The sweet sorghum SbWRKY50 is negatively involved in salt response by regulating ion homeostasis

Poplar glutathione S-transferase PtrGSTF8 contributes to reactive oxygen species scavenging and salt tolerance

Glutathione S-transferases (GSTs) constitute a protein superfamily encoded by a large gene family and play a crucial role in plant growth and development. However, their precise functions in wood plant responses to abiotic stress are not fully understood. In this study, we isolated a Phi class glutathione S-transferase-encoding gene, PtrGSTF8, from poplar (Populus alba × P. glandulosa), which is significantly up-regulated under salt stress. Moreover, compared with wild-type (WT) plants, transgenic tobacco plants exhibited significant salt stress tolerance. Under salt stress, PtrGSTF8-overexpressing tobacco plants showed a significant increase in plant height and root length, and less accumulation of reactive oxygen species. In addition, these transgenic tobacco plants exhibited higher superoxide dismutase, peroxidase, and catalase activities and reduced malondialdehyde content compared with WT plants. Quantitative real-time PCR experiments showed that the overexpression of PtrGSTF8 increased the expression of numerous genes related to salt stress. Furthermore, PtrMYB108, a MYB transcription factor involved in salt resistance in poplar, was found to directly activate the promoter of PtrGSTF8, as demonstrated by yeast one-hybrid assays and luciferase complementation assays. Taken together, these findings suggest that poplar PtrGSTF8 contributes to enhanced salt tolerance and confers multiple growth advantages when overexpressed in tobacco.

The PbbHLH62/PbVHA-B1 module confers salt tolerance through modulating intracellular Na+/K+ homeostasis and reactive oxygen species removal in pear

The vacuolar H+-ATPase (V-ATPase) is a multi-subunit membrane protein complex, which plays pivotal roles in building up an electrochemical H+-gradient across tonoplast, energizing Na+ sequestration into the central vacuole, and enhancing salt stress tolerance in plants. In this study, a B subunit of V-ATPase gene, PbVHA-B1 was discovered and isolated from stress-induced P. betulaefolia combining with RT-PCR method. The RT-qPCR analysis revealed that the expression level of PbVHA-B1 was upregulated by salt, drought, cold, and exogenous ABA treatment. Subcellular localization analyses showed that PbVHA-B1 was located in the cytoplasm and nucleus. Moreover, overexpression of PbVHA-B1 gene noticeably increased the ATPase activity and the tolerance to salt in transgenic Arabidopsis plants. In contrast, knockdown of PbVHA-B1 gene in P.betulaefolia by virus-induced gene silencing had reduced resistance to salt stress. In addition, using yeast one-hybride (Y1H) and yeast two-hybride (Y2H) screens, PbbHLH62, a bHLH transcription factor, was identified as a partner of the PbVHA-B1 promoter and protein. Then, we also found that PbbHLH62 positively regulate the expression of PbVHA-B1 and the ATPase activity after salt stress treatment. These findings provide evidence that PbbHLH62 played a critical role in the salt response. Collectively, our results demonstrate that a PbbHLH62/PbVHA-B1 module plays a positive role in salt tolerance by maintain intracellular ion and ROS homeostasis in pear.

Inroads into saline-alkaline stress response in plants: unravelling morphological, physiological, biochemical, and molecular mechanisms

MAIN CONCLUSION

This article discusses the complex network of ion transporters, genes, microRNAs, and transcription factors that regulate crop tolerance to saline-alkaline stress. The framework aids scientists produce stress-tolerant crops for smart agriculture. Salinity and alkalinity are frequently coexisting abiotic limitations that have emerged as archetypal mediators of low yield in many semi-arid and arid regions throughout the world. Saline-alkaline stress, which occurs in an environment with high concentrations of salts and a high pH, negatively impacts plant metabolism to a greater extent than either stress alone. Of late, saline stress has been the focus of the majority of investigations, and saline-alkaline mixed studies are largely lacking. Therefore, a thorough understanding and integration of how plants and crops rewire metabolic pathways to repair damage caused by saline-alkaline stress is of particular interest. This review discusses the multitude of resistance mechanisms that plants develop to cope with saline-alkaline stress, including morphological and physiological adaptations as well as molecular regulation. We examine the role of various ion transporters, transcription factors (TFs), differentially expressed genes (DEGs), microRNAs (miRNAs), or quantitative trait loci (QTLs) activated under saline-alkaline stress in achieving opportunistic modes of growth, development, and survival. The review provides a background for understanding the transport of micronutrients, specifically iron (Fe), in conditions of iron deficiency produced by high pH. Additionally, it discusses the role of calcium in enhancing stress tolerance. The review highlights that to encourage biomolecular architects to reconsider molecular responses as auxiliary for developing tolerant crops and raising crop production, it is essential to (a) close the major gaps in our understanding of saline-alkaline resistance genes, (b) identify and take into account crop-specific responses, and (c) target stress-tolerant genes to specific crops.

The role of WRKY transcription factors in exogenous potassium (K+) response to NaCl stress in Tamarix ramosissima

Introduction: Soil salinization poses a significant challenge to plant growth and vitality. Plants like Tamarix ramosissima Ledeb (T. ramosissima), which are halophytes, are often integrated into planting schemes tailored for saline environments. Yet, the role of WRKY transcription factors in T. ramosissima, especially under sodium chloride (NaCl) stress mitigated by exogenous K+ application, is not well-understood. This research endeavors to bridge this knowledge gap. Methods: Using Pfam protein domain prediction and physicochemical property analysis, we delved into the WRKY genes in T. ramosissima roots that are implicated in counteracting NaCl stress when aided by exogenous K+ applications. By observing shifts in the expression levels of WRKY genes annotated to the KEGG pathway under NaCl stress at 0, 48, and 168 h, we aimed to identify potential key WRKY genes. Results: We found that the expression of 56 WRKY genes in T. ramosissima roots responded to exogenous K+ application during NaCl stress at the indicated time points. Particularly, the expression levels of these genes were primarily upregulated within 168 h. From these, 10 WRKY genes were found to be relevant in the KEGG pathways. Moreover, six genes, namely Unigene0024962, Unigene0024963, Unigene0010090, Unigene0007135, Unigene0070215, and Unigene0077293, were annotated to the Plant-pathogen interaction pathway or the MAPK signaling pathway in plants. These genes exhibited dynamic expression regulation at 48 h with the application of exogenous K+ under NaCl stress. Discussion: Our research highlights that WRKY transcription factors can modulate the activation or inhibition of related genes during NaCl stress with the application of exogenous K+. This regulation enhances the plant's adaptability to saline environments and mitigates the damage induced by NaCl. These findings provide valuable gene resources for future salt-tolerant Tamarix breeding and expand our understanding of the molecular mechanisms of WRKY transcription factors in alleviating NaCl toxicity.

