[Elicitor activity of chitosan and arachidonic acid: their similarity and distinction]

Two elicitors-chitosan and arachidonic acid-induced the same defense responses in potatoes, stimulating the processes of wound reparation and inducing the formation of phytoalexins, inhibitors of proteinase, and active forms of oxygen. However, chitosan induced the defense potential of plant tissues at concentrations higher than those of arachidonic acid. The protective action of chitosan was defined by two parameters, i.e., the ability to induce the immune responses in plant tissues and to exhibit a toxic effect on the pathogen development, causing late blight and seedling blight, whereas the elicitor effect of arachidonic acid depended on its ability to induce the defense potential of plant tissues only.

Nonsense-mediated mRNA decay factors, UPF1 and UPF3, contribute to plant defense

In Arabidopsis, the NMD-defective mutants upf1-5 and upf3-1 are characterized by dwarfism, curly leaves and late flowering. These phenotypes are similar to those of mutants showing constitutive pathogenesis-related (PR) gene expression, salicylic acid (SA) accumulation and, subsequently, resistance to pathogens. The disease symptoms of upf1-5 and upf3-1 mutants were observed following infection with the virulent pathogen Pst DC3000 with the aim of determining whether the loss of nonsense-mediated mRNA decay (NMD) is involved in disease resistance. These mutant plants showed not only enhanced resistance to Pst DC3000, but also elevated levels of endogenous SA, PR gene transcripts and WRKY transcripts. UPF1 and UPF3 expression was down-regulated in Pst DC3000-infected Arabidopsis plants, but the expression of various NMD target genes was up-regulated. The expression of 10 defense-related genes was elevated in cycloheximide (CHX)-treated plants. The transcriptional ratios of eight of these 10 defense-related genes in CHX-treated to non-treated plants were lower in NMD-defective mutants than in the wild-type plants. These eight defense-related genes are possibly regulated by the NMD mechanism, and it is clear that an alternatively spliced transcript of WRKY62, which contains a premature termination codon, was regulated by this mechanism. Taken together, our results suggest that UPF1 and UPF3, which are key NMD factors, may act as defense-related regulators associated with plant immunity.

Alkamides activate jasmonic acid biosynthesis and signaling pathways and confer resistance to Botrytis cinerea in Arabidopsis thaliana

Alkamides are fatty acid amides of wide distribution in plants, structurally related to N-acyl-L-homoserine lactones (AHLs) from Gram-negative bacteria and to N- acylethanolamines (NAEs) from plants and mammals. Global analysis of gene expression changes in Arabidopsis thaliana in response to N-isobutyl decanamide, the most highly active alkamide identified to date, revealed an overrepresentation of defense-responsive transcriptional networks. In particular, genes encoding enzymes for jasmonic acid (JA) biosynthesis increased their expression, which occurred in parallel with JA, nitric oxide (NO) and H₂O₂ accumulation. The activity of the alkamide to confer resistance against the necrotizing fungus Botrytis cinerea was tested by inoculating Arabidopsis detached leaves with conidiospores and evaluating disease symptoms and fungal proliferation. N-isobutyl decanamide application significantly reduced necrosis caused by the pathogen and inhibited fungal proliferation. Arabidopsis mutants jar1 and coi1 altered in JA signaling and a MAP kinase mutant (mpk6), unlike salicylic acid- (SA) related mutant eds16/sid2-1, were unable to defend from fungal attack even when N-isobutyl decanamide was supplied, indicating that alkamides could modulate some necrotrophic-associated defense responses through JA-dependent and MPK6-regulated signaling pathways. Our results suggest a role of alkamides in plant immunity induction.

