Defense-related transcription factors WRKY70 and WRKY54 modulate osmotic stress tolerance by regulating stomatal aperture in Arabidopsis

Physiological performance and differential expression profiling of genes associated with drought tolerance in contrasting varieties of two Gossypium species

Cotton is mostly cultivated under rain-fed conditions in India, thus faces frequent drought conditions during its life cycle. Drought being a major stress factor responsible for yield penalty, there has always been a high priority to generate knowledge on adaptation and tolerance of cotton. In the present study, four cotton varieties, JKC-770 and KC-2 (Gossypium hirsutum), and JKC-717 and RAHS-187(Gossypium herbaceum), were imposed to drought. Under drought condition, differential changes in physiological characters like net photosynthesis, transpiration, stomatal conductance, chlorophyll fluorescence, relative water content (RWC), and predawn water potential (ψ 0) showed a change. While proline, malondialdehyde (MDA), and glutathione-S-transferase (GST) content increased along with a concomitant change in the expression of their associated genes. Under moderate stress, tolerant varieties maintain lower ψ 0 probably due to higher proline content as compared to sensitive varieties. Cyclic electron flow (CEF) also plays an important role in tolerance under mild water stress in G. hirsutum varieties. CEF not only activates at high light but also initiates at a very low light intensity. Expression analysis of genes reveals that drought-tolerant varieties showed enhanced detoxifying mechanism by up-regulation of asparagine synthase (AS), glutathione-S-transferase (GST), and methyl glyoxalase (GlyI) genes under drought stress. Up-regulation of Δ(1)-pyrroline-5-carboxylase synthase (Δ(1)P5CS) enhanced accumulation of proline, an osmolyte, under drought in tolerant varieties. While the drought-sensitive varieties showed up-regulation of ethylene responsive factor (ERF) and down-regulation of WRKY70 responsible for senescence of the leaf which correlated well with the high rate of leaf fall in sensitive varieties under water stress.

The interaction of Arabidopsis with Piriformospora indica shifts from initial transient stress induced by fungus-released chemical mediators to a mutualistic interaction after physical contact of the two symbionts

BACKGROUND

Piriformospora indica, an endophytic fungus of Sebacinales, colonizes the roots of many plant species including Arabidopsis thaliana. The symbiotic interaction promotes plant performance, growth and resistance/tolerance against abiotic and biotic stress.

RESULTS

We demonstrate that exudated compounds from the fungus activate stress and defense responses in the Arabidopsis roots and shoots before the two partners are in physical contact. They induce stomata closure, stimulate reactive oxygen species (ROS) production, stress-related phytohormone accumulation and activate defense and stress genes in the roots and/or shoots. Once a physical contact is established, the stomata re-open, ROS and phytohormone levels decline, and the number and expression level of defense/stress-related genes decreases.

CONCLUSIONS

We propose that exudated compounds from P. indica induce stress and defense responses in the host. Root colonization results in the down-regulation of defense responses and the activation of genes involved in promoting plant growth, metabolism and performance.

The F-box protein MAX2 contributes to resistance to bacterial phytopathogens in Arabidopsis thaliana

BACKGROUND

The Arabidopsis thaliana F-box protein MORE AXILLARY GROWTH2 (MAX2) has previously been characterized for its role in plant development. MAX2 appears essential for the perception of the newly characterized phytohormone strigolactone, a negative regulator of polar auxin transport in Arabidopsis.

RESULTS

A reverse genetic screen for F-box protein mutants altered in their stress responses identified MAX2 as a component of plant defense. Here we show that MAX2 contributes to plant resistance against pathogenic bacteria. Interestingly, max2 mutant plants showed increased susceptibility to the bacterial necrotroph Pectobacterium carotovorum as well as to the hemi-biotroph Pseudomonas syringae but not to the fungal necrotroph Botrytis cinerea. max2 mutant phenotype was associated with constitutively increased stomatal conductance and decreased tolerance to apoplastic ROS but also with alterations in hormonal balance.

CONCLUSIONS

Our results suggest that MAX2 previously characterized for its role in regulation of polar auxin transport in Arabidopsis, and thus plant development also significantly influences plant disease resistance. We conclude that the increased susceptibility to P. syringae and P. carotovorum is due to increased stomatal conductance in max2 mutants promoting pathogen entry into the plant apoplast. Additional factors contributing to pathogen susceptibility in max2 plants include decreased tolerance to pathogen-triggered apoplastic ROS and alterations in hormonal signaling.

