Adaptive Mechanisms of Halophytes and Their Potential in Improving Salinity Tolerance in Plants

Inroads into saline-alkaline stress response in plants: unravelling morphological, physiological, biochemical, and molecular mechanisms

MAIN CONCLUSION

This article discusses the complex network of ion transporters, genes, microRNAs, and transcription factors that regulate crop tolerance to saline-alkaline stress. The framework aids scientists produce stress-tolerant crops for smart agriculture. Salinity and alkalinity are frequently coexisting abiotic limitations that have emerged as archetypal mediators of low yield in many semi-arid and arid regions throughout the world. Saline-alkaline stress, which occurs in an environment with high concentrations of salts and a high pH, negatively impacts plant metabolism to a greater extent than either stress alone. Of late, saline stress has been the focus of the majority of investigations, and saline-alkaline mixed studies are largely lacking. Therefore, a thorough understanding and integration of how plants and crops rewire metabolic pathways to repair damage caused by saline-alkaline stress is of particular interest. This review discusses the multitude of resistance mechanisms that plants develop to cope with saline-alkaline stress, including morphological and physiological adaptations as well as molecular regulation. We examine the role of various ion transporters, transcription factors (TFs), differentially expressed genes (DEGs), microRNAs (miRNAs), or quantitative trait loci (QTLs) activated under saline-alkaline stress in achieving opportunistic modes of growth, development, and survival. The review provides a background for understanding the transport of micronutrients, specifically iron (Fe), in conditions of iron deficiency produced by high pH. Additionally, it discusses the role of calcium in enhancing stress tolerance. The review highlights that to encourage biomolecular architects to reconsider molecular responses as auxiliary for developing tolerant crops and raising crop production, it is essential to (a) close the major gaps in our understanding of saline-alkaline resistance genes, (b) identify and take into account crop-specific responses, and (c) target stress-tolerant genes to specific crops.

Copper Oxide Nanoparticles Induced Growth and Physio-Biochemical Changes in Maize (Zea mays L.) in Saline Soil

Research on nanoparticles (NPs) is gaining great attention in modulating abiotic stress tolerance and improving crop productivity. Therefore, this investigation was carried out to evaluate the effects of copper oxide nanoparticles (CuO-NPs) on growth and biochemical characteristics in two maize hybrids (YH-5427 and FH-1046) grown under normal conditions or subjected to saline stress. A pot-culture experiment was carried out in the Botanical Research Area of "the University of Lahore", Lahore, Pakistan, in a completely randomized design. At two phenological stages, both maize hybrids were irrigated with the same amount of distilled water or NaCl solution (EC = 5 dS m-1) and subjected or not to foliar treatment with a suspension of CuO-NPs. The salt stress significantly reduced the photosynthetic parameters (photosynthetic rate, transpiration, stomatal conductance), while the sodium content in the shoot and root increased. The foliar spray with CuO-NPs improved the growth and photosynthetic attributes, along with the N, P, K, Ca, and Mg content in the roots and shoots. However, the maize hybrid YH-5427 responded better than the other hybrid to the saline stress when sprayed with CuO-NPs. Overall, the findings of the current investigation demonstrated that CuO-NPs can help to reduce the adverse effects of salinity stress on maize plants by improving growth and physio-biochemical attributes.

Temporal Changes in Biochemical Responses to Salt Stress in Three Salicornia Species

Halophytes adapt to salinity using different biochemical response mechanisms. Temporal measurements of biochemical parameters over a period of exposure to salinity may clarify the patterns and kinetics of stress responses in halophytes. This study aimed to evaluate short-term temporal changes in shoot biomass and several biochemical variables, including the contents of photosynthetic pigments, ions (Na+, K+, Ca2+, and Mg2+), osmolytes (proline and glycine betaine), oxidative stress markers (H2O2 and malondialdehyde), and antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) activities of three halophytic Salicornia species (S. persica, S. europaea, and S. bigelovii) in response to non-saline, moderate (300 mM NaCl), and high (500 mM NaCl) salinity treatments at three sampling times. Salicornia plants showed maximum shoot biomass under moderate salinity conditions. The results indicated that high Na+ accumulation in the shoots, coupled with the relative retention of K+ and Ca2+ under salt stress conditions, contributed significantly to ionic and osmotic balance and salinity tolerance in the tested Salicornia species. Glycine betaine accumulation, both constitutive and salt-induced, also seems to play a crucial role in osmotic adjustment in Salicornia plants subjected to salinity treatments. Salicornia species possess an efficient antioxidant enzyme system that largely relies on the ascorbate peroxidase and peroxidase activities to partly counteract salt-induced oxidative stress. The results also revealed that S. persica exhibited higher salinity tolerance than S. europaea and S. bigelovii, as shown by better plant growth under moderate and high salinity. This higher tolerance was associated with higher peroxidase activities and increased glycine betaine and proline accumulation in S. persica. Taking all the data together, this study allowed the identification of the biochemical mechanisms contributing significantly to salinity tolerance of Salicornia through the maintenance of ion and osmotic homeostasis and protection against oxidative stress.

Early-responsive molecular signatures associated with halophytic adaptation in Sesuvium portulacastrum (L.)

Sesuvium portulacastrum (L.) is a halophyte, adapted to grow naturally under saline environments. The ability to use Na and K interchangeably indicated its facultative halophyte nature. No significant growth reduction occurs in seedlings up to 250 mM NaCl, except for curling of the youngest leaf. Within 8 h of salt treatment, seedlings accumulate proline, glycine betaine and other amino acids in both root and shoot. Despite a continued increase of tissue Na content, the number of differentially expressed genes (DEGs) decreases between 8 and 24 h of salt exposure, indicating transcriptional restoration after the initial osmotic challenge. At 8 h, upregulated genes mainly encode transporters and transcription factors, while genes in growth-related pathways such as photosynthesis and ribosome-associated biogenesis are suppressed. Overexpression of SpRAB18 (an ABA-responsive dehydrin), one of the most strongly induced DEGs, in soybean was found to increase biomass in control conditions and the growth benefit was maintained when plants were grown in 100 mM NaCl, indicating conservation of function in halophyte and glycophyte. An open-access transcriptome database "SesuviumKB" (https://cb.imsc.res.in/sesuviumkb/) was developed to involve the scientific community in wide-scale functional studies of S. portulacastrum genes, that could pave the way to engineer salt tolerance in crops.

Halotolerant Pseudomonas koreensis S4T10 mitigate salt and drought stress in Arabidopsis thaliana

Salt and drought are documented among the most detrimental and persistent abiotic stresses for crop production. Here, we investigated the impact of Pseudomonas koreensis strain S4T10 on plant performance under salt and drought stress. Arabidopsis thaliana Col-0 wild type and atnced3 mutant plants were inoculated with P. koreensis or tap water and exposed to NaCl (100 mM) for five days and drought stress by withholding water for seven days. P. koreensis significantly enhanced plant biomass and photosynthetic pigments under salt and drought stress conditions. Moreover, P. koreensis activated the antioxidant defence by modulating glutathione (GSH), superoxide dismutase (SOD), peroxidase (POD), and polyphenol oxidase (PPO) activities to scavenge the reactive oxygen species produced due to the stress. In addition, the application of P. koreensis upregulated the expression of genes associated with antioxidant responses, such as AtCAT1, AtCAT3, and AtSOD. Similarly, genes linked to salt stress, such as AtSOS1, AtSOS2, AtSOS3, AtNHX1, and AtHKT1, were also upregulated, affirming the positive role of P. koreensis S4T10 in streamlining the cellular influx and efflux transport systems during salt stress. Likewise, the PGPB inoculation was observed to regulate the expression of drought-responsive genes AtDREB2A, AtDREB2B, and ABA-responsive genes AtAO3, AtABA3 indicating that S4T10 enhanced drought tolerance via modulation of the ABA pathway. The results of this study affirm that P. koreensis S4T10 could be further developed as a biofertilizer to mitigate salt and drought stress at the same time.