N-acyl-homoserine lactone confers resistance toward biotrophic and hemibiotrophic pathogens via altered activation of AtMPK6

Pathogenic and symbiotic bacteria rely on quorum sensing to coordinate the collective behavior during the interactions with their eukaryotic hosts. Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) as signals in such communication. Here we show that plants have evolved means to perceive AHLs and that the length of acyl moiety and the functional group at the γ position specify the plant's response. Root treatment with the N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) reinforced the systemic resistance to the obligate biotrophic fungi Golovinomyces orontii in Arabidopsis (Arabidopsis thaliana) and Blumeria graminis f. sp. hordei in barley (Hordeum vulgare) plants. In addition, oxo-C14-HSL-treated Arabidopsis plants were more resistant toward the hemibiotrophic bacterial pathogen Pseudomonas syringae pv tomato DC3000. Oxo-C14-HSL promoted a stronger activation of mitogen-activated protein kinases AtMPK3 and AtMPK6 when challenged with flg22, followed by a higher expression of the defense-related transcription factors WRKY22 and WRKY29, as well as the PATHOGENESIS-RELATED1 gene. In contrast to wild-type Arabidopsis and mpk3 mutant, the mpk6 mutant is compromised in the AHL effect, suggesting that AtMPK6 is required for AHL-induced resistance. Results of this study show that AHLs commonly produced in the rhizosphere are crucial factors in plant pathology and could be an agronomic issue whose full impact has to be elucidated in future analyses.

A jacalin-related lectin-like gene in wheat is a component of the plant defence system

Jacalin-related lectins (JRLs) are a subgroup of proteins with one or more jacalin-like lectin domains. Although JRLs are often associated with biotic or abiotic stimuli, their biological functions in plants, as well as their relationships to plant disease resistance, are poorly understood. A mannose-specific JRL (mJRL)-like gene (TaJRLL1) that is mainly expressed in stem and spike and encodes a protein with two jacalin-like lectin domains was identified in wheat. Pathogen infection and phytohormone treatments induced its expression; while application of the salicylic acid (SA) biosynthesis inhibitor paclobutrazol and the jasmonic acid (JA) biosynthesis inhibitor diethyldithiocarbamic acid, respectively, substantially inhibited its expression. Attenuating TaJRLL1 through virus-induced gene silencing increased susceptibility to the facultative fungal pathogen Fusarium graminearum and the biotrophic fungal pathogen Blumeria graminis. Arabidopsis thaliana transformed with TaJRLL1 displayed increased resistance to F. graminearum and Botrytis cinerea. JA and SA levels in transgenic Arabidopsis increased significantly. A loss or increase of disease resistance due to an alteration in TaJRLL1 function was correlated with attenuation or enhancement of the SA- and JA-dependent defence signalling pathways. These results suggest that TaJRLL1 could be a component of the SA- and JA-dependent defence signalling pathways.

D-Psicose induces upregulation of defense-related genes and resistance in rice against bacterial blight

We examined rice responses to a rare sugar, d-psicose. Rice growth was inhibited by d-psicose but not by common sugars. Microarray analysis revealed that d-psicose treatment caused an upregulation of many defense-related genes in rice, and dose-dependent upregulation of these genes was confirmed by quantitative reverse-transcription polymerase chain reaction. The level of upregulation of defense-related genes by d-psicose was low compared with that by d-allose, which is another rare sugar known to confer induction of resistance to rice bacterial blight in rice. Treatment with d-psicose conferred resistance to bacterial blight in rice in a dose-dependent manner, and the results indicate that d-psicose might be a candidate plant activator for reducing disease development in rice.

The composite effect of transgenic plant volatiles for acquired immunity to herbivory caused by inter-plant communications

A blend of volatile organic compounds (VOCs) emitted from plants induced by herbivory enables the priming of defensive responses in neighboring plants. These effects may provide insights useful for pest control achieved with transgenic-plant-emitted volatiles. We therefore investigated, under both laboratory and greenhouse conditions, the priming of defense responses in plants (lima bean and corn) by exposing them to transgenic-plant-volatiles (VOCos) including (E)-β-ocimene, emitted from transgenic tobacco plants (NtOS2) that were constitutively overexpressing (E)-β-ocimene synthase. When lima bean plants that had previously been placed downwind of NtOS2 in an open-flow tunnel were infested by spider mites, they were more defensive to spider mites and more attractive to predatory mites, in comparison to the infested plants that had been placed downwind of wild-type tobacco plants. This was similarly observed when the NtOS2-downwind maize plants were infested with Mythimna separata larvae, resulting in reduced larval growth and greater attraction of parasitic wasps (Cotesia kariyai). In a greenhouse experiment, we also found that lima bean plants (VOCos-receiver plants) placed near NtOS2 were more attractive when damaged by spider mites, in comparison to the infested plants that had been placed near the wild-type plants. More intriguingly, VOCs emitted from infested VOCos-receiver plants affected their conspecific neighboring plants to prime indirect defenses in response to herbivory. Altogether, these data suggest that transgenic-plant-emitted volatiles can enhance the ability to prime indirect defenses via both plant-plant and plant-plant-plant communications.

