Beneficial Effects of Salt on Halophyte Growth: Morphology, Cells, and Genes

Exogenous Sodium and Calcium Alleviate Drought Stress by Promoting the Succulence of Suaeda salsa

Succulence is a key trait involved in the response of Suaeda salsa to salt stress. However, few studies have investigated the effects of the interaction between salt and drought stress on S. salsa growth and succulence. In this study, the morphology and physiology of S. salsa were examined under different salt ions (Na+, Ca2+, Mg2+, Cl-, and SO42-) and simulated drought conditions using different polyethylene glycol concentrations (PEG; 0%, 5%, 10%, and 15%). The results demonstrate that Na+ and Ca2+ significantly increased leaf succulence by increasing leaf water content and enlarging epidermal cell size compared to Mg2+, Cl-, and SO42-. Under drought (PEG) stress, with an increase in drought stress, the biomass, degree of leaf succulence, and water content of S. salsa decreased significantly in the non-salt treatment. However, with salt treatment, the results indicated that Na+ and Ca2+ could reduce water stress due to drought by stimulating the succulence of S. salsa. In addition, Na+ and Ca2+ promoted the activity of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), which could reduce oxidative stress. In conclusion, Na+ and Ca2+ are the main factors promoting succulence and can effectively alleviate drought stress in S. salsa.

Effect of Na, K and Ca Salts on Growth, Physiological Performance, Ion Accumulation and Mineral Nutrition of Mesembryanthemum crystallinum

Mesembryanthemum crystallinum L. is an obligatory halophyte species showing optimum growth at elevated soil salinity levels, but the ionic requirements for growth stimulation are not known. The aim of the present study was to compare the effects of sodium, potassium and calcium in the form of chloride and nitrate salts on the growth, physiological performance, ion accumulation and mineral nutrition of M. crystallinum plants in controlled conditions. In a paradoxical way, while sodium and potassium had comparable stimulative effect on plant growth, the effect of calcium was strongly negative even at a relatively low concentration, eventually leading to plant death. Moreover, the effect of Ca nitrate was less negative in comparison to that of Ca chloride, but K in the form of nitrate had some negative effects. There were three components of the stimulation of biomass accumulation by NaCl and KCl salinity in M. crsytallinum: the increase in tissue water content, increase in ion accumulation, and growth activation. As optimum growth was in a salinity range from 20 to 100 mM, the increase in the dry biomass of plants at a moderate (200 mM) and high (400 mM) salinity in comparison to control plants was mostly due to ion accumulation. Among physiological indicators, changes in leaf chlorophyll concentration appeared relatively late, but the chlorophyll a fluorescence parameter, Performance Index Total, was the most sensitive to the effect of salts. In conclusion, both sodium and potassium in the form of chloride salts are efficient in promoting the optimum growth of M. crystallinum plants. However, mechanisms leading to the negative effect of calcium on plants need to be assessed further.

Nocardiopsis suaedae sp. nov. and Nocardiopsis endophytica sp. nov., two novel halophilic actinobacteria isolated from halophytes

