Induction of Arabidopsis defense genes by virulent and avirulent Pseudomonas syringae strains and by a cloned avirulence gene

Mediation of pathogen resistance by exudation of antimicrobials from roots

Most plant species are resistant to most potential pathogens. It is not known why most plant-microbe interactions do not lead to disease, although recent work indicates that this basic disease resistance is multi-factorial. Here we show that the exudation of root-derived antimicrobial metabolites by Arabidopsis thaliana confers tissue-specific resistance to a wide range of bacterial pathogens. However, a Pseudomonas syringae strain that is both at least partly resistant to these compounds and capable of blocking their synthesis/exudation is able to infect the roots and cause disease. We also show that the ability of this P. syringae strain to block antimicrobial exudation is dependent on the type III secretory system.

Pseudomonas syringae manipulates systemic plant defenses against pathogens and herbivores

Many pathogens are virulent because they specifically interfere with host defense responses and therefore can proliferate. Here, we report that virulent strains of the bacterial phytopathogen Pseudomonas syringae induce systemic susceptibility to secondary P. syringae infection in the host plant Arabidopsis thaliana. This systemic induced susceptibility (SIS) is in direct contrast to the well studied avirulence/R gene-dependent resistance response known as the hypersensitive response that elicits systemic acquired resistance. We show that P. syringae-elicited SIS is caused by the production of coronatine (COR), a pathogen-derived functional and structural mimic of the phytohormone jasmonic acid (JA). These data suggest that SIS may be a consequence of the previously described mutually antagonistic interaction between the salicylic acid and JA signaling pathways. Virulent P. syringae also has the potential to induce net systemic susceptibility to herbivory by an insect (Trichoplusia ni, cabbage looper), but this susceptibility is not caused by COR. Rather, consistent with its role as a JA mimic, COR induces systemic resistance to T. ni. These data highlight the complexity of defense signaling interactions among plants, pathogens, and herbivores.

Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

Evolution of the core genome of Pseudomonas syringae, a highly clonal, endemic plant pathogen

Pseudomonas syringae is a common foliar bacterium responsible for many important plant diseases. We studied the population structure and dynamics of the core genome of P. syringae via multilocus sequencing typing (MLST) of 60 strains, representing 21 pathovars and 2 nonpathogens, isolated from a variety of plant hosts. Seven housekeeping genes, dispersed around the P. syringae genome, were sequenced to obtain 400 to 500 nucleotides per gene. Forty unique sequence types were identified, with most strains falling into one of four major clades. Phylogenetic and maximum-likelihood analyses revealed a remarkable degree of congruence among the seven genes, indicating a common evolutionary history for the seven loci. MLST and population genetic analyses also found a very low level of recombination. Overall, mutation was found to be approximately four times more likely than recombination to change any single nucleotide. A skyline plot was used to study the demographic history of P. syringae. The species was found to have maintained a constant population size over time. Strains were also found to remain genetically homogeneous over many years, and when isolated from sites as widespread as the United States and Japan. An analysis of molecular variance found that host association explains only a small proportion of the total genetic variation in the sample. These analyses reveal that with respect to the core genome, P. syringae is a highly clonal and stable species that is endemic within plant populations, yet the genetic variation seen in these genes only weakly predicts host association.

The use of vapor phase extraction in metabolic profiling of phytohormones and other metabolites

Through complex networks of signaling interactions, phytohormones regulate growth, development, reproduction and responses to biotic and abiotic stress. Comprehensive metabolomic approaches, seeking to quantify changes in vast numbers of plant metabolites, may ultimately clarify these complex signaling interactions and consequently explain pleiotropic effects on plant metabolism. Synergistic and antagonistic phytohormone signaling interactions, referred to as crosstalk, are often considered at the level of transduction without proper consideration of synthesis or accumulation of phytohormones because of the limitation and difficulty in quantifying numerous signals. Significant progress has recently been made in the expansion of metabolic profiling and analysis of multiple phytohormones [Birkemeyer et al. (J. Chromatogr. A, 2003, 993, 89); Chiwocha et al. (Plant J., 2003, 35, 405); Müller et al. (Planta, 2002, 216, 44); Schmelz et al. (Proc. Natl Acad. Sci. USA, 2003, 100, 10552)]. We recently presented a novel metabolic profiling approach to the analysis of acidic phytohormones and other metabolites based on a simplistic preparation scheme and analysis by chemical ionization-gas chromatography/mass spectrometry. We now provide a detailed description of this vapor phase extraction technique and use pathogen infection of Arabidopsis with Pseudomonas syringae DC3000 to illustrate metabolic changes in salicylic acid, cinnamic acid, jasmonic acid, indole-3-acetic acid, abscisic acid, unsaturated C(18) fatty acids, 12-oxo-phytodienoic acid, and phytotoxin coronatine. Directions for further method expansion are provided and include issues of recovery, derivatization, range of accessible analytes, optimization, reproducibility and future directions.

Magnaporthe grisea interactions with the model grass Brachypodium distachyon closely resemble those with rice (Oryza sativa)

SUMMARY Germplasm of Brachypodium distachyon was inoculated with Magnaporthe grisea using either rice- (Guy11) or grass-adapted (FAG1.1.1, PA19w-06, PA31v-01) host-limited forms of the fungus, and interactions with varying degrees of susceptibility and resistance were identified. Ecotype ABR5 was resistant to each M. grisea strain whereas ABR1 was susceptible to all but P31vi-01. Mendelian segregation in ABR1 x ABR5 crosses suggested that a single dominant resistance gene conferred resistance to Guy11. Microscopic analyses revealed that the aetiology of Guy11 fungal development and disease progression in ABR1 closely resembled that of rice infections. In ABR5, Guy11 pathogenesis was first suppressed at 48 h post-inoculation, at the secondary hyphal formation stage and was coincident with cytoplasmic granulation. Resistance to strains PA31v-01 and FAG1.1.1 was associated with a localized cell death with little callose deposition. 3,3-Diaminobenzidine staining indicated the elicitation of cell death in B. distachyon was preceded by oxidative stress in the interacting epidermal cells and the underlying mesophyll cells. Northern blot hybridization using probes for barley genes (PR1, PR5 and PAL) indicated that each was more rapidly expressed in ABR5 challenged with Guy11 although the B. distachyon defence genes BD1 and BD8 were more quickly induced in ABR1. Such data show that B. distachyon is an appropriate host for functional genomic investigations into M. grisea pathology and plant responses.

The timing of senescence and response to pathogens is altered in the ascorbate-deficient Arabidopsis mutant vitamin c-1

The ozone-sensitive Arabidopsis mutant vitamin c-1 (vtc1) is deficient in l-ascorbic acid (AsA) due to a mutation in GDP-Man pyrophosphorylase (Conklin et al., 1999), an enzyme involved in the AsA biosynthetic pathway (Smirnoff et al., 2001). In this study, the physiology of this AsA deficiency was initially investigated in response to biotic (virulent pathogens) stress and subsequently with regards to the onset of senescence. Infection with either virulent Pseudomonas syringae or Peronospora parasitica resulted in largely reduced bacterial and hyphal growth in the vtc1 mutant in comparison to the wild type. When vitamin c-2 (vtc2), another AsA-deficient mutant, was challenged with P. parasitica, growth of the fungus was also reduced, indicating that the two AsA-deficient mutants are more resistant to these pathogens. Induction of pathogenesis-related proteins PR-1 and PR-5 is significantly higher in vtc1 than in the wild type when challenged with virulent P. syringae. In addition, the vtc1 mutant exhibits elevated levels of some senescence-associated gene (SAG) transcripts as well as heightened salicylic acid levels. Presumably, therefore, low AsA is causing vtc1 to enter at least some stage(s) of senescence prematurely with an accompanying increase in salicylic acid levels that results in a faster induction of defense responses.

