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PubMed2002

Arabidopsis SON1 is an F-box protein that regulates a novel induced defense response independent of both salicylic acid and systemic acquired resistance.

One of several induced defense responses in plants is systemic acquired resistance (SAR), which is regulated by salicylic acid and in Arabidopsis by the NIM1/NPR1 protein. To identify additional components of the SAR pathway or other genes that regulate SAR-independent resistance, we performed genetic suppressor screens of mutagenized nim1-1 seedlings, which are highly susceptible to infection by Peronospora parasitica. We isolated the son1 (suppressor of nim1-1) mutant, which shows full restoration of pathogen resistance without the induction of SAR-associated genes and expresses resistance when combined with a salicylate hydroxylase (nahG) transgene. These features indicate that son1-mediated resistance is distinct from SAR. Resistance is effective against both the virulent oomycete Peronospora and the bacterial pathogen Pseudomonas syringae pv tomato strain DC3000. We cloned SON1 and found it to encode a novel protein containing an F-box motif, an element found within the specificity determinant in the E3 ubiquitin-ligase complex. We propose the existence of a novel defense response that is independent of SAR and negatively regulated in Arabidopsis by SON1 through the ubiquitin-proteosome pathway.

Amino Acid Motifs↗
PubMed2003

HLM1, an essential signaling component in the hypersensitive response, is a member of the cyclic nucleotide-gated channel ion channel family.

The hypersensitive response (HR) in plants is a programmed cell death that is commonly associated with disease resistance. A novel mutation in Arabidopsis, hlm1, which causes aberrant regulation of cell death, manifested by a lesion-mimic phenotype and an altered HR, segregated as a single recessive allele. Broad-spectrum defense mechanisms remained functional or were constitutive in the mutant plants, which also exhibited increased resistance to a virulent strain of Pseudomonas syringae pv tomato. In response to avirulent strains of the same pathogen, the hlm1 mutant showed differential abilities to restrict bacterial growth, depending on the avirulence gene expressed by the pathogen. The HLM1 gene encodes a cyclic nucleotide-gated channel, CNGC4. Preliminary study of the HLM1/CNGC4 gene pro-duct in Xenopus oocytes (inside-out patch-clamp technique) showed that CNGC4 is permeable to both K(+) and Na(+) and is activated by both cGMP and cAMP. HLM1 gene expression is induced in response to pathogen infection and some pathogen-related signals. Thus, HLM1 might constitute a common downstream component of the signaling pathways leading to HR/resistance.

Alleles↗
PubMed2003

NPR1 modulates cross-talk between salicylate- and jasmonate-dependent defense pathways through a novel function in the cytosol.

Plant defenses against pathogens and insects are regulated differentially by cross-communicating signal transduction pathways in which salicylic acid (SA) and jasmonic acid (JA) play key roles. In this study, we investigated the molecular mechanism of the antagonistic effect of SA on JA signaling. Arabidopsis plants unable to accumulate SA produced 25-fold higher levels of JA and showed enhanced expression of the JA-responsive genes LOX2, PDF1.2, and VSP in response to infection by Pseudomonas syringae pv tomato DC3000, indicating that in wild-type plants, pathogen-induced SA accumulation is associated with the suppression of JA signaling. Analysis of the Arabidopsis mutant npr1, which is impaired in SA signal transduction, revealed that the antagonistic effect of SA on JA signaling requires the regulatory protein NPR1. Nuclear localization of NPR1, which is essential for SA-mediated defense gene expression, is not required for the suppression of JA signaling, indicating that cross-talk between SA and JA is modulated through a novel function of NPR1 in the cytosol.

Arabidopsis Proteins↗
PubMed2001

A humidity-sensitive Arabidopsis copine mutant exhibits precocious cell death and increased disease resistance.

