The overexpression of a maize mitogen-activated protein kinase gene (ZmMPK5) confers salt stress tolerance and induces defence responses in tobacco

Designing salt stress-resilient crops: Current progress and future challenges

Excess soil salinity affects large regions of land and is a major hindrance to crop production worldwide. Therefore, understanding the molecular mechanisms of plant salt tolerance has scientific importance and practical significance. In recent decades, studies have characterized hundreds of genes associated with plant responses to salt stress in different plant species. These studies have substantially advanced our molecular and genetic understanding of salt tolerance in plants and have introduced an era of molecular design breeding of salt-tolerant crops. This review summarizes our current knowledge of plant salt tolerance, emphasizing advances in elucidating the molecular mechanisms of osmotic stress tolerance, salt-ion transport and compartmentalization, oxidative stress tolerance, alkaline stress tolerance, and the trade-off between growth and salt tolerance. We also examine recent advances in understanding natural variation in the salt tolerance of crops and discuss possible strategies and challenges for designing salt stress-resilient crops. We focus on the model plant Arabidopsis (Arabidopsis thaliana) and the four most-studied crops: rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), and soybean (Glycine max).

Transcriptome Expression Profiling Reveals the Molecular Response to Salt Stress in Gossypium anomalum Seedlings

Some wild cotton species are remarkably tolerant to salt stress, and hence represent valuable resources for improving salt tolerance of the domesticated allotetraploid species Gossypium hirsutum L. Here, we first detected salt-induced stress changes in physiological and biochemical indexes of G. anomalum, a wild African diploid cotton species. Under 350 mmol/L NaCl treatment, the photosynthetic parameters declined significantly, whereas hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents increased. Catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activity and proline (PRO) content also significantly increased, reaching peak values at different stages of salt stress. We used RNA-Seq to characterize 15,476 differentially expressed genes in G. anomalum roots after 6, 12, 24, 72, and 144 h of salt stress. Gene Ontology enrichment analysis revealed these genes to be related to sequence-specific DNA and iron ion binding and oxidoreductase, peroxidase, antioxidant, and transferase activity; meanwhile, the top enriched pathways from the Kyoto Encyclopedia of Genes and Genomes database were plant hormone signal transduction, phenylpropanoid biosynthesis, fatty acid degradation, carotenoid biosynthesis, zeatin biosynthesis, starch and sucrose metabolism, and MAPK signaling. A total of 1231 transcription factors were found to be expressed in response to salt stress, representing ERF, MYB, WRKY, NAC, C2H2, bZIP, and HD-ZIP families. Nine candidate genes were validated by quantitative real-time PCR and their expression patterns were found to be consistent with the RNA-Seq data. These data promise to significantly advance our understanding of the molecular response to salt stress in Gossypium spp., with potential value for breeding applications.

Molecular Regulatory Mechanism of Exogenous Hydrogen Sulfide in Alleviating Low-Temperature Stress in Pepper Seedlings

Pepper (Capsicum annuum L.) is sensitive to low temperatures, with low-temperature stress affecting its plant growth, yield, and quality. In this study, we analyzed the effects of exogenous hydrogen sulfide (H2S) on pepper seedlings subjected to low-temperature stress. Exogenous H2S increased the content of endogenous H2S and its synthetase activity, enhanced the antioxidant capacity of membrane lipids, and protected the integrity of the membrane system. Exogenous H2S also promoted the Calvin cycle to protect the integrity of photosynthetic organs; enhanced the photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and photosynthesis; and reduced the intercellular CO2 concentration (Ci). Moreover, the activities of superoxide dismutase, peroxidase, catalase, and anti-cyclic glutathione (ASA-GSH) oxidase were improved to decompose excess reactive oxygen species (ROS), enhance the oxidative stress and detoxification ability of pepper seedlings, and improve the resistance to low-temperature chilling injury in 'Long Yun2' pepper seedlings. In addition, the H2S scavenger hypotaurine (HT) aggravated the ROS imbalance by reducing the endogenous H2S content, partially eliminating the beneficial effects of H2S on the oxidative stress and antioxidant defense system, indicating that H2S can effectively alleviate the damage of low temperature on pepper seedlings. The results of transcriptome analysis showed that H2S could induce the MAPK-signaling pathway and plant hormone signal transduction; upregulate the expression of transcription factors WRKY22 and PTI6; induce defense genes; and activate the ethylene and gibberellin synthesis receptors ERF1, GDI2, and DELLA, enhancing the resistance to low-temperature chilling injury of pepper seedlings. The plant-pathogen interaction was also significantly enriched, suggesting that exogenous H2S also promotes the expression of genes related to plant-pathogen interaction. The results of this study provide novel insights into the molecular mechanisms and genetic modifications of H2S that mitigate the hypothermic response.

How Plants Tolerate Salt Stress

Soil salinization inhibits plant growth and seriously restricts food security and agricultural development. Excessive salt can cause ionic stress, osmotic stress, and ultimately oxidative stress in plants. Plants exclude excess salt from their cells to help maintain ionic homeostasis and stimulate phytohormone signaling pathways, thereby balancing growth and stress tolerance to enhance their survival. Continuous innovations in scientific research techniques have allowed great strides in understanding how plants actively resist salt stress. Here, we briefly summarize recent achievements in elucidating ionic homeostasis, osmotic stress regulation, oxidative stress regulation, and plant hormonal responses under salt stress. Such achievements lay the foundation for a comprehensive understanding of plant salt-tolerance mechanisms.