Technical advances: genome-wide cDNA-AFLP analysis of the Arabidopsis transcriptome

cDNA-AFLP, a technology historically used to identify small numbers of differentially expressed genes, was adapted as a genome-wide transcript profiling method. mRNA levels were assayed in a diverse range of tissues from Arabidopsis thaliana plants grown under a variety of environmental conditions. The resulting cDNA-AFLP fragments were sequenced. By linking cDNA-AFLP fragments to their corresponding mRNAs via these sequences, a database was generated that contained quantitative expression information for up to two-thirds of gene loci in A. thaliana, ecotype Ws. Using this resource, the expression levels of genes, including those with high nucleotide sequence similarity, could be determined in a high-throughput manner merely by comparing cDNA-AFLP profiles with the database. The lengths of cDNA-AFLP fragments inferred from their electrophoretic mobilities correlated well with actual fragment lengths determined by sequencing. In addition, the concentrations of AFLP fragments from single cDNAs were highly correlated, illustrating the validity of cDNA-AFLP as a quantitative, genome-wide, transcript profiling method. cDNA-AFLP profiles were also qualitatively consistent with mRNA profiles obtained from parallel microarray analysis, and with data from previous studies.

The Arabidopsis cytosolic thioredoxin h5 gene induction by oxidative stress and its W-box-mediated response to pathogen elicitor

The AtTRXh5 protein belongs to the cytosolic thioredoxins h family that, in Arabidopsis, contains eight members showing very distinct patterns and levels of expression. Here, we show that the AtTRXh5 gene is up-regulated during wounding, abscission, and senescence, as well as during incompatible interactions with the bacterial pathogen Pseudomonas syringae. By electrophoretic mobility shift assays, a binding activity on a W-box in the AtTRXh5 promoter region was found induced by treatments with the P. syringae-derived elicitor peptide flg22, suggesting that a WRKY transcription factor controls AtTRXh5 induction upon elicitor treatment. Remarkably, AtTRXh5 was up-regulated in plants overexpressing WRKY6. More generally, AtTRXh5 is induced in response to oxidative stress conditions. Collectively, our data indicate a possible implication of the cytosolic thioredoxin AtTRXh5 in response to pathogens and to oxidative stresses. In addition, this regulation is unique to AtTRXh5 among the thioredoxin h family, arguing in favor of a speciation rather than to a redundancy of the members of this multigenic family.

An extracellular aspartic protease functions in Arabidopsis disease resistance signaling

We have used activation tagging with T-DNA carrying cauliflower mosaic virus 35S enhancers to investigate the complex signaling networks underlying disease resistance in Arabidopsis. From a screen of approximately 5000 lines, we identified constitutive disease resistance (CDR1) encoding an apoplastic aspartic protease, the overexpression of which causes dwarfing and resistance to virulent Pseudomonas syringae. These phenotypes reflect salicylic-acid-dependent activation of micro-oxidative bursts and various defense-related genes. Antisense CDR1 plants were compromised for resistance to avirulent P. syringae and more susceptible to virulent strains than wild type. CDR1 accumulates in intercellular fluid in response to pathogen attacks. Induction of CDR1 generates a small mobile signal, and CDR1 action is blocked by the protease inhibitor pepstatin and by mutations in the protease active sites. We propose that CDR1 mediates a peptide signal system involved in the activation of inducible resistance mechanisms.

Activation of a COI1-dependent pathway in Arabidopsis by Pseudomonas syringae type III effectors and coronatine

Gram-negative bacteria use a variety of virulence factors including phytotoxins, exopolysaccharides, effectors secreted by the type III secretion system, and cell-wall-degrading enzymes to promote parasitism in plants. However, little is known about how these virulence factors alter plant cellular responses to promote disease. In this study, we show that virulent Pseudomonas syringae strains activate the transcription of an Arabidopsis ethylene response factor (ERF) gene, RAP2.6, in a coronatine insensitive 1 (COI1)-dependent manner. A highly sensitive RAP2.6 promoter-firefly luciferase (RAP2.6-LUC) reporter line was developed to monitor activities of various bacterial virulence genes. Analyses of P. syringae pv. tomato DC3000 mutants indicated that both type III secretion system and the phytotoxin coronatine are required for RAP2.6 induction. We show that at least five individual type III effectors, avirulence B (AvrB), AvrRpt2, AvrPphB, HopPtoK, and AvrPphEPto, contributed to RAP2.6 induction. Gene-for-gene recognition was not involved in RAP2.6 induction because plants lacking RPM1 and RPS2 responded normally to AvrB and AvrRpt2 in RAP2.6 expression. Interestingly, the role of coronatine in RAP2.6 induction can be partially substituted by the addition of avrB in DC3000, suggesting that AvrB may mimic coronatine. These results suggest that P. syringae type III effectors and coronatine act by augmenting a COI1-dependent pathway to promote parasitism.

The Pseudomonas syringae type III effector AvrRpt2 functions downstream or independently of SA to promote virulence on Arabidopsis thaliana

AvrRpt2, a Pseudomonas syringae type III effector protein, functions from inside plant cells to promote the virulence of P. syringae pv. tomato strain DC3000 (PstDC3000) on Arabidopsis thaliana plants lacking a functional copy of the corresponding RPS2 resistance gene. In this study, we extended our understanding of AvrRpt2 virulence activity by exploring the hypothesis that AvrRpt2 promotes PstDC3000 virulence by suppressing plant defenses. When delivered by PstDC3000, AvrRpt2 suppresses pathogen-related (PR) gene expression during infection, suggesting that AvrRpt2 suppresses defenses mediated by salicylic acid (SA). However, AvrRpt2 promotes PstDC3000 growth on transgenic plants expressing the SA-degrading enzyme NahG, indicating that AvrRpt2 does not promote bacterial virulence by modulating SA levels during infection. AvrRpt2 general virulence activity does not depend on the RPM1 resistance gene, as mutations in RPM1 had no effect on AvrRpt2-induced phenotypes. Transgenic plants expressing AvrRpt2 displayed enhanced susceptibility to PstDC3000 strains defective in type III secretion, indicating that enhanced susceptibility of these plants is not because of suppression of defense responses elicited by other type III effectors. Additionally, avrRpt2 transgenic plants did not exhibit increased susceptibility to Peronospora parasitica and Erysiphe cichoracearum, suggesting that AvrRpt2 virulence activity is specific to P. syringae.

