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PubMed2005

Molecular genetic evidence for the role of SGT1 in the intramolecular complementation of Bs2 protein activity in Nicotiana benthamiana.

Pepper plants (Capsicum annuum) containing the Bs2 resistance gene are resistant to strains of Xanthomonas campestris pv vesicatoria (Xcv) expressing the bacterial effector protein AvrBs2. AvrBs2 is delivered directly to the plant cell via the type III protein secretion system (TTSS) of Xcv. Upon recognition of AvrBs2 by plants expressing the Bs2 gene, a signal transduction cascade is activated leading to a bacterial disease resistance response. Here, we describe a novel pathosystem that consists of epitope-tagged Bs2-expressing transgenic Nicotiana benthamiana plants and engineered strains of Pseudomonas syringae pv tabaci that deliver the effector domain of the Xcv AvrBs2 protein via the TTSS of P. syringae. This pathosystem has allowed us to exploit N. benthamiana as a model host plant to use Agrobacterium tumefaciens-mediated transient protein expression in conjunction with virus-induced gene silencing to validate genes and to identify protein interactions required for the expression of plant host resistance. In this study, we demonstrate that two genes, NbSGT1 and NbNPK1, are required for the Bs2/AvrBs2-mediated resistance responses but that NbRAR1 is not. Protein localization studies in these plants indicate that full-length Bs2 is primarily localized in the plant cytoplasm. Three protein domains of Bs2 have been identified: the N terminus, a central nucleotide binding site, and a C-terminal Leu-rich repeat (LRR). Co-immunoprecipitation studies demonstrate that separate epitope-tagged Bs2 domain constructs interact in trans specifically in the plant cell. Co-immunoprecipitation studies also demonstrate that an NbSGT1-dependent intramolecular interaction is required for Bs2 function. Additionally, Bs2 has been shown to associate with SGT1 via the LRR domain of Bs2. These data suggest a role for SGT1 in the proper folding of Bs2 or the formation of a Bs2-SGT1-containing protein complex that is required for the expression of bacterial disease resistance.

Arabidopsis Proteins↗

PubMed2004

A key role for ALD1 in activation of local and systemic defenses in Arabidopsis.

The Arabidopsis thaliana agd2-like defense response protein1 (ald1) mutant was previously found to be hypersusceptible to the virulent bacterial pathogen Pseudomonas syringae and had reduced accumulation of the defense signal salicylic acid (SA). ALD1 was shown to possess aminotransferase activity in vitro, suggesting it generates an amino acid-derived defense signal. We now find ALD1 to be a key defense component that acts in multiple contexts and partially requires the PHYTOALEXIN DEFICIENT4 (PAD4) defense regulatory gene for its expression in response to infection. ald1 plants have increased susceptibility to avirulent P. syringae strains, are unable to activate systemic acquired resistance and are compromised for resistance to the oomycete pathogen Peronospora parasitica in mutants with constitutively active defenses. ALD1 and PAD4 can act additively to control SA, PATHOGENESIS RELATED GENE1 (PR1) transcript and camalexin (an antimicrobial metabolite) accumulation as well as disease resistance. Finally, ALD1 and PAD4 can mutually affect each other's expression in a constitutive defense mutant, suggesting that these two genes can act in a signal amplification loop.

Arabidopsis↗

PubMed2004

Two Arabidopsis srfr (suppressor of rps4-RLD) mutants exhibit avrRps4-specific disease resistance independent of RPS4.

