Plant Defense Signaling Pathways Research Articles

The Role of Plant Defense Signaling Pathways in Phytoplasma-Infected and Uninfected Aster Leafhoppers' Oviposition, Development, and Settling Behavior.

In plant-microbe-insect systems, plant-mediated responses involve the regulation and interactions of plant defense signaling pathways of phytohormones jasmonic acid (JA), ethylene (ET), and salicylic acid (SA). Phytoplasma subgroup 16SrI is the causal agent of Aster Yellows (AY) disease and is primarily transmitted by populations of aster leafhoppers (Macrosteles quadrilineatus Forbes). Aster Yellows infection in plants is associated with the downregulation of the JA pathway and increased leafhopper oviposition. The extent to which the presence of intact phytohormone-mediated defensive pathways regulates aster leafhopper behavioral responses, such as oviposition or settling preferences, remains unknown. We conducted no-choice and two-choice bioassays using a selection of Arabidopsis thaliana lines that vary in their defense pathways and repeated the experiments using AY-infected aster leafhoppers to evaluate possible differences associated with phytoplasma infection. While nymphal development was similar among the different lines and groups of AY-uninfected and AY-infected insects, the number of offspring and individual female egg load of AY-uninfected and AY-infected insects differed in lines with mutated components of the JA and SA signaling pathways. In most cases, AY-uninfected insects preferred to settle on wild-type (WT) plants over mutant lines; no clear pattern was observed in the settling preference of AY-infected insects. These findings support previous observations in other plant pathosystems and suggest that plant signaling pathways and infection with a plant pathogen can affect insect behavioral responses in more than one manner. Potential differences with previous work on AY could be related to the specific subgroup of phytoplasma involved in each case.

Sequence similarity networks bear out hierarchical relationships of green cytochrome P450.

Land plants have diversified enzyme families. One of the most prominent is the cytochrome P450 (CYP or CYP450) family. With over 443,000 CYP proteins sequenced across the tree of life, CYPs are ubiquitous in archaea, bacteria, and eukaryotes. Here, we focused on land plants and algae to study the role of CYP diversification. CYPs, acting as monooxygenases, catalyze hydroxylation reactions crucial for specialized plant metabolic pathways, including detoxification and phytohormone production; the CYPome consists of one enormous superfamily that is divided into clans and families. Their evolutionary history speaks of high substrate promiscuity; radiation and functional diversification have yielded numerous CYP families. To understand the evolutionary relationships within the CYPs, we employed sequence similarity network analyses. We recovered distinct clusters representing different CYP families, reflecting their diversified sequences that we link to the prediction of functionalities. Hierarchical clustering and phylogenetic analysis further elucidated relationships between CYP clans, uncovering their shared deep evolutionary history. We explored the distribution and diversification of CYP subfamilies across plant and algal lineages, uncovering novel candidates and providing insights into the evolution of these enzyme families. This identified unexpected relationships between CYP families, such as the link between CYP82 and CYP74, shedding light on their roles in plant defense signaling pathways. Our approach provides a methodology that brings insights into the emergence of new functions within the CYP450 family, contributing to the evolutionary history of plants and algae. These insights can be further validated and implemented via experimental setups under various external conditions.

The beneficial endophytic microbes enhanced tobacco defense system to resist bacterial wilt disease

