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

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.

Compost and compost water extract-induced systemic acquired resistance in cucumber and Arabidopsis

ABSTRACT A biocontrol agent-fortified compost mix, suppressive to several diseases caused by soilborne plant pathogens, induced systemic acquired resistance (SAR) in cucumber against anthracnose caused by Colletotrichum orbiculare and in Arabidopsis against bacterial speck caused by Pseudomonas syringae pv. maculicola KD4326. A peat mix conducive to soilborne diseases did not induce SAR. The population size of P. syringae pv. maculicola KD4326 was significantly lower in leaves of Arabidopsis plants grown in the compost mix compared to those grown in the peat mix. Autoclaving destroyed the SAR-inducing effect of the compost mix, and inoculation of the autoclaved mix with nonautoclaved compost mix or Pantoea agglomerans 278A restored the effect, suggesting the SAR-inducing activity of the compost mix was biological in nature. Topical sprays with water extract prepared from the compost mix reduced symptoms of bacterial speck and the population size of pathogenic KD4326 in Arabidopsis grown in the peat mix but not in the compost mix. The peat mix water extract applied as a spray did not control bacterial speck on plants grown in either mix. Topical sprays with salicylic acid (SA) reduced the severity of bacterial speck on plants in the peat mix but did not further reduce the severity of symptoms on plants in the compost mix. The activity of the compost water extract was heat-stable and passed through a 0.2-mum membrane filter. beta-1,3-Glucanase activity was low in cucumber plants grown in either mix, but when infected with C. orbiculare, this activity was induced to significantly higher levels in plants grown in the compost mix than in plants grown in the peat mix. Similar results were obtained for beta-D-glucuronidase (GUS) activity driven by a PR2 (beta-1,3-glucanase) gene promoter in transgenic Arabidopsis plants grown in the compost or peat mix. GUS activity was induced with topical sprays of the compost water extract or SA in plants not inoculated with the pathogen, suggesting that compost-induced disease suppression more than likely involved the potentiation of resistance responses rather than their activation and that compost-induced SAR differed from SAR induced by pathogens, SA, or compost water extract.

Use of Arabidopsis for genetic dissection of plant defense responses

Arabidopsis thaliana (Arabidopsis) is proving to be an ideal model system for studies of host defense responses to pathogen attack. The Arabidopsis genetic system is significantly more tractable than those of other plant species, and Arabidopsis exhibits all of the major kinds of defense responses described in other plants. A large number of virulent and avirulent bacterial, fungal, and viral pathogens of Arabidopsis have been collected. In the last few years, a large number of mutations have been identified in Arabidopsis that cause a wide variety of specific defense-related phenotypes. Analysis of these mutant phenotypes is beginning to give glimpses into the complex signal transduction pathways leading to the induction of the defense responses involved in protecting plants from pathogen infection.

Use of model plant hosts to identify Pseudomonas aeruginosa virulence factors

We used plants as an in vivo pathogenesis model for the identification of virulence factors of the human opportunistic pathogen Pseudomonas aeruginosa. Nine of nine TnphoA mutant derivatives of P. aeruginosa strain UCBPP-PA14 that were identified in a plant leaf assay for less pathogenic mutants also exhibited significantly reduced pathogenicity in a burned mouse pathogenicity model, suggesting that P. aeruginosa utilizes common strategies to infect both hosts. Seven of these nine mutants contain TnphoA insertions in previously unknown genes. These results demonstrate that an alternative nonvertebrate host of a human bacterial pathogen can be used in an in vivo high throughput screen to identify novel bacterial virulence factors involved in mammalian pathogenesis.

PROGRAMMED CELL DEATH IN PLANT-PATHOGEN INTERACTIONS

Plants cope with pathogen attacks by using mechanisms of resistance that rely both on preformed protective defenses and on inducible defenses. The latter are the most well studied, and progress is being made in determining which induced responses are responsible for limiting pathogen growth. Many plant-pathogen interactions are accompanied by plant cell death. Recent evidence suggests that this cell death is often programmed and results from an active process on the part of the host. The review considers the roles and possible mechanisms of plant cell death in response to pathogens.

Phytoalexin-deficient mutants of Arabidopsis reveal that PAD4 encodes a regulatory factor and that four PAD genes contribute to downy mildew resistance

We are working to determine the role of the Arabidopsis phytoalexin, camalexin, in protecting the plant from pathogen attack by isolating phytoalexin-deficient (pad) mutants in the accession Columbia (Col-0) and examining their response to pathogens. Mutations in PAD1, PAD2, and PAD4 caused enhanced susceptibility to the bacterial pathogen Pseudomonas syringae pv. maculicola strain ES4326 (PsmES4326), while mutations in PAD3 or PAD5 did not. Camalexin was not detected in any of the double mutants pad1-1 pad2-1, pad1-1 pad3-1 or pad2-1 pad3-1. Growth of PsmES4326 in pad1-1 pad2-1 was greater than that in pad1-1 or pad2-1 plants, while growth in pad1-1 pad3-1 and pad2-1 pad3-1 plants was similar to that in pad1-1 and pad2-1 plants, respectively. The pad4-1 mutation caused reduced camalexin synthesis in response to PsmES4326 infection, but not in response to Cochliobolus carbonum infection, indicating that PAD4 has a regulatory function. PAD1, PAD2, PAD3 and PAD4 are all required for resistance to the eukaryotic biotroph Peronospora parasitica. The pad4-1 mutation caused the most dramatic change, exhibiting full susceptibility to four of six Col-incompatible parasite isolates. Interestingly, each combination of double mutants between pad1-1, pad2-1 and pad3-1 exhibited additive shifts to moderate or full susceptibility to most of the isolates.