Transcriptome analysis reveals the key pathways and candidate genes involved in salt stress responses in Cymbidium ensifolium leaves

BACKGROUND

Cymbidium ensifolium L. is known for its ornamental value and is frequently used in cosmetics. Information about the salt stress response of C. ensifolium is scarce. In this study, we reported the physiological and transcriptomic responses of C. ensifolium leaves under the influence of 100 mM NaCl stress for 48 (T48) and 96 (T96) hours.

RESULTS

Leaf Na⁺ content, activities of the antioxidant enzymes i.e., superoxide dismutase, glutathione S-transferase, and ascorbate peroxidase, and malondialdehyde content were increased in salt-stressed leaves of C. ensifolium. Transcriptome analysis revealed that a relatively high number of genes were differentially expressed in CKvsT48 (17,249) compared to CKvsT96 (5,376). Several genes related to salt stress sensing (calcium signaling, stomata closure, cell-wall remodeling, and ROS scavenging), ion balance (Na⁺ and H⁺), ion homeostasis (Na⁺/K⁺ ratios), and phytohormone signaling (abscisic acid and brassinosteroid) were differentially expressed in CKvsT48, CKvsT96, and T48vsT96. In general, the expression of genes enriched in these pathways was increased in T48 compared to CK while reduced in T96 compared to T48. Transcription factors (TFs) belonging to more than 70 families were differentially expressed; the major families of differentially expressed TFs included bHLH, NAC, MYB, WRKY, MYB-related, and C3H. A Myb-like gene (CenREV3) was further characterized by overexpressing it in Arabidopsis thaliana. CenREV3's expression was decreased with the prolongation of salt stress. As a result, the CenREV3-overexpression lines showed reduced root length, germination %, and survival % suggesting that this TF is a negative regulator of salt stress tolerance.

CONCLUSION

These results provide the basis for future studies to explore the salt stress response-related pathways in C. ensifolium.

TaMPK2B, a member of the MAPK family in T. aestivum, enhances plant low-Pi stress tolerance through modulating physiological processes associated with phosphorus starvation defensiveness

The mitogen-activated protein kinase (MAPK) cascades are present in plant species and modulate plant growth and stress responses. This study characterizes TaMPK2B, a MAPK family gene in T. aestivum that regulates plant adaptation to low-Pi stress. TaMPK2B harbors the conserved domains involving protein phosphorylation and protein-protein interaction. A yeast two-hybrid assay reveals an interaction between TaMPK2B and TaMPKK2 and between the latter and TaMPKKK;A, suggesting that all comprise a MAPK signaling cascade TaMPKKK;A-TaMPKK2-TaMPK2B. TaMPK2B expression levels were elevated in roots and leaves under a Pi starvation (PS) condition. Additionally, the induced TaMPK2B transcripts under PS in tissues were gradually restored following the Pi normal recovery condition. TaMPK2B overexpression conferred on plants improved PS adaptation; the tobacco lines with TaMPK2B overexpression enhanced the plant's dry mass production, Pi uptake capacity, root system architecture (RSA) establishment, and ROS homeostasis relative to wild type under PS treatment. Moreover, the transcripts of genes in phosphate transporter (PT), PIN-FORMED, and antioxidant enzyme (AE) families, including NtPT3 and NtPT4, NtPIN9, and NtMnSOD1 and NtPOD1;7, were elevated in Pi-deprived lines overexpressing TaMPK2B. Transgene analyses validated their functions in regulating Pi uptake, RSA establishment, and AE activities of plants treated by PS. These results suggest that TaMPK2B-mediated plant PS adaptation is correlated with the modified transcription of distinct PT, PIN, and AE genes. Our investigation suggests that TaMPK2B is one of the crucial regulators in plant low-Pi adaptation by improving Pi uptake, RSA formation, and ROS homeostasis via transcriptionally regulating genes associated with the above physiological processes.

Transcriptomic profiling revealed the role of 24-epibrassinolide in alleviating salt stress damage in tall fescue (Festuca arundinacea)

Soil salinization is a major problem all over the world. The accumulation of salt in soil reduces the root water uptake and directly affects plant growth and metabolic activities. Brassinosteroid is a plant hormone that plays an important role in regulation of plant growth and physiological process, including promotion of cell expansion and elongation, signal transduction and stress response. Exogenous 24-epibrassinolide (EBL) has been proved to alleviate various environmental stress in plants. However, the role that EBL plays in salt stress response is still unknown in tall fescue (Festuca arundinacea). In this study, the physiology and molecular mechanisms regulated by exogenous EBL of salt stress response in tall fescue was investigated. Tall fescue plants were divided into four groups, including control (CK), NaCl solution (SALT), 24-epibrassinolide (EBL), NaCl solution + 24-epibrassinolide (SE). During the growth period of tall fescue, we found that electrolyte leakage (EL) and malondialdehyde (MDA) were decreased, chlorophyll (Chl) content and antioxidant enzyme activity were increased in leaves of tall fescue in SE group compared with SALT group, indicating that EBL improved the salt tolerance in grasses. Transcriptomic profiling analysis showed that after 12 h of treatments, 10,265, 13,830 and 10,537 differential genes were expressed in EBL, SALT, and SE groups compared with control, respectively. These differentially expressed genes (DEGs) mainly focused on binding, catalytic activity, cellular process, metabolic process, cellular anatomical entity. Moreover, most of the differential genes were expressed in the plant hormone signal transduction pathway. These results helped us to better understand the mechanism of exogenous 24-epibrassinolide to improve the salt tolerance of tall fescue.