Cellular interactions during tracheary elements formation and function

The survival of higher plant species on land depends on the development and function of an efficient vascular system distributing water and minerals absorbed by roots to all aerial organs. This conduction and distribution of plant sap relies on specialized cells named tracheary elements (TEs). In contrast to many other cell types in plants, TEs are functionalized by cell death that hollows the cell protoplast to make way for the sap. To maintain a stable conducting function during plant development, recovery from vascular damages as well as to adapt to environmental changes, TEs are completely dependent on direct cellular interactions with neighboring xylem parenchyma cells (XPs).

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.

Genome-wide identification and characterization of the Populus WRKY transcription factor family and analysis of their expression in response to biotic and abiotic stresses

WRKY proteins are a large family of regulators involved in various developmental and physiological processes, especially in coping with diverse biotic and abiotic stresses. In this study, 100 putative PtrWRKY genes encoded the proteins contained in the complete WRKY domain in Populus. Phylogenetic analysis revealed that the members of this superfamily among poplar, Arabidopsis, and other species were divided into three groups with several subgroups based on the structures of the WRKY protein sequences. Various cis-acting elements related to stress and defence responses were found in the promoter regions of PtrWRKY genes by promoter analysis. High-throughput transcriptomic analyses identified that 61 of the PtrWRKY genes were induced by biotic and abiotic treatments, such as Marssonina brunnea, salicylic acid (SA), methyl jasmonate (MeJA), wounding, cold, and salinity. Among these PtrWRKY genes, transcripts of 46 selected genes were observed in different tissues, including roots, stems, and leaves. Quantitative RT-PCR analysis further confirmed the induced expression of 18 PtrWRKY genes by one or more stress treatments. The overexpression of an SA-inducible gene, PtrWRKY89, accelerated expression of PR protein genes and improved resistance to pathogens in transgenic poplar, suggesting that PtrWRKY89 is a regulator of an SA-dependent defence-signalling pathway in poplar. Taken together, our results provided significant information for improving the resistance and stress tolerance of woody plants.

The cotton WRKY transcription factor GhWRKY17 functions in drought and salt stress in transgenic Nicotiana benthamiana through ABA signaling and the modulation of reactive oxygen species production

Drought and high salinity are two major environmental factors that significantly limit the productivity of agricultural crops worldwide. WRKY transcription factors play essential roles in the adaptation of plants to abiotic stresses. However, WRKY genes involved in drought and salt tolerance in cotton (Gossypium hirsutum) are largely unknown. Here, a group IId WRKY gene, GhWRKY17, was isolated and characterized. GhWRKY17 was found to be induced after exposure to drought, salt, H2O2 and ABA. The constitutive expression of GhWRKY17 in Nicotiana benthamiana remarkably reduced plant tolerance to drought and salt stress, as determined through physiological analyses of the germination rate, root growth, survival rate, leaf water loss and Chl content. GhWRKY17 transgenic plants were observed to be more sensitive to ABA-mediated seed germination and root growth. However, overexpressing GhWRKY17 in N. benthamiana impaired ABA-induced stomatal closure. Furthermore, we found that GhWRKY17 modulated the increased sensitivity of plants to drought by reducing the level of ABA, and transcript levels of ABA-inducible genes, including AREB, DREB, NCED, ERD and LEA, were clearly repressed under drought and salt stress conditions. Consistent with the accumulation of reactive oxygen species (ROS), reduced proline contents and enzyme activities, elevated electrolyte leakage and malondialdehyde, and lower expression of ROS-scavenging genes, including APX, CAT and SOD, the GhWRKY17 transgenic plants exhibited reduced tolerance to oxidative stress compared with wild-type plants. These results therefore indicate that GhWRKY17 responds to drought and salt stress through ABA signaling and the regulation of cellular ROS production in plants.

The receptor kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 attenuates abscisic acid responses in Arabidopsis

In plants, membrane-bound receptor kinases are essential for developmental processes, immune responses to pathogens and the establishment of symbiosis. We previously identified the Arabidopsis (Arabidopsis thaliana) receptor kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as required for successful infection with the downy mildew pathogen Hyaloperonospora arabidopsidis. We report here that IOS1 is also required for full susceptibility of Arabidopsis to unrelated (hemi)biotrophic filamentous oomycete and fungal pathogens. Impaired susceptibility in the absence of IOS1 appeared to be independent of plant defense mechanism. Instead, we found that ios1-1 plants were hypersensitive to the plant hormone abscisic acid (ABA), displaying enhanced ABA-mediated inhibition of seed germination, root elongation, and stomatal opening. These findings suggest that IOS1 negatively regulates ABA signaling in Arabidopsis. The expression of ABA-sensitive COLD REGULATED and RESISTANCE TO DESICCATION genes was diminished in Arabidopsis during infection. This effect on ABA signaling was alleviated in the ios1-1 mutant background. Accordingly, ABA-insensitive and ABA-hypersensitive mutants were more susceptible and resistant to oomycete infection, respectively, showing that the intensity of ABA signaling affects the outcome of downy mildew disease. Taken together, our findings suggest that filamentous (hemi)biotrophs attenuate ABA signaling in Arabidopsis during the infection process and that IOS1 participates in this pathogen-mediated reprogramming of the host.