The copines are a newly identified class of calcium-dependent, phospholipid binding proteins that are present in a wide range of organisms, including Paramecium, plants, Caenorhabditis elegans, mouse, and human. However, the biological functions of the copines are unknown. Here, we describe a humidity-sensitive copine mutant in Arabidopsis. Under nonpermissive, low-humidity conditions, the cpn1-1 mutant displayed aberrant regulation of cell death that included a lesion mimic phenotype and an accelerated hypersensitive response (HR). However, the HR in cpn1-1 showed no increase in sensitivity to low pathogen titers. Low-humidity-grown cpn1-1 mutants also exhibited morphological abnormalities, increased resistance to virulent strains of Pseudomonas syringae and Peronospora parasitica, and constitutive expression of pathogenesis-related (PR) genes. Growth of cpn1-1 under permissive, high-humidity conditions abolished the increased disease resistance, lesion mimic, and morphological mutant phenotypes but only partially alleviated the accelerated HR and constitutive PR gene expression phenotypes. The disease resistance phenotype of cpn1-1 suggests that the CPN1 gene regulates defense responses. Alternatively, the primary function of CPN1 may be the regulation of plant responses to low humidity, and the effect of the cpn1-1 mutation on disease resistance may be indirect.

Amino Acid Sequence↗
PubMed2002

EDS5, an essential component of salicylic acid-dependent signaling for disease resistance in Arabidopsis, is a member of the MATE transporter family.

The eds5 mutant of Arabidopsis (earlier named sid1) was shown previously to accumulate very little salicylic acid and PR-1 transcript after pathogen inoculation and to be hypersusceptible to pathogens. We have isolated EDS5 by positional cloning and show that it encodes a protein with a predicted series of nine to 11 membrane-spanning domains and a coil domain at the N terminus. EDS5 is homologous with members of the MATE (multidrug and toxin extrusion) transporter family. EDS5 expression is very low in unstressed plants and strongly induced by pathogens and UV-C light. The transcript starts to accumulate 2 hr after inoculation of Arabidopsis with an avirulent strain of Pseudomonas syringae or UV-C light exposure, and it stays induced for approximately 2 days. EDS5 also is expressed after treatments with salicylic acid, indicating a possible positive feedback regulation. EDS5 expression after infection by certain pathogens as well as after UV-C light exposure depends on the pathogen response proteins EDS1, PAD4, and NDR1, indicating that the signal transduction pathways after UV-C light exposure and pathogen inoculation share common elements.

Amino Acid Sequence↗
PubMed2002

Large-scale structure-function analysis of the Arabidopsis RPM1 disease resistance protein.

The Arabidopsis RPM1 gene confers resistance against Pseudomonas syringae expressing either the AvrRpm1 or the AvrB type III effector protein. We present an exhaustive genetic screen for mutants that no longer recognize avrRpm1. Using an inducible avrRpm1 expression system, we identified 110 independent mutations. These mutations represent six complementation groups. None discriminates between avrRpm1 and avrB recognition. We identified 95 rpm1 alleles and present a detailed structure--function analysis of the RPM1 protein. Several rpm1 mutants retain partial function, and we deduce that their residual activity is dependent on the level of avrRpm1 signal. In these mutants, the hypersensitive response remains activated if the signal goes above a certain threshold. Missense mutations in rpm1 are highly enriched in the nucleotide binding domain, suggesting that this region plays a key role either in the hypersensitive response associated with RPM1 activation or in RPM1 stability. Cluster analysis of rpm1 alleles defines functionally important residues that are highly conserved between nucleotide binding site leucine-rich repeat R proteins and those that are unique to RPM1. Regions of RPM1 to which no loss-of-function alleles map may represent domains in which variation is tolerated and may contribute to the evolution of new R gene specificities.

Alleles↗
PubMed2003

The BOTRYTIS SUSCEPTIBLE1 gene encodes an R2R3MYB transcription factor protein that is required for biotic and abiotic stress responses in Arabidopsis.

The molecular and cellular mechanisms involved in plant resistance to the necrotrophic fungal pathogen Botrytis cinerea and their genetic control are poorly understood. Botrytis causes severe disease in a wide range of plant species, both in the field and in postharvest situations, resulting in significant economic losses. We have isolated the BOS1 (BOTRYTIS-SUSCEPTIBLE1) gene of Arabidopsis based on a T-DNA insertion allele that resulted in increased susceptibility to Botrytis infection. The BOS1 gene is required to restrict the spread of another necrotrophic pathogen, Alternaria brassicicola, suggesting a common host response strategy against these pathogens. In the case of the biotrophic pathogens Pseudomonas syringae pv tomato and the oomycete parasite Peronospora parasitica, bos1 exhibits enhanced disease symptoms, but pathogen growth is similar in bos1 and wild-type plants. Strikingly, bos1 plants have impaired tolerance to water deficit, increased salinity, and oxidative stress. Botrytis infection induces the expression of the BOS1 gene. This increased expression is severely impaired in the coi1 mutant, suggesting an interaction of BOS1 with the jasmonate signaling pathway. BOS1 encodes an R2R3MYB transcription factor protein, and our results suggest that it mediates responses to signals, possibly mediated by reactive oxygen intermediates from both biotic and abiotic stress agents.