A polyphasic approach was used to describe two halophilic actinobacterial strains, designated LSu2-4^T and RSe5-2^T, which were isolated from halophytes [Suaeda maritima (L.) Dum. and Sesuvium portulacastrum (L.) L.] collected from Prachuap Khiri Khan province, Thailand. Comparative analysis of 16S rRNA gene sequences showed that strains LSu2-4^T and RSe5-2^T were assigned to the genus Nocardiopsis, with Nocardiopsis chromatogenes YIM 90109^T(99.2 and 99.2 % similarities, respectively) and Nocardiopsis halophila DSM 44494^T(99.0 and 98.8 % similarities, respectively) being their closely related strains. Whereas the 16S rRNA gene sequence similarity between LSu2-4^T and RSe5-2^T was 99.4 %. Phylogenetic and phylogenomic analyses based on 16S rRNA gene and whole-genome sequences revealed that both strains clustered with N. chromatogenes YIM 90109^T and N. halophila DSM 44494^T. The average nucleotide identity (ANI) based on blast, ANI based on MUMmer and digital DNA-DNA hybridization (dDDH) relatedness values between the two strains and their closest type strains were below the threshold values for identifying a novel species. Morphological characteristics and chemotaxonomic features of both strains were typical for the genus Nocardiopsis by formed well-developed substrate mycelia and aerial mycelia which fragmented into rod-shaped spores. Whole-cell hydrolysates contained meso-diaminopimelic acid as the diagnostic diamino acid. The predominant menaquinones were variously hydrogenated with 10 isoprene units and contained phosphatidylcholine in their polar lipid profiles. Major fatty acids were iso-C_16:0 and 10-methyl C_18:0. In silico analysis predicted that the genomes of LSu2-4^T and RSe5-2^T contained genes associated with stress responses and biosynthetic gene clusters encoding diverse bioactive metabolites. Characterization based on chemotaxonomic, phenotypic, genotypic and phylogenetic evidence demonstrated that strains LSu2-4^T and RSe5-2^T represents two novel species of the genus Nocardiopsis, for which the names Nocardiopsis suaedae sp. nov. (type strain LSu2-4^T=TBRC 16415^T=NBRC 115855^T) and Nocardiopsis endophytica sp. nov. (type strain RSe5-2^T=TBRC 16416^T=NBRC 115856^T) are proposed.

Response to Hypersalinity of Four Halophytes Growing in Hydroponic Floating Systems: Prospects in the Phytomanagement of High Saline Wastewaters and Extreme Environments

Hypersaline environments occur naturally worldwide in arid and semiarid regions or in artificial areas where the discharge of highly saline wastewaters, such as produced water (PW) from oil and gas industrial setups, has concentrated salt (NaCl). Halophytes can tolerate high NaCl concentrations by adopting ion extrusion and inclusion mechanisms at cell, tissue, and organ levels; however, there is still much that is not clear in the response of these plants to salinity and completely unknown issues in hypersaline conditions. Mechanisms of tolerance to saline and hypersaline conditions of four different halophytes (Suaeda fruticosa (L.) Forssk, Halocnemum strobilaceum (Pall.) M. Bieb., Juncus maritimus Lam. and Phragmites australis (Cav.) Trin. ex Steudel) were assessed by analysing growth, chlorophyll fluorescence and photosynthetic pigment parameters, nutrients, and sodium (Na) uptake and distribution in different organs. Plants were exposed to high saline (257 mM or 15 g L^-1 NaCl) and extremely high or hypersaline (514, 856, and 1712 mM or 30, 50, and 100 g L^-1 NaCl) salt concentrations in a hydroponic floating culture system for 28 days. The two dicotyledonous S. fruticosa and H. strobilaceum resulted in greater tolerance to hypersaline concentrations than the two monocotyledonous species J. maritimus and P. australis. Plant biomass and major cation (K, Ca, and Mg) distributions among above- and below-ground organs evidenced the osmoprotectant roles of K in the leaves of S. fruticosa, and of Ca and Mg in the leaves and stem of H. strobilaceum. In J. maritimus and P. australis the rhizome modulated the reduced uptake and translocation of nutrients and Na to shoot with increasing salinity levels. S. fruticosa and H. strobilaceum absorbed and accumulated elevated Na amounts in the aerial parts at all the NaCl doses tested, with high bioaccumulation (from 0.5 to 8.3) and translocation (1.7-16.2) factors. In the two monocotyledons, Na increased in the root and rhizome with the increasing concentration of external NaCl, dramatically reducing the growth in J. maritimus at both 50 and 100 g L^-1 NaCl and compromising the survival of P. australis at 30 g L^-1 NaCl and over after two weeks of treatment.