Sulfur-Oxidizing Bacteria Alleviate Salt and Cadmium Stress in Halophyte Tripolium pannonicum (Jacq.) Dobrocz

The aim of this study was to investigate how introducing halophilic sulfur-oxidizing bacteria (SOB) Halothiobacillus halophilus to the growth substrate affects the physiological and biochemical responses of the halophyte Tripolium pannonicum (also known as sea aster or seashore aster) under salt and cadmium stress conditions. This study assessed the plant's response to these stressors and bacterial inoculation by analyzing various factors including the accumulation of elements such as sodium (Na), chloride (Cl), cadmium (Cd) and sulfur (S); growth parameters; levels of photosynthetic pigments, proline and phenolic compounds; the formation of malondialdehyde (MDA); and the plant's potential to scavenge 2,2-Diphenyl-1-picrylhydrazyl (DPPH). The results revealed that bacterial inoculation was effective in mitigating the deleterious effect of cadmium stress on some growth criteria. For instance, stem length was 2-hold higher, the growth tolerance index was 3-fold higher and there was a 20% increase in the content of photosynthetic pigments compared to non-inoculated plants. Furthermore, the SOB contributed to enhancing cadmium tolerance in Tripolium pannonicum by increasing the availability of sulfur in the plant's leaves, which led to the maintenance of an appropriate, about 2-fold-higher level of phenolic compounds (phenylpropanoids and flavonols), as well as chloride ions. The level of MDA decreased after bacterial application in all experimental variants except when both salt and cadmium stress were present. These findings provide novel insights into how halophytes respond to abiotic stress following inoculation of the growth medium with sulfur-oxidizing bacteria. The data suggest that inoculating the substrate with SOB has a beneficial effect on T. pannonicum's tolerance to cadmium stress.

The potential and prospects of modified biochar for comprehensive management of salt-affected soils and plants: A critical review

Soil salinization has become a global problem that threatens farmland health and restricts crop production. Salt-affected soils seriously restrict the development of agricultural, mainly because of sodium ion (Na+) toxicity, nutrient deficiency, and structural changes in the soil. Biochar is a carbon (C)-based substance produced by heating typical biomass waste at high temperatures in anaerobic circumstances. It has high cation exchange capacity (CEC), adsorption capacity, and C content, which is often used as a soil amendment. Biochar generally reduces the concentration of Na+ in soil colloids through its strong adsorption, or uses the calcium (Ca) or magnesium (Mg) rich on its surface to exchange sodium ions (Ex-Na) from soil colloids through cation exchange to accelerate salt leaching during irrigation. Nowadays, biochar is widely used for acidic soils improvement due to its alkaline properties. Although the fact that biochar has gained increasing attention for its significant role in saline alkali soil remediation, there is currently a lack of systematic research on biochar improvers and their potential mechanisms for identifying physical, chemical, and biological indicators of soil eco-environment assessment and plant growth conditions affected by salt stress. This paper reviews the preparation, modification, and activation of biochar, the effects of biochar and its combination with beneficial salt-tolerant strains on salt-affected soils and plant growth. Finally, the limitations, benefits, and future needs of biochar-based soil health assessment technology in salt-affected soils and plant were discussed. This article elaborates on the future opportunities and challenges of biochar in the treatment of saline land, and a green method was provided for the integrate control to salt-affected soils.

Comparative transcriptome analysis reveals the adaptive mechanisms of halophyte Suaeda dendroides encountering high saline environment

Suaeda dendroides, a succulent euhalophyte of the Chenopodiaceae family, intermittently spread around northern Xinjiang, China, has the ability to grow and develop in saline and alkali environments. The objective of this study was therefore to investigate the underlying molecular mechanisms of S. dendroides response to high salt conditions. 27 sequencing libraries prepared from low salt (200 mM NaCl) and high salt (800 mM NaCl) treated plants at 5 different stages were sequenced using Illumina Hiseq 2000. A total of 133,107 unigenes were obtained, of which 4,758 were DEGs. The number of DEGs in the high salt group (3,189) was more than the low salt treatment group (733) compared with the control. GO and KEGG analysis of the DEGs at different time points of the high salt treatment group showed that the genes related to cell wall biosynthesis and modification, plant hormone signal transduction, ion homeostasis, organic osmolyte accumulation, and reactive oxygen species (ROS) detoxification were significantly expressed, which indicated that these could be the main mechanisms of S. dendroides acclimate to high salt stress. The study provides a new perspective for understanding the molecular mechanisms of halophytes adapting to high salinity. It also provides a basis for future investigations of key salt-responsive genes in S. dendroides.

Characterization and transformation of TtMYB1 transcription factor from Tritipyrum to improve salt tolerance in wheat

BACKGROUND

Common wheat (Triticum aestivum L.) is a worldwide cereal crop, which is an integral part of the diets of many countries. In addition, the MYB gene of wheat plays a role in the response to salt stress.

RESULTS

"Y1805" is a Tritipyrum variety that is relatively tolerant to salt. We used transcriptome analysis to show that the "Y1805" MYB gene was both highly expressed and sensitive to salt stress. Compared with control roots, the level of MYB expression during salt stress was higher, which rapidly decreased to control levels during the recovery process. MYB gene relative expression showed the highest levels in "Y1805" roots during salt stress, with the stems and then leaves being the next highest stressed tissues. The novel MYB gene (TtMYB1) was successfully cloned from "Y1805". It showed a coding sequence length of 783 bp with 95.79% homology with Tel2E01G633100 from Thinopyrum elongatum. TtMYB1 and MYB from Th. elongatum were clustered in the same branch using phylogenetic analysis, which indicated high similarities. The TtMYB1 gene is located in the nucleus. The coleoptile method was employed when a TtMYB1 overexpression vector was used during transformation into "1718" (common wheat). Under high salt stress, TtMYB1 leaves of overexpression lines had decreased wilting, when compared with wild-type (WT) plants. During normal conditions, salt stress, and recovery, the lengths of the roots and the heights of seedlings from the overexpression lines were found to be significantly greater than roots and seedlings of WT plants. In addition, during high salt stress, the overexpression lines showed that proline and soluble sugar levels were higher than that of WT plants, but with lower malondialdehyde levels. Forty-three proteins that interacted with TtMYB1 were identified using the yeast two-hybrid assay. Protein-protein interaction analyses indicated that most were SANT domain-containing and Wd repeat region domain-containing proteins. Among these proteins, ribosomal proteins were the main node. Abiotic stress-related terms (such as "carbonate dehydratase activity", "protein targeting peroxisomes", and "glutathione peroxidase activity") were enriched in GO analysis. In KEGG analysis, "carbohydrate metabolism", "environmental information processing", "genetic information processing", "signaling and cell precursors", and "energy metabolism" pathways were enriched.

CONCLUSION

The TtMYB1 gene might enhance salt tolerance by increasing proline and soluble sugar content and antioxidase activity in transgenic wheat. It therefore has the potential to enhance high salt tolerance in plants.