Arabidopsis enhanced disease susceptibility mutants exhibit enhanced susceptibility to several bacterial pathogens and alterations in PR-1 gene expression

To identify plant defense responses that limit pathogen attack, Arabidopsis eds mutants that exhibit enhanced disease susceptibility to the virulent bacterial pathogen Pseudomonas syringae pv maculicola ES4326 were previously identified. In this study, we show that each of four eds mutants (eds5-1, eds6-1, eds7-1, and eds9-1) has a distinguishable phenotype with respect to the degree of susceptibility to a panel of bacterial phytopathogens and the ability to activate pathogenesis-related PR-1 gene expression after pathogen attack. None of the four eds mutants exhibited observable defects in mounting a hypersensitive response. Although all four eds mutants were also capable of mounting a systemic acquired resistance response, enhanced growth of P. s. maculicola ES4326 was still apparent in the secondarily infected leaves of three of the eds mutants. These data indicate that eds genes define a diverse set of previously unknown defense responses that affect resistance to virulent pathogens.

Differences in susceptibility of Arabidopsis ecotypes to crown gall disease may result from a deficiency in T-DNA integration

We show that among ecotypes of Arabidopsis, there is considerable variation in their susceptibility to crown gall disease. Differences in susceptibility are heritable and, in one ecotype, segregate as a single major contributing locus. In several ecotypes, recalcitrance to tumorigenesis results from decreased binding of Agrobacterium to inoculated root explants. The recalcitrance of another ecotype occurs at a late step in T-DNA transfer. Transient expression of a T-DNA-encoded beta-glucuronidase gusA gene is efficient, but the ecotype is deficient in crown gall tumorigenesis, transformation to kanamycin resistance, and stable GUS expression. This ecotype is also more sensitive to gamma radiation than is a susceptible ecotype. DNA gel blot analysis showed that after infection by Agrobacterium, less T-DNA was integrated into the genome of the recalcitrant ecotype than was integrated into the genome of a highly susceptible ecotype.

The Arabidopsis NPR1 gene that controls systemic acquired resistance encodes a novel protein containing ankyrin repeats

The Arabidopsis NPR1 gene controls the onset of systemic acquired resistance (SAR), a plant immunity, to a broad spectrum of pathogens that is normally established after a primary exposure to avirulent pathogens. Mutants with defects in NPR1 fail to respond to various SAR-inducing treatments, displaying little expression of pathogenesis-related (PR) genes and exhibiting increased susceptibility to infections. NPR1 was cloned using a map-based approach and was found to encode a novel protein containing ankyrin repeats. The lesion in one npr1 mutant allele disrupted the ankyrin consensus sequence, suggesting that these repeats are important for NPR1 function. Furthermore, transformation of the cloned wild-type NPR1 gene into npr1 mutants not only complemented the mutations, restoring the responsiveness to SAR induction with respect to PR-gene expression and resistance to infections, but also rendered the transgenic plants more resistant to infection by P. syringae in the absence of SAR induction.

Rapid induction by wounding and bacterial infection of an S gene family receptor-like kinase gene in Brassica oleracea

A receptor-like kinase, SRK, has been implicated in the autoincompatible response that leads to the rejection of self-pollen in Brassica plants. SRK is encoded by one member of a multigene family, which includes several receptor-like kinase genes with patterns of expression very different from that of SRK but of unknown function. Here, we report the characterization of a novel member of the Brassica S gene family, SFR2. RNA gel blot analysis demonstrated that SFR2 mRNA accumulated rapidly in response both to wounding and to infiltration with either of two bacteria: Xanthomonas campestris, a pathogen, and Escherichia coli, a saprophyte. SFR2 mRNA also accumulated rapidly after treatment with salicylic acid, a molecule that has been implicated in plant defense response signaling pathways. A SFR2 promoter and reporter gene fusion was introduced into tobacco and was shown to be induced by bacteria of another genus, Ralstonia (Pseudomonas) solanacearum. The accumulation of SFR2 mRNA in response to wounding and pathogen invasion is typical of a gene involved in the defense responses of the plant. The rapidity of SFR2 mRNA accumulation is consistent with SFR2 playing a role in the signal transduction pathway that leads to induction of plant defense proteins, such as pathogenesis-related proteins or enzymes of phenylpropanoid metabolism.

Molecular recognition of pathogen attack occurs inside of plant cells in plant disease resistance specified by the Arabidopsis genes RPS2 and RPM1

The Arabidopsis thaliana disease resistance genes RPS2 and RPM1 belong to a class of plant disease resistance genes that encode proteins that contain an N-terminal tripartite nucleotide binding site (NBS) and a C-terminal tandem array of leucine-rich repeats. RPS2 and RPM1 confer resistance to strains of the bacterial phytopathogen Pseudomonas syringae carrying the avirulence genes avrRpt2 and avrB, respectively. In these gene-for-gene relationships, it has been proposed that pathogen avirulence genes generate specific ligands that are recognized by cognate receptors encoded by the corresponding plant resistance genes. To test this hypothesis, it is crucial to know the site of the potential molecular recognition. Mutational analysis of RPS2 protein and in vitro translation/translocation studies indicated that RPS2 protein is localized in the plant cytoplasm. To determine whether avirulence gene products themselves are the ligands for resistance proteins, we expressed the avrRpt2 and avrB genes directly in plant cell using a novel quantitative transient expression assay, and found that expression of avrRpt2 and avrB elicited a resistance response in plants carrying the corresponding resistance genes. This observation indicates that no bacterial factors other than the avirulence gene products are required for the specific resistance response as long as the avirulence gene products are correctly localized. We propose that molecular recognition of P. syringae in RPS2- and RPM1-specified resistance occurs inside of plant cells.

