The Tomato Kinase Pti1 Contributes to Production of Reactive Oxygen Species in Response to Two Flagellin-Derived Peptides and Promotes Resistance to Pseudomonas syringae Infection

The Pti1 kinase was identified from a reverse genetic screen as contributing to pattern-triggered immunity (PTI) against Pseudomonas syringae pv. tomato (Pst). The tomato genome has two Pti1 genes, referred to as Pti1a and Pti1b. A hairpin-Pti1 (hpPti1) construct was developed and was used to generate two independent stable transgenic tomato lines that had reduced transcript abundance of both genes. In response to P. syringae pv. tomato inoculation, these hpPti1 plants developed more severe disease symptoms, supported higher bacterial populations, and had reduced transcript accumulation of PTI-associated genes, as compared with wild-type plants. In response to two flagellin-derived peptides, the hpPti1 plants produced lesser amounts of reactive oxygen species (ROS) but showed no difference in mitogen-activated protein kinase (MAPK). Synthetic Pti1a and Pti1b genes designed to avoid silencing were transiently expressed in the hpPti1 plants and restored the ability of the plants to produce wild-type levels of ROS. Our results identify a new component of PTI in tomato that, because it affects ROS production but not MAPK signaling, appears to act early in the immune response.

GENERAL CONTROL NONREPRESSIBLE4 Degrades 14-3-3 and the RIN4 Complex to Regulate Stomatal Aperture with Implications on Nonhost Disease Resistance and Drought Tolerance

Plants have complex and adaptive innate immune responses against pathogen infections. Stomata are key entry points for many plant pathogens. Both pathogens and plants regulate stomatal aperture for pathogen entry and defense, respectively. Not all plant proteins involved in stomatal aperture regulation have been identified. Here, we report GENERAL CONTROL NONREPRESSIBLE4 (GCN4), an AAA+-ATPase family protein, as one of the key proteins regulating stomatal aperture during biotic and abiotic stress. Silencing of GCN4 in Nicotiana benthamiana and Arabidopsis thaliana compromises host and nonhost disease resistance due to open stomata during pathogen infection. AtGCN4 overexpression plants have reduced H+-ATPase activity, stomata that are less responsive to pathogen virulence factors such as coronatine (phytotoxin produced by the bacterium Pseudomonas syringae) or fusicoccin (a fungal toxin produced by the fungus Fusicoccum amygdali), reduced pathogen entry, and enhanced drought tolerance. This study also demonstrates that AtGCN4 interacts with RIN4 and 14-3-3 proteins and suggests that GCN4 degrades RIN4 and 14-3-3 proteins via a proteasome-mediated pathway and thereby reduces the activity of the plasma membrane H+-ATPase complex, thus reducing proton pump activity to close stomata.

Cu-chitosan nanoparticle boost defense responses and plant growth in maize (Zea mays L.)

In agriculture, search for biopolymer derived materials are in high demand to replace the synthetic agrochemicals. In the present investigation, the efficacy of Cu-chitosan nanoparticles (NPs) to boost defense responses against Curvularia leaf spot (CLS) disease of maize and plant growth promotry activity were evaluated. Cu-chitosan NPs treated plants showed significant defense response through higher activities of antioxidant (superoxide dismutase and peroxidase) and defense enzymes (polyphenol oxidase and phenylalanine ammonia-lyase). Significant control of CLS disease of maize was recorded at 0.04 to 0.16% of Cu-chitosan NPs treatments in pot and 0.12 to 0.16% of NPs treatments in field condition. Further, NPs treatments exhibited growth promotry effect in terms of plant height, stem diameter, root length, root number and chlorophyll content in pot experiments. In field experiment, plant height, ear length, ear weight/plot, grain yield/plot and 100 grain weight were enhanced in NPs treatments. Disease control and enhancement of plant growth was further enlightened through Cu release profile of Cu-chitosan NPs. This is an important development in agriculture nanomaterial research where biodegradable Cu-chitosan NPs are better compatible with biological control as NPs "mimic" the natural elicitation of the plant defense and antioxidant system for disease protection and sustainable growth.