Salt Tolerance in Machilus faberi: Elucidating Growth and Physiological Adaptations to Saline Environments

Adversity stress is the main environmental factor limiting plant growth and development, including salt and other stress factors. This study delves into the adaptability and salt tolerance mechanisms of Machilus faberi Hemsl, a species with potential for cultivation in salinized areas. We subjected the plants to various salt concentrations to observe their growth responses and to assess key physiological and biochemical indicators. The results revealed that under high salt concentrations (500 and 700 mmol-1/L), symptoms such as leaf yellowing, wilting, and eventual death were observed. Notably, plant height and shoot growth ceased on the 14th day of exposure. Chlorophyll content (a, b, total a + b, and the a/b ratio) initially increased but subsequently decreased under varying levels of salt stress. Similarly, the net photosynthetic rate, stomatal conductance, leaf water content, and root activity significantly declined under these conditions. Moreover, we observed an increase in malondialdehyde levels and relative conductivity, indicative of cellular damage and stress. The activity of superoxide dismutase and ascorbate peroxidase initially increased and then diminished with prolonged stress, whereas peroxidase activity consistently increased. Levels of proline and soluble protein exhibited an upward trend, contrasting with the fluctuating pattern of soluble sugars, which decreased initially but increased subsequently. In conclusion, M. faberi exhibits a degree of tolerance to salt stress, albeit with growth limitations when concentrations exceed 300 mmol-1/L. These results shed light on the plant's mechanisms of responding to salt stress and provide a theoretical foundation for its cultivation and application in salt-affected regions.

Halophyte Elymus dahuricus colonization regulates microbial community succession by mediating saline-alkaline and biogenic organic matter in bauxite residue

Alkalinity regulation and nutrient accumulation are critical factors in the construction of plant and microbial communities and soil formation in bauxite residue, and are extremely important for sustainable vegetation restoration in bauxite residue disposal areas. However, the establishment and succession of microbial communities driven by plant colonization-mediated improvements in the physicochemical properties of bauxite residues remain poorly understood. Thus, in this study, we determined the saline-alkali properties and dissolved organic matter (DOM) components under plant growth conditions and explored the microbial community diversity and structure using Illumina high-throughput sequencing. The planting of Elymus dahuricus (E. dahuricus) in the bauxite residue resulted in a significant decrease in total alkalinity (TA), exchangeable Na, and electrical conductivity (EC) as well as the release of more tryptophan-like protein compounds and low-molecular-weight humic substances associated with biological activities into the bauxite residue substrate. Taxonomical analysis revealed an initial-stage bacterial and fungal community dominated by alkaline-tolerant Actinobacteriota, Firmicutes, and Ascomycota, and an increase in the relative abundances of the phyla Bacteroidota, Cyanobacteria, Chloroflexi, and Gemmatimonadota. The biological activities of phylum Actinobacteriota, Bacteroidota, and Gemmatimonadota were significantly associated with protein-like and UVA-like humic substances. As eutrophic bacteria, Proteobacteria participate in the transformation of humic substances and can not only utilize small molecules of organic matter and convert them into humic substances but also promote the gradual conversion of humic acids into simple molecular compounds. Our results suggest that plant roots secrete organic matter and microbial metabolites as the main biogenic organic matter that participates in the establishment and succession of the microbial community in bauxite residues. Root length affects bacterial and fungal diversity by mediating the production of protein-like substances.

Chasing the mechanisms of ecologically adaptive salinity tolerance

Plants adapted to challenging environments offer fascinating models of evolutionary change. Importantly, they also give information to meet our pressing need to develop resilient, low-input crops. With mounting environmental fluctuation-including temperature, rainfall, and soil salinity and degradation-this is more urgent than ever. Happily, solutions are hiding in plain sight: the adaptive mechanisms from natural adapted populations, once understood, can then be leveraged. Much recent insight has come from the study of salinity, a widespread factor limiting productivity, with estimates of 20% of all cultivated lands affected. This is an expanding problem, given increasing climate volatility, rising sea levels, and poor irrigation practices. We therefore highlight recent benchmark studies of ecologically adaptive salt tolerance in plants, assessing macro- and microevolutionary mechanisms, and the recently recognized role of ploidy and the microbiome on salinity adaptation. We synthesize insight specifically on naturally evolved adaptive salt-tolerance mechanisms, as these works move substantially beyond traditional mutant or knockout studies, to show how evolution can nimbly "tweak" plant physiology to optimize function. We then point to future directions to advance this field that intersect evolutionary biology, abiotic-stress tolerance, breeding, and molecular plant physiology.

Leaf nitrogen and phosphorus stoichiometry of the halophytes across China

Halophytes play a crucial role in the ecological restoration of saline and alkaline land and hold promising benefits to food security in China. Although a variety of aspects of halophytes have been extensively addressed, there is still a lack of overall understanding of the leaf nitrogen (N) and phosphorus (P) stoichiometric characteristics, especially at a national scale. We compiled a national dataset of 311 observations from 113 sampling sites across China to explore the changing trends and influencing factors on leaf N and P concentrations, and N:P ratio of halophytes. The results showed that leaf N concentration decreased significantly with increasing latitude (LAT), which was mainly driven by the mean annual temperature (MAT) and mean annual precipitation (MAP). The leaf P concentration increased remarkably with increasing longitude (LON), which was induced by the variation in soil total P (TP) content. The leaf N:P ratio increased as LAT increased and LON decreased, which was potentially regulated by the MAT, MAP, and soil TP content. The scaling exponents of the N-P relationship differed significantly among halophyte types and were 0.40, 0.87, and 1.39 for euhalophyte, pseudohalophyte, and recretohalophyte, respectively. The leaf N concentration exhibited significant differences among ecosystem types and halophyte types, whereas the leaf P concentration and N:P ratio remained relatively stable. In summary, the leaf N concentration and N-P scaling exponent might be the classification criteria for halophyte types from the perspective of plant nutrient resource allocation. Moreover, this study characterized the spatial distribution and allocation strategy of leaf N and P stoichiometry in halophytes by data integration analysis, providing the basic information for nutrient management in the processes of the future domestication and introduction of halophytes.

Analysis of aquaporins in northern grasses reveal functional importance of Puccinellia nuttalliana PIP2;2 in salt tolerance

To better understand the roles of aquaporins in salt tolerance, we cloned PIP2;1, PIP2;2, PIP2;3, PIP1;1, PIP1;3, and TIP1;1 aquaporins from three northern grasses varying is salt tolerance including the halophytic grass Puccinellia nuttalliana, moderately salt tolerant Poa juncifolia, and relatively salt sensitive Poa pratensis. We analysed aquaporin expression in roots by exposing the plants to 0 and 150 mM for 6 days in hydroponic culture. NaCl treatment upregulated several PIP transcripts in P. nuttalliana while decreasing PnuTIP1;1. The PnuPIP2;2 transcripts increased by about six-fold in P. nuttalliana, two-fold in Poa juncifolia, and did not change in Poa pratensis. The NaCl treatment enhanced the rate of water transport in yeast expressing PnuPIP2;2 by 56% compared with control. PnuPIP2,2 expression also resulted in a higher Na⁺ uptake in yeast cells compared with an empty vector suggesting that PnuPIP2;2 may have both water and ion transporting functions. Structural analysis revealed that the transport properties of PnuPIP2;2 could be affected by its unique pore characteristics, which include a combination of hourglass, cylindrical, and increasing diameter conical entrance shape with pore hydropathy of -0.22.