Expression of genes for a defensin and a proteinase inhibitor in specific areas of the shoot apex and the developing flower in tomato

Two genes that are highly expressed in the tomato shoot apex have been cloned by differential hybridization. One of the deduced polypeptides (AT1) shows significant similarities to class II proteinase inhibitors, while the other (AT2) displays similarities to defensins. Transcripts of both genes are also detectable in the developing flower and are present only in minor amounts in other tissues tested. In situ hybridization analysis revealed that both genes are expressed in non-overlapping subsets of cells in the shoot apex, as well as in the developing flower. The potential use of these genes as markers for certain cell types and the possible biological function of the encoded proteins are discussed.

Systemic resistance in Arabidopsis induced by biocontrol bacteria is independent of salicylic acid accumulation and pathogenesis-related gene expression

Systemic acquired resistance is a pathogen-inducible defense mechanism in plants. The resistant state is dependent on endogenous accumulation of salicylic acid (SA) and is characterized by the activation of genes encoding pathogenesis-related (PR) proteins. Recently, selected nonpathogenic, root-colonizing biocontrol bacteria have been shown to trigger a systemic resistance response as well. To study the molecular basis underlying this type of systemic resistance, we developed an Arabidopsis-based model system using Fusarium oxysporum f sp raphani and Pseudomonas syringae pv tomato as challenging pathogens. Colonization of the rhizosphere by the biological control strain WCS417r of P. fluorescens resulted in a plant-mediated resistance response that significantly reduced symptoms elicited by both challenging pathogens. Moreover, growth of P. syringae in infected leaves was strongly inhibited in P. fluorescens WCS417r-treated plants. Transgenic Arabidopsis NahG plants, unable to accumulate SA, and wild-type plants were equally responsive to P. fluorescens WCS417r-mediated induction of resistance. Furthermore, P. fluorescens WCS417r-mediated systemic resistance did not coincide with the accumulation of PR mRNAs before challenge inoculation. These results indicate that P. fluorescens WCS417r induces a pathway different from the one that controls classic systemic acquired resistance and that this pathway leads to a form of systemic resistance independent of SA accumulation and PR gene expression.

Systemic resistance in Arabidopsis induced by biocontrol bacteria is independent of salicylic acid accumulation and pathogenesis-related gene expression

Systemic acquired resistance is a pathogen-inducible defense mechanism in plants. The resistant state is dependent on endogenous accumulation of salicylic acid (SA) and is characterized by the activation of genes encoding pathogenesis-related (PR) proteins. Recently, selected nonpathogenic, root-colonizing biocontrol bacteria have been shown to trigger a systemic resistance response as well. To study the molecular basis underlying this type of systemic resistance, we developed an Arabidopsis-based model system using Fusarium oxysporum f sp raphani and Pseudomonas syringae pv tomato as challenging pathogens. Colonization of the rhizosphere by the biological control strain WCS417r of P. fluorescens resulted in a plant-mediated resistance response that significantly reduced symptoms elicited by both challenging pathogens. Moreover, growth of P. syringae in infected leaves was strongly inhibited in P. fluorescens WCS417r-treated plants. Transgenic Arabidopsis NahG plants, unable to accumulate SA, and wild-type plants were equally responsive to P. fluorescens WCS417r-mediated induction of resistance. Furthermore, P. fluorescens WCS417r-mediated systemic resistance did not coincide with the accumulation of PR mRNAs before challenge inoculation. These results indicate that P. fluorescens WCS417r induces a pathway different from the one that controls classic systemic acquired resistance and that this pathway leads to a form of systemic resistance independent of SA accumulation and PR gene expression.

Isolation of Arabidopsis mutants with enhanced disease susceptibility by direct screening

To discover which components of plant defense responses make significant contributions to limiting pathogen attack, we screened a mutagenized population of Arabidopsis thaliana for individuals that exhibit increased susceptibility to the moderately virulent bacterial pathogen Pseudomonas syringae pv. maculicola ES4326 (Psm ES4326). The 12 enhanced disease susceptibility (eds) mutants isolated included alleles of two genes involved in phytoalexin biosynthesis (pad2, which had been identified previously, and pad4, which had not been identified previously), two alleles of the previously identified npr1 gene, which affects expression of other defense genes, and alleles of seven previously unidentified genes of unknown function. The npr1 mutations caused greatly reduced expression of the PR1 gene in response to PsmES4326 infection, but had little effect on expression of two other defense genes, BGL2 and PR5, suggesting that PR1 expression may be important for limiting growth of PsmES4326. While direct screens for mutants with quantitative pathogen-susceptibility phenotypes have not been reported previously, our finding that mutants isolated in this way include those affected in known defense responses supports the notion that this type of screening strategy allows genetic dissection of the roles of various plant defense responses in disease resistance.

Molecular cloning of a sulfotransferase in Arabidopsis thaliana and regulation during development and in response to infection with pathogenic bacteria

A cDNA clone (RaRO47) encoding a sulfotransferase (ST) has been isolated from Arabidopsis cell suspensions. The deduced polypeptide of 302 amino acids is highly related to plant flavonol sulfotransferases (FSTs), characterized for the first time in Flaveria, and also to STs from animal tissue. The expression of the Arabidopsis ST gene(s) corresponding to RaR047 was examined during different developmental stages. It was found that, at the level of steady-state mRNA, expression of gene(s) encoding this ST was rapidly induced in the aerial parts of young seedlings, and during growth of Arabidopsis cell cultures. No expression could be detected in roots. Treatment of Arabidopsis seedlings with hormonal or stress-related compounds, showed that RaR047 mRNA accumulation was more particularly induced in response to salicylic acid and methyl jasmonate. Furthermore, in the leaves of mature plants or in cell suspensions, accumulation of RaR047 mRNA was observed upon infection with bacterial pathogens. This expression was observed preferentially in response to avirulent pathogens causing an hypersensitive reaction, as compared to virulent pathogens, which lead to disease.

