Roles of salicylic acid, jasmonic acid, and ethylene in cpr-induced resistance in arabidopsis

Targeted activation tagging of the Arabidopsis NBS-LRR gene, ADR1, conveys resistance to virulent pathogens

A transgenic Arabidopsis line containing a chimeric PR-1::luciferase (LUC) reporter gene was subjected to mutagenesis with activation tags. Screening of lines via high-throughput LUC imaging identified a number of dominant Arabidopsis mutants that exhibited enhanced PR-1 gene expression. Here, we report the characterization of one of these mutants, designated activated disease resistance (adr) 1. This line showed constitutive expression of a number of key defense marker genes and accumulated salicylic acid but not ethylene or jasmonic acid. Furthermore, adr1 plants exhibited resistance against the biotrophic pathogens Peronospora parasitica and Erysiphe cichoracearum but not the necrotrophic fungus Botrytis cinerea. Analysis of a series of adr1 double mutants suggested that adr1-mediated resistance against P. parasitica was salicylic acid (SA)-dependent, while resistance against E. cichoracearum was both SA-dependent and partially NPR1-dependent. The ADR1 gene encoded a protein possessing a number of key features, including homology to subdomains of protein kinases, a nucleotide binding domain, and leucine-rich repeats. The controlled, transient expression of ADR1 conveyed striking disease resistance in the absence of yield penalty, highlighting the potential utility of this gene in crop protection.

The Arabidopsis NHL3 gene encodes a plasma membrane protein and its overexpression correlates with increased resistance to Pseudomonas syringae pv. tomato DC3000

The Arabidopsis genome contains a family of NDR1/HIN1-like (NHL) genes that show homology to the nonrace-specific disease resistance (NDR1) and the tobacco (Nicotiana tabacum) harpin-induced (HIN1) genes. NHL3 is a pathogen-responsive member of this NHL gene family that is potentially involved in defense. In independent transgenic NHL3-overexpressing plant lines, a clear correlation between increased resistance to virulent Pseudomonas syringae pv. tomato DC3000 and enhanced NHL3 transcript levels was seen. These transgenic plants did not show enhanced pathogenesis-related gene expression or reactive oxygen species accumulation. Biochemical and localization experiments were performed to assist elucidation of how NHL3 may confer enhanced disease resistance. Gene constructs expressing amino-terminal c-myc-tagged or carboxyl-terminal hemagglutinin epitope (HA)-tagged NHL3 demonstrated membrane localization in transiently transformed tobacco leaves. Stable Arabidopsis transformants containing the NHL3-HA construct corroborated the findings observed in tobacco. The detected immunoreactive proteins were 10 kD larger than the calculated size and could be partially accounted for by the glycosylation state. However, the expected size was not attained with deglycosylation, suggesting possibly additional posttranslational modification. Detergent treatment, but not chemicals used to strip membrane-associated proteins, could displace the immunoreactive signal from microsomal fractions, showing that NHL3 is tightly membrane associated. Furthermore, immunofluorescence and immunogold labeling, coupled with two-phase partitioning techniques, revealed plasma membrane localization of NHL3-HA. This subcellular localization of NHL3 positions it at an initial contact site to pathogens and may be important in facilitating interception of pathogen-derived signals.

Ethylene and jasmonic acid signaling affect the NPR1-independent expression of defense genes without impacting resistance to Pseudomonas syringae and Peronospora parasitica in the Arabidopsis ssi1 mutant

Salicylic acid (SA), ethylene, and jasmonic acid (JA) are important signaling molecules in plant defense to biotic stress. An intricate signaling network involving SA, ethylene, and JA fine tunes plant defense responses. SA-dependent defense responses in Arabidopsis thaliana are mediated through NPR1-dependent and -independent mechanisms. We have previously shown that activation of an NPR1-independent defense mechanism confers enhanced disease resistance and constitutive expression of the pathogenesis-related (PR) genes in the Arabidopsis ssi1 mutant. In addition, the ssi1 mutant constitutively expresses the defensin gene PDF1.2. Moreover, SA is required for the ssi1-conferred constitutive expression of PDF1.2 in addition to PR genes. Hence, the ssi1 mutant appears to target a step common to SA- and ethylene- or JA-regulated defense pathways. In the present study, we show that, in addition to SA, ethylene and JA signaling also are required for the ssi1-conferred constitutive expression of PDF1.2 and the NPR1-independent expression of PR-1. Furthermore, the ethylene-insensitive ein2 and JA-insensitive jar1 mutants enhance susceptibility of ssi1 plants to the necrotrophic fungus Botrytis cinerea. However, defects in either the ethylene- or JA-signaling pathways do not compromise ssi1-conferred resistance to the bacterial pathogen Pseudomonas synringae pv. maculicola and the oomycete pathogen Peronospora parasitica. Interestingly, ssi1 exhibits a marginal increase in the levels of ethylene and JA, suggesting that low endogenous levels of these phytohormones are sufficient to activate expression of defense genes. Taken together, our results indicate that although cross talk in ssi1 renders expression of ethylene- or JA-responsive defense genes sensitive to SA and vice versa, it does not affect downstream signaling leading to resistance.