Tomato WRKY transcriptional factor SlDRW1 is required for disease resistance against Botrytis cinerea and tolerance to oxidative stress

WRKY proteins comprise a large family of transcription factors that play important roles in plant responses to biotic and abiotic stresses; however, only a few of tomato WRKYs have been studied for their biological functions. In the present study, we identified a Botrytis cinerea-responsive WRKY gene SlDRW1 (Solanum lycopersicumdefense-related WRKY1) from tomato. SlDRW1 is a nucleus localized protein with transactivation activity in yeast. Expression of SlDRW1 was significantly induced by B. cinerea, leading to 10-13 folds of increase than that in the mock-inoculated plants but not by Pseudomonas syringae pv. tomato (Pst) DC3000. Silencing of SlDRW1 resulted in increased severity of disease caused by B. cinerea, but did not affect the phenotype of disease caused by Pst DC3000. In addition, silencing of SlDRW1 also resulted in decreased tolerance against oxidative stress but did not affect drought stress tolerance. Furthermore, silencing of SlDRW1 attenuated defense response such as expression of defense-related genes after infection by B. cinerea. Our results demonstrate that SlDRW1 is a positive regulator of defense response in tomato against B. cinerea and oxidative stress.

Endogenous salicylic acid accumulation is required for chilling tolerance in cucumber (Cucumis sativus L.) seedlings

Salicylic acid (SA) is an important plant hormone, and its exogenous application can induce tolerance to multiple environmental stresses in plants. In this study, we examine the potential involvement of endogenous SA in response to chilling in cucumber (Cucumis sativus L.) seedlings. A low temperature of 8 °C induces a moderate increase in endogenous SA levels. Chilling stimulates the enzymatic activities and the expression of genes for phenylalanine ammonia-lyase (PAL) and benzoic acid-2-hydroxylase rather than isochorismate synthase. This indicates that the PAL enzymatic pathway contributes to chilling-induced SA production. Cucumber seedlings pretreated with SA biosynthesis inhibitors accumulate less endogenous SA and suffer more from chilling damage. The expression of cold-responsive genes is also repressed by SA inhibitors. The reduction in stress tolerance and in gene expression can be restored by the exogenous application of SA, confirming the critical roles of SA in chilling responses in cucumber seedlings. Furthermore, the inhibition of SA biosynthesis under chilling stress results in a prolonged and enhanced hydrogen peroxide (H2O2) accumulation. The application of exogenous SA and the chemical scavenger of H2O2 reduces the excess H2O2 and alleviates chilling injury. In contrast, the protective effects of SA are negated by foliar spraying with high concentrations of H2O2 and an inhibitor of the antioxidant enzyme. These results suggest that endogenous SA is required in response to chilling stress in cucumber seedlings, by modulating the expression of cold-responsive genes and the precise induction of cellular H2O2 levels.

Expression-based network biology identifies immune-related functional modules involved in plant defense

Background

Plants respond to diverse environmental cues including microbial perturbations by coordinated regulation of thousands of genes. These intricate transcriptional regulatory interactions depend on the recognition of specific promoter sequences by regulatory transcription factors. The combinatorial and cooperative action of multiple transcription factors defines a regulatory network that enables plant cells to respond to distinct biological signals. The identification of immune-related modules in large-scale transcriptional regulatory networks can reveal the mechanisms by which exposure to a pathogen elicits a precise phenotypic immune response.

Results

We have generated a large-scale immune co-expression network using a comprehensive set of Arabidopsis thaliana (hereafter Arabidopsis) transcriptomic data, which consists of a wide spectrum of immune responses to pathogens or pathogen-mimicking stimuli treatments. We employed both linear and non-linear models to generate Arabidopsis immune co-expression regulatory (AICR) network. We computed network topological properties and ascertained that this newly constructed immune network is densely connected, possesses hubs, exhibits high modularity, and displays hallmarks of a “real” biological network. We partitioned the network and identified 156 novel modules related to immune functions. Gene Ontology (GO) enrichment analyses provided insight into the key biological processes involved in determining finely tuned immune responses. We also developed novel software called OCCEAN (One Click Cis-regulatory Elements ANalysis) to discover statistically enriched promoter elements in the upstream regulatory regions of Arabidopsis at a whole genome level. We demonstrated that OCCEAN exhibits higher precision than the existing promoter element discovery tools. In light of known and newly discovered cis-regulatory elements, we evaluated biological significance of two key immune-related functional modules and proposed mechanism(s) to explain how large sets of diverse GO genes coherently function to mount effective immune responses.

