jasmonic-acid and camalexin

Topics: Cyclopentanes; Disease Resistance; Ethylenes; Gene Expression Regulation, Plant; Indoles; Oxylipins; Plant Diseases; Signal Transduction; Thiazoles

The large WRKY transcription factor family is mainly involved in regulating plant immune responses. Arabidopsis WRKY33 is a key transcriptional regulator of hormonal and metabolic processes towards Botrytis cinerea strain 2100 infection and is essential for resistance. In contrast to B. cinerea strain 2100, the strain B05.10 is virulent on wild-type (WT) Col-0 Arabidopsis plants highlighting the genetic diversity within this pathogen species. We analysed how early WRKY33-dependent responses are affected upon infection with strain B05.10 and found that most of these responses were strongly dampened during this interaction. Ectopic expression of WRKY33 resulted in complete resistance towards this strain indicating that virulence of B05.10, at least partly, depends on suppressing WRKY33 expression/protein accumulation. As a consequence, the expression levels of direct WRKY33 target genes, including those involved in the biosynthesis of camalexin, were also reduced upon infection. Concomitantly, elevated levels of the phytohormone abscisic acid (ABA) were observed. Molecular and genetic studies revealed that ABA negatively influences defence to B05.10 and effects jasmonic acid/ethylene (JA/ET) and salicylic acid (SA) levels. Susceptibility/resistance was determined by the antagonistic effect of ABA on JA, and this crosstalk required suppressing WRKY33 functions at early infection stages. This indicates that B. cinerea B05.10 promotes disease by suppressing WRKY33-mediated host defences.

Topics: Abscisic Acid; Arabidopsis; Arabidopsis Proteins; Botrytis; Cyclopentanes; DNA, Plant; Ecotype; Gene Expression Regulation, Plant; Genes, Plant; Genotype; Indoles; Mutation; Oxylipins; Phenotype; Plant Diseases; Plant Growth Regulators; Plant Immunity; Protein Binding; RNA, Messenger; Thiazoles; Transcription Factors

Topics: Arabidopsis; Arabidopsis Proteins; Botrytis; Cyclopentanes; Disease Resistance; Gene Expression Regulation, Plant; Gene Regulatory Networks; Genotype; Host-Pathogen Interactions; Indoles; Mutation; Oxylipins; Plant Diseases; Salicylic Acid; Signal Transduction; Thiazoles; Transcriptome

Beneficial soil microbes can promote plant growth and induce systemic resistance (ISR) in aboveground tissues against pathogens and herbivorous insects. Despite the increasing interest in microbial-ISR against herbivores, the underlying molecular and chemical mechanisms of this phenomenon remain elusive. Using Arabidopsis thaliana and the rhizobacterium Pseudomonas simiae WCS417r (formerly known as P. fluorescens WCS417r), we here evaluate the role of the JA-regulated MYC2-branch and the JA/ET-regulated ORA59-branch in modulating rhizobacteria-ISR to Mamestra brassicae by combining gene transcriptional, phytochemical, and herbivore performance assays. Our data show a consistent negative effect of rhizobacteria-mediated ISR on the performance of M. brassicae. Functional JA- and ET-signaling pathways are required for this effect, as shown by investigating the knock-out mutants dde2-2 and ein2-1. Additionally, whereas herbivory mainly induces the MYC2-branch, rhizobacterial colonization alone or in combination with herbivore infestation induces the ORA59-branch of the JA signaling pathway. Rhizobacterial colonization enhances the synthesis of camalexin and aliphatic glucosinolates (GLS) compared to the control, while it suppresses the herbivore-induced levels of indole GLS. These changes are associated with modulation of the JA-/ET-signaling pathways. Our data show that the colonization of plant roots by rhizobacteria modulates plant-insect interactions by prioritizing the JA/ET-regulated ORA59-branch over the JA-regulated MYC2-branch. This study elucidates how microbial plant symbionts can modulate the plant immune system to mount an effective defense response against herbivorous plant attackers.

Topics: Animals; Arabidopsis; Arabidopsis Proteins; Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; Cyclopentanes; Ethylenes; Gene Expression Regulation, Plant; Glucosinolates; Herbivory; Indoles; Lepidoptera; Oxylipins; Plant Growth Regulators; Plant Roots; Pseudomonas; Rhizobium; Signal Transduction; Symbiosis; Thiazoles; Transcription Factors

Shade-intolerant plants respond to low red : far-red (R : FR) ratios, which signal the proximity of potential competitors, by down-regulating immune responses. Here we investigated the mechanisms underlying this immune suppression in Arabidopsis. We used genetic, transcriptomic and metabolomic approaches to examine the functional connections between R : FR ratio and Arabidopsis resistance to the fungus Botrytis cinerea. Low R : FR ratios reduced the concentration of indol-3-ylmethyl glucosinolate (I3M) (an indolic glucosinolate, iGS) and camalexin in plants inoculated with B. cinerea, and attenuated the I3M response triggered by jasmonate elicitation. These effects on metabolite abundance correlated with reduced expression of iGS and camalexin biosynthetic genes. Furthermore, the effect of low R : FR increasing Arabidopsis susceptibility to B. cinerea was not present in mutants deficient in the biosynthesis of camalexin (pad3) or metabolism of iGS (pen2). Finally, in a mutant deficient in the JASMONATE ZIM DOMAIN-10 (JAZ10) protein, which does not respond to low R : FR with increased susceptibility to B. cinerea, supplemental FR failed to down-regulate iGS production. These results indicate that suppression of Arabidopsis immunity against B. cinerea by low R : FR ratios is mediated by reduced levels of Trp-derived defenses, and provide further evidence for a functional role of JAZ10 in the link between phytochrome and jasmonate signaling.

Topics: Arabidopsis; Arabidopsis Proteins; Biosynthetic Pathways; Botrytis; Cyclopentanes; Disease Susceptibility; Gene Expression Regulation, Plant; Glucosinolates; Indoleacetic Acids; Indoles; Mutation; Nuclear Proteins; Oxylipins; Phytochrome; Plant Diseases; Plant Immunity; Plant Leaves; Signal Transduction; Thiazoles

· Small RNAs play important roles in resistance to plant viruses and the complex responses against pathogens and leaf-chewing insects. · We investigated whether small RNA pathways are involved in Arabidopsis resistance against a phloem-feeding insect, the green peach aphid (Myzus persicae). We used a 2-wk fecundity assay to assess aphid performance on Arabidopsis RNA silencing and defence pathway mutants. Quantitative real-time polymerase chain reaction was used to monitor the transcriptional activity of defence-related genes in plants of varying aphid susceptibility. High-performance liquid chromatography-mass spectrometry was employed to measure the accumulation of the antimicrobial compound camalexin. Artificial diet assays allowed the assessment of the effect of camalexin on aphid performance. · Myzus persicae produces significantly less progeny on Arabidopsis microRNA (miRNA) pathway mutants. Plants unable to process miRNAs respond to aphid infestation with increased induction of PHYTOALEXIN DEFICIENT3 (PAD3) and production of camalexin. Aphids ingest camalexin when feeding on Arabidopsis and are more successful on pad3 and cyp79b2/cyp79b3 mutants defective in camalexin production. Aphids produce less progeny on artificial diets containing camalexin. · Our data indicate that camalexin functions beyond antimicrobial defence to also include hemipteran insects. This work also highlights the extensive role of the miRNA-mediated regulation of secondary metabolic defence pathways with relevance to resistance against a hemipteran pest.