Amino Acid Sequence↗
PubMed2003

A gain-of-function mutation in a plant disease resistance gene leads to constitutive activation of downstream signal transduction pathways in suppressor of npr1-1, constitutive 1.

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive 1 (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR-type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.

Amino Acid Sequence↗
PubMed1998

Ethylene regulates the susceptible response to pathogen infection in tomato.

Ethylene evolution occurs concomitantly with the progression of disease symptoms in response to many virulent pathogen infections in plants. A tomato mutant impaired in ethylene perception-Never ripe-exhibited a significant reduction in disease symptoms in comparison to the wild type after inoculations of both genotypes with virulent bacterial (Xanthomonas campestris pv vesicatoria and Pseudomonas syringae pv tomato) and fungal (Fusarium oxysporum f sp lycopersici) pathogens. Bacterial spot disease symptoms were also reduced in tomato genotypes impaired in ethylene synthesis (1-aminocyclopropane-1-carboxylic acid deaminase) and perception (14893), thereby corroborating a reducing effect for ethylene insensitivity on foliar disease development. The reduction in foliar disease symptoms in Never ripe plants was a specific effect of ethylene insensitivity and was not due to reductions in bacterial populations or decreased ethylene synthesis. PR-1B1 mRNA accumulation in response to X. c. vesicatoria infection was not affected by ethylene insensitivity, indicating that ethylene is not required for defense gene induction. Our findings suggest that broad tolerance of diverse vegetative diseases may be achieved via engineering of ethylene insensitivity in tomato.

Cloning, Molecular↗
PubMed1998

Uncoupling PR gene expression from NPR1 and bacterial resistance: characterization of the dominant Arabidopsis cpr6-1 mutant.

In Arabidopsis, NPR1 mediates the salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance (SAR). Here, we report the identification of another component, CPR 6, that may function with NPR1 in regulating PR gene expression. The dominant CPR 6-1 mutant expresses the SA/NPR1-regulated PR genes (PR-1, BGL 2, and PR-5) and displays enhanced resistance to Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2 in the absence of SAR induction. cpr 6-1-induced PR gene expression is not suppressed in the cpr 6-1 npr1-1 double mutant but is suppressed when SA is removed by salicylate hydroxylase. Thus, constitutive PR gene expression in cpr 6-1 requires SA but not NPR1. In addition, resistance to P. s. maculicola ES4326 is suppressed in the cpr 6-1 npr1-1 double mutant, despite expression of PR-1, BGL 2, and PR-5. Resistance to P. s. maculicola ES4326 must therefore be accomplished through unidentified antibacterial gene products that are regulated through NPR1. These results show that CPR 6 is an important regulator of multiple signal transduction pathways involved in plant defense.

Arabidopsis↗
PubMed1998

PAD4 functions upstream from salicylic acid to control defense responses in Arabidopsis.

The Arabidopsis PAD4 gene was previously shown to be required for synthesis of camalexin in response to infection by the virulent bacterial pathogen Pseudomonas syringae pv maculicola ES4326 but not in response to challenge by the non-host fungal pathogen Cochliobolus carbonum. In this study, we show that pad4 mutants exhibit defects in defense responses, including camalexin synthesis and pathogenesis-related PR-1 gene expression, when infected by P. s. maculicola ES4 326. No such defects were observed in response to infection by an isogenic avirulent strain carrying the avirulence gene avrRpt2. In P. s. maculicola ES4 326-infected pad4 plants, synthesis of salicylic acid (SA) was found to be reduced and delayed when compared with SA synthesis in wild-type plants. Moreover, treatment of pad4 plants with SA partially reversed the camalexin deficiency and PR-1 gene expression phenotypes of P. s. maculicola ES4 326-infected pad4 plants. These findings support the hypothesis that PAD4 acts upstream from SA accumulation in regulating defense response expression in plants infected with P. s. maculicola ES4 326. A working model of the role of PAD4 in governing expression of defense responses is presented.