Quantitative genetic analysis of salicylic acid perception in Arabidopsis

Salicylic acid (SA) is a phytohormone required for a full resistance against some pathogens in Arabidopsis, and NPR1 (Non-Expressor of Pathogenesis Related Genes 1) is the only gene with a strong effect on resistance induced by SA which has been described. There can be additional components of SA perception that escape the traditional approach of mutagenesis. An alternative to that approach is searching in the natural variation of Arabidopsis. Different methods of analyzing the variation between ecotypes have been tried and it has been found that measuring the growth of a virulent isolate of Pseudomonas syringae after the exogenous application of SA is the most effective one. Two ecotypes, Edi-0 and Stw-0, have been crossed, and their F2 has been studied. There are two significant quantitative trait loci (QTLs) in this population, and there is one QTL in each one of the existing mapping populations Col-4 × Laer-0 and Laer-0 × No-0. They have different characteristics: while one QTL is only detectable at low concentrations of SA, the other acts after the point of crosstalk with methyl jasmonate signalling. Three of the QTLs have candidates described in SA perception as NPR1, its interactors, and a calmodulin binding protein.

Trichoderma-induced plant immunity likely involves both hormonal- and camalexin-dependent mechanisms in Arabidopsis thaliana and confers resistance against necrotrophic fungi Botrytis cinerea

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.

Transcript profiling of chitosan-treated Arabidopsis seedlings

In nature, plants can recognize potential pathogens, thus activating intricate networks of defense signals and reactions. Inducible defense is often mediated by the detection of microbe or pathogen associated molecular pattern elicitors, such as flagellin and chitin. Chitosan, the deacetylated form of chitin, plays a role in inducing protection against pathogens in many plant species. We evaluated the ability of chitosan to confer resistance to Botrytis cinerea in Arabidopsis leaves. We subsequently treated Arabidopsis seedlings with chitosan and carried out a transcript profiling analysis using both ATH1 GeneChip microarrays and quantitative RT-PCR. The results showed that defense response genes, including camalexin biosynthesis genes, were up-regulated by chitosan, both in wild-type and in the chitin-insensitive cerk1 mutant, indicating that chitosan is perceived through a CERK1-independent pathway.

Metabolomic approaches reveal that cell wall modifications play a major role in ethylene-mediated resistance against Botrytis cinerea

In Arabidopsis, resistance to the necrotrophic fungus Botrytis cinerea is conferred by ethylene via poorly understood mechanisms. Metabolomic approaches compared the responses of the wild-type, the ethylene-insensitive mutant etr1-1, which showed increased susceptibility, and the constitutively active ethylene mutants ctr1-1 and eto2 both exhibited decreased susceptibility to B. cinerea. Fourier transform-infrared (FT-IR) spectroscopy demonstrated reproducible biochemical differences between treatments and genotypes. To identify discriminatory mass-to-charge ratios (m/z) associated with resistance, discriminant function analysis was employed on spectra derived from direct injection electrospray ionisation-mass spectrometry on the derived principal components of these data. Ethylene-modulated m/z were mapped onto Arabidopsis biochemical pathways and many were associated with hydroxycinnamate and monolignol biosynthesis, both linked to cell wall modification. A high-resolution linear triple quadrupole-Orbitrap hybrid system confirmed the identity of key metabolites in these pathways. The contribution of these pathways to defence against B. cinerea was validated through the use of multiple Arabidopsis mutants. The FT-IR microspectroscopy indicated that spatial accumulation of hydroxycinnamates and monolignols at the cell wall to confine disease was linked ot ethylene. These data demonstrate the power of metabolomic approaches in elucidating novel biological phenomena, especially when coupled to validation steps exploiting relevant mutant genotypes.