Melatonin enhances KCl salinity tolerance by maintaining K+ homeostasis in Malus hupehensis

Large amounts of potash fertilizer are often applied to apple (Malus domestica) orchards to enhance fruit quality and yields, but this treatment aggravates KCl-based salinity stress. Melatonin (MT) is involved in a variety of abiotic stress responses in plants. However, its role in KCl stress tolerance is still unknown. In the present study, we determined that an appropriate concentration (100 μm) of MT significantly alleviated KCl stress in Malus hupehensis by enhancing K+ efflux out of cells and compartmentalizing K+ in vacuoles. Transcriptome deep-sequencing analysis identified the core transcription factor gene MdWRKY53, whose expression responded to both KCl and MT treatment. Overexpressing MdWRKY53 enhanced KCl tolerance in transgenic apple plants by increasing K+ efflux and K+ compartmentalization. Subsequently, we characterized the transporter genes MdGORK1 and MdNHX2 as downstream targets of MdWRKY53 by ChIP-seq. MdGORK1 localized to the plasma membrane and enhanced K+ efflux to increase KCl tolerance in transgenic apple plants. Moreover, overexpressing MdNHX2 enhanced the KCl tolerance of transgenic apple plants/callus by compartmentalizing K+ into the vacuole. RT-qPCR and LUC activity analyses indicated that MdWRKY53 binds to the promoters of MdGORK1 and MdNHX2 and induces their transcription. Taken together, our findings reveal that the MT-WRKY53-GORK1/NHX2-K+ module regulates K+ homeostasis to enhance KCl stress tolerance in apple. These findings shed light on the molecular mechanism of apple response to KCl-based salinity stress and lay the foundation for the practical application of MT in salt stress.

High-affinity potassium transporter from a mangrove tree Avicennia officinalis increases salinity tolerance of Arabidopsis thaliana

Salinity reduces the growth and productivity of crop plants worldwide. Mangroves have evolved efficient ion homeostasis mechanisms to survive under their natural saline growth habitat. Information obtained from them may be utilized for increasing the salt tolerance of crop plants. We identified and characterized a high-affinity potassium transporter gene (AoHKT1) from Avicennia officinalis. The expression of AoHKT1 was induced by NaCl mainly in the leaves. Functional study by heterologous expression of AoHKT1 in Arabidopsis T-DNA insertional mutants athkt1-1 and athkt1-4 revealed that it could enhance the salt tolerance of the mutant plants. This was accompanied by an increase in K+ accumulation in the leaves. AoHKT1 was localized to the plasma membrane in Arabidopsis, and when expressed in yeast, it could complement the functions of both Na+ and K+ transporters. An attempt was made to identify the upstream regulator of AtHKT1, a close homolog of AoHKT1. Using chromatin immunoprecipitation, luciferase assay and yeast one-hybrid assays, WRKY9 was identified as the main transcription factor in the process. Furthermore, this was corroborated by the observation that AtHKT1 levels were significantly reduced in the atwrky9 seedlings. These findings revealed a part of the molecular regulatory mechanism of HKT1 induction in response to salt treatment in Arabidopsis. Our study suggests that AoHKT1 is a potential candidate for generating crop plants with increased salt tolerance.

Plant salinity stress, sensing, and its mitigation through WRKY

Salinity or salt stress has deleterious effects on plant growth and development. It imposes osmotic, ionic, and secondary stresses, including oxidative stress on the plants and is responsible for the reduction of overall crop productivity and therefore challenges global food security. Plants respond to salinity, by triggering homoeostatic mechanisms that counter salt-triggered disturbances in the physiology and biochemistry of plants. This involves the activation of many signaling components such as SOS pathway, ABA pathway, and ROS and osmotic stress signaling. These biochemical responses are accompanied by transcriptional modulation of stress-responsive genes, which is mostly mediated by salt-induced transcription factor (TF) activity. Among the TFs, the multifaceted significance of WRKY proteins has been realized in many diverse avenues of plants' life including regulation of plant stress response. Therefore, in this review, we aimed to highlight the significance of salinity in a global perspective, the mechanism of salt sensing in plants, and the contribution of WRKYs in the modulation of plants' response to salinity stress. This review will be a substantial tool to investigate this problem in different perspectives, targeting WRKY and offering directions to better manage salinity stress in the field to ensure food security.

Functional Characterisation of the Transcription Factor GsWRKY23 Gene from Glycine soja in Overexpressed Soybean Composite Plants and Arabidopsis under Salt Stress

