Comparative transcriptome analysis of developmental stages of the Limonium bicolor leaf generates insights into salt gland differentiation

Global analysis of key post-transcriptional regulation in early leaf development of Limonium bicolor identifies a long non-coding RNA that promotes salt gland development and salt resistance

Limonium bicolor, known horticulturally as sea lavender, is a typical recretohalophyte with salt glands in its leaf epidermis that secrete excess Na+ out of the plant. Although many genes have been proposed to contribute to salt gland initiation and development, a detailed analysis of alternative splicing, alternative polyadenylation patterns, and long non-coding RNAs (lncRNAs) has been lacking. Here, we applied single-molecule long-read mRNA isoform sequencing (Iso-seq) to explore the complexity of the L. bicolor transcriptome in leaves during salt gland initiation (stage A) and salt gland differentiation (stage B) based on the reference genome. We identified alternative splicing events and the use of alternative poly(A) sites unique to stage A or stage B, leading to the hypothesis that they might contribute to the differentiation of salt glands. Based on the Iso-seq data and RNA in situ hybridization of candidate genes, we selected the lncRNA Btranscript_153392 for gene editing and virus-induced gene silencing to dissect its function. In the absence of this transcript, we observed fewer salt glands on the leaf epidermis, leading to diminished salt secretion and salt tolerance. Our data provide transcriptome resources for unraveling the mechanisms behind salt gland development and furthering crop transformation efforts towards enhanced survivability in saline soils.

Global dynamics and cytokinin participation of salt gland development trajectory in recretohalophyte Limonium bicolor

Salt gland is an epidermal Na+ secretory structure that enhances salt resistance in the recretohalophyte sea lavender (Limonium bicolor). To elucidate the salt gland development trajectory and related molecular mechanisms, we performed single-cell RNA sequencing of L. bicolor protoplasts from young leaves at salt gland initiation and differentiation stages. Dimensionality reduction analyses defined 19 transcriptionally distinct cell clusters, which were assigned into 4 broad populations-promeristem, epidermis, mesophyll, and vascular tissue-verified by in situ hybridization. Cytokinin was further proposed to participate in salt gland development by the expression patterns of related genes and cytological evidence. By comparison analyses of Single-cell RNA sequencing with exogenous application of 6-benzylaminopurine, we delineated 5 salt gland development-associated subclusters and defined salt gland-specific differentiation trajectories from Subclusters 8, 4, and 6 to Subcluster 3 and 1. Additionally, we validated the participation of TRIPTYCHON and the interacting protein Lb7G34824 in salt gland development, which regulated the expression of cytokinin metabolism and signaling-related genes such as GLABROUS INFLORESCENCE STEMS 2 to maintain cytokinin homeostasis during salt gland development. Our results generated a gene expression map of young leaves at single-cell resolution for the comprehensive investigation of salt gland determinants and cytokinin participation that helps elucidate cell fate determination during epidermis formation and evolution in recretohalophytes.

The RING zinc finger protein LbRZF1 promotes salt gland development and salt tolerance in Limonium bicolor

The recretohalophyte Limonium bicolor thrives in high-salinity environments because salt glands on the above-ground parts of the plant help to expel excess salt. Here, we characterize a nucleus-localized C3HC4 (RING-HC)-type zinc finger protein of L. bicolor named  RING  ZINC  FINGER PROTEIN  1 (LbRZF1). LbRZF1 was expressed in salt glands and in response to NaCl treatment. LbRZF1 showed no E3 ubiquitin ligase activity. The phenotypes of overexpression and knockout lines for LbRZF1 indicated that LbRZF1 positively regulated salt gland development and salt tolerance in L. bicolor. lbrzf1 mutants had fewer salt glands and secreted less salt than did the wild-type, whereas LbRZF1-overexpressing lines had opposite phenotypes, in keeping with the overall salt tolerance of these plants. A yeast two-hybrid screen revealed that LbRZF1 interacted with LbCATALASE2 (LbCAT2) and the transcription factor LbMYB113, leading to their stabilization. Silencing of LbCAT2 or LbMYB113 decreased salt gland density and salt tolerance. The heterologous expression of LbRZF1 in Arabidopsis thaliana conferred salt tolerance to this non-halophyte. We also identified the transcription factor LbMYB48 as an upstream regulator of LbRZF1 transcription. The study of LbRZF1 in the regulation network of salt gland development also provides a good foundation for transforming crops and improving their salt resistance.

Genome-wide identification of bHLH transcription factors and functional analysis in salt gland development of the recretohalophyte sea lavender (Limonium bicolor)

Transcription factors with basic helix-loop-helix (bHLH) structures regulate plant growth, epidermal structure development, metabolic processes, and responses to stress extensively. Sea lavender (Limonium bicolor) is a recretohalophyte with unique salt glands in the epidermis that make it highly resistant to salt stress, contributing to the improvement of saline lands. However, the features of the bHLH transcription factor family in L. bicolor are largely unknown. Here, we systematically analyzed the characteristics, localization, and phylogenetic relationships of 187 identified bHLH family genes throughout the L. bicolor genome, as well as their cis-regulatory promoter elements, expression patterns, and key roles in salt gland development or salt tolerance by genetic analysis. Nine verified L. bicolor bHLH genes are expressed and the encoded proteins function in the nucleus, among which the proteins encoded by Lb2G14060 and Lb1G07934 also localize to salt glands. Analysis of CRISPR-Cas9-generated knockout mutants and overexpression lines indicated that the protein encoded by Lb1G07934 is involved in the formation of salt glands, salt secretion, and salt resistance, indicating that bHLH genes strongly influence epidermal structure development and stress responses. The current study lays the foundation for further investigation of the effects and functional mechanisms of bHLH genes in L. bicolor and paves the way for selecting salt-tolerance genes that will enhance salt resistance in crops and for the improvement of saline soils.

The NAC gene family in the halophyte Limonium bicolor: Identification, expression analysis, and regulation of abiotic stress tolerance

NAC transcription factors regulate plant growth, development, and stress responses. However, the number, types, and biological functions of Limonium bicolor LbNAC genes have remained elusive. L. bicolor secretes excessive salt ions through salt glands on its stems and leaves to reduce salt-induced damage. Here, we identified 63 NAC members (LbNAC1-63) in L. bicolor, which were unevenly distributed across eight chromosomes. Cis-elements in the LbNAC promoters were related to growth and development, stress responses, and phytohormone responses. We observed strong colinearity between LbNACs and GmNACs from soybean (Glycine max). Thus, LbNAC genes may share similar functions with GmNAC genes. Expression analysis indicated that 16 LbNAC genes are highly expressed in roots, stems, leaves, and flowers, whereas 17 LbNAC genes were highly expressed throughout salt gland development, suggesting that they may regulate this developmental stage. Silencing LbNAC54 in L. bicolor decreased salt gland density, salt secretion from leaves, and overall salt tolerance. In agreement, genes related to salt gland development were significantly downregulated in LbNAC54-silenced lines. Our findings shed light on LbNAC genes and help elucidate salt gland development and salt secretion in L. bicolor. Our data also provide insight into NAC functions in halophytes.

