Two pathways act in an additive rather than obligatorily synergistic fashion to induce systemic acquired resistance and PRgene expression
© Zhang and Shapiro; licensee BioMed Central Ltd. 2002
Received: 15 July 2002
Accepted: 15 October 2002
Published: 15 October 2002
Local infection with necrotizing pathogens induces whole plant immunity to secondary challenge. Pathogenesis-related genes are induced in parallel with this systemic acquired resistance response and thought to be co-regulated. The hypothesis of co-regulation has been challenged by induction of Arabidopsis PR-1 but not systemic acquired resistance in npr1 mutant plants responding to Pseudomonas syringae carrying the avirulence gene avrRpt2. However, experiments with ndr1 mutant plants have revealed major differences between avirulence genes. The ndr1-1 mutation prevents hypersensitive cell death, systemic acquired resistance and PR-1 induction elicited by bacteria carrying avrRpt2. This mutation does not prevent these responses to bacteria carrying avrB.
Systemic acquired resistance, PR-1 induction and PR-5 induction were assessed in comparisons of npr1-2 and ndr1-1 mutant plants, double mutant plants, and wild-type plants. Systemic acquired resistance was displayed by all four plant lines in response to Pseudomonas syringae bacteria carrying avrB. PR-1 induction was partially impaired by either single mutation in response to either bacterial strain, but only fully impaired in the double mutant in response to avrRpt2. PR-5 induction was not fully impaired in any of the mutants in response to either avirulence gene.
Two pathways act additively, rather than in an obligatorily synergistic fashion, to induce systemic acquired resistance, PR-1 and PR-5. One of these pathways is NPR1-independent and depends on signals associated with hypersensitive cell death. The other pathway is dependent on salicylic acid accumulation and acts through NPR1. At least two other pathways also contribute additively to PR-5 induction.
In response to local infection with necrotizing pathogen, plants display a whole plant enhanced immunity to secondary challenge (systemic acquired resistance or SAR) . A characteristic set of genes termed pathogenesis-related (PR) is induced both locally and in distal tissues in parallel with SAR induction . Accumulation of salicylic acid (SA) has been shown to be necessary for induction of SAR .
The Arabidopsis ndr1-1 mutant showed drastically impaired induction of SAR and PR-1 gene expression in response to infection with Pseudomonas syringae bacteria carrying the avirulence (avr) gene avrRpt2. This mutant is also incapable of showing a hypersensitive response (HR) to these bacteria [5, 6] except at very high levels of inoculum . The HR is a macroscopic leaf collapse. The HR occurs in response to infection with avirulent bacteria at levels of inoculum that cause a large percentage of leaf cells to undergo programmed cell death (PCD) . In contrast to results with avrRpt2, P. syringae bacteria carrying the avirulence genes avrB, avrRpm1, avrPphB or avrRps4 elicit an exaggerated HR relative to that on the wild-type Columbia parent [5, 7]. In a study in which bacteria carrying avrB were chosen as a representative member of this class, in further contrast to results with avrRpt2, these bacteria were capable of eliciting SAR and PR-1 induction on ndr1-1 mutant plants .
However, induction of SA accumulation by ndr1-1 mutant plants was impaired at early time points to a similar extent in response to bacteria carrying either avr gene . These results suggested that at least two pathways control SAR and PR-1 gene expression. One pathway is directly dependent on SA accumulation. The second pathway is not directly dependent on SA levels. The correlation between ability to elicit the HR and to elicit SAR and PR-1 induction led to the model that the second pathway was caused by signals associated with hypersensitive cell death. Despite the severe impairment in SA accumulation within the first 12 hours post-inoculation of ndr1 plants, bacteria carrying avrB could nonetheless induce PR gene expression and SAR because of the HR-associated pathway. Bacteria carrying avrRpt2 could not because both pathways were impaired by the ndr1 mutation. This study could not address the question as to whether the HR-associated pathway alone was sufficient to induce SAR and PR-1 because SA accumulation did occur late in the response.