Induction of systemic stress tolerance by brassinosteroid in Cucumis sativus

• Brassinosteroids (BRs) are a new class of plant hormones that are essential for plant growth and development. Here, the involvement of BRs in plant systemic tolerance to biotic and abiotic stresses was studied. • The effects of 24-epibrassinolide (EBR) on plant stress tolerance were studied through the assessment of symptoms of photooxidative stress by chlorophyll fluorescence imaging pulse amplitude modulation, the analysis of gene expression using quantitative real-time PCR and the measurement of hydrogen peroxide (H₂O₂) production using a spectrophotometric assay or confocal laser scanning microscopy. • Treatment of primary leaves with EBR induced systemic tolerance to photooxidative stress in untreated upper and lower leaves. This was accompanied by the systemic accumulation of H₂O₂ and the systemic induction of genes associated with stress responses. Foliar treatment of EBR also enhanced root resistance to Fusarium wilt pathogen. Pharmacological study showed that EBR-induced systemic tolerance was dependent on local and systemic H₂O₂ accumulation. The expression of BR biosynthetic genes was repressed in EBR-treated leaves, but elevated significantly in untreated systemic leaves. Further analysis indicated that EBR-induced systemic induction of BR biosynthetic genes was mediated by systemically elevated H₂O₂. • These results strongly argue that local EBR treatment can activate the continuous production of H₂O₂, and the autopropagative nature of the reactive oxygen species signal, in turn, mediates EBR-induced systemic tolerance.

Expression profile of jasmonic acid-induced genes and the induced resistance against the root-knot nematode (Meloidogyne incognita) in tomato plants (Solanum lycopersicum) after foliar treatment with methyl jasmonate

We investigated what gene(s) in the plant roots have the positive role against repressing root-knot nematode (RKN) infection. We investigated the interaction between RKN infection and gene expression in the plant roots induced by methyl jasmonate (MeJA). We focused on the induced resistance response and the duration after foliar treatment with MeJA of 0.1, 0.5, 1.0, and 5.0mM at 1, 24, 48, and 72h prior to the inoculation of RKN. As a result, the foliar treatment with MeJA at 0.5mM or higher concentrations significantly reduced the infection of RKN in plants and the effect lasted for about 1 week. The repressing effect on RKN population declined to the lowest level in two weeks after MeJA treatment. The expression of proteinase inhibitors (PIs) and multicystatin (MC) were induced while the repressing effect on RKN was valid and a negative correlation was found between the expression of PIs or MC and RKN infection. In addition, when tomato plants no longer expressing MC and PIs were treated again with MeJA, the repressing effect revived. These phenomena appeared to be regardless of the existence of Mi-genes or isolate of RKN. Our results indicate that the expression level of MC and PIs may be effective as marker genes for estimating the induced resistance response against RKN infection.

A survey on basal resistance and riboflavin-induced defense responses of sugar beet against Rhizoctonia solani

We examined basal defense responses and cytomolecular aspects of riboflavin-induced resistance (IR) in sugar beet-Rhizoctonia solani pathsystem by investigating H(2)O(2) burst, phenolics accumulation and analyzing the expression of phenylalanine ammonia-lyase (PAL) and peroxidase (cprx1) genes. Riboflavin was capable of priming plant defense responses via timely induction of H(2)O(2) production and phenolics accumulation. A correlation was found between induction of resistance by riboflavin and upregulation of PAL and cprx1 which are involved in phenylpropanoid signaling and phenolics metabolism. Application of peroxidase and PAL inhibitors suppressed not only basal resistance, but also riboflavin-IR of sugar beet to the pathogen. Treatment of the leaves with each inhibitor alone or together with riboflavin reduced phenolics accumulation which was correlated with higher level of disease progress. Together, these results demonstrate the indispensability of rapid H(2)O(2) accumulation, phenylpropanoid pathway and phenolics metabolism in basal defense and riboflavin-IR of sugar beet against R. solani.