WRKY proteins are a superfamily of transcription factors (TFs) that play multiple roles in plants' growth, development, and environmental stress response. In this study, a novel WRKY gene called GsWRKY23 that is specifically upregulated in salt-tolerant Glycine soja accession BB52 seedlings was identified by transcriptomic analysis under salt stress. How the physiological functions and mechanisms of the GsWRKY23 gene affect salt tolerance was investigated using transformations of soybean hairy roots and Arabidopsis, including wild-type (WT) and atwrky23-mutant plants. The results showed that GsWRKY23 in the roots, stems, and leaves of BB52, along with its promoter in the cotyledons and root tips of GsWRKY23pro::GUS Arabidopsis seedlings, displayed enhanced induction under salt stress. GsWRKY23 localises to the nucleus and shows transcriptional activation ability in yeast cells. Compared to GsWRKY23-RNAi wild soybean hairy-root composite plants under salt stress, obvious improvements, such as superior growth appearance, plant height and fresh weight (FW), and leaf chlorophyll and relative water content (RWC), were displayed by GsWRKY23-overexpressing (OE) composite plants. Moreover, their relative electrolytic leakage (REL) values and malondialdehyde (MDA) contents in the roots and leaves declined significantly. Most of the contents of Na+ and Cl- in the roots, stems, and leaves of GsWRKY23-OE plants decreased significantly, while the content of K+ in the roots increased, and the content of NO3- displayed no obvious change. Ultimately, the Na+/K+ ratios of roots, stems, and leaves, along with the Cl-/NO3- ratios of roots and stems, decreased significantly. In the transgenic WT-GsWRKY23 and atwrky23-GsWRKY23 Arabidopsis seedlings, the salt-induced reduction in seed germination rate and seedling growth was markedly ameliorated; plant FW, leaf chlorophyll content, and RWC increased, and the REL value and MDA content in shoots decreased significantly. In addition, the accumulation of Na+ and Cl- decreased, and the K+ and NO3- levels increased markedly to maintain lower Na+/K+ and Cl-/NO3- ratios in the roots and shoots. Taken together, these results highlight the role of GsWRKY23 in regulating ionic homeostasis in NaCl-stressed overexpressed soybean composite plants and Arabidopsis seedlings to maintain lower Na+/K+ and Cl-/NO3- ratios in the roots and shoots, thus conferring improved salt tolerance.

Physiological and Transcriptional Analyses Provide Insight into Maintaining Ion Homeostasis of Sweet Sorghum under Salt Stress

Sweet sorghum is an important bioenergy grass and valuable forage with a strong adaptability to saline environments. However, little is known about the mechanisms of sweet sorghum coping with ion toxicity under salt stresses. Here, we first evaluated the salt tolerance of a sweet sorghum cultivar "Lvjuren" and determined its ion accumulation traits under NaCl treatments; then, we explored key genes involved in Na⁺, Cl^-, K⁺ and NO₃^- transport using transcriptome profiling and the qRT-PCR method. The results showed that growth and photosynthesis of sweet sorghum were unaffected by 50 and 100 mM NaCl treatments, indicative of a strong salt tolerance of this species. Under NaCl treatments, sweet sorghum could efficiently exclude Na⁺ from shoots and accumulate Cl^- in leaf sheaths to avoid their overaccumulation in leaf blades; meanwhile, it possessed a prominent ability to sustain NO₃^- homeostasis in leaf blades. Transcriptome profiling identified several differentially expressed genes associated with Na⁺, Cl^-, K⁺ and NO₃^- transport in roots, leaf sheaths and leaf blades after 200 mM NaCl treatment for 6 and 48 h. Moreover, transcriptome data and qRT-PCR results indicated that HKT1;5, CLCc and NPF7.3-1 should be key genes involved in Na⁺ retention in roots, Cl^- accumulation in leaf sheaths and maintenance of NO₃^- homeostasis in leaf blades, respectively. Many TFs were also identified after NaCl treatment, which should play important regulatory roles in salt tolerance of sweet sorghum. In addition, GO analysis identified candidate genes involved in maintaining membrane stability and photosynthetic capacity under salt stresses. This work lays a preliminary foundation for clarifying the molecular basis underlying the adaptation of sweet sorghum to adverse environments.

Ethylene-responsive SbWRKY50 suppresses leaf senescence by inhibition of chlorophyll degradation in sorghum

The onset of leaf de-greening and senescence is governed by a complex regulatory network including environmental cues and internal factors such as transcription factors (TFs) and phytohormones, in which ethylene (ET) is one key inducer. However, the detailed mechanism of ET signalling for senescence regulation is still largely unknown. Here, we found that the WRKY TF SbWRKY50 from Sorghum bicolor L., a direct target of the key component ETHYLENE INSENSITIVE 3 in ET signalling, functioned for leaf senescence repression. The clustered regularly interspaced short palindromic repeats/CRISPR-associated protein9-edited SbWRKY50 mutant (SbWRKY5O-KO) of sorghum displayed precocious senescent phenotypes, while SbWRKY50 overexpression delayed age-dependent and dark-induced senescence in sorghum. SbWRKY50 negatively regulated chlorophyll degradation through direct binding to the promoters of several chlorophyll catabolic genes. In addition, SbWRKY50 recruited the Polycomb repressive complex 1 through direct interaction with SbBMI1A, to induce histone 2A mono-ubiquitination accumulation on the chlorophyll catabolic genes for epigenetic silencing and thus delayed leaf senescence. Especially, SbWRKY50 can suppress early steps of chlorophyll catabolic pathway via directly repressing SbNYC1 (NON-YELLOW COLORING 1). Other senescence-related hormones could also influence leaf senescence through repression of SbWRKY50. Hence, our work shows that SbWRKY50 is an essential regulator downstream of ET and SbWRKY50 also responds to other phytohormones for senescence regulation in sorghum.

Abiotic stress tolerance in plants: a fascinating action of defense mechanisms

Climate fluctuation mediated abiotic stress consequences loss in crop yields. These stresses have a negative impact on plant growth and development by causing physiological and molecular changes. In this review, we have attempted to outline recent studies (5 years) associated with abiotic stress resistance in plants. We investigated the various factors that contribute to coping with abiotic challenges, such as transcription factors (TFs), microRNAs (miRNAs), epigenetic changes, chemical priming, transgenic breeding, autophagy, and non-coding RNAs. Stress responsive genes are regulated mostly by TFs, and these can be used to enhance stress resistance in plants. Plants express some miRNA during stress imposition that act on stress-related target genes to help them survive. Epigenetic alterations govern gene expression and facilitate stress tolerance. Chemical priming enhances growth in plants by modulating physiological parameters. Transgenic breeding enables identification of genes involved in precise plant responses during stressful situations. In addition to protein coding genes, non-coding RNAs also influence the growth of the plant by causing alterations at gene expression levels. For achieving sustainable agriculture for a rising world population, it is crucial to develop abiotic-resistant crops with anticipated agronomical traits. To achieve this objective, understanding the diverse mechanisms by which plants protect themselves against abiotic stresses is imperative. This review emphasizes on recent progress and future prospects for abiotic stress tolerance and productivity in plants.