ACD6, a novel ankyrin protein, is a regulator and an effector of salicylic acid signaling in the Arabidopsis defense response

The previously reported Arabidopsis dominant gain-of-function mutant accelerated cell death6-1 (acd6-1) shows spontaneous cell death and increased disease resistance. acd6-1 also confers increased responsiveness to the major defense signal salicylic acid (SA). To further explore the role of ACD6 in the defense response, we cloned and characterized the gene. ACD6 encodes a novel protein with putative ankyrin and transmembrane regions. It is a member of one of the largest uncharacterized gene families in higher plants. Steady state basal expression of ACD6 mRNA required light, SA, and an intact SA signaling pathway. Additionally, ACD6 mRNA levels were increased in the systemic, uninfected tissue of Pseudomonas syringae-infected plants as well as in plants treated with the SA agonist benzothiazole (BTH). A newly isolated ACD6 loss-of-function mutant was less responsive to BTH and upon P. syringae infection had reduced SA levels and increased susceptibility. Conversely, plants overexpressing ACD6 showed modestly increased SA levels, increased resistance to P. syringae, and BTH-inducible and/or a low level of spontaneous cell death. Thus, ACD6 is a necessary and dose-dependent activator of the defense response against virulent bacteria and can activate SA-dependent cell death.

Differential volatile emissions and salicylic acid levels from tobacco plants in response to different strains of Pseudomonas syringae

Pathogen-induced plant responses include changes in both volatile and non-volatile secondary metabolites. To characterize the role of bacterial pathogenesis in plant volatile emissions, tobacco plants, Nicotiana tabacum L. K326, were inoculated with virulent, avirulent, and mutant strains of Pseudomonas syringae. Volatile compounds released by pathogen-inoculated tobacco plants were collected, identified, and quantified. Tobacco plants infected with the avirulent strains P. syringae pv. maculicola ES4326 (Psm ES4326) or pv. tomato DC3000 (Pst DC3000), emitted quantitatively different, but qualitatively similar volatile blends of (E)-beta-ocimene, linalool, methyl salicylate (MeSA), indole, caryophyllene, beta-elemene, alpha-farnesene, and two unidentified sesquiterpenes. Plants treated with the hrcC mutant of Pst DC3000 (hrcC, deficient in the type-III secretion system) released low levels of many of the same volatile compounds as in Psm ES4326- or Pst DC3000-infected plants, with the exception of MeSA, which occurred only in trace amounts. Interaction of the virulent pathogen P. syringae pv. tabaci (Pstb), with tobacco plants resulted in a different volatile blend, consisting of MeSA and two unidentified sesquiterpenes. Overall, maximum volatile emissions occurred within 36 h post-inoculation in all the treatments except for the Pstb infection that produced peak volatile emissions about 60 h post-inoculation. (E)-beta-Ocimene was released in a diurnal pattern with the greatest emissions during the day and reduced emissions at night. Both avirulent strains, Psm ES4326 and Pst DC3000, induced accumulation of free salicylic acid (SA) within 6 h after inoculation and conjugated SA within 60 h and 36 h respectively. In contrast, SA inductions by the virulent strain Pstb occurred much later and conjugated SA increased slowly for a longer period of time, while the hrcC mutant strain did not trigger free and conjugated SA accumulations in amounts significantly different from control plants. Jasmonic acid, known to induce plant volatile emissions, was not produced in significantly higher levels in inoculated plants compared to the control plants in any treatments, indicating that induced volatile emissions from tobacco plants in response to P. syringae are not linked to changes in jasmonic acid.

Defense Gene Expression Analysis of Arabidopsis thaliana Parasitized by Orobanche ramosa

ABSTRACT The infection of Arabidopsis thaliana roots with the obligate parasite Orobanche ramosa represents a useful model for a study of the molecular events involved in the host plant response to a parasitic plant attack. To avoid analysis problems due to the subterranean development of O. ramosa, we developed two in vitro co-culture systems: O. ramosa seedlings infesting Arabidopsis plantlet roots and callus tissues. We were then able to investigate the expression patterns of some host plant genes selected among genes known to be involved in metabolic pathways and resistance mechanisms activated during several plant-pathogen interactions including ethylene, isoprenoid, phenylpropanoid, and jasmonate biosynthesis pathways, oxidative stress responses, and pathogenesis-related proteins. Molecular analyses were carried out using polymerase chain reaction amplification methods allowing semiquantitative evaluation of transcript accumulation during early (first hours) and late (15 days) stages of infestation, in whole roots or parts close to the parasite attachment site. In A. thaliana, O. ramosa induced most of the general response signaling pathways in a transient manner even before its attachment to A. thaliana roots. However, no salicylic acid-dependent defense is observed because no activation of systemic acquired resistance markers is detectable, whereas genes, co-regulated by jasmonate and ethylene, do display enhanced expression.

Quantitative nature of Arabidopsis responses during compatible and incompatible interactions with the bacterial pathogen Pseudomonas syringae

We performed large-scale mRNA expression profiling using an Affymetrix GeneChip to study Arabidopsis responses to the bacterial pathogen Pseudomonas syringae. The interactions were compatible (virulent bacteria) or incompatible (avirulent bacteria), including a nonhost interaction and interactions mediated by two different avirulence gene-resistance (R) gene combinations. Approximately 2000 of the approximately 8000 genes monitored showed reproducible significant expression level changes in at least one of the interactions. Analysis of biological variation suggested that the system behavior of the plant response in an incompatible interaction was robust but that of a compatible interaction was not. A large part of the difference between incompatible and compatible interactions can be explained quantitatively. Despite high similarity between responses mediated by the R genes RPS2 and RPM1 in wild-type plants, RPS2-mediated responses were strongly suppressed by the ndr1 mutation and the NahG transgene, whereas RPM1-mediated responses were not. This finding is consistent with the resistance phenotypes of these plants. We propose a simple quantitative model with a saturating response curve that approximates the overall behavior of this plant-pathogen system.

Direct delivery of bacterial avirulence proteins into resistant Arabidopsis protoplasts leads to hypersensitive cell death

Many bacterial avirulence (Avr) proteins, including the Pseudomonas syringae proteins, AvrRpt2 and AvrB, appear to be recognized inside the host plant cell by resistance mechanisms mediated by the cognate resistance (R) genes. It is thought that Avr proteins are either delivered directly into the host cell via the bacterial type III secretion system (TTSS) or taken up by the plant cell following secretion into the apoplast through the TTSS. Recently, it was shown that the Xanthomonas campestris AvrBs2 protein can be delivered directly into the host plant cell by the TTSS. However, it is not known whether other type III effectors of phytopathogens behave similarly. Here, using a novel protein transfection method, we demonstrate that AvrRpt2 and AvrB must enter the plant cell to be recognized by R gene-mediated mechanisms. First, we established a hypersensitive cell death assay for protoplasts using the membrane-impermeable, nuclear-staining dye, YO-PRO-1, and transgenic Arabidopsis plants that carry an inducible avrRpt2 gene. Second, we transfected E. coli-produced AvrRpt2 or AvrB proteins into Arabidopsis protoplasts using a protein transfection kit based on the carrier peptide Pep-1, and demonstrated that hypersensitive cell death occurs in a gene-for-gene-specific manner. In contrast, these Avr proteins failed to elicit hypersensitive cell death when they were applied to protoplasts without the carrier peptide. We conclude that our preparations of E. coli-produced AvrRpt2 and AvrB are active, that AvrRpt2 and AvrB must be delivered into the plant cell to be recognized, and that a method based on a carrier peptide can be used to introduce proteins into plant cells.

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.