Structural, developmental and functional analyses of leaf salt glands of mangrove recretohalophyte Aegiceras corniculatum

Salt secretion is an important strategy used by the mangrove plant Aegiceras corniculatum to adapt to the coastal intertidal environment. However, the structural, developmental and functional analyses on the leaf salt glands, particularly the salt secretion mechanism, are not well documented. In this study, we investigated the structural, developmental and degenerative characteristics and the salt secretion mechanisms of salt glands to further elucidate the mechanisms of salt tolerance of A. corniculatum. The results showed that the salt gland cells have a large number of mitochondria and vesicles, and plenty of plasmodesmata as well, while chloroplasts were found in the collecting cells. The salt glands developed early and began to differentiate at the leaf primordium stage. We observed and defined three stages of salt gland degradation for the first time in A. corniculatum, where the secretory cells gradually twisted and wrinkled inward and collapsed downward as the salt gland degeneration increased and the intensity of salt gland autofluorescence gradually diminished. In addition, we found that the salt secretion rate of the salt glands increased when the treated concentration of NaCl increased, reaching the maximum at 400 mM NaCl. The salt-secreting capacity of the salt glands of the adaxial epidermis is significantly greater than that of the abaxial epidermis. The real-time quantitative PCR results indicate that SAD2, TTG1, GL2 and RBR1 may be involved in regulating the development of the salt glands of A. corniculatum. Moreover, Na+/H+ antiporter, H+-ATPase, K+ channel and Cl- channel may play important roles in the salt secretion of salt glands. In sum mary, this study strengthens the understanding of the structural, developmental and degenerative patterns of salt glands and salt secretion mechanisms in mangrove recretohalophyte A. corniculatum, providing an important reference for further studies at the molecular level.

The transmembrane protein LbRSG from the recretohalophyte Limonium bicolor enhances salt gland development and salt tolerance

Salt glands are the unique epidermal structures present in recretohalophytes, plants that actively excrete excess Na+ by salt secretory structures to avoid salt damage. Here, we describe a transmembrane protein that localizes to the plasma membrane of the recretohalophyte Limonium bicolor. As virus-induced gene silencing of the corresponding gene LbRSG in L. bicolor decreased the number of salt glands, we named the gene Reduced Salt Gland. We detected LbRSG transcripts in salt glands by in situ hybridization and transient transformation. Overexpression and silencing of LbRSG in L. bicolor pointed to a positive role in salt gland development and salt secretion by interacting with Lb3G16832. Heterologous LbRSG expression in Arabidopsis enhanced salt tolerance during germination and the seedling stage by alleviating NaCl-induced ion stress and osmotic stress after replacing or deleting the (highly) negatively charged region of extramembranous loop. After screened by immunoprecipitation-mass spectrometry and verified using yeast two-hybrid, PGK1 and BGLU18 were proposed to interact with LbRSG to strengthen salt tolerance. Therefore, we identified (highly) negatively charged regions in the extramembrane loop that may play an essential role in salt tolerance, offering hints about LbRSG function and its potential to confer salt resistance.

Exogenous 6-BA enhances salt tolerance of Limonium bicolor by increasing the number of salt glands

KEY MESSAGE

Exogenous 6-BA can increase endogenous hormone content, improve photosynthesis, decrease Na+ by increasing leaf salt gland density and salt secretion ability, and reduce ROS content so that it can promote L. bicolor growth. 6-benzyl adenine (6-BA) is an artificial cytokinin and has been widely applied to improving plant adaptation to stress. However, it is rarely reported that 6-BA alleviates salt damage of halophytes. In this paper, we treated Limonium bicolor seedlings, a recretohalophyte with high medicinal and ornamental values, with 300 mM NaCl and different concentrations of 6-BA (0.5, 1.0, and 1.5 mg/L) and measured plant growth, physiological index, the density of salt gland, and the salt secretion ability of leaves. The results showed that exogenous applications 1.0 mg/L 6-BA significantly improved plant growth and photosynthesis, increased cytokinin and auxins contents, K+ and organic soluble matter contents, the activities of SOD, CAT, APX, and POD, and decreased Na+, H2O2, and O2- contents compared to that treated with 300 mM NaCl. Further research showed that exogenous 6-BA significantly increased the density of salt gland and the salt secretion ability of leaves by upregulating the expression of the salt gland developmental genes, therefore, can secrete more excess Na+, and thus reduces the Na+ concentration in leaves, which can alleviate Na+ damage to the species. In all, exogenous 1.0 mg/L 6-BA can increase endogenous hormone, improve photosynthesis, decrease Na+ by increasing secretion ability, and reduce ROS content of L. bicolor so that it can improve the growth. These results above systematically prove the new role of 6-BA in salt tolerance of L. bicolor.

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.

Physiological and transcriptomic analysis of salt tolerant Glaux maritima grown under high saline condition

Land salinization considerably limits crop production. Biological improvement of saline and alkaline land is an important way to achieve efficient land use. It is crucial to study the salt tolerance of halophyte resources in order to explore and improve plant resources through biological improvement. Glaux maritima is a mesophyte halophyte with strong salt tolerance. In this study, we conducted research on the salt tolerance mechanism of G. maritima through phenotypic, physiological, and transcriptomic aspects. The results indicate that leaf cross-sections revealed that G. maritima has a salt gland tissue composed of stalk, collecting, and secretory cells, which are trapped in epidermal cells. At the physiological level, the maximum salt tolerance threshold of G. maritima leaves was 600 mM/L. At this concentration, proline content, relative conductivity, and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities were maximum. At the transcriptional level, transcriptome data of three experimental groups (N0: 0 mM/L, N3: 600 mM/L, and N4: 800 mM/L) were analyzed, and six essential genes related to proline synthesis and five essential genes related to SOD and CAT enzyme activities were identified. Two genes involved in CAT enzyme activity were also found to play an important role in the MAPK signaling pathway. Trend analysis revealed that the MAPK signaling regulation (37 differentially expressed genes (DEGs)), phytohormone regulation (48 DEGs), glutathione metabolism (8 DEGs), flavonoid and flavonoid biosynthesis (2DEGs), and flavonoid biosynthesis (24 DEGs) pathways played important roles in regulating the salt tolerance of G. maritima. These findings provide valuable information for further studies on the functional characteristics of G. maritima in response to abiotic stress and may contribute to salt resistance breeding of fodder crops for cultivation in saline alkali land.

Leaf sodium homeostasis controlled by salt gland is associated with salt tolerance in mangrove plant Avicennia marina

Avicennia marina, a mangrove plant growing in coastal wetland habitats, is frequently affected by tidal salinity. To understand its salinity tolerance, the seedlings of A. marina were treated with 0, 200, 400 and 600 mM NaCl. We found the whole-plant dry weight and photosynthetic parameters increased at 200 mM NaCl but decreased over 400 mM NaCl. The maximum quantum yield of primary photochemistry (Fv/Fm) significantly decreased at 600 mM NaCl. Transmission electron microscopy observations showed high salinity caused the reduction in starch grain size, swelling of the thylakoids and separation of the granal stacks, and even destruction of the envelope. In addition, the dense protoplasm and abundant mitochondria in the secretory and stalk cells, and abundant plasmodesmata between salt gland cells were observed in the salt glands of the adaxial epidermis. At all salinities, Na+ content was higher in leaves than in stems and roots; however, Na+ content increased in the roots while it remained at a constant level in the leaves over 400 mM NaCl treatment, due to salt secretion from the salt glands. As a result, salt crystals on the leaf adaxial surface increased with salinity. On the other hand, salt treatment increased Na+ and K+ efflux and decreased H+ efflux from the salt glands by the non-invasive micro-test technology, although Na+ efflux reached the maximum at 400 mM NaCl. Further real-time quantitative PCR analysis indicated that the expression of Na+/H+ antiporter (SOS1 and NHX1), H+-ATPase (AHA1 and VHA-c1) and K+ channel (AKT1, HAK5 and GORK) were up-regulated, and only the only Na+ inward transporter (HKT1) was down-regulated in the salt glands enriched adaxial epidermis of the leaves under 400 mM NaCl treatment. In conclusion, salinity below 200 mM NaCl was beneficial to the growth of A. marina, and below 400 mM, the salt glands could excrete Na+ effectively, thus improving its salt tolerance.

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.