BackgroundThe vital role of rhizosphere microbiome in protecting plants against pathogen infection has been well characterized. By contrast, beneficial effects of the plant endophytic microbiome and interactions with plants remain poorly understood. Here, we integrated microbial community analysis, culture-based methods and plant defense gene quantification to systematically investigate the responses of endophytic root and stem microbiomes and the defense system of the tobacco plant to Ralstonia solanacearum, a bacterial pathogen causing bacterial wilt disease worldwide.ResultsAn obvious reduction of community diversity and changes of microbial composition were observed in tobacco root and stem compartments upon R. solanacearum infection. Beneficial endophytic microbes were enriched in both diseased root and stem compartments, and beneficial microbes enriched in roots were more diverse than those in stems. Several beneficial bacteria enriched in diseased roots showed positive correlations with the pathogen abundance and exhibited great antagonistic activities against R. solanacearum. At the same time, the expression of key genes participating in plant defense signaling pathways and activities of plant defense enzymes were activated with R. solanacearum infection. Inoculation of beneficial endophyte Burkholderia ASV_550 in soils could greatly enhance the tobacco defense system in the presence of pathogen and decrease the disease incidence.ConclusionsThe recruited beneficial microbes exhibited an enhancement of the defense system of host plants. This work provides endophyte evidence for the “cry for help” strategy in plants and advances the current understanding of plant–microbe interactions in resisting plant disease.Graphical

Whitefly-Plant Interactions: An Integrated Molecular Perspective.

The rapid advances in available transcriptomic and genomic data and our understanding of the physiology and biochemistry of whitefly-plant interactions have allowed us to gain new and significant insights into the biology of whiteflies and their successful adaptation to host plants. In this review, we provide a comprehensive overview of the mechanisms that whiteflies have evolved to overcome the challenges of feeding on phloem sap. We also highlight the evolution and functions of gene families involved in host perception, evaluation, and manipulation; primary metabolism; and metabolite detoxification. We discuss the emerging themes in plant immunity to whiteflies, focusing on whitefly effectors and their sites of action in plant defense-signaling pathways. We conclude with a discussion of advances in the genetic manipulation of whiteflies and the potential that they hold for exploring the interactions between whiteflies and their host plants, as well as the development of novel strategies for the genetic control of whiteflies.

Bio-priming of banana tissue culture plantlets with endophytic Bacillus velezensis EB1 to improve Fusarium wilt resistance.

Tissue culture techniques have been routinely used for banana propagation and offered rapid production of planting materials with favorable genotypes and free of pathogenic microorganisms in the banana industry. Meanwhile, extensive scientific work suggests that micropropagated plantlets are more susceptible to Fusarium oxysporum f. sp. cubense (Foc), the deadly strain that causes Fusarium wilt of bananas than conventional planting material due to the loss of indigenous endophytes. In this study, an endophytic bacterium Bacillus velezensis EB1 was isolated and characterized. EB1 shows remarkable in vitro antagonistic activity against Foc with an inhibition rate of 75.43% and induces significant morphological and ultrastructural changes and alterations in the hyphae of Foc. Colony-forming unit (c.f.u.) counting and scanning electron microscopy (SEM) revealed that EB1 could colonize both the surface and inner tissues of banana tissue culture plantlets. Banana tissue culture plantlets of late rooting stage bioprimed with EB1 could efficiently ward off the invasive of Foc. The bio-priming effect could maintain in the acclimatized banana plants and significantly decrease the disease severity of Fusarium wilt and induce strong disease resistance by manipulating plant defense signaling pathways in a pot experiment. Our results provide the adaptability and potential of native endophyte EB1 in protecting plants from pathogens and infer that banana tissue culture plantlets bio-priming with endophytic microbiota could be a promising biological solution in the fight against the Fusarium wilt of banana.

Role Of Pathogenesis-Related (PR) Proteins in Plant Microbes Defence Mechanism

A group of diverse substances called PR proteins and antimicrobial peptides (AMPs) are produced by phytopathogens and signalling molecules connected to defence. They are crucial elements of the plant's innate immune system, particularly in the case of molecular markers for defensive signalling pathways called systemic acquired resistance (SAR). Although PR proteins and peptides were discovered prior to the development of modern scientific tools, little is known about their biological significance. One of the most promising methods for creating disease-resistant transgenic crops uses plant genetic engineering, which makes use of several antimicrobial genes like PR genes. Overexpression of the PR genes (chitinase, glucanase, thaumatin, defensin, and thionin) singly or in combination has a major impact on the level of plant defence against many diseases. However, enhancing agricultural plants' resistance to a variety of stresses requires a detailed understanding of the signalling pathways that regulate the expression of these adaptive proteins. This is the topic of plant stress biology, which will be covered in the future. Since PR proteins are involved in both biotic and abiotic stress, as well as plant defence signalling pathways, this analysis gives a thorough review of PR proteins. Additionally, we talked about the advantages and disadvantages of transgenic plants, PR proteins, and peptide expression.