Conclusions

We used a network-based, top-down approach to discover immune-related modules from transcriptomic data in Arabidopsis. Detailed analyses of these functional modules reveal new insight into the topological properties of immune co-expression networks and a comprehensive understanding of multifaceted plant defense responses. We present evidence that our newly developed software, OCCEAN, could become a popular tool for the Arabidopsis research community as well as potentially expand to analyze other eukaryotic genomes.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-421) contains supplementary material, which is available to authorized users.

Related to ABA-Insensitive3(ABI3)/Viviparous1 and AtABI5 transcription factor coexpression in cotton enhances drought stress adaptation

Drought tolerance is an important trait being pursued by the agbiotech industry. Abscisic acid (ABA) is a stress hormone that mediates a multitude of processes in growth and development, water use efficiency (WUE) and gene expression during seed development and in response to environmental stresses. Arabidopsis B3-domain transcription factor Related to ABA-Insensitive3 (ABI3)/Viviparous1 (namely AtRAV2) and basic leucine zipper (bZIPs) AtABI5 or AtABF3 transactivated ABA-inducible promoter:GUS reporter expression in a maize mesophyll protoplast transient assay and showed synergies in reporter transactivation when coexpressed. Transgenic cotton (Gossypium hirsutum) expressing AtRAV1/2 and/or AtABI5 showed resistance to imposed drought stress under field and greenhouse conditions and exhibited improved photosynthesis and WUEs associated with absorption through larger root system and greater leaf area. We observed synergy for root biomass accumulation in the greenhouse, intrinsic WUE in the field and drought tolerance in stacked AtRAV and AtABI5 double-transgenic cotton. We assessed AtABI5 and AtRAV1/2 involvement in drought stress adaptations through reactive oxygen species scavenging and osmotic adjustment by marker gene expression in cotton. Deficit irrigation-grown AtRAV1/2 and AtABI5 transgenics had 'less-stressed' molecular and physiological phenotypes under drought, likely due to improved photoassimilation and root and shoot sink strengths and enhanced expression of endogenous GhRAV and genes for antioxidant and osmolyte biosynthesis. Overexpression of bZIP and RAV TFs could impact sustainable cotton agriculture and potentially other crops under limited irrigation conditions.

Regulation of water, salinity, and cold stress responses by salicylic acid

Salicylic acid (SA) is a naturally occurring phenolic compound. SA plays an important role in the regulation of plant growth, development, ripening, and defense responses. The role of SA in the plant-pathogen relationship has been extensively investigated. In addition to defense responses, SA plays an important role in the response to abiotic stresses, including drought, low temperature, and salinity stresses. It has been suggested that SA has great agronomic potential to improve the stress tolerance of agriculturally important crops. However, the utility of SA is dependent on the concentration of the applied SA, the mode of application, and the state of the plants (e.g., developmental stage and acclimation). Generally, low concentrations of applied SA alleviate the sensitivity to abiotic stresses, and high concentrations of applied induce high levels of oxidative stress, leading to a decreased tolerance to abiotic stresses. In this article, the effects of SA on the water stress responses and regulation of stomatal closure are reviewed.

Genome and transcriptome analysis of the grapevine (Vitis vinifera L.) WRKY gene family

The plant WRKY gene family represents an ancient and complex class of zinc-finger transcription factors (TFs) that are involved in the regulation of various physiological processes, such as development and senescence, and in plant response to many biotic and abiotic stresses. Despite the growing number of studies on the genomic organisation of WRKY gene family in different species, little information is available about this family in grapevine (Vitis vinifera L.). In the present study, a total number of 59 putative grapevine WRKY transcription factors (VvWRKYs) were identified based on the analysis of various genomic and proteomic grapevine databases. According to their structural and phylogentic features, the identified grapevine WRKY transcription factors were classified into three main groups. In order to shed light into their regulatory roles in growth and development as well as in response to biotic and abiotic stress in grapevine, the VvWRKYs expression profiles were examined in publicly available microarray data. Bioinformatics analysis of these data revealed distinct temporal and spatial expression patterns of VvWRKYs in various tissues, organs and developmental stages, as well as in response to biotic and abiotic stresses. To also extend our analysis to situations not covered by the arrays and to validate our results, the expression profiles of selected VvWRKYs in response to drought stress, Erysiphe necator (powdery mildew) infection, and hormone treatments (salicilic acid and ethylene), were investigated by quantitative real-time reverse transcription PCR (qRT-PCR). The present study provides a foundation for further comparative genomics and functional studies of this important class of transcriptional regulators in grapevine.