The rapid induction of glutathione S-transferases AtGSTF2 and AtGSTF6 by avirulent Pseudomonas syringae is the result of combined salicylic acid and ethylene signaling

The expression of two members of the glutathione S-transferase (GST) multigene family was studied in Arabidopsis plants inoculated with an avirulent strain of Pseudomonas syringae pv. tomato (Pst). Accumulation of AtGSTF2 and AtGSTF6 transcripts started 4 and 2 h after inoculation, respectively, and clearly preceded the induction of the pathogenesis-related PR-1 gene. The aim of this work was to find the reason for the faster induction of the two GSTs compared with classical salicylic acid (SA)-regulated PR-proteins. Expression studies in Pst-inoculated SA-signaling mutants NahG and npr1 revealed that induction of both GSTs was SA-dependent and partially NPR1-independent. The induction of AtGSTF2 by Pst was also strongly repressed in the ethylene insensitive etr1 mutant. Both GSTs were induced by low amounts of SA (0.1 mM) and ethylene (0.1 ppm) while PR-1 gene expression was unaffected by ethylene. Interestingly, ethylene was about 50-fold less effective in NahG compared with wild-type plants thus suggesting a potentiation effect of SA on ethylene-induced accumulation of AtGST transcripts. Increased AtGST expression in plants inoculated with Pst correlated with increased production of SA and ethylene. However, the initial phase of AtGSTF6 induction was independent of SA- and ethylene-signaling. The jasmonate (JA)-insensitive mutant jar1 showed normal induction kinetics for both GSTs. Our data support the hypothesis that full expression of the pathogen-induced AtGSTF2 and, to a lesser extent AtGSTF6, is the result of combined SA- and ethylene-signaling and that early AtGSTF6 expression depends on additional unknown signaling mechanisms.

Arabidopsis local resistance to Botrytis cinerea involves salicylic acid and camalexin and requires EDS4 and PAD2, but not SID2, EDS5 or PAD4

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

Systemic gene expression in Arabidopsis during an incompatible interaction with Alternaria brassicicola

Pathogen challenge can trigger an integrated set of signal transduction pathways, which ultimately leads to a state of "high alert," otherwise known as systemic or induced resistance in tissue remote to the initial infection. Although large-scale gene expression during systemic acquired resistance, which is induced by salicylic acid or necrotizing pathogens has been previously reported using a bacterial pathogen, the nature of systemic defense responses triggered by an incompatible necrotrophic fungal pathogen is not known. We examined transcriptional changes that occur during systemic defense responses in Arabidopsis plants inoculated with the incompatible fungal pathogen Alternaria brassicicola. Substantial changes (2.00-fold and statistically significant) were demonstrated in distal tissue of inoculated plants for 35 genes (25 up-regulated and 10 down-regulated), and expression of a selected subset of systemically expressed genes was confirmed using real-time quantitative polymerase chain reaction. Genes with altered expression in distal tissue included those with putative functions in cellular housekeeping, indicating that plants modify these vital processes to facilitate a coordinated response to pathogen attack. Transcriptional up-regulation of genes encoding enzymes functioning in the beta-oxidation pathway of fatty acids was particularly interesting. Transcriptional up-regulation was also observed for genes involved in cell wall synthesis and modification and genes putatively involved in signal transduction. The results of this study, therefore, confirm the notion that distal tissue of a pathogen-challenged plant has a heightened preparedness for subsequent pathogen attacks.

Downy mildew of Arabidopsis thaliana caused by Hyaloperonospora parasitica (formerly Peronospora parasitica)

SUMMARY Downy mildew of Arabidopsis is not a hugely destructive disease of an important crop plant, neither is it of any economic importance. The most obvious symptom, the aerial conidiophores, might, at a glance to the casual observer, be mistaken for the trichomes normally present on the leaves. However, a huge research effort is being devoted to this humble pathosystem which became established as a laboratory model in the 1990s. Since then, enormous progress has been made in cloning and characterizing major genes for resistance (RPP genes) and in defining many of their downstream signalling components, some of them RPP-gene specific. Resistance is generally associated with an oxidative burst and a salicylic acid dependent hypersensitive reaction type of programmed cell death. Biological and chemical induction of systemic acquired resistance (SAR) in Arabidopsis protecting against downy mildew were demonstrated early on, and investigations of mutants have contributed fundamentally to our understanding of host-pathogen interactions and the mechanisms of plant defence. This review will attempt to collate the wealth of information which has accrued with this pathosystem in the last decade and will attempt to predict future research directions by drawing attention to some still unanswered questions.