RPS4 specifies the Arabidopsis disease resistance response to Pseudomonas syringae pv. tomato expressing avrRps4 and was cloned based on the identification of RLD as a naturally occurring susceptible accession. To dissect the molecular and genetic basis of disease resistance, we used a genetic approach to identify suppressor mutations that reactivate the avrRps4-triggered defense response in RLD. In this report, we describe two non-allelic srfr (suppressor of rps4-RLD) mutants, srfr1 and srfr3, that were susceptible to virulent P. syringae pv. tomato strain DC3000, but resistant to DC3000 expressing avrRps4. In quantitative bacterial growth assays, growth of DC3000 was similar in wild-type control and both mutant lines, indicating that basal resistance was not enhanced in srfr1 and srfr3. Growth of DC3000 (avrRps4) was approximately 30-fold lower in srfr1 and srfr3 than in RLD, but intermediate compared with fully resistant Col-0 and transgenic RLD containing RPS4-Col. The srfr1 and srfr3 mutants did not develop spontaneous lesions prior to inoculation or constitutively express the pathogenesis-related gene PR-1. Therefore, srfr1 and srfr3 constitute novel avr-specific mutants that differ from previously described Arabidopsis mutants with elevated disease resistance. The srfr1 and srfr3 mutations were recessive, and both mapped to the bottom of chromosome IV. Genetic analysis indicated that resistance in srfr1 and srfr3 was independent of the rps4-RLD allele, but dependent on a second gene in RLD. We propose that SRFR1 and SRFR3 are negative regulators of avrRps4-triggered gene-for-gene disease resistance.

Arabidopsis↗

PubMed2004

The BOS loci of Arabidopsis are required for resistance to Botrytis cinerea infection.

Three Botrytis-susceptible mutants bos2, bos3, and bos4 which define independent and novel genetic loci required for Arabidopsis resistance to Botrytis cinerea were isolated. The bos2 mutant is susceptible to B. cinerea but retains wild-type levels of resistance to other pathogens tested, indicative of a defect in a response pathway more specific to B. cinerea. The bos3 and bos4 mutants also show increased susceptibility to Alternaria brassicicola, another necrotrophic pathogen, suggesting a broader role for these loci in resistance. bos4 shows the broadest range of effects on resistance, being more susceptible to avirulent strain of Pseudomonas syringae pv. tomato. Interestingly, bos3 is more resistant than wild-type plants to virulent strains of the biotrophic pathogen Peronospora parasitica and the bacterial pathogen P. syringae pv. tomato. The Pathogenesis Related gene 1 (PR-1), a molecular marker of the salicylic acid (SA)-dependent resistance pathway, shows a wild-type pattern of expression in bos2, while in bos3 this gene was expressed at elevated levels, both constitutively and in response to pathogen challenge. In bos4 plants, PR-1 expression was reduced compared with wild type in response to B. cinerea and SA. In bos3, the mutant most susceptible to B. cinerea and with the highest expression of PR-1, removal of SA resulted in reduced PR-1 expression but no change to the B. cinerea response. Expression of the plant defensin gene PDF1-2 was generally lower in bos mutants compared with wild-type plants, with a particularly strong reduction in bos3. Production of the phytoalexin camalexin is another well-characterized plant defense response. The bos2 and bos4 mutants accumulate reduced levels of camalexin whereas bos3 accumulates significantly higher levels of camalexin than wild-type plants in response to B. cinerea. The BOS2, BOS3, and BOS4 loci may affect camalexin levels and responsiveness to ethylene and jasmonate. The three new mutants appear to mediate disease responses through mechanisms independent of the previously described BOS1 gene. Based on the differences in the phenotypes of the bos mutants, it appears that they affect different points in defense response pathways.

Alternaria↗

PubMed2004

Role of chloroplast trienoic fatty acids in plant disease defense responses.

Trienoic fatty acids (TAs) are the major polyunsaturated fatty acid species in the membrane lipids in plant cells. TAs are crucial for the adaptation to abiotic stresses, especially low- or high-temperature stress. We show that TAs in chloroplast membrane lipids are involved in defense responses against avirulent bacterial pathogens. Avirulent pathogen invasion of plants induces a transient production of reactive oxygen intermediates (ROI), programmed cell death and subsequent disease resistance. The Arabidopsis fad7fad8 mutation, which prevents the synthesis of TAs in chloroplast lipids, caused the reduction in ROI accumulation in leaves inoculated with Pseudomonas syringae pv. tomato DC3000 (avrRpm1). Linolenic acid, the most abundant TA, activated the NADPH oxidase that is responsible for ROI generation. TAs were transferred from chloroplast lipids to extrachloroplast lipids coincident with ROI accumulation after inoculation with Pst DC3000 (avrRpm1). Furthermore, the fad7fad8 mutant exhibited reduced cell death and was compromised in its resistance to several avirulent P. syringae strains. These results suggest that TAs derived from chloroplast lipids play an important role in the regulation of plant defense responses.

