Comprehensive transcriptional profiling of NaHCO3-stressed Tamarix hispida roots reveals networks of responsive genes

Transcriptome analysis reveals the mechanism of NaHCO3 promoting tobacco leaf maturation

NaHCO3 accelerates the aging of tobacco leaves; however, the underlying molecular mechanisms have not been elucidated. This study aimed to explore the mechanism of NaHCO3 in the promotion of tobacco leaf maturation using transcriptome analysis. Leaves on plants or detached leaves of the tobacco variety, Honghua Dajinyuan, were sprayed with or without 1% NaHCO3. The leaf yellowing was observed, the pigment content and enzyme activities were determined and RNA sequencing (RNA-seq) was performed. Spraying NaHCO3 onto detached leaves was found to promote leaf yellowing. Pigment content, catalase activity, and superoxide dismutase activity significantly decreased, whereas peroxidase activity and malondialdehyde content significantly increased. RNA-seq demonstrated that spraying with NaHCO3 upregulated genes associated with cysteine and methionine metabolism; alpha-linolenic acid metabolism; and phenylalanine, tyrosine, and tryptophan biosynthesis and downregulated genes related to photosynthesis and carotenoid biosynthesis. Genes correlated with autophagy-other, valine, leucine, and isoleucine degradation, and the MAPK signaling pathway were upregulated while those correlated with DNA replication, phenylalanine, and tyrosine and tryptophan biosynthesis were downregulated in detached leaves sprayed with NaHCO3 compared with the plant leaves sprayed with NaHCO3. Overall, this study is the first to elucidate the molecular and metabolic mechanisms of NaHCO3 in the promotion of tobacco leaf maturation.

Exploration of the Regulatory Pathways and Key Genes Involved in the Response to Saline-Alkali Stress in Betula platyphylla via RNA-Seq Analysis

The pH of saline-alkali soil is high because of carbonate salts, and the deleterious effects of saline-alkali soil on the growth of plants are greater than those of saline soil. Few studies have examined the saline-alkali tolerance of Betula platyphylla at the molecular level. To clarify the regulatory mechanism underlying saline-alkali tolerance in B. platyphylla, RNA sequencing analysis of B. platyphylla seedlings treated with NaHCO₃ was conducted. Differences in gene expression in the roots of B. platyphylla seedlings under saline-alkali stress (induced via NaHCO₃) for 3 h and 6 h were characterized, and a total of 595 and 607 alkali stress-responsive genes were identified, respectively. Most differentially expressed genes were involved in stress, signal transduction, secondary metabolic process, regulation of jasmonic acid, and the abiotic stimulus signaling pathway. The single nucleotide polymorphism loci in the differentially expressed genes were associated with the alkaline-salt tolerance in birch germplasm. In addition, birch plants overexpressing WRKY70 and NAC9 were obtained using the A. tumefaciens-mediated transient transformation method, and these two genes were found to play key roles in saline-alkali tolerance. Additional study revealed that WRKY70 and NAC9 can increase resistance to saline-alkali stress by enhancing reactive oxygen species scavenging and inhibiting cell death in birch plants. The results of this study enhance our understanding of the saline-alkali stress tolerance of B. platyphylla at the molecular level, and provide several key genes that could be used in the breeding of birch plants in the future.

Transcriptome and metabolome analysis reveals key genes and secondary metabolites of Casuarina equisetifolia ssp. incana in response to drought stress

Casuarina equisetifolia is drought tolerant, salt tolerant, and able to grow in barren environments. It is often used to reduce wind damage, to prevent sand erosion, and to help establish plant communities in tropical and subtropical coastal zones. To determine the basis for its drought tolerance, we conducted transcriptomic and metabolic analyses of young branchlets under a non-drought treatment (D_0h) and 2-, 12-, and 24-h-long drought treatments (D_2h, D_12h, and D_24h). A total of 5033 and 8159 differentially expressed genes (DEGs) were identified in D_2h/D_0h and D_24h/D_0h. These DEGs were involved in plant hormone signal transduction, jasmonic acid (JA) biosynthesis, flavonoid biosynthesis, and phenylpropanoid biosynthesis. A total of 148 and 168 differentially accumulated metabolites (DAMs) were identified in D_12h/D_0h and D_24h/D_0h, which were mainly amino acids, phenolic acids, and flavonoids. In conclusion, C. equisetifolia responds to drought by regulating plant hormone signal transduction and the biosynthesis of JA, flavonoid, and phenylpropanoid. These results increase the understanding of drought tolerance in C. equisetifolia at both transcriptional and metabolic levels and provide new insights into coastal vegetation reconstruction and management.

MicroRNAs balance growth and salt stress responses in sweet sorghum

Salt stress is one of the major causes of reduced crop production, limiting agricultural development globally. Plants have evolved with complex systems to maintain the balance between growth and stress responses, where signaling pathways such as hormone signaling play key roles. Recent studies revealed that hormones are modulated by microRNAs (miRNAs). Previously, two sweet sorghum (Sorghum bicolor) inbred lines with different salt tolerance were identified: the salt-tolerant M-81E and the salt-sensitive Roma. The levels of endogenous hormones in M-81E and Roma varied differently under salt stress, showing a different balance between growth and stress responses. miRNA and degradome sequencing showed that the expression of many upstream transcription factors regulating signal transduction and hormone-responsive genes was directly induced by differentially expressed miRNAs, whose levels were very different between the two sweet sorghum lines. Furthermore, the effects of representative miRNAs on salt tolerance in sorghum were verified through a transformation system mediated by Agrobacterium rhizogenes. Also, miR-6225-5p reduced the level of Ca²⁺ in the miR-6225-5p-overexpressing line by inhibiting the expression of the Ca²⁺ uptake gene SbGLR3.1 in the root epidermis and affected salt tolerance in sorghum. This study provides evidence for miRNA-mediated growth and stress responses in sweet sorghum.

ThASR3 confers salt and osmotic stress tolerances in transgenic Tamarix and Arabidopsis

BACKGROUND

ASR (abscisic acid-, stress-, and ripening-induced) gene family plays a crucial role in responding to abiotic stresses in plants. However, the roles of ASR genes protecting plants against high salt and drought stresses remain unknown in Tamarix hispida.