Enhanced Na+ and Cl- sequestration and secretion selectivity contribute to high salt tolerance in the tetraploid recretohalophyte Plumbago auriculata Lam

Enhanced secretion of Na⁺ and Cl^- in leaf glands and leaf vacuolar sequestration of Na⁺ or root retention of Cl^-, combined with K⁺ retention, contribute to the improved salt tolerance of tetraploid recretohalophyte P. auriculata. Salt stress is one of the major abiotic factors threatening plant growth and development, and polyploids generally exhibit higher salt stress resistance than diploids. In recretohalophytes, which secrete ions from the salt gland in leaf epidermal cells, the effects of polyploidization on ion homeostasis and secretion remain unknown. In this study, we compared the morphology, physiology, and ion homeostasis regulation of diploid and autotetraploid accessions of the recretohalophyte Plumbago auriculata Lam. after treatment with 300 mM NaCl for 0, 2, 4, 6, and 8 days. The results showed that salt stress altered the morphology, photosynthetic efficiency, and chloroplast structure of diploid P. auriculata to a greater extent than those of its tetraploid counterpart. Moreover, the contents of organic osmoregulatory substances (proline and soluble sugars) were significantly higher in the tetraploid than in the diploid, while those of H₂O₂ and malondialdehyde (MDA) were significantly lower. Analysis of ion homeostasis revealed that the tetraploid cytotype accumulated more Na⁺ in stems and leaves and more Cl^- in roots but less K⁺ loss in roots compared with diploid P. auriculata. Additionally, the rate of Na⁺ and Cl^- secretion from the leaf surface was higher, while that of K⁺, Mg²⁺, and Ca²⁺ secretion was lower in tetraploid plants. X-ray microanalysis of mesophyll cells revealed that Na⁺ mainly accumulated in different cellular compartments in the tetraploid (vacuole) and diploid (cytoplasm) plants. Our results suggest that polyploid recretohalophytes require the ability to sequester Na⁺ and Cl^-(via accumulation in leaf cell vacuoles or unloading by roots) and selectively secrete these ions (through salt glands) together with the ability to prevent K⁺ loss (by roots). This mechanism required to maintain K⁺/Na⁺ homeostasis in polyploid recretohalophytes under high salinity provides new insights in the improved maintenance of ion homeostasis in polyploids under salt stress.

A first look at sea-lavenders genomics - can genome wide SNP information tip the scales of controversy in the Limonium vulgare species complex?

BACKGROUND

Sea-lavenders (Limonium Mill., Plumbaginaceae) are a cosmopolitan group of diploid and polyploid plants often adapted to extreme saline environments, with a mostly Tethyan distribution, occurring in the Mediterranean, Irano-Turanian, Euro-Siberian and in the New World. The halophylic Limonium vulgare polyploid complex in particular, presents a large distribution throughout extreme salt-marsh habitats and shows little morphological but high taximetric variation, frequently blurring species delimitation. In this work we pursue three main goals: assert whether SNP data from polyploid individuals has the resolution to distinguish the seven sampled species, to better understand how genetically structured Limonium vulgare is, and attempt to identify specific molecular mechanisms for the differentiation between L. maritimum and L. vulgare. For this purpose, 95 individuals were genotyped using Genotyping by Sequencing (GBS), which were assembled as two independent datasets using IPYRAD. All analyses performed downstream of assembly were fully automated. Phylogenetic inference, PCA, and admixture plots were used to infer answers to the study's main goals.

RESULTS

Close to 10,000 SNPs were obtained for each dataset. Phylogenetic analyses reveal that polyploid data can be used to infer species relationships. Population structure analyses suggest a genetically structured L. vulgare. A set of 34 SNPs were found to be fully segregated between L. vulgare and L. maritimum, two of which are potentially linked to proteins that might be involved in the speciation process.

CONCLUSION

Despite polyploid data analyses shortcomings, GBS generated SNPs have the resolution to discern all seven included species. Limonium vulgare revealed pronounced genetic structure along a geographical north-south cline. L. maritimum always appears as a distinct genetic entity. Segregated SNPs between L. vulgare and L. maritimum indicate salinity response and morphological trait control genes as potentially interesting to follow up for studying these species' divergence process.

An uncharacterized gene Lb1G04794 from Limonium bicolor promotes salt tolerance and trichome development in Arabidopsis

Halophytes can grow and reproduce in high-salinity environments, making them an important reservoir of genes conferring salt tolerance. With the expansion of saline soils worldwide, exploring the mechanisms of salt tolerance in halophytes and improving the salt tolerance of crops have become increasingly urgent. Limonium bicolor is a halophyte with salt glands that secrete excess Na⁺ through leaves. Here, we identified an uncharacterized gene Lb1G04794, which showed increased expression after NaCl treatment and was high during salt gland development in L. bicolor. Overexpression of Lb1G04794 in L. bicolor showed promoted salt gland development, indicating that this gene may promote salt gland differentiation. Transgenic Arabidopsis strains overexpressing Lb1G04794 showed increased trichomes and decreased root hairs under normal conditions. Compared with wild type (WT), root growth in the transgenic lines was less inhibited by NaCl treatment. Transgenic seedlings accumulated less fresh/dry weight reductions under long-term salt treatment, accompanied by lower Na⁺ and malondialdehyde accumulation than WT, indicating that these transgenic lines behave better growth and undergo less cellular damage under NaCl stress. These results were consistent with the low expression levels of salt-tolerance marker genes in the transgenic lines upon salt stress. We conclude that the unknown gene Lb1G04794 positively regulated salt gland development, and promoted salt tolerance of Arabidopsis, offering a new direction for improving salt tolerance of non-halophytes and crops.

Mining for salt-tolerant genes from halophyte Zoysia matrella using FOX system and functional analysis of ZmGnTL

Zoysia matrella is a salt-tolerant turfgrass grown in areas with high soil salinity irrigated with effluent water. Previous studies focused on explaining the regulatory mechanism of Z. matrella salt-tolerance at phenotypic and physiological levels. However, the molecular mechanism associated with salt tolerance of Z. matrella remained unclear. In this study, a high-efficient method named FOX (full-length cDNA overexpression) hunting system was used to search for salt-tolerant genes in Z. matrella. Eleven candidate genes, including several known or novel salt-tolerant genes involved in different metabolism pathways, were identified. These genes exhibited inducible expression under salt stress condition. Furthermore, a novel salt-inducible candidate gene ZmGnTL was transformed into Arabidopsis for functional analysis. ZmGnTL improved salt-tolerance through regulating ion homeostasis, reactive oxygen species scavenging, and osmotic adjustment. In summary, we demonstrated that FOX is a reliable system for discovering novel genes relevant to salt tolerance and several candidate genes were identified from Z. matrella that can assist molecular breeding for plant salt-tolerance improvement.

Proteomics of Salt Gland-Secreted Sap Indicates a Pivotal Role for Vesicle Transport and Energy Metabolism in Plant Salt Secretion

Soil salinization is one of the major factors restricting crop growth and agricultural production worldwide. Recretohalophytes have developed unique epidermal structures in their aboveground tissues, such as salt glands or salt bladders, to secrete excess salt out of the plant body as a protective mechanism from ion damage. Three hypotheses were proposed to explain how salt glands secrete salts: the osmotic hypothesis, a hypothesis similar to animal fluid transport, and vesicle-mediated exocytosis. However, there is no direct evidence to show whether the salt gland-secreted liquid contains landmark proteins or peptides which would elucidate the salt secretion mechanism. In this study, we collected the secreted liquid of salt glands from Limonium bicolor, followed by extraction and identification of its constituent proteins and peptides by SDS-PAGE and mass spectrometry. We detected 214 proteins and 440 polypeptides in the salt gland-secreted droplets of plants grown under control conditions. Unexpectedly, the proportion of energy metabolism-related proteins increased significantly though only 16 proteins and 35 polypeptides in the droplets of salt-treated plants were detected. In addition, vesicle transport proteins such as the Golgi marker enzyme glycosyltransferase were present in the secreted sap of salt glands from both control and salt-treated plants. These results suggest that trans-Golgi network-mediated vesicular transport and energy production contributes to salt secretion in salt glands.

