Constitutive expression of a salinity-induced wheat WRKY transcription factor enhances salinity and ionic stress tolerance in transgenic Arabidopsis thaliana

Salt-Induced Modulation of Ion Transport and PSII Photoprotection Determine the Salinity Tolerance of Amphidiploid Brassicas

Brassica species show varying levels of resistance to salt stress. To understand the genetics underlying these differential stress tolerance patterns in Brassicas, we exposed two widely cultivated amphidiploid Brassica species having different genomes, Brassica juncea (AABB, n = 18) and Brassica napus (AACC, n = 19), to elevated levels of NaCl concentration (300 mM, half the salinity of seawater). B. juncea produced more biomass, an increased chlorophyll content, and fewer accumulated sodium (Na+) and chloride (Cl-) ions in its photosynthesizing tissues. Chlorophyll fluorescence assays revealed that the reaction centers of PSII of B. juncea were more photoprotected and hence more active than those of B. napus under NaCl stress, which, in turn, resulted in a better PSII quantum efficiency, better utilization of photochemical energy with significantly reduced energy loss, and higher electron transport rates, even under stressful conditions. The expression of key genes responsible for salt tolerance (NHX1 and AVP1, which are nuclear-encoded) and photosynthesis (psbA, psaA, petB, and rbcL, which are chloroplast-encoded) were monitored for their genetic differences underlying stress tolerance. Under NaCl stress, the expression of NHX1, D1, and Rubisco increased several folds in B. juncea plants compared to B. napus, highlighting differences in genetics between these two Brassicas. The higher photosynthetic potential under stress suggests that B. juncea is a promising candidate for genetic modifications and its cultivation on marginal lands.

Transcriptomic analysis provides insight into the genetic regulation of shade avoidance in Aegilops tauschii

BACKGROUND

Weeds are not only economically important but also fascinating models for studying the adaptation of species in human-mediated environments. Aegilops tauschii is the D-genome donor species of common wheat but is also a weed that influences wheat production. How shading stress caused by adjacent wheat plants affects Ae. tauschii growth is a fundamental scientific question but is also important in agriculture, such as for weed control and wheat breeding.

RESULT

The present study indicated that shade avoidance is a strategy of Ae. tauschii in response to shading stress. Ae. tauschii plants exhibited growth increases in specific organs, such as stem and leaf elongation, to avoid shading. However, these changes were accompanied by sacrificing the growth of other parts of the plants, such as a reduction in tiller number. The two reverse phenotype responses seem to be formed by systemically regulating the expression of different genes. Fifty-six genes involved in the regulation of cell division and cell expansion were found to be downregulated, and one key upstream negative regulator (RPK2) of cell division was upregulated under shading stress. On the other hand, the upregulated genes under shading stress were mainly enriched in protein serine/threonine kinase activity and carbon metabolism, which are associated with cell enlargement, signal transduction and energy supply. The transcription factor WRKY72 may be important in regulating genes in response to shading stress, which can be used as a prior candidate gene for further study on the genetic regulation of shade avoidance.

CONCLUSIONS

This study sheds new light on the gene expression changes and molecular processes involved in the response and avoidance of Ae. tauschii to shading stress, which may aid more effective development of shading stress avoidance or cultivars in wheat and other crops in the future.

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.

Dioscorea composita WRKY3 positively regulates salt-stress tolerance in transgenic Arabidopsis thaliana