TAXONOMY

Hyaloperonospora Constant. parasitica (Pers.:Fr) Fr. (formerly Peronospora parasitica), Kingdom Chromista, Phylum Oomycota, Order Peronosporales, Family Peronosporaceae, Genus Hyaloperonospora, of which it is the type species. The taxonomy of the group of organisms causing downy mildew of brassicas has undergone a number of revisions since Corda (1837) originally coined the genus Peronospora. All isolates pathogenic on brassicas were described initially as P. parasitica but Gäumann (1918) classified isolates from different brassicaceous hosts distinctly and thus defined 52 new species based on conidial dimensions and host range. After much debate it was decided to revert to the aggregate species of P. parasitica for all brassica-infecting downy mildews, whilst recognizing that these show some isolate-specific differences (Yerkes and Shaw, 1959). The latest re-examination of P. parasitica by Constantinescu and Fatehi (2002) has placed isolates of P. parasitica and five other downy mildew species in a clear new subgroup on the basis of their hyaline conidiospores, recurved conidiophore branch tips and ITS1, ITS2 and 5.8S rDNA sequence comparisons; meriting the coining of the new genus 'Hyaloperonospora Constant'. The class Oomycetes in the Kingdom Chromista (Straminipila) comprises fungus-like organisms with heterokont zoospores (i.e. possessing two types of flagellae, whiplash and tinsel). The Oomycetes have non-septate hyphae with cellulose-based walls containing very little or no chitin. The latter is regarded as a major distinction separating the Oomycetes from the true fungi, and reports of the presence of chitin had generally been regarded as due to small amounts of contamination (Gams et al., 1998). However, in view of recent studies by Werner et al. (2002) showing a chitin synthase gene in an Oomycete and demonstrating the presence of the polymer itself by an interaction with wheat germ agglutinin (WGA), it is perhaps safe to say that we have not seen the last taxonomic revision which will affect this group! The families within the Oomycetes show a clear evolutionary trend to a lesser absolute dependence on an aqueous environment and some members of the Peronosporales, e.g. H. parasitica, have no zoosporic stage in the life cycle.

HOST RANGE

Isolates infecting Arabidopsis thaliana have so far proven to be non-pathogenic on other crucifers tested but exist in a clear gene-for-gene relationship with different host ecotypes. Disease symptoms: Infections are first apparent to the naked eye as a carpet or 'down' of conidiophores covering the upper and lower surfaces of leaves and petioles. This symptom is characteristic of this group of diseases and lends it its name.

USEFUL WEBSITES

<http://ppathw3.cals.cornell.edu/PP644/ references.htm> (links to references on Oomycetes), <http://www.arabidopsis.org/> (TAIR, The Arabidopsis Information Resource).

Susceptible to intolerance--a range of hormonal actions in a susceptible Arabidopsis pathogen response

Ethylene and salicylic acid (SA) are key intermediates in a host's response to pathogens. Previously, we have shown using a tomato compatible interaction that ethylene and SA act sequentially and are essential for disease symptom production. Here, we have examined the relationship between the two signals in the Arabidopsis-Xanthomonas campestris pv. campestris (Xcc) compatible interaction. Preventing SA accumulation by expression of the nahG gene reduced subsequent ethylene production and altered the development of disease symptoms, with plants showing no visible chlorosis. The ethylene insensitive lines, etr1-1 and etr2-1, on the other hand, accumulated SA and exhibited normal but precocious symptom development. Therefore, Arabidopsis, like tomato, was found to exhibit co-operative ethylene and SA action for the production of disease symptoms. However, in Arabidopsis, SA was found to act upstream of ethylene. Jasmonic acid and indole-3-acetic acid levels were also found to increase in response to Xcc. In contrast to ethylene, accumulation of these hormones was not found to be dependent on SA action. These results indicate that the plants response to a virulent pathogen is a composite of multiple signaling pathways.

A gain-of-function mutation in an Arabidopsis Toll Interleukin1 receptor-nucleotide binding site-leucine-rich repeat type R gene triggers defense responses and results in enhanced disease resistance

In a screen for suppressors of npr1-5-based salicylic acid (SA) insensitivity, we isolated a semidominant gain-of-function mutation, designated ssi4, that confers constitutive expression of several PR (pathogenesis-related) genes, induces SA accumulation, triggers programmed cell death, and enhances resistance to bacterial and oomycete pathogens. Through map-based cloning, ssi4 was identified and found to encode a putative protein belonging to the TIR-NBS-LRR (Toll Interleukin1 Receptor-Nucleotide Binding Site-Leu-Rich Repeat) class of R (resistance) proteins. Comparison between ssi4 and the corresponding wild-type sequence revealed a single amino acid substitution in the NBS. Epistasis analysis indicated that SA and EDS1 are required for ssi4-induced PR-1 expression and enhanced disease resistance; they also are required for the increased accumulation of SSI4 and EDS1 transcripts detected in the ssi4 mutant. Although high levels of ssi4 transcripts correlate with the appearance of the mutant phenotype, overexpression of the wild-type SSI4 gene failed to induce stunting, spontaneous lesion formation, or increased PR-1 expression associated with the ssi4 mutation. Thus, the ssi4 phenotype does not appear to be caused by overexpression of this R gene; rather, we propose that the NBS substitution generates a constitutively activated R protein. Furthermore, because SA treatment induced the expression of SSI4 and the closely related TIR-NBS-LRR genes RPP1 and RPS4 but had little effect on the expression of the coiled-coil NBS-LRR genes RPM1 and RPS2, we suggest that SA not only functions as a critical signal for downstream resistance events but also upregulates the expression of certain R genes.

