1 and 5 of orthologous StSOS1 genes were paired with those in tomato, pepper, tobacco, and Arabidopsis, respectively. Furthermore, RT-qPCR results revealed that the expression of StSOS1s were significant modulated by various abiotic stresses, in particular salt and abscisic acid stress. Furthermore, subcellular localization in Nicotiana benthamiana suggested that StSOS1-13 was located on the plasma membrane. These results extend the comprehensive overview of the StSOS1 gene family and set the stage for further analysis of the function of genes in SOS and hormone signaling pathways.

CycC1;1-WRKY75 complex-mediated transcriptional regulation of SOS1 controls salt stress tolerance in Arabidopsis

SALT OVERLY SENSITIVE1 (SOS1) is a key component of plant salt tolerance. However, how SOS1 transcription is dynamically regulated in plant response to different salinity conditions remains elusive. Here, we report that C-type Cyclin1;1 (CycC1;1) negatively regulates salt tolerance by interfering with WRKY75-mediated transcriptional activation of SOS1 in Arabidopsis (Arabidopsis thaliana). Disruption of CycC1;1 promotes SOS1 expression and salt tolerance in Arabidopsis because CycC1;1 interferes with RNA polymerase II recruitment by occupying the SOS1 promoter. Enhanced salt tolerance of the cycc1;1 mutant was completely compromised by an SOS1 mutation. Moreover, CycC1;1 physically interacts with the transcription factor WRKY75, which can bind to the SOS1 promoter and activate SOS1 expression. In contrast to the cycc1;1 mutant, the wrky75 mutant has attenuated SOS1 expression and salt tolerance, whereas overexpression of SOS1 rescues the salt sensitivity of wrky75. Intriguingly, CycC1;1 inhibits WRKY75-mediated transcriptional activation of SOS1 via their interaction. Thus, increased SOS1 expression and salt tolerance in cycc1;1 were abolished by WRKY75 mutation. Our findings demonstrate that CycC1;1 forms a complex with WRKY75 to inactivate SOS1 transcription under low salinity conditions. By contrast, under high salinity conditions, SOS1 transcription and plant salt tolerance are activated at least partially by increased WRKY75 expression but decreased CycC1;1 expression.

A Na+/H+ antiporter-encoding salt overly sensitive 1 gene, LpSOS1, involved in positively regulating the salt tolerance in Lilium pumilum

Lilium pumilum has a strong salt tolerance. However, the molecular mechanism underlying its salt tolerance remains unexplored. Here, LpSOS1 was cloned from L. pumilum and found to be significantly enriched at high NaCl concentrations (100 mM). In tobacco epidermal cells, localization analysis showed that the LpSOS1 protein was primarily located in the plasma membrane. Overexpression of LpSOS1 resulted in up-regulation of salt stress tolerance in Arabidopsis, as indicated by reduced malondialdehyde levels and Na⁺/K⁺ ratio, and increased activity of antioxidant reductases (including superoxide dismutase, peroxidase, and catalase). Treatment with NaCl resulted in improved growth, as evidenced by increased biomass, root length, and lateral root growth, in both sos1 mutant (atsos1) and wild-type (WT) Arabidopsis plants that overexpressed LpSOS1,Under NaCl treatment,atsos1 and WT Arabidopsis plants overexpressing LpSOS1 exhibited better growth, with higher biomass, root length, and lateral root quantity, whereas in the absence of LpSOS1 overexpression, the plants of both lines were wilted and chlorotic and even died under salt stress. When exposed to salt stress, the expression of stress-related genes was notably upregulated in the LpSOS1 overexpression line of Arabidopsis as compared to the WT. Our findings indicate that LpSOS1 enhances salt tolerance in plants by regulating ion homeostasis, reducing Na⁺/K⁺ ratio, thereby protecting the plasma membrane from oxidative damage caused by salt stress, and enhancing the activity of antioxidant enzymes. Therefore, the increased salt tolerance conferred by LpSOS1 in plants makes it a potential bioresource for breeding salt-tolerant crops. Further investigation into the mechanisms underlying lily's resistance to salt stress would be advantageous and could serve as a foundation for future molecular improvements.

Five Post-Translational Modification Residues of CmPT2 Play Key Roles in Yeast and Rice