Induced instability of two Arabidopsis constitutive pathogen-response alleles

Paramutation is an example of a non-Mendelian-directed allelic interaction that results in the epigenetic alteration of one allele. We describe a paramutation-like interaction between two alleles, bal and cpr1-1 (constitutive expressor of PR genes 1), which map to a complex R-like gene cluster on Arabidopsis chromosome 4. Both alleles cause dwarfing and constitutive defense responses, similar to another dwarf variant, ssi1 (suppressor of SA-insensitivity 1). Previous work has demonstrated that the bal and ssi1 phenotypes are caused by overexpression of an R-like gene from the cluster, which activates an salicylic acid-dependent defense pathway. Here, we show that the cpr1-1 variant does not alter gene expression from the R-like gene cluster. The bal and cpr1-1 alleles did not complement each other in F(1) hybrids, but F(2) populations that segregated bal and cpr1-1 alleles contained plants with normal morphology at a frequency of 20%. By using molecularly marked bal and cpr1-1 lines, we found that the majority of the normal phenotypes were correlated with inheritance of an altered cpr1-1 allele. Our observation that cpr1-1 is a metastable allele suggests that cpr1-1 is an epigenetic allele. The cpr1-1 allele is the third candidate epigenetic allele originating from this R-like gene cluster, making the region a possible hotspot of epigenetic variation.

Isolation of differential genes in suspension cultures of Taxus cuspidata induced by additional taxol

Addition of taxol into suspension cultures of Taxus cuspidata induced cell apoptosis, which was confirmed by gel electrophoresis of the DNA ladders indicating the progressive delineation of fragmented nuclear DNA (nDNA) into distinct bodies. The additional taxol not only changed the microtubule assembly of cells, but also affected the gene expression. Fourteen cDNA fragments, named as TIGT9-22, were isolated after addition of taxol and their GenBank accession numbers were given as BF704560-BF704573, respectively. Among them, TIGT13 and TIGT21 were apparently homogeneous with apbE and carbamoylphosphate synthetase, respectively. Other cDNA fragments showed no significant analogy with the known sequences in GenBank.

Induction of pepper cDNA encoding a lipid transfer protein during the resistance response to tobacco mosaic virus

Pepper (Capsicum annuum) plants exhibit hypersensitive response (HR) against infection by many tobamoviruses. A clone encoding a putative nonspecific lipid transfer protein (CaLTP1) was isolated by differential screening of a cDNA library from resistant pepper leaves when inoculated with tobacco mosaic virus (TMV) pathotype P0. The predicted amino acid sequence of CaLTP1 is highly similar to that of the other plant LTPs. Southern blot analysis showed that a small gene family of LTP-related sequences was present in the pepper genome. Transcripts homologous to CaLTP1 accumulated abundantly in old leaves and flowers. CaLTP1 expression was induced in the incompatible interaction with TMV-P0 but was not induced in the compatible interaction with TMV-P1.2. In correlation with the temporal progression of HR in the inoculated leaves, CaLTP1 transcripts started to accumulate at 24 h after TMV-P0 inoculation, reaching a maximal level at 48 h. A strain of Xanthomonas campestris pv. vesicatoria (Xcv) that carries the bacterial avirulence gene, avrBs2, was infiltrated into leaves of a pepper cultivar containing the Bs2 resistance gene. A marked induction of CaLTP1 expression was observed in Xcv-infiltrated leaves. Effects of exogenously applied abiotic elicitors on CaLTP1 expression were also examined. Salicylic acid caused a rapid accumulation of CaLTP1 transcripts in pepper leaves and ethephon treatment also induced the expression of the CaLTP1 gene. Transient expression in the detached pepper leaves by biolistic gene bombardment indicated that CaLTP1 is localized mostly at the plant cell surface, possibly in the cell wall. These results suggest possible role(s) for LTPs in plant defense against pathogens including viruses.

Epigenetic variation in Arabidopsis disease resistance

Plant pathogen resistance is mediated by a large repertoire of resistance (R) genes, which are often clustered in the genome and show a high degree of genetic variation. Here, we show that an Arabidopsis thaliana R-gene cluster is also subject to epigenetic variation. We describe a heritable but metastable epigenetic variant bal that overexpresses the R-like gene At4g16890 from a gene cluster on Chromosome 4. The bal variant and Arabidopsis transgenics overexpressing the At4g16890 gene are dwarfed and constitutively activate the salicylic acid (SA)-dependent defense response pathway. Overexpression of a related R-like gene also occurs in the ssi1 (suppressor of SA insensitivity 1) background, suggesting that ssi1 is mechanistically related to bal.

Differential roles of the Pseudomonas aeruginosa PA14 rpoN gene in pathogenicity in plants, nematodes, insects, and mice

We cloned the rpoN (ntrA, glnF) gene encoding the alternate sigma factor sigma(54) from the opportunistic multihost pathogen Pseudomonas aeruginosa strain PA14. A marker exchange protocol was used to construct the PA14 rpoN insertional mutation rpoN::Gen(r). PA14 rpoN::Gen(r) synthesized reduced levels of pyocyanin and displayed a variety of phenotypes typical of rpoN mutants, including a lack of motility and the failure to grow on nitrate, glutamate, or histidine as the sole nitrogen source. Compared to wild-type PA14, rpoN::Gen(r) was ca. 100-fold less virulent in a mouse thermal injury model and was significantly impaired in its ability to kill the nematode Caenorhabditis elegans. In an Arabidopsis thaliana leaf infectivity assay, although rpoN::Gen(r) exhibited significantly reduced attachment to trichomes, stomata, and the epidermal cell surface, did not attach perpendicularly to or perforate mesophyll cell walls, and proliferated less rapidly in Arabidopsis leaves, it nevertheless elicited similar disease symptoms to wild-type P. aeruginosa PA14 at later stages of infection. rpoN::Gen(r) was not impaired in virulence in a Galleria mellonella (greater wax moth) pathogenicity model. These data indicate that rpoN does not regulate the expression of any genes that encode virulence factors universally required for P. aeruginosa pathogenicity in diverse hosts.

A high-throughput method for quantifying growth of phytopathogenic bacteria in Arabidopsis thaliana

Measuring the growth of pathogenic bacteria in leaves is a mainstay of plant pathology studies. We have made significant improvements to standard methods that will not only increase the throughput but also reduce the space limitations. Additionally, the method described here is as accurate as the standard method. Briefly, we infected leaves by dipping whole seedlings of Arabidopsis into a bacterial solution containing a surfactant. After harvest, the seedlings were then simply shaken in buffer. The resulting bacterial solutions were diluted in microtitre plates and spotted onto agar plates. Colony-forming units were then counted 40 h after plating. Therefore, we have eliminated most of the labour-intensive steps involved in measuring the growth of bacteria in Arabidopsis, and describe a method that could be automated. The assay is sensitive enough to detect small differences between pathogens or ecotypes.

The roles of mucD and alginate in the virulence of Pseudomonas aeruginosa in plants, nematodes and mice

We are exploiting the broad host range of the human opportunistic pathogen Pseudomonas aeruginosa strain PA14 to elucidate the molecular basis of bacterial virulence in plants, nematodes, insects and mice. In this report, we characterize the role that two PA14 gene products, MucD and AlgD, play in virulence. MucD is orthologous to the Escherichia coli periplasmic protease and chaperone DegP. DegP homologues are known virulence factors that play a protective role in stress responses in various species. AlgD is an enzyme involved in the biosynthesis of the exopolysaccharide alginate, which is hyperinduced in mucD mutants. A PA14 mucD mutant was significantly impaired in its ability to cause disease in Arabidopsis thaliana and mice and to kill the nematode Caenorhabditis elegans. Moreover, MucD was found to be required for the production of an extracellular toxin involved in C. elegans killing. In contrast, a PA14 algD mutant was not impaired in virulence in plants, nematodes or mice. A mucDalgD double mutant had the same phenotype as the mucD single mutant in the plant and nematode pathogenesis models. However, the mucDalgD double mutant was synergistically reduced in virulence in mice, suggesting that alginate can partially compensate for the loss of MucD function in mouse pathogenesis.