Cross talk between signaling pathways in pathogen defense

Plant defense in response to microbial attack is regulated through a complex network of signaling pathways that involve three signaling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene. The SA and JA signaling pathways are mutually antagonistic. This regulatory cross talk may have evolved to allow plants to fine-tune the induction of their defenses in response to different plant pathogens.

Overexpression of Pto induces a salicylate-independent cell death but inhibits necrotic lesions caused by salicylate-deficiency in tomato plants

Tomato plants overexpressing the disease resistance gene Pto (35S::Pto) exhibit spontaneous cell death, accumulation of salicylic acid (SA), elevated expression of pathogenesis-related genes, and enhanced resistance to a broad range of pathogens. Because salicylate plays an important role in the cell death and defense activation in many lesion mimic mutants, we investigated the interaction of SA-mediated processes and the 35S::Pto-mediated defense pathway by introducing the nahG transgene that encodes salicylate hydroxylase. Here, we show that SA is not required for the 35S::Pto-activated microscopic cell death and plays a minor role in defense gene activation and general disease resistance in 35S::Pto plants. In contrast, temperature greatly affects the spontaneous cell death and general resistance in 35S::Pto plants, and high temperature inhibits the cell death. The NahG tomato plants develop spontaneous, unconstrained necrotic lesions on leaves. These lesions also are initiated by the inoculation of a virulent strain of Pseudomonas syringae pv. tomato. However, the NahG-dependent necrotic lesions are inhibited in the NahG/35S::Pto plants. This inhibition is most pronounced under conditions favoring the 35S::Pto-mediated spontaneous cell death development. These results indicate that the signaling pathways activated by Pto overexpression suppress the cellular damage that is caused by SA depletion. We also found that ethylene is dispensable for the 35S::Pto-mediated general defense.

Dominant alleles of the basic helix-loop-helix transcription factor ATR2 activate stress-responsive genes in Arabidopsis

Members of the R/B basic helix-loop-helix (bHLH) family of plant transcription factors are involved in a variety of growth and differentiation processes. We isolated a dominant mutation in an R/B-related bHLH transcription factor in the course of studying Arabidopsis tryptophan pathway regulation. This mutant, atr2D, displayed increased expression of several tryptophan genes as well as a subset of other stress-responsive genes. The atr2D mutation creates an aspartate to asparagine change at a position that is highly conserved in R/B factors. Substitutions of other residues with uncharged side chains at this position also conferred dominant phenotypes. Moreover, overexpression of mutant atr2D, but not wild-type ATR2, conferred pleiotropic effects, including reduced size, dark pigmentation, and sterility. Therefore, atr2D is likely to be an altered-function allele that identifies a key regulatory site in the R/B factor coding sequence. Double-mutant analysis with atr1D, an overexpression allele of the ATR1 Myb factor previously isolated in tryptophan regulation screens, showed that atr2D and atr1D have additive effects on tryptophan regulation and are likely to act through distinct mechanisms to activate tryptophan genes. The dominant atr mutations thus provide tools for altering tryptophan metabolism in plants.

The role of ethylene and wound signaling in resistance of tomato to Botrytis cinerea

Ethylene, jasmonate, and salicylate play important roles in plant defense responses to pathogens. To investigate the contributions of these compounds in resistance of tomato (Lycopersicon esculentum) to the fungal pathogen Botrytis cinerea, three types of experiments were conducted: (a) quantitative disease assays with plants pretreated with ethylene, inhibitors of ethylene perception, or salicylate; (b) quantitative disease assays with mutants or transgenes affected in the production of or the response to either ethylene or jasmonate; and (c) expression analysis of defense-related genes before and after inoculation of plants with B. cinerea. Plants pretreated with ethylene showed a decreased susceptibility toward B. cinerea, whereas pretreatment with 1-methylcyclopropene, an inhibitor of ethylene perception, resulted in increased susceptibility. Ethylene pretreatment induced expression of several pathogenesis-related protein genes before B. cinerea infection. Proteinase inhibitor I expression was repressed by ethylene and induced by 1-methylcyclopropene. Ethylene also induced resistance in the mutant Never ripe. RNA analysis showed that Never ripe retained some ethylene sensitivity. The mutant Epinastic, constitutively activated in a subset of ethylene responses, and a transgenic line producing negligible ethylene were also tested. The results confirmed that ethylene responses are important for resistance of tomato to B. cinerea. The mutant Defenseless, impaired in jasmonate biosynthesis, showed increased susceptibility to B. cinerea. A transgenic line with reduced prosystemin expression showed similar susceptibility as Defenseless, whereas a prosystemin-overexpressing transgene was highly resistant. Ethylene and wound signaling acted independently on resistance. Salicylate and ethylene acted synergistically on defense gene expression, but antagonistically on resistance.