Toxicological implications of selenium nanoparticles with different coatings along with Se4+ on Lemna minor

Nanoparticles have potential high risks for living organisms in the environment due to their specific qualities and their easy access. In the present study, selenium nanoparticles (Se NPs) with two different coatings (l-cysteine and tannic acid) were synthesized. The characteristics of particles were analyzed using XRD, FT-IR and SEM. The impact of the nanoparticles besides Se⁴⁺, on the aquatic higher plant Lemna minor was evaluated and compared. Entrance of l-cysteine and tannic acid capped Se NPs in the roots of Lemna minor was proved by TEM and fluorescence microscopy. Adverse effects of mentioned NPs and differences of these effects from those by sodium selenite as the ionic form were assessed by a range of biophysicochemical tests. Altogether, the results asserted that Lemna minor was notably poisoned by both capped Se NPs and Se⁴⁺. Thus, growth and photosynthetic pigments were decreased while lipid peroxidation along with total phenol and flavonoid contents were raised. Eventually some changes in enzymatic activities were presented. To sum up the consequences, it can be concluded that all changes occurred due to the plant defense system especially in order to remove reactive oxygen species (ROS) and possible phytotoxicity originated by l- cysteine and tannic acid capped Se NPs in addition to Se⁴⁺. The influence of tannic acid capped Se NPs after sodium selenite is stronger by the means of antioxidant enzymes activity in comparison with l-cysteine capped Se NPs.

MYC2 Orchestrates a Hierarchical Transcriptional Cascade That Regulates Jasmonate-Mediated Plant Immunity in Tomato

The hormone jasmonate (JA), which functions in plant immunity, regulates resistance to pathogen infection and insect attack through triggering genome-wide transcriptional reprogramming in plants. We show that the basic helix-loop-helix transcription factor (TF) MYC2 in tomato (Solanum lycopersicum) acts downstream of the JA receptor to orchestrate JA-mediated activation of both the wounding and pathogen responses. Using chromatin immunoprecipitation sequencing (ChIP-seq) coupled with RNA sequencing (RNA-seq) assays, we identified 655 MYC2-targeted JA-responsive genes. These genes are highly enriched in Gene Ontology categories related to TFs and the early response to JA, indicating that MYC2 functions at a high hierarchical level to regulate JA-mediated gene transcription. We also identified a group of MYC2-targeted TFs (MTFs) that may directly regulate the JA-induced transcription of late defense genes. Our findings suggest that MYC2 and its downstream MTFs form a hierarchical transcriptional cascade during JA-mediated plant immunity that initiates and amplifies transcriptional output. As proof of concept, we showed that during plant resistance to the necrotrophic pathogen Botrytis cinerea, MYC2 and the MTF JA2-Like form a transcription module that preferentially regulates wounding-responsive genes, whereas MYC2 and the MTF ETHYLENE RESPONSE FACTOR.C3 form a transcription module that preferentially regulates pathogen-responsive genes.

Apoplastic ROS signaling in plant immunity

Reactive oxygen species (ROS) are widely produced in different cellular compartments under both biotic and abiotic stress conditions. ROS play a central role in plant signaling and regulate diverse cellular processes. Recent advances are shedding new light on sophisticated mechanisms controlling ROS biogenesis and signaling in plant immunity. In this review, we summarize our current understanding of the regulation of apoplastic ROS production in response to microbial molecular patterns and draw comparison with abscisic acid (ABA)-induced apoplastic ROS. We also discuss how ROS act as signal molecules to regulate cellular activities using stomatal movement as an example.