Dioscorea composita (D. composita) is a perennial dioecious herb with strong biotic and abiotic stress tolerance. However, what roles WRKY transcription factors might play in regulating abiotic stress responses in this medicinal plant is unknown. Here, we isolated DcWRKY3 from D. composita and analyzed its role in stress tolerance. DcWRKY3 is a group I WRKY transcription factor that localized to the nucleus and specifically bound to the W-box cis-elements, but lacked transcriptional activation activity in yeast cells. The expression of DcWRKY3 was strongly affected by salt stress. The heterologous expression of DcWRKY3 strongly enhanced the seed germination rate and root length of Arabidopsis thaliana under salt stress. The DcWRKY3-expressing transgenic lines (DcWRKY3-OEs) also showed higher proline content and antioxidant enzyme activity but lower malondialdehyde and reactive oxygen (ROS) levels compared with the wild type. Moreover, these plants showed upregulated expression of genes related to the salt-stress response and ROS clearance. These findings indicate that DcWRKY3 plays a positive role in the salt-stress response by improving the ROS scavenging ability and maintaining the balance of osmotic pressure in plants. Further studies showed that DcWRKY3 binds to the promoter of AtP5CS1, but not AtSOD and AtRD22, suggesting that DcWRKY3 improves salt tolerance in plants by directly or indirectly regulating the expression of downstream genes. This functional characterization of DcWRKY3 provides new insight into the molecular mechanism underlying the response of D. composita to salt stress.

Discovery of DNA polymorphisms via whole genome resequencing and their functional relevance in salinity stress response in chickpea

Salinity stress is one of the major constraints for plant growth and yield. The salinity stress response of different genotypes of crop plants may largely be governed by DNA polymorphisms. To determine the molecular genetic factors involved in salinity stress tolerance in chickpea, we performed a whole genome resequencing data analysis of three each of salinity-sensitive and salinity-tolerant genotypes. A total of 6173 single nucleotide polymorphisms and 920 insertions and deletions differentiating the chickpea genotypes with contrasting salinity stress responses were identified. Gene ontology analysis revealed the enrichment of functional terms related to stress response and development among the genes harboring DNA polymorphisms in their promoter and/or coding regions. DNA polymorphisms located within the cis-regulatory motifs of the quantitative trait loci (QTL)-associated and abiotic stress related genes were identified, which may influence salinity stress response via modulating binding affinity of the transcription factors. Several genes including QTL-associated and abiotic stress response related genes harboring DNA polymorphisms exhibited differential expression in response to salinity stress especially at the reproductive stage of development in the salinity-tolerant genotype. Furthermore, effects of non-synonymous DNA polymorphisms on mutational sensitivity and structural integrity of the encoded proteins by the candidate QTL-associated and abiotic stress response related genes were revealed. The results suggest that DNA polymorphisms may determine salinity stress response via influencing differential gene expression in genotype and/or stage-dependent manner. Altogether, we provide a high-quality set of DNA polymorphisms and candidate genes that may govern salinity stress tolerance in chickpea.

A novel SAPK10-WRKY87-ABF1 biological pathway synergistically enhance abiotic stress tolerance in transgenic rice (Oryza sativa)

WRKY transcription factors play a role in a variety of biological processes. Several studies have revealed that abiotic stress regulates the transcription of a large number of WRKY genes. In this study, we report the identification of a novel 'SAPK10-WRKY87-ABF1' biological pathway, through which they harmoniously enhance drought and salinity tolerance. We generated OsWRKY87-overexpressing transgenic rice and found that the transgenic seedlings exhibited significantly improved drought and salinity stress tolerance. Subcellular localization in rice seedling protoplast revealed that OsWRKY87-GFP fusion protein mostly accumulated in the nucleus, suggesting that OsWRKY87 is a nucleus-localized protein, in line with the predicted function of OsWRKY87 as a transcription factor. In vivo interaction between SAPK10 and WRKY87 was demonstrated by Yeast two-hybrid-assay. In addition, phosphorylation assays showed that SAPK10 exhibits autophosphorylation activity on the 177th serine, enabling it to phosphorylate WRKY87. OsWRKY87 functioned as a transcriptional initiator, according to a yeast one-hybrid assay and a luciferase assay. Remarkably, gel mobility shift assay showed that phosphorylated WRKY87 enhances its DNA-binding ability to the W-box cis-element of ABF1 promoter and activated its transcription, thereby elevating the ABF1 transcription and improving drought and salinity tolerance. Overall, our findings revealed a novel 'SAPK10- WRKY87-ABF1' module, which synergistically interacts to improve drought and salt tolerance in rice (Oryza sativa).