Roles of S-Adenosylmethionine and Its Derivatives in Salt Tolerance of Cotton

Salinity is a major abiotic stress that restricts cotton growth and affects fiber yield and quality. Although studies on salt tolerance have achieved great progress in cotton since the completion of cotton genome sequencing, knowledge about how cotton copes with salt stress is still scant. S-adenosylmethionine (SAM) plays important roles in many organelles with the help of the SAM transporter, and it is also a synthetic precursor for substances such as ethylene (ET), polyamines (PAs), betaine, and lignin, which often accumulate in plants in response to stresses. This review focused on the biosynthesis and signal transduction pathways of ET and PAs. The current progress of ET and PAs in regulating plant growth and development under salt stress has been summarized. Moreover, we verified the function of a cotton SAM transporter and suggested that it can regulate salt stress response in cotton. At last, an improved regulatory pathway of ET and PAs under salt stress in cotton is proposed for the breeding of salt-tolerant varieties.

Halophytes as new model plant species for salt tolerance strategies

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

Salt Tolerance of Limonium gmelinii subsp. hungaricum as a Potential Ornamental Plant for Secondary Salinized Soils

Secondary salinization caused by climate change is a growing global problem. Searching for plants that can survive in areas with high salt content and even have decorative value was the focus of our research. Thirty plants of Limonium gmelinii subsp. hungaricum were planted in clear river sand; another thirty plants were planted in Pindstrup, a growing substrate enriched with 40% clay. With the latter, we modeled the natural soil. In addition to the control tap-water treatment, plants received 50, 125, 250, 375, and 500 mM NaCl solution irrigation twice a week. The leaf sizes of plants planted in sand decreased proportionally with the increasing NaCl concentration, and their dry matter content increased. In the clay-containing medium, leaf sizes increased, even at a concentration of 375 mM, although the dry matter content increased only at high concentrations. Carotene content in both media became higher, due to the higher NaCl concentrations, while proline content in the plants grown in sandy media increased, even with the 125 mM concentration. With our present experiment we proved the salt tolerance of the taxon, and even the soil's great importance in supporting the plant's salt tolerance.

Biochemical Profile and In Vitro Therapeutic Properties of Two Euhalophytes, Halocnemum strobilaceum Pall. and Suaeda fruticosa (L.) Forske., Grown in the Sabkha Ecosystem in the Algerian Sahara

This study reports the biochemical profile and in vitro biological activities of the aerial part of two shrubs: Halocnemum strobilaceum and Suaeda fruticosa, a halophytes species native to saline habitats. The biomass was evaluated by determining its physiological properties and approximate composition. Hydro-methanolic extracts from Halocnemum strobilaceum and Suaeda fruticosa have been investigated for the inhibition of bacterial growth, the protection of proteins (albumin) from denaturation, and cytotoxicity to hepatocellular carcinomas (Huh-7 and HepG2). Their antioxidant activity was evaluated by five tests, including one that examined their ability to inhibit hydrogen peroxide (H₂O₂)-induced hemolysis. The profile of their phenolic compounds was also determined. These two euhalophytes had a high moisture content, high levels of photosynthetic pigments, elevated levels of ash and protein, low oxidative damage indices, MDA (Malondialdehyde) and proline, and low lipids levels. Their content was also characterized by a moderate acidity with good electrical conductivity. They contained abundant levels of phytochemicals and varied phenolic contents. Reverse phase high performance liquid chromatography (RP-HPLC) analysis revealed the presence of caffeic acid, p-coumaric acid, rutin, and quercetin in both plant extracts. On the pharmaceutical level, the two euhalophytes had anti-inflammatory, antibacterial, antioxidant, and cytotoxic properties, and therefore it was recommended to isolate and identify biologically active compounds from these plants and evaluate them in vivo.

Moderate Salinity Stress Increases the Seedling Biomass in Oilseed Rape (Brassica napus L.)

Oilseed rape (Brassica napus L.), an important oil crop of the world, suffers various abiotic stresses including salinity stress during the growth stage. While most of the previous studies paid attention to the adverse effects of high salinity stress on plant growth and development, as well as their underlying physiological and molecular mechanisms, less attention was paid to the effects of moderate or low salinity stress. In this study, we first tested the effects of different concentrations of NaCl solution on the seedling growth performance of two oilseed rape varieties (CH336, a semi-winter type, and Bruttor, a spring type) in pot cultures. We found that moderate salt concentrations (25 and 50 mmol L^-1 NaCl) can stimulate seedling growth by a significant increase (10~20%, compared to controls) in both above- and underground biomasses, as estimated at the early flowering stage. We then performed RNA-seq analyses of shoot apical meristems (SAMs) from six-leaf-aged seedlings under control (CK), low (LS, 25 mmol L^-1), and high (HS, 180 mmol L^-1) salinity treatments in the two varieties. The GO and KEGG enrichment analyses of differentially expressed genes (DEGs) demonstrated that such a stimulating effect on seedling growth by low salinity stress may be caused by a more efficient capacity for photosynthesis as compensation, accompanied by a reduced energy loss for the biosynthesis of secondary metabolites and redirecting of energy to biomass formation. Our study provides a new perspective on the cultivation of oilseed rape in saline regions and new insights into the molecular mechanisms of salt tolerance in Brassica crops. The candidate genes identified in this study can serve as targets for molecular breeding selection and genetic engineering toward enhancing salt tolerance in B. napus.

Detergent-Resistant Membranes in Chloroplasts and Mitochondria of the Halophyte Salicornia perennans under Salt Stress

Halophytes represent important models for studying the key mechanisms of salt tolerance. One approach to the development of new knowledge of salt tolerance is to study the properties of detergent-resistant membranes (DRMs). In this work, the lipid profiles of DRMs of chloroplasts and mitochondria of euhalophyte Salicornia perennans Willd, before and after their exposure to shock concentrations of NaCl, have been investigated. We found that DRMs of chloroplasts are enriched in cerebrosides (CERs) and that sterols (STs) dominate the mass of mitochondrial DRMs. Also, it has been proven that (i) the impact of salinity provokes obvious growth in the content of CERs in DRMs of chloroplasts; (ii) the content of STs in DRMs of chloroplasts does not change under the influence of NaCl; (iii) salinity also causes some elevation in the content of monounsaturated and saturated fatty acids (FAs). Considering the fact that DRMs represent integral parts of both chloroplast and mitochondrial membranes, the authors have come to the conclusion that the cells of euhalophyte S. perennans, under the impact of salinity, presumes the choice (by the cell) of some specific composition of lipids and FAs in the membrane. This may be considered as a specific protection reaction of the plant cell against salinity.

Re-thinking the environment in landscape genomics

Detecting the extrinsic selective pressures shaping genomic variation is critical for a better understanding of adaptation and for forecasting evolutionary responses of natural populations to changing environmental conditions. With increasing availability of geo-referenced environmental data, landscape genomics provides unprecedented insights into how genomic variation and underlying gene functions affect traits potentially under selection. Yet, the robustness of genotype-environment associations used in landscape genomics remains tempered due to various limitations, including the characteristics of environmental data used, sampling designs employed, and statistical frameworks applied. Here, we argue that using complementary or new environmental data sources and well-informed sampling designs may help improve the detection of selective pressures underlying patterns of local adaptation in various organisms and environments.