Constitutively Active Arabidopsis MAP Kinase 3 Triggers Defense Responses Involving Salicylic Acid and SUMM2 Resistance Protein

Mitogen-activated protein kinases (MAPKs) are important regulators of plant immunity. Most of the knowledge about the function of these pathways is derived from loss-of-function approaches. Using a gain-of-function approach, we investigated the responses controlled by a constitutively active (CA) MPK3 in Arabidopsis thalianaCA-MPK3 plants are dwarfed and display a massive derepression of defense genes associated with spontaneous cell death as well as the accumulation of reactive oxygen species, phytoalexins, and the stress-related hormones ethylene and salicylic acid (SA). Remarkably CA-MPK3/sid2 and CA-MPK3/ein2-50 lines, which are impaired in SA synthesis and ethylene signaling, respectively, retain most of the CA-MPK3-associated phenotypes, indicating that the constitutive activity of MPK3 can bypass SA and ethylene signaling to activate defense responses. A comparative analysis of the molecular phenotypes of CA-MPK3 and mpk4 autoimmunity suggested convergence between the MPK3- and MPK4-guarding modules. In support of this model, CA-MPK3 crosses with summ1 and summ2, two known suppressors of mpk4, resulted in a partial reversion of the CA-MPK3 phenotypes. Overall, our data unravel a novel mechanism by which the MAPK signaling network contributes to a robust defense-response system.

Conservation of Chitin-Induced MAPK Signaling Pathways in Rice and Arabidopsis

Perception of microbe-associated molecular patterns (MAMPs) including chitin by pattern recognition receptors (PRRs) rapidly induces activation of mitogen-activated protein kinase (MAPK) cascades. However, how PRRs transmit immune signals to the MAPK cascade is largely unknown. Recently, Arabidopsis receptor-like cytoplasmic kinase PBL27 has been reported to activate MAPKs through phosphorylation of AtMAPKKK5 in the chitin signaling pathway. In this study, we found that OsRLCK185, a rice ortholog of PBL27, regulates chitin-induced MAPK activation in a similar fashion to PBL27 in rice. Upon chitin perception, OsRLCK185 is phosphorylated by OsCERK1, a component of the chitin receptor complex. OsRLCK185 interacted with OsMAPKKK11 and OsMAPKKK18, rice orthologs of AtMAPKKK5, in yeast two-hybrid assays. Silencing of both OsMAPKKK11 and OsMAPKKK18 significantly reduced chitin-induced activation of OsMPK3 and OsMPK6. Expression levels of OsMAPKKK18 were much higher than that of OsMAPKKK11 in rice cells, which was consistent with the fact that the Osmapkkk11 single mutation did not affect MAPK activation. This result suggested that OsMAPKKK18 plays a more important role than OsMAPKKK11 in the chitin-induced activation of OsMPK3 and OsMPK6. The bimolecular fluorescence complementation (BiFC) experiment indicated that OsRLCK185 interacted with OsMAPKKK18 at the plasma membrane in planta. In vitro phosphorylation experiments showed that OsRLCK185 directly phosphorylates OsMAPKKK18. Furthermore, OsMAPKKK18 interacted with the MAPKK OsMKK4, the upstream component of OsMPK3/6. These results suggested that OsRLCK185 connects the chitin receptor to the MAPK cascade consisting of OsMAPKKK18-OsMKK4-OsMPK3/6. Our data revealed that chitin-induced MAPK activation in rice and Arabidopsis is regulated by common homologous elements.

Chitosan-induced immunity in Camellia sinensis (L.) O. Kuntze against blister blight disease is mediated by nitric-oxide

Blister blight disease, caused by an obligate biotrophic fungal pathogen, Exobasidium vexans Massee is posing a serious threat for tea cultivation in Asia. As the use of chemical pesticides on tea leaves substantially increases the toxic risks of tea consumption, serious attempts are being made to control such pathogens by boosting the intrinsic natural defense responses against invading pathogens in tea plants. In this study, the nature and durability of resistance offered by chitosan and the possible mechanism of chitosan-induced defense induction in Camellia sinensis (L.) O. Kuntze plants against blister blight disease were investigated. Foliar application of 0.01% chitosan solution at 15 days interval not only reduced the blister blight incidence for two seasons, but also maintained the induced expressions of different defense related enzymes and total phenol content compared to the control. Defense responses induced by chitosan were found to be down regulated under nitric oxide (NO) deficient conditions in vivo, indicating that the observed chitosan-induced resistance is probably activated via NO signaling. Such role of NO in host defense response was further established by application of the NO donor, sodium nitroprusside (SNP), which produced similar defense responses accomplished through chitosan treatment. Taken together, our results suggest that increased production of NO in chitosan-treated tea plants may play a critical role in triggering the innate defense responses effective against plant pathogens, including that causing the blister blight disease.

