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

Comparative study on chloroplast genome of Tamarix species

Tamaricaceae comprises about 120 species and has a long evolutionary history, Tamarix Linn accounts for approximately 75% of the total species in this family. It is the most widely distributed and diverse genus in the family. They have important ecological significance for transforming deserts and improving climate conditions. However, Tamarix is the most poorly classified genera among flowering plants owing to its large variability and high susceptibility to interspecific hybridization. In this study, the complete chloroplast genomes of three Tamarix species and one draft chloroplast genome were obtained in this study. Combined with eight chloroplast genomes deposited in GenBank, complete chloroplast sequences of 12 Tamarix species were used for further analysis. There are 176 non-SSR-related indels and 681 non-indel-related SSRs in the 12 Tamarix chloroplast genomes. The mononucleotide SSRs are the most prevalent among all types of SSRs. The mVISTA results indicate high sequence similarities across the chloroplast genome, suggesting that the chloroplast genomes are highly conserved, except for sample Tamarix androssowii (ENC850343). The IR regions and the coding regions are more conserved than the single-copy and noncoding regions. The trnF-ndhJ, ndhC-trnM-CAU, ycf1, and trnL-UAG-ndhF regions are the most variable and have higher variability than those of the universal DNA markers. Finally, the first phylogenetic tree of Tamaricaceae was constructed which confirmed the monophyly of Tamarix in Tamaricaceae. The first phylogenetic tree of Tamarix was based on the complete chloroplast genome to date, the changes in branch length and support rate can potentially help us clarify the phylogenetic relationships of Tamarix. All the obtained genetic resources will facilitate future studies in population genetics, species identification, and conservation biology of Tamarix.

A chromosome-level genome assembly provides insights into the local adaptation of Tamarix austromongolica in the Yellow River Basin, China

Tamarix austromongolica is endemic to the Yellow River Basin and has adapted to diverse ecological settings in the region, including the arid areas of northwestern China and the saline soil regions of the Yellow River Delta. However, the genetic basis of its local adaptation remains unclear. We report a chromosome-level assembly of the T. austromongolica genome based on PacBio high-fidelity sequencing and Hi-C technology. The 12 pseudochromosomes cover 98.44% of the 1.32 Gb assembly, with a contig N50 of 52.57 Mb and a BUSCO score of 98.2%. The genome comprises 913.6 Mb (68.83%) of repetitive sequences and 22,374 protein-coding genes. Genome evolution analyses suggest that genes under positive selection and significantly expanded gene families have facilitated T. austromongolica's adaptability to diverse environmental factors and high resistance to diseases. Using genotyping-by-sequencing, we conducted population structure and selection analyses of 114 samples from 15 sites. Two genetic groups were identified, and 114 and 289 candidate genes were assigned to the populations of the northwestern and eastern parts of the Yellow River, respectively. Furthermore, we discovered numerous candidate genes associated with high-altitude adaptability and salt tolerance. This research provides valuable genomic resources for the evolutionary study and genetic breeding of tamarisk.

Genome-wide identification of the mitogen-activated kinase gene family from Limonium bicolor and functional characterization of LbMAPK2 under salt stress

BACKGROUND

Mitogen-activated protein kinases (MAPKs) are ubiquitous signal transduction components in eukaryotes. In plants, MAPKs play an essential role in growth and development, phytohormone regulation, and abiotic stress responses. The typical recretohalophyte Limonium bicolor (Bunge) Kuntze has multicellular salt glands on its stems and leaves; these glands secrete excess salt ions from its cells to mitigate salt damage. The number, type, and biological function of L. bicolor MAPK genes are unknown.