Coping with salt stress-interaction of halotolerant bacteria in crop plants: A mini review

Salinity is one of the major environmental abiotic stress factors that limit the growth and yield of crop plants worldwide. It is crucial to understand the importance of several adaptive mechanisms in plants toward salt stress so as to increase agricultural productivity. Plant resilience toward salinity stress is improved by cohabiting with diverse microorganisms, especially bacteria. In the last few decades, increasing attention of researchers has focused on bacterial communities for promoting plant growth and fitness. The biotechnological applications of salt-tolerant plant growth-promoting rhizobacteria (PGPR) gained widespread interest for their numerous metabolites. This review provides novel insights into the importance of halotolerant (HT) bacteria associated with crop plants in enhancing plant tolerance toward salinity stress. Furthermore, the present review highlights several challenges of using HT-PGPR in the agricultural field and possible solutions to overcome those challenges for sustainable agriculture development in the future.

Low iron ameliorates the salinity-induced growth cessation of seminal roots in wheat seedlings

Wheat plants are ubiquitously simultaneously exposed to salinity and limited iron availability caused by soil saline-alkalisation. Through this study, we found that both low Fe and NaCl severely inhibited the growth of seminal roots in wheat seedlings; however, sufficient Fe caused greater growth cessation of seminal roots than low Fe under salt stress. Low Fe improved the root meristematic division activity, not altering the mature cell sizes compared with sufficient Fe under salt stress. Foliar Fe spray and split-root experiments showed that low Fe-alleviating the salinity-induced growth cessation of seminal roots was dependent on local low Fe signals in the roots. Ionomics combined with TEM/X-ray few differences in the root Na⁺ uptake and vacuolar Na⁺ sequestration between two Fe levels under salt stress. Phytohormone profiling and metabolomics revealed salinity-induced overaccumulation of ACC/ethylene and tryptophan/auxin in the roots under sufficient Fe than under low Fe. Differential gene expression, pharmacological inhibitor addition and the root growth performance of transgenic wheat plants revealed that the rootward auxin efflux and was responsible for the low Fe-mediated amelioration of the salinity-induced growth cessation of seminal roots. Our findings will provide novel insights into the modulation of crop root growth under salt stress.

Plant growth, salt removal capacity, and forage nutritive value of the annual euhalophyte Suaeda salsa irrigated with saline water

Sustainable agricultural development in semiarid and arid regions is severely restricted by soil and water salinization. Cultivation of the representative halophyte Suaeda salsa, which can be irrigated with saline water and cultivated on saline soils, is considered to be a potential solution to the issues of freshwater scarcity, soil salinization, and fodder shortage. However, the salt removal capacity and differences in the forage nutritive value of S. salsa under different saline water treatments remain unknown. Using the methods of field trials and randomized blocks design, we quantified salt accumulation in the aboveground biomass, and the biochemical and nutritive value of field-cultivated S. salsa in arid northwestern China under irrigation with water of different salinities [i.e., freshwater or water containing10, 20, 30, or 40 g/L NaCl). The fresh and dry weights of S. salsa increased, then decreased, with increase in salinity. The salt content of the plant's aboveground biomass increased to a constant range and, thus, the salt extraction of S. salsa was relatively stable under different salinities of irrigation water. Under the experimental conditions, the crude protein content significantly increased to 9.45% dry weight (DW) and then decreased to 6.85% DW, with an increase in salinity (p < 0.05). The neutral detergent fiber (42.93%-50.00% DW) and acid detergent fiber (34.76%-39.70% DW) contents were suitable for forage. The contents of trace elements, such as copper and zinc, were significantly increased after irrigation with saline water (p < 0.05). The forage of S. salsa is of high nutritive value for livestock, and contains low concentrations of anti-nutrients. Therefore, S. salsa can be considered for cultivation in saline soils irrigated with saline water. In addition, it provides a viable additional source of fodder in arid regions, where the availability of freshwater and non-saline arable land is limited.

Ranunculus sceleratus as a Model Species to Decrypt the Role of Ethylene in Plant Adaptation to Salinity

The aim of the present study was to develop an experimental system for an exploration of ethylene-dependent responses using intact growing Ranunculus sceleratus plants and to approbate the system for assessing the role of ethylene in salinity tolerance and ion accumulation. Plants were cultivated in sealed plastic containers in a modified gaseous atmosphere by introducing ethylene or 1-methylcyclopropene (1-MCP), a competitive inhibitor of ethylene action. High humidity inside the containers induced a fast elongation of the leaf petioles of R. sceleratus. The effect was ethylene-dependent, as 1-MCP completely blocked it, but exogenous ethylene further promoted petiole elongation. Exogenous ethylene decreased (by 48%) but 1-MCP increased (by 48%) the Na⁺ accumulation in leaf blades of NaCl-treated plants. The experimental system was further calibrated with ethylene and silica xerogel, and the optimum concentrations were found for inducing leaf petiole elongation (10 μL L^-1 ethylene) and preventing leaf petiole elongation (200 g silica xerogel per 24 L), respectively. The second experiment involved a treatment with NaCl in the presence of 1-MCP, ethylene, or 1-MCP + ethylene, both in normal and high air humidity conditions. In high humidity conditions, NaCl inhibited petiole elongation by 25% and ethylene treatment fully reversed this inhibition and stimulated elongation by 12% in comparison to the response of the control plants. Treatment with 1-MCP fully prevented this ethylene effect. In normal humidity conditions, NaCl inhibited petiole elongation by 20%, which was reversed by ethylene without additional elongation stimulation. However, 1-MCP only partially inhibited the ethylene effect on petiole elongation. In high humidity conditions, ethylene inhibited Na⁺ accumulation in NaCl-treated plants by 14%, but 1-MCP reversed this effect. In conclusion, the stimulation of endogenous ethylene production in R. sceleratus plants at a high air humidity or in flooded conditions reverses the inhibitory effect of salinity on plant growth and concomitantly inhibits the accumulation of Na⁺ in tissues. R. sceleratus is a highly promising model species for use in studies regarding ethylene-dependent salinity responses and ion accumulation potential involving the manipulation of a gaseous environment.

Analysis of the main antioxidant enzymes in the roots of Tamarix ramosissima under NaCl stress by applying exogenous potassium (K+)

Introduction

Salinization affects more than 25% of the world's arable land, and Tamarix ramosissima Ledeb (T. ramosissima), the representative of Tamarix plants, is widely grown in salinized soil. In contrast, less is known about the mechanism of potassium's antioxidative enzyme activity in preventing NaCl stress damage to plants.

Method

This study examined changes in root growth for T. ramosissima at 0h, 48h, and 168h, performed antioxidant enzyme activity assays, transcriptome sequencing, and non-targeted metabolite analysis to understand changes in their roots as well as changes in the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Quantitative real-time PCR (qRT-PCR) was used to identify differentially expressed genes (DEGs) and differential metabolites associated with antioxidant enzyme activities.

Result

As the time increased, the results showed that compared with the 200 Mm NaCl group, the root growth of the 200 mM NaCl + 10 mM KCl group increased, the activities of SOD, POD and CAT increased the most, but the contents of hydrogen peroxide (H2O2) and Malondialdehyde (MDA) increased less. Meanwhile, 58 DEGs related to SOD, POD and CAT activities were changed during the application of exogenous K+ for 48h and 168h in T. ramosissima. Based on association analysis of transcriptomic and metabolomic data, we found coniferyl alcohol, which can act as a substrate to label catalytic POD. It is worth noting that Unigene0013825 and Unigene0014843, as POD-related genes, have positively regulated the downstream of coniferyl alcohol, and they have a significant correlation with coniferyl alcohol.