Topics: Animals; Aphids; Arabidopsis; Arabidopsis Proteins; Cyclopentanes; Disease Resistance; Ethylenes; Feeding Behavior; Fertility; Gene Expression Regulation, Plant; Indoles; MicroRNAs; Mutation; Oxylipins; Phloem; Plant Diseases; Prunus; Reproduction; Signal Transduction; Survival Analysis; Thiazoles; Up-Regulation

Simultaneous mutation of two WRKY-type transcription factors, WRKY18 and WRKY40, renders otherwise susceptible wild-type Arabidopsis plants resistant towards the biotrophic powdery mildew fungus Golovinomyces orontii. Resistance in wrky18 wrky40 double mutant plants is accompanied by massive transcriptional reprogramming, imbalance in salicylic acid (SA) and jasmonic acid (JA) signaling, altered ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) expression, and accumulation of the phytoalexin camalexin. Genetic analyses identified SA biosynthesis and EDS1 signaling as well as biosynthesis of the indole-glucosinolate 4MI3G as essential components required for loss-of-WRKY18 WRKY40-mediated resistance towards G. orontii. The analysis of wrky18 wrky40 pad3 mutant plants impaired in camalexin biosynthesis revealed an uncoupling of pre- from postinvasive resistance against G. orontii. Comprehensive infection studies demonstrated the specificity of wrky18 wrky40-mediated G. orontii resistance. Interestingly, WRKY18 and WRKY40 act as positive regulators in effector-triggered immunity, as the wrky18 wrky40 double mutant was found to be strongly susceptible towards the bacterial pathogen Pseudomonas syringae DC3000 expressing the effector AvrRPS4 but not against other tested Pseudomonas strains. We hypothesize that G. orontii depends on the function of WRKY18 and WRKY40 to successfully infect Arabidopsis wild-type plants while, in the interaction with P. syringae AvrRPS4, they are required to mediate effector-triggered immunity.

Topics: Arabidopsis; Arabidopsis Proteins; Ascomycota; Botrytis; Cyclopentanes; Disease Resistance; DNA-Binding Proteins; Gene Expression Regulation, Plant; Glucosinolates; Indoles; Mutation; Oomycetes; Oxylipins; Plant Diseases; Plant Growth Regulators; Plant Leaves; Plants, Genetically Modified; Pseudomonas syringae; Salicylic Acid; Signal Transduction; Thiazoles; Transcription Factors

Phytophthora capsici causes devastating diseases on a broad range of plant species. To better understand the interaction with its host plants, knowledge obtained from a model pathosystem can be instrumental. Here, we describe the interaction between P. capsici and Arabidopsis and the exploitation of this novel pathosystem to assign metabolic pathways involved in defence against P. capsici. Inoculation assays on Arabidopsis accessions with different P. capsici isolates revealed interaction specificity among accession-isolate combinations. In a compatible interaction, appressorium-mediated penetration was followed by the formation of invasive hyphae, haustoria and sporangia in leaves and roots. In contrast, in an incompatible interaction, P. capsici infection elicited callose deposition, accumulation of active oxygen species and cell death, resulting in early pathogen encasement in leaves. Moreover, Arabidopsis mutants with defects in salicylic acid signalling, camalexin or indole glucosinolates biosynthesis pathways displayed severely compromised resistance to P. capsici. It is anticipated that this model pathosystem will facilitate the genetic dissection of complex traits responsible for resistance against P. capsici.

Topics: Arabidopsis; Cyclopentanes; Ethylenes; Glucosinolates; Host-Pathogen Interactions; Indoles; Oxylipins; Phenotype; Phytophthora; Plant Diseases; Salicylic Acid; Thiazoles

Auxin is a pivotal plant hormone that regulates many aspects of plant growth and development. Auxin signaling is also known to promote plant disease caused by plant pathogens. However, the mechanism by which this hormone confers susceptibility to pathogens is not well understood. Here, we present evidence that fungal and bacterial plant pathogens hijack the host auxin metabolism in Arabidopsis thaliana, leading to the accumulation of a conjugated form of the hormone, indole-3-acetic acid (IAA)-Asp, to promote disease development. We also show that IAA-Asp increases pathogen progression in the plant by regulating the transcription of virulence genes. These data highlight a novel mechanism to promote plant susceptibility to pathogens through auxin conjugation.

Topics: Arabidopsis; Arabidopsis Proteins; Aspartic Acid; Botrytis; Cyclopentanes; Gene Expression Regulation, Plant; Host-Pathogen Interactions; Indoleacetic Acids; Indoles; Oxylipins; Plant Diseases; Plant Growth Regulators; Pseudomonas syringae; Salicylic Acid; Signal Transduction; Thiazoles; Virulence

The Arabidopsis (Arabidopsis thaliana) transcription factor WRKY33 is essential for defense toward the necrotrophic fungus Botrytis cinerea. Here, we aimed at identifying early transcriptional responses mediated by WRKY33. Global expression profiling on susceptible wrky33 and resistant wild-type plants uncovered massive differential transcriptional reprogramming upon B. cinerea infection. Subsequent detailed kinetic analyses revealed that loss of WRKY33 function results in inappropriate activation of the salicylic acid (SA)-related host response and elevated SA levels post infection and in the down-regulation of jasmonic acid (JA)-associated responses at later stages. This down-regulation appears to involve direct activation of several jasmonate ZIM-domain genes, encoding repressors of the JA-response pathway, by loss of WRKY33 function and by additional SA-dependent WRKY factors. Moreover, genes involved in redox homeostasis, SA signaling, ethylene-JA-mediated cross-communication, and camalexin biosynthesis were identified as direct targets of WRKY33. Genetic studies indicate that although SA-mediated repression of the JA pathway may contribute to the susceptibility of wrky33 plants to B. cinerea, it is insufficient for WRKY33-mediated resistance. Thus, WRKY33 apparently directly targets other still unidentified components that are also critical for establishing full resistance toward this necrotroph.

Topics: Agrobacterium tumefaciens; Arabidopsis; Arabidopsis Proteins; Botrytis; Cloning, Molecular; Cyclopentanes; Disease Resistance; Gene Expression Profiling; Gene Expression Regulation, Plant; Genes, Plant; Indoles; Oxidation-Reduction; Oxylipins; Plant Diseases; Plant Growth Regulators; Plant Leaves; Promoter Regions, Genetic; Salicylic Acid; Signal Transduction; Thiazoles; Transcription Factors; Transcription, Genetic; Transformation, Genetic

Phosphite (Phi), a phloem-mobile oxyanion of phosphorous acid (H(3)PO(3)), protects plants against diseases caused by oomycetes. Its mode of action is unclear, as evidence indicates both direct antibiotic effects on pathogens as well as inhibition through enhanced plant defense responses, and its target(s) in the plants is unknown. Here, we demonstrate that the biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) exhibits an unusual biphasic dose-dependent response to Phi after inoculation of Arabidopsis (Arabidopsis thaliana), with characteristics of indirect activity at low doses (10 mm or less) and direct inhibition at high doses (50 mm or greater). The effect of low doses of Phi on Hpa infection was nullified in salicylic acid (SA)-defective plants (sid2-1, NahG) and in a mutant impaired in SA signaling (npr1-1). Compromised jasmonate (jar1-1) and ethylene (ein2-1) signaling or abscisic acid (aba1-5) biosynthesis, reactive oxygen generation (atrbohD), or accumulation of the phytoalexins camalexin (pad3-1) and scopoletin (f6'h1-1) did not affect Phi activity. Low doses of Phi primed the accumulation of SA and Pathogenesis-Related protein1 transcripts and mobilized two essential components of basal resistance, Enhanced Disease Susceptibility1 and Phytoalexin Deficient4, following pathogen challenge. Compared with inoculated, Phi-untreated plants, the gene expression, accumulation, and phosphorylation of the mitogen-activated protein kinase MPK4, a negative regulator of SA-dependent defenses, were reduced in plants treated with low doses of Phi. We propose that Phi negatively regulates MPK4, thus priming SA-dependent defense responses following Hpa infection.

