Salt-Enhanced Reproductive Development of Suaeda salsa L. Coincided With Ion Transporter Gene Upregulation in Flowers and Increased Pollen K+ Content

High-throughput 16S rRNA gene-based amplicon sequencing reveals the functional divergence of halophilic bacterial communities in the Suaeda salsa root compartments on the eastern coast of China

The rhizosphere environment of plants, which harbors halophilic bacterial communities, faces significant challenges in coping with environmental stressors, particularly saline soil properties. This study utilizes a high-throughput 16S rRNA gene-based amplicon sequencing to investigate the variations in bacterial community dynamics in rhizosphere soil (RH), root surface soil (RS), root endophytic bacteria (PE) compartments of Suaeda salsa roots, and adjoining soils (CK) across six locations along the eastern coast of China: Nantong (NT), Yancheng (YC), Dalian (DL), Tianjin (TJ), Dongying (DY), and Qingdao (QD), all characterized by chloride-type saline soil. Variations in the physicochemical properties of the RH compartment were also evaluated. The results revealed significant changes in pH, electrical conductivity, total salt content, and ion concentrations in RH samples from different locations. Notably, the NT location exhibited the highest alkalinity and nitrogen availability. The pH variations were linked to HCO3- accumulation in S. salsa roots, while salinity stress influenced soil pH through H+ discharge. Despite salinity stress, enzymatic activities such as catalase and urease were higher in soils from various locations. The diversity and richness of bacterial communities were higher in specific locations, with Proteobacteria dominating PE samples from the DL location. Additionally, Vibrio and Marinobacter were prevalent in RH samples. Significant correlations were found between soil pH, salinity, nutrient content, and the abundance and diversity of bacterial taxa in RH samples. Bioinformatics analysis revealed the prevalence of halophilic bacteria, such as Bacillus, Halomonas, and Streptomyces, with diverse metabolic functions, including amino acid and carbohydrate metabolisms. Essential genes, such as auxin response factor (ARF) and GTPase-encoding genes, were abundant in RH samples, suggesting adaptive strategies for harsh environments. Likewise, proline/betaine transport protein genes were enriched, indicating potential bioremediation mechanisms against high salt stress. These findings provide insight into the metabolic adaptations facilitating resilience in saline ecosystems and contribute to understanding the complex interplay between soil conditions, bacterial communities, and plant adaptation.

Utilization of halophytes in saline agriculture and restoration of contaminated salinized soils from genes to ecosystem: Suaeda salsa as an example

Halophytes can be used to screen genes for breeding salt-tolerant crops and are of great value in the restoration of salinized or contaminated soils. However, the potential of halophytes in improving saline soils remains limited. In this paper, based on the latest research progress, we use Suaeda salsa L. as an example to evaluate the value of halophytes in developing saline agriculture including: 1) some defined salt-resistance genes and high-affinity nitrate transporter genes in the species for breeding salt-tolerance and nitrogen efficiency crops; 2) the value of S. salsa and microorganisms from S. salsa in remediation of heavy metal contaminated and organic polluted saline soils; and 3) the capacity to remove salts from soils and the application of the species. In conclusion, S. salsa has high value as a candidate to explore the theoretical base and practical application for utilizing halophytes to improve salinized soils from genes to ecosystem.

The HKT1 Na+ transporter protects plant fertility by decreasing Na+ content in stamen filaments

Salinity stress can greatly reduce seed production because plants are especially sensitive to salt during their reproductive stage. Here, we show that the sodium ion transporter AtHKT1;1 is specifically expressed around the phloem and xylem of the stamen in Arabidopsis thaliana to prevent a marked decrease in seed production caused by salt stress. The stamens of AtHKT1;1 mutant under salt stress overaccumulate Na+, limiting their elongation and resulting in male sterility. Specifically restricting AtHKT1;1 expression to the phloem leads to a 1.5-fold increase in the seed yield upon sodium ion stress. Expanding phloem expression of AtHKT1;1 throughout the entire plant is a promising strategy for increasing plant productivity under salinity stress.