Cloning and characterization of a coronatine-regulated tyrosine aminotransferase from Arabidopsis

In plants, the phytotoxin coronatine, which is an analog of the octadecanoids 12-oxo-phytodienoic acid and/or jasmonic acid, gives rise to a number of physiological responses similar to those of octadecanoids. To further elucidate the physiological role of these compounds, the differential RNA display technique was used to isolate a number of novel octadecanoid-inducible genes expressed in coronatine-treated Arabidopsis. Among these, a cDNA clone was identified that was similar to known tyrosine aminotransferases (TATs). The function was verified with the expressed recombinant protein. In Arabidopsis, the protein is present as a multimer of 98 kD, with a monomer of an apparent molecular mass of 47 kD. TAT mRNA could be induced within 2 h by various octadecanoids and by wounding of the plants. Accumulation of the TAT protein and a 5- to 7-fold increase in its enzymatic activity was observed 7 to 9 h after application of octadecanoids, coronatine, or wounding. The potential role of TAT in the defense response to herbivores and pathogens is discussed.

Two cinnamoyl-CoA reductase (CCR) genes from Arabidopsis thaliana are differentially expressed during development and in response to infection with pathogenic bacteria

Cinnamoyl-CoA reductase (CCR; EC 1.2.1.44) catalyses the conversion of cinnamoyl-CoAs into their corresponding cinnamaldehydes, i.e. the first step of the phenylpropanoid pathway specifically dedicated to the monolignol biosynthetic branch. In previous work, we described the isolation and characterisation of the first cDNA encoding CCR in Eucalyptus (Lacombe, E., Hawkins, S., Van Dorsselaere, J., Piquemal, J., Goffner, D., Poeydomenge, O., Boudet, A.M., Grima-Pettenati, J., 1997. Cinnamoyl CoA reductase, the first committed enzyme of the lignin branch biosynthetic pathway: cloning, expression and phylogenetic relationships. Plant Journal 11, 429--441) and shown the role of this enzyme in controlling the carbon flux into lignins (Piquemal, J., Lapierre, C., Myton, K., O'Connell, A., Schuch, W., Grima-Pettenati, J., Boudet, A.M., 1998. Down-regulation of cinnamoyl-CoA reductase induces significant changes of lignin profiles in transgenic tobacco plants. Plant Journal 13, 71--83). Here, we report the characterisation of two functionally and structurally distinct cDNA clones, AtCCR1 and AtCCR2 (81.6% protein sequence identity) in Arabidopsis thaliana. The two recombinant proteins expressed in Escherichia coli are able to use the three cinnamoyl-CoAs tested but with different levels of efficiency. AtCCR1 is five times more efficient with feruloyl-CoA and sinapoyl-CoA than AtCCR2. In addition, the two genes are differentially expressed during development and in response to infection. AtCCR1 is preferentially expressed in tissues undergoing lignification. In contrast, AtCCR2, which is poorly expressed during development, is strongly and transiently induced during the incompatible interaction with Xanthomonas campestris pv. campestris leading to a hypersensitive response. Altogether, these data suggest that AtCCR1 is involved in constitutive lignification whereas AtCCR2 is involved in the biosynthesis of phenolics whose accumulation may lead to resistance.

Reduced expression of the tomato ethylene receptor gene LeETR4 enhances the hypersensitive response to Xanthomonas campestris pv. vesicatoria

The hypersensitive response (HR) involves rapid death of cells at the site of pathogen infection and is thought to limit pathogen growth through the plant. Ethylene regulates senescence and developmental programmed cell death, but its role in hypersensitive cell death is less clear. Expression of two ethylene receptor genes, NR and LeETR4, is induced in tomato (Lycopersicon esculentum cv. Mill) leaves during an HR to Xanthomonas campestris pv. vesicatoria, with the greatest increase observed in LeETR4. LeETR4 antisense plants previously were shown to exhibit increased sensitivity to ethylene. These plants also exhibit greatly reduced induction of LeETR4 expression during infection and an accelerated HR at inoculum concentrations ranging from 10(5) to 10(7) CFU/ml. Increases in ethylene synthesis and pathogenesis-related gene expression are greater and more rapid in infected LeETR4 antisense plants, indicating an enhanced defense response. Populations of avirulent X. campestris pv. vesicatoria decrease more quickly and to a lower level in the transgenic plants, indicating a greater resistance to this pathogen. Because the ethylene action inhibitor 1-methylcyclopropene alleviates the enhanced HR phenotype in LeETR4 antisense plants, these changes in pathogen response are a result of increased ethylene sensitivity.

Functional analysis of the type III effectors AvrRpt2 and AvrRpm1 of Pseudomonas syringae with the use of a single-copy genomic integration system

Gram-negative phytopathogenic bacteria require a type III secretion apparatus for pathogenesis, presumably to deliver Avr effector proteins directly into plant cells. To extend previous studies of Avr effectors that employed plasmids encoding Avr proteins, we developed a system that permits the integration of any gene into the Pseudomonas syringae genome in single copy. With this system, we confirmed earlier findings showing that P. syringae pv. maculicola strain PsmES4326 expressing the AvrRpt2 effector induces a resistance response in plants with the cognate R gene, RPS2. Chromosomally located avrRpt2, however, provoked a stronger resistance response than that observed with plasmid-expressed AvrRpt2 in RPS2+ plants. Additionally, chromosomal expression of AvrRpt2 conferred a fitness advantage on P. syringae grown in rps2- plants, aiding in growth within leaves and escape to leaf surfaces that was difficult to detect with plasmid-borne avrRpt2. Finally, with the use of the genomic integration system, we found that a chimeric protein composed of the N terminus of the heterologous AvrRpml effector and the C-terminal effector region of AvrRpt2 was delivered to plant cells. Because the C terminus of AvrRpt2 cannot translocate into plant cells on its own, this indicates that the N-terminal region can direct secretion and translocation during an infection, which supports the view that Avr proteins have a modular design. This work establishes a readily manipulatable system to study type III effectors in a biologically realistic context.

Molecular responses to aphid feeding in Arabidopsis in relation to plant defense pathways

Little is known about molecular responses in plants to phloem feeding by insects. The induction of genes associated with wound and pathogen response pathways was investigated following green peach aphid (Myzus persicae) feeding on Arabidopsis. Aphid feeding on rosette leaves induced transcription of two genes associated with salicylic acid (SA)-dependent responses to pathogens (PR-1 and BGL2) 10- and 23-fold, respectively. Induction of PR-1 and BGL2 mRNA was reduced in npr1 mutant plants, which are deficient in SA signaling. Application of the SA analog benzothiadiazole led to decreases in aphid reproduction on leaves of both wild-type plants and mutant plants deficient in responsiveness to SA, suggesting that wild-type SA-dependent responses do not influence resistance to aphids. Two-fold increases occurred in mRNA levels of PDF1.2, which encodes defensin, a peptide involved in the jasmonate (JA)-/ethylene-dependent response pathway. Transcripts encoding JA-inducible lipoxygenase (LOX2) and SA/JA-inducible Phe-ammonia lyase increased 1.5- to 2-fold. PDF1.2 and LOX2 induction by aphids did not occur in infested leaves of the JA-resistant coi1-1 mutant. Aphid feeding induced 10-fold increases in mRNA levels of a stress-related monosaccharide symporter gene, STP4. Phloem feeding on Arabidopsis leads to stimulation of response pathways associated with both pathogen infection and wounding.