Induction of SAR has been reported to be dependent upon the NPR1 gene [9–11]. In these studies, neither SA/SA analogs nor avirulent bacteria were able to induce SAR on npr1 mutant plants. The biological inducer of SAR tested in these experiments was P. syringae bacteria carrying avrRpt2. In contrast to ndr1 plants, npr1 plants do show the HR in response to P. syringae carrying avrRpt2 (data not shown). These results could be taken to suggest that the HR-associated pathway is not sufficient to induce SAR in the absence of the NPR1-dependent, SA-associated pathway (but see below for why this conclusion is not correct).
In contrast to SAR induction, induction of PR-1 by avirulent bacteria has been shown to occur in npr1 mutants . It is, however, reduced in extent and delayed. If npr1 mutants are indeed fully blocked in SAR induction, then the pathways leading to PR-1 induction can not be the same as those leading to SAR. However, we have argued on the basis of results with ndr1 mutant plants that HR "triggering" in response to the avrRpt2/RPS2 combination is weaker than that in response to avrB/RPM1. Weaker "triggering" results in a lower probability of individual cells undergoing PCD early in the HR, prior to the onset of potentiation caused by increased SA levels [4, 12]. Signaling elicited by avrRpt2 clearly differs in many respects from that directed by the other avr genes. Use of bacteria carrying avrB to elicit SAR would allow a more rigorous test of whether the NPR1-dependent pathway is essential for SAR induction.
To clarify the signaling circuitry governing SAR and PR gene induction, we have constructed a ndr1/npr1 double mutant. This line was compared with wild-type Columbia Arabidopsis and both single mutant lines for the ability to display biological induction of SAR and PR gene expression. We conclude that SAR, PR-1 induction and PR-5 induction are regulated in parallel by the two pathways we have previously delineated , but the effects of these pathways are additive rather than obligatorily synergistic. Moreover, additional pathways also contribute additively to PR-5 induction.
Systemic acquired resistance elicited by DC3000•avrB
To test epistasis relationships between these mutants, a double mutant was constructed (see Methods). The double mutant was also tested for SAR in response to DC3000•avrB (Figure 1). Statistically significant SAR was seen at the two day time point but not at the four day time point (Student's t test, P < 0.05). In a replicate experiment, SAR was statistically significant only at the four day time point. The combination of the two mutations was thus sufficient to abolish most of the SAR response. Residual SAR may have been due to HR-associated signals in that the double mutant did display a HR to DC3000•avrB. The macroscopic HR phenotype was exaggerated, like that of the ndr1-1 single mutant (data not shown).
Induction of PR-1 gene expression in response to DC3000•avrB and DC3000•avrRpt2
We have previously argued that PR-1 gene expression is controlled by two separate pathways . One pathway results from SA accumulation and is dependent upon the NPR1 gene product. The second pathway results from signals correlated with the HR and is NPR1-independent. The ndr1-1/npr1-2 double mutant was used to further test this hypothesis.
Induction of PR-5 gene expression in response to DC3000•avrB and DC3000•avrRpt2
PR genes are thought to be tightly associated with SAR because they are co-regulated with SAR [13, 14] and because many PR gene products have been shown to have antimicrobial activity . The complete impairment of npr1 plants in SAR induction  as contrasted with a partial impairment in PR-1 induction  in response to P. syringae carrying avrRpt2 challenged the hypothesis of co-regulation. However, avrRpt2 was shown to differ markedly from the other characterized P. syringae avr genes in the responses it elicited on ndr1 mutant plants [4, 5, 7]. The apparent challenge to the hypothesis of co-regulation could have been because of the choice of an atypical avr gene. Therefore, SAR and PR gene expression of npr1 mutant plants were assessed in response to a second avr gene (avrB). Markedly different results were obtained. Mutant npr1 plants were not fully impaired in induction of SAR, PR-1 gene expression or PR-5 gene expression in response to P. syringae carrying avrB. These results are consistent with the hypothesis of co-regulation. The apparent NPR1-dependence of SAR induction in response to bacteria carrying avrRpt2 is likely quantitative rather than qualitative.