Conserved hierarchical gene regulatory networks for drought and cold stress response in Myrica rubra

Stress response in plant is regulated by a large number of genes co-operating in diverse networks that serve multiple adaptive process. To understand how gene regulatory networks (GRNs) modulating abiotic stress responses, we compare the GRNs underlying drought and cold stresses using samples collected at 4 or 6 h intervals within 48 h in Chinese bayberry (Myrica rubra). We detected 7,583 and 8,840 differentially expressed genes (DEGs) under drought and cold stress respectively, which might be responsive to environmental stresses. Drought- and cold-responsive GRNs, which have been built according to the timing of transcription under both abiotic stresses, have a conserved trans-regulator and a common regulatory network. In both GRNs, basic helix-loop-helix family transcription factor (bHLH) serve as central nodes. MrbHLHp10 transcripts exhibited continuous increase in the two abiotic stresses and acts upstream regulator of ASCORBATE PEROXIDASE (APX) gene. To examine the potential biological functions of MrbHLH10, we generated a transgenic Arabidopsis plant that constitutively overexpresses the MrbHLH10 gene. Compared to wild-type (WT) plants, overexpressing transgenic Arabidopsis plants maintained higher APX activity and biomass accumulation under drought and cold stress. Consistently, RNAi plants had elevated susceptibility to both stresses. Taken together, these results suggested that MrbHLH10 mitigates abiotic stresses through the modulation of ROS scavenging.

Research Advances on Molecular Mechanism of Salt Tolerance in Suaeda

Plant growth and development are inevitably affected by various environmental factors. High salinity is the main factor leading to the reduction of cultivated land area, which seriously affects the growth and yield of plants. The genus Suaeda is a kind of euhalophyte herb, with seedlings that grow rapidly in moderately saline environments and can even survive in conditions of extreme salinity. Its fresh branches can be used as vegetables and the seed oil is rich in unsaturated fatty acids, which has important economic value and usually grows in a saline environment. This paper reviews the progress of research in recent years into the salt tolerance of several Suaeda species (for example, S. salsa, S. japonica, S. glauca, S. corniculata), focusing on ion regulation and compartmentation, osmotic regulation of organic solutes, antioxidant regulation, plant hormones, photosynthetic systems, and omics (transcriptomics, proteomics, and metabolomics). It helps us to understand the salt tolerance mechanism of the genus Suaeda, and provides a theoretical foundation for effectively improving crop resistance to salt stress environments.

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

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

Salt glands play a pivotal role in the salt resistance of four recretohalophyte Limonium Mill. species

Limonium Mill. plants are typical recretohalophytes, as they withstand salt stress by secreting excess salt onto the leaf surface through salt glands. However, little is known on the salinity thresholds of these plants and the function of salt glands in salt tolerance. Here, we investigated the salinity thresholds of salt tolerance of the Limonium species L. aureum (Linn.) Hill, L. gmelinii (Willd.) Kuntze, L. otolepis (Schrenk) Kuntze and L. sinuatum (L.) Mill grown with various concentrations of NaCl. The salinity thresholds of L. otolepis, L. aureum, L. sinuatum and L. gmelinii were 300, 350, 400 and 420 mm NaCl, respectively. Correlation analysis indicated that total dry weight, chlorophyll content and intercellular CO₂ concentration were highly positively correlated with the total fresh weights of all four Limonium species and could therefore be used as indicators of plant salt tolerance. Furthermore, as the salt gland density on the leaf surface increased, the rate of salt secretion per salt gland also increased, allowing more Na⁺ to be secreted from the plant. Redundancy discriminant analysis indicated that salt gland density, Na⁺ content and Na⁺ secretion rate per salt gland were positively correlated with salt concentration. These observations support the notion that salt glands play important roles in the adaptation of Limonium species to high salinity conditions.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Current Understanding of bHLH Transcription Factors in Plant Abiotic Stress Tolerance