TMT-based comparative proteomics reveals the role of acyl-CoA oxidase 4 in enhancing the drought stress tolerance in common buckwheat (Fagopyrum esculentum)

Drought stress has been the main abiotic factor affecting the growth, development and production of common buckwheat (Fagopyrum esculentum). To explore the response mechanisms of regulating buckwheat drought stress on the post-transcriptional and translational levels, a comparative proteomic analysis was applied to monitor the short-term proteomic variations under the drought stress in the seedling stage. From which 593 differentially abundant proteins (DAPs) were identified using the TMT-based proteomics analysis. A number of DAPs were found to be intimately correlated with the styrene degradation, phenylpropanoid biosynthesis and stimulus response, within which. The acyl-CoA oxidase 4 (ACX4), a key regulator in plant abiotic stress response, was selected for further elucidation. Overexpression of the FeACX4 not only conferred drought and salt tolerance in the Arabidopsis, but also significantly increased the root length and fresh weight in the overexpression lines plant relative to the wild type (WT) plant, accompanied by the elevated activities of catalase (CAT) and lowered malonaldehyde (MDA) and H₂O₂ contents, therefore allowing plants to better adapt to adverse environments. Our results provided information in the exploring of the molecular regulation mechanism responding to drought tolerance in common buckwheat.

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.

Overexpression of a Zea mays Brassinosteroid-Signaling Kinase Gene ZmBSK1 Confers Salt Stress Tolerance in Maize

Salinity has become a crucial environmental factor seriously restricting maize (Zea mays L.) growth, development and productivity. However, how plants respond to salt stress is still poorly understood. In this study, we report that a maize brassinosteroid-signaling kinase gene ZmBSK1 plays a significant role in salt stress response. Expression pattern analysis revealed that the transcript level of ZmBSK1 was upregulated by NaCl treatment both in maize leaves, roots, and stems. Phenotypic and physiological analysis showed that overexpression of ZmBSK1 in maize improved salt tolerance by reducing the malondialdehyde (MDA) content, the percentage of electrolyte leakage, O₂ ^- and H₂O₂ accumulation under salt stress, relying on the increases of antioxidant defense enzyme activities and proline content. qRT-PCR analysis showed that overexpression of ZmBSK1 also positively modulated the expression levels of reactive oxygen species (ROS)-scavenging and proline biosynthesis-related genes under salt stress. Moreover, immunoprecipitation-mass spectrometry (IP-MS) assay and firefly luciferase complementation imaging (LCI) assay showed that ZmBSK1 could associate with heat shock protein ZmHSP8 and 14-3-3-like protein ZmGF14-6, and their gene expression levels could be significantly induced by NaCl treatment in different maize tissues. Our findings unravel the new function of ZmBSK1 in salt stress response, which provides the theoretical bases for the improvement of maize salt resistance.

MPK homolog GhNTF6 was involved in cotton against Verticillium wilt by interacted with VdEPG1

Mitogen-activated protein kinases (MPKs) are important in regulating plant development and stress response. Rapid activation of MPKs in plants usually depends on its phosphorylated. In view of this situation, a phosphorylated GhNTF6 belonged to MPKs family was screened in cotton roots under Verticillium dahliae challenge by phosphoproteomics analysis. Expression of GhNTF6 in cotton plants was did not induce by V. dahliae infection, while, silencing GhNTF6 results to enhance cotton plants susceptibility to V. dahliae, overexpression - GhNTF6 enhance Arabidopsis plants survivability to V. dahliae. Moreover, the mutation of GhNTF6 at site Thr195 and Thy197 with the phosphorylation decreased the plant resistance to V. dahliae. Therefore, GhNTF6 phosphorylation is important in plants against V. dahliae. Further analysis demonstrated that GhNTF6 interacted with a V. dahliae endopolygalacturonase (VdEPG1) on the cell nucleus. We propose that GhNTF6 is a potential molecular target for improving resistance to Verticillium wilt in cotton.

ZmMPK5 phosphorylates ZmNAC49 to enhance oxidative stress tolerance in maize

Mitogen-activated protein kinase (MPK) is a critical regulator of the antioxidant defence system in response to various stimuli. However, how MPK directly and exactly regulates antioxidant enzyme activities is still unclear. Here, we demonstrated that a NAC transcription factor ZmNAC49 mediated the regulation of antioxidant enzyme activities by ZmMPK5. ZmNAC49 expression is induced by oxidative stress. ZmNAC49 enhances oxidative stress tolerance in maize, and it also reduces superoxide anion generation and increases superoxide dismutase (SOD) activity. A detailed study showed that ZmMPK5 directly interacts with and phosphorylates ZmNAC49 in vitro and in vivo. ZmMPK5 directly phosphorylates Thr-26 in NAC subdomain A of ZmNAC49. Mutation at Thr-26 of ZmNAC49 does not affect the interaction with ZmMPK5 and its subcellular localisation. Further analysis found that ZmNAC49 activates the ZmSOD3 expression by directly binding to its promoter. ZmMPK5-mediated ZmNAC49 phosphorylation improves its ability to bind to the ZmSOD3 promoter. Thr-26 of ZmNAC49 is essential for its transcriptional activity. In addition, ZmSOD3 enhances oxidative stress tolerance in maize. Our results show that phosphorylation of Thr-26 in ZmNAC49 by ZmMPK5 increased its DNA-binding activity to the ZmSOD3 promoter, enhanced SOD activity and thereby improved oxidative stress tolerance in maize.