A potential alternative explanation for these results might be residual function of mutant gene products in the plant lines used. The ndr1-1 mutant is an unambiguous null mutant in that a deletion removes most of the coding region and the proximal promoter region . However, none of the characterized npr1 mutants have a large deletion [15, 16]. As such, attempts to define allelic series have relied upon the level of residual function associated with these mutants in assays of gene expression or fungal disease resistance [15, 16]. However, the complexity of signaling leading to these outputs complicates analyses of these phenotypes. A "null" mutant might still show gene expression or disease resistance caused by pathways that do not require the corresponding gene product.
What is required is an assay where the output is due solely to an NPR1-dependent pathway. The experiment presented in Figure 2 provided such data. The combination of npr1-2 with ndr1-1 in a double mutant prevented the avrRpt2-elicited HR under the conditions used. Under these conditions, DC3000•avrRpt2 failed to elicit any PR-1 gene expression (the level was statistically indistinguishable from that seen with inoculation of a MgCl2 blank). Impairment of SA accumulation in the double mutant (Zhang, Gutsche and Shapiro, manuscript in preparation) resembled that in the ndr1 single mutant  at early time points prior to the onset of NPR1-dependent negative feedback on SA accumulation [10, 11]. Nonetheless, SA accumulation likely occurred by the time points taken in SAR and PR gene expression experiments reported herein.
This assay depends solely on NPR1 function. The complete lack of PR-1 induction supports our contention that npr1-2 is indeed a null mutant or perhaps a severe hypomorph. This contention is further supported by published data showing that the npr1-2 allele failed to interact with putative target TGA transcription factor genes in a two-hybrid assay  or in an in vivo, reporter gene-based assay . The treatment of npr1-2 seedlings with exogenous SA failed to induce increases in PR-1 gene expression under some experimental conditions  albeit not under all conditions . The potential alternative explanation that results presented herein were caused by residual gene product function in the mutant lines used is thus not correct.
The idea that SAR and PR gene expression are co-regulated has also been challenged by experiments with the eds5 and sid2 mutants . These mutants have been implicated in SA biosynthesis. They showed impairments in PR-1 induction in response to bacteria that were very similar to those displayed by ndr1 plants but marginal if any impairment in PR-2 or PR-5 induction. Direct comparison with results presented here required addressing an issue with the potential to affect interpretation of that study. In that study, PR gene expression was quantitated from uninfected parts of leaves that had been infected with 1 × 108 mL-1 DC3000•avrRpt2. This level of inoculum was sufficiently high to cause "false positive" spurious HR-like responses to DC3000•empty vector . Indeed, when sid2 mutant plants were inoculated with 2 × 107 mL-1 DC3000•avrRpt2, no HR was seen (data not shown).
The high levels of inoculum used in the cited study can only be used without causing "false positive" HRs if a bacterial strain that is not a pathogen of Arabidopsis is used to deliver the avr gene product [5, 7]. When sid2 plants were inoculated with 1 × 108 mL-1 of a nonpathogen strain (P. syringae pv. glycinea Race 5) carrying avrRpt2, HRs were seen (data not shown). The identical results with the two bacterial strain backgrounds imply that the cited results with sid2 mutant plants were obtained under conditions in which high levels of bacterial inoculum overcame some effects of the mutation. The ndr1-1 mutation can also be overcome at high levels of inoculum. However, the level at which ndr1-1 phenotypes are overcome is higher than that for sid2 phenotypes in that 1 × 109 mL-1 of a nonpathogen strain carrying avrRpt2 is required to yield reproducible HRs on ndr1-1 plants .
Clearly, the interpretation of results with PR-5 induction depends critically on the experimental design. The relatively unimpaired PR-2 and PR-5 induction in sid2 plants might imply that the HR-associated signals were quantitatively more important than the SA-associated signals. The same two pathways invoked to explain SAR and PR-1 induction could potentially account for PR-2 and PR-5 induction if this assumption were correct.