Plant Immunity Inducer Development and Application

Plant immunity inducers represent a new and rapidly developing field in plant-protection research. In this paper, we discuss recent research on plant immunity inducers and their development and applications in China. Plant immunity inducers include plant immunity-inducing proteins, chitosan oligosaccharides, and microbial inducers. These compounds and microorganisms can trigger defense responses and confer disease resistance in plants. We also describe the mechanisms of plant immunity inducers and how they promote plant health. Furthermore, we summarize the current situation in plant immunity inducer development in China and the global marketplace. Finally, we also deeply analyze the development trends and application prospects of plant immunity inducers in environmental protection and food safety.

Immune responses induced by oligogalacturonides are differentially affected by AvrPto and loss of BAK1/BKK1 and PEPR1/PEPR2

Plants possess an innate immune system capable of restricting invasion by most potential pathogens. At the cell surface, the recognition of microbe-associated molecular patterns (MAMPs) and/or damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs) represents the first event for the prompt mounting of an effective immune response. Pathogens have evolved effectors that block MAMP-triggered immunity. The Pseudomonas syringae effector AvrPto abolishes immunity triggered by the peptide MAMPs flg22 and elf18, derived from the bacterial flagellin and elongation factor Tu, respectively, by inhibiting the kinase function of the corresponding receptors FLS2 and EFR, as well as their co-receptors BAK1 and BKK1. Oligogalacturonides (OGs), a well-known class of DAMPs, are oligomers of α-1,4-linked galacturonosyl residues, released on partial degradation of the plant cell wall homogalacturonan. We show here that AvrPto affects only a subset of the OG-triggered immune responses and that, among these responses, only a subset is affected by the concomitant loss of BAK1 and BKK1. However, the antagonistic effect on auxin-related responses is not affected by either AvrPto or the loss of BAK1/BKK1. These observations reveal an unprecedented complexity among the MAMP/DAMP response cascades. We also show that the signalling system mediated by Peps, another class of DAMPs, and their receptors PEPRs, contributes to OG-activated immunity. We hypothesize that OGs are sensed through multiple and partially redundant perception/transduction complexes, some targeted by AvrPto, but not necessarily comprising BAK1 and BKK1.

Jasmonic acid-mediated defense suppresses brassinosteroid-mediated susceptibility to Rice black streaked dwarf virus infection in rice

Plant hormones play a vital role in plant immune responses. However, in contrast to the relative wealth of information on hormone-mediated immunity in dicot plants, little information is available on monocot-virus defense systems. We used a high-throughput-sequencing approach to compare the global gene expression of Rice black-streaked dwarf virus (RBSDV)-infected rice plants with that of healthy plants. Exogenous hormone applications and transgenic rice were used to test RBSDV infectivity and pathogenicity. Our results revealed that the jasmonic acid (JA) pathway was induced while the brassinosteroid (BR) pathway was suppressed in infected plants. Foliar application of methyl jasmonate (MeJA) or brassinazole (BRZ) resulted in a significant reduction in RBSDV incidence, while epibrassinolide (BL) treatment increased RBSDV infection. Infection studies using coi1-13 and Go mutants demonstrated JA-mediated resistance and BR-mediated susceptibility to RBSDV infection. A mixture of MeJA and BL treatment resulted in a significant reduction in RBSDV infection compared with a single BL treatment. MeJA application efficiently suppressed the expression of BR pathway genes, and this inhibition depended on the JA coreceptor OsCOI1. Collectively, our results reveal that JA-mediated defense can suppress the BR-mediated susceptibility to RBSDV infection.