Anti-Infective Agents↗
PubMed1998

The root knot nematode resistance gene Mi from tomato is a member of the leucine zipper, nucleotide binding, leucine-rich repeat family of plant genes.

The Mi locus of tomato confers resistance to root knot nematodes. Tomato DNA spanning the locus was isolated as bacterial artificial chromosome clones, and 52 kb of contiguous DNA was sequenced. Three open reading frames were identified with similarity to cloned plant disease resistance genes. Two of them, Mi-1.1 and Mi-1.2, appear to be intact genes; the third is a pseudogene. A 4-kb mRNA hybridizing with these genes is present in tomato roots. Complementation studies using cloned copies of Mi-1.1 and Mi-1.2 indicated that Mi-1.2, but not Mi-1.1, is sufficient to confer resistance to a susceptible tomato line with the progeny of transformants segregating for resistance. The cloned gene most similar to Mi-1.2 is Prf, a tomato gene required for resistance to Pseudomonas syringae. Prf and Mi-1.2 share several structural motifs, including a nucleotide binding site and a leucine-rich repeat region, that are characteristic of a family of plant proteins, including several that are required for resistance against viruses, bacteria, fungi, and now, nematodes.

Amino Acid Sequence↗
PubMed1998

A novel signaling pathway controlling induced systemic resistance in Arabidopsis.

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1, the ethylene response mutant etr1, and the SAR regulatory mutant npr1, we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1, etr1, and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance.

Acetates↗
PubMed2004

Arabidopsis nonsymbiotic hemoglobin AHb1 modulates nitric oxide bioactivity.

Nitric oxide (NO) is a widespread signaling molecule, and numerous targets of its action exist in plants. Whereas the activity of NO in erythrocytes, microorganisms, and invertebrates has been shown to be regulated by several hemoglobins, the function of plant hemoglobins in NO detoxification has not yet been elucidated. Here, we show that Arabidopsis thaliana nonsymbiotic hemoglobin AHb1 scavenges NO through production of S-nitrosohemoglobin and reduces NO emission under hypoxic stress, indicating its role in NO detoxification. However, AHb1 does not affect NO-mediated hypersensitive cell death in response to avirulent Pseudomonas syringae, suggesting that it is not involved in the removal of NO bursts originated from acute responses when NO mediates crucial defense signaling functions.

Arabidopsis↗
PubMed2005

The membrane-anchored BOTRYTIS-INDUCED KINASE1 plays distinct roles in Arabidopsis resistance to necrotrophic and biotrophic pathogens.

Plant resistance to disease is controlled by the combination of defense response pathways that are activated depending on the nature of the pathogen. We identified the Arabidopsis thaliana BOTRYTIS-INDUCED KINASE1 (BIK1) gene that is transcriptionally regulated by Botrytis cinerea infection. Inactivation of BIK1 causes severe susceptibility to necrotrophic fungal pathogens but enhances resistance to a virulent strain of the bacterial pathogen Pseudomonas syringae pv tomato. The response to an avirulent bacterial strain is unchanged, limiting the role of BIK1 to basal defense rather than race-specific resistance. The jasmonate- and ethylene-regulated defense response, generally associated with resistance to necrotrophic fungi, is attenuated in the bik1 mutant based on the expression of the plant defensin PDF1.2 gene. bik1 mutants show altered root growth, producing more and longer root hairs, demonstrating that BIK1 is also required for normal plant growth and development. Whereas the pathogen responses of bik1 are mostly dependent on salicylic acid (SA) levels, the nondefense responses are independent of SA. BIK1 is membrane-localized, suggesting possible involvement in early stages of the recognition or transduction of pathogen response. Our data suggest that BIK1 modulates the signaling of cellular factors required for defense responses to pathogen infection and normal root hair growth, linking defense response regulation with that of growth and development.

Alternaria↗
PubMed2006

Within-species flagellin polymorphism in Xanthomonas campestris pv campestris and its impact on elicitation of Arabidopsis FLAGELLIN SENSING2-dependent defenses.