The results presented herein, however, argue strongly that PR-5 induction is indeed regulated differently than SAR or PR-1 induction. The two pathways identified as making the major contributions to PR-1 and SAR induction were shown to contribute to PR-5 induction. However, significant PR-5 induction occurred in the double mutant in response to DC3000•avrRpt2 (Figure 3). DC3000•avrRpt2 elicited HRs on Columbia or npr1-2 plants but not on ndr1-1 plants or double mutant plants (data not shown). As such, these results can not be explained by HR-associated signals or SA-associated, NPR1-dependent signals. There must be additional pathways contributing to PR-5 induction. This conclusion is consistent with the original interpretation of PR gene expression data in eds5 and sid2 mutants that PR-1 and PR-5 are regulated differently .
The data presented in Figures 2 and 3 suggest that there must be at least two additional pathways contributing to PR-5 induction. DC3000•empty vector did not elicit significant PR-1 induction on the double mutant at either time point (Student's t test, P > 0.05). However, the analogous experiment with PR-5 induction gave the opposite results. Moreover, at both time points, the PR-5 induction elicited by infection with DC3000•empty vector was to a similar extent regardless of allele status at the NDR1 or NPR1 loci (no statistically significant differences, Student's t test, P > 0.05). These bacteria elicited PR-5 gene expression in spite of the absence of a functional avr gene, the absence of PCD, and mutations in both the NPR1 and NDR1 genes. As even E. coli has been shown to induce plant defense-related gene expression , it should not be surprising that bacteria can elicit changes in plant gene expression via many different signaling pathways. Indeed, conserved features of bacteria can act as elicitors of gene expression through pathways distinct from those associated with gene-for-gene disease resistance .
However, an additional pathway leading to PR-5 gene expression that stems from avrRpt2 function must also exist. DC3000•avrRpt2 elicited no statistically significant induction of PR-1 by the double mutant at either time point (Student's t test, P > 0.05). However, clear induction of PR-5 was seen in the analogous experiment. The level of PR-5 induction was higher than in response to DC3000•empty vector (Student's t test, P < 0.05). As the double mutant was susceptible to disease caused by this bacterial strain (data not shown), perhaps this residual gene induction was due to the virulence function of avrRpt2. Mutant npr1 plants have previously been shown to be impaired in induction of PR-1 but not PR-5 in response to virulent bacterial pathogen .
The HR progresses more slowly in response to avrRpt2 than in response to avrB[4, 25, 26]. Evidence has been presented, however, that differences between HRs are not merely differences in kinetics . As such, the avrRpt2-elicited HR and the avrB-elicited HR were pictured as two separate components. When assayed under standard conditions, ndr1-1 mutant plants did not show the avrRpt2-elicited HR. An "and" gate was used to show that both infection with bacteria carrying avrRpt2 and early accumulation of SA were needed to give the characteristic, "slow HR". By contrast, ndr1-1 mutant plants did show the HR to bacteria carrying avrB. A straight line that is not connected via a gate to SA accumulation thus leads to "Fast HR-associated signals".
SAR is gated by an "or" gate with three possible inputs. As DC3000•avrB elicited SAR on the ndr1-1/npr1-2 double mutant, neither of these gene products was essential for SAR induction. The early SA accumulation was not likely to have been essential either, because that was markedly impaired by the ndr1-1 mutation. However, the characteristic avrB-elicited, "fast HR" did occur in the double mutant. As such, a straight line deriving from "Fast HR-associated signals" is one of the inputs to this "or" gate. Another input is an "and" gate. The evidence requiring this gate is that SAR elicited by DC3000•avrRpt2 was prevented by either a npr1 mutation  or a ndr1 mutation . We conclude that the characteristic avrRpt2-elicited "slow HR" can only contribute to SAR in combination with NPR1-dependent, early SA-elicited signaling, necessitating the "and" gate. The final input is SA-elicited, NPR1-dependent signaling acting alone .