Discussion

In summary, 48h and 168h of exogenous K⁺ applied to the roots of T. ramosissima under NaCl stress can resist NaCl stress by scavenging the reactive oxygen species (ROS) generated by high salt stress by enhancing the mechanism of antioxidant enzyme activity, relieving NaCl toxicity and maintaining growth. This study provides genetic resources and a scientific theoretical basis for further breeding of salt-tolerant Tamarix plants and the molecular mechanism of K⁺ alleviating NaCl toxicity.

Genomic analyses provide insights into the evolution and salinity adaptation of halophyte Tamarix chinensis

BACKGROUND

The woody halophyte Tamarix chinensis is a pioneer tree species in the coastal wetland ecosystem of northern China, exhibiting high resistance to salt stress. However, the genetic information underlying salt tolerance in T. chinensis remains to be seen. Here we present a genomic investigation of T. chinensis to elucidate the underlying mechanism of its high resistance to salinity.

RESULTS

Using a combination of PacBio and high-throughput chromosome conformation capture data, a chromosome-level T. chinensis genome was assembled with a size of 1.32 Gb and scaffold N50 of 110.03 Mb. Genome evolution analyses revealed that T. chinensis significantly expanded families of HAT and LIMYB genes. Whole-genome and tandem duplications contributed to the expansion of genes associated with the salinity adaptation of T. chinensis. Transcriptome analyses were performed on root and shoot tissues during salt stress and recovery, and several hub genes responding to salt stress were identified. WRKY33/40, MPK3/4, and XBAT31 were critical in responding to salt stress during early exposure, while WRKY40, ZAT10, AHK4, IRX9, and CESA4/8 were involved in responding to salt stress during late stress and recovery. In addition, PER7/27/57/73 encoding class III peroxidase and MCM3/4/5/7 encoding DNA replication licensing factor maintained up/downregulation during salt stress and recovery stages.

CONCLUSIONS

The results presented here reveal the genetic mechanisms underlying salt adaptation in T. chinensis, thus providing important genomic resources for evolutionary studies on tamarisk and plant salt tolerance genetic improvement.

Evaluation of Halophyte Biopotential as an Unused Natural Resource: The Case of Lobularia maritima

Halophytes are plant species widely distributed in saline habitats, such as beaches, postindustrial wastelands, irrigated lands, salt flats, and others. Excessive salt level, known to limit plant growth, is not harmful to halophytes, which have developed a variety of defense mechanisms allowing them to colonize harsh environments. Plants under stress are known to respond with several morpho-anatomical adaptations, but also to enhance the production of secondary metabolites to better cope with difficult conditions. Owing to these adaptations, halophytes are an interesting group of undemanding plants with a high potential for application in the food and pharmaceutical industries. Therefore, this review aims to present the characteristics of halophytes, describe changes in their gene expression, and discuss their synthesized metabolites of pharmacognostic and pharmacological significance. Lobularia maritima is characterized as a widely spread halophyte that has been shown to exhibit various pharmacological properties in vitro and in vivo. It is concluded that halophytes may become important sources of natural products for the treatment of various ailments and for supplementing the human diet with necessary non-nutrients and minerals. However, extensive studies are needed to deepen the knowledge of their biological potential in vivo, so that they can be introduced to the pharmaceutical and food industries.

Comprehensive Analysis of Transcriptome and Metabolome Elucidates the Molecular Regulatory Mechanism of Salt Resistance in Roots of Achnatherum inebrians Mediated by Epichloë gansuensis

Salinization of soil is a major environmental risk factor to plant functions, leading to a reduction of productivity of crops and forage. Epichloë gansuensis, seed-borne endophytic fungi, establishes a mutualistic symbiotic relationship with Achnatherum inebrians and confers salt tolerance in the host plants. In this study, analysis of transcriptome and metabolome was used to explore the potential molecular mechanism underlying the salt-adaptation of A. inebrians roots mediated by E. gansuensis. We found that E. gansuensis played an important role in the gene expression of the host’s roots and regulated multiple pathways involved in amino acid metabolism, carbohydrate metabolism, TCA cycle, secondary metabolism, and lipid metabolism in the roots of A. inebrians. Importantly, E. gansuensis significantly induced the biological processes, including exocytosis, glycolytic process, fructose metabolic process, and potassium ion transport in roots of host plants at transcriptional levels, and altered the pathways, including inositol phosphate metabolism, galactose metabolism, starch, and sucrose metabolism at metabolite levels under NaCl stress. These findings provided insight into the molecular mechanism of salt resistance in roots of A. inebrians mediated by E. gansuensis and could drive progress in the cultivation of new salt-resistance breeds with endophytes.

Similar Responses of Relatively Salt-Tolerant Plants to Na and K during Chloride Salinity: Comparison of Growth, Water Content and Ion Accumulation

The aim of the present study was to compare changes in growth, ion accumulation and tissue water content in relatively salt-tolerant plant taxa—Beta vulgaris subsp. maritima, Beta vulgaris subsp. vulgaris var. cicla, Cochlearia officinalis, Mentha aquatica and Plantago maritima—as a result of NaCl and KCl salinity in controlled conditions. Similar growth responses to Na⁺ and K⁺ salinity in a form of chloride salts were found for all model plants, including growth stimulation at low concentrations, an increase in water content in leaves, and growth inhibition at high salinity for less salt-resistant taxa. All plant taxa were cultivated in soil except M. aquatica, which was cultivated in hydroponics. While the morphological responses of B. vulgaris subsp. vulgaris var. cicla, B. vulgaris subsp. maritima and P. maritima plants to NaCl and KCl were rather similar, C. officinalis plants tended to perform worse when treated with KCl, but the opposite was evident for M. aquatica. Plants treated with KCl accumulated higher concentrations of K⁺ in comparison to the accumulation of Na⁺ in plants treated with equimolar concentrations of NaCl. KCl-treated plants also had higher tissue levels of electrical conductivity than NaCl-treated plants. Based on the results of the present study, it seems that both positive and negative effects of Na⁺ and K⁺ on plant growth were due to unspecific ionic effects of monovalent cations or/and the specific effect of Cl⁻.

Genome-wide analysis of tandem duplicated genes and their expression under salt stress in seashore paspalum

Seashore paspalum (Paspalum vaginatum) is a halophytic, warm-season grass which is closely related to various grain crops. Gene duplication plays an important role in plant evolution, conferring significant plant adaptation at the genomic level. Here, we identified 2,542 tandem duplicated genes (TDGs) in the P. vaginatum genome and estimated the divergence time of pairs of TDGs based on synonymous substitution rates (Ks). Expression of P. vaginatum TDGs resulted in enrichment in many GO terms and KEGG pathways when compared to four other closely-related species. The GO terms included: "ion transmembrane transporter activity," "anion transmembrane transporter activity" and "cation transmembrane transport," and KEGG pathways included "ABC transport." RNA-seq analysis of TDGs showed tissue-specific expression under salt stress, and we speculated that P. vaginatum leaves became adapted to salt stress in the earlier whole-genome duplication (WGD; ~83.3 million years ago; Ma), whereas the entire P. vaginatum plant acquired a large number of TDGs related to salt stress in the second WGD (~23.3 Ma). These results can be used as a reference resource to accelerate salt-resistance research in other grasses and crops.