Arabidopsis↗

PubMed2005

A bacterial inhibitor of host programmed cell death defenses is an E3 ubiquitin ligase.

The Pseudomonas syringae protein AvrPtoB is translocated into plant cells, where it inhibits immunity-associated programmed cell death (PCD). The structure of a C-terminal domain of AvrPtoB that is essential for anti-PCD activity reveals an unexpected homology to the U-box and RING-finger components of eukaryotic E3 ubiquitin ligases, and we show that AvrPtoB has ubiquitin ligase activity. Mutation of conserved residues involved in the binding of E2 ubiquitin-conjugating enzymes abolishes this activity in vitro, as well as anti-PCD activity in tomato leaves, which dramatically decreases virulence. These results show that Pseudomonas syringae uses a mimic of host E3 ubiquitin ligases to inactivate plant defenses.

Apoptosis↗

PubMed2006

A plant miRNA contributes to antibacterial resistance by repressing auxin signaling.

Plants and animals activate defenses after perceiving pathogen-associated molecular patterns (PAMPs) such as bacterial flagellin. In Arabidopsis, perception of flagellin increases resistance to the bacterium Pseudomonas syringae, although the molecular mechanisms involved remain elusive. Here, we show that a flagellin-derived peptide induces a plant microRNA (miRNA) that negatively regulates messenger RNAs for the F-box auxin receptors TIR1, AFB2, and AFB3. Repression of auxin signaling restricts P. syringae growth, implicating auxin in disease susceptibility and miRNA-mediated suppression of auxin signaling in resistance.

Arabidopsis↗

PubMed2006

A bacterial virulence protein suppresses host innate immunity to cause plant disease.

Plants have evolved a powerful immune system to defend against infection by most microbial organisms. However, successful pathogens, such as Pseudomonas syringae, have developed countermeasures and inject virulence proteins into the host plant cell to suppress immunity and cause devastating diseases. Despite intensive research efforts, the molecular targets of bacterial virulence proteins that are important for plant disease development have remained obscure. Here, we show that a conserved P. syringae virulence protein, HopM1, targets an immunity-associated protein, AtMIN7, in Arabidopsis thaliana. HopM1 mediates the destruction of AtMIN7 via the host proteasome. Our results illustrate a strategy by which a bacterial pathogen exploits the host proteasome to subvert host immunity and causes infection in plants.

ADP-Ribosylation Factors↗

PubMed2004

Frequency, size, and localization of bacterial aggregates on bean leaf surfaces.

Using epifluorescence microscopy and image analysis, we have quantitatively described the frequency, size, and spatial distribution of bacterial aggregates on leaf surfaces of greenhouse-grown bean plants inoculated with the plant-pathogenic bacterium Pseudomonas syringae pv. syringae strain B728a. Bacterial cells were not randomly distributed on the leaf surface but occurred in a wide range of cluster sizes, ranging from single cells to over 10(4) cells per aggregate. The average cluster size increased through time, and aggregates were more numerous and larger when plants were maintained under conditions of high relative humidity levels than under dry conditions. The large majority of aggregates observed were small (less than 100 cells), and aggregate sizes exhibited a strong right-hand-skewed frequency distribution. While large aggregates are not frequent on a given leaf, they often accounted for the majority of cells present. We observed that up to 50% of cells present on a leaf were located in aggregates containing 10(3) cells or more. Aggregates were associated with several different anatomical features of the leaf surface but not with stomates. Aggregates were preferentially associated with glandular trichomes and veins. The biological and ecological significance of aggregate formation by epiphytic bacteria is discussed.