Comparative Genomic Analysis of Quantitative Trait Loci Associated With Micronutrient Contents, Grain Quality, and Agronomic Traits in Wheat (Triticum aestivum L.)

Comparative genomics and meta-quantitative trait loci (MQTLs) analysis are important tools for the identification of reliable and stable QTLs and functional genes controlling quantitative traits. We conducted a meta-analysis to identify the most stable QTLs for grain yield (GY), grain quality traits, and micronutrient contents in wheat. A total of 735 QTLs retrieved from 27 independent mapping populations reported in the last 13 years were used for the meta-analysis. The results showed that 449 QTLs were successfully projected onto the genetic consensus map which condensed to 100 MQTLs distributed on wheat chromosomes. This consolidation of MQTLs resulted in a three-fold reduction in the confidence interval (CI) compared with the CI for the initial QTLs. Projection of QTLs revealed that the majority of QTLs and MQTLs were in the non-telomeric regions of chromosomes. The majority of micronutrient MQTLs were located on the A and D genomes. The QTLs of thousand kernel weight (TKW) were frequently associated with QTLs for GY and grain protein content (GPC) with co-localization occurring at 55 and 63%, respectively. The co- localization of QTLs for GY and grain Fe was found to be 52% and for QTLs of grain Fe and Zn, it was found to be 66%. The genomic collinearity within Poaceae allowed us to identify 16 orthologous MQTLs (OrMQTLs) in wheat, rice, and maize. Annotation of promising candidate genes (CGs) located in the genomic intervals of the stable MQTLs indicated that several CGs (e.g., TraesCS2A02G141400, TraesCS3B02G040900, TraesCS4D02G323700, TraesCS3B02G077100, and TraesCS4D02G290900) had effects on micronutrients contents, yield, and yield-related traits. The mapping refinements leading to the identification of these CGs provide an opportunity to understand the genetic mechanisms driving quantitative variation for these traits and apply this information for crop improvement programs.

Molecular dissection of maize seedling salt tolerance using a genome-wide association analysis method

Salt stress is a major devastating abiotic factor that affects the yield and quality of maize. However, knowledge of the molecular mechanisms of the responses to salt stress in maize is limited. To elucidate the genetic basis of salt tolerance traits, a genome-wide association study was performed on 348 maize inbred lines under normal and salt stress conditions using 557 894 single nucleotide polymorphisms (SNPs). The phenotypic data for 27 traits revealed coefficients of variation of >25%. In total, 149 significant SNPs explaining 6.6%-11.2% of the phenotypic variation for each SNP were identified. Of the 104 identified quantitative trait loci (QTLs), 83 were related to salt tolerance and 21 to normal traits. Additionally, 13 QTLs were associated with two to five traits. Eleven and six QTLs controlling salt tolerance traits and normal root growth, respectively, co-localized with QTL intervals reported previously. Based on functional annotations, 13 candidate genes were predicted. Expression levels analysis of 12 candidate genes revealed that they were all responsive to salt stress. The CRISPR/Cas9 technology targeting three sites was applied in maize, and its editing efficiency reached 70%. By comparing the biomass of three CRISPR/Cas9 mutants of ZmCLCg and one zmpmp3 EMS mutant with their wild-type plants under salt stress, the salt tolerance function of candidate genes ZmCLCg and ZmPMP3 were confirmed. Chloride content analysis revealed that ZmCLCg regulated chloride transport under sodium chloride stress. These results help to explain genetic variations in salt tolerance and provide novel loci for generating salt-tolerant maize lines.

A Review on Plant Responses to Salt Stress and Their Mechanisms of Salt Resistance

Nowadays, crop insufficiency resulting from soil salinization is threatening the world. On the basis that soil salinization has become a worldwide problem, studying the mechanisms of plant salt tolerance is of great theoretical and practical significance to improve crop yield, to cultivate new salt-tolerant varieties, and to make full use of saline land. Based on previous studies, this paper reviews the damage of salt stress to plants, including suppression of photosynthesis, disturbance of ion homeostasis, and membrane peroxidation. We have also summarized the physiological mechanisms of salt tolerance, including reactive oxygen species (ROS) scavenging and osmotic adjustment. Four main stress-related signaling pathways, salt overly sensitive (SOS) pathway, calcium-dependent protein kinase (CDPK) pathway, mitogen-activated protein kinase (MAPKs) pathway, and abscisic acid (ABA) pathway, are included. We have also enumerated some salt stress-responsive genes that correspond to physiological mechanisms. In the end, we have outlined the present approaches and techniques to improve salt tolerance of plants. All in all, we reviewed those aspects above, in the hope of providing valuable background knowledge for the future cultivation of agricultural and forestry plants.

Regulation of Plant Responses to Salt Stress

Salt stress is a major environmental stress that affects plant growth and development. Plants are sessile and thus have to develop suitable mechanisms to adapt to high-salt environments. Salt stress increases the intracellular osmotic pressure and can cause the accumulation of sodium to toxic levels. Thus, in response to salt stress signals, plants adapt via various mechanisms, including regulating ion homeostasis, activating the osmotic stress pathway, mediating plant hormone signaling, and regulating cytoskeleton dynamics and the cell wall composition. Unraveling the mechanisms underlying these physiological and biochemical responses to salt stress could provide valuable strategies to improve agricultural crop yields. In this review, we summarize recent developments in our understanding of the regulation of plant salt stress.

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.