Bracelet salt glands of the recretohalophyte Limonium bicolor: Distribution, morphology, and induction

Halophytes complete their life cycles in saline environments. The recretohalophyte Limonium bicolor has evolved a specialized salt secretory structure, the salt gland, which excretes Na⁺ to avoid salt damage. Typical L. bicolor salt glands consist of 16 cells with four fluorescent foci and four secretory pores. Here, we describe a special type of salt gland at the base of the L. bicolor leaf petiole named bracelet salt glands due to their beaded-bracelet-like shape of blue auto-fluorescence. Bracelet salt glands contain more than 16 cells and more than four secretory pores. Leaf disc secretion measurements and non-invasive micro-test techniques indicated that bracelet salt glands secrete more salt than normal salt glands, which helps maintain low Na⁺ levels at the leaf blade to protect the leaf. Cytokinin treatment induced bracelet salt gland differentiation, and the developed ones showed no further differentiation when traced with a living fluorescence microscopy imager, even though new salt gland development and leaf expansion were observed. Transcriptome revealed a NAC transcription factor gene that participates in bracelet salt gland development, as confirmed by its genome editing and overexpression in L. bicolor. These findings shed light on bracelet salt gland development and may facilitate the engineering of salt-tolerant crops.

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.

Type of Anion Largely Determines Salinity Tolerance in Four Rumex Species

The aim of the present study was to compare the effect of various salts composed of different cations (Na⁺, K⁺) and anions (chloride, nitrate, nitrite) on growth, development and ion accumulation in three Rumex species with accessions from sea coast habitats (Rumex hydrolapathum, Rumex longifolius and Rumex maritimus) and Rumex confertus from an inland habitat. Plants were cultivated in soil in an experimental automated greenhouse during the autumn-winter season. Nitrite salts strongly inhibited growth of all Rumex species, but R. maritimus was the least sensitive. Negative effects of chloride salts were rather little-pronounced, but nitrates resulted in significant growth stimulation, plant growth and development. Effects of Na⁺ and K⁺ at the morphological level were relatively similar, but treatment with K⁺ salts resulted in both higher tissue electrolyte levels and proportion of senescent leaves, especially for chloride salts. Increases in tissue water content in leaves were associated with anion type, and were most pronounced in nitrate-treated plants, resulting in dilution of electrolyte concentration. At the morphological level, salinity responses of R. confertus and R. hydrolapathum were similar, but at the developmental and physiological level, R. hydrolapathum and R. maritimus showed more similar salinity effects. In conclusion, the salinity tolerance of all coastal Rumex species was high, but the inland species R. confertus was the least tolerant to salinity. Similarity in effects between Na⁺ and K⁺ could be related to the fact that surplus Na⁺ and K⁺ has similar fate (including mechanisms of uptake, translocation and compartmentation) in relatively salt-tolerant species. However, differences between various anions are most likely related to differences in physiological functions and metabolic fate of particular ions.

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⁻.

Effect of Salinity on Leaf Functional Traits and Chloroplast Lipids Composition in Two C3 and C4 Chenopodiaceae Halophytes

Salt stress is one of the most common abiotic kinds of stress. Understanding the key mechanisms of salt tolerance in plants involves the study of halophytes. The effect of salinity was studied in two halophytic annuals of Chenopodiaceae Salicornia perennans Willd. and Climacoptera crassa (Bied.) Botsch. These species are plants with C₃ and C₄-metabolism, respectively. We performed a comprehensive analysis of the photosynthetic apparatus of these halophyte species at different levels of integration. The C₃ species S. perennans showed larger variation in leaf functional traits-both at the level of cell morphology and membrane system (chloroplast envelope and thylakoid). S. perennans also had larger photosynthetic cells, by 10-15 times, and more effective mechanisms of osmoregulation and protecting cells against the toxic effect of Na⁺. Salinity caused changes in photosynthetic tissues of C. crassa such as an increase of the mesophyll cell surface, the expansion of the interface area between mesophyll and bundle sheath cells, and an increase of the volume of the latter. These functional changes compensated for scarce CO₂ supply when salinity increased. Overall, we concluded that these C₃ and C₄ Chenopodiaceae species demonstrated different responses to salinity, both at the cellular and subcellular levels.