Bacterial Adhesion↗

PubMed2005

Production of an engineered killer peptide in Nicotiana benthamiana by using a potato virus X expression system.

The decapeptide killer peptide (KP) derived from the sequence of a single-chain, anti-idiotypic antibody acting as a functional internal image of a microbicidal, broad-spectrum yeast killer toxin (KT) was shown to exert a strong microbicidal activity against human pathogens. With the aim to exploit this peptide to confer resistance to plant pathogens, we assayed its antimicrobial activity against a broad spectrum of phytopathogenic bacteria and fungi. Synthetic KP exhibited antimicrobial activity in vitro towards Pseudomonas syringae, Erwinia carotovora, Botrytis cinerea, and Fusarium oxysporum. KP was also expressed in plants by using a Potato virus X (PVX)-derived vector as a fusion to the viral coat protein, yielding chimeric virus particles (CVPs) displaying the heterologous peptide. Purified CVPs showed enhanced antimicrobial activity against the above-mentioned plant pathogens and human pathogens such as Staphylococcus aureus and Candida albicans. Moreover, in vivo assays designed to challenge KP-expressing plants (as CVPs) with Pseudomonas syringae pv. tabaci showed enhanced resistance to bacterial attack. The results indicate that the PVX-based display system is a high-yield, rapid, and efficient method to produce and evaluate antimicrobial peptides in plants, representing a milestone for the large-scale production of high-added-value peptides through molecular farming. Moreover, KP is a promising molecule to be stably engineered in plants to confer broad-spectrum resistance to phytopathogens.

Anti-Bacterial Agents↗

PubMed2004

Characterization of Cfa1, a monofunctional acyl carrier protein involved in the biosynthesis of the phytotoxin coronatine.

Cfa1 was overproduced in Escherichia coli and Pseudomonas syringae, and the degree of 4'-phosphopantetheinylation was determined. The malonyl-coenzyme A:acyl carrier protein transacylase (FabD) of P. syringae was overproduced and shown to catalyze malonylation of Cfa1, suggesting that FabD plays a role in coronatine biosynthesis. Highly purified Cfa1 did not exhibit self-malonylation activity.

Acyl Carrier Protein↗

PubMed2006

Formation of functional Tat translocases from heterologous components.

BACKGROUND: The Tat pathway transports folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of plants. In Eschericha coli, Tat transport requires the integral membrane proteins TatA, TatB and TatC. In this study we have tested the ability of tat genes from the eubacterial species Pseudomonas syringae, Streptomyces coelicolor and Aquifex aeolicus, to compensate for the absence of the cognate E. coli tat gene, and thus to form functional Tat translocases with E. coli Tat components. RESULTS: All three subunits of the Tat system from the Gram positive organism Streptomyces coelicolor were able to form heterologous translocases with substantive Tat transport activity. However, only the TatA and TatB proteins of Pseudomonas syringae were able to functionally interact with the E. coli Tat system even though the two organisms are closely related. Of the Tat components from the phylogenetically distant hyperthermophillic bacterium Aquifex aeolicus only the TatA proteins showed any detectable level of heterologous functionality. The heterologously expressed TatA proteins of S. coelicolor and A. aeolicus were found exclusively in the membrane fraction. CONCLUSION: Our results show that of the three Tat proteins, TatA is most likely to show cross-species complementation. By contrast, TatB and TatC do not always show cross-complementation, probably because they must recognise heterologous signal peptides. Since heterologously-expressed S. coelicolor TatA protein was functional and found only in the membrane fraction, it suggests that soluble forms of Streptomyces TatA reported by others do not play a role in protein export.

Bacterial Proteins↗

PubMed2002

Two pathways act in an additive rather than obligatorily synergistic fashion to induce systemic acquired resistance and PR gene expression.