Discovery of a small peptide from combinatorial libraries that can activate the plant immune system by a jasmonic acid signaling pathway

Plants defend themselves by using an innate immune system that is activated in response to a variety of molecules derived from pathogens. These molecules have provided profound insights into the mechanisms of pathogen recognition and subsequent signaling pathways in plants. In the present study, we screened a combinatorial random hexapeptide library for peptides that activate the plant immune system, by using a cell-based high-throughput screening system in which H(2)O(2) generation was monitored. We discovered a novel small peptide (YGIHTH-amide, PIP-1) that triggered H(2)O(2) production in tobacco and tomato cells, but not in Arabidopsis cells. PIP-1 induced significant levels of phytoalexin biosynthesis and defense-related gene expression in tobacco cells; this is likely to be activated by a jasmonic acid pathway.

Broad-spectrum suppression of innate immunity is required for colonization of Arabidopsis roots by the fungus Piriformospora indica

Piriformospora indica is a root-colonizing basidiomycete that confers a wide range of beneficial traits to its host. The fungus shows a biotrophic growth phase in Arabidopsis (Arabidopsis thaliana) roots followed by a cell death-associated colonization phase, a colonization strategy that, to our knowledge, has not yet been reported for this plant. P. indica has evolved an extraordinary capacity for plant root colonization. Its broad host spectrum encompasses gymnosperms and monocotyledonous as well as dicotyledonous angiosperms, which suggests that it has an effective mechanism(s) for bypassing or suppressing host immunity. The results of our work argue that P. indica is confronted with a functional root immune system. Moreover, the fungus does not evade detection but rather suppresses immunity triggered by various microbe-associated molecular patterns. This ability to suppress host immunity is compromised in the jasmonate mutants jasmonate insensitive1-1 and jasmonate resistant1-1. A quintuple-DELLA mutant displaying constitutive gibberellin (GA) responses and the GA biosynthesis mutant ga1-6 (for GA requiring 1) showed higher and lower degrees of colonization, respectively, in the cell death-associated stage, suggesting that P. indica recruits GA signaling to help establish proapoptotic root cell colonization. Our study demonstrates that mutualists, like pathogens, are confronted with an effective innate immune system in roots and that colonization success essentially depends on the evolution of strategies for immunosuppression.

Broad-spectrum suppression of innate immunity is required for colonization of Arabidopsis roots by the fungus Piriformospora indica

Piriformospora indica is a root-colonizing basidiomycete that confers a wide range of beneficial traits to its host. The fungus shows a biotrophic growth phase in Arabidopsis (Arabidopsis thaliana) roots followed by a cell death-associated colonization phase, a colonization strategy that, to our knowledge, has not yet been reported for this plant. P. indica has evolved an extraordinary capacity for plant root colonization. Its broad host spectrum encompasses gymnosperms and monocotyledonous as well as dicotyledonous angiosperms, which suggests that it has an effective mechanism(s) for bypassing or suppressing host immunity. The results of our work argue that P. indica is confronted with a functional root immune system. Moreover, the fungus does not evade detection but rather suppresses immunity triggered by various microbe-associated molecular patterns. This ability to suppress host immunity is compromised in the jasmonate mutants jasmonate insensitive1-1 and jasmonate resistant1-1. A quintuple-DELLA mutant displaying constitutive gibberellin (GA) responses and the GA biosynthesis mutant ga1-6 (for GA requiring 1) showed higher and lower degrees of colonization, respectively, in the cell death-associated stage, suggesting that P. indica recruits GA signaling to help establish proapoptotic root cell colonization. Our study demonstrates that mutualists, like pathogens, are confronted with an effective innate immune system in roots and that colonization success essentially depends on the evolution of strategies for immunosuppression.