Topics: Abscisic Acid; Arabidopsis; Arabidopsis Proteins; Cyclopentanes; Disease Resistance; DNA-Binding Proteins; Dose-Response Relationship, Drug; Ethylenes; Gene Expression Regulation, Plant; Indoles; Mitogen-Activated Protein Kinases; Oomycetes; Oxylipins; Phosphites; Phosphorylation; Plant Diseases; Plant Immunity; Salicylic Acid; Scopoletin; Signal Transduction; Thiazoles

Light is emerging as a central regulator of plant immune responses against herbivores and pathogens. Solar UV-B radiation plays an important role as a positive modulator of plant defense. However, since UV-B photons can interact with a wide spectrum of molecular targets in plant tissues, the mechanisms that mediate their effects on plant defense have remained elusive. Here, we show that ecologically meaningful doses of UV-B radiation increase Arabidopsis resistance to the necrotrophic fungus Botrytis cinerea and that this effect is mediated by the photoreceptor UVR8. The UV-B effect on plant resistance was conserved in mutants impaired in jasmonate (JA) signaling (jar1-1 and P35S:JAZ10.4) or metabolism of tryptophan-derived defense compounds (pen2-1, pad3-1, pen2 pad3), suggesting that neither regulation of the JA pathway nor changes in levels of indolic glucosinolates (iGS) or camalexin are involved in this response. UV-B radiation, acting through UVR8, increased the levels of flavonoids and sinapates in leaf tissue. The UV-B effect on pathogen resistance was still detectable in tt4-1, a mutant deficient in chalcone synthase and therefore impaired in the synthesis of flavonoids, but was absent in fah1-7, a mutant deficient in ferulic acid 5-hydroxylase, which is essential for sinapate biosynthesis. Collectively, these results indicate that UVR8 plays an important role in mediating the effects of UV-B radiation on pathogen resistance by controlling the expression of the sinapate biosynthetic pathway.

Topics: Arabidopsis; Arabidopsis Proteins; Botrytis; Chromosomal Proteins, Non-Histone; Coumaric Acids; Cyclopentanes; Disease Resistance; Gene Expression Regulation, Plant; Glucosinolates; Indoles; Mutation; Oxylipins; Phenols; Plant Diseases; Signal Transduction; Thiazoles; Ultraviolet Rays

Fusarium oxysporum is a root-infecting fungal pathogen that causes wilt disease on a broad range of plant species, including the model plant Arabidopsis thaliana. Currently, very little is known about the molecular or physiological processes that are activated in the host during infection and the roles these processes play in resistance and susceptibility to F. oxysporum. In this study, we analyzed global gene expression profiles of F. oxysporum-infected Arabidopsis plants. Genes involved in jasmonate biosynthesis as well as jasmonate-dependent defense were coordinately induced by F. oxysporum. Similarly, tryptophan pathway genes, including those involved in both indole-glucosinolate and auxin biosynthesis, were upregulated in both the leaves and the roots of inoculated plants. Analysis of plants expressing the DR5:GUS construct suggested that root auxin homeostasis was altered during F. oxysporum infection. However, Arabidopsis mutants with altered auxin and tryptophan-derived metabolites such as indole-glucosinolates and camalexin did not show an altered resistance to this pathogen. In contrast, several auxin-signaling mutants were more resistant to F. oxysporum. Chemical or genetic alteration of polar auxin transport also conferred increased pathogen resistance. Our results suggest that, similarly to many other pathogenic and nonpathogenic or beneficial soil organisms, F. oxysporum requires components of auxin signaling and transport to colonize the plant more effectively. Potential mechanisms of auxin signaling and transport-mediated F. oxysporum susceptibility are discussed.

Topics: Arabidopsis; Arabidopsis Proteins; Biological Transport; Cyclopentanes; Fusarium; Gene Expression Regulation, Plant; Indoleacetic Acids; Indoles; Mutation; Oxylipins; Plant Diseases; Plant Roots; Salicylic Acid; Signal Transduction; Thiazoles

Jasmonic acid and its derived metabolites (JAs) orchestrate plant defense against insects and fungi. 12-Oxo-phytodienoic acid (OPDA), a JA precursor, has also been implicated in plant defense. We sought to define JAs and OPDA functions through comparative defense susceptibility characteristics of three Arabidopsis (Arabidopsis thaliana) genotypes: aos, lacking JAs and OPDA; opda reductase3 (opr3), deficient in JA production but can accumulate OPDA; and transgenics that overexpress OPR3. opr3, like aos, is susceptible to cabbage loopers (Trichoplusia ni) but, relative to aos, opr3 has enhanced resistance to a necrotrophic fungus. Gas chromatography-mass spectrometry reveals that opr3 produces OPDA but no detectable JAs following wounding and looper infestation; unexpectedly, substantial levels of JAs accumulate in opr3 upon fungal infection. Full-length OPR3 transcripts accumulate in fungal-infected opr3, potentially through splicing of the T-DNA containing intron. Fungal resistance correlates with levels of JAs not OPDA; therefore, opr3 resistance to some pests is likely due to JA accumulation, and signaling activities ascribed to OPDA should be reassessed because opr3 can produce JAs. Together these data (1) reinforce the primary role JAs play in plant defense against insects and necrotrophic fungi, (2) argue for a reassessment of signaling activities ascribed to OPDA, and (3) provide evidence that mutants with intron insertions can retain gene function.

Topics: Animals; Arabidopsis; Arabidopsis Proteins; Botrytis; Brassica; Cyclopentanes; DNA, Bacterial; Fatty Acids, Unsaturated; Fertility; Gene Expression Regulation, Plant; Immunity, Innate; Indoles; Introns; Molecular Sequence Data; Moths; Mutagenesis, Insertional; Mutation; Oxidoreductases; Oxylipins; Plant Diseases; Thiazoles

Filamentous fungi belonging to the genus Trichoderma have long been recognized as agents for the biocontrol of plant diseases. In this work, we investigated the mechanisms involved in the defense responses of Arabidopsis thaliana seedlings elicited by co-culture with Trichoderma virens and Trichoderma atroviride. Interaction of plant roots with fungal mycelium induced growth and defense responses, indicating that both processes are not inherently antagonist. Expression studies of the pathogenesis-related reporter markers pPr1a:uidA and pLox2:uidA in response to T. virens or T. atroviride provided evidence that the defense signaling pathway activated by these fungi involves salicylic acid (SA) and/or jasmonic acid (JA) depending on the amount of conidia inoculated. Moreover, we found that Arabidopsis seedlings colonized by Trichoderma accumulated hydrogen peroxide and camalexin in leaves. When grown under axenic conditions, T. virens produced indole-3-carboxaldehyde (ICAld) a tryptophan-derived compound with activity in plant development. In Arabidopsis seedlings whose roots are in contact with T. virens or T. atroviride, and challenged with Botrytis cinerea in leaves, disease severity was significantly reduced compared to axenically grown seedlings. Our results indicate that the defense responses elicited by Trichoderma in Arabidopsis are complex and involve the canonical defense hormones SA and JA as well as camalexin, which may be important factors in boosting plant immunity.

Topics: Arabidopsis; Biomass; Botrytis; Cyclopentanes; Disease Resistance; Gas Chromatography-Mass Spectrometry; Gene Expression Regulation, Plant; Hydrogen Peroxide; Indoles; Oxylipins; Plant Diseases; Plant Growth Regulators; Plant Immunity; Plant Leaves; Plant Roots; Salicylic Acid; Seedlings; Thiazoles; Trichoderma

Despite the fact that roots are the organs most subject to microbial interactions, very little is known about the response of roots to microbe-associated molecular patterns (MAMPs). By monitoring transcriptional activation of beta-glucuronidase reporters and MAMP-elicited callose deposition, we show that three MAMPs, the flagellar peptide Flg22, peptidoglycan, and chitin, trigger a strong tissue-specific response in Arabidopsis thaliana roots, either at the elongation zone for Flg22 and peptidoglycan or in the mature parts of the roots for chitin. Ethylene signaling, the 4-methoxy-indole-3-ylmethylglucosinolate biosynthetic pathway, and the PEN2 myrosinase, but not salicylic acid or jasmonic acid signaling, play major roles in this MAMP response. We also show that Flg22 induces the cytochrome P450 CYP71A12-dependent exudation of the phytoalexin camalexin by Arabidopsis roots. The phytotoxin coronatine, an Ile-jasmonic acid mimic produced by Pseudomonas syringae pathovars, suppresses MAMP-activated responses in the roots. This suppression requires the E3 ubiquitin ligase COI1 as well as the transcription factor JIN1/MYC2 but does not rely on salicylic acid-jasmonic acid antagonism. These experiments demonstrate the presence of highly orchestrated and tissue-specific MAMP responses in roots and potential pathogen-encoded mechanisms to block these MAMP-elicited signaling pathways.