Pathogenesis of the human opportunistic pathogen Pseudomonas aeruginosa PA14 in Arabidopsis

The human opportunistic pathogen Pseudomonas aeruginosa strain PA14 is a multihost pathogen that can infect Arabidopsis. We found that PA14 pathogenesis in Arabidopsis involves the following steps: attachment to the leaf surface, congregation of bacteria at and invasion through stomata or wounds, colonization of intercellular spaces, and concomitant disruption of plant cell wall and membrane structures, basipetal movement along the vascular parenchyma, and maceration and rotting of the petiole and central bud. Distinctive features of P. aeruginosa pathogenesis are that the surface of mesophyll cell walls adopt an unusual convoluted or undulated appearance, that PA14 cells orient themselves perpendicularly to the outer surface of mesophyll cell walls, and that PA14 cells make circular perforations, approximately equal to the diameter of P. aeruginosa, in mesophyll cell walls. Taken together, our data show that P. aeruginosa strain PA14 is a facultative pathogen of Arabidopsis that is capable of causing local and systemic infection, which can result in the death of the infected plant.

The Pseudomonas syringae avrRpt2 gene product promotes pathogen virulence from inside plant cells

Several bacterial avr genes have been shown to contribute to virulence on susceptible plants lacking the corresponding resistance (R) gene. The mechanisms by which avr genes promote parasitism and disease, however, are not well understood. We investigated the role of the Pseudomonas syringae pv. tomato avrRpt2 gene in pathogenesis by studying the interaction of P. syringae pv. tomato strain PstDC3000 expressing avrRpt2 with several Arabidopsis thaliana lines lacking the corresponding R gene, RPS2. We found that PstDC3000 expressing avrRpt2 grew to significantly higher levels and often resulted in the formation of more severe disease symptoms in ecotype No-0 plants carrying a mutant RPS2 allele, as well as in two Col-0 mutant lines, cpr5 rps2 and coil rps2, that exhibit enhanced resistance. We also generated transgenic A. thaliana lines expressing avrRpt2 and demonstrated, by using several different assays, that expression of avrRpt2 within the plant also promotes virulence of PstDC3000. Thus, AvrRpt2 appears to promote pathogen virulence from within the plant cell.

Coordinated plant defense responses in Arabidopsis revealed by microarray analysis

Disease resistance is associated with a plant defense response that involves an integrated set of signal transduction pathways. Changes in the expression patterns of 2,375 selected genes were examined simultaneously by cDNA microarray analysis in Arabidopsis thaliana after inoculation with an incompatible fungal pathogen Alternaria brassicicola or treatment with the defense-related signaling molecules salicylic acid (SA), methyl jasmonate (MJ), or ethylene. Substantial changes (up- and down-regulation) in the steady-state abundance of 705 mRNAs were observed in response to one or more of the treatments, including known and putative defense-related genes and 106 genes with no previously described function or homology. In leaf tissue inoculated with A. brassicicola, the abundance of 168 mRNAs was increased more than 2.5-fold, whereas that of 39 mRNAs was reduced. Similarly, the abundance of 192, 221, and 55 mRNAs was highly (>2.5-fold) increased after treatment with SA, MJ, and ethylene, respectively. Data analysis revealed a surprising level of coordinated defense responses, including 169 mRNAs regulated by multiple treatments/defense pathways. The largest number of genes coinduced (one of four induced genes) and corepressed was found after treatments with SA and MJ. In addition, 50% of the genes induced by ethylene treatment were also induced by MJ treatment. These results indicated the existence of a substantial network of regulatory interactions and coordination occurring during plant defense among the different defense signaling pathways, notably between the salicylate and jasmonate pathways that were previously thought to act in an antagonistic fashion.

Three unique mutants of Arabidopsis identify eds loci required for limiting growth of a biotrophic fungal pathogen

To identify components of the defense response that limit growth of a biotrophic fungal pathogen, we isolated Arabidopsis mutants with enhanced disease susceptibility to Erysiphe orontii. Our initial characterization focused on three mutants, eds14, eds15, and eds16. None of these is considerably more susceptible to a virulent strain of the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm). All three mutants develop a hypersensitive response when infiltrated with Psm expressing the avirulence gene avrRpt2, which activates resistance via the LZ-NBS/LRR resistance protein encoded by RPS2. The growth of Psm(avrRpt2), while somewhat greater in the mutants than in the wild type, is less than growth of the isogenic virulent strain. These results indicate that resistance mediated via LZ-NBS/LRR R genes is functional. Analysis of the growth of avirulent Peronospora parasitica strains showed that the resistance pathway utilized by TIR-NBS/LRR R genes is also operative in all three mutants. Surprisingly, only eds14 and eds16 were more susceptible to Erysiphe cichoracearum. Analysis of the expression profiles of PR-1, BGL2, PR-5 and PDF1.2 in eds14, eds15, and eds16 revealed differences from the wild type for all the lines. In contrast, these mutants were not significantly different from wild type in the deposition of callose at sites of E. orontii penetration. All three mutants have reduced levels of salicylic acid after infection. eds16 was mapped to the lower arm of chromosome I and found by complementation tests to be allelic to the salicylic acid-deficient mutant sid2.

The alternative sigma factor RpoN is required for hrp activity in Pseudomonas syringae pv. maculicola and acts at the level of hrpL transcription

beta-Glucuronidase (uidA) reporter gene fusions were constructed for the hrpZ, hrpL, and hrpS genes from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. These reporters, as well as an avrRpt2-uidA fusion, were used to measure transcriptional activity in ES4326 and a ES4326 rpoN mutant. rpoN was required for the expression of avrRpt2, hrpZ, and hrpL in vitro in minimal media and in vivo when infiltrated into Arabidopsis thaliana leaves. In contrast, the expression of hrpS was essentially the same in wild-type and rpoN mutant strains. Constitutive expression of hrpL in an rpoN mutant restored hrpZ transcription to wild-type levels, restored the hypersensitive response when infiltrated into tobacco (Nicotiana tobacum), and partially restored the elicitation of virulence-related symptoms but not growth when infiltrated into Arabidopsis leaves. These data indicate that rpoN-mediated control of hrp gene expression acts at the level of hrpL and that in planta growth of P. syringae is not required for the elicitation of disease symptoms.