Impact of phyto-oxylipins in plant defense

Phyto-oxylipins are metabolites produced in plants by the oxidative transformation of unsaturated fatty acids via a series of diverging metabolic pathways. Biochemical dissection and genetic approaches have provided compelling evidence that these oxygenated derivatives actively participate in plant defense mechanisms. During the past decade, interest in this field was focused on the biosynthesis of jasmonic acid (one branch of C18 polyunsaturated fatty acid metabolism) and on its relationship to the other plant defense-signaling pathways. However, recently, antisense strategies have revealed that oxylipins other than jasmonates are probably also essential for the resistance of plants to pathogens.

Signals involved in Arabidopsis resistance to Trichoplusia ni caterpillars induced by virulent and avirulent strains of the phytopathogen Pseudomonas syringae

Plants have evolved different but interconnected strategies to defend themselves against herbivorous insects and microbial pathogens. We used an Arabidopsis/Pseudomonas syringae pathosystem to investigate the impact of pathogen-induced defense responses on cabbage looper (Trichoplusia ni) larval feeding. Arabidopsis mutants [npr1, pad4, eds5, and sid2(eds16)] or transgenic plants (nahG) that are more susceptible to microbial pathogens and are compromised in salicylic acid (SA)-dependent defense responses exhibited reduced levels of feeding by T. ni compared with wild-type plants. Consistent with these results, Arabidopsis mutants that are more resistant to microbial pathogens and have elevated levels of SA (cpr1 and cpr6) exhibited enhanced levels of T. ni feeding. These experiments suggested an inverse relationship between an active SA defense pathway and insect feeding. In contrast to these results, there was increased resistance to T. ni in wild-type Arabidopsis ecotype Columbia plants that were infected with P. syringae pv. maculicola strain ES4326 (Psm ES4326) expressing the avirulence genes avrRpt2 or avrB, which elicit a hypersensitive response, high levels of SA accumulation, and systemic acquired resistance to bacterial infection. Similar results were obtained with other ecotypes, including Landsberg erecta, Cape Verdi Islands, and Shakdara. When infected with Psm ES4326(avrRpt2) or Psm ES4326(avrB), nahG transgenic and npr1 mutant plants (which are more susceptible to virulent and avirulent P. syringae strains) failed to show the increased insect resistance exhibited by wild-type plants. It was surprising that wild-type plants, as well as nahG and npr1 plants, infected with Psm ES4326 not expressing avrRpt2 or avrB, which elicits disease, became more susceptible to T. ni. Our results suggest two potentially novel systemic signaling pathways: a systemic response elicited by HR that leads to enhanced T. ni resistance and overrides the SA-mediated increase in T. ni susceptibility, and a SA-independent systemic response induced by virulent pathogens that leads to enhanced susceptibility to T. ni.

Jasmonate response locus JAR1 and several related Arabidopsis genes encode enzymes of the firefly luciferase superfamily that show activity on jasmonic, salicylic, and indole-3-acetic acids in an assay for adenylation

Jasmonic acid (JA) and related cyclopentanones are critical plant signaling molecules, but their mode of action at the molecular level is unclear. A map-based approach was used to identify the defective gene in the Arabidopsis JA response mutant jar1-1. JAR1 is 1 of 19 closely related Arabidopsis genes that are similar to the auxin-induced soybean GH3 gene. Analysis of fold predictions for this protein family suggested that JAR1 might belong to the acyl adenylate-forming firefly luciferase superfamily. These enzymes activate the carboxyl groups of a variety of substrates for their subsequent biochemical modification. An ATP-PPi isotope exchange assay was used to demonstrate adenylation activity in a glutathione S-transferase-JAR1 fusion protein. Activity was specific for JA, suggesting that covalent modification of JA is important for its function. Six other Arabidopsis genes were specifically active on indole-3-acetic acid (IAA), and one was active on both IAA and salicylic acid. These findings suggest that the JAR1 gene family is involved in multiple important plant signaling pathways.

Role of salicylic acid and NIM1/NPR1 in race-specific resistance in arabidopsis

Salicylic acid (SA) and the NIM1/NPR1 protein have both been demonstrated to be required for systemic acquired resistance (SAR) and implicated in expression of race-specific resistance. In this work, we analyzed the role that each of these molecules play in the resistance response triggered by members of two subclasses of resistance (R) genes, members of which recognize unrelated pathogens. We tested the ability of TIR and coiled-coil-class (also known as leucine-zipper-class) R genes to confer resistance to Pseudomonas syringae pv. tomato or Peronospora parasitica in SA-depleted (NahG) and nim1/npr1 plants. We found that all of the P. syringae pv. tomato-specific R genes tested were dependent upon SA accumulation, while none showed strong dependence upon NIM1/NPR1 activity. A similar SA dependence was observed for the P. parasitica TIR and CC-class R genes RPP5 and RPP8, respectively. However, the P. parasitica-specific R genes differed in their requirement for NIM1/NPR1, with just RPP5 depending upon NIM1/NPR1 activity for effectiveness. These data are consistent with the hypothesis that at least in Arabidopsis, SA accumulation is necessary for the majority of R-gene-triggered resistance, while the role of NIM1/NPR in race-specific resistance is limited to resistance to P. parasitica mediated by TIR-class R genes.