Plants' Physio-Biochemical and Phyto-Hormonal Responses to Alleviate the Adverse Effects of Drought Stress: A Comprehensive Review.

Water, a necessary component of cell protoplasm, plays an essential role in supporting life on Earth; nevertheless, extreme changes in climatic conditions limit water availability, causing numerous issues, such as the current water-scarce regimes in many regions of the biome. This review aims to collect data from various published studies in the literature to understand and critically analyze plants’ morphological, growth, yield, and physio-biochemical responses to drought stress and their potential to modulate and nullify the damaging effects of drought stress via activating natural physiological and biochemical mechanisms. In addition, the review described current breakthroughs in understanding how plant hormones influence drought stress responses and phytohormonal interaction through signaling under water stress regimes. The information for this review was systematically gathered from different global search engines and the scientific literature databases Science Direct, including Google Scholar, Web of Science, related studies, published books, and articles. Drought stress is a significant obstacle to meeting food demand for the world’s constantly growing population. Plants cope with stress regimes through changes to cellular osmotic potential, water potential, and activation of natural defense systems in the form of antioxidant enzymes and accumulation of osmolytes including proteins, proline, glycine betaine, phenolic compounds, and soluble sugars. Phytohormones modulate developmental processes and signaling networks, which aid in acclimating plants to biotic and abiotic challenges and, consequently, their survival. Significant progress has been made for jasmonates, salicylic acid, and ethylene in identifying important components and understanding their roles in plant responses to abiotic stress. Other plant hormones, such as abscisic acid, auxin, gibberellic acid, brassinosteroids, and peptide hormones, have been linked to plant defense signaling pathways in various ways.

Burkholderia sp. Strain AU4i Promotes Arabidopsis Growth and Increases Plant Defence Response to Necrotrophic Pathogens

Burkholderia sp. AU4i is a strain previously isolated from pea, which can act as plant growth-promoting rhizobacteria (PGPR). Here, we demonstrate that AU4i is able to promote Arabidopsis growth and increase the number of lateral roots as well as shoot and root dry weight. The inoculation of Arabidopsis leaves with AU4i significantly reduced the lesion’s areas provoked by the necrotrophic pathogens Botrytis cinerea and Sclerotinia sclerotiorum. Bacterial inoculation induced the expression of the jasmonate responsive marker gene AtPDF1.2, but not the expression of salicylic acid marker gene AtPR-1. Notwithstanding, similar AU4i protective effect was observed in Arabidopsis mutants coi1 and sid2-2 (defective in jasmonate perception and salicylic acid synthesis, respectively) compared with Col-0 plants. Thus, the independence of the AU4i protective effect and plant defence signalling pathways is suggested. AU4i inoculation was able to induced anthocyanins and phenolic compounds accumulation, as well as phenylalanine ammonia-lyase activity. Moreover, the effect of AU4i inoculation on the metabolism of plant cell wall was evaluated. As a result, B. cinerea and S. sclerotiorum mycelial growth was reduced in culture media containing cell wall polysaccharides from inoculated Arabidopsis leaves. Additionally, leaves colonized by AU4i showed the downregulation of genes involved in hemicelluloses and pectins degradation (AtExp8 and AtAra1, respectively) and the upregulation of AtPME3, a gene encoding a putative pectin methylesterase involved in pectin main chain de-esterification. Taken together, our results suggest Burkholderia sp. AU4i not only as potential biocontrol agent, but also as a good candidate for better understanding the mechanisms used by PGPRs to protect plants from fungal attacks.