Analysis of Amino Acids in the Roots of Tamarix ramosissima by Application of Exogenous Potassium (K+) under NaCl Stress

Soil salinization is one of the main environmental factors affecting plant growth worldwide. Tamarix ramosissima Ledeb. (T. ramosissima) is a halophyte representative that is widely grown in salinized soils. As an important nutrient element for plant growth, K⁺ plays an important role in improving the tolerance to salt stress, but the mechanism of reducing the damage caused by NaCl stress to T. ramosissima is less reported. Our results show that the proline content and the Log₂ fold-change of proline's relative quantification in the roots of T. ramosissima increased over time with the application of exogenous potassium (K⁺) for 48 h and 168 h under NaCl stress. Moreover, 13 amino-acid-related metabolic pathways were involved in the resistance of T. ramosissima to salt stress. Mainly, the aldehyde dehydrogenase family genes and tryptophan-synthase-related genes were found at 48 h and 168 h with exogenous potassium applied to the roots of T. ramosissima under NaCl stress, and they regulated their related metabolic accumulation in the arginine and proline metabolism pathways, increasing the effectiveness of inducing NaCl tolerance of T. ramosissima. It is noteworthy that alpha-ketobutyric was produced in the roots of T. ramosissima under NaCl stress for 48 h with the application of exogenous potassium, which is one of the most effective mechanisms for inducing salt tolerance in plants. Meanwhile, we found three DEGs regulating alpha-ketobutyric acid. This study provides a scientific theoretical basis for further understanding the molecular mechanism of K⁺ alleviating the salinity damage to T. ramosissima caused by NaCl.

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.

Transcriptome and Metabolome Analyses Reveal Potential Salt Tolerance Mechanisms Contributing to Maintenance of Water Balance by the Halophytic Grass Puccinellia nuttalliana

Elevated soil salinity exacerbated by human activities and global climate change poses serious threats to plant survival. Although halophytes provide many important clues concerning salt tolerance in plants, some unanswered questions remain to be addressed, including the processes of water and solute transport regulation. We performed high-throughput RNA-sequencing in roots and metabolome characterizations in roots and leaves of Puccinellia nuttalliana halophytic grass subjected to 0 (control) and 150 mM NaCl. In RNAseq, a total of 31 Gb clean bases generated were de novo assembled into 941,894 transcripts. The PIP2;2 and HKT1;5 transcript levels increased in response to the NaCl treatment implying their roles in water and ion homeostasis. Several transcription factors, including WRKY39, DEK3, HY5, and ABF2, were also overexpressed in response to NaCl. The metabolomic analysis revealed that proline and dopamine significantly increased due to the upregulation of the pathway genes under salt stress, likely contributing to salt tolerance mechanisms. Several phosphatidylcholines significantly increased in roots suggesting that the alterations of membrane lipid composition may be an important strategy in P. nuttalliana for maintaining cellular homeostasis and membrane integrity under salt stress. In leaves, the TCA cycle was enriched suggesting enhanced energy metabolism to cope with salt stress. Other features contributing to the ability of P. nuttalliana to survive under high salinity conditions include salt secretion by the salt glands and enhanced cell wall lignification of the root cells. While most of the reported transcriptomic, metabolomics, and structural alterations may have consequences to water balance maintenance by plants under salinity stress, the key processes that need to be further addressed include the role of the changes in the aquaporin gene expression profiles in the earlier reported enhancement of the aquaporin-mediated root water transport.

Comparative genome-wide analysis of WRKY, MADS-box and MYB transcription factor families in Arabidopsis and rice

Transcription factors (TFs) form the major class of regulatory genes and play key roles in multiple plant stress responses. In most eukaryotic plants, transcription factor (TF) families (WRKY, MADS-box and MYB) activate unique cellular-level abiotic and biotic stress-responsive strategies, which are considered as key determinants for defense and developmental processes. Arabidopsis and rice are two important representative model systems for dicot and monocot plants, respectively. A comprehensive comparative study on 101 OsWRKY, 34 OsMADS box and 122 OsMYB genes (rice genome) and, 71 AtWRKY, 66 AtMADS box and 144 AtMYB genes (Arabidopsis genome) showed various relationships among TFs across species. The phylogenetic analysis clustered WRKY, MADS-box and MYB TF family members into 10, 7 and 14 clades, respectively. All clades in WRKY and MYB TF families and almost half of the total number of clades in the MADS-box TF family are shared between both species. Chromosomal and gene structure analysis showed that the Arabidopsis-rice orthologous TF gene pairs were unevenly localized within their chromosomes whilst the distribution of exon–intron gene structure and motif conservation indicated plausible functional similarity in both species. The abiotic and biotic stress-responsive cis-regulatory element type and distribution patterns in the promoter regions of Arabidopsis and rice WRKY, MADS-box and MYB orthologous gene pairs provide better knowledge on their role as conserved regulators in both species. Co-expression network analysis showed the correlation between WRKY, MADs-box and MYB genes in each independent rice and Arabidopsis network indicating their role in stress responsiveness and developmental processes.