RESULTS

In this study, a salt and drought-induced ASR gene, ThASR3, was isolated from Tamarix hispida. Transgenic Arabidopsis overexpressing ThASR3 exhibited stimulating root growth and increasing fresh weight compared with wild-type (WT) plants under both salt and water deficit stresses. To further analyze the gain- and loss-of-function of ThASR3, the transgenic T. hispida plants overexpressing or RNA interference (RNAi)-silencing ThASR3 were generated using transient transformation. The overexpression of ThASR3 in Tamarix and Arabidopsis plants displayed enhanced reactive oxygen species (ROS) scavenging capability under high salt and osmotic stress conditions, including increasing the activities of antioxidant enzymes and the contents of proline and betaine, and reducing malondialdehyde (MDA) content and electrolyte leakage rates.

CONCLUSION

Our results indicate that ThASR3 functions as a positive regulator in Tamarix responses to salt and osmotic stresses and confers multiple abiotic stress tolerances in transgenic plants, which may have an important application value in the genetic improvement of forest tree resistance.

Integrated Analyses of Transcriptome and Chlorophyll Fluorescence Characteristics Reveal the Mechanism Underlying Saline-Alkali Stress Tolerance in Kosteletzkya pentacarpos

In recent years, soil salinization has become increasingly severe, and the ecological functions of saline-alkali soils have deteriorated because of the lack of plants. Therefore, understanding the tolerance mechanisms of saline-alkali-tolerant plants has become crucial to restore the ecological functions of saline-alkali soils. In this study, we evaluated the molecular mechanism underlying the tolerance of Kosteletzkya pentacarpos L. (seashore mallow) seedlings treated with 0.05 or 0.5% saline-alkali solution (NaCl: NaHCO₃ = 4:1 mass ratio) for 1 and 7 days. We identified the key genes involved in tolerance to saline-alkali stress using orthogonal partial least squares regression analysis (OPLS-RA) based on both chlorophyll fluorescence indexes and stress-responsive genes using transcriptome analysis, and, finally, validated their expression using qRT-PCR. We observed minor changes in the maximum photochemical efficiency of the stressed seedlings, whose photosynthetic performance remained stable. Moreover, compared to the control, other indicators varied more evidently on day 7 of 0.5% saline-alkali treatment, but no variations were observed in other treatments. Transcriptome analysis revealed a total of 54,601 full-length sequences, with predominantly downregulated differentially expressed gene (DEG) expression. In the high concentration treatment, the expression of 89.11 and 88.38% of DEGs was downregulated on days 1 and 7, respectively. Furthermore, nine key genes, including KpAGO4, KpLARP1C, and KpPUB33, were involved in negative regulatory pathways, such as siRNA-mediated DNA methylation, inhibition of 5'-terminal oligopyrimidine mRNA translation, ubiquitin/proteasome degradation, and other pathways, including programmed cell death. Finally, quantitative analysis suggested that the expression of key genes was essentially downregulated. Thus, these genes can be used in plant molecular breeding in the future to generate efficient saline-alkali-tolerant plant germplasm resources to improve the ecological functions of saline-alkali landscapes.

Transcriptomic Analysis of Cadmium Stressed Tamarix hispida Revealed Novel Transcripts and the Importance of Abscisic Acid Network

Cadmium (Cd) pollution is widely detected in soil and has been recognized as a major environmental problem. Tamarix hispida is a woody halophyte, which can form natural forest on the desert and soil with 0.5 to 1% salt content, making it an ideal plant for the research on response to abiotic stresses. However, no systematic study has investigated the molecular mechanism of Cd tolerance in T. hispida. In the study, RNA-seq technique was applied to analyze the transcriptomic changes in T. hispida treated with 150 μmol L^-1 CdCl₂ for 24, 48, and 72 h compared with control. In total, 72,764 unigenes exhibited similar sequences in the Non-redundant nucleic acid database (NR database), while 36.3% of all these unigenes may be new transcripts. In addition, 6,778, 8,282, and 8,601 DEGs were detected at 24, 48, and 72 h, respectively. Functional annotation analysis indicated that many genes may be involved in Cd stress response, including ion bonding, signal transduction, stress sensing, hormone responses and ROS metabolism. A ThUGT gene from the abscisic acid (ABA) signaling pathway can enhance Cd resistance ability of T. hispida by regulating the production of ROS under Cd stress and inhibit absorption of Cd. The new transcriptome resources and data that we present in this study for T. hispida may facilitate investigation of molecular mechanisms governing Cd resistance.

Comprehensive analysis of the stress associated protein (SAP) gene family in Tamarix hispida and the function of ThSAP6 in salt tolerance

Stress associated proteins (SAPs), a class of A20/AN1 zinc finger domain-containing proteins, are involved in a variety of biotic and abiotic stress responses in plants. However, little is known about the SAP gene family and their functions in Tamarix hispida. In this study, we isolated and characterized 11 SAPs from T. hispida. The expression patterns of ThSAPs were analyzed under various stresses (salt and drought) and phytohormone treatment (SA, ABA and MeJA) using real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR). Most ThSAPs exhibited transcriptional responses to abiotic stresses and phytohormones. Among these ThSAPs, ThSAP6 was significantly induced by salt stress. Gain-and loss-of-function analyses revealed that ThSAP6 was a positive regulator of salt stress response. Overexpression of ThSAP6 in T. hispida increased antioxidant enzymes activity and proline content and decreased reactive oxygen species (ROS) accumulation and cell membrane damage under salt stress, while the opposite physiological changes were observed in ThSAP6-RNAi (RNA interference) lines. This study provides a comprehensive description of the SAP gene family in T. hispida, and demonstrates that ThSAP6 is a potential candidate for biotechnological approaches to improve salt tolerance in plants.