CATION-CHLORIDE CO-TRANSPORTER 1 (CCC1) Mediates Plant Resistance against Pseudomonas syringae.

Plasma membrane (PM) depolarization functions as an initial step in plant defense signaling pathways. However, only a few ion channels/transporters have been characterized in the context of plant immunity. Here, we show that the Arabidopsis (Arabidopsis thaliana) Na+:K+:2Cl- (NKCC) cotransporter CCC1 has a dual function in plant immunity. CCC1 functions independently of PM depolarization and negatively regulates pathogen-associated molecular pattern-triggered immunity. However, CCC1 positively regulates plant basal and effector-triggered resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. In line with the compromised immunity to Pst DC3000, ccc1 mutants show reduced expression of genes encoding enzymes involved in the biosynthesis of antimicrobial peptides, camalexin, and 4-OH-ICN, as well as pathogenesis-related proteins. Moreover, genes involved in cell wall and cuticle biosynthesis are constitutively down-regulated in ccc1 mutants, and the cell walls of these mutants exhibit major changes in monosaccharide composition. The role of CCC1 ion transporter activity in the regulation of plant immunity is corroborated by experiments using the specific NKCC inhibitor bumetanide. These results reveal a function for ion transporters in immunity-related cell wall fortification and antimicrobial biosynthesis.

Molecular Regulation of Host Defense Responses Mediated by Biological Anti-TMV Agent Ningnanmycin.

Ningnanmycin (NNM) belongs to microbial pesticides that display comprehensive antiviral activity against plant viruses. NNM treatment has been shown to efficiently delay or suppress the disease symptoms caused by tobacco mosaic virus (TMV) infection in local-inoculated or systemic-uninoculated tobacco leaves, respectively. However, the underlying molecular mechanism of NNM-mediated antiviral activity remains to be further elucidated. In this study, 414 differentially expressed genes (DEGs), including 383 which were up-regulated and 31 down-regulated, caused by NNM treatment in TMV-infected BY-2 protoplasts, were discovered by RNA-seq. In addition, KEGG analysis indicated significant enrichment of DEGs in the plant–pathogen interaction and MAPK signaling pathway. The up-regulated expression of crucial DEGs, including defense-responsive genes, such as the receptor-like kinase FLS2, RLK1, and the mitogen-activated protein kinase kinase kinase MAPKKK, calcium signaling genes, such as the calcium-binding protein CML19, as well as phytohormone responsive genes, such as the WRKY transcription factors WRKY40 and WRKY70, were confirmed by RT-qPCR. These findings provided valuable insights into the antiviral mechanisms of NNM, which indicated that the agent induces tobacco systemic resistance against TMV via activating multiple plant defense signaling pathways.

Plant defense factors involved in Olea europaea resistance against Xylella fastidiosa infection.

Olive quick decline syndrome (OQDS) is a dangerous plant disease, caused by the bacterium Xylella fastidiosa, which targets olive (Olea europaea). Since field observations suggested that some olive cultivars (i.e. Leccino) were more resistant to OQDS than others (i.e. Cellina di Nardò), the plant defense strategies adopted by olive to contrast X. fastidiosa infection were investigated. In the present study, ELISA and genetic approaches were used to confirm plant infection, while microbial colonization mechanism and distribution in host plant tissues and reactive oxygen species (ROS) levels were examined by light, scanning electron and confocal microscopy analyses. Spectrophotometric and chromatographic techniques were performed to measure secondary metabolites content and qPCR assay was carried out for monitoring plant gene expression variation. Our analysis showed that X. fastidiosa caused accumulation of ROS in Leccino samples compared to Cellina di Nardò. Moreover, the infection induced the up-regulation of defense-related genes, such as NADPH oxidase, some protein kinases, pathogen plant response factors and metabolic enzymes. We also found that Leccino plants enhanced the production of specific antioxidant and antimicrobial molecules, to fight the pathogen and avoid its spreading into xylem vessels. We provided new information on OQDS resistance mechanism applied by Leccino cultivar. In particular, we evidenced that high concentrations of ROS, switching on plant defence signalling pathways, may represent a key factor in fighting X. fastidiosa infection.