Topics: Arabidopsis; Arabidopsis Proteins; Chitin; Cyclopentanes; Cytochrome P-450 Enzyme System; Ethylenes; Flagella; Glucans; Host-Pathogen Interactions; Indoles; N-Glycosyl Hydrolases; Oxylipins; Peptidoglycan; Plant Roots; Plants, Genetically Modified; Pseudomonas; RNA, Plant; Salicylic Acid; Signal Transduction; Thiazoles

Despite the described central role of jasmonate signaling in plant defense against necrotrophic pathogens, the existence of intraspecific variation in pathogen capacity to activate or evade plant jasmonate-mediated defenses is rarely considered. Experimental infection of jasmonate-deficient and jasmonate-insensitive Arabidopsis thaliana with diverse isolates of the necrotrophic fungal pathogen Botrytis cinerea revealed pathogen variation for virulence inhibition by jasmonate-mediated plant defenses and induction of plant defense metabolites. Comparison of the transcriptional effects of infection by two distinct B. cinerea isolates showed only minor differences in transcriptional responses of wild-type plants, but notable isolate-specific transcript differences in jasmonate-insensitive plants. These transcriptional differences suggest B. cinerea activation of plant defenses that require plant jasmonate signaling for activity in response to only one of the two B. cinerea isolates tested. Thus, similar infection phenotypes observed in wild-type plants result from different signaling interactions with the plant that are likely integrated by jasmonate signaling.

Topics: Arabidopsis; Arabidopsis Proteins; Botrytis; Cyclopentanes; Gene Expression; Gene Expression Profiling; Gene Expression Regulation, Plant; Host-Parasite Interactions; Indoles; Mycoses; Oxylipins; Phenotype; Plant Diseases; Reverse Transcriptase Polymerase Chain Reaction; Signal Transduction; Thiazoles

As sessile organisms, plants have to endure a wide variety of biotic and abiotic stresses, and accordingly they have evolved intricate and rapidly inducible defense strategies associated with the activation of a battery of genes. Among other mechanisms, changes in chromatin structure are thought to provide a flexible, global, and stable means for the regulation of gene transcription. In support of this idea, we demonstrate here that the Arabidopsis (Arabidopsis thaliana) histone methyltransferase SET DOMAIN GROUP8 (SDG8) plays a crucial role in plant defense against fungal pathogens by regulating a subset of genes within the jasmonic acid (JA) and/or ethylene signaling pathway. We show that the loss-of-function mutant sdg8-1 displays reduced resistance to the necrotrophic fungal pathogens Alternaria brassicicola and Botrytis cinerea. While levels of JA, a primary phytohormone involved in plant defense, and camalexin, a major phytoalexin against fungal pathogens, remain unchanged or even above normal in sdg8-1, induction of several defense genes within the JA/ethylene signaling pathway is severely compromised in response to fungal infection or JA treatment in mutant plants. Both downstream genes and, remarkably, also upstream mitogen-activated protein kinase kinase genes MKK3 and MKK5 are misregulated in sdg8-1. Accordingly, chromatin immunoprecipitation analysis shows that sdg8-1 impairs dynamic changes of histone H3 lysine 36 methylation at defense marker genes as well as at MKK3 and MKK5, which normally occurs upon infection with fungal pathogens or methyl JA treatment in wild-type plants. Our data indicate that SDG8-mediated histone H3 lysine 36 methylation may serve as a memory of permissive transcription for a subset of defense genes, allowing rapid establishment of transcriptional induction.

Topics: Alternaria; Arabidopsis; Arabidopsis Proteins; Botrytis; Cyclopentanes; Ethylenes; Gene Expression Regulation, Plant; Histone Methyltransferases; Histone-Lysine N-Methyltransferase; Histones; Indoles; Methylation; Oxylipins; Plant Diseases; Plant Growth Regulators; Plant Immunity; Promoter Regions, Genetic; RNA, Plant; Thiazoles

The two closely related Arabidopsis transcription factors, WRKY18 and WRKY40, play a major and partly redundant role in PAMP-triggered basal defense. We monitored the transcriptional reprogramming induced by the powdery mildew fungus, Golovinomyces orontii, during early stages of infection with respect to the role of WRKY18/40. Expression of >1300 Arabidopsis genes was differentially altered already 8 hours post infection (hpi), indicating rapid pre-penetration signaling between the pathogen and the host. We found that WRKY18/40 negatively affects pre-invasion host defenses and deduced a subset of genes that appear to be under WRKY18/40 control. A mutant lacking the WRKY18/40 repressors executes pathogen-dependent but exaggerated expression of some defense genes leading, for example, to strongly elevated levels of camalexin. This implies that WRKY18/40 act in a feedback repression system controlling basal defense. Moreover, using chromatin immunoprecipitation (ChIP), direct in vivo interactions of WRKY40 to promoter regions containing W box elements of the regulatory gene EDS1, the AP2-type transcription factor gene RRTF1 and to JAZ8, a member of the JA-signaling repressor gene family were demonstrated. Our data support a model in which WRKY18/40 negatively modulate the expression of positive regulators of defense such as CYP71A13, EDS1 and PAD4, but positively modulate the expression of some key JA-signaling genes by partly suppressing the expression of JAZ repressors.

Topics: Arabidopsis; Arabidopsis Proteins; Ascomycota; Cyclopentanes; DNA-Binding Proteins; Gene Expression Profiling; Gene Expression Regulation, Plant; Host-Pathogen Interactions; Indoles; Mutation; Oxylipins; Plant Diseases; Promoter Regions, Genetic; Signal Transduction; Thiazoles; Transcription Factors

Pathogen infection leads to the activation of defense signaling networks in plants. To study these networks and the relationships between their components, we introduced various defense mutations into acd6-1, a constitutive gain-of-function Arabidopsis mutant that is highly disease resistant. acd6-1 plants show spontaneous cell death, reduced stature, and accumulate high levels of camalexin (an anti-fungal compound) and salicylic acid (SA; a signaling molecule). Disruption of several defense genes revealed that in acd6-1, SA levels/signaling were positively correlated with the degree of disease resistance and defense gene expression. Salicylic acid also modulates the severity of cell death. However, accumulation of camalexin in acd6-1 is largely unaffected by reducing the level of SA. In addition, acd6-1 shows ethylene- and jasmonic acid-mediated signaling that is antagonized and therefore masked by the presence of SA. Mutant analysis revealed a new relationship between the signaling components NPR1 and PAD4 and also indicated that multiple defense pathways were required for phenotypes conferred by acd6-1. In addition, our data confirmed that the size of acd6-1 was inversely correlated with SA levels/signaling. We exploited this unique feature of acd6-1 to identify two genes disrupted in acd6-1 suppressor (sup) mutants: one encodes a known SA biosynthetic component (SID2) and the other encodes an uncharacterized putative metalloprotease (At5g20660). Taken together, acd6-1 is a powerful tool not only for dissecting defense regulatory networks but also for discovering novel defense genes.