BACKGROUND: Local infection with necrotizing pathogens induces whole plant immunity to secondary challenge. Pathogenesis-related genes are induced in parallel with this systemic acquired resistance response and thought to be co-regulated. The hypothesis of co-regulation has been challenged by induction of Arabidopsis PR-1 but not systemic acquired resistance in npr1 mutant plants responding to Pseudomonas syringae carrying the avirulence gene avrRpt2. However, experiments with ndr1 mutant plants have revealed major differences between avirulence genes. The ndr1-1 mutation prevents hypersensitive cell death, systemic acquired resistance and PR-1 induction elicited by bacteria carrying avrRpt2. This mutation does not prevent these responses to bacteria carrying avrB. RESULTS: Systemic acquired resistance, PR-1 induction and PR-5 induction were assessed in comparisons of npr1-2 and ndr1-1 mutant plants, double mutant plants, and wild-type plants. Systemic acquired resistance was displayed by all four plant lines in response to Pseudomonas syringae bacteria carrying avrB. PR-1 induction was partially impaired by either single mutation in response to either bacterial strain, but only fully impaired in the double mutant in response to avrRpt2. PR-5 induction was not fully impaired in any of the mutants in response to either avirulence gene. CONCLUSION: Two pathways act additively, rather than in an obligatorily synergistic fashion, to induce systemic acquired resistance, PR-1 and PR-5. One of these pathways is NPR1-independent and depends on signals associated with hypersensitive cell death. The other pathway is dependent on salicylic acid accumulation and acts through NPR1. At least two other pathways also contribute additively to PR-5 induction.

Arabidopsis↗

PubMed2005

[Kinetic parameters of single ion channels and stationary conductivities of phytotoxin modified lipid bilayers].

As shown earlier, phytotoxins produced by Pseudomonas syringae pv. syringae form ion channels of "small" and "large" conductance when incorporated into planar lipid membranes. The multilevel conductance is due to cluster organization of the channels (Kaulin et al., 1998; Gurnev et al., 2002). In this study the kinetic parameters of syringomycin E (SRE) and syringostatin A (SSA) channels in negatively charged bilayer lipid membranes were estimated. The average time of open state of the small channels (t(s)(open)) did not depend on transmembrane voltage (in the range of +/- 200 mV). The channel characteristics differed between two phytotoxins: the t(s)(open) for the SRE-channels was much larger than that for SSA-channels. An energetic diagram with two non-conducting states illustrating the formation of the small channel is proposed to explain the voltage independence of the kinetic parameters. The probability for synchronous functioning of small channels with SSA was higher than that with SRE. To analyse the role of the clusters in the biological activities of SRE and SSA, we estimated the cluster contribution to a net transmembrane currents to be 60 and 90%, respectively.

Bacterial Toxins↗

PubMed2006

An Arabidopsis homologue of human seven-in-absentia-interacting protein is involved in pathogen resistance.

Human seven-in-absentia (SIAH)-interacting protein (SIP) is a component of the E3 ligase complex targeting beta-catenin for destruction. Arabidopsis has one SIP protein (AtSIP) with 32% amino acid sequence identity to SIP. To investigate the functions of AtSIP, we isolated an atsip knockout mutant, and generated transgenic plants overexpressing AtSIP. The growth rates and morphologies of the atsip and transgenic plants were indistinguishable from those of wild type. However, atsip plants were more susceptible to Pseudomonas syringae infection, and the transgenic plants overexpressing AtSIP were more resistant. Consistent with this, RNA blot analysis showed that the AtSIP gene is strongly induced by wounding and hydrogen peroxide treatment. In addition, when plants were infected with P. syringae, AtSIP was transiently induced prior to PR-1 induction. These observations show that Arabidopsis AtSIP plays a role in resistance to pathogenic infection.

Arabidopsis↗

PubMed2006

CaWRKY2, a chili pepper transcription factor, is rapidly induced by incompatible plant pathogens.