Chrysanthemum (Chrysanthemum morifolium Ramat.) is one of the largest cut flowers in the world. Phosphate transporter Pht1 family member CmPht1;2 protein (CmPT2) plays an important role in response to low-phosphate (LP) stress in chrysanthemum. Post-translational modification (PTM) can modulate the function of proteins in multiple ways. Here, we used yeast and rice systems to study the role of putative PTM in CmPT2 by determining the effect of mutation of key amino acid residues of putative glycosylation, phosphorylation, and myristoylation sites. We chose nine amino acid residues in the putative PTM sites and mutated them to alanine (A) (Cmphts). CmPT2 recovered the growth of yeast strain MB192 under LP conditions. However, G84A, G222A, T239A, Y242A, and N422A mutants could not grow normally under LP conditions. Analysis of phosphorus absorption kinetics showed that the Km of CmPT2 was 65.7 μM. Among the nine Cmphts, the expression of five with larger Km (124.4-397.5 μM) than CmPT2 was further evaluated in rice. Overexpression of CmPT2-OE increased plant height, effective panicle numbers, branch numbers, and yield compared with that of wild type 'Wuyunjing No. 7' (W7). Overexpression of Cmphts-OE led to decreased plant height and effective panicle numbers compared with that of the CmPT2-OE strain. The Pi content in roots of CmPT2-OE was higher than that of the W7 under both high (normal) phosphate (HP) and LP conditions. However, the Pi content in the leaves and roots was significantly lower in the N422A-OE strain than in the CmPT2-OE strain under both HP and LP conditions. Under LP conditions, the phosphorus starvation response (PSR) genes in CmPT2-OE were inhibited at the transcription level. The expression patterns of phosphorus-related genes in T239A, Y242A, and N422A-OE under LP conditions were different from those of CmPT2-OE. In conclusion, these five post-translational modification residues of CmPT2 play key roles in modulating the function of CmPT2. This work boosters our understanding of the function of phosphate transporters and provides genetic resources for improving the efficiency of phosphorus utilization in crop plants.

Analysis of the NAC Gene Family in Salix and the Identification of SpsNAC005 Gene Contributing to Salt and Drought Tolerance

The NAC gene family is of great value for plant stress resistance and development. In this study, five NAC genes with a typical NAM domain were isolated from Salix psammophila, which is a stress-resistant willow endemic to western China. Two hundred sixty-two NAC genes from Salix psammophila, Salix purpurea, and Arabidopsis were used to construct the phylogenetic tree to examine the phylogenetic relationship. Five NAC genes in Salix psammophila were the focus of bioinformatics analysis and conserved structural domain analysis. The SpsNAC005 gene was overexpressed in Populus hopeiensis, and the transgenic lines were subjected to salt and simulated drought stress to analyze their phenotype changes and tolerance to stress. The results showed that transgenic poplar height and leaf area increased by 29.73% and 76.36%, respectively, compared with those of wild-type plants. Under stress treatment, the height growth rates and ground diameter growth rates of the transgenic lines were significantly higher than those of the wild-type, whereas their fresh weight and dry weight were decreased compared to those of the wild-type. The SOD activities, POD activities, and Pro contents of the transgenic plants were significantly increased, and the accumulation of MDA was significantly lower than that in the wild-type, and the transgenic lines showed clear tolerance to salt and drought. The expressions of the SOS1, MPK6, HKT1, and P5CS1 genes were downregulated in the transgenic lines. The expression of the PRODH1 gene was downregulated in the transgenic lines. These results indicate that overexpression of the SpsNAC005 gene in transgenic plants can promote plant growth and development and improve tolerance to salt and drought.

How chrysanthemum (Chrysanthemum × grandiflorum) 'Palisade White' deals with long-term salt stress

Salinity is a serious problem in the cultivation of ornamental plants. Chrysanthemum (Chrysanthemum × grandiflorum) 'Palisade White' was evaluated in order to examine its responses to long-term salt stress. Plants were grown in substrate supplemented with NaCl doses (g dm^-3 of substrate) 0, 0.44, 0.96, 1.47, 1.98, 2.48 and 2.99. The initial electrical conductivity (EC) of the substrates was 0.3, 0.9, 1.4, 1.9, 2.6, 3.1 and 3.9 dS m^-1, respectively. Plant growth, relative water content (RWC), Na, Cl, K, N and P concentrations, membrane injury (MI), chlorophyll and proline levels, as well as gas exchange parameters in leaves of chrysanthemum were determined. A dose-dependent significant reduction of growth and minor decrease of leaf RWC were observed. Foliar Na and Cl concentrations increased with the highest NaCl dose up to 6-fold. However, the concentration of K increased by about 14 %, N by about 5 % but P decreased by about 23 %. Membrane injury was rather low (11 %) even at the highest NaCl dose. Statistically significant decreases of stomatal conductance (20 %), transpiration rate (32 %) and photosynthesis (25 %) were already observed at the lowest NaCl dose and about 40 % decrease of all these parameters with the highest dose. A significant reduction in the intercellular CO₂ concentration occurred at the lower NaCl doses and no changes with the highest dose. These results show that in plants grown with the highest NaCl dose, non-stomatal limitation of photosynthesis may occur. According to Maas and Hoffman tolerance assessment (1977) chrysanthemum 'Palisade White' may be considered as moderately sensitive to salt stress in terms of growth inhibition. However, it is able to cope with long-term salt stress without any signs of damage, such as chlorophyll depletion, leaf browning or necrotic spots probably due to maintenance of K homeostasis and proline accumulation, which alleviate the toxic effect of chloride.