Candidate Genes for Salt Tolerance in Forage Sorghum under Saline Conditions from Germination to Harvest Maturity

To address the plant adaptability of sorghum (Sorghum bicolor) in salinity, the research focus should shift from only selecting tolerant varieties to understanding the precise whole-plant genetic coping mechanisms with long-term influence on various phenotypes of interest to expanding salinity, improving water use, and ensuring nutrient use efficiency. In this review, we discovered that multiple genes may play pleiotropic regulatory roles in sorghum germination, growth, and development, salt stress response, forage value, and the web of signaling networks. The conserved domain and gene family analysis reveals a remarkable functional overlap among members of the bHLH (basic helix loop helix), WRKY (WRKY DNA-binding domain), and NAC (NAM, ATAF1/2, and CUC2) superfamilies. Shoot water and carbon partitioning, for example, are dominated by genes from the aquaporins and SWEET families, respectively. The gibberellin (GA) family of genes is prevalent during pre-saline exposure seed dormancy breaking and early embryo development at post-saline exposure. To improve the precision of the conventional method of determining silage harvest maturity time, we propose three phenotypes and their underlying genetic mechanisms: (i) the precise timing of transcriptional repression of cytokinin biosynthesis (IPT) and stay green (stg1 and stg2) genes; (ii) the transcriptional upregulation of the SbY1 gene and (iii) the transcriptional upregulation of the HSP90-6 gene responsible for grain filling with nutritive biochemicals. This work presents a potential resource for sorghum salt tolerance and genetic studies for forage and breeding.

Salinity pretreatment synergies heat shock toxicity in cyanobacterium Anabaena PCC7120

This study was undertaken to bridge the knowledge gap pertaining to cyanobacteria's response to pretreatment. The result elucidates the synergistic effect of pretreatment toxicity in cyanobacterium Anabaena PCC7120 on morphological and biochemical attributes. Chemical (salt) and physical (heat) stress-pretreated cells exhibited significant and reproducible changes in terms of growth pattern, morphology, pigments, lipid peroxidation, and antioxidant activity. Salinity pretreatment showed more than a five-fold decrease in the phycocyanin content but a six-fold and five-fold increase in carotenoid, lipid peroxidation (MDA content), and antioxidant activity (SOD and CAT) at 1 h and on 3rd day of treatment, respectively, giving the impression of stress-induced free radicals that are scavenged by antioxidants when compared to heat shock pretreatment. Furthermore, quantitative analysis of transcript (qRT-PCR) for FeSOD and MnSOD displayed a 3.6- and 1.8-fold increase in salt-pretreated (S-H) samples. The upregulation of transcript corresponding to salt pretreatment suggests a toxic role of salinity in synergizing heat shock. However, heat pretreatment suggests a protective role in mitigating salt toxicity. It could be inferred that pretreatment enhances the deleterious effect. However, it further showed that salinity (chemical stress) augments the damaging effect of heat shock (physical stress) more profoundly than physical stress on chemical stress possibly by modulating redox balance via activation of antioxidant responses. Our study reveals that upon pretreatment of heat, the negative effect of salt can be mitigated in filamentous cyanobacteria, thus providing a foundation for improved cyanobacterial tolerance to salt stress.

Genome-wide analysis of the Tritipyrum WRKY gene family and the response of TtWRKY256 in salt-tolerance

INTRODUCTION

The transcription factor WRKY is widespread in the plant kingdom and plays a crucial role in diverse abiotic stress responses in plant species. Tritipyrum, an octoploid derived from an intergeneric cross between Triticum aestivum (AABBDD) and Thinopyrum elongatum (EE), is a valuable germplasm resource for introducing superior traits of Th. elongatum into T. aestivum. The recent release of the complete genome sequences of T. aestivum and Th. elongatum enabled us to investigate the organization and expression profiling of Tritipyrum WRKY genes across the entire genome.

RESULTS

In this study, 346 WRKY genes, from TtWRKY1 to TtWRKY346, were identified in Tritipyrum. The phylogenetic analysis grouped these genes into three subfamilies (I-III), and members of the same subfamilies shared a conserved motif composition. The 346 TtWRKY genes were dispersed unevenly across 28 chromosomes, with 218 duplicates. Analysis of synteny suggests that the WRKY gene family may have a common ancestor. Expression profiles derived from transcriptome data and qPCR demonstrated that 54 TtWRKY genes exhibited relatively high levels of expression across various salt stresses and recovery treatments. Tel1E01T143800 (TtWRKY256) is extremely sensitive to salt stress and is on the same evolutionary branch as the salt-tolerant A. thaliana genes AtWRKY25 and AtWRKY33. From 'Y1805', the novel AtWRKY25 was cloned. The Pearson correlation analysis identified 181 genes that were positively correlated (R>0.9) with the expression of TtWRKY256, and these genes were mainly enriched in metabolic processes, cellular processes, response to stimulus, biological regulation, and regulation of biological. Subcellular localization and qRT-PCR analysis revealed that TtWRKY256 was located in the nucleus and was highly expressed in roots, stems, and leaves under salt stress.

DISCUSSION

The above results suggest that TtWRKY256 may be associated with salt stress tolerance in plants and may be a valuable alien gene for improving salt tolerance in wheat.

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.

WRKY transcription factors (TFs): Molecular switches to regulate drought, temperature, and salinity stresses in plants

The WRKY transcription factor (TF) belongs to one of the major plant protein superfamilies. The WRKY TF gene family plays an important role in the regulation of transcriptional reprogramming associated with plant stress responses. Change in the expression patterns of WRKY genes or the modifications in their action; participate in the elaboration of numerous signaling pathways and regulatory networks. WRKY proteins contribute to plant growth, for example, gamete formation, seed germination, post-germination growth, stem elongation, root hair growth, leaf senescence, flowering time, and plant height. Moreover, they play a key role in many types of environmental signals, including drought, temperature, salinity, cold, and biotic stresses. This review summarizes the current progress made in unraveling the functions of numerous WRKY TFs under drought, salinity, temperature, and cold stresses as well as their role in plant growth and development.