Topics: Ankyrins; Arabidopsis; Arabidopsis Proteins; Cyclopentanes; Ethylenes; Genes, Plant; Immunity, Innate; Indoles; Intramolecular Transferases; Mutagenesis, Insertional; Mutation; Oxylipins; Salicylic Acid; Signal Transduction; Thiazoles

Plants have evolved different defense components to counteract pathogen attacks. The resistance locus resistance to Leptosphaeria maculans 1 (RLM1) is a key factor for Arabidopsis thaliana resistance to L. maculans. The present work aimed to reveal downstream defense responses regulated by RLM1. Quantitative assessment of fungal colonization in the host was carried out using quantitative polymerase chain reaction (qPCR) and GUS expression analyses, to further characterize RLM1 resistance and the role of salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) in disease development. Additional assessments of A. thaliana mutants were performed to expand our understanding of this pathosystem. Resistance responses such as lignification and the formation of vascular plugs were found to occur in an RLM1-dependent manner, in contrast to the RLM1-independent increase in reactive oxygen species at the stomata and hydathodes. Analyses of mutants defective in hormone signaling in the camalexin-free rlm1(Ler)pad3 background revealed a significant influence of JA and ET on symptom development and pathogen colonization. The overall results indicate that the defense responses of primary importance induced by RLM1 are all associated with physical barriers, and that responses of secondary importance involve complex cross-talk among SA, JA and ET. Our observations further suggest that ET positively affects fungal colonization.

Topics: Arabidopsis; Arabidopsis Proteins; Cyclopentanes; Cytochrome P-450 Enzyme System; Ethylenes; Fungi; Gene Expression Regulation, Plant; Genes, Plant; Host-Pathogen Interactions; Indoles; Lignin; Oxylipins; Plant Diseases; Plant Growth Regulators; Salicylic Acid; Signal Transduction; Thiazoles; Virulence Factors

Many inducible plant-defense responses are activated by jasmonates (JAs), C(6)-aldehydes, and their corresponding derivatives, produced by the two main competing branches of the oxylipin pathway, the allene oxide synthase (AOS) and hydroperoxide lyase (HPL) branches, respectively. In addition to competition for substrates, these branch-pathway-derived metabolites have substantial overlap in regulation of gene expression. Past experiments to define the role of C(6)-aldehydes in plant defense responses were biased towards the exogenous application of the synthetic metabolites or the use of genetic manipulation of HPL expression levels in plant genotypes with intact ability to produce the competing AOS-derived metabolites. To uncouple the roles of the C(6)-aldehydes and jasmonates in mediating direct and indirect plant-defense responses, we generated Arabidopsis genotypes lacking either one or both of these metabolites. These genotypes were subsequently challenged with a phloem-feeding insect (aphids: Myzus persicae), an insect herbivore (leafminers: Liriomyza trifolii), and two different necrotrophic fungal pathogens (Botrytis cinerea and Alternaria brassicicola). We also characterized the volatiles emitted by these plants upon aphid infestation or mechanical wounding and identified hexenyl acetate as the predominant compound in these volatile blends. Subsequently, we examined the signaling role of this compound in attracting the parasitoid wasp (Aphidius colemani), a natural enemy of aphids.. This study conclusively establishes that jasmonates and C(6)-aldehydes play distinct roles in plant defense responses. The jasmonates are indispensable metabolites in mediating the activation of direct plant-defense responses, whereas the C(6)-aldehyes are not. On the other hand, hexenyl acetate, an acetylated C(6)-aldehyde, is the predominant wound-inducible volatile signal that mediates indirect defense responses by directing tritrophic (plant-herbivore-natural enemy) interactions.. The data suggest that jasmonates and hexenyl acetate play distinct roles in mediating direct and indirect plant-defense responses. The potential advantage of this "division of labor" is to ensure the most effective defense strategy that minimizes incurred damages at a reduced metabolic cost.

Topics: Aldehyde-Lyases; Aldehydes; Animals; Aphids; Arabidopsis; Cyclopentanes; Cytochrome P-450 Enzyme System; Gene Expression Regulation, Plant; Genotype; Indoles; Intramolecular Oxidoreductases; Models, Biological; Oxylipins; Plant Diseases; Plants; Signal Transduction; Species Specificity; Thiazoles

Insect feeding on plants causes a complex series of coordinated defence responses. Little is known, however, about the time-dependent aspect of induced changes. Here we present a time series-based investigation of Arabidopsis thaliana Ler subjected to attack by a specialist pest of Brassicaceae species, Brevicoryne brassicae. Transcriptome and metabolome changes were studied at 6, 12, 24 and 48 h after infestation to monitor the progress of early induced responses. The use of full-genome oligonucleotide microarrays revealed the initiation of extensive gene expression changes already during the first 6 h of infestation. Data indicated the involvement of reactive oxygen species (ROS) and calcium in early signalling, and salicylic acid (SA) and jasmonic acid (JA) in the regulation of defence responses. Transcripts related to senescence, biosynthesis of anti-insect proteins, indolyl glucosinolates (GS) and camalexin, as well as several uncharacterized to date WRKY transcription factors, were induced. Follow-up studies of defence-involved secondary metabolites revealed depositions of callose at the insects' feeding sites, a decrease in the total level of aliphatic GS, particularly 3-hydroxypropyl glucosinolate, and accumulation of 4-methoxyindol-3-ylmethyl glucosinolate 48 h after the attack. The novel role of camalexin, induced as a part of defence against aphids, was verified in fitness experiments. Fecundity of B. brassicae was reduced on camalexin-accumulating wild-type (WT) plants as compared with camalexin-deficient pad3-1 mutants. Based on experimental data, a model of plant-aphid interactions at the early phase of infestation was proposed.

Topics: Animals; Aphids; Arabidopsis; Arabidopsis Proteins; Brassica; Calcium Signaling; Cell Wall; Cyclopentanes; Ethylenes; Fertility; Gene Expression Regulation, Plant; Genes, Plant; Glucosinolates; Hydrogen Peroxide; Indoles; Models, Biological; Oxidative Stress; Oxylipins; Plant Leaves; Salicylic Acid; Thiazoles; Time Factors; Transcription Factors; Transcription, Genetic

Although interactions of plants with virulent and avirulent host pathogens are under intensive study, relatively little is known about plant interactions with non-adapted pathogens and the molecular events underlying non-host resistance. Here we show that two Pseudomonas syringae strains for which Arabidopsis is a non-host plant, P. syringae pathovar (pv.) glycinea (Psg) and P. syringae pv. phaseolicola (Psp),induce salicylic acid (SA) accumulation and pathogenesis-related gene expression at inoculation sites, and that induction of these defences is largely dependent on bacterial type III secretion. The defence signalling components activated by non-adapted bacteria resemble those initiated by host pathogens, including SA, non-expressor of PR-1, non-race specific disease resistance 1, phytoalexin-deficient 4 and enhanced disease susceptibility 1. However, some differences in individual defence pathways induced by Psg and Psp exist, suggesting that for each strain, distinct sets of type III effectors are recognized by the plant. Although induction of SA-related defences occurs, it does not directly contribute to bacterial non-host resistance, because Arabidopsis mutants compromised in SA signalling and other classical defence pathways do not permit enhanced survival of Psg or Psp in leaves. The finding that numbers of non-adapted bacteria in leaf extracellular spaces rapidly decline after inoculation suggests that they fail to overcome toxic or structural defence barriers preceding SA-related responses. Consistent with this hypothesis, rapid, type III secretion system-independent upregulation of the lignin biosynthesis genes, PAL1 and BCB, which might contribute to an early induced, cell wall-based defence mechanism, occurs in response to non-adapted bacteria. Moreover, knockout of PAL1 permits increased leaf survival of non-host bacteria. In addition, different survival rates of non-adapted bacteria in leaves from Arabidopsis accessions and mutants with distinct glucosinolate composition or hydrolysis exist. Possible roles for early inducible, cell wall-based defences and the glucosinolate/myrosinase system in bacterial non-host resistance are discussed.