A Putative Plasma Membrane Na+/H+ Antiporter GmSOS1 Is Critical for Salt Stress Tolerance in Glycine max

Soybean (Glycine max) is a staple crop and a major source of vegetable protein and vegetable oil. The growth of soybean is dramatically inhibited by salt stress, especially by the excessive toxic Na⁺. Salt Overly Sensitive 1 (SOS1) is the only extensively characterized Na⁺ efflux transporter in multiple plant species so far. However, the role of GmSOS1 in soybean salt stress responses remains unclear. Herein, we created three gmsos1 mutants using the CRISPR-Cas9 system in soybean. We found a significant accumulation of Na⁺ in the roots of the gmsos1 mutants, resulting in the imbalance of Na⁺ and K⁺, which links to impaired Na⁺ efflux and increased K⁺ efflux in the roots of the gmsos1 mutants under salt stress. Compared to the wild type, our RNA-seq analysis revealed that the roots of the gmsos1-1 showed preferential up and downregulation of ion transporters under salt stress, supporting impaired stress detection or an inability to develop a comprehensive response to salinity in the gmsos1 mutants. Our findings indicate that the plasma membrane Na⁺/H⁺ exchanger GmSOS1 plays a critical role in soybean salt tolerance by maintaining Na⁺ homeostasis and provides evidence for molecular breeding to improve salt tolerance in soybean and other crops.

Mechanisms of Salt Tolerance and Molecular Breeding of Salt-Tolerant Ornamental Plants

As the area of salinized soils increases, and freshwater becomes more scarcer worldwide, an urgent measure for agricultural production is to use salinized land and conserve freshwater resources. Ornamental flowering plants, such as carnations, roses, chrysanthemums, and gerberas, are found around the world and have high economic, ornamental, ecological, and edible value. It is therefore prudent to improve the salt tolerance of these important horticultural crops. Here, we summarize the salt-adaptive mechanisms, genes, and molecular breeding of ornamental flowering crops. We also review the genome editing technologies that provide us with the means to obtain novel varieties with high salinity tolerance and improved utility value, and discuss future directions of research into ornamental plants like salt exclusion mechanism. We considered that the salt exclusion mechanism in ornamental flowering plants, the acquisition of flowers with high quality and novel color under salinity condition through gene editing techniques should be focused on for the future research.

Salt stress proteins in plants: An overview

Salinity stress is considered the most devastating abiotic stress for crop productivity. Accumulating different types of soluble proteins has evolved as a vital strategy that plays a central regulatory role in the growth and development of plants subjected to salt stress. In the last two decades, efforts have been undertaken to critically examine the genome structure and functions of the transcriptome in plants subjected to salinity stress. Although genomics and transcriptomics studies indicate physiological and biochemical alterations in plants, it do not reflect changes in the amount and type of proteins corresponding to gene expression at the transcriptome level. In addition, proteins are a more reliable determinant of salt tolerance than simple gene expression as they play major roles in shaping physiological traits in salt-tolerant phenotypes. However, little information is available on salt stress-responsive proteins and their possible modes of action in conferring salinity stress tolerance. In addition, a complete proteome profile under normal or stress conditions has not been established yet for any model plant species. Similarly, a complete set of low abundant and key stress regulatory proteins in plants has not been identified. Furthermore, insufficient information on post-translational modifications in salt stress regulatory proteins is available. Therefore, in recent past, studies focused on exploring changes in protein expression under salt stress, which will complement genomic, transcriptomic, and physiological studies in understanding mechanism of salt tolerance in plants. This review focused on recent studies on proteome profiling in plants subjected to salinity stress, and provide synthesis of updated literature about how salinity regulates various salt stress proteins involved in the plant salt tolerance mechanism. This review also highlights the recent reports on regulation of salt stress proteins using transgenic approaches with enhanced salt stress tolerance in crops.

Gene regulation at transcriptional and post-transcriptional levels to combat salt stress in plants

Soil salinity is a major challenge that will be faced more and more by human population in the near future. Higher salt concentrations in the soil limit the growth and production of crops, which poses serious threats to global food production. Various plant breeding approaches have been followed in the past which are reported to reduce the effect of salt stress by inducing the level of protective metabolites like osmolytes and antioxidants. Conventional breeding approaches are time-consuming and not cost-effective. In recent times, genetic engineering has been largely followed to confer salt tolerance through introgressions of single transgenes or stacking multiple transgenes. However, most of such works are limited only at the laboratory level and field trials are still awaited to prove the long-term efficacy of such transgenics. In this review, we attempt to present a broad overview of the current strategies undertaken to develop halophytic and salt-tolerant crops. The salt-induced damages in the plants are highlighted, followed by representing the novel traits, associated with salt stress, which can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of transcriptional and epigenetic regulation in plants for amelioration of salt-induced damages has been reviewed. The role of post-transcriptional mechanisms such as microRNA regulation, genome editing and alternative splicing, during salt stress, and their implications in the development of salt-tolerant crops are also discussed. Finally, we present a short overview about the role of ion transporters and rhizobacteria in the engineering of salt tolerance in crop species.

Understanding the Integrated Pathways and Mechanisms of Transporters, Protein Kinases, and Transcription Factors in Plants under Salt Stress

Abiotic stress is the major threat confronted by modern-day agriculture. Salinity is one of the major abiotic stresses that influence geographical distribution, survival, and productivity of various crops across the globe. Plants perceive salt stress cues and communicate specific signals, which lead to the initiation of defence response against it. Stress signalling involves the transporters, which are critical for water transport and ion homeostasis. Various cytoplasmic components like calcium and kinases are critical for any type of signalling within the cell which elicits molecular responses. Stress signalling instils regulatory proteins and transcription factors (TFs), which induce stress-responsive genes. In this review, we discuss the role of ion transporters, protein kinases, and TFs in plants to overcome the salt stress. Understanding stress responses by components collectively will enhance our ability in understanding the underlying mechanism, which could be utilized for crop improvement strategies for achieving food security.