PR-1 induction is also gated by an "or" gate with three alternative inputs. The "Fast HR" alone (this study), the "slow HR" alone , or SA-elicited, NPR1-dependent signaling [9–11] were each sufficient by themselves to give PR-1 induction. Induction of PR-5 gene expression can also be via any of these three inputs acting independently. However, we (this study) and others [19, 20] have presented evidence that other pathways can also contribute to PR-5 induction. As such, PR-5 is gated by an "or" gate. One input is a branch point from the "or" gate leading to PR-1 induction. The second input is pictured as a question mark. Evidence has been presented herein that this second input represents at least two pathways. However, the details of these pathways are not known and are thus not pictured.
No attempt has been made to represent the kinetics of signaling events in Figure 4. Boolean formalism can incorporate kinetic information through use of gating delays. Differential equations-based kinetic modeling is an alternative means of capturing the rich dynamics of signaling networks [28, 29]. As time-resolved data sets become available and feedback loops are delineated, it may be necessary to use these methods to understand system dynamics and control . However, the authors agree with Genoud et. al. (2001) that the scheme used in that work and in Figure 4 of this paper has advantages for representing the topology of signaling networks. This formalism allows clear differentiation of additive from obligatorily synergistic signaling interactions through use of "or" and "and" gates, respectively.
In conclusion, the additive effects of two pathways explain most if not all induction of SAR and PR-1 gene expression by the avirulent bacteria examined. One of these pathways stems from signals associated with hypersensitive cell death and is NPR1-independent. The other pathway stems from SA accumulation and signals through NPR1. By contrast, PR-5 induction by avirulent bacteria involves at least two additional pathways that also act in an additive fashion.
Bacteria and plant growth
Pseudomonas syringae was cultured according to published methods . Arabidopsis was grown in a HTPC walk-in chamber (Conviron, Winnipeg, MN) or a MTR-30 reach-in chamber (Conviron, Winnipeg, MN) as previously described . Columbia, ndr1-1 and ndr1-1/gl-1 seed used were previously described [4, 5]. Seeds of npr1-2 and sid2-1 mutants were generously provided by Dr. Jane Glazebrook (Torrey Mesa Research Institute) and Dr. Christiane Nawrath (University of Fribourg), respectively.
Isolation and validation of the ndr1-1/npr1-2double mutant line
The ndr1-1/npr1-2 double mutant was generated using pollen from a npr1-2 plant to fertilize flowers of a ndr1-1/gl-1 plant. The success of the cross was validated by the hairy (non-glabrous) phenotype of leaves of the F1 plant. DNA was prepared from leaves of 5-week old F2 progeny from this cross for npr1-2 detection or cultured roots grown from F3 seeds for ndr1-1 detection.
The ndr1-1 deletion was detected by Southern blotting done essentially as previously described . To make a radiolabeled probe, part of the NDR1 locus was amplified using ndr1-1 genomic DNA as template. The primers used in the amplification were as follows: 5'-TCCTAAGGTTTCGTTTTGGGTC-3' and 5'-GGGACGGTTTCAATTCTGTGATAG-3'. The resulting PCR product was gel purified. This PCR product was subsequently used as template DNA in a labeling reaction using the Radprime DNA labeling system (Invitrogen, Carlsbad, CA).
The npr1-2 point mutation  was detected using a CAPS (cleaved co-dominant amplified polymorphic sequence) marker . The primers used in PCR were as follows: 5'-CCTGATGTATCTGCTCT-3' and 5'-GCTTAATGCAGATGGTG-3'. The amplified products were cut with FspI to yield a 330 bp fragment and a 134 bp fragment if wild type (NPR1) DNA was used as the template. FspI does not cut DNA amplified from npr1-2 mutant plants.