Virulence of Pseudomonas syringae pv. tomato DC3000 Is Influenced by the Catabolite Repression Control Protein Crc

Pseudomonas syringae infects diverse plant species and is widely used as a model system in the study of effector function and the molecular basis of plant diseases. Although the relationship between bacterial metabolism, nutrient acquisition, and virulence has attracted increasing attention in bacterial pathology, it is largely unexplored in P. syringae. The Crc (catabolite repression control) protein is a putative RNA-binding protein that regulates carbon metabolism as well as a number of other factors in the pseudomonads. Here, we show that deletion of crc increased bacterial swarming motility and biofilm formation. The crc mutant showed reduced growth and symptoms in Arabidopsis and tomato when compared with the wild-type strain. We have evidence that the crc mutant shows delayed hypersensitive response (HR) when infiltrated into Nicotiana benthamiana and tobacco. Interestingly, the crc mutant was more susceptible to hydrogen peroxide, suggesting that, in planta, the mutant may be sensitive to reactive oxygen species generated during pathogen-associated molecular pattern-triggered immunity (PTI). Indeed, HR was further delayed when PTI-induced tissues were challenged with the crc mutant. The crc mutant did not elicit an altered PTI response in plants compared with the wild-type strain. We conclude that Crc plays an important role in growth and survival during infection.

A core function of EDS1 with PAD4 is to protect the salicylic acid defense sector in Arabidopsis immunity

Plant defenses induced by salicylic acid (SA) are vital for resistance against biotrophic pathogens. In basal and receptor-triggered immunity, SA accumulation is promoted by Enhanced Disease Susceptibility1 with its co-regulator Phytoalexin Deficient4 (EDS1/PAD4). Current models position EDS1/PAD4 upstream of SA but their functional relationship remains unclear. In a genetic and transcriptomic analysis of Arabidopsis autoimmunity caused by constitutive or conditional EDS1/PAD4 overexpression, intrinsic EDS1/PAD4 signaling properties and their relation to SA were uncovered. A core EDS1/PAD4 pathway works in parallel with SA in basal and effector-triggered bacterial immunity. It protects against disabled SA-regulated gene expression and pathogen resistance, and is distinct from a known SA-compensatory route involving MAPK signaling. Results help to explain previously identified EDS1/PAD4 regulated SA-dependent and SA-independent gene expression sectors. Plants have evolved an alternative route for preserving SA-regulated defenses against pathogen or genetic perturbations. In a proposed signaling framework, EDS1 with PAD4, besides promoting SA biosynthesis, maintains important SA-related resistance programs, thereby increasing robustness of the innate immune system.

Capping Protein Modulates Actin Remodeling in Response to Reactive Oxygen Species during Plant Innate Immunity

Plants perceive microbe-associated molecular patterns and damage-associated molecular patterns to activate innate immune signaling events, such as bursts of reactive oxygen species (ROS). The actin cytoskeleton remodels during the first 5 min of innate immune signaling in Arabidopsis (Arabidopsis thaliana) epidermal cells; however, the immune signals that impinge on actin cytoskeleton and its response regulators remain largely unknown. Here, we demonstrate that rapid actin remodeling upon elicitation with diverse microbe-associated molecular patterns and damage-associated molecular patterns represent a conserved plant immune response. Actin remodeling requires ROS generated by the defense-associated NADPH oxidase, RBOHD. Moreover, perception of flg22 by its cognate receptor complex triggers actin remodeling through the activation of RBOHD-dependent ROS production. Our genetic studies reveal that the ubiquitous heterodimeric capping protein transduces ROS signaling to the actin cytoskeleton during innate immunity. Additionally, we uncover a negative feedback loop between actin remodeling and flg22-induced ROS production.

Induced Genome-Wide Binding of Three Arabidopsis WRKY Transcription Factors during Early MAMP-Triggered Immunity

During microbial-associated molecular pattern-triggered immunity (MTI), molecules derived from microbes are perceived by cell surface receptors and upon signaling to the nucleus initiate a massive transcriptional reprogramming critical to mount an appropriate host defense response. WRKY transcription factors play an important role in regulating these transcriptional processes. Here, we determined on a genome-wide scale the flg22-induced in vivo DNA binding dynamics of three of the most prominent WRKY factors, WRKY18, WRKY40, and WRKY33. The three WRKY factors each bound to more than 1000 gene loci predominantly at W-box elements, the known WRKY binding motif. Binding occurred mainly in the 500-bp promoter regions of these genes. Many of the targeted genes are involved in signal perception and transduction not only during MTI but also upon damage-associated molecular pattern-triggered immunity, providing a mechanistic link between these functionally interconnected basal defense pathways. Among the additional targets were genes involved in the production of indolic secondary metabolites and in modulating distinct plant hormone pathways. Importantly, among the targeted genes were numerous transcription factors, encoding predominantly ethylene response factors, active during early MTI, and WRKY factors, supporting the previously hypothesized existence of a WRKY subregulatory network. Transcriptional analysis revealed that WRKY18 and WRKY40 function redundantly as negative regulators of flg22-induced genes often to prevent exaggerated defense responses.