The genome of the recretohalophyte Limonium bicolor provides insights into salt gland development and salinity adaptation during terrestrial evolution

Halophytes have evolved specialized strategies to cope with high salinity. The extreme halophyte sea lavender (Limonium bicolor) lacks trichomes but possesses salt glands on its epidermis that can excrete harmful ions, such as sodium, to avoid salt damage. Here, we report a high-quality, 2.92-Gb, chromosome-scale L. bicolor genome assembly based on a combination of Illumina short reads, single-molecule, real-time long reads, chromosome conformation capture (Hi-C) data, and Bionano genome maps, greatly enriching the genomic information on recretohalophytes with multicellular salt glands. Although the L. bicolor genome contains genes that show similarity to trichome fate genes from Arabidopsis thaliana, it lacks homologs of the decision fate genes GLABRA3, ENHANCER OF GLABRA3, GLABRA2, TRANSPARENT TESTA GLABRA2, and SIAMESE, providing a molecular explanation for the absence of trichomes in this species. We identified key genes (LbHLH and LbTTG1) controlling salt gland development among classical trichome homologous genes and confirmed their roles by showing that their mutations markedly disrupted salt gland initiation, salt secretion, and salt tolerance, thus offering genetic support for the long-standing hypothesis that salt glands and trichomes may share a common origin. In addition, a whole-genome duplication event occurred in the L. bicolor genome after its divergence from Tartary buckwheat and may have contributed to its adaptation to high salinity. The L. bicolor genome resource and genetic evidence reported in this study provide profound insights into plant salt tolerance mechanisms that may facilitate the engineering of salt-tolerant crops.

Molecular Mechanisms of Plant Trichome Development

Plant trichomes, protrusions formed from specialized aboveground epidermal cells, provide protection against various biotic and abiotic stresses. Trichomes can be unicellular, bicellular or multicellular, with multiple branches or no branches at all. Unicellular trichomes are generally not secretory, whereas multicellular trichomes include both secretory and non-secretory hairs. The secretory trichomes release secondary metabolites such as artemisinin, which is valuable as an antimalarial agent. Cotton trichomes, also known as cotton fibers, are an important natural product for the textile industry. In recent years, much progress has been made in unraveling the molecular mechanisms of trichome formation in Arabidopsis thaliana, Gossypium hirsutum, Oryza sativa, Cucumis sativus, Solanum lycopersicum, Nicotiana tabacum, and Artemisia annua. Here, we review current knowledge of the molecular mechanisms underlying fate determination and initiation, elongation, and maturation of unicellular, bicellular and multicellular trichomes in several representative plants. We emphasize the regulatory roles of plant hormones, transcription factors, the cell cycle and epigenetic modifications in different stages of trichome development. Finally, we identify the obstacles and key points for future research on plant trichome development, and speculated the development relationship between the salt glands of halophytes and the trichomes of non-halophytes, which provides a reference for future studying the development of plant epidermal cells.

Lb1G04202, an Uncharacterized Protein from Recretohalophyte Limonium bicolor, Is Important in Salt Tolerance

With global increases in saline soil, it has become increasingly important to decipher salt-tolerance mechanisms and identify strategies to improve salt tolerance in crops. Halophytes complete their life cycles in environments containing ≥200 mM NaCl; these remarkable plants provide a potential source of genes for improving crop salt tolerance. Recretohalophytes such as Limonium bicolor have salt glands that secrete Na+ on their leaf epidermis. Here, we identified Lb1G04202, an uncharacterized gene with no conserved domains, from L. bicolor, which was highly expressed after NaCl treatment. We confirmed its expression in the salt gland by in situ hybridization, and then heterologously expressed Lb1G04202 in Arabidopsis thaliana. The transgenic lines had a higher germination rate, greater cotyledon growth percentage, and longer roots than the wild type (WT) under NaCl treatments (50, 100 and 150 mM). At the seedling stage, the transgenic lines grew better than the WT and had lower Na+ and malonyldialdehyde accumulation, and higher K+ and proline contents. This corresponded with the high expression of the key proline biosynthesis genes AtP5CS1 and AtP5CS2 under NaCl treatment. Isotonic mannitol treatment showed that Lb1G04202 overexpression significantly relieved osmotic stress. Therefore, this novel gene provides a potential target for improving salt tolerance.

Cell biology of the leaf epidermis: Fate specification, morphogenesis, and coordination

As the outermost layer of plants, the epidermis serves as a critical interface between plants and the environment. During leaf development, the differentiation of specialized epidermal cell types, including stomatal guard cells, pavement cells, and trichomes, occurs simultaneously, each providing unique and pivotal functions for plant growth and survival. Decades of molecular-genetic and physiological studies have unraveled key players and hormone signaling specifying epidermal differentiation. However, most studies focus on only one cell type at a time, and how these distinct cell types coordinate as a unit is far from well-comprehended. Here we provide a review on the current knowledge of regulatory mechanisms underpinning the fate specification, differentiation, morphogenesis, and positioning of these specialized cell types. Emphasis is given to their shared developmental origins, fate flexibility, as well as cell cycle and hormonal controls. Furthermore, we discuss computational modeling approaches to integrate how mechanical properties of individual epidermal cell types and entire tissue/organ properties mutually influence each other. We hope to illuminate the underlying mechanisms coordinating the cell differentiation that ultimately generate a functional leaf epidermis.

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.

Integrative transcriptome and proteome analyses provide deep insights into the molecular mechanism of salt tolerance in Limonium bicolor

Integrative transcriptome and proteome analyses revealed many candidate members that may involve in salt secretion from salt glands in Limonium bicolor. Limonium bicolor, a typical recretohalophyte, protects itself from salt damage by excreting excess salt out of its cells through salt glands. Here, to provide an overview of the salt-tolerance mechanism of L. bicolor, we conducted integrative transcriptome and proteome analyses of this species under salt treatment. We identified numerous differentially expressed transcripts and proteins that may be related to the salt-tolerance mechanism of L. bicolor. By measuring the Na⁺ secretion rate, were found that this cation secretion rate of a single salt gland was significantly increased after high salinity treatment compared with that in control and then reached the maximum in a short time. Interestingly, transcripts and proteins involved in transmembrane transport of ions were differentially expressed in response to high salinity treatment, suggesting a number of genes and proteins they may play important roles in the salt-stress response. Correlation between differentially expressed transcript and protein profiles revealed several transcripts and proteins that may be responsible for salt tolerance, such as cellulose synthases and annexins. Our findings uncovered many candidate transcripts and proteins in response to the salt tolerance of L. bicolor, providing deep insights into the molecular mechanisms of this important process in recretohalophytes.

Coupled Development of Salt Glands, Stomata, and Pavement Cells in Limonium bicolor

Salt-resistant plants have different mechanisms to limit the deleterious effects of high salt in soil; for example, recretohalophytes secrete salt from unique structures called salt glands. Salt glands are the first differentiated epidermal structure of the recretohalophyte sea lavender (Limonium bicolor), followed by stomata and pavement cells. While salt glands and stomata develop prior to leaf expansion, it is not clear whether these steps are connected. Here, we explored the effects of the five phytohormones salicylic acid, brassinolide, methyl jasmonate, gibberellic acid, and abscisic acid on the development of the first expanded leaf of L. bicolor and its potential connection to salt gland, stomata, and pavement cell differentiation. We calculated the total number of salt glands, stomata, and pavement cells, as well as leaf area and pavement cell area, and assessed the correlations between these parameters. We detected strong and positive correlations between salt gland number and pavement cell area, between stomatal number and pavement cell area, and between salt gland number and stomatal number. We observed evidence of coupling between the development of salt glands, stomata, and pavement cells in L. bicolor, which lays the foundation for further investigation of the mechanism behind salt gland development.