Characterization of a novel, defense-related Arabidopsis mutant, cir1, isolated by luciferase imaging

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.

The Arabidopsis hrl1 mutation reveals novel overlapping roles for salicylic acid, jasmonic acid and ethylene signalling in cell death and defence against pathogens

Defence against pathogens in Arabidopsis is orchestrated by at least three signalling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). The hrl1 (hypersensitive response-like lesions 1) mutant of Arabidopsis is characterized by spontaneous necrotic lesions, accumulation of reactive oxygen species, constitutive expression of SA- and ET/JA-responsive defence genes, and enhanced resistance to virulent bacterial and oomycete pathogens. Epistasis analyses of hrl1 with npr1, etr1, coi1 and SA-depleted nahG plants revealed novel interactions between SA and ET/JA signalling pathways in regulating defence gene expression and cell death. RNA gel-blot analysis of RNA isolated separately from the lesion+ and the lesion- leaves of double mutants of hrl1 revealed different signalling requirements for the expression of defence genes in these tissues. Expression of the ET/JA-responsive PDF1.2 gene was markedly reduced in hrl1 npr1 and in SA-depleted hrl1 nahG plants. In hrl1 nahG plants, expression of PDF1.2 was regulated by benzathiadiazole in a concentration-dependent manner: induced at low concentration and suppressed at high concentration. The hrl1 etr1 plants lacked systemic PR-1 expression, and exhibited compromised resistance to virulent Pseudomonas syringae and Peronospora parasitica. Inhibiting JA responses in hrl1 coi1 plants lead to exaggerated cell death and severe stunting of plants. Finally, the hrl1 mutation lead to elevated expression of AtrbohD, which encodes a major subunit of the NADPH oxidase complex. Our results indicate that defence gene expression and resistance against pathogens in hrl1 is regulated synergistically by SA and ET/JA defence pathways.

RAR1 and NDR1 contribute quantitatively to disease resistance in Arabidopsis, and their relative contributions are dependent on the R gene assayed

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.

Isolation and characterization of broad-spectrum disease-resistant Arabidopsis mutants

To identify Arabidopsis mutants that constitutively express systemic acquired resistance (SAR), we constructed reporter lines expressing the firefly luciferase gene under the control of the SAR-inducible PR-1 promoter (PR-1/luc). After EMS mutagenesis of a well-characterized transgenic line, we screened 250,000 M(2) plants for constitutive expression of the reporter gene in vivo. From a mutant collection containing several hundred putative mutants, we concentrated on 16 mutants lacking spontaneous hypersensitive response (HR) cell death. We mapped 4 of these constitutive immunity (cim) mutants to chromosome arms. Constitutive expression of disease resistance was established by analyzing responses to virulent Peronospora parasitica and Pseudomonas syringae strains, by RNA blot analysis for endogenous marker genes, and by determination of salicylic acid levels in the mutants. The variety of the cim phenotypes allowed us to define distinct steps in both the canonical SAR signaling pathway and a separate pathway for resistance to Erysiphe cichoracearum, active in only a subset of the mutants.

Identification of Arabidopsis ethylene-responsive element binding factors with distinct induction kinetics after pathogen infection

Ethylene-responsive element binding factors (ERF) proteins are plant-specific transcription factors, many of which have been linked to stress responses. We have identified four Arabidopsis ERF genes whose expression was specifically induced by avirulent and virulent strains of the bacterial pathogen Pseudomonas syringae pv tomato, with overlapping but distinct induction kinetics. However, a delay in ERF mRNA accumulation after infection with the virulent strain was observed when compared with the avirulent strain. The induction of ERF gene expression in most cases preceded the mRNA accumulation of a basic chitinase gene, a potential downstream target for one or more of these ERFs. The expression of the ERF genes was examined among different Arabidopsis tissues, in response to the signaling molecules ethylene, methyl jasmonate, and salicylic acid (SA), and in Arabidopsis mutants with decreased or enhanced susceptibility to pathogens, and significant differences were observed. For example, in seedlings, some of the ERF genes were not induced by SA in the wild-type but were SA responsive in the pad4-1 mutant, suggesting that PAD4-1, which acts upstream of SA accumulation, is also involved in repressing the SA-induced expression of specific ERF genes. The four ERF proteins were shown to contain transcriptional activation domains. These results suggest that transcriptional activation cascades involving ERF proteins may be important for plant defense to pathogen attack and that some ERF family members could be involved in the cross-talk between SA- and jasmonic acid-signaling pathways.