RESULTS

We identified 20 candidate L. bicolor MAPK genes, which can be divided into four groups. Of these 20 genes, 17 were anchored to 7 chromosomes, while LbMAPK18, LbMAPK19, and LbMAPK20 mapped to distinct scaffolds. Structure analysis showed that the predicted protein LbMAPK19 contains the special structural motif TNY in its activation loop, whereas the other LbMAPK members harbor the conserved TEY or TDY motif. The promoters of most LbMAPK genes carry cis-acting elements related to growth and development, phytohormones, and abiotic stress. LbMAPK1, LbMAPK2, LbMAPK16, and LbMAPK20 are highly expressed in the early stages of salt gland development, whereas LbMAPK4, LbMAPK5, LbMAPK6, LbMAPK7, LbMAPK11, LbMAPK14, and LbMAPK15 are highly expressed during the late stages. These 20 LbMAPK genes all responded to salt, drought and ABA stress. We explored the function of LbMAPK2 via virus-induced gene silencing: knocking down LbMAPK2 transcript levels in L. bicolor resulted in fewer salt glands, lower salt secretion ability from leaves, and decreased salt tolerance. The expression of several genes [LbTTG1 (TRANSPARENT TESTA OF GL1), LbCPC (CAPRICE), and LbGL2 (GLABRA2)] related to salt gland development was significantly upregulated in LbMAPK2 knockdown lines, while the expression of LbEGL3 (ENHANCER OF GL3) was significantly downregulated.

CONCLUSION

These findings increase our understanding of the LbMAPK gene family and will be useful for in-depth studies of the molecular mechanisms behind salt gland development and salt secretion in L. bicolor. In addition, our analysis lays the foundation for exploring the biological functions of MAPKs in an extreme halophyte.

Effects of Arbuscular Mycorrhizal Fungus on Sodium and Chloride Ion Channels of Casuarina glauca under Salt Stress

Casuarina glauca is an important coastal protection forest species, which is exposed to high salt stress all year round. Arbuscular mycorrhizal fungi (AMF) can promote the growth and salt tolerance of C. glauca under salt stress. However, the effects of AMF on the distribution of Na⁺ and Cl^- and the expression of related genes in C. glauca under salt stress need to be further explored. This study explored the effects of Rhizophagus irregularis on plant biomass, the distribution of Na⁺ and Cl^-, and the expression of related genes in C. glauca under NaCl stress through pot simulation experiments. The results revealed that the mechanisms of Na⁺ and Cl^- transport of C. glauca under NaCl stress were different. C. glauca took a salt accumulation approach to Na⁺, transferring Na⁺ from roots to shoots. Salt accumulation of Na⁺ promoted by AMF was associated with CgNHX7. The transport mechanism of C. glauca to Cl^- might involve salt exclusion rather than salt accumulation, and Cl^- was no longer transferred to shoots in large quantities but started to accumulate in roots. However, AMF alleviated Na⁺ and Cl^- stress by similar mechanisms. AMF could promote salt dilution of C. glauca by increasing biomass and the content of K⁺, compartmentalizing Na⁺ and Cl^- in vacuoles. These processes were associated with the expression of CgNHX1, CgNHX2-1, CgCLCD, CgCLCF, and CgCLCG. Our study will provide a theoretical basis for the application of AMF to improve salt tolerance in plants.

Exogenous hydrogen sulfide mediates Na+ and K+ fluxes of salt gland in salt-secreting mangrove plant Avicennia marina

Hydrogen sulfide (H2S), is a crucial biological player in plants. Here, we primarily explored the interaction between sodium hydrosulfide (NaHS, a H2S donor) and the fluxes of Na+ and K+ from the salt glands of mangrove species Avicennia marina (Forsk.) Vierh. with non-invasive micro-test technology (NMT) and quantitative real-time PCR (qRT-PCR) approaches under salinity treatments. The results showed that under 400-mM NaCl treatment, the addition of 200-μM NaHS markedly increased the quantity of salt crystals in the adaxial epidermis of A. marina leaves, accompanied by an increase in the K+/Na+ ratio. Meanwhile, the endogenous content of H2S was dramatically elevated in this process. The NMT result revealed that the Na+ efflux was increased from salt glands, whereas K+ efflux was decreased with NaHS application. On the contrary, the effects of NaHS were reversed by H2S scavenger hypotaurine (HT), and DL-propargylglycine (PAG), an inhibitor of cystathionine-γ-lyase (CES, a H2S synthase). Moreover, enzymic assay revealed that NaHS increased the activities of plasma membrane and tonoplast H+-ATPase. qRT-PCR analysis revealed that NaHS significantly increased the genes transcript levels of tonoplast Na+/H+ antiporter (NHX1), plasma membrane Na+/H+ antiporter (SOS1), plasma membrane H+-ATPase (AHA1) and tonoplast H+-ATPase subunit c (VHA-c1), while suppressed above-mentioned gene expressions by the application of HT and PAG. Overall, H2S promotes Na+ secretion from the salt glands of A. marina by up-regulating the plasma membrane and tonoplast Na+/H+ antiporter and H+-ATPase.