Interactions between plant defence signalling pathways: Evidence from bioassays with insect herbivores and plant pathogens

Abstract Sequential damage by attackers is hypothesized to result in reciprocal antagonism (crosstalk) between jasmonic acid (JA) and salicylic acid (SA) defence signalling pathways in plants. However, evidence for this crosstalk is not universal and several studies have found positive interactions (i.e. synergism) or no interaction whatsoever between JA and SA pathways. Here we conducted a meta‐analysis of studies on plant‐mediated effects of initial attackers on performance of subsequent attackers to test the hypothesis of crosstalk between plant hormonal signalling pathways. We found a significant negative mean effect size of JA‐inducing initial attackers on both JA‐ and SA‐inducing subsequent attackers, but a non‐significant effect of SA‐inducing initial attackers on both JA‐ and SA‐inducing subsequent attackers. Effects on subsequent herbivores were contingent on the biology of the initial attacker, with negative effects of JA‐inducing initial herbivores but no effect of SA‐inducing initial herbivores, whereas pathogens on average did not influence subsequent herbivores. Furthermore, negative effects of JA‐inducing initial herbivores on JA‐ and SA‐inducing subsequent herbivores held when both attacks occurred on the same plant part, but when attacks were on different plant parts the effect of JA‐inducing herbivores on SA‐inducing herbivores was non‐significant. Synthesis. These results indicate that reciprocal antagonism between jasmonic acid and salicylic acid pathways is not universal, and suggest assymetries and specificity in the strength of plant‐mediated interactions.

The Multifunctional Role of Chitosan in Horticultural Crops; A Review.

Chitosan is a naturally occurring compound and is commercially produced from seafood shells. It has been utilized in the induction of the defense system in both pre and post-harvest fruits and vegetables against fungi, bacteria, viruses, and other abiotic stresses. In addition to that, chitosan effectively improves the physiological properties of plants and also enhances the shelf life of post-harvest produces. Moreover, chitosan treatment regulates several genes in plants, particularly the activation of plant defense signaling pathways. That includes the elicitation of phytoalexins and pathogenesis-related (PR) protein. Besides that, chitosan has been employed in soil as a plant nutrient and has shown great efficacy in combination with other industrial fertilizers without affecting the soil’s beneficial microbes. Furthermore, it is helpful in reducing the fertilizer losses due to its coating ability, which is important in keeping the environmental pollution under check. Based on exhibiting such excellent properties, there is a striking interest in using chitosan biopolymers in agriculture systems. Therefore, our current review has been centered upon the multiple roles of chitosan in horticultural crops that could be useful in future crop improvement programs.

Biowaste-derived hydrolysates as plant disease suppressants for oilseed rape

Abstract Water soluble substances, obtained by aqueous alkaline hydrolysis of fermented urban kitchen and garden waste, have been reported enhancing the productivity of several food and ornamental plants. The present work reports unknown new property of these substances as plant disease suppressants. It describes a case study where oilseed rape cotyledons were protected by pre-treatment with two different types of soluble substances against a fungal pathogen Leptosphaeria maculans. One type was isolated from the digestate of a biogas production reactor fed with kitchen wastes. The second type was obtained from a compost made from a mix of biogas digestate, gardening residues and sewage sludge. The results demonstrate that the alkaline hydrolysate exhibited the following effects: (i) decreased the necrosis development by L. maculans by 42–56% when applied to leaves and by 31–37% when applied to roots; (ii) reduced cotyledon leaf area by 13–17%; (iii) increased biomass dry weight by 9% compared to the distilled-water-treated control. The soluble substances induced a plant defence involving an ethylene-dependent signalling pathway, which was assessed through the expression of marker genes involved in the main plant defence signalling pathways. Considering the performance, cost and origin of the alkaline hydrolysates they seem a promising alternative to commercial synthetic fungicides. The benefits for the agriculture and waste management sectors, which may potentially derive from the market allocation of the alkaline hydrolysates for use in agriculture, environmental technology, and in the chemical industry for manufacturing consumer goods and products, are discussed.