Multiple treatment meta-analysis of products evaluated for control of fire blight in the eastern United States

The aim of this analysis was to estimate the effect sizes and consistency of products evaluated for fire blight control in the eastern United States over the last decade. Because only 3% of the 69 studies published from 2000 to 2008 explicitly presented a measure of within-study variability, a method for estimating the least significant difference (LSD) and, hence the sampling variance, for studies with at least two significant mean separations in the presented mean multiple comparisons was developed. Lin's concordance analysis indicated that the estimated LSD was an accurate predictor of the actual LSD based on 35 studies in a calibration evaluation (ρ(c) = 0.997). Separate multi-treatment random-effects meta-analyses were performed for three control categories: antibiotics, biological control, and plant defense-activating products and mean log response ratios relative to the nontreated controls ([Formula: see text]) were computed for each treatment and then back-transformed to obtain the mean percent disease control. None of the products evaluated performed as well as streptomycin, the standard product for fire blight control, for which the mean disease control was 68.6%. As a group, experimental antibiotics provided the best fire blight control with mean effect sizes ranging from 59.7 to 61.7%. Among the biological controls, the best control was noted for treatments combining the antibiotic streptomycin with a product based on Pantoea agglomerans (55.0% mean disease reduction) or Bacillus subtilis (53.9%). Mean disease control was 31.9, 25.7, and 22.6%, respectively, for products based on B. subtilis, Pantoea agglomerans, and Pseudomonas fluorescens without an antibiotic, suggesting that the higher efficacy of the combination treatments was due to the antibiotic. Among the plant defense-activating products, prohexadione calcium had the highest and most consistent effect size (50.7% control), while other products provided modest mean disease control of between 6.1 and 25.8%. Percent control values were significantly moderated by study location and cultivar used in the study, and were smaller, but more variable, when products were tested under high disease intensity compared with low disease intensity. Results indicate that wide-scale use of biological control and plant defense-activating products in the eastern United States is likely to remain low.

Active suppression of early immune response in tobacco by the human pathogen Salmonella Typhimurium

The persistence of enteric pathogens on plants has been studied extensively, mainly due to the potential hazard of human pathogens such as Salmonella enterica being able to invade and survive in/on plants. Factors involved in the interactions between enteric bacteria and plants have been identified and consequently it was hypothesized that plants may be vectors or alternative hosts for enteric pathogens. To survive, endophytic bacteria have to escape the plant immune systems, which function at different levels through the plant-bacteria interactions. To understand how S. enterica survives endophyticaly we conducted a detailed analysis on its ability to elicit or evade the plant immune response. The models of this study were Nicotiana tabacum plants and cells suspension exposed to S. enterica serovar Typhimurium. The plant immune response was analyzed by looking at tissue damage and by testing oxidative burst and pH changes. It was found that S. Typhimurium did not promote disease symptoms in the contaminated plants. Live S. Typhimurium did not trigger the production of an oxidative burst and pH changes by the plant cells, while heat killed or chloramphenicol treated S. Typhimurium and purified LPS of Salmonella were significant elicitors, indicating that S. Typhimurium actively suppress the plant response. By looking at the plant response to mutants defective in virulence factors we showed that the suppression depends on secreted factors. Deletion of invA reduced the ability of S. Typhimurium to suppress oxidative burst and pH changes, indicating that a functional SPI1 TTSS is required for the suppression. This study demonstrates that plant colonization by S. Typhimurium is indeed an active process. S. Typhimurium utilizes adaptive strategies of altering innate plant perception systems to improve its fitness in the plant habitat. All together these results suggest a complex mechanism for perception of S. Typhimurium by plants.

Priming for JA-dependent defenses using hexanoic acid is an effective mechanism to protect Arabidopsis against B. cinerea