Topics: Arabidopsis; Arabidopsis Proteins; Blotting, Northern; Carrier Proteins; Cyclopentanes; Gene Expression Regulation, Plant; Host-Pathogen Interactions; Indoles; Lignin; Oxylipins; Phenylalanine Ammonia-Lyase; Plant Leaves; Plants, Genetically Modified; Pseudomonas syringae; Salicylic Acid; Thiazoles

Cross-talk between signal transduction pathways is a central feature of the tightly regulated plant defense signaling network. The potential synergism or antagonism between defense pathways is determined by recognition of the type of pathogen or pathogen-derived elicitor. Our studies have identified WRKY70 as a node of convergence for integrating salicylic acid (SA)- and jasmonic acid (JA)-mediated signaling events during plant response to bacterial pathogens. Here, we challenged transgenic plants altered in WRKY70 expression as well as WRKY70 knockout mutants of Arabidopsis with the fungal pathogens Alternaria brassicicola and Erysiphe cichoracearum to elucidate the role of WRKY70 in modulating the balance between distinct defense responses. Gain or loss of WRKY70 function causes opposite effects on JA-mediated resistance to A. brassicicola and the SA-mediated resistance to E. cichoracearum. While the up-regulation of WRKY70 caused enhanced resistance to E. cichoracearum, it compromised plant resistance to A. brassicicola. Conversely, down-regulation or insertional inactivation of WRKY70 impaired plant resistance to E. cichoracearum. Over-expression of WRKY70 resulted in the suppression of several JA responses including expression of a subset of JA- and A. brassicicola-responsive genes. We show that this WRKY70-controlled suppression of JA-signaling is partly executed by NPR1. The results indicate that WRKY70 has a pivotal role in determining the balance between SA-dependent and JA-dependent defense pathways.

Topics: Alternaria; Anthocyanins; Arabidopsis; Arabidopsis Proteins; Ascomycota; Cyclopentanes; Gene Expression Regulation, Plant; Glucosinolates; Immunity, Innate; Indoles; Mutation; Oxylipins; Phenotype; Plant Leaves; Plant Roots; Plants, Genetically Modified; Salicylic Acid; Signal Transduction; Thiazoles; Transcription Factors

Upon localized attack by necrotizing pathogens, plants gradually develop increased resistance against subsequent infections at the whole-plant level, a phenomenon known as systemic acquired resistance (SAR). To identify genes involved in the establishment of SAR, we pursued a strategy that combined gene expression information from microarray data with pathological characterization of selected Arabidopsis (Arabidopsis thaliana) T-DNA insertion lines. A gene that is up-regulated in Arabidopsis leaves inoculated with avirulent or virulent strains of the bacterial pathogen Pseudomonas syringae pv maculicola (Psm) showed homology to flavin-dependent monooxygenases (FMO) and was designated as FMO1. An Arabidopsis knockout line of FMO1 proved to be fully impaired in the establishment of SAR triggered by avirulent (Psm avrRpm1) or virulent (Psm) bacteria. Loss of SAR in the fmo1 mutants was accompanied by the inability to initiate systemic accumulation of salicylic acid (SA) and systemic expression of diverse defense-related genes. In contrast, responses at the site of pathogen attack, including increases in the levels of the defense signals SA and jasmonic acid, camalexin accumulation, and expression of various defense genes, were induced in a similar manner in both fmo1 mutant and wild-type plants. Consistently, the fmo1 mutation did not significantly affect local disease resistance toward virulent or avirulent bacteria in naive plants. Induction of FMO1 expression at the site of pathogen inoculation is independent of SA signaling, but attenuated in the Arabidopsis eds1 and pad4 defense mutants. Importantly, FMO1 expression is also systemically induced upon localized P. syringae infection. This systemic up-regulation is missing in the SAR-defective SA pathway mutants sid2 and npr1, as well as in the defense mutant ndr1, indicating a close correlation between systemic FMO1 expression and SAR establishment. Our findings suggest that the presence of the FMO1 gene product in systemic tissue is critical for the development of SAR, possibly by synthesis of a metabolite required for the transduction or amplification of a signal during the early phases of SAR establishment in systemic leaves.

Topics: Arabidopsis; Arabidopsis Proteins; Cyclopentanes; Gene Expression Regulation, Plant; Immunity, Innate; Indoles; Mutation; Oxygenases; Oxylipins; Plant Leaves; Pseudomonas syringae; Salicylic Acid; Signal Transduction; Thiazoles

Cell-wall and glucopeptide components of yeast have been reported to exhibit elicitor activity. The mode of action of defense activation by yeast is not known so far. In this study, we used the model plant Arabidopsis to investigate the activation of defense responses by yeast, the effect on resistance against different pathogens, and the mode of action. Treatment of Arabidopsis plants with an autoclaved yeast suspension induced the expression of systemic acquired resistance-related genes and accumulation of the phytoalexin camalexin. Symptom development and bacterial growth after infection with a virulent strain of the pathogen Pseudomonas syringae was reduced in yeast-pretreated plants. No protection was detectable in mutants affected in the salicylate pathway, while mutants in the jasmonate or camalexin pathway were protected by yeast, indicating that the salicylate pathway is necessary for the yeast-induced resistance against P. syringae. Yeast also reduced symptom development after challenge with Botrytis cinerea. This protection was detectable in all mutants tested, indicating that it is independent of the salicylate, jasmonate, and camalexin pathway.

Topics: Arabidopsis; Botrytis; Cyclopentanes; Immunity, Innate; Indoles; Oxylipins; Plant Leaves; Pseudomonas syringae; Saccharomyces cerevisiae; Salicylic Acid; Signal Transduction; Thiazoles

Tocopherols (vitamin E) are lipophilic antioxidants that are synthesized by all plants and are particularly abundant in seeds. Two tocopherol-deficient mutant loci in Arabidopsis thaliana were used to examine the functions of tocopherols in seedlings: vitamin e1 (vte1), which accumulates the pathway intermediate 2,3-dimethyl-5-phytyl-1,4-benzoquinone (DMPBQ); and vte2, which lacks all tocopherols and pathway intermediates. Only vte2 displayed severe seedling growth defects, which corresponded with massively increased levels of the major classes of nonenzymatic lipid peroxidation products: hydroxy fatty acids, malondialdehyde, and phytoprostanes. In the absence of pathogens, the phytoalexin camalexin accumulated in vte2 seedlings to levels 100-fold higher than in wild-type or vte1 seedlings. Similarly, gene expression profiling in wild-type, vte1, and vte2 seedlings indicated that increased levels of nonenzymatic lipid peroxidation in vte2 corresponded to increased expression of many defense-related genes, which were not induced in vte1. Both biochemical and transcriptional analyses of vte2 seedlings indicate that nonenzymatic lipid peroxidation plays a significant role in modulating plant defense responses. Together, these results establish that tocopherols in wild-type plants or DMPBQ in vte1 plants limit nonenzymatic lipid peroxidation during germination and early seedling development, thereby preventing the inappropriate activation of transcriptional and biochemical defense responses.