Virulence of the phytopathogen Pseudomonas syringae pv. maculicola is rpoN dependent

We cloned the rpoN (ntrA and glnF) gene encoding sigma(54) from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. The P. syringae ES4326 rpoN gene complemented Pseudomonas aeruginosa, Escherichia coli, and Klebsiella aerogenes rpoN mutants for a variety of rpoN mutant phenotypes, including the inability to utilize nitrate as sole nitrogen source. DNA sequence analysis of the P. syringae ES4326 rpoN gene revealed that the deduced amino acid sequence was most similar (86% identity; 95% similarity) to the sigma(54) protein encoded by the Pseudomonas putida rpoN gene. A marker exchange protocol was used to construct an ES4326 rpoN insertional mutation, rpoN::Km(r). In contrast to wild-type ES4326, ES4326 rpoN::Km(r) was nonmotile and could not utilize nitrate, urea, C(4)-dicarboxylic acids, several amino acids, or concentrations of ammonia below 2 mM as nitrogen sources. rpoN was essential for production of the phytotoxin coronatine and for expression of the structural genes encoding coronamic acid. In addition, ES4326 rpoN::Km(r) did not multiply or elicit disease symptoms when infiltrated into Arabidopsis thaliana leaves, did not elicit the accumulation of several Arabidopsis defense-related mRNAs, and did not elicit a hypersensitive response (HR) when infiltrated into tobacco (Nicotiana tabacum) leaves. Furthermore, whereas P. syringae ES4326 carrying the avirulence gene avrRpt2 elicited an HR when infiltrated into Arabidopsis ecotype Columbia leaves, ES4326 rpoN::Km(r) carrying avrRpt2 elicited no response. Constitutive expression of ES4326 hrpL in ES4326 rpoN::Km(r) partially restored defense-related mRNA accumulation, showing a direct role for the hrp cluster in host defense gene induction in a compatible host-pathogen interaction. However, constitutive expression of hrpL in ES4326 rpoN::Km(r) did not restore coronatine production, showing that coronatine biosynthesis requires factors other than hrpL.

Arabidopsis thaliana EDS4 contributes to salicylic acid (SA)-dependent expression of defense responses: evidence for inhibition of jasmonic acid signaling by SA

The Arabidopsis enhanced disease susceptibility 4 (eds4) mutation causes enhanced susceptibility to infection by the bacterial pathogen Pseudomonas syringae pv. maculicola ES4326 (Psm ES4326). Gene-for-gene resistance to bacteria carrying the avirulence gene avrRpt2 is not significantly affected by eds4. Plants homozygous for eds4 exhibit reduced expression of the pathogenesis-related gene PR-1 after infection by Psm ES4326, weakened responses to treatment with the signal molecule salicylic acid (SA), impairment of the systemic acquired resistance response, and reduced accumulation of SA after infection with Psm ES4326. These phenotypes indicate that EDS4 plays a role in SA-dependent signaling. SA has been shown to have a negative effect on activation of gene expression by the signal molecule jasmonic acid (JA). Two mutations that cause reduced SA levels, eds4 and pad4, cause heightened responses to inducers of JA-dependent gene expression, providing genetic evidence to support the idea that SA interferes with JA-dependent signaling. Two possible working models of the role of EDS4 in governing activation of defense responses are presented.

Identification of Arabidopsis mutants exhibiting an altered hypersensitive response in gene-for-gene disease resistance

A mutational study was carried out to isolate Arabidopsis thaliana plants that exhibit full or partial disruption of the RPS2-mediated hypersensitive response (HR) to Pseudomonas syringae that express avrRpt2. Five classes of mutants were identified including mutations at RPS2, dnd mutations causing a "defense, no death" loss-of-HR phenotype, a lesion-mimic mutant that also exhibited an HR- phenotype, and a number of intermediate or partial-loss-of-HR mutants. Surprisingly, many of these mutants displayed elevated resistance to virulent P. syringae and, in some cases, to Peronospora parasitica. Constitutively elevated levels of pathogenesis-related (PR) gene expression and salicylic acid were also observed. In the lesion-mimic mutant, appearance of elevated resistance was temporally correlated with appearance of lesions. For one of the intermediate lines, resistance was shown to be dependent on elevated levels of salicylic acid. A new locus was identified and named IHR1, after the mutant phenotype of "intermediate HR." Genetic analysis of the intermediate-HR plant lines was difficult due to uncertainties in distinguishing the partial/intermediate mutant phenotypes from wild type. Despite this difficulty, the intermediate-HR mutants remain of interest because they reveal potential new defense-related loci and because many of these lines exhibit partially elevated disease resistance without dwarfing or other apparent growth defects.

Isolation, partial sequencing, and expression of pathogenesis-related cDNA genes from pepper leaves infected by Xanthomonas campestris pv. vesicatoria

Specific cDNAs showing differential expression in bacteria-infected pepper leaves as opposed to healthy leaves were isolated from a pepper cDNA library from hypersensitive response (HR) lesions of leaves infected with an avirulent strain of Xanthomonas campestris pv. vesicatoria. Among a total of 282 cDNA clones tested, 36 individual cDNA genes (13%) hybridized strongly or differentially to the cDNA probes from bacteria-infected leaves. Ten Capsicum Annuum-Induced (CAI) genes encoding putative thionin, lipid transfer protein I and II, osmotin (PR-5), class I chitinase, beta-1,3-glucanase, SAR 8.2, stellacyanin, leucine-rich repeat protein, and auxin-repressed protein were identified. Two CAI genes showed little or no sequence homology to the previously sequenced plant genes. Transcripts of the CAI genes were strongly or preferentially induced in pepper tissues by infection with X. campestris pv. vesicatoria or Phytophthora capsici, and by abiotic elicitor treatment. In particular, most of the CAI genes were strongly induced in pepper tissues by ethephon and methyl jasmonate.

Arabidopsis PAD3, a gene required for camalexin biosynthesis, encodes a putative cytochrome P450 monooxygenase

Phytoalexins are low molecular weight antimicrobial compounds that are synthesized in response to pathogen attack. The phytoalexin camalexin, an indole derivative, is produced by Arabidopsis in response to infection with the bacterial pathogen Pseudomonas syringae. The phytoalexin deficient 3 (pad3) mutation, which causes a defect in camalexin production, has no effect on resistance to P. syringae but compromises resistance to the fungal pathogen Alternaria brassicicola. We have now isolated PAD3 by map-based cloning. The predicted PAD3 protein appears to be a cytochrome P450 monooxygenase, similar to those from maize that catalyze synthesis of the indole-derived secondary metabolite 2,4-dihydroxy-1, 4-benzoxazin-3-one. The expression of PAD3 is tightly correlated with camalexin synthesis and is regulated by PAD4 and PAD1. On the basis of these findings, we conclude that PAD3 almost certainly encodes an enzyme required for camalexin biosynthesis. Moreover, these results strongly support the idea that camalexin does not play a major role in plant resistance to P. syringae infection, although it is involved in resistance to a fungal pathogen.

Identification of new early markers of the hypersensitive response in Arabidopsis thaliana(1)

New molecular markers of the hypersensitive response (HR) of Arabidopsis thaliana to the bacterial pathogen Xanthomonas campestris pv. campestris (X.c.c.) have been identified by differential screening of a cDNA library constructed from suspension cells inoculated by an HR-inducing strain in the presence of cycloheximide. Seven families of genes (called Athsr) have been isolated, show similarities to voltage-dependent anion channels (VDAC) and alternative oxidases, or are novel proteins. Athsr genes have shown to be specifically or preferentially expressed during the HR. These data suggest that Athsr genes might be involved in early events conditioning the establishment of the HR.