Ion homeostasis for salinity tolerance in plants: a molecular approach

Soil salinity is one of the major environmental stresses faced by the plants. Sodium chloride is the most important salt responsible for inducing salt stress by disrupting the osmotic potential. Due to various innate mechanisms, plants adapt to the sodic niche around them. Genes and transcription factors regulating ion transport and exclusion such as salt overly sensitive (SOS), Na⁺ /H⁺ exchangers (NHXs), high sodium affinity transporter (HKT) and plasma membrane protein (PMP) are activated during salinity stress and help in alleviating cells of ion toxicity. For salt tolerance in plants signal transduction and gene expression is regulated via transcription factors such as NAM (no apical meristem), ATAF (Arabidopsis transcription activation factor), CUC (cup-shaped cotyledon), Apetala 2/ethylene responsive factor (AP2/ERF), W-box binding factor (WRKY) and basic leucine zipper domain (bZIP). Cross-talk between all these transcription factors and genes aid in developing the tolerance mechanisms adopted by plants against salt stress. These genes and transcription factors regulate the movement of ions out of the cells by opening various membrane ion channels. Mutants or knockouts of all these genes are known to be less salt-tolerant compared to wild-types. Using novel molecular techniques such as analysis of genome, transcriptome, ionome and metabolome of a plant, can help in expanding the understanding of salt tolerance mechanism in plants. In this review, we discuss the genes responsible for imparting salt tolerance under salinity stress through transport dynamics of ion balance and need to integrate high-throughput molecular biology techniques to delineate the issue.

Characterization of the Salt Overly Sensitive pathway genes in sugarcane under salinity stress

The Salt Overly Sensitive (SOS) pathway is a crucial ion homeostasis process in crop plants trafficking excess Na⁺ ions for elimination/sequestration. The SOS pathway genes SOS1 (Na⁺ /H⁺ antiporter), SOS2 (CIPK), and SOS3 (CBL) associated with ion homeostasis were isolated and characterized in the sugarcane clone Co 85019. The isolated genes had a coding region of 1086, 904, and 636 bp, respectively. A nucleotide blast analysis of the isolated SOS gene sequences showed strong similarity with previous genes found to be involved in the active functioning of the SOS pathway for ion homeostasis conferring salinity tolerance in sugarcane. The analysis of tissue specific gene expression of the identified SOS genes revealed a significant linear increase in the leaves under the first 96 h of salt stress (2.5- to 21.6-fold) in the tolerant genotype Co 85019, while the expression in the roots showed a linear increase up to 48 h and thereafter a gradual decline. The expression of SOS genes in the susceptible genotype (Co 97010) was significantly lower than in the tolerant genotype. Tissue ion content analysis also revealed a differential accumulation of Na⁺ and K⁺ ions in the contrasting sugarcane genotypes (Co 85019 and Co 97010) and this corroborates the varied expressions of SOS genes between the tolerant and susceptible varieties under salinity. Genome-wide analysis of identified SOS family genes showed the homologs in Saccharum complex members, Sorghum bicolor and Zea mays, and this verifies a close genetic similarity among these genera.

Transcriptome analysis and differential gene expression profiling of two contrasting quinoa genotypes in response to salt stress

BACKGROUND

Soil salinity is one of the major abiotic stress factors that affect crop growth and yield, which seriously restricts the sustainable development of agriculture. Quinoa is considered as one of the most promising crops in the future for its high nutrition value and strong adaptability to extreme weather and soil conditions. However, the molecular mechanisms underlying the adaptive response to salinity stress of quinoa remain poorly understood. To identify candidate genes related to salt tolerance, we performed reference-guided assembly and compared the gene expression in roots treated with 300 mM NaCl for 0, 0.5, 2, and 24 h of two contrasting quinoa genotypes differing in salt tolerance.

RESULTS

The salt-tolerant (ST) genotype displayed higher seed germination rate and plant survival rate, and stronger seedling growth potential as well than the salt-sensitive (SS) genotype under salt stress. An average of 38,510,203 high-quality clean reads were generated. Significant Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified to deeper understand the differential response. Transcriptome analysis indicated that salt-responsive genes in quinoa were mainly related to biosynthesis of secondary metabolites, alpha-Linolenic acid metabolism, plant hormone signal transduction, and metabolic pathways. Moreover, several pathways were significantly enriched amongst the differentially expressed genes (DEGs) in ST genotypes, such as phenylpropanoid biosynthesis, plant-pathogen interaction, isoquinoline alkaloid biosynthesis, and tyrosine metabolism. One hundred seventeen DEGs were common to various stages of both genotypes, identified as core salt-responsive genes, including some transcription factor members, like MYB, WRKY and NAC, and some plant hormone signal transduction related genes, like PYL, PP2C and TIFY10A, which play an important role in the adaptation to salt conditions of this species. The expression patterns of 21 DEGs were detected by quantitative real-time PCR (qRT-PCR) and confirmed the reliability of the RNA-Seq results.