Analysis of PAMP-Triggered ROS Burst in Plant Immunity

The plant perception of pathogen-associated molecular patterns triggers a plethora of cellular immune responses. One of these responses is a rapid and transient burst of reactive oxygen species (ROS) mediated by plasma membrane-localized NADPH oxidases. The ROS burst requires a functional receptor complex and the contribution of several additional regulatory components. In laboratory conditions, the ROS burst can be detected a few minutes after the treatment with an immunogenic microbial elicitor. For these reasons, the elicitor-triggered ROS burst has been often exploited as readout to probe the contribution of plant components to early immune responses. Here, we describe a detailed protocol for the measurement of elicitor-triggered ROS burst in a simple, fast, and easy manner.

Endoplasmic reticulum stress responses function in the HRT-mediated hypersensitive response in Nicotiana benthamiana

HRT is a plant coiled-coil, nucleotide-binding and leucine-rich repeat (CC-NB-LRR) disease resistance protein that triggers the hypersensitive response (HR) on recognition of Turnip crinkle virus (TCV) coat protein (CP). The molecular mechanism and significance of HR-mediated cell death for TCV resistance have not been fully elucidated. To identify the genes involved in HRT/TCV CP-mediated HR in Nicotiana benthamiana, we performed virus-induced gene silencing (VIGS) of 459 expressed sequence tags (ESTs) of pathogen-responsive Capsicum annuum genes. VIGS of CaBLP5, which encodes an endoplasmic reticulum (ER)-associated immunoglobulin-binding protein (BiP), silenced NbBiP4 and NbBiP5 and significantly reduced HRT-mediated HR. The induction of ER stress-responsive genes and the accumulation of ER-targeted BiPs in response to HRT-mediated HR suggest that ER is involved in HR in N. benthamiana. BiP4/5 silencing significantly down-regulated HRT at the mRNA and protein levels, and affected SGT1 and HSP90 expression. Co-expression of TCV CP in BiP4/5-silenced plants completely abolished HRT induction. Transient expression of TCV CP alone induced selected ER stress-responsive gene transcripts only in Tobacco rattle virus (TRV)-infected plants, and most of these genes were induced by HRT/TCV CP, except for bZIP60, which was induced specifically in response to HRT/TCV CP. TCV CP-mediated induction of ER stress-responsive genes still occurred in BiP4/5-silenced plants, but HRT/TCV CP-mediated induction of these genes was defective. Tunicamycin, a chemical that inhibits protein N-glycosylation, inhibited HRT-mediated HR, suggesting that ER has a role in HR regulation. These results indicate that BiP and ER, which modulate pattern recognition receptors in innate immunity, also regulate R protein-mediated resistance.

Loss-of-function of Arabidopsis receptor-like kinase BIR1 activates cell death and defense responses mediated by BAK1 and SOBIR1

The Arabidopsis receptor-like kinase (RLK) BIR1 (BAK1-INTERACTING RECEPTOR-LIKE KINASE 1) functions as a negative regulator of plant immunity. Previous work showed that loss-of-function of BIR1 leads to constitutive activation of cell death and defense responses. These autoimmune phenotypes are partially dependent on another RLK, SOBIR1. In order to identify additional components involved in the BIR1-regulated plant defense signaling pathway, a suppressor screen was carried out in the bir1-1 pad4-1 mutant background. Mutations in the suppressor mutants were identified by genetic mapping and re-sequencing of the mutant genomes. A number of suppressor mutants were found to carry mutations in an additional RLK, BAK1, indicating that BAK1 is required for activation of cell death and defense responses in bir1-1. Co-immunoprecipitation analysis revealed that BAK1 and SOBIR1 associate with each other in planta when the function of BIR1 is compromised. Although BAK1 was previously characterized as a negative regulator of cell death, our study highlights a novel role of BAK1 in promoting cell death and defense responses in conjunction with SOBIR1.