Topics: Arabidopsis; Arabidopsis Proteins; Biomarkers; Cyclopentanes; Fatty Acids, Unsaturated; Gene Expression Profiling; Gene Expression Regulation, Plant; Germination; Immunity, Innate; Indoles; Lipid Peroxidation; Malondialdehyde; Mutation; Oxylipins; Plant Diseases; RNA, Messenger; Seedlings; Thiazoles; Tocopherols; Up-Regulation

Out of 168 Arabidopsis accessions screened with isolates of Leptosphaeria maculans, one (An-1) showed clear disease symptoms. In order to identify additional components involved in containment of L. maculans in Arabidopsis, a screen for L. maculans-susceptible (lms) mutants was performed. Eleven lms mutants were isolated, which displayed differential susceptibility responses to L. maculans. lms1 was crossed with Columbia (Col-0) and Ws-0, and mapping data for both populations showed the highest linkage to a region on chromosome 2. Reduced levels of PR-1 and PDF1.2 expression were found in lms1 compared to wild-type plants 48 h after pathogen inoculation. In contrast, the lms1 mutant displayed upregulation of either marker gene upon chemical treatment, possibly as an effect of an altered ethylene (ET) response. To assess the contribution of different defence pathways, genotypes implicated in salicylic acid (SA) signalling plants expressing the bacterial salicylate hydroxylase (nahG) gene, non-expressor of PR1 (npr1)-1 and phytoalexin-deficient (pad4-1), jasmonic acid (JA) signalling (coronatine insensitive (coi)1-16, enhanced disease susceptibility (eds)8-1 and jasmonic acid resistant (jar)1-1) and ET signalling (eds4-1, ethylene insensitive (ein)2, ein3-1 and ethylene resistant (etr)1-1) were screened. All the genotypes screened were as resistant as wild-type plants, demonstrating the dispensability of the pathways in L. maculans resistance. When mutants implicated in cell death responses were assayed, responsive to antagonist 1 (ran1)-1 exhibited a weak susceptible phenotype, whereas accelerated cell death (acd)1-20 showed a rapid lesion development. Camalexin is only partially responsible for L. maculans containment in Arabidopsis, as pad3-1 and enhanced susceptibility to Alternaria (esa)1 clearly showed a susceptible response while wild-type levels of camalexin were present in An-1 and lms1. The data presented point to the existence of multiple defence mechanisms controlling the containment of L. maculans in Arabidopsis.

Topics: Arabidopsis; Ascomycota; Copper Sulfate; Cyclopentanes; Ethylenes; Immunity, Innate; Indoles; Mutation; Oxylipins; Plant Diseases; Plant Growth Regulators; Salicylic Acid; Signal Transduction; Silver Nitrate; Thiazoles

The non-protein amino acid beta-amino-butyric acid (BABA) protects plants against a wide range of pathogens. We have examined the effectiveness and mode of action of BABA on resistance against two necrotrophic pathogens. Treatment of Arabidopsis with BABA induced resistance against Alternaria brassicicola and Plectosphaerella cucumerina to a similar level by jasmonic acid (JA). Conversely, treatment with benzothiadiazole (BTH), a functional analogue of salicylic acid (SA), had no significant effect on the resistance against both pathogens. BABA-induced resistance against A. brassicicola and P. cucumerina was unaffected in the JA-insensitive mutant coi1-1 and the camalexin-deficient mutant pad3-1. Moreover, the expression of BABA-induced resistance was not associated with enhanced accumulation of camalexin or enhanced transcription of the JA-inducible PDF1.2 gene. The expression of BABA-induced resistance against P. cucumerina was unaffected in mutants impaired in ethylene (ET) and SA signalling, but was blocked in the abscisic acid (ABA)-deficient mutant aba1-5, the ABA-insensitive mutant abi4-1 and the callose-deficient mutant pmr4-1. Upon infection by both pathogens, BABA-treated plants showed an earlier and more pronounced accumulation of callose. Treatment with the callose-inhibitor 2-deoxy-D-glucose (2-DDG) reversed the BABA-induced resistance against A. brassicicola. Furthermore, primed callose deposition was absent in BABA-treated abi4-1 and pmr4-1 plants upon infection by P. cucumerina. Although the expression of BABA-induced resistance was not associated with enhanced transcription of the ABA-inducible RAB18 gene, application of ABA mimicked the effect of BABA on the level of callose accumulation and resistance. Hence, BABA-induced resistance against necrotrophic pathogens is based on primed callose accumulation, which is controlled by an ABA-dependent defence pathway.

Topics: Abscisic Acid; Alternaria; Aminobutyrates; Arabidopsis; Cyclopentanes; Genes, Plant; Glucans; Indoles; Mutation; Oxylipins; Phyllachorales; Plant Diseases; Plants, Genetically Modified; Salicylic Acid; Signal Transduction; Thiadiazoles; Thiazoles

When challenged with the crucifer pathogen Colletotrichum higginsianum, Arabidopsis thaliana ecotype Columbia (Col-0) was colonized by the fungus within 2 to 3 days, developing brown necrotic lesions surrounded by a yellow halo. Lesions spread from the inoculation site within 3 to 4 days, and subsequently continued to expand until they covered the entire leaf. Electron microscopy confirmed that C. higginsianum is a hemibiotroph on Arabidopsis, feeding initially on living cells as a biotroph before switching to a necrotrophic mode of growth. A collection of 37 ecotypes of Arabidopsis varied in their responses to infection by C. higginsianum. The ecotype Eil-0 was highly resistant, with symptoms limited to necrotic flecking and with only very limited fungal colonization. Analyses suggested that the hypersensitive response and reactive oxygen species may be important in this defense response. Expression analyses with cDNA microarrays indicated that the defense reaction depends primarily on the jasmonic acid- and ethylene-dependent signaling pathways and, to a lesser extent, on the salicylate-dependent pathway. Crosses between the Eil-0 and Col-0 ecotypes suggested that the resistance in Eil-0 was dominant and was conferred by a single locus, which we named RCH1. RCH1 is the first resistance locus to be identified from Arabidopsis against the hemibiotrophic fungus genus Colletotrichum.

Topics: Arabidopsis; Arabidopsis Proteins; Colletotrichum; Cyclopentanes; Ethylenes; Immunity, Innate; Indoles; Microscopy, Electron; Oligonucleotide Array Sequence Analysis; Oxylipins; Phylogeny; Plant Diseases; Plant Growth Regulators; Plant Leaves; Reactive Oxygen Species; Salicylic Acid; Signal Transduction; Thiazoles

Salicylic acid (SA) is an important regulator of plant defense responses, and a variety of Arabidopsis mutants impaired in resistance against bacterial and fungal pathogens show defects in SA accumulation, perception, or signal transduction. Nevertheless, the role of SA-dependent defense responses against necrotrophic fungi is currently unclear. We determined the susceptibility of a set of previously identified Arabidopsis mutants impaired in defense responses to the necrotrophic fungal pathogen Botrytis cinerea. The rate of development of B. cinerea disease symptoms on primary infected leaves was affected by responses mediated by the genes EIN2, JAR1, EDS4, PAD2, and PAD3, but was largely independent of EDS5, SID2/ICS1, and PAD4. Furthermore, plants expressing a nahG transgene or treated with a phenylalanine ammonia lyase (PAL) inhibitor showed enhanced symptoms, suggesting that SA synthesized via PAL, and not via isochorismate synthase (ICS), mediates lesion development. In addition, the degree of lesion development did not correlate with defensin or PR1 expression, although it was partially dependent upon camalexin accumulation. Although npr1 mutant leaves were normally susceptible to B. cinerea infection, a double ein2 npr1 mutant was significantly more susceptible than ein2 plants, and exogenous application of SA decreased B. cinerea lesion size through an NPR1-dependent mechanism that could be mimicked by the cpr1 mutation. These data indicate that local resistance to B. cinerea requires ethylene-, jasmonate-, and SA-mediated signaling, that the SA affecting this resistance does not require ICS1 and is likely synthesized via PAL, and that camalexin limits lesion development.