CONCLUSIONS

We identified candidate genes involved in salt tolerance in quinoa, as well as some DEGs exclusively expressed in ST genotype. The DEGs common to both genotypes under salt stress may be the key genes for quinoa to adapt to salinity environment. These candidate genes regulate salt tolerance primarily by participating in reactive oxygen species (ROS) scavenging system, protein kinases biosynthesis, plant hormone signal transduction and other important biological processes. These findings provide theoretical basis for further understanding the regulation mechanism underlying salt tolerance network of quinoa, as well establish foundation for improving its tolerance to salinity in future breeding programs.

Improving crop salt tolerance using transgenic approaches: An update and physiological analysis

Salinization of land is likely to increase due to climate change with impact on agricultural production. Since most species used as crops are sensitive to salinity, improvement of salt tolerance is needed to maintain global food production. This review summarises successes and failures of transgenic approaches in improving salt tolerance in crop species. A conceptual model of coordinated physiological mechanisms in roots and shoots required for salt tolerance is presented. Transgenic plants overexpressing genes of key proteins contributing to Na⁺ 'exclusion' (PM-ATPases with SOS1 antiporter, and HKT1 transporter) and Na⁺ compartmentation in vacuoles (V-H⁺ ATPase and V-H⁺ PPase with NHX antiporter), as well as two proteins potentially involved in alleviating water deficit during salt stress (aquaporins and dehydrins), were evaluated. Of the 51 transformations, with gene(s) involved in Na⁺ 'exclusion' or Na⁺ vacuolar compartmentation that contained quantitative data on growth and include a non-saline control, 48 showed improvements in salt tolerance (less impact on plant mass) of transgenic plants, but with only two tested in field conditions. Of these 51 transformations, 26 involved crop species. Tissue ion concentrations were altered, but not always in the same way. Although glasshouse data are promising, field studies are required to assess crop salinity tolerance.

Overexpression of CmSOS1 confers waterlogging tolerance in Chrysanthemum

The Na⁺ /H⁺ antiporter SOS1 enhances the salinity tolerance of a number of plant species, but its involvement in the response to hypoxia is less well known. We presented chrysanthemum homologs CmSOS1 and CmRCD1 coordinately mediate waterlogging tolerance by maintaining membrane integrity and minimizing the level of reactive oxygen species.

Engineering salinity tolerance in plants: progress and prospects

There is a need to integrate conceptual framework based on the current understanding of salt stress responses with different approaches for manipulating and improving salt tolerance in crop plants. Soil salinity exerts significant constraints on global crop production, posing a serious challenge for plant breeders and biotechnologists. The classical transgenic approach for enhancing salinity tolerance in plants revolves by boosting endogenous defence mechanisms, often via a single-gene approach, and usually involves the enhanced synthesis of compatible osmolytes, antioxidants, polyamines, maintenance of hormone homeostasis, modification of transporters and/or regulatory proteins, including transcription factors and alternative splicing events. Occasionally, genetic manipulation of regulatory proteins or phytohormone levels confers salinity tolerance, but all these may cause undesired reduction in plant growth and/or yields. In this review, we present and evaluate novel and cutting-edge approaches for engineering salt tolerance in crop plants. First, we cover recent findings regarding the importance of regulatory proteins and transporters, and how they can be used to enhance salt tolerance in crop plants. We also evaluate the importance of halobiomes as a reservoir of genes that can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of microRNAs as critical post-transcriptional regulators in plant adaptive responses to salt stress is reviewed and their use for engineering salt-tolerant crop plants is critically assessed. The potentials of alternative splicing mechanisms and targeted gene-editing technologies in understanding plant salt stress responses and developing salt-tolerant crop plants are also discussed.

Unraveling salinity stress responses in ancestral and neglected wheat species at early growth stage: A baseline for utilization in future wheat improvement programs

In this study, we analyzed the behavior of several neglected, ancestral, and domesticated wheat genotypes, including Ae. triuncialis, Ae. neglecta, Ae. caudata, Ae. umbellulata, Ae. tauschii, Ae. speltoides, T. boeoticum, T. urartu, T. durum, and T. aestivum under control and salinity stress to assess the mechanisms involved in salinity tolerance. Physiological and biochemical traits including root/shoot biomasses, root/shoot ion concentrations, activity of antioxidant enzymes APX, SOD, and GXP, and the relative expression of TaHKT1;5, TaSOS1, APX, GXP, and MnSOD genes were measured. Analysis of variance (ANOVA) revealed significant effects of the salinity treatments and genotypes for all evaluated traits. Salinity stress (350 mM NaCl) significantly decreased root/shoot biomasses, K+ concentration in root/shoot, and root/shoot K+/Na+ ratios. In contrast, salinity stress significantly increased Na+ concentration in root and shoot, activity of antioxidant enzymes (APX, SOD, and GPX) and relative expression of salt tolerance-related genes (TaHKT1;5, TaSOS1, APX, GPX, and MnSOD). Based on heat map and principal component analysis, the relationships among physiological traits and relative expression of salt-responsive genes were investigated. Remarkably, we observed a significant association between the relative expression of TaHKT1;5 with root K+ concentration and K+/Na+ ratio and with TaSOS1. Taken together, our study revealed that two neglected (Ae. triuncialis) and ancestral (Ae. tauschii) wheat genotypes responded better to salinity stress than other genotypes. Further molecular tasks are therefore essential to specify the pathways linked with salinity tolerance in these genotypes.