Integration of mRNA and miRNA analysis reveals the molecular mechanism of potato (Solanum tuberosum L.) response to alkali stress

The continuing increase in the global saline-alkali land area has made saline-alkali stress the principal abiotic stress limiting plant growth. Potato is the most important non-grain crop, and its production is also severely limited by saline-alkali stress. However, few studies have addressed the mechanism of saline-alkali tolerance of potato with a focus on its response to neutral salt NaCl stress, or its response to alkali stress. Recently, miRNA-mRNA analyses have helped advance our understanding of how plants respond to stress. Here, we have characterized the morphological, physiological, and transcriptome changes of tissue culture seedlings of potato variety "Qingshu No. 9" treated with NaHCO₃ (for 0, 2, 6, and 24 h). We found that the leaves of tissue culture seedlings wilted and withered under alkali stress, and the contents of ABA, BRs, trehalose, and lignin in roots increased significantly. The contents of GAs decreased significantly. Subsequently, miRNA-seq analysis results identified 168 differentially expressed miRNAs (DEMIs) under alkali stress, including 21 exist miRNAs and 37 known miRNAs from 47 families and 110 novel miRNAs. The mRNA-seq results identified 5731 differentially expressed mRNAs (DEMs) under alkali stress. By miRNA-mRNA integrated analysis, were obtained 33 miRNA-target gene pairs composed of 20 DEMIs and 33 DEMs. Next, we identified the "phenylpropanoid biosynthesis", "plant hormone signal transduction", and "starch and sucrose metabolism" pathways as necessary for potato to respond to alkali stress. miR4243-x and novel-m064-5p were involved in the response of potato to alkali stress by their negative regulatory effects on shikimate O-hydroxycinnamoyltransferase (HCT) and sucrose-phosphate synthase (SPS) genes, respectively. The expression results of miRNA and mRNA were verified by quantitative real-time PCR (qRT-PCR). Our results clarify the mechanism of potato response to alkali stress at the miRNA level, providing new insights into the molecular mechanisms of potato's response to alkali stress. We report many candidate miRNAs and mRNAs for molecular-assisted screening and salt-alkali resistance breeding.

Transcriptomics Reveals Fast Changes in Salicylate and Jasmonate Signaling Pathways in Shoots of Carbonate-Tolerant Arabidopsis thaliana under Bicarbonate Exposure

High bicarbonate concentrations of calcareous soils with high pH can affect crop performance due to different constraints. Among these, Fe deficiency has mostly been studied. The ability to mobilize sparingly soluble Fe is a key factor for tolerance. Here, a comparative transcriptomic analysis was performed with two naturally selected Arabidopsis thaliana demes, the carbonate-tolerant A1(c+) and the sensitive T6(c-). Analyses of plants exposed to either pH stress alone (pH 5.9 vs. pH 8.3) or to alkalinity caused by 10 mM NaHCO3 (pH 8.3) confirmed better growth and nutrient homeostasis of A1(c+) under alkaline conditions. RNA-sequencing (RNA-seq) revealed that bicarbonate quickly (3 h) induced Fe deficiency-related genes in T6(c-) leaves. Contrastingly, in A1(c+), initial changes concerned receptor-like proteins (RLP), jasmonate (JA) and salicylate (SA) pathways, methionine-derived glucosinolates (GS), sulfur starvation, starch degradation, and cell cycle. Our results suggest that leaves of carbonate-tolerant plants do not sense iron deficiency as fast as sensitive ones. This is in line with a more efficient Fe translocation to aerial parts. In A1(c+) leaves, the activation of other genes related to stress perception, signal transduction, GS, sulfur acquisition, and cell cycle precedes the induction of iron homeostasis mechanisms yielding an efficient response to bicarbonate stress.

Bacillus amyloliquefaciens PDR1 from root of karst adaptive plant enhances Arabidopsis thaliana resistance to alkaline stress through modulation of plasma membrane H+-ATPase activity

Exploration of native microbes is a feasible way to develop microbial agents for ecological restoration. This study was aimed to explore the impact of Bacillus amyloliquefaciens PDR1 from karst adaptive plant on the activity of root plasma membrane H⁺-ATPase in Arabidopsis thaliana. A. thaliana was cultured in presence or absence of B. amyloliquefaciens PDR1 and its effects on the growth were evaluated by measuring the taproot length and dry weight. The rhizosphere acidification capacity was detected by a pH indicator, a pH meter and non-invasive micro-test techniques (NMT). The nutrient uptake was performed using appropriate methods. A combination of transcriptome sequencing and real-time quantitative polymerase chain reaction (qRT-PCR) was used to measure the expression of functional genes that regulate the plasma membrane H⁺-ATPase activity in A. thaliana roots. Functional analysis was performed to understand how B. amyloliquefaciens regulates biological processes and metabolic pathways to strengthen A. thaliana resistance to alkaline stress. Here, we show that volatile organic compounds (VOCs) from B. amyloliquefaciens PDR1 promoted the growth and development of A. thaliana, enhanced the plasma membrane H⁺-ATPase activity, and affected ion absorption in Arabidopsis roots. Moreover, B. amyloliquefaciens PDR1 VOCs did not affect the expression of the gene coding for plasma membrane H⁺-ATPase, but affected the expression of genes regulating the activity of plasma membrane H⁺-ATPase. Our findings illuminate the mechanism by which B. amyloliquefaciens regulates the growth and alkaline stress resistance of A. thaliana, and lay a foundation for wide and efficient application for agricultural production and ecological protection.

Water Deficit Transcriptomic Responses Differ in the Invasive Tamarix chinensis and T. ramosissima Established in the Southern and Northern United States

Tamarix spp. (saltcedar) were introduced from Asia to the southern United States as windbreak and ornamental plants and have spread into natural areas. This study determined differential gene expression responses to water deficit (WD) in seedlings of T. chinensis and T. ramosissima from established invasive stands in New Mexico and Montana, respectively. A reference de novo transcriptome was developed using RNA sequences from WD and well-watered samples. Blast2GO analysis of the resulting 271,872 transcripts yielded 89,389 homologs. The reference Tamarix (Tamaricaceae, Carophyllales order) transcriptome showed homology with 14,247 predicted genes of the Beta vulgaris subsp. vulgaris (Amaranthaceae, Carophyllales order) genome assembly. T. ramosissima took longer to show water stress symptoms than T. chinensis. There were 2068 and 669 differentially expressed genes (DEG) in T. chinensis and T. ramosissima, respectively; 332 were DEG in common between the two species. Network analysis showed large biological process networks of similar gene content for each of the species under water deficit. Two distinct molecular function gene ontology networks (binding and transcription factor-related) encompassing multiple up-regulated transcription factors (MYB, NAC, and WRKY) and a cellular components network containing many down-regulated photosynthesis-related genes were identified in T. chinensis, in contrast to one small molecular function network in T. ramosissima.