Cd Phytoextraction Potential in Halophyte Salicornia fruticosa: Salinity Impact

The phytoextraction potential of halophytes has been broadly recognized. Nevertheless, the impact of salt on the accumulation proprieties of cadmium (Cd) in different halophytic species, likely linked to their salt tolerance, remains unclear. A hydroponic culture was used to investigate the impact of salinity on Cd tolerance as well as accumulation in the distinct halophyte Salicornia fruticosa (S. fruticosa). The plant was subjected to 0, 25, and 50 μg L⁻¹ Cd (0-Cd, L-Cd, and H-Cd, respectively), with or without 50, 100, and 200 mM NaCl in the nutrient solution. Data demonstrated that Cd individually induced depletion in biomass accumulation. NaCl amplified the Cd tolerance induced by enhanced biomass gaining and root length, which was associated with adequate transpiration, leaf succulence, elevated levels of ascorbic acid (ASA), reduced glutathione (GSH), phytochelatins (PCs), and proline as well as antioxidant enzymatic capacity via upregulation of peroxidases (PO), glutathione peroxidase, ascorbate peroxidase, and superoxide dismutase. All Cd treatments decreased the uptake of calcium (Ca) as well as potassium (K) and transport to the shoots; however, sodium (Na) accumulation in the shoots was not influenced by Cd. Consequently, S. fruticosa retained its halophytic properties. Based on the low transfer efficiency and high enrichment coefficient at 0–50 mM NaCl, an examination of Cd accumulation characteristics revealed that phytostabilization was the selected phytoremediation strategy. At 100–200 mM NaCl, the high aboveground Cd-translocation and high absorption efficiency encourage phytoremediation via phytoextraction. The results revealed that S. fruticosa might be also potentially utilized to renovate saline soils tainted with heavy metals (HMs) because of its maximized capacity for Cd tolerance magnified by NaCl. Cd accumulation in S. fruticosa is mainly depending on the NaCl concentration. Future studies may be established for other heavy metal pollutants screening, to detect which could be extracted and/or stabilized by the S. fruticosa plant; moreover, other substrates presenting high electrical conductivity should be identified for reclamation.

Genotypic-specific hormonal reprogramming and crosstalk are crucial for root growth and salt tolerance in bermudagrass (Cynodon dactylon)

Salt stress is one of the major abiotic factors limiting the productivity of bermudagrass (Cynodon dactylon). However, the role of hormonal reprogramming and crosstalk in regulating root growth and salt tolerance in bermudagrass was not reported. Here, we examined the physiological and hormonal responses of two contrasting bermudagrass genotypes; 'C43,' salt-tolerant 'C198' salt-sensitive. Under salt stress, 'C43' had better membrane stability and higher photosynthetic activity than the 'C198.' Salt stress promoted root growth and improved root/shoot ratio and root activity in 'C43,' but the root growth of 'C198' was inhibited by salt stress, leading to diminished root activity. The two bermudagrass genotypes also showed critical differences in hormonal responses, especially in the roots. The root contents of indole-3-acetic acid (IAA), cytokinin derivatives, such as trans-zeatin riboside (tZR) and dihydrozeatin riboside (DHZR) were increased in 'C43,' but decreased in 'C198' when exposed to salt stress. The root growth rate was positively correlated with the root IAA, tZR and DHZR, indicating their crucial role in root growth under salt stress. The expressions of TAA/YUCCA and CYP735A involved in IAA and tZR biosynthesis were induced by salt stress in 'C43,' but inhibited in 'C198,' leading to reduced hormone accumulations. Salt stress decreased the iP, tZ, and DHZ content in the roots of both genotypes, and no significant difference was observed between the two genotypes. Salt stress reduced the content of GA₃ in both genotypes by inhibiting GA20ox and GA2ox genes, which could be attributed to the reduced shoot growth in both genotypes. The increased ABA level by salt stress was significantly higher in 'C198' than 'C43.' Furthermore, there were positive and negative correlations between different hormones and root growth, suggesting that root growth could be regulated by complex hormonal reprogramming and crosstalk. This study provides a foundation for understanding the underlying mechanisms of hormonal-mediated root growth and salt tolerance in bermudagrass.

DNA Methylation in Plant Responses and Adaption to Abiotic Stresses

Due to their sessile state, plants are inevitably affected by and respond to the external environment. So far, plants have developed multiple adaptation and regulation strategies to abiotic stresses. One such system is epigenetic regulation, among which DNA methylation is one of the earliest and most studied regulatory mechanisms, which can regulate genome functioning and induce plant resistance and adaption to abiotic stresses. In this review, we outline the most recent findings on plant DNA methylation responses to drought, high temperature, cold, salt, and heavy metal stresses. In addition, we discuss stress memory regulated by DNA methylation, both in a transient way and the long-term memory that could pass to next generations. To sum up, the present review furnishes an updated account of DNA methylation in plant responses and adaptations to abiotic stresses.

Plant Growth-Promoting Rhizobacteria Alleviate High Salinity Impact on the Halophyte Suaeda fruticosa by Modulating Antioxidant Defense and Soil Biological Activity

Plant growth-promoting rhizobacteria (PGPR) are considered as bio-ameliorators that confer better salt resistance to host plants while improving soil biological activity. Despite their importance, data about the likely synergisms between PGPR and halophytes in their native environments are scarce. The objective of this study was to assess the effect of PGPR (Glutamicibacter sp. and Pseudomonas sp.) inoculation on biomass, nutrient uptake, and antioxidant enzymes of Suaeda fruticosa, an obligate halophyte native in salt marshes and arid areas in Tunisia. Besides, the activity of rhizospheric soil enzyme activities upon plant inoculation was determined. Plants were grown in pots filled with soil and irrigated with 600 mM NaCl for 1 month. Inoculation (either with Pseudomonas sp. or Glutamicibacter sp.) resulted in significantly higher shoot dry weight and less accumulation of Na+ and Cl- in shoots of salt-treated plants. Glutamicibacter sp. inoculation significantly reduced malondialdehyde (MDA) concentration, while increasing the activity of antioxidant enzymes (superoxide dismutase; catalase; ascorbate peroxidase; and glutathione reductase) by up to 100%. This provides strong arguments in favor of a boosting effect of this strain on S. fruticosa challenged with high salinity. Pseudomonas sp. inoculation increased shoot K+ and Ca2+ content and lowered shoot MDA concentration. Regarding the soil biological activity, Pseudomonas sp. significantly enhanced the activities of three rhizospheric soil enzymes (urease, ß-glucosidase, and dehydrogenase) as compared to their respective non-inoculated saline treatment. Hence, Pseudomonas sp. could have a great potential to be used as bio-inoculants in order to improve plant growth and soil nutrient uptake under salt stress. Indole-3-acetic acid concentration in the soil increased in both bacterial treatments under saline conditions, especially with Glutamicibacter sp. (up to +214%). As a whole, Glutamicibacter sp. and Pseudomonas sp. strains are promising candidates as part of biological solutions aiming at the phytoremediation and reclamation of saline-degraded areas.