Bacterial flagellins have been portrayed as a relatively invariant pathogen-associated molecular pattern. We have found within-species, within-pathovar variation for defense-eliciting activity of flagellins among Xanthomonas campestris pv campestris (Xcc) strains. Arabidopsis thaliana FLAGELLIN SENSING2 (FLS2), a transmembrane leucine-rich repeat kinase, confers flagellin responsiveness. The flg22 region was the only Xcc flagellin region responsible for detectable elicitation of Arabidopsis defense responses. A Val-43/Asp polymorphism determined the eliciting/noneliciting nature of Xcc flagellins (structural gene fliC). Arabidopsis detected flagellins carrying Asp-43 or Asn-43 but not Val-43 or Ala-43, and it responded minimally for Glu-43. Wild-type Xcc strains carrying nonrecognized flagellin were more virulent than those carrying a recognized flagellin when infiltrated into Arabidopsis leaf mesophyll, but this correlation was misleading. Isogenic Xcc fliC gene replacement strains expressing eliciting or noneliciting flagellins grew similarly, both in leaf mesophyll and in hydathode/vascular colonization assays. The plant FLS2 genotype also had no detectable effect on disease outcome when previously untreated plants were infected by Xcc. However, resistance against Xcc was enhanced if FLS2-dependent responses were elicited 1 d before Xcc infection. Prior immunization was not required for FLS2-dependent restriction of Pseudomonas syringae pv tomato. We conclude that plant immune systems do not uniformly detect all flagellins of a particular pathogen species and that Xcc can evade Arabidopsis FLS2-mediated defenses unless the FLS2 system has been activated by previous infections.

Amino Acid Sequence↗
PubMed2006

The E3 ubiquitin ligase activity of arabidopsis PLANT U-BOX17 and its functional tobacco homolog ACRE276 are required for cell death and defense.

Previous analysis of transcriptional changes after elicitation of Cf-9 transgenic tobacco (Nicotiana tabacum) by Avr9 peptide revealed a rapidly upregulated gene, ACRE276. We show that ACRE276 is transiently induced in wounded leaves within 15 min, but upon Avr9 elicitor treatment, this upregulation is enhanced and maintained until cell death onset in Cf-9 tobacco. ACRE276 RNA interference (RNAi) silencing in tobacco results in loss of hypersensitive response (HR) specified by Cf resistance genes. ACRE276 RNAi plants are also compromised for HR mediated by the tobacco mosaic virus defense elicitor p50. Silencing tomato (Lycopersicon esculentum) ACRE276 leads to breakdown of Cf-9-specified resistance against Cladosporium fulvum leaf mold. We confirmed that tobacco ACRE276 is an E3 ubiquitin ligase requiring an intact U-box domain. Bioinformatic analyses revealed Arabidopsis thaliana PLANT U-BOX17 (PUB17) and Brassica napus ARC1 as the closest homologs of tobacco ACRE276. Transiently expressing PUB17 in Cf-9 tobacco silenced for ACRE276 restores HR, while mutant PUB17 lacking E3 ligase activity fails to do so, demonstrating that PUB17 ligase activity is crucial for defense signaling. Arabidopsis PUB17 knockout plants are compromised in RPM1- and RPS4-mediated resistance against Pseudomonas syringae pv tomato containing avirulence genes AvrB and AvrRPS4, respectively. We identify a conserved class of U-box ARMADILLO repeat E3 ligases that are positive regulators of cell death and defense across the Solanaceae and Brassicaceae.

Amino Acid Sequence↗
PubMed1999

Overexpression of Pto activates defense responses and confers broad resistance.

The tomato disease resistance (R) gene Pto specifies race-specific resistance to the bacterial pathogen Pseudomonas syringae pv tomato carrying the avrPto gene. Pto encodes a serine/threonine protein kinase that is postulated to be activated by a physical interaction with the AvrPto protein. Here, we report that overexpression of Pto in tomato activates defense responses in the absence of the Pto-AvrPto interaction. Leaves of three transgenic tomato lines carrying the cauliflower mosaic virus 35S::Pto transgene exhibited microscopic cell death, salicylic acid accumulation, and increased expression of pathogenesis-related genes. Cell death in these plants was limited to palisade mesophyll cells and required light for induction. Mesophyll cells of 35S::Pto plants showed the accumulation of autofluorescent compounds, callose deposition, and lignification. When inoculated with P. s. tomato without avrPto, all three 35S::Pto lines displayed significant resistance and supported less bacterial growth than did nontransgenic lines. Similarly, the 35S::Pto lines also were more resistant to Xanthomonas campestris pv vesicatoria and Cladosporium fulvum. These results demonstrate that defense responses and general resistance can be activated by the overexpression of an R gene.

Cell Death↗