Recent Progress on the Salt Tolerance Mechanisms and Application of Tamarisk

Salinized soil is a major environmental stress affecting plant growth and development. Excessive salt in the soil inhibits the growth of most plants and even threatens their survival. Halophytes are plants that can grow and develop normally on saline-alkali soil due to salt tolerance mechanisms that emerged during evolution. For this reason, halophytes are used as pioneer plants for improving and utilizing saline land. Tamarisk, a family of woody halophytes, is highly salt tolerant and has high economic value. Understanding the mechanisms of salt tolerance in tamarisk and identifying the key genes involved are important for improving saline land and increasing the salt tolerance of crops. Here, we review recent advances in our understanding of the salt tolerance mechanisms of tamarisk and the economic and medicinal value of this halophyte.

Treatment wetlands and phyto-technologies for remediation of winery effluent: Challenges and opportunities

The composition and concentration of contaminants present in winery wastewater fluctuate through space and time, presenting a challenge for traditional remediation methods. Bio-hydrogeochemical engineered systems, such as treatment wetlands, have been demonstrated to effectively reduce contaminant loads prior to disposal or reuse of the effluent. This review identifies and details the status quo and challenges associated with (i) the characteristics of winery wastewater, and the (ii) functional components, (iii) operational parameters, and (iv) performance of treatment wetlands for remediation of winery effluent. Potential solutions to challenges associated with these aspects are presented, based on the latest literature. A particular emphasis has been placed on the phytoremediation of winery wastewater, and the rationale for selection of plant species for niche bioremediatory roles. This is attributed to previously reported low-to-negative removal percentages of persistent contaminants, such as salts and heavy metals that may be present in winery wastewater. A case for the inclusion of selected terrestrial halophytes in treatment wetlands and in areas irrigated using winery effluent is discussed. These are plant species that have an elevated ability to accumulate, cross-tolerate and potentially remove a range of persistent contaminants from winery effluent via various phytotechnologies (e.g., phytodesalination).

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

Soil salinization, which is aggravated by climate change and inappropriate anthropogenic activities, has emerged as a serious environmental problem, threatening sustainable agriculture and future food security. Although there has been considerable progress in developing crop varieties by introducing salt tolerance-associated traits, most crop cultivars grown in saline soils still exhibit a decline in yield, necessitating the search for alternatives. Halophytes, with their intrinsic salt tolerance characteristics, are known to have great potential in rehabilitating salt-contaminated soils to support plant growth in saline soils by employing various strategies, including phytoremediation. In addition, the recent identification and characterization of salt tolerance-related genes encoding signaling components from halophytes, which are naturally grown under high salinity, have paved the way for the development of transgenic crops with improved salt tolerance. In this review, we aim to provide a comprehensive update on salinity-induced negative effects on soils and plants, including alterations of physicochemical properties in soils, and changes in physiological and biochemical processes and ion disparities in plants. We also review the physiological and biochemical adaptation strategies that help halophytes grow and survive in salinity-affected areas. Furthermore, we illustrate the halophyte-mediated phytoremediation process in salinity-affected areas, as well as their potential impacts on soil properties. Importantly, based on the recent findings on salt tolerance mechanisms in halophytes, we also comprehensively discuss the potential of improving salt tolerance in crop plants by introducing candidate genes related to antiporters, ion transporters, antioxidants, and defense proteins from halophytes for conserving sustainable agriculture in salinity-prone areas.

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.

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.