Septoria Leaf Blotch and Reduced Nitrogen Availability Alter WRKY Transcription Factor Expression in a Codependent Manner

A major cause of yield loss in wheat worldwide is the fungal pathogen Zymoseptoria tritici, a hemibiotrophic fungus which causes Septoria leaf blotch, the most destructive wheat disease in Europe. Resistance in commercial wheat varieties is poor, however, a link between reduced nitrogen availability and increased Septoria tolerance has been observed. We have shown that Septoria load is not affected by nitrogen, whilst the fungus is in its first, symptomless stage of growth. This suggests that a link between nitrogen and Septoria is only present during the necrotrophic phase of Septoria infection. Quantitative real-time PCR data demonstrated that WRKYs, a superfamily of plant-specific transcription factors, are differentially expressed in response to both reduced nitrogen and Septoria. WRKY39 was downregulated over 30-fold in response to necrotrophic stage Septoria, whilst changes in the expression of WRKY68a during the late biotrophic phase were dependent on the concentration of nitrogen under which wheat is grown. WRKY68a may therefore mediate a link between nitrogen and Septoria. The potential remains to identify key regulators in the link between nitrogen and Septoria, and as such, elucidate molecular markers for wheat breeding, or targets for molecular-based breeding approaches.

Comparative Analysis of Root Transcriptome Reveals Candidate Genes and Expression Divergence of Homoeologous Genes in Response to Water Stress in Wheat

Crop cultivars with larger root systems have an increased ability to absorb water and nutrients under conditions of water deficit. To unravel the molecular mechanism of water-stress tolerance in wheat, we performed RNA-seq analysis on the two genotypes, Colotana 296-52 (Colotana) and Tincurrin, contrasting the root growth under polyethylene-glycol-induced water-stress treatment. Out of a total of 35,047 differentially expressed genes, 3692 were specifically upregulated in drought-tolerant Colotana under water stress. Transcription factors, pyrroline-5-carboxylate reductase and late-embryogenesis-abundant proteins were among upregulated genes in Colotana. Variant calling between Colotana and Tincurrin detected 15,207 SNPs and Indels, which may affect protein function and mediate the contrasting root length phenotype. Finally, the expression patterns of five triads in response to water, high-salinity, heat, and cold stresses were analyzed using qRT-PCR to see if there were differences in homoeologous gene expression in response to those conditions. The five examined triads showed variation in the contribution of homoeologous genes to water, high-salinity, heat, and cold stresses in the two genotypes. The variation of homoeologous gene expression in response to environmental stresses may enable plants to better cope with stresses in their natural environments.

Populus euphratica WRKY1 binds the promoter of H+-ATPase gene to enhance gene expression and salt tolerance

Plasma membrane proton pumps play a crucial role in maintaining ionic homeostasis in salt-resistant Populus euphratica under saline conditions. High levels of NaCl (200 mM) induced PeHA1 expression in P. euphratica roots and leaves. We isolated a 2022 bp promoter fragment upstream of the translational start of PeHA1 from P. euphratica. The promoter-reporter construct PeHA1-pro::GUS was transferred to tobacco plants, demonstrating that β-glucuronidase activities increased in root, leaf, and stem tissues under salt stress. DNA affinity purification sequencing revealed that PeWRKY1 protein targeted the PeHA1 gene. We assessed the salt-induced transcriptional response of PeWRKY1 and its interaction with PeHA1 in P. euphratica. PeWRKY1 binding to the PeHA1 W-box in the promoter region was verified by a yeast one-hybrid assay, EMSA, luciferase reporter assay, and virus-induced gene silencing. Transgenic tobacco plants overexpressing PeWRKY1 had improved expression of NtHA4, which has a cis-acting W-box in the regulatory region, and improved H+ pumping activity in both in vivo and in vitro assays. We conclude that salt stress up-regulated PeHA1 transcription due to the binding of PeWRKY1 to the W-box in the promoter region of PeHA1. Thus, we conclude that enhanced H+ pumping activity enabled salt-stressed plants to retain Na+ homeostasis.