Plant Mitogen-Activated Protein Kinase Cascades in Environmental Stresses

Due to global warming and population growth, plants need to rescue themselves, especially in unfavorable environments, to fulfill food requirements because they are sessile organisms. Stress signal sensing is a crucial step that determines the appropriate response which, ultimately, determines the survival of plants. As important signaling modules in eukaryotes, plant mitogen-activated protein kinase (MAPK) cascades play a key role in regulating responses to the following four major environmental stresses: high salinity, drought, extreme temperature and insect and pathogen infections. MAPK cascades are involved in responses to these environmental stresses by regulating the expression of related genes, plant hormone production and crosstalk with other environmental stresses. In this review, we describe recent major studies investigating MAPK-mediated environmental stress responses. We also highlight the diverse function of MAPK cascades in environmental stress. These findings help us understand the regulatory network of MAPKs under environmental stress and provide another strategy to improve stress resistance in crops to ensure food security.

Embryogenic Calli Induction and Salt Stress Response Revealed by RNA-Seq in Diploid Wild Species Gossypium sturtianum and Gossypium raimondii

Wild cotton species can contribute to a valuable gene pool for genetic improvement, such as genes related to salt tolerance. However, reproductive isolation of different species poses an obstacle to produce hybrids through conventional breeding. Protoplast fusion technology for somatic cell hybridization provides an opportunity for genetic manipulation and targeting of agronomic traits. Transcriptome sequencing analysis of callus under salt stress is conducive to study salt tolerance genes. In this study, calli were induced to provide materials for extracting protoplasts and also for screening salt tolerance genes. Calli were successfully induced from leaves of Gossypium sturtianum (C₁ genome) and hypocotyls of G. raimondii (D₅ genome), and embryogenic calli of G. sturtianum and G. raimondii were induced on a differentiation medium with different concentrations of 2, 4-D, KT, and IBA, respectively. In addition, embryogenic calli were also induced successfully from G. raimondii through suspension cultivation. Transcriptome sequencing analysis was performed on the calli of G. raimondii and G. sturtianum, which were treated with 200 mM NaCl at 0, 6, 12, 24, and 48 h, and a total of 12,524 genes were detected with different expression patterns under salt stress. Functional analysis showed that 3,482 genes, which were differentially expressed in calli of G. raimondii and G. sturtianum, were associated with biological processes of nucleic acid binding, plant hormone (such as ABA) biosynthesis, and signal transduction. We demonstrated that DEGs or TFs which related to ABA metabolism were involved in the response to salt stress, including xanthoxin dehydrogenase genes (ABA2), sucrose non-fermenting 1-related protein kinases (SnRK2), NAM, ATAT1/2, and CUC2 transcription factors (NAC), and WRKY class of zinc-finger proteins (WRKY). This research has successfully induced calli from two diploid cotton species and revealed new genes responding to salt stress in callus tissue, which will lay the foundation for protoplast fusion for further understanding of salt stress responses in cotton.

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.

Isolation and functional validation of stress tolerant EaMYB18 gene and its comparative physio-biochemical analysis with transgenic tobacco plants overexpressing SoMYB18 and SsMYB18

MYB transcription factors are one of the most important mediators for the survival of plants under multiple stress responses. In the present study, EaMYB18, encoding a single R3 repeat MYB DNA binding domain was isolated from stress-tolerant wild relative species of sugarcane Erianthus arundinaceus. In silico analysis of 948 bp coding mRNA sequence of EaMYB18 exhibited the presence of four exons and three introns. Further, the EaMYB18 gene was transformed in tobacco and its stable inheritance was confirmed through antibiotic resistance screening, PCR amplification and Southern hybridization blotting. Results of the estimation of MDA, proline, total chlorophyll and antioxidant activities of EaMYB18 transgenic tobacco lines exhibited least oxidative damage under drought and cold stress over the untransformed ones, the over-expression of EaMYB18 has improved drought and cold stress tolerance ability in tobacco. The comparative physiological and biochemical analysis of transgenic tobacco plants overexpressing SoMYB18, SsMYB18 and EaMYB18, revealed that the EaMYB18 and SsMYB18 transgenic plants demonstrated effective tolerance to drought and cold stresses, while SoMYB18 showed improved tolerance to salt stress alone. Amongst these three genes, EaMYB18 displayed the highest potential for drought and cold stress tolerances as compared to SoMYB18 and SsMYB18 genes.

Salinity tolerance in barley during germination- homologs and potential genes

Salinity affects more than 6% of the world's total land area, causing massive losses in crop yield. Salinity inhibits plant growth and development through osmotic and ionic stresses; however, some plants exhibit adaptations through osmotic regulation, exclusion, and translocation of accumulated Na+ or Cl-. Currently, there are no practical, economically viable methods for managing salinity, so the best practice is to grow crops with improved tolerance. Germination is the stage in a plant's life cycle most adversely affected by salinity. Barley, the fourth most important cereal crop in the world, has outstanding salinity tolerance, relative to other cereal crops. Here, we review the genetics of salinity tolerance in barley during germination by summarizing reported quantitative trait loci (QTLs) and functional genes. The homologs of candidate genes for salinity tolerance in Arabidopsis, soybean, maize, wheat, and rice have been blasted and mapped on the barley reference genome. The genetic diversity of three reported functional gene families for salt tolerance during barley germination, namely dehydration-responsive element-binding (DREB) protein, somatic embryogenesis receptor-like kinase and aquaporin genes, is discussed. While all three gene families show great diversity in most plant species, the DREB gene family is more diverse in barley than in wheat and rice. Further to this review, a convenient method for screening for salinity tolerance at germination is needed, and the mechanisms of action of the genes involved in salt tolerance need to be identified, validated, and transferred to commercial cultivars for field production in saline soil.