Named for the characteristic basic helix-loop-helix (bHLH) region in their protein structure, bHLH proteins are a widespread transcription factor class in eukaryotes. bHLHs transcriptionally regulate their target genes by binding to specific positions on their promoters and thereby direct a variety of plant developmental and metabolic processes, such as photomorphogenesis, flowering induction, shade avoidance, and secondary metabolite biosynthesis, which are important for promoting plant tolerance or adaptation to adverse environments. In this review, we discuss the vital roles of bHLHs in plant responses to abiotic stresses, such as drought, salinity, cold, and iron deficiency. We suggest directions for future studies into the roles of bHLH genes in plant and discuss their potential applications in crop breeding.

Halophytes and other molecular strategies for the generation of salt-tolerant crops

The current increase in salinity can intensify the disparity between potential and actual crop yields, thus affecting economies and food security. One of the mitigating alternatives is plant breeding via biotechnology, where advances achieved so far are significant. Considering certain aspects when developing studies related to plant breeding can determine the success and accuracy of experimental design. Besides this strategy, halophytes with intrinsic and efficient abilities against salinity can be used as models for improving the response of crops to salinity stress. As crops are mostly glycophytes, it is crucial to point out the molecular differences between these two groups of plants, which may be the key to guiding and optimizing the transformation of glycophytes with halophytic tolerance genes. Therefore, this can broaden perspectives in the trajectory of research towards the cultivation, commercialization, and consumption of salt-tolerant crops on a large scale.

Role of Suaeda salsa SsNRT2.1 in nitrate uptake under low nitrate and high saline conditions

The global annual loss in agricultural production resulting from soil salinization is significant. Although nitrate (NO₃^-) is known to play both nutritional and osmotic roles in the salt tolerance of halophytes, it remains unclear how halophytes such as Suaeda salsa L. take up NO₃^- under saline conditions. In the present study, the gene of nitrate transporter 2.1 (SsNRT2.1) was cloned from S. salsa and its function was identified in both S. salsa and Arabidopsis thaliana under salinity and low NO₃^--N (0.5 mM NO₃^-) conditions. The results revealed that SsNRT2.1 expression and NO₃^- concentration in the roots of S. salsa were higher at 200 mM NaCl, compared with that at 0 and 500 mM NaCl after 24 h treatment. The Arabidopsis overexpression lines showed a higher NO₃^- content compared to the WT lines at 0 and 50 mM NaCl. A similar trend was observed in the root length. In conclusion, salinity promoted the SsNRT2.1 expression in S. salsa, suggesting that this gene may contribute to the efficient NO₃^- uptake in S. salsa under low NO₃^- and high salinity conditions. This trait may explain why S. salsa can tolerate high salinity and produce the highest biomass at about 200 mM NaCl.

Cation and Zn Accumulation in Brown Seeds of the Euhalophyte Suaeda salsa Improves Germination Under Saline Conditions

Salinity inhibits plant growth due to salt ion accumulation in plant cells and reduced absorption of other nutrients such as metal ions; however halophyte plants have evolved mechanisms to survive and thrive in high-salt conditions. The euhalophyte Suaeda salsa generates dimorphic seeds (black and brown), which show marked differences in germination and seedling growth under high-salt conditions. However, it is unclear whether their ionic status differs. Here, to provide insight on the role of ions in salt tolerance, we used inductively coupled plasma mass spectrometry to measure the ion contents in the dimorphic seeds from S. salsa plants treated with or without NaCl. We measured the macroelements Na, K, Mg, and Ca, and the microelements Mn, Fe, Zn, Cu, and Mo. NaCl-treated S. salsa plants produced seeds with significantly reduced metallic element contents and significantly increased Na⁺ contents. The brown seeds of S. salsa plants treated with 0 and 200 mM NaCl had much higher contents of K⁺, Ca²⁺, and Fe²⁺ compared with the black seeds. However, the S. salsa seeds (both black and brown) from NaCl-treated plants were significantly larger, and had higher germination rate and higher seedling salt tolerance compared with seeds from plants not treated with NaCl. Interestingly, we measured significantly higher Zn²⁺ contents in the brown seeds from plants treated with NaCl compared with the black seeds. This suggests that the high contents of Zn²⁺ and other cations affected seed development and salt tolerance during germination under high-salt conditions. These observations provide insight into the mechanisms of salt tolerance in this halophyte and inform efforts to increase salt tolerance in salt-sensitive species.