De novo transcriptome analysis of gene responses to pest feeding in leaves of Panax ginseng C. A. Meyer

The aim of the present study was to investigate the transcriptomic differences between Panax ginseng [Renshen (RS)] plants bitten by pests (n=3, test group; samples defined as RS11-13) or not (n=3, control group; samples defined as RS1-3) using de novo RNA sequencing on an Illumina HiSeq™ 2000 platform. A total of 51,097,386 (99.6%), 49,310,564 (99.5%), 59,192,372 (99.6%), 60,338,540 (99.5%), 56,976,410 (99.6%) and 54,226,588 (99.6%) clean reads were obtained for RS11, RS12, RS13, RS1, RS2 and RS3, respectively. De novo assembly generated 370,267 unigenes, 927 of which were differentially expressed genes (DEGs), including 782 significantly upregulated and 145 significantly downregulated genes. Function enrichment analysis revealed that these DEGs were located in 28 significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways, including phenylpropanoid biosynthesis (for example, TRINITY_DN30766_c0_g2_i1, encoding peroxidase 20) and mitogen-activated protein kinase (MAPK) signaling (TRINITY_DN85589_c0_g1_i1, encoding WRKY transcription factor 75). Weighted gene co-expression network analysis identified modules including TRINITY_DN85589_c0_g1_i1, TRINITY_DN58279_c0_g1_i1 [encoding aspartyl protease (AP)] and TRINITY_DN74866_c0_g2_i1 [encoding 12-oxophytodienoate reductase (OPR)] that may be the most significantly associated with pest responses. In this module, TRINITY_DN85589_c0_g1_i1 may co-express with TRINITY_DN58279_c0_g1_i1 or TRINITY_DN74866_c0_g2_i1. WRYK and AP have been suggested to promote the activity of antioxidant peroxidase. Collectively, the findings from the present study suggested that a MAPK-WRKY-OPR/AP-peroxidase signaling pathway may be a potentially important mechanism underlying defense responses against pests in ginseng plants.

Overexpression of LcSABP, an Orthologous Gene for Salicylic Acid Binding Protein 2, Enhances Drought Stress Tolerance in Transgenic Tobacco

Salicylic acid (SA) plays an essential role in the growth and development of plants, and in their response to abiotic stress. Previous studies have mostly focused on the effects of exogenously applied SA on the physiological response of plants to abiotic stresses; however, the underlying genetic mechanisms for the regulatory functions of endogenous SA in the defense response of plants remain unclear. In plants, SA binding protein 2 (SABP2), possessing methyl salicylate (MeSA) esterase activity, catalyzes the conversion of MeSA to SA. Herein, a SABP2-like gene, LcSABP, was cloned from Lycium chinense, which contained a complete open reading frame of 795 bp and encoded a protein of 264 amino acids that shared high sequence similarities with SABP2 orthologs from other plants. Overexpression of LcSABP enhanced the drought tolerance of transgenic tobacco plants. The results indicated that increased levels of LcSABP transcripts and endogenous SA content were involved in the enhanced drought tolerance. Physiological and biochemical studies further demonstrated that higher chlorophyll content, increased photosynthetic capacity, lower malondialdehyde content, and higher activities of superoxide dismutase, peroxidase, and catalase enhanced the drought tolerance of transgenic plants. Moreover, overexpression of LcSABP also increased the expression of reactive oxygen species (ROS)- and stress-responsive genes under drought stress. Overall, our results demonstrate that LcSABP plays a positive regulatory role in drought stress response by enhancing the endogenous SA content, promoting the scavenging of ROS, and regulating of the expression of stress-related transcription factor genes. Our findings indicate that LcSABP functions as a major regulator of the plant's response to drought stress through a SA-dependent defense pathway.