Physiological and transcriptomic analyses reveal novel insights into the cultivar-specific response to alkaline stress in alfalfa (Medicago sativa L.)

Soil alkalization severely limits plant growth and development, however, the mechanisms of alkaline response in plants remain largely unknown. In this study, we performed physiological and transcriptomic analyses using two alfalfa cultivars (Medicago sativa L.) with different sensitivities to alkaline conditions. The chlorophyll content and shoot fresh mass drastically declined in the alkaline-sensitive cultivar Algonquin (AG) following alkaline treatment (0-25 mM Na2CO3 solution), while the alkaline-tolerant cultivar Gongnong NO.1 (GN) maintained relatively stable growth and chlorophyll content. Compared with AG, GN had higher contents of Ca2+ and Mg2+; the ratios of Ca2+ and Mg2+ to Na+, proline and soluble sugar, as well as higher enzyme activities of peroxidase (POD) and catalase (CAT) under the alkaline conditions. Furthermore, transcriptomic analysis identified three categories of alkaline-responsive differentially expressed genes (DEGs) between the two cultivars: 48 genes commonly induced in both the cultivars (CAR), 574 genes from the tolerant cultivar (TAR), and 493 genes from the sensitive cultivar (SAR). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that CAR genes were mostly involved in phenylpropanoid biosynthesis, lipid metabolism, and DNA replication and repair; TAR genes were significantly enriched in metabolic pathways, such as biosynthesis of amino acids and secondary metabolites including flavonoids, and the MAPK signaling pathway; SAR genes were specifically enriched in vitamin B6 metabolism. Taken together, the results identified candidate pathways associated with genetic variation in response to alkaline stress, providing novel insights into the mechanisms underlying alkaline tolerance in alfalfa.

Morus alba L. Leaves - Integration of Their Transcriptome and Metabolomics Dataset: Investigating Potential Genes Involved in Flavonoid Biosynthesis at Different Harvest Times

The mulberry leaf is a classic herb commonly used in traditional Chinese medicine. It has also been used as animal feed for livestock and its fruits have been made into a variety of food products. Traditionally, mulberry (Morus alba L.) leaf harvesting after frost is thought to have better medicinal properties, but the underlying mechanism remains largely unsolved. To elucidate the biological basis of mulberry leaves after frost, we first explored the content changes of various compounds in mulberry leaves at different harvest times. Significant enrichment of flavonoids was observed with a total of 224 differential metabolites after frost. Subsequently, we analyzed the transcriptomic data of mulberry leaves collected at different harvest times and successfully annotated 22,939 unigenes containing 1,695 new genes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed 26, 20, and 59 unigenes related to flavonoids synthesis in three different groups harvested at different times. We found that the expression levels of flavonoid biosynthesis-related unigenes also increased when harvested at a delayed time, which was consistent with the flavonoid accumulation discovered by the metabolomic analysis. The results indicated that low temperature may be a key trigger in flavonoid biosynthesis of mulberry leaves by increasing the expression of flavonoid biosynthesis-related genes. This study also provided a theoretical basis for the optimal harvest time of mulberry leaves.

Post-translational regulation of the membrane transporters contributing to salt tolerance in plants

This review article summarises the role of membrane transporters and their regulatory kinases in minimising the toxicity of Na+ in the plant under salt stress. The salt-tolerant plants keep their cytosolic level of Na+ up to 10-50mM. The first line of action in this context is the generation of proton motive force by the plasma membrane H+-ATPase. The generated proton motive force repolarises the membrane that gets depolarised due to passive uptake of Na+ under salt stress. The proton motive force generated also drives the plasma membrane Na+/H+ antiporter, SOS1 that effluxes the cytosolic Na+ back into the environment. At the intracellular level, Na+ is sequestered by the vacuole. Vacuolar Na+ uptake is mediated by Na+/H+ antiporter, NHX, driven by the electrochemical gradient for H+, generated by tonoplast H+ pumps, both H+ATPase and PPase. However, it is the expression of the regulatory kinases that make these transporters active through post-translational modification enabling them to effectively manage the cytosolic level of Na+, which is essential for tolerance to salinity in plants. Yet our knowledge of the expression and functioning of the regulatory kinases in plant species differing in tolerance to salinity is scant. Bioinformatics-based identification of the kinases like OsCIPK24 in crop plants, which are mostly salt-sensitive, may enable biotechnological intervention in making the crop cultivar more salt-tolerant, and effectively increasing its annual yield.

Advances in the Regulation of Epidermal Cell Development by C2H2 Zinc Finger Proteins in Plants

Plant epidermal cells, such as trichomes, root hairs, salt glands, and stomata, play pivotal roles in the growth, development, and environmental adaptation of terrestrial plants. Cell fate determination, differentiation, and the formation of epidermal structures represent basic developmental processes in multicellular organisms. Increasing evidence indicates that C2H2 zinc finger proteins play important roles in regulating the development of epidermal structures in plants and plant adaptation to unfavorable environments. Here, we systematically summarize the molecular mechanism underlying the roles of C2H2 zinc finger proteins in controlling epidermal cell formation in plants, with an emphasis on trichomes, root hairs, and salt glands and their roles in plant adaptation to environmental stress. In addition, we discuss the possible roles of homologous C2H2 zinc finger proteins in trichome development in non-halophytes and salt gland development in halophytes based on bioinformatic analysis. This review provides a foundation for further study of epidermal cell development and abiotic stress responses in plants.

A poplar B-box protein PtrBBX23 modulates the accumulation of anthocyanins and proanthocyanidins in response to high light

Flavonoids, which modulate plant resistance to various stresses, can be induced by high light. B-box (BBX) transcription factors (TFs) play crucial roles in the transcriptional regulation of flavonoids biosynthesis, but limited information is available on the association of BBX proteins with high light. We present a detailed overview of 45 Populus trichocarpa BBX TFs. Phylogenetic relationships, gene structure, tissue-specific expression patterns and expression profiles were determined under 10 stress or phytohormone treatments to screen candidate BBX proteins associated with the flavonoid pathway. Sixteen candidate genes were identified, of which five were expressed predominantly in young leaves and roots, and BBX23 showed the most distinct response to high light. Overexpression of BBX23 in poplar activated expression of MYB TFs and structural genes in the flavonoid pathway, thereby promoting the accumulation of proanthocyanidins and anthocyanins. CRISPR/Cas9-generated knockout of BBX23 resulted in the opposite trend. Furthermore, the phenotype induced by BBX23 overexpression was enhanced under exposure to high light. BBX23 was capable of binding directly to the promoters of proanthocyanidin- and anthocyanin-specific genes, and its interaction with HY5 enhanced activation activity. We identified novel regulators of flavonoid biosynthesis in poplar, thereby enhancing our general understanding of the transcriptional regulatory mechanisms involved.

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.

A novel gene LbHLH from the halophyte Limonium bicolor enhances salt tolerance via reducing root hair development and enhancing osmotic resistance

BACKGROUND

Identifying genes involved in salt tolerance in the recretohalophyte Limonium bicolor could facilitate the breeding of crops with enhanced salt tolerance. Here we cloned the previously uncharacterized gene LbHLH and explored its role in salt tolerance.

RESULTS

The 2,067-bp open reading frame of LbHLH encodes a 688-amino-acid protein with a typical helix-loop-helix (HLH) domain. In situ hybridization showed that LbHLH is expressed in salt glands of L. bicolor. LbHLH localizes to the nucleus, and LbHLH is highly expressed during salt gland development and in response to NaCl treatment. To further explore its function, we heterologously expressed LbHLH in Arabidopsis thaliana under the 35S promoter. The overexpression lines showed significantly increased trichome number and reduced root hair number. LbHLH might interact with GLABRA1 to influence trichome and root hair development, as revealed by yeast two-hybrid analysis. The transgenic lines showed higher germination percentages and longer roots than the wild type under NaCl treatment. Analysis of seedlings grown on medium containing sorbitol with the same osmotic pressure as 100 mM NaCl demonstrated that overexpressing LbHLH enhanced osmotic resistance.