A novel myb oncogene homologue in Arabidopsis thaliana related to hypersensitive cell death

A novel myb oncogene homologue (AtMYB30) has been isolated by differential screening of a cDNA library prepared from Xanthomonas campestris pv. campestris (X. campestris)-inoculated Arabidopsis thaliana cells cultured in the presence of cycloheximide. AtMYB30 is a single-copy gene, and the encoded protein contains a MYB domain highly homologous to other plant and animal MYB proteins. Analyses of transcript levels in A. thaliana plants, or in cultured A. thaliana cells infected with either virulent or avirulent strains of the pathogens X. campestris and Pseudomonas syringae pv. tomato, showed that maximal levels of transcription of this gene occurred during the hypersensitive response. Furthermore, in A. thaliana mutants affected in the control of cell death initiation (lsd3, lsd4 and lsd5), constitutive expression or expression in lesion-positive plants was observed, while in suppressors of the mutations lsd5 and lsd4, AtMYB30 transcripts did not accumulate. However, AtMYB30 expression could not be detected in the lsd1 mutant, which was hyper-responsive to cell death initiators and unable to limit the extent of cell death, whatever the environmental conditions. The results presented here suggest a strong correlation between AtMYB30 and genetically controlled cell death, with a role in the initiation of cell death rather than in the limitation of its extent. Our results further indicate that the lsd mutants constitute an appropriate genetic model for studying the role of this gene in hypersensitive cell death, and their relation to different steps of the pathway(s) leading to cell death.

Rapid transcript accumulation of pathogenesis-related genes during an incompatible interaction in bacterial speck disease-resistant tomato plants

In the yeast two-hybrid system, the Pto kinase interacts with three putative transcription factors Pti4, Pti5 and Pti6. The Pti4/5/6 proteins contain a DNA binding domain that recognizes and binds a DNA sequence (5'-AGCCGCC3'; the 'PR box') present in the promoter region of a large number of genes encoding 'pathogenesis-related' (PR) proteins. We have now investigated the pathogen-induced expression of PR box-containing genes in tomato. We isolated a tomato osmotin gene that contains two PR boxes in its promoter region and demonstrated that the abundance of the osmotin transcript rapidly increases during an incompatible interaction involving Pro-containing tomato plants and the bacterial pathogen Pseudomonas syringae pv. tomato expressing the avrPto gene. In addition, we found that transcripts of two other tomato PR genes (encoding endochitinase and beta-1,3-glucanase B) and at least one ACC oxidase gene, all of which contain PR boxes in their promoter regions, rapidly accumulate in the incompatible interaction. These data support the hypothesis that the tomato Pto kinase regulates the expression of certain defense genes in tomato by interaction with transcription factors that bind the PR box.

Enhanced expression and activation of the alternative oxidase during infection of Arabidopsis with Pseudomonas syringae pv tomato

Cyanide-resistant ("alternative") respiration was studied in Arabidopsis during incompatible and compatible infection with Pseudomonas syringae pv tomato DC3000. Total leaf respiration increased as the leaves became necrotic, as did the cyanide-resistant component that was sensitive to salicylhydroxamic acid. Infiltration of leaves with an avirulent strain rapidly induced alternative oxidase (AOX) mRNA, whereas the increase was delayed in the compatible combination. The increase in mRNA correlated with the increase in AOX protein. Increased expression was confined to the infected leaves, in contrast to the pathogenesis-related protein-1, which was induced systemically. Virtually all of the AOX protein was in the reduced (high-activity) form. Using transgenic NahG and mutant npr1-1 and etr1-1 plants, we established that the rapid induction of the AOX was associated with necrosis and that ethylene, but not salicylic acid, was required for its induction. Increased pyruvate levels in the infected leaves suggested that increased substrate levels were respired through the alternative pathway; however, in the control leaves and the infected leaves, respiration was not inhibited by salicylhydroxamic acid alone. Increased respiration appeared to be associated primarily with symptom expression rather than resistance reactions.

Characterization of an Arabidopsis thaliana receptor-like protein kinase gene activated by oxidative stress and pathogen attack

An Arabidopsis thaliana cDNA clone that encodes a putative receptor-like protein kinase gene (At-RLK3) was characterized. The deduced 667-amino acid protein consists of an amino-terminal signal sequence, an extracellular domain, a single transmembrane domain, and a cytoplasmic domain with characteristics of serine/threonine protein kinase. Because of the original features of its extracellular domain, the At-RLK3 protein is a member of a new class of receptor-like protein kinases. The At-RLK3 gene is present as a single copy within the Arabidopsis genome and its transcripts are detected in root, stem, leaf and flower. In cultured cells, the At-RLK3 gene is activated upon oxidative stress and salicylic acid treatment. In plants, the gene appears to be differentially regulated during various plant-pathogen interactions: upon inoculation with strains of Pseudomonas syringae pv. tomato harboring or not, different avr genes, At-RLK3 transcripts accumulate transiently at similar levels during both compatible and incompatible interactions. This gene is, however, preferentially expressed during the incompatible interaction induced by the soil-borne vascular bacteria, Ralstonia solanacearum. The involvement of At-RLK3 in signal transduction pathways during pathogen attack is discussed.

A novel flower-specific Arabidopsis gene related to both pathogen-induced and developmentally regulated plant beta-1,3-glucanase genes

Beta-1,3-glucanases are usually associated with plant defense responses, although some are also developmentally or hormonally regulated. We characterized two Arabidopsis genes linked in a tandem array, BG4 and BG5, encoding putative novel isoforms of beta-1,3-glucanase. The deduced polypeptides, BG4 and BG5, were highly similar to each other (89% amino acid identity) but only moderately related (32 to 41% amino acid identity) to the different categories of previously characterized beta-1,3-glucanases, suggesting that BG4 and BG5 may represent a novel class of beta-1,3-glucanases in plants. Neither of the genes was responsive to pathogen or SA induction in contrast to the previously identified Arabidopsis beta-1,3-glucanases, nor could we detect any developmental or hormonally induced expression in the vegetative parts of the plants. Both RNA blot and in situ hybridization data demonstrated that the BG4 gene was specifically expressed in the style and septum of the ovary, suggesting that the corresponding protein is involved in the reproductive process of the plant.

Differential expression of a senescence-enhanced metallothionein gene in Arabidopsis in response to isolates of Peronospora parasitica and Pseudomonas syringae

The metallothionein gene, LSC54, shows increased expression during leaf senescence in Brassica napus and Arabidopsis thaliana. A number of abiotic and biotic stresses have been shown to induce senescence-like symptoms in plants and, to investigate this further, the promoter of the LSC54 gene was cloned and fused to the GUS gene and transformed into Arabidopsis. The promoter was highly induced during leaf senescence and also in response to wounding; histochemical analysis indicated that this induction was localised to a few cells close to the wound site. The transgenic Arabidopsis tissue was infected with compatible and incompatible isolates of both the fungal biotroph, Peronospora parasitica and the bacterial necrotroph, Pseudomonas syringae. Incompatible isolates induced rapid cell death (the hypersensitive response) at the site of infection and, with both pathogens, early, localised expression of the GUS gene was observed. In contrast, relatively slow induction of the GUS gene was seen in the compatible interaction and this was correlated with the appearance of senescence-like symptoms in the biotrophic interaction and cell death by necrosis that occurred in response to the necrotrophic pathogen. These results suggest that there are common steps in the signalling pathways that lead to cell death in the hypersensitive response, pathogen induced necrosis and senescence.

Jasmonate and salicylate as global signals for defense gene expression

Remarkably, only a few low molecular mass signals, including jasmonic acid, ethylene and salicylic acid, upregulate the expression of scores of defense-related genes. Using these regulators, the plant fine-tunes its defense gene expression against aggressors which, in some cases, may be able to disrupt or amplify plant defense signal pathways to their own ends.

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.