Soil drench treatments with hexanoic acid can effectively protect Arabidopsis plants against Botrytis cinerea through a mechanism based on a stronger and faster accumulation of JA-dependent defenses. Plants impaired in ethylene, salicylic acid, abscisic acid or glutathion pathways showed intact protection by hexanoic acid upon B. cinerea infection. Accordingly, no significant changes in the SA marker gene PR-1 in either the SA or ABA hormone balance were observed in the infected and treated plants. In contrast, the JA signaling pathway showed dramatic changes after hexanoic acid treatment, mainly when the pathogen was present. The impaired JA mutants, jin1-2 and jar1, were unable to display hexanoic acid priming against the necrotroph. In addition, hexanoic acid-treated plants infected with B. cinerea showed priming in the expression of the PDF1.2, PR-4 and VSP1 genes implicated in the JA pathways. Moreover, JA and OPDA levels were primed at early stages by hexanoic acid. Treatments also stimulated increased callose accumulation in response to the pathogen. Although callose accumulation has proved an effective IR mechanism against B. cinerea, it is apparently not essential to express hexanoic acid-induced resistance (HxAc-IR) because the mutant pmr4.1 (callose synthesis defective mutant) is protected by treatment. We recently described how hexanoic acid treatments can protect tomato plants against B. cinerea by stimulating ABA-dependent callose deposition and by priming OPDA and JA-Ile production. We clearly demonstrate here that Hx-IR is a dependent plant species, since this acid protects Arabidopsis plants against the same necrotroph by priming JA-dependent defenses without enhancing callose accumulation.

Priming for enhanced defence responses by specific inhibition of the Arabidopsis response to coronatine

The priming agent β-aminobutyric acid (BABA) is known to enhance Arabidopsis resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 by potentiating salicylic acid (SA) defence signalling, notably PR1 expression. The molecular mechanisms underlying this phenomenon remain unknown. A genome-wide microarray analysis of BABA priming during Pst DC3000 infection revealed direct and primed up-regulation of genes that are responsive to SA, the SA analogue benzothiadiazole and pathogens. In addition, BABA was found to inhibit the Arabidopsis response to the bacterial effector coronatine (COR). COR is known to promote bacterial virulence by inducing the jasmonic acid (JA) response to antagonize SA signalling activation. BABA specifically repressed the JA response induced by COR without affecting other plant JA responses. This repression was largely SA-independent, suggesting that it is not caused by negative cross-talk between SA and JA signalling cascades. Treatment with relatively high concentrations of purified COR counteracted BABA inhibition. Under these conditions, BABA failed to protect Arabidopsis against Pst DC3000. BABA did not induce priming and resistance in plants inoculated with a COR-deficient strain of Pst DC3000 or in the COR-insensitive mutant coi1-16. In addition, BABA blocked the COR-dependent re-opening of stomata during Pst DC3000 infection. Our data suggest that BABA primes for enhanced resistance to Pst DC3000 by interfering with the bacterial suppression of Arabidopsis SA-dependent defences. This study also suggests the existence of a signalling node that distinguishes COR from other JA responses.

OsEDR1 negatively regulates rice bacterial resistance via activation of ethylene biosynthesis

Rice OsEDR1 is a sequence ortholog of Arabidopsis EDR1. However, its molecular function is unknown. We show here that OsEDR1-suppressing/knockout (KO) plants, which developed spontaneous lesions on the leaves, have enhanced resistance to Xanthomonas oryzae pv. oryzae (Xoo) causing bacterial blight disease. This resistance was associated with increased accumulation of salicylic acid (SA) and jasmonic acid (JA), induced expression of SA- and JA-related genes and suppressed accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC), the direct precursor of ethylene, and expression of ethylene-related genes. OsEDR1-KO plants also showed suppressed production of ethylene. Knockout of OsEDR1 suppressed the ACC synthase (ACS) gene family, which encodes the rate-limiting enzymes of ethylene biosynthesis by catalysing the formation of ACC. The lesion phenotype and enhanced bacterial resistance of the OsEDR1-KO plants was partly complemented by the treatment with ACC. ACC treatment was associated with decreased SA and JA biosynthesis in OsEDR1-KO plants. In contrast, aminoethoxyvinylglycine, the inhibitor of ethylene biosynthesis, promoted expression of SA and JA synthesis-related genes in OsEDR1-KO plants. These results suggest that ethylene is a negative signalling molecule in rice bacterial resistance. In the rice-Xoo interaction, OsEDR1 transcriptionally promotes the synthesis of ethylene that, in turn, suppresses SA- and JA-associated defence signalling.