CONCLUSION

These results indicate that LbHLH enhances salt tolerance by reducing root hair development and enhancing osmotic resistance under NaCl stress.

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.

Current Understanding of Role of Vesicular Transport in Salt Secretion by Salt Glands in Recretohalophytes

Soil salinization is a serious and growing problem around the world. Some plants, recognized as the recretohalophytes, can normally grow on saline-alkali soil without adverse effects by secreting excessive salt out of the body. The elucidation of the salt secretion process is of great significance for understanding the salt tolerance mechanism adopted by the recretohalophytes. Between the 1950s and the 1970s, three hypotheses, including the osmotic potential hypothesis, the transfer system similar to liquid flow in animals, and vesicle-mediated exocytosis, were proposed to explain the salt secretion process of plant salt glands. More recently, increasing evidence has indicated that vesicular transport plays vital roles in salt secretion of recretohalophytes. Here, we summarize recent findings, especially regarding the molecular evidence on the functional roles of vesicular trafficking in the salt secretion process of plant salt glands. A model of salt secretion in salt gland is also proposed.

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.

Insulin negatively regulates dedifferentiation of mouse adipocytes in vitro

Insulin plays an important role during adipogenic differentiation of animal preadipocytes and the maintenance of mature phenotypes. However, its role and mechanism in dedifferentiation of adipocyte remains unclear. This study investigated the effects of insulin on dedifferentiation of mice adipocytes, and the potential mechanisms. The preadipocytes were isolated from the subcutaneous white adipose tissue of wild type (WT), TNFα gene mutant (TNFα-/-), leptin gene spontaneous point mutant (db/db) and TNFα-/-/db/db mice and were then induced for differentiation. Interestingly, dedifferentiation of these adipocytes occurred once removing exogenous insulin from the adipogenic medium. As characteristics of dedifferentiation of the adipocytes, downregulation of adipogenic markers, upregulation of stemness markers and loss of intracellular lipids were observed from the four genotypes. Notably, dedifferentiation was occurring earlier if the insulin signal was blocked. These dedifferentiated cells regained the potentials of the stem cell-like characteristics. There is no significant difference in the characteristics of the dedifferentiation between the adipocytes. Overall, the study provided evidence that insulin plays a negative regulatory role in the dedifferentiation of adipocytes. We also confirmed that both dedifferentiation of mouse adipocytes, and effect of the insulin on this process were independent of the cell genotypes, while it is a widespread phenomenon in the adipocytes.

Exogenous melatonin enhances salt secretion from salt glands by upregulating the expression of ion transporter and vesicle transport genes in Limonium bicolor

BACKGROUND

Salt, a common environmental stress factor, inhibits plant growth and reduces yields. Melatonin is a pleiotropic molecule that regulates plant growth and can alleviate environmental stress in plants. All previous research on this topic has focused on the use of melatonin to improve the relatively low salt tolerance of glycophytes by promoting growth and enhancing antioxidant ability. It is unclear whether exogenous melatonin can increase the salt tolerance of halophytes, particularly recretohalophytes, by enhancing salt secretion from the salt glands.

RESULTS

To examine the mechanisms of melatonin-mediated salt tolerance, we explored the effects of exogenous applications of melatonin on the secretion of salt from the salt glands of Limonium bicolor (a kind of recretohalophyte) seedlings and on the expression of associated genes. A pretreatment with 5 μM melatonin significantly improved the growth of L. bicolor seedlings under 300 mM NaCl. Furthermore, exogenous melatonin significantly increased the dry weight and endogenous melatonin content of L. bicolor. In addition, this treatment reduced the content of Na⁺ and Cl^- in leaves, but increased the K⁺ content. Both the salt secretion rate of the salt glands and the expression level of genes encoding ion transporters (LbHTK1, LbSOS1, LbPMA, and LbNHX1) and vesicular transport proteins (LbVAMP721, LbVAP27, and LbVAMP12) were significantly increased by exogenous melatonin treatment. These results indicate that melatonin improves the salt tolerance of the recretohalophyte L. bicolor via the upregulation of salt secretion by the salt glands.

CONCLUSIONS

Our results showed that melatonin can upregulate the expression of genes encoding ion transporters and vesicle transport proteins to enhance salt secretion from the salt glands. Combining the results of the current study with previous research, we formulated a novel mechanism by which melatonin increases salt secretion in L. bicolor. Ions in mesophyll cells are transported to the salt glands through ion transporters located at the plasma membrane. After the ions enter the salt glands, they are transported to the collecting chamber adjacent to the secretory pore through vesicle transport and ions transporter and then are secreted from the secretory pore of salt glands, which maintain ionic homeostasis in the cells and alleviate NaCl-induced growth inhibition.

Photosynthetic and transcriptomic responses of two C4 grass species with different NaCl tolerance

This report reveals the effects of salt on the photosynthetic electron transport and transcriptome of the glycophyte Setaria viridis (S. viridis) and its salt-tolerant close relative halophyte Spartina alterniflora (S. alterniflora). S. viridis was unable to survive exposed to sodium chloride (NaCl) levels higher than 100 mM, in contrast, S. alterniflora could tolerate NaCl up to 550 mM, with negligible effect on gas exchange related parameters and conductance of electrons transport chain (g_ETC). Under salt, the prompt fluorescence (OJIP-curves) exhibits an increase in the O- and J-steps in S. viridis and much less for S. alterniflora. Flowing NaCl stress, a dramatic decline in the photosystem II (PSII) primary photochemistry was observed for S. viridis, as reflected by the drastic drop in F_v/F_m, F_v/F_o and Φ_PSII; however, no substantial change was recorded for these parameters in S. alterniflora. Interestingly, we found an increase in the primary PSII photochemistry (Φ_PSII) for S. alterniflora with increasing either NaCl concentration or NaCl treatment duration. The NPQ magnitude was strongly enhanced for S. viridis even at a low NaCl (50 mM); however, it remains unchangeable or slightly increased for S. alterniflora at NaCl levels above 400 mM. After NaCl treatment, we found an increase in both the proportion of oxidized P₇₀₀ and the amount of active P₇₀₀ in S. viridis and almost no change for S. alterniflora. Under salt, the net photosynthetic rate (A) and stomatal conductance (gs) measurements demonstrate that A decreases earlier in S. viridis, even after one week exposure to only 50 mM NaCl; in contrast, in S. alterniflora, the effect of NaCl on A and g_s was minor even after exposure for two weeks to high NaCl levels. For S. viridis exposed to 50 mM NaCl for 12 d, carbon dioxide (CO₂) at a concentration of 2000 μL L^-1 could not fully restore A to the control (Ctrl) level. Conversely, in S. alterniflora, high CO₂ can fully restore A for all NaCl treatments except at 550 mM. RNA-seq data shows a major impact of NaCl on metabolic pathways in S. viridis and we found a number of transcription factors potentially related to NaCl responses. For S. alterniflora, no major changes in the transcriptomic levels were recorded under NaCl stress. To confirm our data analysis of RNA-seq, we performed quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis for randomly selected four genes for each species (8 genes in total) and we found that our results (up- and/or down-regulated genes) are fully consistent and match well our RNA-seq data. Overall, this study showed drastically different photosynthetic and transcriptomic responses of a salt-tolerant C₄ grass species and one salt-sensitive C₄ grass species to NaCl stress, which suggests that S. alterniflora could be used as a promising model species to study salt tolerance in C₄ or monocot species.