PsnWRKY70 Negatively Regulates NaHCO3 Tolerance in Populus

Poplar is an important afforestation and ornamental tree species in Northeast China. The distribution area of saline-alkali land is approximately 765 hm2 in Northeast China. The breeding of saline-alkali-resistant transgenic trees could be an effective method of afforestation in saline-alkali land. WRKY transcription factors play a crucial role in abiotic stress. In this study, we analyzed the genetic stability of the two-year-old PsnWRKY70 transgenic poplars. The results showed that PsnWRKY70 of transgenic poplars had been expressed stably and normally at the mRNA level. The gene interference expression (RE) lines had no significant effect on the growth of PsnWRKY70 under NaHCO3 stress, and the alkali damage index of RE lines was significantly lower than that of WT and overexpression (OE) lines at day 15 under NaHCO3 stress. POD activity was significantly higher in RE lines than in WT. The MDA content of the RE line was lower than that of the WT line. Transcriptome analysis showed that RE lines up-regulated genes enriched in cell wall organization or biogenesis pathway-related genes such as EXPA8, EXPA4, EXPA3, EXPA1, EXPB3, EXP10, PME53, PME34, PME36, XTH9, XTH6, XTH23, CESA1, CESA3, CES9; FLA11, FLA16 and FLA7 genes. These genes play an important role in NaHCO3 stress. Our study showed that the interference expression of the PsnWRKY70 gene can enhance the tolerance of NaHCO3 in poplar.

WRKY transcription factor family in lettuce plant (Lactuca sativa): Genome-wide characterization, chromosome location, phylogeny structures, and expression patterns

WRKY transcription factors (TF) have been identified in many plant species and play critical roles in multiple stages of growth and development and under various stress conditions. As one of the most popular vegetable crops, asparagus lettuce has important medicinal and nutritional value. However, study of WRKY TFs family in asparagus lettuce is limited. With the lettuce (Lactuca sativa L.) genome publication, we identified 76 WRKY TFs and analyzed structural characteristics, phylogenetic relationships, chromosomal distribution, interaction network, and expression profiles. The 76 LsWRKY TFs were phylogenetically classified as Groups I, II (IIa-IIe), and III. Cis element analysis revealed complex regulatory relationships of LsWRKY genes in response to different biological progresses. Interaction network analysis indicated that LsWRKY TFs could interact with other proteins, such as SIB (sigma factor binding protein), WRKY TFs, and MPK. The WRKYIII subfamily genes showed different expression patterns during the progress of asparagus lettuce stem enlargement. According to qRT-PCR analysis, abiotic stresses (drought, salt, low temperature, and high temperature) and phytohormone treatment could induce specific LsWRKYIII gene expression. These results will provide systematic and comprehensive information on LsWRKY TFs and lay the foundation for further clarification of the regulatory mechanism of LsWRKY, especially LsWRKYIII TFs, involved in stress response and the progress of plant growth and development.

Molecular Pathways of WRKY Genes in Regulating Plant Salinity Tolerance

Salinity is a natural and anthropogenic process that plants overcome using various responses. Salinity imposes a two-phase effect, simplified into the initial osmotic challenges and subsequent salinity-specific ion toxicities from continual exposure to sodium and chloride ions. Plant responses to salinity encompass a complex gene network involving osmotic balance, ion transport, antioxidant response, and hormone signaling pathways typically mediated by transcription factors. One particular transcription factor mega family, WRKY, is a principal regulator of salinity responses. Here, we categorize a collection of known salinity-responding WRKYs and summarize their molecular pathways. WRKYs collectively play a part in regulating osmotic balance, ion transport response, antioxidant response, and hormone signaling pathways in plants. Particular attention is given to the hormone signaling pathway to illuminate the relationship between WRKYs and abscisic acid signaling. Observed trends among WRKYs are highlighted, including group II WRKYs as major regulators of the salinity response. We recommend renaming existing WRKYs and adopting a naming system to a standardized format based on protein structure.

Transcription factor ZmWRKY20 interacts with ZmWRKY115 to repress expression of ZmbZIP111 for salt tolerance in maize

Maize (Zea mays) is an important cereal crop worldwide. However, its yield and quality are adversely affected by salt stress resulting from soil hypersalinity. Exploring the regulatory mechanisms of stress responses is of vital importance to increase maize seed production. In the present study, we screened ethyl methanesulfonate-induced maize mutants and identified a salt-tolerant mutant. A single base was mutated in ZmWRKY20, leading to the formation of a truncated protein variant. A detailed phenotypic analysis revealed that this mutant had significantly higher resistance to wilting and lower reactive oxygen species levels than the inbred line B73. ZmWRKY20 showed transcriptional activity in yeast and specifically bound W-boxes according to the results of our yeast one-hybrid, electrophoretic mobility shift, and dual-luciferase assays. Overexpression of ZmWRKY20 decreased salt tolerance in maize. Transcriptome profiling revealed that ZmWRKY20 overexpression extensively reprogrammed genes involved in regulating defense and oxidation-reduction responses. The results substantiate that ZmWRKY20 is directly targeted to the basic leucine zipper (bZIP) motif in the transcription factor ZmbZIP111. It was also verified that ZmWRKY20 interacts with ZmWRKY115 and both proteins act jointly to enhance ZmbZIP111 repression. The results indicate that the ZmWRKY20 and ZmWRKY115 transcription factors interact in the nucleus, leading to repression of ZmbZIP111 expression by directly binding its promoter, and increase the sensitivity of maize seedlings to salt stress. The current study improves our understanding of the complicated responses of maize to salt stress.