Overexpression of a WRKY Transcription Factor TaWRKY2 Enhances Drought Stress Tolerance in Transgenic Wheat

Drought is a major environmental stress that severely restricts plant growth and crop productivity. A previous study showed that TaWRKY2 from wheat (Triticum aestivum) plays an important role in drought stress tolerance. In the present study, we isolated the promoter of TaWRKY2 and identified multiple regulatory cis-elements in the promoter region. The activity of the TaWRKY2 promoter was induced by drought, salt, heat, and abscisic acid (ABA). We also generated TaWRKY2-overexpressing transgenic wheat, and found that the transgenic seedlings exhibited significantly enhanced tolerance to drought stress, as evidenced by a higher survival rate and lower water loss rate of detached leaves compared with wild type (WT) plants. In addition, the transgenic lines had higher contents of free proline, soluble sugar, and chlorophyll. During a prolonged period of drought stress before the heading stage, the growth of WT plants was inhibited, whereas the TaWRKY2-overexpressing lines progressed to the heading stage. The increased grain yield of the transgenic wheat lines reflected the cumulative effects of longer panicle length, more kernels per spike, and greater aboveground biomass. Our findings show that TaWRKY2 can enhance drought tolerance and increase grain yield in wheat, thus providing a promising candidate target for improving the drought tolerance of wheat cultivars through genetic engineering.

The Reaumuria trigyna transcription factor RtWRKY1 confers tolerance to salt stress in transgenic Arabidopsis

Reaumuria trigyna (R. trigyna) is an endangered small shrub endemic to the Eastern Alxa-Western Ordos area in Inner Mongolia, China. Based on R. trigyna transcriptome data, the Group I WRKY transcription factor gene RtWRKY1 was cloned from R. trigyna. The full-length RtWRKY1 gene was 2100bp, including a 1261-bp open reading frame (ORF) encoding 573 amino acids. RtWRKY1 was mainly expressed in the stem and was induced by salt, cold stress, and ABA treatment. Overexpression of RtWRKY1 in Arabidopsis significantly enhanced the chlorophyll content, root length, and fresh weight of the transgenic lines under salt stress. RtWRKY1 transgenic Arabidopsis exhibited higher proline content, GSH-PX, POD, SOD, and CAT activities, and lower MDA content, Na⁺ content, and Na⁺/K⁺ ratio than wild-type Arabidopsis under salt stress conditions. Salt stress affected the expression of ion transport, proline biosynthesis, and antioxidant related genes, including AtAPX1, AtCAT1, AtSOD1, AtP5CS1, AtP5CS2, AtPRODH1, AtPRODH2, and AtSOS1 in transgenic lines. RtWRKY1 confers tolerance to salt stress in transgenic Arabidopsis by regulating plant growth, osmotic balance, Na⁺/K⁺ homeostasis, and the antioxidant system.

Integration of Hormonal and Nutritional Cues Orchestrates Progressive Corolla Opening

Flower opening is essential for pollination and thus successful sexual reproduction; however, the underlying mechanisms of its timing control remain largely elusive. We identify a unique cucumber (Cucumis sativus) line '6457' that produces normal ovaries when nutrients are under-supplied, and super ovaries (87%) with delayed corolla opening when nutrients are oversupplied. Corolla opening in both normal and super ovaries is divided into four distinct phases, namely the green bud, green-yellow bud, yellow bud, and flowering stages, along with progressive color transition, cytological tuning, and differential expression of 14,282 genes. In the super ovary, cell division and cell expansion persisted for a significantly longer period of time; the expressions of genes related to photosynthesis, protein degradation, and signaling kinases were dramatically up-regulated, whereas the activities of most transcription factors and stress-related genes were significantly down-regulated; concentrations of cytokinins (CKs) and gibberellins were higher in accordance with reduced cytokinin conjugation and degradation and increased expression of gibberellin biosynthesis genes. Exogenous CK application was sufficient for the genesis of super ovaries, suggesting a decisive role of CKs in controlling the timing of corolla opening. Furthermore, 194 out of 11,127 differentially expressed genes identified in pairwise comparisons, including critical developmental, signaling, and cytological regulators, contained all three types of cis-elements for CK, nitrate, and phosphorus responses in their promoter regions, indicating that the integration of hormone modulation and nutritional regulation orchestrated the precise control of corolla opening in cucumber. Our findings provide a valuable framework for dissecting the regulatory pathways for flower opening in plants.

