Differential effectiveness of salicylate-dependent and jasmonate/ethylene-dependent induced resistance in Arabidopsis.
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Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are each involved in the regulation of basal resistance against different pathogens. These three signals play important roles in induced resistance as well. SA is a key regulator of pathogen-induced systemic acquired resistance (SAR), whereas JA and ET are required for rhizobacteria-mediated induced systemic resistance (ISR). Both types of induced resistance are effective against a broad spectrum of pathogens. In this study, we compared the spectrum of effectiveness of SAR and ISR using an oomycete, a fungal, a bacterial, and a viral pathogen. In noninduced Arabidopsis plants, these pathogens are primarily resisted through either SA-dependent basal resistance (Peronospora parasitica and Turnip crinkle virus [TCV]), JA/ET-dependent basal resistance responses (Alternaria brassicicola), or a combination of SA-, JA-, and ET-dependent defenses (Xanthomonas campestris pv. armoraciae). Activation of ISR resulted in a significant level of protection against A. brassicicola, whereas SAR was ineffective against this pathogen. Conversely, activation of SAR resulted in a high level of protection against P. parasitica and TCV, whereas ISR conferred only weak and no protection against P. parasitica and TCV, respectively. Induction of SAR and ISR was equally effective against X. campestris pv. armoraciae. These results indicate that SAR is effective against pathogens that in noninduced plants are resisted through SA-dependent defenses, whereas ISR is effective against pathogens that in noninduced plants are resisted through JA/ET-dependent defenses. This suggests that SAR and ISR constitute a reinforcement of extant SA- or JA/ET-dependent basal defense responses, respectively. (+info)
A tobacco S-like RNase inhibits hyphal elongation of plant pathogens.
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Ribonuclease (RNase) NE gene expression is induced in tobacco leaves in response to Phytophthora parasitica. Using antibodies directed against RNase NE, we demonstrate that RNase NE is extracellular at the early steps of the interaction, while the fungal tip growth is initiated in the apoplastic compartment. After production in Pichia pastoris and biochemical purification, we show that the S-like RNase NE inhibits hyphal growth from P. parasitica zoospores and from Fusarium oxysporum conidia in vitro. Conversion into an enzymatically inactive form after mutagenesis of the active site-histidine 97 residue to phenylalanine leads to the suppression of this activity, suggesting that RNase NE inhibits the elongation of germ tubes by degradation of microbial RNAs. Exogenous application of RNase NE in the extracellular space of leaves inhibits the development of P. parasitica. Based on its induction by inoculation, its localization, and its activity against two plant pathogens, we propose that RNase NE participates in tobacco defense mechanisms by a direct action on hyphal development in the extracellular space. The RNase activity-dependent antimicrobial activity of the S-like RNase NE shares similarities with the only other biological activity demonstrated for plant RNases, the inhibition of elongation of pollen tubes by the S-RNase in gametophytic self-incompatibility, suggesting a functional link between self and nonself interactions in plants. (+info)
Synthetic plant promoters containing defined regulatory elements provide novel insights into pathogen- and wound-induced signaling.
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Pathogen-inducible plant promoters contain multiple cis-acting elements, only some of which may contribute to pathogen inducibility. Therefore, we made defined synthetic promoters containing tetramers of only a single type of element and present evidence that a range of cis-acting elements (boxes W1, W2, GCC, JERE, S, Gst1, and D) can mediate local gene expression in planta after pathogen attack. The expression patterns of the promoters were monitored during interactions with a number of pathogens, including compatible, incompatible, and nonhost interactions. Interestingly, there were major differences in the inducibilities of the various promoters with the pathogens tested as well as differences in the speed of induction and in the basal expression levels. We also show that defense signaling is largely conserved across species boundaries at the cis-acting element level. Many of these promoters also direct local wound-induced expression, and this provides evidence for the convergence of resistance gene, nonhost, and wound responses at the level of the promoter elements. We have used these cis-acting elements to construct improved synthetic promoters and show the effects of varying the number, order, and spacing of such elements. These promoters are valuable additions to the study of signaling and transcriptional activation during plant-pathogen interactions. (+info)
Arabidopsis RAR1 exerts rate-limiting control of R gene-mediated defenses against multiple pathogens.
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We have identified the Arabidopsis ortholog of barley RAR1 as a component of resistance specified by multiple nucleotide binding/Leu-rich repeat resistance (R) genes recognizing different bacterial and oomycete pathogen isolates. Characterization of partially and fully defective rar1 mutations revealed that wild-type RAR1 acts as a rate-limiting regulator of early R gene-triggered defenses, determining the extent of pathogen containment, hypersensitive plant cell death, and an oxidative burst at primary infection sites. We conclude that RAR1 defense signaling function is conserved between plant species that are separated evolutionarily by 150 million years. RAR1 encodes a protein with two zinc binding (CHORD) domains that are highly conserved across eukaryotic phyla, and the single nematode CHORD-containing homolog, Chp, was found previously to be essential for embryo viability. An absence of obvious developmental defects in null Arabidopsis rar1 mutants favors the notion that, in contrast, RAR1 does not play a fundamental role in plant development. (+info)
RAR1 and NDR1 contribute quantitatively to disease resistance in Arabidopsis, and their relative contributions are dependent on the R gene assayed.