Isolation of Arabidopsis genes that differentiate between resistance responses mediated by the RPS2 and RPM1 disease resistance genes

The Arabidopsis disease resistance gene RPS2 is involved in recognition of bacterial pathogens carrying the avirulence gene avrRpt2, and the RPM1 resistance gene is involved in recognition of pathogens carrying avrRpm1 or avrB. We identified and cloned two Arabidopsis genes, AIG1 and AIG2 (for avrRpt2-induced gene), that exhibit RPS2- and avrRpt2-dependent induction early after infection with Pseudomonas syringae pv maculicola strain ES4326 carrying avrRpt2. However, ES4326 carrying avrRpm1 or avrB did not induce early expression of AIG1 and AIG2. Conversely, ES4326 carrying avrRpm1 or avrB induced early expression of the previously isolated defense-related gene ELI3, whereas ES4326 carrying avrRpt2 did not. The induction patterns of the AIG genes and ELI3 demonstrate that different resistance gene-avr gene combinations can elicit distinct defense responses. Furthermore, by examining the expression of AIG1 and ELI3 in plants infiltrated with a mixed inoculum of ES4326 carrying avrRpt2 and ES4326 carrying avrRpm1, we found that there is interference between the RPS2- and RPM1-mediated resistance responses.

Bacterial avirulence genes

Although more than 30 bacterial avirulence genes have been cloned and characterized, the function of the gene products in the elictitation of resistance is unknown in all cases but one. The product of avrD from Pseudomonas syringae pv. glycinea likely functions indirectly to elicit resistance in soybean, that is, evidence suggests the gene product is an enzyme involved in elicitor production. In most if not all cases, bacterial avirulence gene function is dependent on interactions with the hypersensitive response and pathogenicity (hrp) genes. Many hrp genes are similar to genes involved in delivery of pathogenicity factors in mammalian bacterial pathogens. Thus, analogies between mammalian and plant pathogens may provide needed clues to elucidate how virulence gene products control induction of resistance.

The Arabidopsis thaliana 4-coumarate:CoA ligase (4CL) gene: stress and developmentally regulated expression and nucleotide sequence of its cDNA

An Arabidopsis cDNA clone encoding 4-coumarate:CoA ligase (4CL), a key enzyme of phenylpropanoid metabolism, was identified and sequenced. The predicted amino acid sequence is similar to those of other cloned 4CL genes. Southern blot analysis indicated that 4CL is single-copy gene in Arabidopsis. Northern blots showed that 4CL expression was activated early during seedling development. The onset of 4CL expression was correlated with the onset of lignin deposition in cotyledons and roots 2-3 days after germination. The timing of the expression of a parsley 4CL1-GUS fusion in transgenic Arabidopsis seedlings was examined in parallel and was very similar to that of endogenous 4CL. In mature plants, highest 4CL expression was observed in bolting stems, where relatively large amounts of lignin accumulate. Both 4CL and 4CL1-GUS mRNA accumulation was strongly and transiently activated by wounding of mature Arabidopsis leaves. 4CL expression was specifically activated within 6 h after infiltration of Arabidopsis ecotype Columbia leaves with a Pseudomonas syringae pv. maculicola strain harboring the bacterial avirulence gene avrB, which causes in incompatible interaction. The timing of 4CL activation was identical to the previously observed activation of PAL gene expression in this interaction. No activation of 4CL expression was observed in a compatible interaction caused by a Pseudomonas syringae pv. maculicola strain without avrB.

NDR1, a locus of Arabidopsis thaliana that is required for disease resistance to both a bacterial and a fungal pathogen

We have employed Arabidopsis thaliana as a model host plant to genetically dissect the molecular pathways leading to disease resistance. A. thaliana accession Col-0 is susceptible to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 but resistant in a race-specific manner to DC3000 carrying any one of the cloned avirulence genes avrB, avrRpm1, avrRpt2, and avrPph3. Fast-neutron-mutagenized Col-0 M2 seed was screened to identify mutants susceptible to DC3000(avrB). Disease assays and analysis of in planta bacterial growth identified one mutant, ndr1-1 (nonrace-specific disease resistance), that was susceptible to DC3000 expressing any one of the four avirulence genes tested. Interestingly, a hypersensitive-like response was still induced by several of the strains. The ndr1-1 mutation also rendered the plant susceptible to several avirulent isolates of the fungal pathogen Peronospora parasitica. Genetic analysis of ndr1-1 demonstrated that the mutation segregated as a single recessive locus, located on chromosome III. Characterization of the ndr1-1 mutation suggests that a common step exists in pathways of resistance to two unrelated pathogens.

Use of Arabidopsis thaliana defense-related mutants to dissect the plant response to pathogens

The plant defense response to microbial pathogens had been studied primarily by using biochemical and physiological techniques. Recently, several laboratories have developed a variety of pathosystems utilizing Arabidopsis thaliana as a model host so that genetic analysis could also be used to study plant defense responses. Utilizing a pathosystem that involves the infection of Arabidopsis with pathogenic pseudomonads, we have cloned the Arabidopsis disease-resistance gene RPS2, which corresponds to the avirulence gene avrRpt2 in a gene-for-gene relationship. RPS2 encodes a 105-kDa protein containing a leucine zipper, a nucleotide binding site, and 14 imperfect leucine-rich repeats. The RPS2 protein is remarkably similar to the product of the tobacco N gene, which confers resistance to tobacco mosaic virus. We have also isolated a series of Arabidopsis mutants that synthesize decreased levels of an Arabidopsis phytoalexin called camalexin. Analysis of these mutants indicated that camalexin does not play a significant role in limiting growth of avirulent Pseudomonas syringae strains during the hypersensitive defense response but that it may play a role in limiting the growth of virulent strains. More generally, we have shown that we can utilize Arabidopsis to systematically dissect the defense response by isolation and characterization of appropriate defense-related mutants.