Autophosphorylation of Specific Threonine and Tyrosine Residues in Arabidopsis CERK1 is Essential for the Activation of Chitin-Induced Immune Signaling

Pattern recognition receptors on the plant cell surface mediate the recognition of microbe/damage-associated molecular patterns (MAMPs/DAMPs) and activate downstream immune signaling. Autophosphorylation of signaling receptor-like kinases is a critical event for the activation of downstream responses but the function of each phosphorylation site in the regulation of immune signaling is not well understood. In this study, 41 Ser/Thr/Tyr and 15 Ser/Thr residues were identified as in vitro and in vivo autophosphorylation sites of Arabidopsis CERK1, which is essential for chitin signaling. Comprehensive analysis of transgenic plants expressing mutated CERK1 genes for each phosphorylation site in the cerk1-2 background indicated that the phosphorylation of T479 in the activation segment and Y428 located upstream of the catalytic loop is important for the activation of chitin-triggered defense responses. Contribution of the phosphorylation of T573 to the chitin responses was also suggested. In vitro evaluation of kinase activities of mutated kinase domains indicated that the phosphorylation of T479 and T573 is directly involved in the regulation of kinase activity of CERK1 but the phosphorylation of Y428 regulates chitin signaling independently of the regulation of kinase activity. These results indicated that the phosphorylation of specific residues in the kinase domain contributes to the regulation of downstream signaling either through the regulation of kinase activity or the different mechanisms, e.g. regulation of protein-protein interactions.

Comprehensive transcriptional and functional analyses of melatonin synthesis genes in cassava reveal their novel role in hypersensitive-like cell death

Melatonin is a widely known hormone in animals. Since melatonin was discovered in plants, more and more studies highlight its involvement in a wide range of physiological processes including plant development and stress responses. Many advances have been made in the terms of melatonin-mediated abiotic stress resistance and innate immunity in plants, focusing on model plants such as rice and Arabidopsis. In this study, 7 melatonin synthesis genes were systematically analyzed in cassava. Quantitative real-time PCR showed that all these genes were commonly regulated by melatonin, flg22, Xanthomonas axonopodis pv manihotis (Xam) and hydrogen peroxide (H2O2). Transient expression in Nicotiana benthamiana revealed the subcellular locations and possible roles of these melatonin synthesis genes. Notably, we highlight novel roles of these genes in hypersensitive-like cell death, as confirmed by the results of several physiological parameters. Moreover, transient expression of these genes had significant effects on the transcripts of reactive oxygen species (ROS) accumulation and defense-related genes, and triggered the burst of callose depositions and papillae-associated plant defense, indicating the possible role of them in plant innate immunity. Taken together, this study reveals the comprehensive transcripts and putative roles of melatonin synthesis genes as well as melatonin in immune responses in cassava.

Glycolipid biosurfactants: main properties and potential applications in agriculture and food industry

Glycolipids, consisting of a carbohydrate moiety linked to fatty acids, are microbial surface active compounds produced by various microorganisms. They are characterized by high structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface, respectively. Rhamnolipids, trehalolipids, mannosylerythritol lipids and cellobiose lipids are among the most popular glycolipids. They have received much practical attention as biopesticides for controlling plant diseases and protecting stored products. As a result of their antifungal activity towards phytopathogenic fungi and larvicidal and mosquitocidal potencies, glycolipid biosurfactants permit the preservation of plants and plant crops from pest invasion. Also, as a result of their emulsifying and antibacterial activities, glycolipids have great potential as food additives and food preservatives. Furthermore, the valorization of food byproducts via the production of glycolipid biosurfactant has received much attention because it permits the bioconversion of byproducts on valuable compounds and decreases the cost of production. Generally, the use of glycolipids in many fields requires their retention from fermentation media. Accordingly, different strategies have been developed to extract and purify glycolipids. © 2016 Society of Chemical Industry.