Chrysanthemum (Chrysanthemum morifolium) CmICE2 conferred freezing tolerance in Arabidopsis

Genes of the ICE (Inducer of CBF Expression) family play a key role in cold and freezing stresses response via the CBF regulatory pathway. In this work, we identified the ICE family gene, CmICE2, from Chrysanthemum morifolium 'Jinba'. CmICE2 encodes a 451-amino acid protein with a conserved nuclear localization domain, a bHLH domain and ACT domain. CmICE2 is expressed in abundance in leaves and flowers, and the expression of CmICE2 is induced by freezing and drought stresses. CmICE2 localized to the nucleus, and has transcriptional activity in yeast cells. After a 24-hour 4 °C acclimation, Arabidopsis plants overexpressing CmICE2 were more tolerant to freezing stress (-9 °C for 6 h) than the Col-0. When exposed to -9 °C for 6 h, the expression levels of genes such as AtCBF1, AtCBF2, AtCBF4, AtCOR 6.6A, AtCOR 414 and AtKIN1 were up-regulated significantly in CmICE2 overexpression plant lines compared to wild type. The proline contents, activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were also increased in plants overexpressing CmICE2. In summary, CmICE2 confers to plant response to freezing stress.

Physiological and Transcriptomic Analysis Reveals Distorted Ion Homeostasis and Responses in the Freshwater Plant Spirodela polyrhiza L. under Salt Stress

Duckweeds are a family of freshwater angiosperms with morphology reduced to fronds and propagation by vegetative budding. Unlike other angiosperm plants such as Arabidopsis and rice that have physical barriers between their photosynthetic organs and soils, the photosynthetic organs of duckweeds face directly to their nutrient suppliers (waters), therefore, their responses to salinity may be distinct. In this research, we found that the duckweed Spirodela polyrhiza L. accumulated high content of sodium and reduced potassium and calcium contents in large amounts under salt stress. Fresh weight, Rubisco and AGPase activities, and starch content were significantly decreaseded in the first day but recovered gradually in the following days and accumulated more starch than control from Day 3 to Day 5 when treated with 100 mM and 150 mM NaCl. A total of 2156 differentially expressed genes were identified. Overall, the genes related to ethylene metabolism, major CHO degradation, lipid degradation, N-metabolism, secondary metabolism of flavonoids, and abiotic stress were significantly increased, while those involved in cell cycle and organization, cell wall, mitochondrial electron transport of ATP synthesis, light reaction of photosynthesis, auxin metabolism, and tetrapyrrole synthesis were greatly inhibited. Moreover, salt stress also significantly influenced the expression of transcription factors that are mainly involved in abiotic stress and cell differentiation. However, most of the osmosensing calcium antiporters (OSCA) and the potassium inward channels were downregulated, Na⁺/H⁺ antiporters (SOS1 and NHX) and a Na⁺/Ca²⁺ exchanger were slightly upregulated, but most of them did not respond significantly to salt stress. These results indicated that the ion homeostasis was strongly disturbed. Finally, the shared and distinct regulatory networks of salt stress responses between duckweeds and other plants were intensively discussed. Taken together, these findings provide novel insights into the underlying mechanisms of salt stress response in duckweeds, and can be served as a useful foundation for salt tolerance improvement of duckweeds for the application in salinity conditions.

The Recretohalophyte Tamarix TrSOS1 Gene Confers Enhanced Salt Tolerance to Transgenic Hairy Root Composite Cotton Seedlings Exhibiting Virus-Induced Gene Silencing of GhSOS1

The salt overly sensitive 1 (SOS1) gene encodes the plasma membrane Na+/H+ antiporter, SOS1, that is mainly responsible for extruding Na+ from the cytoplasm and reducing the Na+ content in plants under salt stress and is considered a vital determinant in conferring salt tolerance to the plant. However, studies on the salt tolerance function of the TrSOS1 gene of recretohalophytes, such as Tamarix, are limited. In this work, the effects of salt stress on cotton seedlings transformed with tobacco-rattle-virus-based virus-induced gene silencing (VIGS) of the endogenous GhSOS1 gene, or Agrobacterium rhizogenes strain K599-mediated TrSOS1-transgenic hairy root composite cotton plants exhibiting VIGS of GhSOS1 were first investigated. Then, with Arabidopsis thaliana AtSOS1 as a reference, differences in the complementation effect of TrSOS1 or GhSOS1 in a yeast mutant were compared under salt treatment. Results showed that compared to empty-vector-transformed plants, GhSOS1-VIGS-transformed cotton plants were more sensitive to salt stress and had reduced growth, insufficient root vigor, and increased Na+ content and Na+/K+ ratio in roots, stems, and leaves. Overexpression of TrSOS1 enhanced the salt tolerance of hairy root composite cotton seedlings exhibiting GhSOS1-VIGS by maintaining higher root vigor and leaf relative water content (RWC), and lower Na+ content and Na+/K+ ratio in roots, stems, and leaves. Transformations of TrSOS1, GhSOS1, or AtSOS1 into yeast NHA1 (Na+/H+ antiporter 1) mutant reduced cellular Na+ content and Na+/K+ ratio, increased K+ level under salt stress, and had good growth complementation in saline conditions. In particular, the ability of TrSOS1 or GhSOS1 to complement the yeast mutant was better than that of AtSOS1. This may indicate that TrSOS1 is an effective substitute and confers enhanced salt tolerance to transgenic hairy root composite cotton seedlings, and even the SOS1 gene from salt-tolerant Tamarix or cotton may have higher efficiency than salt-sensitive Arabidopsis in regulating Na+ efflux, maintaining Na+ and K+ homeostasis, and therefore contributing to stronger salt tolerance.