The phenylalanine ammonia-lyase gene family in Arabidopsis thaliana

Phenylpropanoid derivatives are a complex class of secondary metabolites that have many important roles in plants during normal growth and in responses to environmental stress. Phenylalanine ammonialyase (PAL) catalyzes the first step in the biosynthesis of phenylpropanoids, and is usually encoded by a multi-gene family. Genomic clones for three Arabidopsis thaliana PAL genes containing the entire protein-coding region and upstream and downstream sequences have been obtained and completely sequenced. Two A. thaliana PAL genes (PAL1 and PAL2) are structurally similar to PAL genes that have been cloned from other plant species, with a single intron at a conserved position, and a long highly conserved second exon. Previously identified promoter motifs plus several additional sequence motifs were found in the promoter regions of PAL1 and PAL2. Expression of PAL1 and PAL2 is both qualitatively and quantitatively similar in different plant organs and under various inductive conditions. A third A. thaliana PAL gene, PAL3, differs significantly from PAL1 and PAL2 and other sequenced plant PAL genes. PAL3 contains an additional intron, and its deduced amino acid sequence is less homologous to other PAL proteins. The PAL3 promoter region lacks several sequence motifs conserved between A. thaliana PAL1 and PAL2, as well as motifs described in other genes involved in phenylpropanoid metabolism. A. thaliana PAL3 was expressed at very low levels under the conditions examined.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

Characterization of an Arabidopsis Mutant That Is Nonresponsive to Inducers of Systemic Acquired Resistance

Systemic acquired resistance (SAR) is a general defense response in plants that is characterized by the expression of pathogenesis-related (PR) genes. SAR can be induced after a hypersensitive response to an avirulent pathogen or by treatment with either salicylic acid (SA) or 2,6-dichloroisonicotinic acid (INA). To dissect the signal transduction pathway of SAR, we isolated an Arabidopsis mutant that lacks the expression of an SA-, INA-, and pathogen-responsive chimeric reporter gene composed of the 5[prime] untranslated region of an Arabidopsis PR gene, [beta]-1,3-glucanase (BGL2), and the coding region of [beta]-glucuronidase (GUS). This mutant, npr1 (nonexpresser of PR genes), carries a single recessive mutation that abolishes the SAR-responsive expression of other PR genes as well. While SA-, INA-, or avirulent pathogen-induced SAR protects wild-type plants from Pseudomonas syringae infection, the mutant cannot be protected by pretreatment with these inducers. The insensitivity of npr1 to SA, INA, and avirulent pathogens in SAR induction indicates that these inducers share a common signal transduction pathway. Moreover, in npr1, the localized expression of PR genes induced by a virulent Pseudomonas pathogen is disrupted, and the lesion formed is less confined. These results suggest a role for PR genes in preventing the proximal spread of pathogens in addition to their suggested role in SAR.

Characterization of an Arabidopsis Mutant That Is Nonresponsive to Inducers of Systemic Acquired Resistance

Systemic acquired resistance (SAR) is a general defense response in plants that is characterized by the expression of pathogenesis-related (PR) genes. SAR can be induced after a hypersensitive response to an avirulent pathogen or by treatment with either salicylic acid (SA) or 2,6-dichloroisonicotinic acid (INA). To dissect the signal transduction pathway of SAR, we isolated an Arabidopsis mutant that lacks the expression of an SA-, INA-, and pathogen-responsive chimeric reporter gene composed of the 5[prime] untranslated region of an Arabidopsis PR gene, [beta]-1,3-glucanase (BGL2), and the coding region of [beta]-glucuronidase (GUS). This mutant, npr1 (nonexpresser of PR genes), carries a single recessive mutation that abolishes the SAR-responsive expression of other PR genes as well. While SA-, INA-, or avirulent pathogen-induced SAR protects wild-type plants from Pseudomonas syringae infection, the mutant cannot be protected by pretreatment with these inducers. The insensitivity of npr1 to SA, INA, and avirulent pathogens in SAR induction indicates that these inducers share a common signal transduction pathway. Moreover, in npr1, the localized expression of PR genes induced by a virulent Pseudomonas pathogen is disrupted, and the lesion formed is less confined. These results suggest a role for PR genes in preventing the proximal spread of pathogens in addition to their suggested role in SAR.

A birch gene family encoding pollen allergens and pathogenesis-related proteins

Bet v I, the major pollen allergen of birch (Betula verrucosa), shows high sequence homology to a family of pathogenesis-related (PR) proteins that have recently been identified in several other plant species. We have used a pollen Bet v I cDNA clone and anti-Bet v I antibodies as probes to study the expression of Bet v I genes in birch cell suspension cultures under different experimental conditions. Induction of Bet v I-related proteins was detected in immunoblots of cell extracts upon co-cultivation with microbial pathogens. Northern analysis revealed the rapid induction of Bet v I transcripts in the presence of bacteria and fungi, but not by stress treatments (heat shock, metal ions) or by chemical elicitors. RNase protection experiments showed that the pathogen-inducible RNAs did not correspond to the pollen cDNA clone but most likely to the products of transcription of other members of the Bet v I gene family, sharing high sequence homology with the pollen-specific gene within the 5'-half of the coding region. We conclude that the Bet v I gene family of pollen allergens includes a subset of defense-related genes that are transcriptionally activated in the presence of microbial pathogens.