Poplar stem transcriptome is massively remodelled in response to single or repeated mechanical stimuli

BackgroundTrees experience mechanical stimuli -like wind- that trigger thigmomorphogenetic syndrome, leading to modifications of plant growth and wood quality. This syndrome affects tree productivity but is also believed to improve tree acclimation to chronic wind. Wind is particularly challenging for trees, because of their stature and perenniality. Climate change forecasts are predicting that the occurrence of high wind will worsen, making it increasingly vital to understand the mechanisms regulating thigmomorphogenesis, especially in perennial plants. By extension, this also implies factoring in the recurring nature of wind episodes. However, data on the molecular processes underpinning mechanoperception and transduction of mechanical signals, and their dynamics, are still dramatically lacking in trees.ResultsHere we performed a genome-wide and time-series analysis of poplar transcriptional responsiveness to transitory and recurring controlled stem bending, mimicking wind. The study revealed that 6% of the poplar genome is differentially expressed after a single transient bending. The combination of clustering, Gene Ontology categorization and time-series expression approaches revealed the diversity of gene expression patterns and biological processes affected by stem bending. Short-term transcriptomic responses entailed a rapid stimulation of plant defence and abiotic stress signalling pathways, including ethylene and jasmonic acid signalling but also photosynthesis process regulation. Late transcriptomic responses affected genes involved in cell wall organization and/or wood development. An analysis of the molecular impact of recurring bending found that the vast majority (96%) of the genes differentially expressed after a first bending presented reduced or even net-zero amplitude regulation after the second exposure to bending.ConclusionThis study constitutes the first dynamic characterization of the molecular processes affected by single or repeated stem bending in poplar. Moreover, the global attenuation of the transcriptional responses, observed from as early as after a second bending, indicates the existence of a mechanism governing a fine tuning of plant responsiveness. This points toward several mechanistic pathways that can now be targeted to elucidate the complex dynamics of wind acclimation.

Negative Autogenous Control of the Master Type III Secretion System Regulator HrpL in Pseudomonas syringae.

ABSTRACT The type III secretion system (T3SS) is a principal virulence determinant of the model bacterial plant pathogen Pseudomonas syringae. T3SS effector proteins inhibit plant defense signaling pathways in susceptible hosts and elicit evolved immunity in resistant plants. The extracytoplasmic function sigma factor HrpL coordinates the expression of most T3SS genes. Transcription of hrpL is dependent on sigma-54 and the codependent enhancer binding proteins HrpR and HrpS for hrpL promoter activation. hrpL is oriented adjacently to and divergently from the HrpL-dependent gene hrpJ, sharing an intergenic upstream regulatory region. We show that association of the RNA polymerase (RNAP)-HrpL complex with the hrpJ promoter element imposes negative autogenous control on hrpL transcription in P. syringae pv. tomato DC3000. The hrpL promoter was upregulated in a ΔhrpL mutant and was repressed by plasmid-borne hrpL. In a minimal Escherichia coli background, the activity of HrpL was sufficient to achieve repression of reconstituted hrpL transcription. This repression was relieved if both the HrpL DNA-binding function and the hrp-box sequence of the hrpJ promoter were compromised, implying dependence upon the hrpJ promoter. DNA-bound RNAP-HrpL entirely occluded the HrpRS and partially occluded the integration host factor (IHF) recognition elements of the hrpL promoter in vitro, implicating inhibition of DNA binding by these factors as a cause of negative autogenous control. A modest increase in the HrpL concentration caused hypersecretion of the HrpA1 pilus protein but intracellular accumulation of later T3SS substrates. We argue that negative feedback on HrpL activity fine-tunes expression of the T3SS regulon to minimize the elicitation of plant defenses.