Heavy metal tolerance and potential for remediation of heavy metal-contaminated saline soils for the euhalophyte Suaeda salsa

Due to irrigation practices and industrial pollution, large areas of the lands in the world are simultaneously affected by salinity and heavy metal contamination. It has been considered that halophytes have adapted to salinity, and can be used to remediate heavy metal-contaminated saline soils. Suaeda salsa L. (S. salsa) is a high salt-resistance plant, which can efficiently absorb and accumulate salt and toxic metals from saline soils, suggesting that this may be potential plant species that can be used for the restoration of saline soils contaminated with heavy metals. The present brief review sheds light on the characteristics of S. salsa in the uptake and accumulation of high levels of heavy metals. Furthermore, the physiological and molecular mechanisms for heavy metal tolerance were highlighted. The potential values of S. salsa in the remediation of saline soils were also summarized.

Exposure to High Salinity During Seed Development Markedly Enhances Seedling Emergence and Fitness of the Progeny of the Extreme Halophyte Suaeda salsa

Irrigation with 200 mM NaCl significantly increases vegetative and reproductive growth of the extreme halophyte Suaeda salsa. However, little is known about how the progeny of S. salsa plants grown under a continuous NaCl supply behave in terms of growth and seed set parameters. We investigated various plant growth and reproductive parameters of the progeny that germinated from seeds harvested from mother plants grown under 0 or 200 mM NaCl over three generations. Seedling emergence, plant height, stem diameter, total branch length, flowering branch length, flowering branch ratio, and seed production were all significantly enhanced in the progeny produced by mother plants grown with 200 mM NaCl compared to progeny of mother plants grown on low salinity conditions. Therefore, irrigation with 200 mM of NaCl is beneficial to seed development in the halophyte S. salsa and possibly contributes to population establishment in high salinity environments. Likewise, the prolonged absence of NaCl in the growth environment inhibits seed development, results in lower seed quality, and thus limits seedling growth of the progeny, thereby restricting S. salsa to a high salinity ecological niche.

NaCl improves reproduction by enhancing starch accumulation in the ovules of the euhalophyte Suaeda salsa

BACKGROUND

Halophytes show optimal reproduction under high-salinity conditions. However, the role of NaCl in reproduction and its possible mechanisms in the euhalophyte Suaeda salsa remain to be elucidated.

RESULTS

We performed transcript profiling of S. salsa flowers and measured starch accumulation in ovules, sugar contents in flowers, and photosynthetic parameters in the leaves of plants supplied with 0 and 200 mM NaCl. Starch accumulation in ovules, sugar contents in flowers and ovules, and net photosynthetic rate and photochemical efficiency in leaves were significantly higher in NaCl-treated plants vs. the control. We identified 14,348 differentially expressed genes in flowers of NaCl-treated vs. control plants. Many of these genes were predicted to be associated with photosynthesis, carbon utilization, and sugar and starch metabolism. These genes are crucial for maintaining photosystem structure, regulating electron transport, and improving photosynthetic efficiency in NaCl-treated plants. In addition, genes encoding fructokinase and sucrose phosphate synthase were upregulated in flowers of NaCl-treated plants.

CONCLUSIONS

The higher starch and sugar contents in the ovules and flowers of S. salsa in response to NaCl treatment are likely due to the upregulation of genes involved in photosynthesis and carbohydrate metabolism, which increase photosynthetic efficiency and accumulation of photosynthetic products under these conditions.