The A. thaliana disease resistance gene RPS2 encodes a protein containing a nucleotide-binding site and leucine-rich repeats

In plants, resistance to a pathogen is frequently correlated with a genetically defined interaction between a plant resistance gene and a corresponding pathogen avirulence gene. A simple model explains these gene-for-gene interactions: avirulence gene products generate signals (ligands), and resistance genes encode cognate receptors. The A. thaliana RPS2 gene confers resistance to the bacterial pathogen P. syringae carrying the avirulence gene avrRpt2. A map-based positional cloning strategy was used to identify RPS2. The identification of RPS2 was verified using a newly developed transient assay for RPS2 function and by genetic complementation in transgenic plants. RPS2 encodes a novel 105 kDa protein containing a leucine zipper, a nucleotide-binding site, and 14 imperfect leucine-rich repeats.

Isolation of phytoalexin-deficient mutants of Arabidopsis thaliana and characterization of their interactions with bacterial pathogens

A genetic approach was used to assess the extent to which a particular plant defense response, phytoalexin biosynthesis, contributes to Arabidopsis thaliana resistance to Pseudomonas syringae pathogens. The A. thaliana phytoalexin, camalexin, accumulated in response to infection by various P. syringae strains. No correlation between pathogen avirulence and camalexin accumulation was observed. A biochemical screen was used to isolate three mutants of A. thaliana ecotype Columbia that were phytoalexin deficient (pad mutants). The mutations pad1, pad2, and pad3 were found to be recessive alleles of three different genes. pad1 and pad2 were mapped to chromosome IV and pad3 was mapped to chromosome III. Infection of pad mutant plants with strains carrying cloned avirulence genes revealed that the pad mutations did not affect the plants' ability to restrict the growth of these strains. This result strongly suggests that in A. thaliana, phytoalexin biosynthesis is not required for resistance to avirulent P. syringae pathogens. Two of the pad mutants displayed enhanced sensitivity to isogenic virulent P. syringae pathogens, suggesting that camalexin may serve to limit the growth of virulent bacteria.

Differential regulation of an auxin-producing nitrilase gene family in Arabidopsis thaliana

Nitrilases (nitrile aminohydrolase, EC 3.5.5.1) convert nitriles to carboxylic acids. We report the cloning, characterization, and expression patterns of four Arabidopsis thaliana nitrilase genes (NIT1-4), one of which was previously described [Bartling, D., Seedorf, M., Mithöfer, A. & Weiler, E. W. (1992) Eur. J. Biochem. 205, 417-424]. The nitrilase genes encode very similar proteins that hydrolyze indole-3-acetonitrile to the phytohormone indole-3-acetic acid in vitro, and three of the four genes are tandemly arranged on chromosome III. Northern analysis using gene-specific probes and analysis of transgenic plants containing promoter-reporter gene fusions indicate that the four genes are differentially regulated. NIT2 expression is specifically induced around lesions caused by bacterial pathogen infiltration. The sites of nitrilase expression may represent sites of auxin biosynthesis in A. thaliana.

Arabidopsis mutants simulating disease resistance response

We describe six Arabidopsis mutants, defining at least four loci, that spontaneously form necrotic lesions on leaves. Lesions resemble those resulting from disease, but occur in the absence of pathogen. In five mutants, lesion formation correlates with expression of histochemical and molecular markers of plant disease resistance responses and with expression of genes activated during development of broad disease resistance in plants (systemic acquired resistance [SAR]). We designate this novel mutant class Isd (for lesions simulating disease resistance response). Strikingly, four Isd mutants express substantial resistance to virulent fungal pathogen isolates. Isd mutants vary in cell type preferences for lesion onset and spread. Lesion formation can be conditional and can be induced specifically by biotic and chemical activators of SAR in Isd1 mutants.

Programmed cell death in plants: a pathogen-triggered response activated coordinately with multiple defense functions

In plants, the hypersensitive response (HR) to pathogens involves rapid cell death, which is hypothesized to arise from the activation of a cell death program. We describe mutant A. thaliana plants that contain lesions in a single accelerated cell death (ACD) gene called ACD2 and that bypass the need for pathogen exposure to induce the HR. acd2 plants that develop spontaneous lesions show typical HR characteristics both within the necrotic tissue and within the healthy part of the plant, including: modification of plant cell walls, resistance to bacterial pathogens, and accumulation of defense-related gene transcripts, the signal molecule salicylic acid and an antimicrobial compound. We propose that the ACD2 gene is involved in a pathway(s) that negatively regulates a genetically programmed HR.

A new, pathogen-inducible gene of Arabidopsis is expressed in an ecotype-specific manner

In this study we describe a novel gene, which was isolated in an attempt to search for specific plant resistance genes of Arabidopsis against isolates of the phytopathogenic bacterium Xanthomonas campestris pv. campestris. The gene was cloned by differential screening of a genomic library of the Xcc 750-resistant ecotype Col-0, using cDNA populations derived from ecotype Col-0 and the Xcc 750-susceptible ecotype Oy-0. The isolated gene, CXc750, is differentially expressed in ecotypes of Arabidopsis thaliana. In addition, although highly expressed in uninfected plants, gene expression increases in response to pathogen attack. CXc750 potentially codes for a small, basic protein of about 10 kDa. The predicted protein product contains a potential signal leader peptide at the amino-terminal end but no ER retention sequence and no further transmembrane domain. This indicates that the gene product is transported to other compartments or out of the cell. The possible function of CXc750 as a member of the plant defense response system is discussed.