The antioxidant system in Suaeda salsa under salt stress

L. is a typical euhalophyte and is widely distributed throughout the world. Suaeda plants are important halophyte resources, and the physiological and biochemical characteristics of their various organsand their response to salt stress have been intensively studied. Leaf succulence, intracellular ion localization, increased osmotic regulation and enhanced antioxidant capacities are important responses for Suaeda plants to adapt to salt stress. Among these responses, scavenging of reactive oxygen species (ROS) is an important mechanism for plants to withstand oxidative stress and improve salt tolerance. The generation and scavenging pathways of ROS, as well as the expression of scavenging enzymes change under salt stress. This article reviews the antioxidant system constitute of S. salsa, and the mechanisms by which S. salsaantioxidant capacity is improved for salt tolerance. In addition, the differences between types of antioxidant mechanisms in S. salsaare reviewed, thereby revealing the adaptation mechanisms of Suaeda to different habitats. The review provides important clues for the comprehensive understanding of the salt tolerance mechanisms of halophytes.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Novel module and hub genes of distinctive breast cancer associated fibroblasts identified by weighted gene co-expression network analysis

BACKGROUND

As abundant and heterogeneous stromal cells in tumor microenvironment, carcinoma-associated fibroblasts (CAFs) are critically involved in cancer progression.

METHODS

To identify co-expression module and hub genes of distinctive breast CAFs, weighted gene co-expression network analysis (WGCNA) was conducted based on the expression array results of CAFs from seven chemo-sensitive breast cancer (BC) patients and seven chemo-resistant ones before neo-adjuvant chemotherapy.

RESULTS

A total of 4916 genes were included in WGCNA, and 12 modules were determined. Module-trait assay showed that the blue module (cor = 0.97, P < 0.001) was associated with CAF-related chemo-resistance, which was enriched mainly as "inflammatory response", "interferon-gamma-mediated signaling" and "NIK/NF-kappaB signaling" pathways. Moreover, CXCL8, CXCL10, CXCL11, PLSCR1, RIPK2 and USP18 were found to be potentially associated with chemo-resistance related to CAFs and prognosis of BC.

CONCLUSIONS

Our current data offered valuable insights into the molecular mechanisms of distinctive breast CAFs, which was beneficial for revealing how chemo-resistance of BC was initiated.

Transcriptomic analysis identifies novel genes and pathways for salt stress responses in Suaeda salsa leaves

Salinity is a critical abiotic stress, which significantly impacts the agricultural yield worldwide. Identification of the molecular mechanisms underlying the salt tolerance in euhalophyte Suaeda salsa is conducive to the development of salt-resistant crops. In the present study, high-throughput RNA sequencing was performed after S. salsa leaves were exposed to 300 mM NaCl for 7 days, and 7,753 unigenes were identified as differently expressed genes (DEGs) in S. salsa, including 3,638 increased and 4,115 decreased unigenes. Moreover, hundreds of pathways were predicted to participate in salt stress response in S. salsa by Gene Ontology (GO), MapMan and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses, including ion transport and sequestration as well as photoprotection of photosystem (PS) II. The GO enrichment analysis indicated that genes related to ion transport, reactive oxygen species (ROS) scavenging and transcriptional factors were highly expressed upon NaCl treatment. The excessive Na+ and Cl- ions were supposed to be absorbed into the vacuole for ion sequestration and balance adjustment by potassium transporters (such as KEA3) with high expressions. Moreover, we predicted that mutiple candidate genes associated with photosynthesis (such as PSB33 and ABA4), ROS (such as TAU9 and PHI8) and transcriptional regulation (HB-7 and MYB78) pathways could mitigate salt stress-caused damage in S. salsa.

The developmental dynamics of the sweet sorghum root transcriptome elucidate the differentiation of apoplastic barriers

Apoplastic barriers in the endodermis, such as Casparian strips and suberin lamellae, control the passage of water and minerals into the stele. Apoplastic barriers are thus thought to contribute to salt exclusion in salt-excluding plants such as sweet sorghum (Sorghum bicolor). However, little is known about the genes involved in the development of the apoplastic barrier. Here, we identified candidate genes involved in Casparian strip and suberin lamella development in the roots of a sweet sorghum line (M-81E). Three distinct developmental regions (no differentiation, developing, and mature) were identified based on Casparian strip and suberin lamella staining in root cross sections. Sequencing of RNA extracted from these distinct sections identified key genes participating in the differentiation of the apoplastic barrier. The different sections were structurally distinct, presumably due to differences in gene expression. Genes controlling the phenylpropanoid pathway, fatty acid elongation, and fatty acid ω-hydroxylation appeared to be directly responsible for the formation of the apoplastic barrier. Our dataset elucidates the molecular processes underpinning apoplastic barrier development and provides a basis for future research on molecular mechanisms of apoplastic barrier formation and salt exclusion.Abbreviations: SHR, SHORTROOT; MYB, MYB DOMAIN PROTEIN; CIFs, Casparian strip integrity factors; CASP, Casparian strip domain proteins; PER, peroxidase; ESB1, ENHANCED SUBERIN1; CS, Casparian strip; RPKM, reads per kilobase per million reads; DEGs, differentially expressed genes; FDR, false discovery rate; GO, Gene Ontology; KEGG, Kyoto Encyclopedia of Genes and Genomes; RNA-seq, RNA sequencing; PAL, phenylalanine ammonia-lyase; CYP, cytochrome P450 monooxygenases; 4CL, 4-coumarate-CoA ligase; AAE5, ACYL-ACTIVATING ENZYME5; CCR, cinnamoyl CoA reductase; TKPR, TETRAKETIDE ALPHA-PYRONE REDUCTASE1; CAD, cinnamyl alcohol dehydrogenase; HST, shikimate O-hydroxycinnamoyltransferase; PMAT2, PHENOLIC GLUCOSIDE MALONYLTRANSFERASE2; CCOAOMT, caffeoyl-CoA O-methyltransferase; KCS, β-ketoacyl-CoA synthase; CUT1, CUTICULAR PROTEIN1; DET2, 5-alpha-reductase; TAX, 3'-N-debenzoyl-2'-deoxytaxol N-benzoyltransferase; CER1, ECERIFERUM1; FAR, fatty acyl reductase; AF-CoA, alcohol-forming fatty acyl-CoA reductase; ABCG, ATP-binding cassette, subfamily G; ERF, ethylene-responsive transcription factor; HSF, heat stress transcription factor; NTF, NUCLEAR TRANSCRIPTION FACTOR Y SUBUNIT B-5; GPAT, glycerol 3-phosphate acyltransferase.

Chlorophyll synthesis and the photoprotective mechanism in leaves of mulberry (Morus alba L.) seedlings under NaCl and NaHCO3 stress revealed by TMT-based proteomics analyses