BcWRKY1 confers salt sensitivity via inhibiting Reactive oxygen species scavenging

WRKY transcription factors play important roles in abiotic stress by directly regulating stress-related genes. However, the molecular mechanism of its involvement in salt stress in pak-choi is still poorly understood. In this study, we elucidated the function of BcWRKY1 from pak-choi (Brassica rapa ssp. chinensis) in salt stress. The expression level of BcWRKY1 showed the highest in rosette leaves among different tissues and was induced by salt and ABA treatment in pak-choi. Subcellular localization showed that BcWRKY1 was located in nucleus. The transgenic Arabidopsis overexpressing BcWRKY1 exhibited enhanced salt sensitivity and higher H₂O₂ contents, which were further confirmed by silencing BcWRKY1 in pak-choi. In addition, the expression of ZAT12 was negatively regulated with BcWRKY1 under salt stress both in pak-choi and Arabidopsis. Yeast one-hybrid and dual luciferase reporter assay showed that BcWRKY1 could bind to the promoter of BcZAT12, and BcsAPX expression was activated by BcZAT12. To sum up, we propose a BcWRKY1-BcZAT12-BcsAPX regulatory model that involves in pak-choi salt stress response.

Identification and Transcriptome Analysis of Genes Related to Membrane Lipid Regulation in Sweet Sorghum under Salt Stress

Sweet sorghum has strong stress resistance and is considered a promising energy crop. In the present study, the effects of salt on the membrane lipid metabolism of two sweet sorghum inbred lines (salt-tolerant M-81E and salt-sensitive Roma) were analyzed. After treatment with 150 mM NaCl, higher levels of fresh weight and chlorophyll fluorescence, as well as lower levels of malondialdehyde (MDA) were found in salt-tolerant M-81E. Concomitantly, 702 and 1339 differentially expression genes (DEGs) in M-81E and Roma were identified in response to salt stress. We determined that most DEGs were related to glycerophospholipid metabolism, glycerolipid metabolism, and other membrane lipid metabolisms. Under NaCl treatment, the expression of the membrane-associated phospholipase A1 was down-regulated at the transcriptional level, along with an increased content of phosphatidylcholine (PC) in both cultivars. The inhibition of triacylglycerol (TAG) mobilization in M-81E delayed salt-induced leaf senescence. Furthermore, enhanced levels of glycerol-3-phosphate acyltransferase (GPAT) expression contributed to improved salt resistance in M-81E. The results of this study demonstrate membrane the role of lipid regulation in mediating salt-defensive responses in sweet sorghum and expand our understanding of the relationship between changes in membrane lipid content and salt resistance.

Comparative Transcriptome Analysis of Two Sweet Sorghum Genotypes with Different Salt Tolerance Abilities to Reveal the Mechanism of Salt Tolerance

Sweet sorghum is a C4 crop that can be grown for silage forage, fiber, syrup and fuel production. It is generally considered a salt-tolerant plant. However, the salt tolerance ability varies among genotypes, and the mechanism is not well known. To further uncover the salt tolerance mechanism, we performed comparative transcriptome analysis with RNA samples in two sweet sorghum genotypes showing different salt tolerance abilities (salt-tolerant line RIO and salt-sensitive line SN005) upon salt treatment. These response processes mainly focused on secondary metabolism, hormone signaling and stress response. The expression pattern cluster analysis showed that RIO-specific response genes were significantly enriched in the categories related to secondary metabolic pathways. GO enrichment analysis indicated that RIO responded earlier than SN005 in the 2 h after treatment. In addition, we identified more transcription factors (TFs) in RIO than SN005 that were specifically expressed differently in the first 2 h of salt treatment, and the pattern of TF change was obviously different. These results indicate that an early response in secondary metabolism might be essential for salt tolerance in sweet sorghum. In conclusion, we found that an early response, especially in secondary metabolism and hormone signaling, might be essential for salt tolerance in sweet sorghum.

SbbHLH85, a bHLH member, modulates resilience to salt stress by regulating root hair growth in sorghum

bHLH family proteins play an important role in plant stress response. However, the molecular mechanism regulating the salt response of bHLH is largely unknown. This study aimed to investigate the function and regulating mechanism of the sweet sorghum SbbHLH85 during salt stress. The results showed that SbbHLH85 was different from its homologs in other species. Also, it was a new atypical bHLH transcription factor and a key gene for root development in sweet sorghum. The overexpression of SbbHLH85 resulted in significantly increased number and length of root hairs via ABA and auxin signaling pathways, increasing the absorption of Na⁺. Thus, SbbHLH85 plays a negative regulatory role in the salt tolerance of sorghum. We identified a potential interaction partner of SbbHLH85, which was phosphate transporter chaperone PHF1 and modulated the distribution of phosphate, through screening a yeast two-hybrid library. Both yeast two-hybrid and BiFC experiments confirmed the interaction between SbbHLH85 and PHF1. The overexpression of SbbHLH85 led to a decrease in the expression of PHF1 as well as the content of Pi. Based on these results, we suggested that the increase in the Na⁺ content and the decrease in the Pi content resulted in the salt sensitivity of transgenic sorghum.

Screening and identification of salt-tolerance genes in Sophora alopecuroides and functional verification of SaAQP

Overexpression of SaAQP can improve the salt tolerance of transgenic soybean hairy roots and A. thaliana. Salt stress severely affects crop yield and food security. There is a need to improve the salt tolerance of crops, but the discovery and utilization of salt-tolerance genes remains limited. Owing to its strong stress tolerance, Sophora alopecuroides is ideal for the identification of salt-tolerance genes. Therefore, we aimed to screen and identify the salt-tolerance genes in S. alopecuroides. With a yeast expression library of seedlings, salt-tolerant genes were screened using a salt-containing medium to simulate salt stress. By combining salt-treatment screening and transcriptome sequencing, 11 candidate genes related to salt tolerance were identified, including genes for peroxidase, inositol methyltransferase, aquaporin, cysteine synthase, pectinesterase, and WRKY. The expression dynamics of candidate genes were analyzed after salt treatment of S. alopecuroides, and salt tolerance was verified in yeast BY4743. The candidate genes participated in the salt-stress response in S. alopecuroides, and their overexpression significantly improved the salt tolerance of yeast. Salt tolerance mediated by SaAQP was further verified in soybean hairy roots and Arabidopsis thaliana, and it was found that SaAQP might enhance the salt tolerance of A. thaliana by participating in a reactive oxygen species scavenging mechanism. This result provides new genetic resources in plant breeding for salt resistance.