WRKY family proteins are a class of plant-specific transcription factors involved in stress response signaling pathways. In this study a gene encoding a putative WRKY protein was isolated from a pepper EST database (http://genepool.kribb.re.kr). The cDNA, named Capsicum annuum WRKY2 (CaWRKY2), encodes a putative polypeptide of 548 amino acids, containing two WRKY domains with zinc finger motifs and two potential nuclear localization signals. Northern blot analyses showed that CaWRKY2 mRNA was preferentially induced during incompatible interactions of pepper plants with PMMoV, Pseudomonas syringae pv. syringae 61, and Xanthomonas axonopodis pv. vesicatoria race 3. Furthermore, CaWRKY2 transcripts were strongly induced by wounding and ethephon treatment, whereas only moderate expression was detected following treatment with salicylic acid and jasmonic acid. CaWRKY2 was translocated to the nucleus when a CaWRKY2-smGFP fusion construct was expressed in onion epidermal cells. CaWRKY2 also had transcriptional activation activity in yeast. Taken together our data suggest that CaWRKY2 is a pathogen-inducible transcription factor that may have a role in early defense responses to biotic and abiotic stresses.

Acetates↗

PubMed2001

Analysis of bacterial lipodepsipeptides by matrix-assisted laser desorption/ionisation time-of-flight and high-performance liquid chromatography with electrospray mass spectrometry.

Strains of certain plant pathogenic bacteria, in particular several pathovars of Pseudomonas syringae, are known to produce cyclic lipodepsipeptides (LDPs) endowed with peculiar structural features and noticeable biological activities. In this study, a mass spectrometry procedure is proposed for screening LDP-producing bacterial strains and for identifying and assessing individual LDPs. After matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) screening of thirteen P. syringae strains for LDP production, the extracts from culture filtrates of eight positive strains were subjected to electrospray mass spectrometry for the identification of LDPs. Five strains were found to produce two forms of syringomycins (SR-E and SR-G) and two forms of syringopeptin 25 (SP25A and SP25B); two strains produced SR-E, SR-G and a new form of SP22; one strain produced syringotoxin (ST) and syringostatin A (SS-A) in addition to SP25A and SP25B. The yield in culture of two major LPDs: SR-G (3.2-13.8 mg x L(-1)) and SP25A (41.6-231.5 mg x L(-1)) was assessed by and high-performance liquid chromatography with electrospray mass spectrometry (HPLC/ESI-MS) in both scan and single ion monitoring (SIM) modes. Results of this investigation showed that the mass spectrometry protocol developed here is a precise and reliable method for screening bacterial strains for LDP production and for assessing the amount of each metabolite under various culture conditions. This could be of practical value in view of potential applications, e.g. biocontrol of post-harvest fungal diseases.

Amino Acid Sequence↗

PubMed1998

Evidence for growth of strains of the plant epiphytic bacterium Erwinia herbicola and transconjugation among the bacterial strains in guts of the silkworm Bombyx mori.

Growth of plant epiphytic bacteria Erwinia herbicola and Pseudomonas syringae in guts of the silkworm, Bombyx mori, was studied. Fifth instar silkworm larvae were fed artificial diets supplemented with these bacteria for 6 to 12 h followed by uncontaminated diets. At 1, 3, and 6 days after feeding, bacteria were isolated from insect guts and feces. A much larger population of E. herbicola was detected in the samples collected 3 and 6 days after the inoculation than in samples collected after 1 day, indicating that these bacteria grew in the insect gut, while P. syringae was unable to survive. Transconjugation between E. herbicola strains in the insect gut was also examined. First, either a donor or a recipient strain was fed to the insects in artificial diets containing the bacteria during 12 h, and then pairing strains were fed during 12 h after starvation for 12 h. The conjugative plasmid pBPW1::Tn7 was transferred into recipient cells at very high frequencies (10(-1)/recipient after 3 days and 10(-3) after 6 days) in insect guts. Indigenous plasmids of E. herbicola mobilized RSF1010 plasmid into recipient cells at frequencies of 10(-4) in insect guts. These transconjugants were detected in the feces of the insects. Thus, plasmid-mediated gene transfer among the epiphytic bacteria in insect guts was demonstrated. The results obtained suggest that in insecta gene transfer may play an important role in the evolution of plant epiphytic bacteria.

Animals↗