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.

Pseudomonas syringae pv. tomato OxyR Is Required for Virulence in Tomato and Arabidopsis.

Reactive oxygen species (ROS) have been shown to have a crucial role in plant defense responses and signaling pathways. In addition, ROS also have direct toxicity against pathogens. However, the molecular mechanisms of plant ROS in the direct effects against pathogens is still unclear. To investigate the function of plant ROS in the interactions of plant and bacterial pathogens, we focused on oxyR, encoding an oxidative stress-regulated transcription factor in Pseudomonas syringae pv. tomato DC3000 (DC3000), and generated an ΔoxyR mutant. The DC3000 ΔoxyR mutant showed high sensitivity to oxidative stress in comparison with wild type and the complemented line. The host plants of DC3000, including tomato and Arabidopsis inoculated with the ΔoxyR mutant, clearly showed reduced disease symptoms as well as reduced bacterial populations. Expression profiles of DC3000 genes revealed that OxyR could regulate the expression of genes encoding ROS-detoxifying enzymes, including catalases (KatB and KatG), in response to ROS. We also demonstrated that the expression of katB could be regulated by OxyR during the infection of DC3000 in Arabidopsis. These results suggest that OxyR has an important role in the virulence of DC3000 by regulating the expression of genes related to oxidative stress.

Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening).

Commercial sweet orange cultivars lack resistance to Huanglongbing (HLB), a serious phloem limited bacterial disease that is usually fatal. In order to develop sustained disease resistance to HLB, transgenic sweet orange cultivars ‘Hamlin’ and ‘Valencia’ expressing an Arabidopsis thaliana NPR1 gene under the control of a constitutive CaMV 35S promoter or a phloem specific Arabidopsis SUC2 (AtSUC2) promoter were produced. Overexpression of AtNPR1 resulted in trees with normal phenotypes that exhibited enhanced resistance to HLB. Phloem specific expression of NPR1 was equally effective for enhancing disease resistance. Transgenic trees exhibited reduced diseased severity and a few lines remained disease-free even after 36 months of planting in a high-disease pressure field site. Expression of the NPR1 gene induced expression of several native genes involved in the plant defense signaling pathways. The AtNPR1 gene being plant derived can serve as a component for the development of an all plant T-DNA derived consumer friendly GM tree.

Synthetic plant defense elicitors.

To defend themselves against invading pathogens plants utilize a complex regulatory network that coordinates extensive transcriptional and metabolic reprogramming. Although many of the key players of this immunity-associated network are known, the details of its topology and dynamics are still poorly understood. As an alternative to forward and reverse genetic studies, chemical genetics-related approaches based on bioactive small molecules have gained substantial popularity in the analysis of biological pathways and networks. Use of such molecular probes can allow researchers to access biological space that was previously inaccessible to genetic analyses due to gene redundancy or lethality of mutations. Synthetic elicitors are small drug-like molecules that induce plant defense responses, but are distinct from known natural elicitors of plant immunity. While the discovery of some synthetic elicitors had already been reported in the 1970s, recent breakthroughs in combinatorial chemical synthesis now allow for inexpensive high-throughput screens for bioactive plant defense-inducing compounds. Along with powerful reverse genetics tools and resources available for model plants and crop systems, comprehensive collections of new synthetic elicitors will likely allow plant scientists to study the intricacies of plant defense signaling pathways and networks in an unparalleled fashion. As synthetic elicitors can protect crops from diseases, without the need to be directly toxic for pathogenic organisms, they may also serve as promising alternatives to conventional biocidal pesticides, which often are harmful for the environment, farmers and consumers. Here we are discussing various types of synthetic elicitors that have been used for studies on the plant immune system, their modes-of-action as well as their application in crop protection.