The Roles of CCCH Zinc-Finger Proteins in Plant Abiotic Stress Tolerance

Zinc-finger proteins, a superfamily of proteins with a typical structural domain that coordinates a zinc ion and binds nucleic acids, participate in the regulation of growth, development, and stress adaptation in plants. Most zinc fingers are C2H2-type or CCCC-type, named after the configuration of cysteine (C) and histidine (H); the less-common CCCH zinc-finger proteins are important in the regulation of plant stress responses. In this review, we introduce the domain structures, classification, and subcellular localization of CCCH zinc-finger proteins in plants and discuss their functions in transcriptional and post-transcriptional regulation via interactions with DNA, RNA, and other proteins. We describe the functions of CCCH zinc-finger proteins in plant development and tolerance to abiotic stresses such as salt, drought, flooding, cold temperatures and oxidative stress. Finally, we summarize the signal transduction pathways and regulatory networks of CCCH zinc-finger proteins in their responses to abiotic stress. CCCH zinc-finger proteins regulate the adaptation of plants to abiotic stress in various ways, but the specific molecular mechanisms need to be further explored, along with other mechanisms such as cytoplasm-to-nucleus shuttling and post-transcriptional regulation. Unraveling the molecular mechanisms by which CCCH zinc-finger proteins improve stress tolerance will facilitate the breeding and genetic engineering of crops with improved traits.

Sorghum under saline conditions: responses, tolerance mechanisms, and management strategies

An overview is presented of recent advances in our knowledge of responses and mechanisms rendering adaptation to saline conditions in sorghum. Different strategies deployed to enhance salinity stress tolerance in sorghum are also pointed out. Salinity stress is a growing problem worldwide. Sorghum is the fifth key crop among cereals. Understanding responses and tolerance strategies in sorghum would be therefore helpful effort for providing biomarkers for designing greatest salinity-tolerant sorghum genotypes. When sorghum exposed to salinity, salinity-tolerant genotypes most probably reprogram their gene expression to activate adaptive biochemical and physiological responses for survival. The review thus discusses the possible physiological and biochemical responses that confer salinity tolerance to sorghum under saline conditions. Although it is not characterized in sorghum, salinity perceiving and transmitting signals to downstream responses via signaling transduction pathways most likely are essential strategy for sorghum adaptation to salinity stress. Sorghum has also shown to withstand moderate saline environments and retain the germination, growth, and photosynthetic activities. Salinity-tolerant sorghum genotypes show the ability to exclude excessive Na⁺ from reaching shoots and induce ion homeostasis. Osmotic homeostasis and ROS detoxification are also evident as salinity tolerance strategies in sorghum. These above mechanisms lead to re-establishment of cellular ionic, osmotic, and redox homeostasis as well as photosynthesis efficiency. It is noteworthy that these mechanisms act individually or co-operatively to minimize the salinity hazards and enhance acclimation in sorghum. We conclude, however, that although these responses contribute to sorghum tolerance to salinity stress, they seem to be not adequate at higher concentrations of salinity, which agrees with sorghum ranking as moderately salinity-tolerant crop. Also, some of these tolerance strategies reported in other crops are not well studied and documented in sorghum, but most probably have roles in sorghum. Further improvement in sorghum salinity tolerance using different approaches is definitely necessary to meet the requirements of its harsh production environments, and therefore, these approaches are addressed.

Identification of the Group III WRKY Subfamily and the Functional Analysis of GhWRKY53 in Gossypium hirsutum L

WRKY transcription factors had multiple functions in plant secondary metabolism, leaf senescence, fruit ripening, adaptation to biotic and abiotic stress, and plant growth and development. However, knowledge of the group III WRKY subfamily in fiber development in upland cotton (Gossypium hirsutum L.) is largely absent. Previous studies have shown that there were 21 putative group III WRKY members in G. hirsutum L. These putative amino acid sequences from the III WRKY group were phylogenetically clustered into three clades. Multiple alignment, conservative motif analysis, and gene structure analysis showed that the members clustered together in the phylogenetic tree had similar motifs and gene structures. Expression pattern analysis revealed that variation in the expression levels of these genes in different tissues and fiber development stages. To better understand the functions of putative group III WRKY genes in G. hirsutum L., we selected the cotton fiber initiation-related gene GhWRKY53 for cloning and functional identification. The subcellular localization experiment of GhWRKY53 in Nicotiana tabacum leaves showed that it was located in the nucleus. The heterologous expression of GhWRKY53 in Arabidopsis thaliana could significantly increase the density of trichomes. Twelve proteins that interacted with GhWRKY53 were screened from the cotton fiber cDNA library by yeast two-hybrid experiment. This study findings lay a foundation for further research on the role of the GhWRKY53 during cotton fiber development and provide a new insight for further studying putative group III WRKY genes in G. hirsutum L. Our research results also provide vital information for the genetic mechanism of high-quality cotton fiber formation and essential genetic resources for cotton fiber quality improvement.

The Importance of Non-Diffusional Factors in Determining Photosynthesis of Two Contrasting Quinoa Ecotypes (Chenopodium quinoa Willd.) Subjected to Salinity Conditions

The broad distribution of quinoa in saline and non-saline environments is reflected in variations in the photosynthesis-associated mechanisms of different ecotypes. The aim of this study was to characterize the photosynthetic response to high salinity (0.4 M NaCl) of two contrasting Chilean genotypes, Amarilla (salt-tolerant, salares ecotype) and Hueque (salt-sensitive, coastal ecotype). Our results show that saline stress induced a significant decrease in the K⁺/Na⁺ ratio in roots and an increase in glycine betaine in leaves, particularly in the sensitive genotype (Hueque). Measurement of the photosynthesis-related parameters showed that maximum CO₂ assimilation (A_max) in control plants was comparable between genotypes (ca. 9–10 μmol CO₂ m⁻² s⁻¹). However, salt treatment produced different responses, with A_max values decreasing by 65.1% in the sensitive ecotype and 37.7% in the tolerant one. Although both genotypes maintained mesophyll conductance when stomatal restrictions were removed, the biochemical components of Amarilla were impaired to a lesser extent under salt stress conditions: for example, the maximum rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO; V_cmax) was not as affected in Amarilla, revealing that this enzyme has a higher affinity for its substrate in this genotype and, thus, a better carboxylation efficiency. The present results show that the higher salinity tolerance of Amarilla was also due to its ability to control non-diffusional components, indicating its superior photosynthetic capacity compared to Hueque, particularly under salt stress conditions.