Role of OsNPR1 in rice defense program as revealed by genome-wide expression analysis

NPR1 is a central regulator of salicylic-acid (SA)-mediated defense signaling in Arabidopsis. Here, we report the characterization of OsNPR1, an Oryzae sativa (rice) ortholog of NPR1, focusing on its role in blast disease resistance and identification of OsNPR1-regulated genes. Blast resistance tests using OsNPR1 knockdown and overexpressing rice lines demonstrated the essential role of OsNPR1 in benzothiadiazole (BTH)-induced blast resistance. Genome-wide transcript profiling using OsNPR1-knockdown lines revealed that 358 genes out of 1,228 BTH-upregulated genes and 724 genes out of 1,069 BTH-downregulated genes were OsNPR1-dependent with respect to BTH responsiveness, thereby indicating that OsNPR1 plays a more vital role in gene downregulation. The OsNPR1-dependently downregulated genes included many of those involved in photosynthesis and in chloroplast translation and transcription. Reduction of photosynthetic activity after BTH treatment and its negation by OsNPR1 knockdown were indeed reflected in the changes in Fv/Fm values in leaves. These results imply the role of OsNPR1 in the reallocation of energy and resources during defense responses. We also examined the OsNPR1-dependence of SA-mediated suppression of ABA-induced genes.

Ethylene signaling regulates accumulation of the FLS2 receptor and is required for the oxidative burst contributing to plant immunity

Reactive oxygen species (ROS) are potent signal molecules rapidly generated in response to stress. Detection of pathogen-associated molecular patterns induces a transient apoplastic ROS through the function of the NADPH respiratory burst oxidase homologs D (RbohD). However, little is known about the regulation of pathogen-associated molecular pattern-elicited ROS or its role in plant immunity. We investigated ROS production triggered by bacterial flagellin (flg22) in Arabidopsis (Arabidopsis thaliana). The oxidative burst was diminished in ethylene-insensitive mutants. Flagellin Sensitive2 (FLS2) accumulation was reduced in etr1 and ein2, indicating a requirement of ethylene signaling for FLS2 expression. Multiplication of virulent bacteria was enhanced in Arabidopsis lines displaying altered ROS production at early but not late stages of infection, suggesting an impairment of preinvasive immunity. Stomatal closure, a mechanism used to reduce bacterial entry into plant tissues, was abolished in etr1, ein2, and rbohD mutants. These results point to the importance of flg22-triggered ROS at an early stage of the plant immune response.

Plant immunity directly or indirectly restricts the injection of type III effectors by the Pseudomonas syringae type III secretion system

Plants perceive microorganisms by recognizing microbial molecules known as pathogen-associated molecular patterns (PAMPs) inducing PAMP-triggered immunity (PTI) or by recognizing pathogen effectors inducing effector-triggered immunity (ETI). The hypersensitive response (HR), a programmed cell death response associated with ETI, is known to be inhibited by PTI. Here, we show that PTI-induced HR inhibition is due to direct or indirect restriction of the type III protein secretion system's (T3SS) ability to inject type III effectors (T3Es). We found that the Pseudomonas syringae T3SS was restricted in its ability to inject a T3E-adenylate cyclase (CyaA) injection reporter into PTI-induced tobacco (Nicotiana tabacum) cells. We confirmed this restriction with a direct injection assay that monitored the in planta processing of the AvrRpt2 T3E. Virulent P. syringae strains were able to overcome a PAMP pretreatment in tobacco or Arabidopsis (Arabidopsis thaliana) and continue to inject a T3E-CyaA reporter into host cells. In contrast, ETI-inducing P. syringae strains were unable to overcome PTI-induced injection restriction. A P. syringae pv tomato DC3000 mutant lacking about one-third of its T3E inventory was less capable of injecting into PTI-induced Arabidopsis plant cells, grew poorly in planta, and did not cause disease symptoms. PTI-induced transgenic Arabidopsis expressing the T3E HopAO1 or HopF2 allowed higher amounts of the T3E-CyaA reporter to be injected into plant cells compared to wild-type plants. Our results show that PTI-induced HR inhibition is due to direct or indirect restriction of T3E injection and that T3Es can relieve this restriction by suppressing PTI.