Transcriptomic and phosphoproteomic profiling and metabolite analyses reveal the mechanism of NaHCO3-induced organic acid secretion in grapevine roots

BACKGROUND

Organic acid secretion is a widespread physiological response of plants to alkalinity. However, the characteristics and underlying mechanism of the alkali-induced secretion of organic acids are poorly understood.

RESULTS

Oxalate was the main organic acid synthesized and secreted in grapevine (a hybrid of Vitis amurensis, V. berlandieri and V. riparia) roots, while acetate synthesis and malate secretion were also promoted under NaHCO₃ stress. NaHCO₃ stress enhanced the H⁺ efflux rate of grapevine roots, which is related to the plasma membrane H⁺-ATPase activity. Transcriptomic profiling revealed that carbohydrate metabolism was the most significantly altered biological process under NaHCO₃ stress; a total of seven genes related to organic acid metabolism were significantly altered, including two phosphoenolpyruvate carboxylases and phosphoenolpyruvate carboxylase kinases. Additionally, the expression levels of five ATP-binding cassette transporters, particularly ATP-binding cassette B19, and two Al-activated malate transporter 2 s were substantially upregulated by NaHCO₃ stress. Phosphoproteomic profiling demonstrated that the altered phosphoproteins were primarily related to binding, catalytic activity and transporter activity in the context of their molecular functions. The phosphorylation levels of phosphoenolpyruvate carboxylase 3, two plasma membrane H⁺-ATPases 4 and ATP-binding cassette B19 and pleiotropic drug resistance 12 were significantly increased. Additionally, the inhibition of ethylene synthesis and perception completely blocked NaHCO₃-induced organic acid secretion, while the inhibition of indoleacetic acid synthesis reduced NaHCO₃-induced organic acid secretion.

CONCLUSIONS

Our results demonstrated that oxalate was the main organic acid produced under alkali stress and revealed the necessity of ethylene in mediating organic acid secretion. Additionally, we further identified several candidate genes and phosphoproteins responsible for organic acid metabolism and secretion.

The NAC Protein from Tamarix hispida, ThNAC7, Confers Salt and Osmotic Stress Tolerance by Increasing Reactive Oxygen Species Scavenging Capability

Plant specific NAC (NAM, ATAF1/2 and CUC2) transcription factors (TFs) play important roles in response to abiotic stress. In this study, we identified and characterized a NAC protein, ThNAC7, from Tamarix hispida. ThNAC7 is a nuclear localized protein and has transcriptional activation activity. ThNAC7 expression was markedly induced by salt and osmotic stresses. Transiently transformed T. hispida seedlings overexpressing ThNAC7 (OE) or with RNA interference (RNAi) silenced ThNAC7 were generated to investigate abiotic stress tolerance via the gain- and loss- of function. Overexpressing ThNAC7 showed an increased reactive oxygen species (ROS) scavenging capabilities and proline content, which was accomplished by enhancing the activities of superoxide dismutase (SOD) and peroxidase (POD) in transiently transformed T. hispida and stably transformed Arabidopsis plants. Additionally, ThNAC7 activated these physiological changes by regulating the transcription level of P5CS, SOD and PODgenes. RNA-sequencing (RNA-seq) comparison between wild-type and ThNAC7-transformed Arabidopsis showed that more than 40 known salt tolerance genes might regulated by ThNAC7, including stress tolerance-related genes and TF genes. The results indicated that ThNAC7 induces the transcription level of genes associated with stress tolerance to enhance salt and osmotic stress tolerance via an increase in osmotic potential and enhanced ROS scavenging.

Physiological and TMT-based proteomic analysis of oat early seedlings in response to alkali stress

Oats are an important cereal crop worldwide, and they also serve as a phytoremediation crop to ameliorate salinized and alkalized soils. However, the mechanism of the oat response to alkali remains unclear. Physiological and tandem mass tag (TMT)-based proteomic analyses were employed to elucidate the mechanism of the oat response to alkali stress. Physiological and phenotypic data showed that oat root growth was inhibited more severely than shoot growth after alkali stress. In total, 164 proteins were up-regulated and 241 proteins were down-regulated in roots, and 93 proteins were up-regulated and 139 proteins were down-regulated in shoots. Under high pH stress, transmembrane proton transporters were down-regulated; conversely, organic acid synthesis related enzymes were increased. Transporters of N, P, Fe, Cu and Ca in addition to N assimilation enzymes in the root were highly increased. This result revealed that higher efficiency of P, Fe, Cu and Ca transport, especially higher efficiency of N intake and assimilation, greatly promoted oat root resistance to alkali stress. Furthermore, many resistance proteins, such as late embryogenesis abundant (LEA) mainly in shoots, GDSL esterase lipase mainly in roots, and WD40-like beta propeller repeat families, greatly accumulated to contribute to oat resistance to alkali stress. SIGNIFICANCE: In this study, physiological and tandem mass tag (TMT)-based proteomic analyses were employed to elucidate oats early seedlings in response to alkali stress. Many difference expression proteins were found involving in oats response to alkali stress. Also, higher efficiency transport of P, Fe, Cu, Ca and N greatly promoted oat resistance to alkali stress.

A Glycine soja group S2 bZIP transcription factor GsbZIP67 conferred bicarbonate alkaline tolerance in Medicago sativa

BACKGROUND

Even though bicarbonate alkaline stress is a serious threat to crop growth and yields, it attracts much fewer researches than high salinity stress. The basic leucine zipper (bZIP) transcription factors have been well demonstrated to function in diverse abiotic stresses; however, their biological role in alkaline tolerance still remains elusive. In this study, we functionally characterized a bZIP gene from Glycine soja GsbZIP67 in bicarbonate alkaline stress responses.

RESULTS

GsbZIP67 was initially identified as a putative bicarbonate responsive gene, on the basis of previous RNA-seq data of 50 mM NaHCO3-treated Glycine soja roots. GsbZIP67 protein possessed a conserved bZIP domain, and belonged to the group S2 bZIP, which is yet less well-studied. Our studies showed that GsbZIP67 targeted to nucleus in Arabidopsis protoplasts, and displayed transcriptional activation activity in yeast cells. The quantitative real-time PCR analyses unraveled the bicarbonate stress responsive expression and tissue specific expression of GsbZIP67 in wild soybean. Further phenotypic analysis illustrated that GsbZIP67 overexpression in alfalfa promoted plant growth under bicarbonate alkaline stress, as evidenced by longer roots and shoots. Furthermore, GsbZIP67 overexpression also modified the physiological indices of transgenic alfalfa under bicarbonate alkaline stress. In addition, the expression levels of several stress responsive genes were also augmented by GsbZIP67 overexpression.