Chlorophyll (Chl) and effective photoprotective mechanism are important prerequisites to ensure the photosynthetic function of plants under stress. In this study, the effects of 100 mmol L^-1 NaCl and NaHCO₃ stress on chlorophyll synthesis and photosynthetic function of mulberry seedlings were studied by physiological combined with proteomics technology. The results show that: NaCl stress had little effect on the expression of Chl synthesis related proteins, and there were no significant changes in Chl content and Chl a:b ratio. However, 13 of the 15 key proteins in the process of Chl synthesis were significantly decreased under NaHCO₃ stress, and the contents of Chl a and Chl b were significantly decreased (especially Chl a). Although stomatal conductance (G_s) decreased significantly under NaCl stress, net photosynthetic rate (P_n), PSII maximum photochemical efficiency (F_v/F_m) and electron transfer rate (ETR) did not change significantly, but under NaHCO₃ stress, not only G_s decreased significantly, PSII activity and photosynthetic carbon were the same. In the photoprotective mechanism under NaCl stress, NAD(P)H dehydrogenase (NDH)-dependent cyclic electron flow (CEF) enhanced, the expression of related proteins subunit, ndhH, ndhI, ndhK, and ndhM, the key enzyme of the xanthophyll cycle, violaxanthin de-epoxidase (VDE) were up-regulated, the ratio of (A + Z)/(V + A + Z) and non-photochemical quenching (NPQ) was increased. The expressions of proteins FTR and Fd-NiR were also significant up-regulated under NaCl stress, Fd-dependent ROS metabolism and nitrogen metabolism can effectively reduce the electronic pressure on Fd. Under NaHCO₃ stress, the expressions of NDH-dependent CEF related proteins subunit (ndhH, ndhI, ndhK, ndhM and ndhN), VDE, ZE, FTR, Fd-NiR and Fd-GOGAT, were significant down-regulated, and ZE, CP26, ndhK, ndhM, Fd-NiR, Fd-GOGAT and FTR genes expression also significantly decreased, the photoprotective mechanism, like the xanthophyll cycle，CEF and Fd-dependent ROS metabolism and nitrogen metabolism might be damaged, resulting in the inhibition of PSII electron transfer and carbon assimilation in mulberry leaves under NaHCO₃ stress.

Advances in salt tolerance molecular mechanism in tobacco plants

Tobacco, an economic crop and important model plant, has received more progress in salt tolerance with the aid of transgenic technique. Salt stress has become a key research field in abiotic stress. The study of tobacco promotes the understanding about the important adjustment for survival in high salinity environments, including cellular ion transport, osmotic regulation, antioxidation, signal transduction and expression regulation, and protection of cells from stress damage. Genes, which response to salt, have been studied using targeted transgenic technologies in tobacco plants to investigate the molecular mechanisms. The transgenic tobacco plants exhibited higher seed germination and survival rates, better root and shoot growth under salt stress treatments. Transgenic approach could be the promising option for enhancing tobacco production under saline condition. This review highlighted the salt tolerance molecular mechanisms of tobacco.

C2H2 Zinc Finger Proteins: Master Regulators of Abiotic Stress Responses in Plants

Abiotic stresses such as drought and salinity are major environmental factors that limit crop yields. Unraveling the molecular mechanisms underlying abiotic stress resistance is crucial for improving crop performance and increasing productivity under adverse environmental conditions. Zinc finger proteins, comprising one of the largest transcription factor families, are known for their finger-like structure and their ability to bind Zn2+. Zinc finger proteins are categorized into nine subfamilies based on their conserved Cys and His motifs, including the Cys2/His2-type (C2H2), C3H, C3HC4, C2HC5, C4HC3, C2HC, C4, C6, and C8 subfamilies. Over the past two decades, much progress has been made in understanding the roles of C2H2 zinc finger proteins in plant growth, development, and stress signal transduction. In this review, we focus on recent progress in elucidating the structures, functions, and classifications of plant C2H2 zinc finger proteins and their roles in abiotic stress responses.

Cell Cycle Regulation in the Plant Response to Stress

As sessile organisms, plants face a variety of environmental challenges. Their reproduction and survival depend on their ability to adapt to these stressors, which include water, heat stress, high salinity, and pathogen infection. Failure to adapt to these stressors results in programmed cell death and decreased viability, as well as reduced productivity in the case of crop plants. The growth and development of plants are maintained by meiosis and mitosis as well as endoreduplication, during which DNA replicates without cytokinesis, leading to polyploidy. As in other eukaryotes, the cell cycle in plants consists of four stages (G1, S, G2, and M) with two major check points, namely, the G1/S check point and G2/M check point, that ensure normal cell division. Progression through these checkpoints involves the activity of cyclin-dependent kinases and their regulatory subunits known as cyclins. In order for plants to survive, cell cycle control must be balanced with adaption to dynamic environmental conditions. In this review, we summarize recent advances in our understanding of cell cycle regulation in plants, with a focus on the molecular interactions of cell cycle machinery in the context of stress tolerance.

Plant Unsaturated Fatty Acids: Multiple Roles in Stress Response

Land plants are exposed to not only biotic stresses such as pathogen infection and herbivore wounding, but abiotic stresses such as cold, heat, drought, and salt. Elaborate strategies have been developed to avoid or abide the adverse effects, with unsaturated fatty acids (UFAs) emerging as general defenders. In higher plants, the most common UFAs are three 18-carbon species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds act as ingredients and modulators of cellular membranes in glycerolipids, reserve of carbon and energy in triacylglycerol, stocks of extracellular barrier constituents (e.g., cutin and suberin), precursors of various bioactive molecules (e.g., jasmonates and nitroalkenes), and regulators of stress signaling. Nevertheless, they are also potential inducers of oxidative stress. In this review, we will present an overview of these roles and then shed light on genetic engineering of FA synthetic genes for improving plant/crop stress tolerance.

Importin-β From the Recretohalophyte Limonium bicolor Enhances Salt Tolerance in Arabidopsis thaliana by Reducing Root Hair Development and Abscisic Acid Sensitivity

AIMS

To elucidate the genetics underlying salt tolerance in recretohalophytes and assess its relevance to non-halophytes, we cloned the Limonium bicolor homolog of Arabidopsis thaliana (Arabidopsis) SUPER SENSITIVE TO ABA AND DROUGHT2 (AtSAD2) and named it LbSAD2, an importin-β gene associated with trichome initiation and reduced abscisic acid (ABA) sensitivity, and then we assessed the heterologously expressed LbSAD2 in Arabidopsis.

METHODS

We examined LbSAD2 expression and assessed the effect of heterologous LbSAD2 expression in Arabidopsis on root hair/trichome induction; the expression levels of possible related genes in trichome/root hair development; some physiological parameters involved in salt tolerance including germination rate, root length, and contents of Na⁺, proline, and malondialdehyde; and the response of ABA at the germination stage.

RESULTS

The LbSAD2 gene is highly expressed in the salt gland development stage and salt treatment, especially located in the salt gland by in situ hybridization, and the LbSAD2 protein contains some special domains compared with AtSAD2, which may suggest the involvement of LbSAD2 in salt tolerance. Compared with the SAD2/GL1 mutant CS65878, which lacks trichomes, CS65878-35S:LbSAD2 had higher trichome abundance but lower root hair abundance. Under 100 mM NaCl treatment, CS65878-35S:LbSAD2 showed enhanced germination and root lengths; improved physiological parameters, including high proline and low contents of Na⁺ and malondialdehyde; higher expression of the salt-tolerance genes Δ1-PYRROLINE-5-CARBOXYLATE SYNTHETASE 1 (P5CS1) and GST CLASS TAU 5 (GSTU5); reduced ABA sensitivity; and increased expression of the ABA signaling genes RESPONSIVE TO ABA 18 (RAB18) and SNF1-RELATED PROTEIN KINASE 2 (SRK2E), but not of the ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3).

CONCLUSION

LbSAD2 enhances salt tolerance in Arabidopsis by specifically reducing root hair development, Na⁺ accumulation, and ABA sensitivity.

Plant Unsaturated Fatty Acids: Biosynthesis and Regulation

In most plants, major unsaturated fatty acids (UFAs) are three C18 species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds play multiple crucial roles in planta and are also important economic traits of oil crops. The enzymatic steps of C18 UFA biosynthesis have been well established. However, the associated FA/lipid trafficking between the plastid and the endoplasmic reticulum remains largely unclear, as does the regulation of the expression and activities of the involved enzymes. In this review, we will revisit the biosynthesis of C18 UFAs with an emphasis on the trafficking, and present an overview of the key enzymes and their regulation. Of particular interest is the emerging regulatory network composed of transcriptional factors and upstream signaling pathways. The review thereby provides the promise of using physical, biochemical and/or genetic means to manipulate FA composition and increase oil yield in crop improvement.

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.