SbCASP4 improves salt exclusion by enhancing the root apoplastic barrier

SbCASP4 improves the salt tolerance of sweet sorghum [Sorghum bicolor (L.) Mocnch] by enhancing the root apoplastic barrier and blocking the transport of sodium ions to the shoot. Sweet sorghum [Sorghum bicolor (L.) Mocnch] is a C4 crop with high biomass and tolerance to abiotic stresses such as salt, drought, and waterlogging. Sweet sorghum is widely used in bioenergy production, as a forage crop, and in liquors and beer. Root salt exclusion has been reported to underlie the salt tolerance of sweet sorghum. The Casparian strip has a key role in root salt exclusion, and the membrane domain protein (CASP) family participates in Casparian strip aggregation. However, the function and the regulatory mechanisms of SbCASP in response to salt stress in sweet sorghum are unclear. In the current study, we cloned SbCASP4 and determined that it is induced by salt stress and expressed in the endodermis cells of sweet sorghum. Histochemical staining and physiological indicators showed that heterologous expression of SbCASP4 significantly increased the tolerance to salt stress in transgenic Arabidopsis thaliana. Compared with wild type and casp5 mutants, under 50 mM NaCl treatment, SbCASP4-expression lines had the less leaf Na⁺, lower PI accumulation in stele, smaller oxidative damage and higher salinity threshold, longer root length and higher expression levels of the genes related to Casparian strip formation.

Understanding the mechanistic basis of adaptation of perennial Sarcocornia quinqueflora species to soil salinity

Succulent halophytes can be used as convenient models for understanding the mechanistic basis of plant adaptation to salt stress. In this work, effects of salinity (0-1000 mM NaCl range) on growth, ion accumulation, and stomatal features were investigated in the succulent halophyte Sarcocornia quinqueflora. Elevated salinity levels up to 400 mM NaCl largely promoted dry matter yield, succulence, shoot surface area, and stomatal characteristics. Plant growth was optimal at 200 mM NaCl and reduced at concentrations exceeding 600 mM NaCl. Osmotic adjustment in a succulent shoot was achieved by a massive accumulation of inorganic ions, with Na⁺ and Cl^- contributing approximately 85% of its osmolality, while organic compatible solutes and K⁺ were responsible for only approximately 15%. Shoot K⁺ was unchanged across the entire range of salinity treatments (200-1000 mM NaCl) and positively correlated with the transpiration rate (R = 0.98). Carbohydrates were not reduced at high salinity compared to plants at optimal conditions, implying that growth retardation at severe salt dosages was attributed to limitations in a vacuolar Na⁺ and Cl^- sequestrations capacity rather than inadequate photosynthesis and/or substrate limitation. It is concluded that the superior salt tolerance of S. quinqueflora is achieved by the effective reliance on Na⁺ and Cl^- accumulation for osmoregulation and turgor maintenance, and efficient K⁺ homeostasis for adequate stomatal functioning over the entire salinity range. The above findings could be instrumental in developing strategies to improve salinity stress tolerance in perennial horticultural crops and optimize their water-use efficiency.