CONCLUSIONS

Collectively, in this study, we demonstrated that GsbZIP67 acted as a positive regulator of plant tolerance to bicarbonate alkaline stress. These results provide direct genetic evidence of group S2 bZIPs in bicarbonate alkaline stress, and will facilitate further studies concerning the cis-elements and/or downstream genes targeted by GsbZIP67 in stress responses.

De novo transcriptome sequencing and analysis of genes related to salt stress response in Glehnia littoralis

Soil salinity is one of the major environmental stresses affecting plant growth, development, and reproduction. Salt stress also affects the accumulation of some secondary metabolites in plants. Glehnia littoralis is an endangered medicinal halophyte that grows in coastal habitats. Peeled and dried Glehnia littoralis roots, named Radix Glehniae, have been used traditionally as a Chinese herbal medicine. Although Glehnia littoralis has great ecological and commercial value, salt-related mechanisms in Glehnia littoralis remain largely unknown. In this study, we analysed the transcriptome of Glehnia littoralis in response to salt stress by RNA-sequencing to identify potential salt tolerance gene networks. After de novo assembly, we obtained 105,875 unigenes, of which 75,559 were annotated in public databases. We identified 10,335 differentially expressed genes (DEGs; false discovery rate <0.05 and |log₂ fold-change| ≥ 1) between NaCl treatment (GL2) and control (GL1), with 5,018 upregulated and 5,317 downregulated DEGs. To further this investigation, we performed Gene Ontology (GO) analysis and the Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis. DEGs involved in secondary metabolite biosynthetic pathways, plant signal transduction pathways, and transcription factors in response to salt stress were analysed. In addition, we tested the gene expression of 15 unigenes by quantitative real-time PCR (qRT-PCR) to confirm the RNA-sequencing results. Our findings represent a large-scale assessment of the Glehnia littoralis gene resource, and provide useful information for exploring its molecular mechanisms of salt tolerance. Moreover, genes enriched in metabolic pathways could be used to investigate potential biosynthetic pathways of active compounds by Glehnia littoralis.

Tamarix microRNA Profiling Reveals New Insight into Salt Tolerance

The halophyte tamarisk (Tamarix) is extremely salt tolerant, making it an ideal material for salt tolerance-related studies. Although many salt-responsive genes of Tamarix were identified in previous studies, there are no reports on the role of post-transcriptional regulation in its salt tolerance. We constructed six small RNA libraries of Tamarix chinensis roots with NaCl treatments. High-throughput sequencing of the six libraries was performed and microRNA expression profiles were constructed. We investigated salt-responsive microRNAs to uncover the microRNA-mediated genes regulation. From these analyses, 251 conserved and 18 novel microRNA were identified from all small RNAs. From 191 differentially expressed microRNAs, 74 co-expressed microRNAs were identified as salt-responsive candidate microRNAs. The most enriched GO (gene ontology) terms for the 157 genes targeted by differentially expressed microRNAs suggested that transcriptions factors were highly active. Two hub microRNAs (miR414, miR5658), which connected by several target genes into an organic microRNA regulatory network, appeared to be the key regulators of post-transcriptional salt-stress responses. As the first survey on the tamarisk small RNAome, this study improves the understanding of tamarisk salt-tolerance mechanisms and will contribute to the molecular-assisted resistance breeding.

An ERF transcription factor from Tamarix hispida, ThCRF1, can adjust osmotic potential and reactive oxygen species scavenging capability to improve salt tolerance

Ethylene-Responsive Factors (ERFs) are plant-specific transcription factors (TFs) involved in multiple biological processes, especially in abiotic stress tolerance. However, the ERFs from woody halophytes that are involved in salt stress have been little studied. In the present investigation, we characterized a subfamily member of ERF TFs from Tamarix hispida, ThCRF1, which responds to salt stress. ThCRF1 is a nuclear protein that binds to the motifs including TTG, DRE and GCC-box. Transient transformation was performed to generate T. hispida overexpressing ThCRF1 and RNA interference (RNAi)-silenced ThCRF1 to analyze its function using gain- and loss-of-function methods. Overexpression of ThCRF1 in T. hispida significantly improved tolerance to salt-shock-induced stress; by contrast, RNAi-silence of ThCRF1 significantly decreased tolerance to salt-shock-induced stress. Further experiments showed that ThCRF1 induces the expression of genes including those encoding pyrroline-5-carboxylate synthetase (P5CS), trehalose-6-phosphate synthase (TPS), trehalose-6-phosphate phosphatase (TPP), superoxide dismutase (SOD) and peroxidase (POD), which lead to enhanced proline and trehalose levels and increased SOD and POD activities. These results were further confirmed by studying transgenic Arabidopsis plants overexpressing ThCRF1. Therefore, the results suggested that ThCRF1 improves tolerance to salt-shock-induced stress by enhancing trehalose and proline biosynthesis to adjust the osmotic potential, and by improving SOD and POD activities to increase reactive oxygen species scavenging capability.

Transcriptome analysis of genes involved in defense against alkaline stress in roots of wild jujube (Ziziphus acidojujuba)

Wild jujube (Ziziphus acidojujuba Mill.) is highly tolerant to alkaline, saline and drought stress; however, no studies have performed transcriptome profiling to study the response of wild jujube to these and other abiotic stresses. In this study, we examined the tolerance of wild jujube to NaHCO3-NaOH solution and analyzed gene expression profiles in response to alkaline stress. Physiological experiments revealed that H2O2 content in leaves increased significantly and root activity decreased quickly during alkaline of pH 9.5 treatment. For transcriptome analysis, wild jujube plants grown hydroponically were treated with NaHCO3-NaOH solution for 0, 1, and 12 h and six transcriptomes from roots were built. In total, 32,758 genes were generated, and 3,604 differentially expressed genes (DEGs) were identified. After 1 h, 853 genes showed significantly different expression between control and treated plants; after 12 h, expression of 2,856 genes was significantly different. The expression pattern of nine genes was validated by quantitative real-time PCR. After gene annotation and gene ontology enrichment analysis, the genes encoding transcriptional factors, serine/threonine-protein kinases, heat shock proteins, cysteine-like kinases, calmodulin-like proteins, and reactive oxygen species (ROS) scavengers were found to be closely involved in alkaline stress response. These results will provide useful insights for elucidating the mechanisms underlying alkaline tolerance in wild jujube.