Functional Validation of JcWRKY2, a Group III Transcription Factor Toward Mitigating Salinity Stress in Transgenic Tobacco

The plants being sessile cannot escape from the adverse environmental stresses, hence get negatively affected in terms of their growth and yield. Transcriptional control simultaneously regulate different cellular processes, minimizing the deleterious effects of these stresses. The salicylic acid (SA)-inducible WRKY family of transcription factors auto or crossregulate the stress signaling in response to abiotic and biotic stresses, facilitating enhanced stress tolerance. In this study, we characterized the group III WRKY gene, JcWRKY2 from ecological and economical valued shrub Jatropha curcas. The JcWRKY2 tobacco transgenics showed improved physiological growth parameters, elevated chlorophyll content, improved antioxidative activities, and increased endogenous SA with both salt and SA stress. Interestingly, the pretreatment with SA and hydrogen peroxide facilitated improved germination of transgenic seeds with salinity stress. The transgenics showed differential regulation of antioxidative enzymes, calcium/calmodulin, dehydrins, and phospholipase genes with salt and SA stress. The increased SA content in transgenics on stress treatments, enhanced the antioxidant capacity leading to reduced susceptibility to stresses. Thus, JcWRKY2 transgenics participate in SA-mediated, improved antioxidative status during salinity stress with reduced reactive oxygen species damage.

Identification of the metallothionein gene family from cucumber and functional characterization of CsMT4 in Escherichia coli under salinity and osmotic stress

Metallothionein (MT) proteins are low-molecular-weight, cysteine-rich and metal-binding proteins that play important roles in the maintenance of metal homeostasis and detoxification, but their roles in abiotic stress tolerance remain largely unknown. In this study, three MT family genes (CsMT2, CsMT3 and CsMT4) were identified in the cucumber genome. CsMT2, CsMT3 and CsMT4 possessed 14, 10, and 18 Cys residues, which were clustered into 2, 2, and 3 Cys-rich regions, respectively. Phylogenetic analysis of MTs from cucumber, Arabidopsis and soybean revealed that these MTs were clustered into four groups in accordance with the MT types (types 1-4). An analysis of the cis-acting regulatory elements revealed that a series of hormone-, stress-, and development-related cis-elements were present in the promoter regions of CsMT genes. Expression pattern analysis by RT-PCR showed that the CsMT genes exhibited different tissue expression patterns. CsMT2 showed relatively higher expression in stem, leaf, and flower; CsMT3 was mainly expressed in leaf, flower, and fruit, while CsMT4 was highly expressed in fruit and leaf. The qRT-PCR results showed that the CsMT genes were induced by various stress treatments including NaCl, PEG, and ABA, while CsMT4 displayed much higher expression levels in response to these stresses than CsMT2 and CsMT3. Escherichia coli cells expressing CsMT4 exhibited higher salinity and osmotic tolerance compared with control cells, indicating the significant function of CsMT4 to confer tolerance to these stresses. These results lay a foundation for further research on the function of MT family genes in plant stress responses.

Melatonin-Mediated Regulation of Growth and Antioxidant Capacity in Salt-Tolerant Naked Oat under Salt Stress

Melatonin (MT; N-acetyl-5-methoxytryptamine) is a pleiotropic signaling molecule that has been demonstrated to play an important role in plant growth, development, and regulation of environmental stress responses. Studies have been conducted on the role of the exogenous application of MT in a few species, but the potential mechanisms of MT-mediated stress tolerance under salt stress are still largely unknown. In this study, naked oat seedlings under salt stress (150 mM NaCl) were pretreated with two different concentrations of MT (50 and 100 μM), and the effects of MT on the growth and antioxidant capacity of naked oat seedlings were analyzed to explore the regulatory effect of MT on salt tolerance. The results showed that pretreating with different concentrations of MT promoted the growth of seedlings in response to 150 mM NaCl. Different concentrations of MT reduced hydrogen peroxide, superoxide anion, and malondialdehyde contents. The exogenous application of MT also increased superoxide dismutase, peroxidase, catalase, and ascorbate peroxide activities. Chlorophyll content, leaf area, leaf volume, and proline increased in the leaves of naked oat seedlings under 150 mM NaCl stress. MT upregulated the expression levels of the lipid peroxidase genes lipoxygenase and peroxygenase, a chlorophyll biosynthase gene (ChlG), the mitogen-activated protein kinase genes Asmap1 and Aspk11, and the transcription factor genes (except DREB2), NAC, WRKY1, WRKY3, and MYB in salt-exposed MT-pretreated seedlings when compared with seedlings exposed to salt stress alone. These results demonstrate an important role of MT in the relief of salt stress and, therefore, provide a reference for managing salinity in naked oat.