Drought-responsive WRKY transcription factor genes TaWRKY1 and TaWRKY33 from wheat confer drought and/or heat resistance in Arabidopsis

BACKGROUND

Drought stress is one of the major causes of crop loss. WRKY transcription factors, as one of the largest transcription factor families, play important roles in regulation of many plant processes, including drought stress response. However, far less information is available on drought-responsive WRKY genes in wheat (Triticum aestivum L.), one of the three staple food crops.

RESULTS

Forty eight putative drought-induced WRKY genes were identified from a comparison between de novo transcriptome sequencing data of wheat without or with drought treatment. TaWRKY1 and TaWRKY33 from WRKY Groups III and II, respectively, were selected for further investigation. Subcellular localization assays revealed that TaWRKY1 and TaWRKY33 were localized in the nuclei in wheat mesophyll protoplasts. Various abiotic stress-related cis-acting elements were observed in the promoters of TaWRKY1 and TaWRKY33. Quantitative real-time PCR (qRT-PCR) analysis showed that TaWRKY1 was slightly up-regulated by high-temperature and abscisic acid (ABA), and down-regulated by low-temperature. TaWRKY33 was involved in high responses to high-temperature, low-temperature, ABA and jasmonic acid methylester (MeJA). Overexpression of TaWRKY1 and TaWRKY33 activated several stress-related downstream genes, increased germination rates, and promoted root growth in Arabidopsis under various stresses. TaWRKY33 transgenic Arabidopsis lines showed lower rates of water loss than TaWRKY1 transgenic Arabidopsis lines and wild type plants during dehydration. Most importantly, TaWRKY33 transgenic lines exhibited enhanced tolerance to heat stress.

CONCLUSIONS

The functional roles highlight the importance of WRKYs in stress response.

Recent Advances in Utilizing Transcription Factors to Improve Plant Abiotic Stress Tolerance by Transgenic Technology

Agricultural production and quality are adversely affected by various abiotic stresses worldwide and this will be exacerbated by the deterioration of global climate. To feed a growing world population, it is very urgent to breed stress-tolerant crops with higher yields and improved qualities against multiple environmental stresses. Since conventional breeding approaches had marginal success due to the complexity of stress tolerance traits, the transgenic approach is now being popularly used to breed stress-tolerant crops. So identifying and characterizing the critical genes involved in plant stress responses is an essential prerequisite for engineering stress-tolerant crops. Far beyond the manipulation of single functional gene, engineering certain regulatory genes has emerged as an effective strategy now for controlling the expression of many stress-responsive genes. Transcription factors (TFs) are good candidates for genetic engineering to breed stress-tolerant crop because of their role as master regulators of many stress-responsive genes. Many TFs belonging to families AP2/EREBP, MYB, WRKY, NAC, bZIP have been found to be involved in various abiotic stresses and some TF genes have also been engineered to improve stress tolerance in model and crop plants. In this review, we take five large families of TFs as examples and review the recent progress of TFs involved in plant abiotic stress responses and their potential utilization to improve multiple stress tolerance of crops in the field conditions.

Phloem transcriptome signatures underpin the physiological differentiation of the pedicel, stalk and fruit of cucumber (Cucumis sativus L.)

Cucumber is one of the most important vegetables grown worldwide due to its important economic and nutritional value. The cucumber fruit consists morphologically of the undesirable stalk and the tasty fruit; however, physiological differentiation of these two parts and the underlying molecular basis remain largely unknown. Here we characterized the physiological differences among the pedicel, stalk and fruit, and compared the respective phloem transcriptomes using laser capture microdissection coupled with RNA sequencing (RNA-Seq). We found that the pedicel was characterized by minor cell expansion and a high concentration of stachyose, the stalk showed rapid cell expansion and high raffinose accumulation, and the fruit featured transition from cell division to cell expansion and high levels of monosaccharides. Analyses of transcriptome data indicated that cell wall- and calcium ion binding-related genes contributed to the cell expansion in the pedicel and stalk, whereas genes implicated in cell cycle and hormone actions regulated the transition from cell division to cell expansion in the fruit. Differential sugar distribution in these three phloem-connected tissues resulted from tissue-specific sugar metabolism and transport. Enrichment of transcription factors in the stalk and fruit may facilitate nutrient accumulation in these sink organs. As such, phloem-located gene expression partially orchestrated physiological differentiation of the pedicel, stalk and fruit in cucumber. In addition, we identified 432 cucumber-unique genes and five phloem markers guiding future functional studies.