Arabidopsis Mutants Selected for Resistance to the Phytotoxin Coronatine Are Male Sterile, Insensitive to Methyl Jasmonate, and Resistant to a Bacterial Pathogen

The phytotoxin coronatine and the plant growth regulator methyl jasmonate (MeJA) caused similar growth-inhibitory effects on Arabidopsis seedlings. To test whether these two compounds have similar action, 14 independent coi1 (coronatine-insensitive) mutants of Arabidopsis were selected. The mutants segregated as single recessive Mendelian markers, and all were alleles at the coi1 locus. All coi1 mutants were also insensitive to MeJA and were male sterile. Both coronatine and MeJA inhibited root growth, stimulated anthocyanin accumulation, and increased the level of two proteins of ~31 and ~29 kD detected in SDS-polyacrylamide gels of wild-type Arabidopsis but caused none of these effects in the coi1 mutant. Coronatine and MeJA also induced the systemic appearance of proteinase inhibitor activity in tomato. The male-sterile flowers of the coi1 mutant produced abnormal pollen and had reduced level of an ~31-kD protein, which was abundant in the wild-type flowers. A coronatine-producing strain of Pseudomonas syringae grew in leaves of wild-type Arabidopsis to a population more than 100 times greater than it reached in the coi1 mutant. We conclude that coronatine mimics the action of MeJA and that coi1 controls a step in MeJA perception/response and in flower development.

Arabidopsis Mutants Selected for Resistance to the Phytotoxin Coronatine Are Male Sterile, Insensitive to Methyl Jasmonate, and Resistant to a Bacterial Pathogen

The phytotoxin coronatine and the plant growth regulator methyl jasmonate (MeJA) caused similar growth-inhibitory effects on Arabidopsis seedlings. To test whether these two compounds have similar action, 14 independent coi1 (coronatine-insensitive) mutants of Arabidopsis were selected. The mutants segregated as single recessive Mendelian markers, and all were alleles at the coi1 locus. All coi1 mutants were also insensitive to MeJA and were male sterile. Both coronatine and MeJA inhibited root growth, stimulated anthocyanin accumulation, and increased the level of two proteins of ~31 and ~29 kD detected in SDS-polyacrylamide gels of wild-type Arabidopsis but caused none of these effects in the coi1 mutant. Coronatine and MeJA also induced the systemic appearance of proteinase inhibitor activity in tomato. The male-sterile flowers of the coi1 mutant produced abnormal pollen and had reduced level of an ~31-kD protein, which was abundant in the wild-type flowers. A coronatine-producing strain of Pseudomonas syringae grew in leaves of wild-type Arabidopsis to a population more than 100 times greater than it reached in the coi1 mutant. We conclude that coronatine mimics the action of MeJA and that coi1 controls a step in MeJA perception/response and in flower development.

Isolation and characterization of the binding sequences for the product of the Arabidopsis floral homeotic gene AGAMOUS

The Arabidopsis floral homeotic gene AGAMOUS (AG) is required for normal flower development. The deduced AG protein contains a region which shares substantial sequence similarity with the DNA-binding domains of known transcription factors, SRF (human) and MCM1 (yeast). Therefore, it is likely that AG is also a DNA-binding protein regulating transcription of floral genes. We describe here several experiments to characterize AG-DNA binding in vitro. We show that AG indeed binds a DNA sequence matching the consensus of SRF targets. Further, we have selected the AG-binding sequences from a pool of random oligonucleotides, and deduced an AG-binding consensus sequence of TT(A/T)CC(A/T)(A/t)2(T/A)NNGG(-G)(A/t)2. We have demonstrated that AG binds to the consensus region of three of the oligonucleotides by footprinting analysis. Finally, we have examined AG's relative binding affinity for different sequences, as compared to SRF, by gel mobility shift analysis. Our results indicate that AG is a sequence-specific DNA-binding protein, and that the AG-binding consensus sequence is similar to those of MCM1 and SRF.

Suppressors of trp1 fluorescence identify a new arabidopsis gene, TRP4, encoding the anthranilate synthase beta subunit

Suppressors of the blue fluorescence phenotype of the Arabidopsis trp1-100 mutant can be used to identify mutations in genes involved in plant tryptophan biosynthesis. Two recessive suppressor mutations define a new gene, TRP4. The trp4 mutant and the trp1-100 mutant are morphologically normal and grow without tryptophan, whereas the trp4; trp1-100 double mutant requires tryptophan for growth. The trp4; trp1-100 double mutant does not segregate at expected frequencies in genetic crosses because of a female-specific defect in transmission of the double mutant genotype, suggesting a role for the tryptophan pathway in female gametophyte development. Genetic and biochemical evidence shows that trp4 mutants are defective in a gene encoding the beta subunit of anthranilate synthase (AS). Arabidopsis AS beta subunit genes were isolated by complementation of an Escherichia coli anthranilate synthase mutation. The trp4 mutation cosegregates with one of the genes, ASB1, located on chromosome 1. Sequence analysis of the ASB1 gene from trp4-1 and trp4-2 plants revealed different single base pair substitutions relative to the wild type. Anthranilate synthase alpha and beta subunit genes are regulated coordinately in response to bacterial pathogen infiltration.