ThDof1.4 and ThZFP1 constitute a transcriptional regulatory cascade involved in salt or osmotic stress in Tamarix hispida

Identification of the upstream regulators of a gene is important to characterize the transcriptional pathway and the function of the gene. Previously, we found that a zinc finger protein (ThZFP1) is involved in abiotic stress tolerance of Tamarix hispida. In the present study, we further investigated the transcriptional pathway of ThZFP1. Dof motifs are abundant in the ThZFP1 promoter; therefore, we used them to screen for transcriptional regulators of ThZFP1. A Dof protein, ThDof1.4, binds to the Dof motif specifically, and was hypothesized as the upstream regulator of ThZFP1. Further study showed that overexpression of ThDof1.4 in T. hispida activated the expression of GUS controlled by the ThZFP1 promoter. In T. hispida, transient overexpression of ThDof1.4 increased the transcripts of ThZFP1; conversely, transient RNAi-silencing of ThDof1.4 reduced the expression of ThZFP1. Chromatin immunoprecipitation indicated that ThDof1.4 binds to the ThZFP1 promoter. Additionally, ThDof1.4 and ThZFP1 share similar expression patterns in response to salt or drought stress. Furthermore, like ThZFP1, ThDof1.4 could increase the proline level and enhance ROS scavenging capability to improve salt and osmotic stress tolerance. Together, these results suggested that ThDof1.4 and ThZFP1 form a transcriptional regulatory cascade involved in abiotic stress resistance in T. hispida.

Making Plants Break a Sweat: the Structure, Function, and Evolution of Plant Salt Glands

Salt stress is a complex trait that poses a grand challenge in developing new crops better adapted to saline environments. Some plants, called recretohalophytes, that have naturally evolved to secrete excess salts through salt glands, offer an underexplored genetic resource for examining how plant development, anatomy, and physiology integrate to prevent excess salt from building up to toxic levels in plant tissue. In this review we examine the structure and evolution of salt glands, salt gland-specific gene expression, and the possibility that all salt glands have originated via evolutionary modifications of trichomes. Salt secretion via salt glands is found in more than 50 species in 14 angiosperm families distributed in caryophyllales, asterids, rosids, and grasses. The salt glands of these distantly related clades can be grouped into four structural classes. Although salt glands appear to have originated independently at least 12 times, they share convergently evolved features that facilitate salt compartmentalization and excretion. We review the structural diversity and evolution of salt glands, major transporters and proteins associated with salt transport and secretion in halophytes, salt gland relevant gene expression regulation, and the prospect for using new genomic and transcriptomic tools in combination with information from model organisms to better understand how salt glands contribute to salt tolerance. Finally, we consider the prospects for using this knowledge to engineer salt glands to increase salt tolerance in model species, and ultimately in crops.

Transcriptome profiling reveals the genetic basis of alkalinity tolerance in wheat

Background

Soil alkalinity shows significant constraints to crop productivity; however, much less attention has been paid to analyze the effect of soil alkalinity on plant growth and development. Shanrong No. 4 (SR4) is an alkalinity tolerant bread wheat cultivar selected from an asymmetric somatic hybridization between the bread wheat cultivar Jinan 177 (JN177) and tall wheatgrass (Thinopyrum ponticum), which is a suitable material for studying alkalinity tolerant associate genes.

Results

The growth of SR4 plant seedlings was less inhibited than that of JN177 when exposed to alkalinity stress conditions. The root cytosolic Na⁺/K⁺ ratio in alkalinity stressed SR4 was lower than in JN177, while alkalinity stressed SR4 contained higher level of nutrient elements than in JN177. SR4 plant seedlings accumulated less malondialdehyde (MDA) and reactive oxygen species (ROS), it also showed higher activity of ROS scavenging enzymes than JN177 under alkalinity stress. The root intracellular pH decreased in both alkalinity stressed JN177 and SR4, however, it was much lower in SR4 than in JN177 under alkalinity stress. The transcriptomes of SR4 and JN177 seedlings exposed to alkalinity stress were analyzed by digital gene expression tag profiling method. Alkalinity stress conditions up- and down-regulated a large number of genes in the seedling roots that play the functions in the categories of transcription regulation, signal transduction and protein modification.

Conclusions

SR4 expresses a superior tolerance to alkaline stress conditions which is due to its strong absorbing ability for nutrient ions, a strong regulating ability for intracellular and rhizosphere pH and a more active ROS scavenging ability.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3421-8) contains supplementary material, which is available to authorized users.

CsWRKY46, a WRKY transcription factor from cucumber, confers cold resistance in transgenic-plant by regulating a set of cold-stress responsive genes in an ABA-dependent manner

Plant WRKY transcription factors are trans-regulatory proteins that are involved in plant immune responses, development and senescence; however, their roles in abiotic stress are still not well understood, especially in the horticultural crop cucumber. In this study, a novel cucumber WRKY gene, CsWRKY46 was cloned and identified, which was up-regulated in response to cold stress and exogenous abscisic acid (ABA) treatment. CsWRKY46 is belonging to group II of the WRKY family, CsWRKY46 was found exclusively in the nucleus, as indicated by a transient expression assay. Yeast one-hybrid assay shown that CsWRKY46 interact with the W-box in the promoter of ABI5. Transgenic Arabidopsis lines over-expressing CsWRKY46, WRK46-OE1 and WRK46-OE5 had higher seedling survival rates upon freezing treatment compared with that of the wild-type. The above over-expression lines also showed much a higher proline accumulation, less electrolyte leakage and lower malondialdehyde (MDA) levels. Furthermore, the CsWRKY46 overexpression lines were hypersensitive to ABA during seed germination, but the seedlings were not. Quantitative RT-PCR analyses revealed that the expression levels of the ABA-responsive transcription factor ABI5 were higher in the WRKY46-OE lines than in wild-type and that the overexpression of CsWRKY46 increased the expression of stress-inducible genes, including RD29A and COR47. Taken together, our results demonstrated that CsWRKY46 from cucumber conferred cold tolerance to transgenic plants and positively regulated the cold signaling pathway in an ABA-dependent manner.