Genome-Wide Identification of Na+/H+ Antiporter (NHX) Genes in Sugar Beet (Beta vulgaris L.) and Their Regulated Expression under Salt Stress

Salinity is one of the major environment factors that limits the growth of plants and the productivity of crops worldwide. It has been shown that Na+ transporters play a central role in salt tolerance and development of plants. The objective of this study was to identify Na+/H+ antiporter (NHX) genes and investigate their expression patterns in sugar beet (Beta vulgaris L.) subjected to various concentrations of NaCl. A total of five putative NHX genes were identified and distributed on four chromosomes in sugar beet. Phylogenetic analysis revealed that these BvNHX genes are grouped into three major classes, viz Vac- (BvNHX1, -2 and -3), Endo- (BvNHX4), and PM-class NHX (BvNHX5/BvSOS1), and within each class the exon/intron structures are conserved. The amiloride-binding site is found in TM3 at N-terminus of Vac-class NHX proteins. Protein-protein interaction (PPI) prediction suggested that only BvNHX5 putatively interacts with calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK), implying it might be the primary NHX involved in CBL-CIPK pathway under saline condition. It was also found that BvNHX5 contains one abscisic acid (ABA)-responsive element (ABRE), suggesting that BvNHX5 might be involved in ABA signal responsiveness. Additionally, the qRT-PCR analysis showed that all the BvNHX genes in both roots and leaves are significantly up-regulated by salt, and the transcription levels under high salinity are significantly higher than those under either low or moderate salinity. Taken together, this work gives a detailed overview of the BvNHX genes and their expression patterns under salt stress. Our findings also provide useful information for elucidating the molecular mechanisms of Na+ homeostasis and further functional identification of the BvNHX genes in sugar beet.

Identification of favorable SNP alleles and candidate genes responsible for inflorescence-related traits via GWAS in chrysanthemum

81 SNPs were identified for three inflorescence-related traits, in which 15 were highly favorable. Two dCAPS markers were developed for future MAS breeding, and six candidate genes were predicted. Chrysanthemum is a leading ornamental species worldwide and demonstrates a wealth of morphological variation. Knowledge about the genetic basis of its phenotypic variation for key horticultural traits can contribute to its effective management and genetic improvement. In this study, we conducted a genome-wide association study (GWAS) based on two years of phenotype data and a set of 92,617 single nucleotide polymorphisms (SNPs) using a panel of 107 diverse cut chrysanthemums to dissect the genetic control of three inflorescence-related traits. A total of 81 SNPs were significantly associated with the three inflorescence-related traits (capitulum diameter, number of ray florets and flowering time) in at least one environment, with an individual allele explaining 22.72-38.67% of the phenotypic variation. Fifteen highly favorable alleles were identified for the three target traits by computing the phenotypic effect values for the stable associations detected in 2 year-long trials at each locus. Dosage pyramiding effects of the highly favorable SNP alleles and significant linear correlations between highly favorable allele numbers and corresponding phenotypic performance were observed. Two highly favorable SNP alleles correlating to flowering time and capitulum diameter were converted to derived cleaved amplified polymorphic sequence (dCAPS) markers to facilitate future breeding. Finally, six putative candidate genes were identified that contribute to flowering time and capitulum diameter. These results serve as a foundation for analyzing the genetic mechanisms underlying important horticultural traits and provide valuable insights into molecular marker-assisted selection (MAS) in chrysanthemum breeding programs.