Effects of Melatonin on Antioxidant Capacity in Naked Oat Seedlings under Drought Stress

Melatonin (N-acetyl-5-methoxytryptamine, MT) is a molecule with pleiotropic effects including antioxidant activity, regulated plant growth, development, and reduced environmental stress in plants. However, only a few studies have analyzed the effect of exogenous MT on drought stress in naked oat seedlings. Therefore, in this study, we studied the effects of exogenous MT on the antioxidant capacity of naked oat under drought stress to understand the possible antioxidant mechanism. The results showed that a pretreatment of 100 μM MT reduced the hydrogen peroxide (H₂O₂) and superoxide anion (O2−•) contents. MT also enhanced superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activities in the leaves of naked oat seedlings under 20% PEG-6000 drought stress. MT upregulated the expression levels of the mitogen-activated protein kinases (MAPKs) Asmap1 and Aspk11, and the transcription factor (TF) genes (except for NAC), WRKY1, DREB2, and MYB increased in drought with MT pretreatment seedlings when compared with seedlings exposed to drought stress alone. These data indicated that the MT-mediated induction of the antioxidant response may require the activation of reactive oxygen species (ROS) and MAPK, followed by triggering a downstream MAPK cascade such as Asmap1 and Aspk11, to regulate the expression of antioxidant-related genes. This study demonstrated that MT could induce the expression of MAPKs and TFs and regulate the expression of downstream stress-responsive genes, thereby increasing the plant’s tolerance. This may provide a new idea for MT modulation in the regulation of plant antioxidant defenses. These results provide a theoretical basis for MT to alleviate drought stress in naked oat.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Brachypodium distachyon T-DNA insertion lines: a model pathosystem to study nonhost resistance to wheat stripe rust

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), is one of the most destructive diseases and can cause severe yield losses in many regions of the world. Because of the large size and complexity of wheat genome, it is difficult to study the molecular mechanism of interaction between wheat and PST. Brachypodium distachyon has become a model system for temperate grasses' functional genomics research. The phenotypic evaluation showed that the response of Brachypodium distachyon to PST was nonhost resistance (NHR), which allowed us to present this plant-pathogen system as a model to explore the immune response and the molecular mechanism underlying wheat and PST. Here we reported the generation of about 7,000 T-DNA insertion lines based on a highly efficient Agrobacterium-mediated transformation system. Hundreds of mutants either more susceptible or more resistant to PST than that of the wild type Bd21 were obtained. The three putative target genes, Bradi5g17540, BdMYB102 and Bradi5g11590, of three T-DNA insertion mutants could be involved in NHR of Brachypodium distachyon to wheat stripe rust. The systemic pathologic study of this T-DNA mutants would broaden our knowledge of NHR, and assist in breeding wheat cultivars with durable resistance.

ZmABA2, an interacting protein of ZmMPK5, is involved in abscisic acid biosynthesis and functions

In maize (Zea mays), the mitogen-activated protein kinase ZmMPK5 has been shown to be involved in abscisic acid (ABA)-induced antioxidant defence and to enhance the tolerance of plants to drought, salt stress and oxidative stress. However, the underlying molecular mechanisms are poorly understood. Here, using ZmMPK5 as bait in yeast two-hybrid screening, a protein interacting with ZmMPK5 named ZmABA2, which belongs to a member of the short-chain dehydrogenase/reductase family, was identified. Pull-down assay and bimolecular fluorescence complementation analysis and co-immunoprecipitation test confirmed that ZmMPK5 interacts with ZmABA2 in vitro and in vivo. Phosphorylation of Ser173 in ZmABA2 by ZmMPK5 was shown to increase the activity of ZmABA2 and the protein stability. Various abiotic stimuli induced the expression of ZmABA2 in leaves of maize plants. Pharmacological, biochemical and molecular biology and genetic analyses showed that both ZmMPK5 and ZmABA2 coordinately regulate the content of ABA. Overexpression of ZmABA2 in tobacco plants was found to elevate the content of ABA, regulate seed germination and root growth under drought and salt stress and enhance the tolerance of tobacco plants to drought and salt stress. These results suggest that ZmABA2 is a direct target of ZmMPK5 and is involved in ABA biosynthesis and functions.

Investigation of the ASR family in foxtail millet and the role of ASR1 in drought/oxidative stress tolerance

KEY MESSAGE

Six foxtail millet ASR genes were regulated by various stress-related signals. Overexpression of ASR1 increased drought and oxidative tolerance by controlling ROS homeostasis and regulating oxidation-related genes in tobacco plants. Abscisic acid stress ripening (ASR) proteins with ABA/WDS domains constituted a class of plant-specific transcription factors, playing important roles in plant development, growth and abiotic stress responses. However, only a few ASRs genes have been characterized in crop plants and none was reported so far in foxtail millet (Setaria italic), an important drought-tolerant crop and model bioenergy grain crop. In the present study, we identified six foxtail millet ASR genes. Gene structure, protein alignments and phylogenetic relationships were analyzed. Transcript expression patterns of ASR genes revealed that ASRs might play important roles in stress-related signaling and abiotic stress responses in diverse tissues in foxtail millet. Subcellular localization assays showed that SiASR1 localized in the nucleus. Overexpression of SiASR1 in tobacco remarkably increased tolerance to drought and oxidative stresses, as determined through developmental and physiological analyses of germination rate, root growth, survival rate, relative water content, ion leakage, chlorophyll content and antioxidant enzyme activities. Furthermore, expression of SiASR1 modulated the transcript levels of oxidation-related genes, including NtSOD, NtAPX, NtCAT, NtRbohA and NtRbohB, under drought and oxidative stress conditions. These results provide a foundation for evolutionary and functional characterization of the ASR gene family in foxtail millet.