Identification and expression analyses of MYB and WRKY transcription factor genes in Papaver somniferum L

Papaver somniferum L. is an herbaceous, annual and diploid plant that is important from pharmacological and strategic point of view. The cDNA clones of two putative MYB and WRKY genes were isolated (GeneBank accession numbers KP411870 and KP203854, respectively) from this plant, via the nested-PCR method, and characterized. The MYB transcription factor (TF) comprises 342 amino acids, and exhibits the structural features of the R2R3MYB protein family. The WRKY TF, a 326 amino acid-long polypeptide, falls structurally into the group II of WRKY protein family. Quantitative real-time PCR (qRT-PCR) analyses indicate the presence of these TFs in all organs of P. somniferum L. and Papaver bracteatum L. Highest expression levels of these two TFs were observed in the leaf tissues of P. somniferum L. while in P. bracteatum L. the espression levels were highest in the root tissues. Promoter analysis of the 10 co-expressed gene clustered involved in noscapine biosynthesis pathway in P. somniferum L. suggested that not only these 10 genes are co-expressed, but also share common regulatory motifs and TFs including MYB and WRKY TFs, and that may explain their common regulation.

Isolation of a WRKY30 gene from Muscadinia rotundifolia (Michx) and validation of its function under biotic and abiotic stresses

WRKY transcription factors (TFs) play important roles in many plant processes, including responses to biotic and abiotic stresses. In the present study, Muscadinia rotundifolia MrWRKY30 dramatically accumulated in grapevine leaves in response to inoculation of Plasmopara viticola, a pathogen causing grapevine downy mildew disease. Similar responses were also found on grapevines treated with exogenous SA/JA/ET. Ectopic expression of MrWRKY30 in Arabidopsis thaliana "COL0" enhanced its resistance to downy mildew pathogen Peronospora parasitica. Pathogenesis-related (PR) genes, including AtPR1, AtPR4, AtPR5, and AtPDF1.2, were significantly upregulated in transgenic A. thaliana after P. parasitica inoculation. In the mean time, two critical genes in SA and JA signaling pathways, AtEDS5 and AtJAR1, were abundantly expressed as well, indicating that MrWRKY30 may enhance disease resistance of A. thaliana through SA and JA defense system. The transgenic A. thaliana plants also enhanced tolerance to cold stress accompanied with upregulation of AtCBF1, AtCBF3, AtICE1, and AtCOR47. MrWRKY30 might protect A. thaliana from cold damage by activating the AtCBF-mediated signaling pathway to induce the downstream AtCOR47 gene. Interestingly, the transgenic seedlings had a negative effect on salt tolerance. Reverse transcription PCR (RT-PCR) analysis revealed that antioxidant enzyme genes AtAPX (ascorbate peroxidase), AtCAT (catalase), and AtGST (glutathione-S-transferase) were suppressed in transgenic plants, which may lead to reactive oxygen species (ROS)-mediated sensitivity to salt stress.

A wheat salinity-induced WRKY transcription factor TaWRKY93 confers multiple abiotic stress tolerance in Arabidopsis thaliana