Salt-tolerance screening in Limonium sinuatum varieties with different flower colors

Limonium sinuatum, a member of Plumbaginaceae commonly known as sea lavender, is widely used as dried flower. Five L. sinuatum varieties with different flower colors (White, Blue, Pink, Yellow, and Purple) are found in saline regions and are widely cultivated in gardens. In the current study, we evaluated the salt tolerance of these varieties under 250 mmol/L NaCl (salt-tolerance threshold) treatment to identify the optimal variety suitable for planting in saline lands. After the measurement of the fresh weight (FW), dry weight (DW), contents of Na⁺, K⁺, Ca²⁺, Cl^-, malondialdehyde (MDA), proline, soluble sugars, hydrogen peroxide (H₂O₂), relative water content, chlorophyll contents, net photosynthetic rate, and osmotic potential of whole plants, the salt-tolerance ability from strongest to weakest is identified as Pink, Yellow, Purple, White, and Blue. Photosynthetic rate was the most reliable and positive indicator of salt tolerance. The density of salt glands showed the greatest increase in Pink under NaCl treatment, indicating that Pink adapts to high-salt levels by enhancing salt gland formation. These results provide a theoretical basis for the large-scale planting of L. sinuatum in saline soils in the future.

Roles of Phytohormones and Their Signaling Pathways in Leaf Development and Stress Responses

Phytohormones participate in various processes over the course of a plant's lifecycle. In addition to the five classical phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and ethylene), phytohormones such as brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and peptides also play important roles in plant growth and stress responses. Given the highly interconnected nature of phytohormones during plant development and stress responses, it is challenging to study the biological function of a single phytohormone in isolation. In the current Review, we describe the combined functions and signaling cascades (especially the shared points and pathways) of various phytohormones in leaf development, in particular, during leaf primordium initiation and the establishment of leaf polarity and leaf morphology as well as leaf development under various stress conditions. We propose a model incorporating the roles of multiple phytohormones in leaf development and stress responses to illustrate the underlying combinatorial signaling pathways. This model provides a reference for breeding stress-resistant crops.

Progress in understanding salt stress response in plants using biotechnological tools

Salinization is a worldwide environmental problem, which is negatively impacting crop yield and thus posing a threat to the world's food security. Considering the rising threat of salinity, it is need of time, to understand the salt tolerant mechanism in plants and find avenues for the development of salinity resistant plants. Several plants tolerate salinity in a different manner, thereby halophytes and glycophytes evolved altered mechanisms to counter the stress. Therefore, in this review article, physiological, metabolic, and molecular aspects of the plant adaptation to salt stress have been discussed. The conventional breeding techniques for developing salt tolerant plants has not been much successful, due to its multigenic trait. The inflow of data from plant sequencing projects and annotation of genes led to the identification of many putative genes having a role in salt stress. The bioinformatics tools provided preliminary information and were helpful for making salt stress-specific databases. The microRNA identification and characterization led to unraveling the finer intricacies of the network. The transgenic approach finally paved a way for overexpressing some important genes viz. DREB, MYB, COMT, SOS, PKE, NHX, etc. conferred salt stress tolerance. In this review, we tried to show the effect of salinity on plants, considering ion homeostasis, antioxidant defense response, proteins involved, possible utilization of transgenic plants, and bioinformatics for coping with this stress factor. An overview of previous studies related to salt stress is presented in order to assist researchers in providing a potential solution for this increasing environmental threat.

Effect of arbuscular mycorrhizal symbiosis on ion homeostasis and salt tolerance-related gene expression in halophyte Suaeda salsa under salt treatments