Genome-Wide Identification and Expression Analysis of the Mitogen-Activated Protein Kinase Gene Family in Cassava

Mitogen-activated protein kinases (MAPKs) play central roles in plant developmental processes, hormone signaling transduction, and responses to abiotic stress. However, no data are currently available about the MAPK family in cassava, an important tropical crop. Herein, 21 MeMAPK genes were identified from cassava. Phylogenetic analysis indicated that MeMAPKs could be classified into four subfamilies. Gene structure analysis demonstrated that the number of introns in MeMAPK genes ranged from 1 to 10, suggesting large variation among cassava MAPK genes. Conserved motif analysis indicated that all MeMAPKs had typical protein kinase domains. Transcriptomic analysis suggested that MeMAPK genes showed differential expression patterns in distinct tissues and in response to drought stress between wild subspecies and cultivated varieties. Interaction networks and co-expression analyses revealed that crucial pathways controlled by MeMAPK networks may be involved in the differential response to drought stress in different accessions of cassava. Expression of nine selected MAPK genes showed that these genes could comprehensively respond to osmotic, salt, cold, oxidative stressors, and abscisic acid (ABA) signaling. These findings yield new insights into the transcriptional control of MAPK gene expression, provide an improved understanding of abiotic stress responses and signaling transduction in cassava, and lead to potential applications in the genetic improvement of cassava cultivars.

Microarray: gateway to unravel the mystery of abiotic stresses in plants

Environmental factors, such as drought, salinity, extreme temperature, ozone poisoning, metal toxicity etc., significantly affect crops. To study these factors and to design a possible remedy, biological experimental data concerning these crops requires the quantification of gene expression and comparative analyses at high throughput level. Development of microarrays is the platform to study the differential expression profiling of the targeted genes. This technology can be applied to gene expression studies, ranging from individual genes to whole genome level. It is now possible to perform the quantification of the differential expression of genes on a glass slide in a single experiment. This review documents recently published reports on the use of microarrays for the identification of genes in different plant species playing their role in different cellular networks under abiotic stresses. The regulation pattern of differentially-expressed genes, individually or in group form, may help us to study different pathways and functions at the cellular and molecular level. These studies can provide us with a lot of useful information to unravel the mystery of abiotic stresses in important crop plants.

Overexpression of ZmMAPK1 enhances drought and heat stress in transgenic Arabidopsis thaliana

Mitogen-activated protein kinase (MAPK) signal transduction cascades play a crucial role in the response to extracellular stimuli in eukaryotes. A number of MAPK family genes have been isolated in plants, but the maize MAPK genes have been little studied. Here, we studied the role of maize MAP kinase 1 (ZmMAPK1) using gene expression, protein subcellular localization, transformation in Arabidopsis, expression patterns of the stress-responsive genes and physiological parameter analysis. Our physiological parameter analysis suggested that over-expression ZmMAPK1 can increase proline content and decrease malondialdehyde content under drought, and prevent chlorophyll loss and the production of scavenger reactive oxygen species under heat stress. The resistance characteristics of the over-expression of ZmMAPK1 were associated with a significant increase in survival rate. These results suggest that ZmMAPK1 plays a positive role in response to drought and heat stress in Arabidopsis, and provide new insights into the mechanisms of action of MAPK in response to abiotic stress in plants.

Genome-wide identification and transcriptional expression analysis of mitogen-activated protein kinase and mitogen-activated protein kinase kinase genes in Capsicum annuum

The tripartite mitogen-activated protein kinase (MAPK) signaling cascades have been implicated in plant growth, development, and environment adaptation, but a comprehensive understanding of MAPK signaling at genome-wide level is limited in Capsicum annuum. Herein, genome-wide identification and transcriptional expression analysis of MAPK and MAPK kinase (MAPKK) were performed in pepper. A total of 19 pepper MAPK (CaMAPKs) genes and five MAPKK (CaMAPKKs) genes were identified. Phylogenetic analysis indicated that CaMAPKs and CaMAPKKs could be classified into four groups and each group contains similar exon-intron structures. However, significant divergences were also found. Notably, five members of the pepper MAPKK family were much less conserved than those found in Arabidopsis, and 9 Arabidopsis MAPKs did not have orthologs in pepper. Additionally, 7 MAPKs in Arabidopsis had either two or three orthologs in the pepper genome, and six pepper MAPKs and one MAPKK differing in sequence were found in three pepper varieties. Quantitative real-time RT-PCR analysis showed that the majority of MAPK and MAPKK genes were ubiquitously expressed and transcriptionally modified in pepper leaves after treatments with heat, salt, and Ralstonia solanacearum inoculation as well as exogenously applied salicylic acid, methyl jasmonate, ethephon, and abscisic acid. The MAPKK-MAPK interactome was tested by yeast two-hybrid assay, the results showed that one MAPKK might interact with multiple MAPKs, one MAPK might also interact with more than one MAPKKs, constituting MAPK signaling networks which may collaborate in transmitting upstream signals into appropriate downstream cellular responses and processes. These results will facilitate future functional characterization of MAPK cascades in pepper.

Improving plant stress tolerance: potential applications of engineered MAPK cascades

The need to develop solutions to the problem of worldwide food crop scarcity under exacerbated climate change will be paramount. This motivates an effort to leverage agricultural biotechnology to improve plant adaptation to severe and multiple, simultaneous environmental stresses. Consequently, engineering synthetic signaling pathways, particularly mitogen activated protein kinase (MAPK) cascades utilizing components already involved in plant stress tolerance, is a worthy focus for research to breed new plant varieties with enhanced stress-tolerance traits.

MAPK transgenic circuit to improve plant stress-tolerance?

Thanks to their distinctive mode of action in a coordinated switch-like way, their multi-tiered signaling cascades and their involvement in cell responses to multiple internal and external stimuli, MAP kinases offer a remarkable possibility to be assembled into what we can call "MAPK transgenic circuits" to improve cell functions. Such circuit could be used to enhance cell signaling efficiency and boost cell functions for several purposes in plant biotechnology, medicine, and pharmaceutical industry.