Topics: Arabidopsis; Arabidopsis Proteins; Botrytis; Carboxylic Ester Hydrolases; Cyclopentanes; Cytochrome P-450 Enzyme System; Defensins; Ethylenes; Gene Expression Regulation, Plant; Immunity, Innate; Indoles; Intramolecular Transferases; Membrane Transport Proteins; Mixed Function Oxygenases; Mutation; Nucleotidyltransferases; Oxylipins; Phenylalanine Ammonia-Lyase; Plant Diseases; Plant Proteins; Receptors, Cell Surface; Salicylic Acid; Signal Transduction; Thiazoles

Botrytis cinerea is a non-specific necrotrophic pathogen that attacks more than 200 plant species. In contrast to biotrophs, the necrotrophs obtain their nutrients by first killing the host cells. Many studies have shown that infection of plants by necrosis-causing pathogens induces a systemic acquired resistance (SAR), which provides protection against successive infections by a range of pathogenic organisms. We analyzed the role of SAR in B. cinerea infection of Arabidopsis. We show that although B. cinerea induced necrotic lesions and camalexin biosynthesis, it did not induce SAR-mediated protection against virulent strains of Pseudomonas syringae, or against subsequent B. cinerea infections. Induction of SAR with avirulent P. syringae or by chemical treatment with salicylic acid (SA) or benzothiadiazole also failed to inhibit B. cinerea growth, although removal of basal SA accumulation by expression of a bacterial salicylate hydroxylase (NahG) gene or by infiltration of 2-aminoindan-2-phosphonic acid, an inhibitor of phenylpropanoid pathway, increased B. cinerea disease symptoms. In addition, we show that B. cinerea induced expression of genes associated with SAR, general stress and ethylene/jasmonate-mediated defense pathways. Thus, B. cinerea does not induce SAR nor is it affected by SAR, making it a rare example of a necrogenic pathogen that does not cause SAR.

Topics: Arabidopsis; Botrytis; Cyclopentanes; Gene Expression Regulation, Plant; Indoles; Oxylipins; Plant Diseases; Plant Leaves; Plant Proteins; Pseudomonas aeruginosa; Salicylic Acid; Thiazoles

An Arabidopsis thaliana mutant, esa1, that shows enhanced susceptibility to the necrotrophic pathogens Alternaria brassicicola, Botrytis cinerea and Plectosphaerella cucumerina, but has wild-type levels of resistance to the biotrophic pathogens Pseudomonas syringae pv. tomato and Peronospora parasitica. The enhanced susceptibility towards necrotrophic pathogens correlated with a delayed induction of phytoalexin accumulation and delayed induction of the plant defensin gene PDF1.2 upon inoculation with pathogens. Two reactive oxygen generating compounds, paraquat and acifluorfen, were found to cause induction of both phytoalexin accumulation and PDF1.2 expression in wild-type plants, but this induction was almost completely abolished in esa1. This finding suggests that esa1 may somehow be involved in transduction of signals generated by reactive oxygen species.

Topics: Alternaria; Arabidopsis; Cyclopentanes; Defensins; Ethylenes; Gene Expression Regulation, Plant; Immunity, Innate; Indoles; Mutation; Nitrobenzoates; Oxylipins; Paraquat; Phytoalexins; Plant Diseases; Plant Extracts; Plant Proteins; Reactive Oxygen Species; Salicylates; Sesquiterpenes; Terpenes; Thiazoles

Arabidopsis accessions were screened with isolates of Phytophthora porri originally isolated from other crucifer species. The described Arabidopsis-Phytophthora pathosystem shows the characteristics of a facultative biotrophic interaction similar to that seen in agronomically important diseases caused by Phytophthora species. In susceptible accessions, extensive colonization of the host tissue occurred and sexual and asexual spores were formed. In incompatible combinations, the plants reacted with a hypersensitive response (HR) and the formation of papillae at the sites of attempted penetration. Defence pathway mutants such as jar1 (jasmonic acid-insensitive), etr1 (ethylene receptor mutant) and ein2 (ethylene-insensitive) remained resistant towards P. porri. However, pad2, a mutant with reduced production of the phytoalexin camalexin, was hyper-susceptible. The accumulation of salicylic acid (SA) and PR1 protein was strongly reduced in pad2. Surprisingly, this lack of SA accumulation does not appear to be the cause of the hyper-susceptibility because interference with SA signalling in nahG plants or sid2 or npr1 mutants had only a minor effect on resistance. In addition, the functional SA analogue benzothiadiazol (BTH) did not induce resistance in susceptible plants including pad2. Similarly, the complete blockage of camalexin biosynthesis in pad3 did not cause susceptibility. Resistance of Arabidopsis against P. porri appears to depend on unknown defence mechanisms that are under the control of PAD2.

Topics: Arabidopsis; Cyclopentanes; Ethylenes; Genes, Plant; Indoles; Mutation; Oxylipins; Phytophthora; Salicylic Acid; Signal Transduction; Thiazoles

To identify components of the defense response that limit growth of a biotrophic fungal pathogen, we isolated Arabidopsis mutants with enhanced disease susceptibility to Erysiphe orontii. Our initial characterization focused on three mutants, eds14, eds15, and eds16. None of these is considerably more susceptible to a virulent strain of the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm). All three mutants develop a hypersensitive response when infiltrated with Psm expressing the avirulence gene avrRpt2, which activates resistance via the LZ-NBS/LRR resistance protein encoded by RPS2. The growth of Psm(avrRpt2), while somewhat greater in the mutants than in the wild type, is less than growth of the isogenic virulent strain. These results indicate that resistance mediated via LZ-NBS/LRR R genes is functional. Analysis of the growth of avirulent Peronospora parasitica strains showed that the resistance pathway utilized by TIR-NBS/LRR R genes is also operative in all three mutants. Surprisingly, only eds14 and eds16 were more susceptible to Erysiphe cichoracearum. Analysis of the expression profiles of PR-1, BGL2, PR-5 and PDF1.2 in eds14, eds15, and eds16 revealed differences from the wild type for all the lines. In contrast, these mutants were not significantly different from wild type in the deposition of callose at sites of E. orontii penetration. All three mutants have reduced levels of salicylic acid after infection. eds16 was mapped to the lower arm of chromosome I and found by complementation tests to be allelic to the salicylic acid-deficient mutant sid2.

Topics: Alleles; Arabidopsis; Ascomycota; Chromosome Mapping; Chromosome Segregation; Cyclopentanes; Ethylenes; Genes, Plant; Genetic Complementation Test; Genetic Predisposition to Disease; Glucans; Indoles; Mutation; Oxylipins; Phenotype; Plant Diseases; Plant Leaves; Salicylic Acid; Signal Transduction; Thiazoles

The phytoalexin-deficient Arabidopsis mutant pad3-1, which is affected in the production of the indole-type phytoalexin camalexin, has previously been shown not to display altered susceptibility to either the bacterium Pseudomonas syringae (Glazebrook & Ausubel 1994; Proc. Natl. Acad. Sci. USA, 91: 8955-8959) or the biotrophic fungi Peronospora parasitica (Glazebrook et al. 1997; Genetics, 146: 381-392) and Erysiphe orontii (Reuber et al. 1998; Plant J. 16: 473-485). We now show that this mutant is markedly more susceptible than its wild-type parental line to infection by the necrotrophic fungus Alternaria brassicicola, but not to Botrytis cinerea. A strong camalexin response was elicited in wild-type plants inoculated with either Alternaria brassicicola or Botrytis cinerea, whereas no camalexin could be detected in pad3-1 challenged with these fungi. Hence, PAD3 appears to be a key determinant in resistance to at least A. brassicicola. The induction of salicylate-dependent and jasmonate/ethylene-dependent defense genes was not reduced in Alternaria-challenged pad3-1 plants compared to similarly treated wild-type plants. Camalexin production could not be triggered by exogenous application of either salicylate, ethylene or jasmonate and was not, or not strongly, reduced in mutants with defects in perception of these defense-related signal molecules. Camalexin-production appears to be controlled by a pathway that exhibits little cross-talk with salicylate-, ethylene- and jasmonate-dependent signalling events.

Topics: Alternaria; Anti-Infective Agents; Antifungal Agents; Arabidopsis; Botrytis; Cyclopentanes; Defensins; Disease Susceptibility; Ethylenes; Gene Expression Regulation, Plant; Indoles; Mutation; Oxylipins; Phytoalexins; Plant Diseases; Plant Extracts; Plant Growth Regulators; Plant Proteins; Salicylic Acid; Sesquiterpenes; Terpenes; Thiazoles