Analysis of N6-methyladenosine reveals a new important mechanism regulating the salt tolerance of sweet sorghum

The N⁶-methyladenosine (m⁶A) modification is the most common internal post-transcriptional modification, with important regulatory effects on RNA export, splicing, stability, and translation. Studies on the m⁶A modifications in plants have focused on Arabidopsis thaliana growth and development. However, A. thaliana is a salt-sensitive and model plant species. Thus, studies aimed at characterizing the role of the m⁶A modification in the salt stress responses of highly salt-tolerant crop species are needed. Sweet sorghum is cultivated as an energy and forage crop, which is highly suitable for growth on saline-alkaline land. Exploring the m⁶A modification in sweet sorghum may be important for elucidating the salt-resistance mechanism of crops. In this study, we mapped the m⁶A modifications in two sorghum genotypes (salt-tolerant M-81E and salt-sensitive Roma) that differ regarding salt tolerance. The m⁶A modification in sweet sorghum under salt stress was drastically altered, especially in Roma, where the m⁶A modification on mRNAs of some salt-resistant related transcripts increased, resulting in enhanced mRNA stability, which in turn was involved in the regulation of salt tolerance in sweet sorghum. Although m⁶A modifications are important for regulating sweet sorghum salt tolerance, the regulatory activity is limited by the initial m⁶A modification level. Additionally, in M-81E and Roma, the differences in the m⁶A modifications were much greater than the differences in gene expression levels and are more sensitive. Our study suggests that the number and extent of m⁶A modifications on the transcripts of salt-resistance genes may be important factors for determining and assessing the salt tolerance of crops.

Response Mechanisms of Plants Under Saline-Alkali Stress

As two coexisting abiotic stresses, salt stress and alkali stress have severely restricted the development of global agriculture. Clarifying the plant resistance mechanism and determining how to improve plant tolerance to salt stress and alkali stress have been popular research topics. At present, most related studies have focused mainly on salt stress, and salt-alkali mixed stress studies are relatively scarce. However, in nature, high concentrations of salt and high pH often occur simultaneously, and their synergistic effects can be more harmful to plant growth and development than the effects of either stress alone. Therefore, it is of great practical importance for the sustainable development of agriculture to study plant resistance mechanisms under saline-alkali mixed stress, screen new saline-alkali stress tolerance genes, and explore new plant salt-alkali tolerance strategies. Herein, we summarized how plants actively respond to saline-alkali stress through morphological adaptation, physiological adaptation and molecular regulation.

Function and Mechanism of WRKY Transcription Factors in Abiotic Stress Responses of Plants

The WRKY gene family is a plant-specific transcription factor (TF) group, playing important roles in many different response pathways of diverse abiotic stresses (drought, saline, alkali, temperature, and ultraviolet radiation, and so forth). In recent years, many studies have explored the role and mechanism of WRKY family members from model plants to agricultural crops and other species. Abiotic stress adversely affects the growth and development of plants. Thus, a review of WRKY with stress responses is important to increase our understanding of abiotic stress responses in plants. Here, we summarize the structural characteristics and regulatory mechanism of WRKY transcription factors and their responses to abiotic stress. We also discuss current issues and future perspectives of WRKY transcription factor research.

TaMYB86B encodes a R2R3-type MYB transcription factor and enhances salt tolerance in wheat

The MYB transcription factor family is important for plant responses to abiotic stresses. In this study, we identified three wheat TaMYB86 genes encoding R2R3-type MYB transcription factors. Analyses of the phylogenetic relationships and gene structures of TaMYB86A, TaMYB86B, and TaMYB86D revealed considerable similarities in gene structures and the encoded amino acid sequences. Additionally, TaMYB86B was highly expressed in the roots, stems, and leaves, suggesting it is critical for regulating salt stress responses in wheat. Moreover, TaMYB86B expression was induced by NaCl, abscisic acid (ABA), methyl jasmonate (MeJA), gibberellin (GA), auxin and low temperature treatments. The TaMYB86B protein localized in the nucleus and exhibited transcriptional activation activity. Under salt stress, TaMYB86B-overexpressing plants had a higher biomass and potassium ion (K⁺) content, but lower MDA, H₂O₂, O₂^-., and sodium ion (Na⁺) contents, when compared with the wild-type plants. Quantitative real-time PCR results indicated that the overexpression of TaMYB86B improved the expression of many stress-related genes. These findings suggest that TaMYB86B influences the salt tolerance of wheat by regulating the ion homeostasis to maintain an appropriate osmotic balance and decrease ROS levels.

Responses of Membranes and the Photosynthetic Apparatus to Salt Stress in Cyanobacteria

Cyanobacteria are autotrophs whose photosynthetic process is similar to that of higher plants, although the photosynthetic apparatus is slightly different. They have been widely used for decades as model systems for studying the principles of photosynthesis, especially the effects of environmental stress on photosynthetic activities. Salt stress, which is the most common abiotic stress in nature, combines ionic and osmotic stresses. High cellular ion concentrations and osmotic stress can alter normal metabolic processes and photosynthesis. Additionally, salt stress increases the intracellular reactive oxygen species (ROS) contents. Excessive amounts of ROS will damage the photosynthetic apparatus, inhibit the synthesis of photosystem-related proteins, including the D1 protein, and destroy the thylakoid membrane structure, leading to inhibited photosynthesis. In this review, we mainly introduce the effects of salt stress on the cyanobacterial membranes and photosynthetic apparatus. We also describe specific salt tolerance mechanisms. A thorough characterization of the responses of membranes and photosynthetic apparatus to salt stress may be relevant for increasing agricultural productivity.

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.