Fusion genetic analysis of jasmonate-signalling mutants in Arabidopsis

Jasmonates induce plant-defence responses and act to regulate defence-related genes including positive feedback of the lipoxygenase 2 (LOX2) gene involved in jasmonate synthesis. To identify jasmonate-signalling mutants, we used a fusion genetic strategy in which the firefly luciferase (FLUC) and Escherichia coli beta-glucuronidase (GUS) reporters were expressed under control of the jasmonate-responsive LOX2 promoter. Spatial and temporal patterns of reporter expression were determined initially, and revealed that JA-responsive expression from the LOX2 promoter required de novo protein synthesis. Reporter activity was also induced by the protein kinase inhibitor staurosporine and antagonized by the protein phosphatase inhibitor okadaic acid. FLUC bio-imaging, RNA gel-blot analysis and progeny analyses identified three recessive mutants that underexpress the FLUC reporter, designated jue1, 2 and 3, as well as two recessive mutants, designated joe1 and 2, that overexpress the reporter. Genetic analysis indicated that reporter overexpression in the joe mutants requires COI. joe1 responded to MeJA with increased anthocyanin accumulation, while joe2 responded with decreased root growth inhibition. In addition, reporter induction and endogenous LOX2 expression by staurosporine was absent in joe2.

Runaway cell death, but not basal disease resistance, in lsd1 is SA- and NIM1/NPR1-dependent

LSD1 was defined as a negative regulator of plant cell death and basal disease resistance based on its null mutant phenotypes. We addressed the relationship between lsd1-mediated runaway cell death and signaling components required for systemic acquired resistance (SAR), namely salicylic acid (SA) accumulation and NIM1/NPR1. We present two important findings. First, SA accumulation and NIM1/NPR1 are required for lsd1-mediated runaway cell death following pathogen infection or application of chemicals that mimic SA action. This implies that lsd1-dependent cell death occurs 'downstream' of the accumulation of SA. As SA application triggers runaway cell death in lsd1 but not wild-type plants, we infer that LSD1 negatively regulates an SA-dependent signal leading to cell death. Thus SA is both a trigger and a required mediator of lsd1 runaway cell death. Second, neither SA accumulation nor NIM1/NPR1 function is required for the basal resistance operating in lsd1. Therefore LSD1 negatively regulates a basal defense pathway that can act upstream or independently of both NIM1/NPR1 function and SA accumulation following avirulent or virulent pathogen challenge. Our data, together with results from other studies, point to the existence of an SA-dependent 'signal potentiation loop' controlling HR. Continued escalation of signaling in the absence of LSD1 leads to runaway cell death. We propose that LSD1 is a key negative regulator of this signal potentiation.

Identification of a novel gene HYS1/CPR5 that has a repressive role in the induction of leaf senescence and pathogen-defence responses in Arabidopsis thaliana

We isolated two allelic mutants of Arabidopsis thaliana that showed an early senescence phenotype under light/dark and continuous dark photo regimes, and designated the mutant hypersenescence1 (hys1). The hys1 mutants decreased chlorophyll and protein content, lowered the efficiency of photosystem II, and accumulated several senescence upregulated gene transcripts earlier than the wild-type plants. In addition to these senescence features, the hys1 seedlings responded more intensely to exogenously applied sugars than did wild-type seedlings in sugar-induced growth inhibition and sugar-mediated transcript accumulation, both of which are known to be regulated by the sugar sensor hexokinase. The hys1 mutant also had abnormal trichomes. Map-based cloning of the HYS1 gene identified a novel gene that encodes a protein with a potential nuclear localization signal in the amino-terminal region, and five putative transmembrane domains in the carboxyl-terminal region. Furthermore, we found that the constitutive expressor of pathogenesis-related genes 5 (cpr5) mutant, which shows spontaneous pathogen-defence responses and abnormal trichomes, has a point mutation in the HYS1 gene, suggesting that these independently isolated mutants are allelic to each other. Although no definite conclusion can be drawn from these results, we suggest that altered sensitivity to sugars and/or enhanced efficiency of sugar signalling in the hys1/cpr5 mutant may have important roles in the initiation processes of leaf senescence and pathogen-defence responses in Arabidopsis.

Constitutive expression of ETHYLENE-RESPONSE-FACTOR1 in Arabidopsis confers resistance to several necrotrophic fungi

Infection of a plant by a pathogen induces a variety of defense responses that imply the action of several signaling molecules, including salicylic acid (SA), jasmonic acid (JA) and ethylene (E). Here we describe the role of ETHYLENE-RESPONSE-FACTOR1 (ERF1) as a regulator of ethylene responses after pathogen attack in Arabidopsis. The ERF1 transcript is induced on infection by Botrytis cinerea, and overexpression of ERF1 in Arabidopsis is sufficient to confer resistance to necrotrophic fungi such as B. cinerea and Plectosphaerella cucumerina. A positive co-operation between E and SA pathways was observed in the plant response to P. cucumerina. Infection by Pseudomonas syringae tomato DC3000, however, does not affect ERF1 expression, and activation of ethylene responses by ERF1 overexpression in Arabidopsis plants reduces tolerance against this pathogen, suggesting negative crosstalk between E and SA signaling pathways, and demonstrating that positive and negative interactions between both pathways can be established depending on the type of pathogen.