Wheat is an important crop in the world. But most of the cultivars are salt sensitive, and often adversely affected by salt stress. WRKY transcription factors play a major role in plant responses to salt stress, but the effective salinity regulatory WRKYs identified in bread wheat are limited and the mechanism of salt stress tolerance is also not well explored. Here, we identified a salt (NaCl) induced class II WRKY transcription factor TaWRKY93. Its transcript level was strongly induced by salt (NaCl) and exogenous abscisic acid (ABA). Over-expression of TaWRKY93 in Arabidopsis thaliana enhanced salt (NaCl), drought, low temperature and osmotic (mannitol) stress tolerance, mainly demonstrated by transgenic plants forming longer primary roots or more lateral roots on MS plates supplemented with NaCl and mannitol individually, higher survival rate under drought and low temperature stress. Further, transgenic plants maintained a more proline content, higher relative water content and less electrolyte leakage than the wild type plants. The transcript abundance of a series of abiotic stress-related genes was up-regulated in the TaWRKY93 transgenic plants. In summary, TaWRKY93 is a new positive regulator of abiotic stress, it may increase salinity, drought and low temperature stress tolerance through enhancing osmotic adjustment, maintaining membrane stability and increasing transcription of stress related genes, and contribute to the superior agricultural traits of SR3 through promoting root development. It can be used as a candidate gene for wheat transgenic engineering breeding against abiotic stress.

Populus euphratica HSF binds the promoter of WRKY1 to enhance salt tolerance

Poplar species increase expressions of transcription factors to deal with salt environments. We assessed the salt-induced transcriptional responses of heat-shock transcription factor (HSF) and WRKY1 in Populus euphratica, and their roles in salt tolerance. High NaCl (200mM) induced PeHSF and PeWRKY1 expressions in P. euphratica, with a rapid rise in roots than in leaves. Moreover, the salt-elicited PeHSF reached its peak level 6h earlier than PeWRKY1 in leaves. PeWRKY1 was down-regulated in salinized P. euphratica when PeHSF was silenced by tobacco rattle virus-based gene silencing. Subcellular assays in onion epidermal cells and Arabidopsis protoplasts revealed that PeHSF and PeWRKY1 were restricted to the nucleus. Transgenic tobacco plants overexpressing PeWRKY1 showed improved salt tolerance in terms of survival rate, root growth, photosynthesis, and ion fluxes. We further isolated an 1182-bp promoter fragment upstream of the translational start of PeWRKY1 from P. euphratica. Promoter sequence analysis revealed that PeWRKY1 harbours four tandem repeats of heat shock element (HSE) in the upstream regulatory region. Yeast one-hybrid assay showed that PeHSF directly binds the cis-acting HSE. To determine whether the HSE cluster was important for salt-induced PeWRKY1 expression, the promoter-reporter construct PeWRKY1-pro::GUS was transferred to tobacco plants. β-glucuronidase activities increased in root, leaf, and stem tissues under salt stress. Therefore, we conclude that salinity increased PeHSF transcription in P. euphratica, and that PeHSF binds the cis-acting HSE of the PeWRKY1 promoter, thus activating PeWRKY1 expression.

The Brachypodium distachyon BdWRKY36 gene confers tolerance to drought stress in transgenic tobacco plants

KEY MESSAGE

The expression of BdWRKY36 was upregulated by drought treatment. BdWRKY36 -overexpressing transgenic tobacco increased drought tolerance by controlling ROS homeostasis and regulating transcription of stress related genes. WRKY transcription factor plays important roles in plant growth, development and stress response. However, the function of group IIe WRKYs is less known. In this study, we cloned and characterized a gene of group IIe WRKY, designated as BdWRKY36, from Brachypodium distachyon. Transient expression of BdWRKY36 in onion epidermal cell suggested its localization in the nucleus. Transactivation assay revealed that the C-terminal region, instead of full length BdWRKY36, had transcriptional activity. BdWRKY36 expression was upregulated by drought. Overexpression of BdWRKY36 in transgenic tobacco plants resulted in enhanced tolerance to drought stress. Physiological-biochemical indices analyses showed that BdWRKY36-overexpressing tobacco lines had lesser ion leakage (IL) and reactive oxygen species (ROS) accumulation, but higher contents of chlorophyll, relative water content (RWC) and activities of antioxidant enzyme than that in control plants under drought condition. Meanwhile expression levels of some ROS-scavenging and stress-responsive genes were upregulated in BdWRKY36-overexpressing tobacco lines under drought stress. These results demonstrate that BdWRKY36 functions as a positive regulator of drought stress response by controlling ROS homeostasis and regulating transcription of stress related genes.