Improved reproductive growth of euhalophyte Suaeda salsa under salinity is correlated with altered phytohormone biosynthesis and signal transduction

Phytohormones are essential for plant reproductive growth. Salinity limits crop reproductive growth and yield, but improves reproductive growth of euhalophytes. However, little is known about the mechanisms underlying salinity's effects on plant reproductive growth. To elucidate the role of plant hormones in flower development of the euhalophyte Suaeda salsa under saline conditions, we analysed endogenous gibberellic acid (GA3,4), indoleacetic acid (IAA), zeatin riboside (ZR), abscisic acid (ABA), and brassinosteroids (BRs) during flowering in control (0 mM) and NaCl-treated (200 mM) plants. At the end of vegetative growth, endogenous GA3, GA4, ABA and BR contents in stems of NaCl-treated plants were significantly higher than those in controls. During flowering, GA3, GA4, IAA and ZR contents showed the most significant enhancement in flower organs of plants treated with NaCl when compared with controls. Additionally, genes related to ZR, IAA, GA, BR and ABA biosynthesis and plant hormone signal transduction, such as those encoding CYP735A, CYP85A, GID1, NCED, PIF4, AHP, TCH4, SnRK2 and ABF, were upregulated in S. salsa flowers from NaCl-treated plants. These results suggest that coordinated upregulation of genes involved in phytohormone biosynthesis and signal transduction contributes to the enhanced reproductive growth of S. salsa under salinity.

NaCl treatment markedly enhanced pollen viability and pollen preservation time of euhalophyte Suaeda salsa via up regulation of pollen development-related genes

Vegetable growth of halophytes has significantly increased through moderate salinity. However, little is known about the reproductive traits of euhalophytes. Male reproduction is pivotal for fertilization and seed production and sensitive to abiotic stressors. The pollen viability and pollen longevity of Suaeda salsa treated with 0 and 200 mM of NaCl were evaluated. It was revealed that the pollen size of S. salsa treated with NaCl was significantly bigger than that in controls. Furthermore, the pollen viability of S. salsa plants treated with NaCl was also significantly higher than that of control after 8 h of the pollens were collected (from 10 to 27 h). The pollen viability of NaCl-treated plants in the field could be maintained for 8 h (from 07:00 to 15:00) in sunny days, which was 1 h longer than that of control plants (from 07:00 to 14:00). Meanwhile, the pollen preservation time of NaCl-treated plants was 16 h at room temperature, which was 8 h longer than that of control plants. Genes related to pollen development, such as SsPRK3, SsPRK4, and SsLRX, exhibited high expression in the flowers of NaCl-treated plants. This indicated that NaCl markedly improved the pollen viability and preservation time via the increased expression of pollen development-related genes, and this benefits the population establishment of halophytes such as S. salsa in saline regions.

Genetic mechanisms of salt stress responses in halophytes

Abiotic stress is a major threat to plant growth and development, resulting in extensive crop loss worldwide. Plants react to abiotic stresses through physiological, biochemical, molecular, and genetic adaptations that promote survival. Exploring the molecular mechanisms involved in abiotic stress responses across various plant species is essential for improving crop yields in unfavorable environments. Halophytes are characterized as plants that survive to reproduce in soils containing high salt concentrations, and thus act as an ideal model to comprehend complicated genetic and physiological mechanisms of salinity stress tolerance. Plant ecologists classify halophytes into three main groups: euhalophytes, recretohalophytes, and pseudo-halophytes. Recent genetic and molecular research has showed complicated regulatory networks by which halophytes coordinate stress adaptation and tolerance. Furthermore, investigation of natural variations in these stress responses has supplied new perspectives on the evolution of mechanisms that regulate tolerance and adaptation. This review discusses the current understanding of the genetic mechanisms that contribute to salt-stress tolerance among different classes of halophytes.