Colletotrichum higginsianum extracellular LysM proteins play dual roles in appressorial function and suppression of chitin-triggered plant immunity

The genome of the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum, encodes a large repertoire of candidate-secreted effectors containing LysM domains, but the role of such proteins in the pathogenicity of any Colletotrichum species is unknown. Here, we characterized the function of two effectors, ChELP1 and ChELP2, which are transcriptionally activated during the initial intracellular biotrophic phase of infection. Using immunocytochemistry, we found that ChELP2 is concentrated on the surface of bulbous biotrophic hyphae at the interface with living host cells but is absent from filamentous necrotrophic hyphae. We show that recombinant ChELP1 and ChELP2 bind chitin and chitin oligomers in vitro with high affinity and specificity and that both proteins suppress the chitin-triggered activation of two immune-related plant mitogen-activated protein kinases in the host Arabidopsis. Using RNAi-mediated gene silencing, we found that ChELP1 and ChELP2 are essential for fungal virulence and appressorium-mediated penetration of both Arabidopsis epidermal cells and cellophane membranes in vitro. The findings suggest a dual role for these LysM proteins as effectors for suppressing chitin-triggered immunity and as proteins required for appressorium function.

Double-stranded RNAs induce a pattern-triggered immune signaling pathway in plants

Pattern-triggered immunity (PTI) is a plant defense response that relies on the perception of conserved microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs, respectively). Recently, it has been recognized that PTI restricts virus infection in plants; however, the nature of the viral or infection-induced PTI elicitors and the underlying signaling pathways are still unknown. As double-stranded RNAs (dsRNAs) are conserved molecular patterns associated with virus replication, we applied dsRNAs or synthetic dsRNA analogs to Arabidopsis thaliana and investigated PTI responses. We show that in vitro-generated dsRNAs, dsRNAs purified from virus-infected plants and the dsRNA analog polyinosinic-polycytidylic acid (poly(I:C)) induce typical PTI responses dependent on the co-receptor SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (SERK1), but independent of dicer-like (DCL) proteins in Arabidopsis. Moreover, dsRNA treatment of Arabidopsis induces SERK1-dependent antiviral resistance. Screening of Arabidopsis wild accessions demonstrates natural variability in dsRNA sensitivity. Our findings suggest that dsRNAs represent genuine PAMPs in plants, which induce a signaling cascade involving SERK1 and a specific dsRNA receptor. The dependence of dsRNA-mediated PTI on SERK1, but not on DCLs, implies that dsRNA-mediated PTI involves membrane-associated processes and operates independently of RNA silencing. dsRNA sensitivity may represent a useful trait to increase antiviral resistance in cultivated plants.

Bismerthiazol Inhibits Xanthomonas citri subsp. citri Growth and Induces Differential Expression of Citrus Defense-Related Genes

Citrus canker, caused by Xanthomonas citri ssp. citri, is a serious disease that causes substantial economic losses to the citrus industry worldwide. The bactericide bismerthiazol has been used to control rice bacterial blight (X. oryzae pv. oryzae). In this paper, we demonstrate that bismerthiazol can effectively control citrus canker by both inhibiting the growth of X. citri ssp. citri and triggering the plant's host defense response through the expression of several pathogenesis-related genes (PR1, PR2, CHI, and RpRd1) and the nonexpresser of PR genes (NPR1, NPR2, and NPR3) in 'Duncan' grapefruit, especially at early treatment times. In addition, we found that bismerthiazol induced the expression of the marker genes CitCHS and CitCHI in the flavonoid pathway and the PAL1 (phenylalanine ammonia lyase 1) gene in the salicylic acid (SA) biosynthesis pathway at different time points. Moreover, bismerthiazol also induced the expression of the priming defense-associated gene AZI1. Taken together, these results indicate that the induction of the defense response in 'Duncan' grapefruit by bismerthiazol may involve the SA signaling pathway and the priming defense and that bismerthiazol may serve as an alternative to copper bactericides for the control of citrus canker.