Characterization of an Arabidopsis-Phytophthora pathosystem: resistance requires a functional PAD2 gene and is independent of salicylic acid, ethylene and jasmonic acid signalling

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

Activation of an EDS1-mediated R-gene pathway in the snc1 mutant leads to constitutive, NPR1-independent pathogen resistance

The Arabidopsis NPR1 protein is an essential regulatory component of systemic acquired resistance (SAR). Mutations in the NPR1 gene completely block the induction of SAR by signals such as salicylic acid (SA). An Arabidopsis mutant, snc1 (suppressor of npr1-1, constitutive 1), was isolated in a screen for suppressors of npr1-1. In the npr1-1 background, the snc1 mutation resulted in constitutive resistance to Pseudomonas syringae maculicola ES4326 and Peronospora parasitica Noco2. High levels of SA were detected in the mutant and shown to be required for manifestation of the snc1 phenotype. The snc1 mutation was mapped to the RPP5 resistance (R) gene cluster and the eds1 mutation that blocks RPP5-mediated resistance suppressed snc1. These data suggest that a RPP5-related resistance pathway is activated constitutively in snc1. This pathway does not employ NPR1 but requires the signal molecule SA and the function of EDS1. Moreover, in snc1, constitutive resistance is conferred in the absence of cell death, which is often associated with R-gene mediated resistance.

Salicylic acid and NIM1/NPR1-independent gene induction by incompatible Peronospora parasitica in arabidopsis

To identify pathogen-induced genes distinct from those involved in systemic acquired resistance, we used cDNA-amplified fragment length polymorphism to examine RNA levels in Arabidopsis thaliana wild type, nim1-1, and salicylate hydroxylase-expressing plants after inoculation with an incompatible isolate of the downy mildew pathogen Peronospora parasitica. Fifteen genes are described, which define three response profiles on the basis of whether their induction requires salicylic acid (SA) accumulation and NIM1/NPR1 activity, SA alone, or neither. Sequence analysis shows that the genes include a calcium binding protein related to TCH3, a protein containing ankyrin repeats and potential transmembrane domains, three glutathione S-transferase gene family members, and a number of small, putatively secreted proteins. We further characterized this set of genes by assessing their expression patterns in each of the three plant lines after inoculation with a compatible P. parasitica isolate and after treatment with the SA analog 2,6-dichloroisonicotinic acid. Some of the genes within subclasses showed different requirements for SA accumulation and NIM1/NPR1 activity, depending upon which elicitor was used, indicating that those genes were not coordinately regulated and that the regulatory pathways are more complex than simple linear models would indicate.

Genetic dissection of systemic acquired resistance

Significant progress has been made in the past year in understanding the mechanism of systemic acquired resistance. Mitogen-activated protein kinase cascades have been implicated as negative regulators of salicyclic acid accumulation and the induction of resistance. The salicylic acid signal is transduced through NPR1, a nuclear-localized protein that interacts with transcription factors that are involved in regulating salicylic-acid-mediated gene expression. Both promoter analyses and genetic studies have shown that gene expression in systemic acquired resistance requires not only the activation of a transcriptional activator(s) but also inhibition of a transcriptional repressor(s). Microarray experiments have been performed to search for those genes whose expression is transcriptionally regulated during systemic acquired resistance and to identify common promoter elements that control these genes.

Genes controlling expression of defense responses in Arabidopsis--2001 status

In the past two years, the focus of studies of the genes controlling expression of defense responses in Arabidopsis has shifted from the identification of mutants to gene isolation and the ordering of genes within branches of the signal transduction networks. It is now clear that gene-for-gene resistance can be mediated through at least three genetically distinguishable pathways. Additional genes affecting salicylic-acid-dependent signaling have been identified, and double-mutant analyses have begun to reveal the order in which they act. Genes required for jasmonic-acid-dependent signaling and for induced systemic resistance have also been identified.

Numeric simulation of plant signaling networks

Plants have evolved an intricate signaling apparatus that integrates relevant information and allows an optimal response to environmental conditions. For instance, the coordination of defense responses against pathogens involves sophisticated molecular detection and communication systems. Multiple protection strategies may be deployed differentially by the plant according to the nature of the invading organism. These responses are also influenced by the environment, metabolism, and developmental stage of the plant. Though the cellular signaling processes traditionally have been described as linear sequences of events, it is now evident that they may be represented more accurately as network-like structures. The emerging paradigm can be represented readily with the use of Boolean language. This digital (numeric) formalism allows an accurate qualitative description of the signal transduction processes, and a dynamic representation through computer simulation. Moreover, it provides the required power to process the increasing amount of information emerging from the fields of genomics and proteomics, and from the use of new technologies such as microarray analysis. In this review, we have used the Boolean language to represent and analyze part of the signaling network of disease resistance in Arabidopsis.

Plant pathogens and integrated defence responses to infection

Plants cannot move to escape environmental challenges. Biotic stresses result from a battery of potential pathogens: fungi, bacteria, nematodes and insects intercept the photosynthate produced by plants, and viruses use replication machinery at the host's expense. Plants, in turn, have evolved sophisticated mechanisms to perceive such attacks, and to translate that perception into an adaptive response. Here, we review the current knowledge of recognition-dependent disease resistance in plants. We include a few crucial concepts to compare and contrast plant innate immunity with that more commonly associated with animals. There are appreciable differences, but also surprising parallels.