A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. II. Clarifying the Roles of SOS1 in the Salt-Stress Response in Arabidopsis

SOS1 transporters play an essential role in plant salt tolerance. Although SOS1 is known to encode a plasma membrane Na⁺/H⁺ antiporter, the transport mechanisms by which these transporters contribute to salt tolerance at the level of the whole root are unclear. Gene expression and flux measurements have provided conflicting evidence for the location of SOS1 transporter activity, making it difficult to determine their function. Whether SOS1 transporters load or unload Na⁺ from the root xylem transpiration stream is also disputed. To address these areas of contention, we applied a mathematical model to answer the question: what is the function of SOS1 transporters in salt-stressed Arabidopsis roots? We used our biophysical model of ion and water transport in a salt-stressed root to simulate a wide range of SOS1 transporter locations in a model Arabidopsis root, providing a level of detail that cannot currently be achieved by experimentation. We compared our simulations with available experimental data to find reasonable parameters for the model and to determine likely locations of SOS1 transporter activity. We found that SOS1 transporters are likely to be operating in at least one tissue of the outer mature root, in the mature stele, and in the epidermis of the root apex. SOS1 transporter activity in the mature outer root cells is essential to maintain low cytosolic Na⁺ levels in the root and also restricts the uptake of Na⁺ to the shoot. SOS1 transporters in the stele actively load Na⁺ into the xylem transpiration stream, enhancing the transport of Na⁺ and water to the shoot. SOS1 transporters acting in the apex restrict cytosolic Na⁺ concentrations in the apex but are unable to maintain low cytosolic Na⁺ levels in the mature root. Our findings suggest that targeted, tissue-specific overexpression or knockout of SOS1 may lead to greater salt tolerance than has been achieved with constitutive gene changes. Tissue-specific changes to the expression of SOS1 could be used to identify the appropriate balance between limiting Na⁺ uptake to the shoot while maintaining water uptake, potentially leading to enhancements in salt tolerance.

Chrysanthemum CmHSFA4 gene positively regulates salt stress tolerance in transgenic chrysanthemum

Salinity-induced Na+ toxicity and oxidative stress hamper plant growth. Here, we showed that expression of the chrysanthemum CmHSFA4, a homologue of the heat-shock factor AtHSFA4a, is inducible by salt and localizes to the nucleus. It is a transcription activator binding with HSE. Chrysanthemum overexpressing CmHSFA4 displayed enhanced salinity tolerance by limiting Na+ accumulation while maintaining K+ concentration, which is consistent with the up-regulation of ion transporters CmSOS1 and CmHKT2. Additionally, the transgenic plants reduced H2 O2 and O2∙- accumulation under salinity, which could be due to up-regulation of ROS scavenger activities such as SOD, APX and CAT as well as CmHSP70, CmHSP90. Together, these results suggest that CmHSFA4 conferred salinity tolerance in chrysanthemum as a consequence of Na+ /K+ ion and ROS homeostasis.

The chrysanthemum leaf and root transcript profiling in response to salinity stress

RNA-Seq was applied to capture the transcriptome of the leaf and root of non-treated and salinity-treated chrysanthemum cv. 'Jinba' plants. A total of 206,868 unigenes of mean length 849 nt and of N50 length 1363 nt was identified; of these about 64% (> 132,000) could be functionally assigned. Depending on the severity of the salinity stress, differential transcription was observed for genes encoding proteins involved in osmotic adjustment, in ion transport, in reactive oxygen species scavenging and in the regulation of abscisic acid (ABA) signaling. The root stress response was dominated by the up-regulation of genes involved in ion transport, while that of the leaf reflected the plant's effort to make osmotic adjustments and to regulate Ca²⁺ transport. An array of known transcription factors (WRKY, AP2/ERF, MYB, bHLH and NAC) were differentially transcribed.

Overexpression of Phosphate Transporter Gene CmPht1;2 Facilitated Pi Uptake and Alternated the Metabolic Profiles of Chrysanthemum Under Phosphate Deficiency

Low availability of phosphorus (P) in the soil is the principal limiting factor for the growth of cut chrysanthemum. Plant phosphate transporters (PTs) facilitate acquisition of inorganic phosphate (Pi) and its homeostasis within the plant. In the present study, CmPht1;2 of the Pht1 family was cloned from chrysanthemum. CmPht1;2 is composed of 12 transmembrane domains and localized to the plasma membrane. Expression of CmPht1;2 in roots was induced by Pi starvation. Chrysanthemum plants with overexpression of CmPht1;2 (Oe) showed higher Pi uptake, as compared to the wild type (WT), both under Pi-starvation and Pi-sufficient conditions, and also showed a higher root biomass compared to WT in the Pi-starvation conditions. Seven days after the P-deficiency treatment, 85 distinct analytes were identified in the roots and 27 in the shoots between the Oe1 plant and WT, in which sophorose, sorbitol (sugars), hydroxybutyric acid (organic acids), and ornithine (amino acid) of CmPht1;2 overexpressing chrysanthemum are specific responses to P-starvation.

Genomics, Physiology, and Molecular Breeding Approaches for Improving Salt Tolerance

Salt stress reduces land and water productivity and contributes to poverty and food insecurity. Increased salinization caused by human practices and climate change is progressively reducing agriculture productivity despite escalating calls for more food. Plant responses to salt stress are well understood, involving numerous critical processes that are each controlled by multiple genes. Knowledge of the critical mechanisms controlling salt uptake and exclusion from functioning tissues, signaling of salt stress, and the arsenal of protective metabolites is advancing. However, little progress has been made in developing salt-tolerant varieties of crop species using standard (but slow) breeding approaches. The genetic diversity available within cultivated crops and their wild relatives provides rich sources for trait and gene discovery that has yet to be sufficiently utilized. Transforming this knowledge into modern approaches using genomics and molecular tools for precision breeding will accelerate the development of tolerant cultivars and help sustain food production.