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Plant disease resistance (R) genes mediate specific pathogen recognition, leading to a successful immune response. Downstream responses include ion fluxes, an oxidative burst, transcriptional reprogramming, and, in many cases, hypersensitive cell death at the infection site. We used a transgenic Arabidopsis line carrying the bacterial avirulence gene avrRpm1 under the control of a steroid-inducible promoter to select for mutations in genes required for RPM1-mediated recognition and signal transduction. We identified an allelic series of eight mutants that also were allelic to the previously identified pbs2 mutation. Positional cloning revealed this gene to be AtRAR1, the Arabidopsis ortholog of barley RAR1, a known mediator of R function. AtRAR1 is required for both full hypersensitive cell death and complete disease resistance mediated by many, but not all, tested R genes. Double mutant analysis of Atrar1 in combination with the R signal intermediate ndr1 suggests that AtRAR1 and NDR1 can operate in both linear and parallel signaling events, depending on the R gene function triggered. In Atrar1 null plants, the levels of RPM1-myc are reduced severely, suggesting that AtRAR1 may regulate R protein stability or accumulation. (+info)
Genomics of phytopathogenic fungi and the development of bioinformatic resources.
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Genomic resources available to researchers studying phytopathogenic fungi are limited. Here, we briefly review the genomic and bioinformatic resources available and the current status of fungal genomics. We also describe a relational database containing sequences of expressed sequence tags (ESTs) from three phytopathogenic fungi, Blumeria graminis, Magnaporthe grisea, and Mycosphaerella graminicola, and the methods and underlying principles required for its construction. The database contains significant annotation for each EST sequence and is accessible at http://cogeme.ex.ac.uk. An easy-to-use interface allows the user to identify gene sequences by using simple text queries or homology searches. New querying functions and large sequence sets from a variety of phytopathogenic species will be incorporated in due course. (+info)
Characterization of a novel, defense-related Arabidopsis mutant, cir1, isolated by luciferase imaging.
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In order to identify components of the defense signaling network engaged following attempted pathogen invasion, we generated a novel PR-1::luciferase (LUC) transgenic line that was deployed in an imaging-based screen to uncover defense-related mutants. The recessive mutant designated cir1 exhibited constitutive expression of salicylic acid (SA), jasmonic acid (JA)/ethylene, and reactive oxygen intermediate-dependent genes. Moreover, this mutation conferred resistance against the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 and a virulent oomycete pathogen Peronospora parasitica Noco2. Epistasis analyses were undertaken between cir1 and mutants that disrupt the SA (nprl, nahG), JA (jar1), and ethylene (ET) (ein2) signaling pathways. While resistance against both P. syringae pv. tomato DC3000 and Peronospora parasitica Noco2 was partially reduced by npr1, resistance against both of these pathogens was lost in an nahG genetic background. Hence, cirl-mediated resistance is established via NPR1-dependent and -independent signaling pathways and SA accumulation is essential for the function of both pathways. While jar1 and ein2 reduced resistance against P. syringae pv. tomato DC3000, these mutations appeared not to impact cir1-mediated resistance against Peronospora parasitica Noco2. Thus, JA and ET sensitivity are required for cir1-mediated resistance against P. syringae pv. tomato DC3000 but not Peronospora parasitica Noco2. Therefore, the cir1 mutation may define a negative regulator of disease resistance that operates upstream of SA, JA, and ET accumulation. (+info)
NHL25 and NHL3, two NDR1/HIN1-1ike genes in Arabidopsis thaliana with potential role(s) in plant defense.
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The Arabidopsis genome contains 28 genes with sequence homology to the Arabidopsis NDR1 gene and the tobacco HIN1 gene. Expression analysis of eight of these genes identified two (NHL25 and NHL3 for NDR1/HIN1-like) that show pathogen-dependent mRNA accumulation. Transcripts did not accumulate during infection with virulent Pseudomonas syringae pv. tomato DC3000 but did accumulate specifically when the bacteria carried any of the four avirulence genes avrRpm1, avrRpt2, avrB, or avrRps4. Furthermore, expression of avrRpt2 in plants containing the corresponding resistance gene, RPS2, was sufficient to induce transcript accumulation. However, during infection with an avirulent oomycete, Peronospora parasitica isolate Cala-2, only NHL25 expression was reproducibly induced. Salicylic acid (SA) treatment can induce expression of NHL25 and NHL3. Studies performed on nahG plants showed that, during interaction with avirulent bacteria, only the expression of NHL25 but not that of NHL3 was affected. This suggests involvement of separate SA-dependent and SA-independent pathways, respectively, in the transcriptional activation of these genes. Bacteria-induced gene expression was not abolished in ethylene- (etrl-3 and ein2-1) and jasmonate- (coil-1) insensitive mutants or in mutants impaired in disease resistance (ndr1-1 and pad4-1). Interestingly, NHL3 transcripts accumulated after infiltration with the avirulent hrcC mutant of Pseudomonas syringae pv. tomato DC3000 and nonhost bacteria but not with the virulent Pseudomonas syringae pv. tomato DC3000, suggesting that virulent bacteria may suppress NHL3 expression during pathogenesis. Hence, the expression patterns and sequence homology to NDR1 and HIN1 suggest one or more potential roles for these genes in plant resistance. (+info)