Molecular analysis of avirulence gene avrRpt2 and identification of a putative regulatory sequence common to all known Pseudomonas syringae avirulence genes

The avrRpt2 locus from Pseudomonas syringae pv. tomato causes virulent strains of P. syringae to be avirulent on some, but not all, lines of Arabidopsis thaliana and Glycine max (soybean). We determined the DNA sequence of the avrRpt2 locus and identified the avrRpt2 gene as a 768-bp open reading frame encoding a putative 28.2-kDa protein. Deletion analysis and transcription studies provided further evidence that this open reading frame encodes AvrRpt2. We found that the avrRpt2 gene also has avirulence activity in P. syringae pathogens of Phaseolus vulgaris (common bean), suggesting that disease resistance genes specific to avrRpt2 are functionally conserved among diverse plant species. The predicted AvrRpt2 protein is hydrophilic and contains no obvious membrane-spanning domains or export signal sequences, and there was no significant similarity of AvrRpt2 to sequences in the GenBank, EMBL, or Swiss PIR data bases. A comparison of the avrRpt2 DNA sequence to nine other P. syringae avirulence genes revealed a highly conserved sequence, GGAACCNA-N14-CCACNNA, upstream of the translation initiation codon. This motif is located 6 to 8 nucleotides upstream of the transcription start site in all four P. syringae avirulence genes for which a transcription start site has been determined, suggesting a role as a binding site for a novel form of RNA polymerase. Regulation of avrRpt2 was similar to other P. syringae avirulence genes; expression was high in minimal medium and low in rich medium and depended on the hrpRS locus and an additional locus at the opposite end of the hrp cluster of P. syringae pv. tomato.

RPS2, an Arabidopsis disease resistance locus specifying recognition of Pseudomonas syringae strains expressing the avirulence gene avrRpt2

A molecular genetic approach was used to identify and characterize plant genes that control bacterial disease resistance in Arabidopsis. A screen for mutants with altered resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) expressing the avirulence gene avrRpt2 resulted in the isolation of four susceptible rps (resistance to P. syringae) mutants. The rps mutants lost resistance specifically to bacterial strains expressing avrRpt2 as they retained resistance to Pst strains expressing the avirulence genes avrB or avrRpm1. Genetic analysis indicated that in each of the four rps mutants, susceptibility was due to a single mutation mapping to the same locus on chromosome 4. Identification of a resistance locus with specificity for a single bacterial avirulence gene suggests that this locus, designated RPS2, controls specific recognition of bacteria expressing the avirulence gene avrRpt2. Ecotype Wü-0, a naturally occurring line that is susceptible to Pst strains expressing avrRpt2, appears to lack a functional allele at RPS2, demonstrating that there is natural variation at the RPS2 locus among wild populations of Arabidopsis.

Transient expression in Arabidopsis thaliana protoplasts derived from rapidly established cell suspension cultures

Arabidopsis thaliana has emerged as a model species for the analysis of genes controlling plant development. However, its small size has impaired biochemical analyses, and the absence of a transient expression system has hampered promoter analysis. Here, we report a method for rapidly establishing A. thaliana suspension cultures that yield protoplasts that can be readily transfected. We have optimized transient expression conditions using a modified polyethylene glycol / calcium nitrate transformation protocol and a Cauliflower Mosaic Virus 35S promoter-luciferase reporter gene construct. Our methods permit isolation of large quantities of rapidly growing cells and analysis of Arabidopsis promoters in vivo in a homologous system.

Suppression of Bean Defense Responses by Pseudomonas syringae

We have developed a model system to examine suppression of defense responses in bean by the compatible bacterium Pseudomonas syringae pv phaseolicola. Previously, we have shown that there is a general mechanism for the induction of the bean defense genes phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI), and chitinase (CHT) by incompatible, compatible, and nonpathogenic bacteria. Here, we show that bean plants infiltrated with isolates of P. s. phaseolicola failed to produce transcripts for PAL, CHS, or CHI up to 120 hr after infiltration and CHT transcript accumulation was significantly delayed when compared to the incompatible P. syringae strains. Infiltration of bean plants with 108 cells per mL of P. s. phaseolicola NPS3121 8 hr prior to infiltration with an equal concentration of incompatible P. s. pv tabaci Pt11528 significantly reduced the typical profile of defense transcript accumulation when compared to plants infiltrated with Pt11528 alone. A corresponding suppression of phytoalexin accumulation was also observed. NPS3121 also suppressed PAL, CHS, CHI, and CHT transcript accumulation and phytoalexin production induced by Escherichia coli DH5[alpha] or the elicitor glutathione. Heat-killed NPS3121 cells or cells treated with protein synthesis inhibitors lost the suppressor activity. Taken together, these experiments suggest that NPS3121 has an active mechanism to suppress the accumulation of defense transcripts and phytoalexin biosynthesis in bean.

Generalized Induction of Defense Responses in Bean Is Not Correlated with the Induction of the Hypersensitive Reaction

Transcripts for phenylalanine ammonia-lyase, chalcone synthase, chalcone isomerase, and chitinase accumulated in common bean after infiltration with the Pseudomonas syringae pv tabaci Hrp- mutant Pt11528::Hrp1, even though a hypersensitive reaction did not occur. The temporal pattern of this transcript accumulation was similar to that seen after infiltration with wild-type P. s. tabaci Pt11528, which resulted in a hypersensitive reaction. Escherichia coli DH5[alpha], P. fluorescens Pf101, heat-killed Pt11528 cells, and Pt11528 cells treated with protein synthesis inhibitors also induced accumulation of defense transcripts but not a hypersensitive reaction. In contrast, these transcripts were not detected in plants infiltrated with water or P.s. pv phaseolicola NPS3121, a compatible pathogen that causes halo blight. Phytoalexins were produced in bean after infiltration with Pt11528, Pt11528::Hrp1, Pt11528 cells treated with neomycin, or Pf101, but not in plants infiltrated with NPS3121 or water. These results suggest that there are unique biochemical events associated with the expression of a hypersensitive reaction which are distinct from other plant defense responses such as phytoalexin biosynthesis. In addition, our results support the hypothesis that there is a general, nonspecific mechanism for the induction of defense transcripts and phytoalexins by pathogenic and saprophytic bacteria that is distinct from the more specific mechanism associated with the induction of the hypersensitive reaction.