Constitutive and Adaptive Traits of Environmental Stress Tolerance in the Threatened Halophyte Limonium angustebracteatum Erben (Plumbaginaceae)

Limonium angustebracteatum is a halophyte endemic to the E and SE Iberian Peninsula with interest in conservation. Salt glands represent an important adaptive trait in recretohalophytes like this and other Limonium species, as they allow the excretion of excess salts, reducing the concentration of toxic ions in foliar tissues. This study included the analysis of the salt gland structure, composed of 12 cells, 4 secretory and 8 accessory. Several anatomical, physiological and biochemical responses to stress were also analysed in adult plants subjected to one month of water stress, complete lack of irrigation, and salt stress, by watering with aqueous solutions of 200, 400, 600 and 800 mM NaCl. Plant growth was inhibited by the severe water deficit and, to a lesser extent, by high NaCl concentrations. A variation in the anatomical structure of the leaves was detected under conditions of salt and water stress; plants from the salt stress treatment showed salt glands sunken between epidermal cells, bordered by very large epidermal cells, whereas in those from the water stress treatment, the epidermal cells were heterogeneous in shape and size. In both, the palisade structure of the leaves was altered. Salt excretion is usually accompanied by the accumulation of salts in the foliar tissue. This was also found in L. angustebracteatum, in which the concentration of all ions analysed was higher in the leaves than in the roots. The increase of K⁺ in the roots of plants subjected to water stress was also remarkable. The multivariate analysis indicated differences in water and salt stress responses, such as the accumulation of Na and Cl, or proline, but K⁺ homeostasis played a relevant role in the mechanism of tolerance to both stressful conditions.

Full-Length Transcriptome Analysis of the Halophyte Nitraria sibirica Pall

Background: Nitraria sibirica Pall. is one of the pioneer tree species in saline–alkali areas due to its extreme salt tolerance. However, the lack of information on its genome limits the further exploration of the molecular mechanisms in N. sibirica under salt stress. Methods: In this study, we used single-molecule real-time (SMRT) technology based on the PacBio Iso-Seq platform to obtain transcriptome data from N. sibirica under salt treatment for the first time, which is helpful for our in-depth analysis of the salt tolerance and molecular characteristics of N. sibirica. Results: Our results suggested that a total of 234,508 circular consensus sequences (CCSs) with a mean read length of 2121 bp were obtained from the 19.26 Gb raw data. Furthermore, based on transcript cluster analysis, 93,713 consensus isoforms were obtained, including 92,116 high-quality isoforms. After removing redundant sequences, 49,240 non-redundant transcripts were obtained from high-quality isoforms. A total of 37,261 SSRs, 1816 LncRNAs and 47,314 CDSs, of which 40,160 carried complete ORFs, were obtained. Based on our transcriptome data, we also analyzed the coding genes of H+-PPase, and the results of both bioinformatics and functional analyses indicated that the gene prediction via full-length transcripts obtained by SMRT technology is reliable and effective. In summary, our research data obtained by SMRT technology provides more reliable and accurate information for the further analysis of the regulatory network and molecular mechanism of N. sibirica under salt stress.

Biological and Agronomic Traits of the Main Halophytes Widespread in the Mediterranean Region as Potential New Vegetable Crops

Salinity is one of the oldest and most serious environmental problems in the world. The increasingly widespread salinization of soils and water resources represents a growing threat to agriculture around the world. A strategy to cope with this problem is to cultivate salt-tolerant crops and, therefore, it is necessary to identify plant species that are naturally adapted to high-salinity conditions. In this review, we focus our attention on some plant species that can be considered among the most representative halophytes of the Mediterranean region; they can be potential resources, such as new or relatively new vegetable crops, to produce raw or minimally processed (or ready-to-eat) products, considering their nutritional properties and nutraceuticals. The main biological and agronomic characteristics of these species and the potential health risks due to mycotoxigenic fungi have been analyzed and summarized in a dedicated section. The objective of this review is to illustrate the main biological and agronomical characteristics of the most common halophytic species in the Mediterranean area, which could expand the range of leafy vegetables on the market.

Ethanol Treatment Enhances Physiological and Biochemical Responses to Mitigate Saline Toxicity in Soybean

Soil salinity, a major environmental concern, significantly reduces plant growth and production all around the world. Finding solutions to reduce the salinity impacts on plants is critical for global food security. In recent years, the priming of plants with organic chemicals has shown to be a viable approach for the alleviation of salinity effects in plants. The current study examined the effects of exogenous ethanol in triggering salinity acclimatization responses in soybean by investigating growth responses, and numerous physiological and biochemical features. Foliar ethanol application to saline water-treated soybean plants resulted in an enhancement of biomass, leaf area, photosynthetic pigment contents, net photosynthetic rate, shoot relative water content, water use efficiency, and K⁺ and Mg²⁺ contents, leading to improved growth performance under salinity. Salt stress significantly enhanced the contents of reactive oxygen species (ROS), malondialdehyde, and electrolyte leakage in the leaves, suggesting salt-induced oxidative stress and membrane damage in soybean plants. In contrast, ethanol treatment of salt-treated soybean plants boosted ROS-detoxification mechanisms by enhancing the activities of antioxidant enzymes, including peroxidase, ascorbate peroxidase, catalase, and glutathione S-transferase. Ethanol application also augmented the levels of proline and total free amino acids in salt-exposed plants, implying a role of ethanol in maintaining osmotic adjustment in response to salt stress. Notably, exogenous ethanol decreased Na⁺ uptake while increasing K⁺ and Mg²⁺ uptake and their partitioning to leaves and roots in salt-stressed plants. Overall, our findings reveal the protective roles of ethanol against salinity in soybean and suggest that the use of this cost-effective and easily accessible ethanol in salinity mitigation could be an effective approach to increase soybean production in salt-affected areas.

Melatonin increases growth and salt tolerance of Limonium bicolor by improving photosynthetic and antioxidant capacity

BACKGROUND

Soil salinization is becoming an increasingly serious problem worldwide, resulting in cultivated land loss and desertification, as well as having a serious impact on agriculture and the economy. The indoleamine melatonin (N-acetyl-5-methoxytryptamine) has a wide array of biological roles in plants, including acting as an auxin analog and an antioxidant. Previous studies have shown that exogenous melatonin application alleviates the salt-induced growth inhibition in non-halophyte plants; however, to our knowledge, melatonin effects have not been examined on halophytes, and it is unclear whether melatonin provides similar protection to salt-exposed halophytic plants.

RESULTS

We exposed the halophyte Limonium bicolor to salt stress (300 mM) and concomitantly treated the plants with 5 μM melatonin to examine the effect of melatonin on salt tolerance. Exogenous melatonin treatment promoted the growth of L. bicolor under salt stress, as reflected by increasing its fresh weight and leaf area. This increased growth was caused by an increase in net photosynthetic rate and water use efficiency. Treatment of salt-stressed L. bicolor seedlings with 5 μM melatonin also enhanced the activities of antioxidants (superoxide dismutase [SOD], peroxidase [POD], catalase [CAT], and ascorbate peroxidase [APX]), while significantly decreasing the contents of hydrogen peroxide (H₂O₂), superoxide anion (O₂^•-), and malondialdehyde (MDA). To screen for L. bicolor genes involved in the above physiological processes, high-throughput RNA sequencing was conducted. A gene ontology enrichment analysis indicated that genes related to photosynthesis, reactive oxygen species scavenging, the auxin-dependent signaling pathway and mitogen-activated protein kinase (MAPK) were highly expressed under melatonin treatment. These data indicated that melatonin improved photosynthesis, decreased reactive oxygen species (ROS) and activated MAPK-mediated antioxidant responses, triggering a downstream MAPK cascade that upregulated the expression of antioxidant-related genes. Thus, melatonin improves the salt tolerance of L. bicolor by increasing photosynthesis and improving cellular redox homeostasis under salt stress.

CONCLUSIONS

Our results showed that melatonin can upregulate the expression of genes related to photosynthesis, reactive oxygen species scavenging and mitogen-activated protein kinase (MAPK) of L. bicolor under salt stress, which can improve photosynthesis and antioxidant enzyme activities. Thus melatonin can promote the growth of the species and maintain the homeostasis of reactive oxygen species to alleviate salt stress.