Na2CO3-responsive mechanisms in halophyte Puccinellia tenuiflora roots revealed by physiological and proteomic analyses

Soil alkalization severely affects crop growth and agricultural productivity. Alkali salts impose ionic, osmotic, and high pH stresses on plants. The alkali tolerance molecular mechanism in roots from halophyte Puccinellia tenuiflora is still unclear. Here, the changes associated with Na₂CO₃ tolerance in P. tenuiflora roots were assessed using physiological and iTRAQ-based quantitative proteomic analyses. We set up the first protein dataset in P. tenuiflora roots containing 2,671 non-redundant proteins. Our results showed that Na₂CO₃ slightly inhibited root growth, caused ROS accumulation, cell membrane damage, and ion imbalance, as well as reduction of transport and protein synthesis/turnover. The Na₂CO₃-responsive patterns of 72 proteins highlighted specific signaling and metabolic pathways in roots. Ca²⁺ signaling was activated to transmit alkali stress signals as inferred by the accumulation of calcium-binding proteins. Additionally, the activities of peroxidase and glutathione peroxidase, and the peroxiredoxin abundance were increased for ROS scavenging. Furthermore, ion toxicity was relieved through Na⁺ influx restriction and compartmentalization, and osmotic homeostasis reestablishment due to glycine betaine accumulation. Importantly, two transcription factors were increased for regulating specific alkali-responsive gene expression. Carbohydrate metabolism-related enzymes were increased for providing energy and carbon skeletons for cellular metabolism. All these provide new insights into alkali-tolerant mechanisms in roots.

De Novo Transcriptional Analysis of Alfalfa in Response to Saline-Alkaline Stress

Saline-alkaline stress, caused by high levels of harmful carbonate salts and high soil pH, is a major abiotic stress that affects crop productivity. Alfalfa is a widely cultivated perennial forage legume with some tolerance to biotic and abiotic stresses, especially to saline-alkaline stress. To elucidate the mechanism underlying plant saline-alkaline tolerance, we conducted transcriptome analysis of whole alfalfa seedlings treated with saline-alkaline solutions for 0 day (control), 1 day (short-term treatment), and 7 days (long-term treatment) using ion torrent sequencing technology. A transcriptome database dataset of 53,853 unigenes was generated, and 2,286 and 2,233 genes were differentially expressed in the short-term and long-term treatment, respectively. Gene ontology analysis revealed 14 highly enriched pathways and demonstrated the differential response of metabolic pathways between the short-term and long-term treatment. The expression levels of 109 and 96 transcription factors were significantly altered significantly after 1 day and 7 days of treatment, respectively. Specific responses of peroxidase, flavonoids, and the light pathway component indicated that the antioxidant capacity was one of the central mechanisms of saline-alkaline stress tolerance response in alfalfa. Among the 18 differentially expressed genes examined by real time PCR, the expression levels of eight genes, including inositol transporter, DNA binding protein, raffinose synthase, ferritin, aldo/keto reductase, glutathione S-transferase, xyloglucan endotrans glucosylase, and a NAC transcription factor, exhibited different patterns in response to saline and alkaline stress. The expression levels of the NAC transcription factor and glutathione S-transferase were altered significantly under saline stress and saline-alkaline stress; they were upregulated under saline-alkaline stress and downregulated under salt stress. Physiology assays showed an increased concentration of reactive oxygen species and malondialdehyde and a decreased content of chlorophyll, indicating that anti-oxidation and detoxification play an important role in response to saline-alkaline stress. Overall, the transcriptome analysis provided novel insights into the saline-alkaline stress tolerance response mechanisms in alfalfa.

Deep-sequencing transcriptome analysis of field-grown Medicago sativa L. crown buds acclimated to freezing stress

Medicago sativa L. (alfalfa) 'Zhaodong' is an important forage legume that can safely survive in northern China where winter temperatures reach as low as -30 °C. Survival of alfalfa following freezing stress depends on the amount and revival ability of crown buds. In order to investigate the molecular mechanisms of frost tolerance in alfalfa, we used transcriptome sequencing technology and bioinformatics strategies to analyze crown buds of field-grown alfalfa during winter. We statistically identified a total of 5605 differentially expressed genes (DEGs) involved in freezing stress including 1900 upregulated and 3705 downregulated DEGs. We validated 36 candidate DEGs using qPCR to confirm the accuracy of the RNA-seq data. Unlike other recent studies, this study employed alfalfa plants grown in the natural environment. Our results indicate that not only the CBF orthologs but also membrane proteins, hormone signal transduction pathways, and ubiquitin-mediated proteolysis pathways indicate the presence of a special freezing adaptation mechanism in alfalfa. The antioxidant defense system may rapidly confer freezing tolerance to alfalfa. Importantly, biosynthesis of secondary metabolites and phenylalanine metabolism, which is of potential importance in coordinating freezing tolerance with growth and development, were downregulated in subzero temperatures. The adaptive mechanism for frost tolerance is a complex multigenic process that is not well understood. This systematic analysis provided an in-depth view of stress tolerance mechanisms in alfalfa.

A bHLH gene from Tamarix hispida improves abiotic stress tolerance by enhancing osmotic potential and decreasing reactive oxygen species accumulation

Basic helix-loop-helix (bHLH) leucine-zipper transcription factors play important roles in abiotic stress responses. However, their specific roles in abiotic stress tolerance are not fully known. Here, we functionally characterized a bHLH gene, ThbHLH1, from Tamarix hispida in abiotic stress tolerance. ThbHLH1 specifically binds to G-box motif with the sequence of 'CACGTG'. Transiently transfected T. hispida plantlets with transiently overexpressed ThbHLH1 and RNAi-silenced ThbHLH1 were generated for gain- and loss-of-function analysis. Transgenic Arabidopsis thaliana lines overexpressing ThbHLH1 were generated to confirm the gain- and loss-of-function analysis. Overexpression of ThbHLH1 significantly elevates glycine betaine and proline levels, increases Ca(2+) concentration and enhances peroxidase (POD) and superoxide dismutase (SOD) activities to decrease reactive oxygen species (ROS) accumulation. Additionally, ThbHLH1 regulates the expression of the genes including P5CS, BADH, CaM, POD and SOD, to activate the above physiological changes, and also induces the expression of stress tolerance-related genes LEAs and HSPs. These data suggest that ThbHLH1 induces the expression of stress tolerance-related genes to improve abiotic stress tolerance by increasing osmotic potential, improving ROS scavenging capability and enhancing second messenger in stress signaling cascades.