Halophytes can remove large quantities of salts from saline soils, so their importance in ecology has received increasing attention. Preliminary studies have shown that arbuscular mycorrhizal (AM) fungi can improve the salt tolerance of halophytes. However, few studies have focused on the molecular mechanisms and effects of AM fungi in halophytes under different salt conditions. A pot experiment was carried out to investigate the effects of Funneliformis mosseae inoculation on growth, nutrient uptake, ion homeostasis and the expression of salt tolerance-related genes in Suaeda salsa under 0, 100, 200 and 400 mM NaCl. The results showed that F. mosseae promoted the growth of S. salsa and increased the shoot Ca²⁺ and Mg²⁺ concentrations under no-salt condition and high-salt condition. In addition, AM fungi increased the K⁺ concentration and maintained a high K⁺/Na⁺ ratio at 400 mM NaCl, while AM fungi decreased the K⁺ concentration and reduced the K⁺/Na⁺ ratio at 0 mM NaCl. AM fungi downregulated the expression of SsNHX1 in shoots and the expression of SsSOS1 in roots at 400 mM NaCl. These effects may decrease the compartmentation of Na⁺ into leaf vacuoles and restrict Na⁺ transport from roots to shoots, leading to an increase in root Na⁺ concentration. AM symbiosis upregulated the expression of SsSOS1 in shoots and downregulated the expression of SsSOS1 and SsNHX1 in roots at 100 mM NaCl. However, regulation of the genes (SsNHX1, SsSOS, SsVHA-B and SsPIP) was not significantly different with AM symbiosis at 0 mM or 200 mM NaCl. The results revealed that AM symbiosis might induce diverse modulation strategies in S. salsa, depending on external Na⁺ concentrations. These findings suggest that AM fungi may play significant ecological roles in the phytoremediation of salinized ecosystems.

Heterologous expression of the Limonium bicolor MYB transcription factor LbTRY in Arabidopsis thaliana increases salt sensitivity by modifying root hair development and osmotic homeostasis

Arabidopsis thaliana TRY is a negative regulator of trichome differentiation that promotes root hair differentiation. Here, we established that LbTRY, from the recretohalophyte Limonium bicolor, is a typical MYB transcription factor that exhibits transcriptional activation activity and locates in nucleus. By in situ hybridization in L. bicolor, LbTRY may be specifically positioned in salt gland of the expanded leaves. LbTRY expression was the highest in mature leaves and lowest under NaCl treatment. For functional assessment, we heterologously expressed LbTRY in wild-type and try29760 mutant Arabidopsis plants. Epidermal differentiation was remarkably affected in the transgenic wild-type line, as was increased root hair development. Complementation of try29760 with LbTRY under both 35S and LbTRY specific promoter restored the wild-type phenotype. qRT-PCR analysis suggested that AtGL3 and AtZFP5 promote root hair cell fate in lines heterologously producing LbTRY. In addition, four genes (AtRHD6, AtRSL1, AtLRL2, and AtLRL3) involved in root hair initiation and elongation were upregulated in the transgenic lines. Furthermore, LbTRY specifically increased the salt sensitivity of the transgenic lines. The transgenic and complementation lines showed poor germination rates and reduced root lengths, whereas the mutant unexpectedly fared the best under a range of NaCl treatments. Under salt stress, the transgenic seedlings accumulated more MDA and Na⁺ and less proline and soluble sugar than try29760. Thus, when heterologously expressed in Arabidopsis, LbTRY participates in hair development, similar to other MYB proteins, and specifically reduces salt tolerance by increasing ion accumulation and reducing osmolytes. The expression of salt-tolerance marker genes (SOS1, SOS2, SOS3 and P5CS1) was significant reduced in the transgenic lines. More will be carried by downregulating expression of TRY homologs in crops to improve salt tolerance.

Salt glands of recretohalophyte Tamarix under salinity: Their evolution and adaptation

Here, we studied the evolution of salt glands in 11 species of Tamarix and determined their role in adaptation to saline environments by measuring the effect of NaCl on plant growth and salt gland characteristics. Cluster analysis divided Tamarix species into three types (types I-III) according to salt-gland characteristics. A phylogenetic tree based on ITS sequences indicated an evolutionary relationship consistent with the geographical distribution of Tamarix. We measured growth under different NaCl conditions (0, 100, 200, and 300 mM) for 40 days in three species (T. gallica, T. ramosissima, and T. laxa) representing the three Tamarix types. With increasing NaCl concentration, the biomass of all species was significantly reduced, especially that of T. gallica. Salt secretion ability and salt-gland density showed similar trends in three types. The order of salt tolerance was type I > type II > type III. We conclude that during Tamarix adaptation to salinity, salt-gland evolution followed two directions: one increasing salt-gland density, and the other increasing salt secretion rate per salt-gland. This study provides a basis for potential mechanisms of recretohalophyte adaptation to salinity.