Molecular mechanisms of foliar water uptake in a desert tree

Water deficits severely affect growth, particularly for the plants in arid and semiarid regions of the world. In addition to precipitation, other subsidiary water, such as dew, fog, clouds and small rain showers, may also be absorbed by leaves in a process known as foliar water uptake. With the severe scarcity of water in desert regions, this process is increasingly becoming a necessity. Studies have reported on physical and physiological processes of foliar water uptake. However, the molecular mechanisms remain less understood. As major channels for water regulation and transport, aquaporins (AQPs) are involved in this process. However, due to the regulatory complexity and functional diversity of AQPs, their molecular mechanism for foliar water uptake remains unclear. In this study, Tamarix ramosissima, a tree species widely distributed in desert regions, was investigated for gene expression patterns of AQPs and for sap flow velocity. Our results suggest that the foliar water uptake of T. ramosissima occurs in natural fields at night when the humidity is over a threshold of 85 %. The diurnal gene expression pattern of AQPs suggests that most AQP gene expressions display a circadian rhythm, and this could affect both photosynthesis and transpiration. At night, the PIP2-1 gene is also upregulated with increased relative air humidity. This gene expression pattern may allow desert plants to regulate foliar water uptake to adapt to extreme drought. This study suggests a molecular basis of foliar water uptake in desert plants.

Tamarix hispida zinc finger protein ThZFP1 participates in salt and osmotic stress tolerance by increasing proline content and SOD and POD activities

Zinc finger proteins (ZFPs) are a large family that play important roles in various biological processes, such as signal transduction, RNA binding, morphogenesis, transcriptional regulation, abiotic or biotic stress response. However, the functions of ZFPs involved in abiotic stress are largely not known. In the present study, we cloned and functionally characterized a ZFP gene, ThZFP1, from Tamarix hispida. The expression of ThZFP1 is highly induced by NaCl, mannitol or ABA treatment. To study the function of ThZFP1 involved in abiotic stress response, transgenic T. hispida plants with overexpression or knockdown of ThZFP1 were generated using a transient transformation system. Gain- and loss-of-function studies of ThZFP1 suggested that ThZFP1 can induce the expression of a series of genes, including delta-pyrroline-5-carboxylate synthetase (P5CS), peroxidase (POD) and superoxide dismutase (SOD), leading to accumulation of proline and enhanced activities of SOD and POD. These physiological changes enhanced proline content and reactive oxygen species (ROS) scavenging capability when exposed to salt or osmotic stress. All the results obtained from T. hispida plants were further confirmed by analyses of the transgenic Arabidopsis plants overexpressing ThZFP1. These data together suggested that ThZFP1 positively regulates proline accumulation and activities of SOD and POD under salt and osmotic stress conditions.

Transcriptional responses of a bicarbonate-tolerant monocot, Puccinellia tenuiflora, and a related bicarbonate-sensitive species, Poa annua, to NaHCO3 stress

Puccinellia tenuiflora is an alkaline salt-tolerant monocot found in saline-alkali soil in China. To identify the genes which are determining the higher tolerance of P. tenuiflora compared to bicarbonate sensitive species, we examined the responses of P. tenuiflora and a related bicarbonate-sensitive Poeae plant, Poa annua, to two days of 20 mM NaHCO₃ stress by RNA-seq analysis. We obtained 28 and 38 million reads for P. tenuiflora and P. annua, respectively. For each species, the reads of both unstressed and stressed samples were combined for de novo assembly of contigs. We obtained 77,329 contigs for P. tenuiflora and 115,335 contigs for P.annua. NaHCO₃ stress resulted in greater than two-fold absolute expression value changes in 157 of the P. tenuiflora contigs and 1090 of P. annua contigs. Homologs of the genes involved in Fe acquisition, which are important for the survival of plants under alkaline stress, were up-regulated in P. tenuiflora and down-regulated in P. annua. The smaller number of the genes differentially regulated in P. tenuiflora suggests that the genes regulating bicarbonate tolerance are constitutively expressed in P. tenuiflora.

Expression profiles of 12 late embryogenesis abundant protein genes from Tamarix hispida in response to abiotic stress

Twelve embryogenesis abundant protein (LEA) genes (named ThLEA-1 to -12) were cloned from Tamarix hispida. The expression profiles of these genes in response to NaCl, PEG, and abscisic acid (ABA) in roots, stems, and leaves of T. hispida were assessed using real-time reverse transcriptase-polymerase chain reaction (RT-PCR). These ThLEAs all showed tissue-specific expression patterns in roots, stems, and leaves under normal growth conditions. However, they shared a high similar expression patterns in the roots, stems, and leaves when exposed to NaCl and PEG stress. Furthermore, ThLEA-1, -2, -3, -4, and -11 were induced by NaCl and PEG, but ThLEA-5, -6, -8, -10, and -12 were downregulated by salt and drought stresses. Under ABA treatment, some ThLEA genes, such as ThLEA-1, -2, and -3, were only slightly differentially expressed in roots, stems, and leaves, indicating that they may be involved in the ABA-independent signaling pathway. These findings provide a basis for the elucidation of the function of LEA genes in future work.

Elucidation of the specific formation of homo- and heterodimeric forms of ThbZIP1 and its role in stress

Protein–protein interactions are important for the molecular understanding of the biological processes of proteins. The dimerization of bZIPs (basic leucine zipper proteins) is involved in modifying binding site specificities, altering dimer stability, and permitting a new set of specific protein-to-protein interactions to occur at the promoter. In the present study, we studied the whether ThbZIP1 form homo- and heterodimers using the yeast two-hybrid method. Five bZIP genes were cloned from Tamarix hispida to investigate their interaction with ThbZIP1. Our results showed that ThbZIP1 can form homodimers with itself, and three out of five bZIPs could interact with the ThbZIP1 protein to form heterodimers. Real-time RT-PCR results suggested that these ThbZIPs can all respond to abiotic stresses and abscisic acid (ABA), and shared very similar expression patterns in response to NaCl, ABA or PEG6000. Subcellular localization studies showed that all ThbZIPs are targeted to the nucleus. Our results showed that ThbZIP1 are dimeric proteins, which can form homo- or heterodimers.