Systemic acquired resistance experiments
SAR experiments were performed as previously described  except that hand inoculation using a syringe was used instead of vacuum infiltration to introduce the challenge pathogen. Triplicate samples of four leaf disks each were used on Day 0, and quintuplicate samples were used on Days 2 and 4. Error bars represent standard deviations of data means. Significance of differences between means at a given time point were determined using Student's t tests. Values of P were read from a t table. A replicate experiment gave comparable results. In accordance with established practice in the field , data was not combined between the two replicate experiments.
Plant growth and inoculations were as described for SAR experiments. Leaf tissue samples were collected at indicated time points. Samples were flash frozen in liquid nitrogen and total RNA was isolated according to Nawrath and Métraux . RNA concentration was determined spectrophotometrically. 25 μg of total RNA was used per lane. Gene specific probes were made from gel-purified DNA fragments using the Radprime DNA labeling system (Invitrogen, Carlsbad, CA). The template DNA used for making probes to detect expression of PR-1, PR-5 and ROC1 (cyclophilin, used to normalize for RNA loading) were PR-1 and PR-5 cDNAs  and an EcoRI fragment of pCG22 , respectively. AGI numbers corresponding to the genes used as probes are AT2G19990, AT1G75040 and AT4G38740, respectively. Northern Blots were performed using standard procedures . Quantitation used a Storm PhosphorImager (Amersham Biosciences, Sunnyvale, CA).
We thank Dr. Jane Glazebrook (Torrey Mesa Research Institute) for seeds of npr1-2, Dr. Christiane Nawrath (University of Fribourg) for seeds of sid2-1, Dr. Charles Gasser (University of California, Davis) for pCG22, Dr. John Ryals (Paradigm Genetics) for the PR-1 and PR-5 cDNAs, Dr. Hui Cao and Dr. Xinnian Dong (Duke University) for developing and suggesting the CAPS marker used to detect the npr1-2 allele and Vikas Agrawal (University of Delaware) for suggestions on the Boolean model of signaling. CZ was supported in part by a predoctoral research assistantship from the College of Agriculture and Natural Resources, University of Delaware. This is paper no. 1725 in the Journal Series of the Delaware Agricultural Experiment Station.
- Métraux J-P, Nawrath C, Genoud T: Systemic Acquired Resistance. Euphytica. 2002, 124: 237-243. 10.1023/A:1015690702313.View ArticleGoogle Scholar
- van Loon LC, van Strien EA: The Families of Pathogenesis-Related Proteins, their Activities, and Comparative Analysis of PR-1 Type Proteins. Physiol Molec Plant Pathol. 1999, 55: 85-97. 10.1006/pmpp.1999.0213.View ArticleGoogle Scholar
- Gaffney T, Friedrich L, Vernooij B, Negrotto D, Nye G, Uknes S, Ward E, Kessmann H, Ryals J: Requirement of Salicylic Acid for the Induction of Systemic Acquired Resistance. Science. 1993, 261: 754-756.PubMedView ArticleGoogle Scholar
- Shapiro AD, Zhang C: The Role of NDR1 in Avirulence Gene-Directed Signaling and Control of Programmed Cell Death in Arabidopsis. Plant Physiol. 2001, 127: 1089-1101. 10.1104/pp.127.3.1089.PubMedPubMed CentralView ArticleGoogle Scholar
- Century KS, Holub EB, Staskawicz BJ: NDR1 a Locus of Arabidopsis thaliana that is Required for Disease Resistance to both a Bacterial and a Fungal Pathogen. Proc Natl Acad Sci USA. 1995, 92: 6597-6601.PubMedPubMed CentralView ArticleGoogle Scholar
- Century KS, Shapiro AD, Repetti PP, Dahlbeck D, Holub E, Staskawicz BJ: NDR1: A Pathogen-Induced Component Required for Arabidopsis Disease Resistance. Science. 1997, 278: 1963-1965. 10.1126/science.278.5345.1963.PubMedView ArticleGoogle Scholar
- Shapiro AD: Using Arabidopsis Mutants to Delineate Disease Resistance Signaling Pathways. Can J Plant Pathol. 2000, 22: 199-216.View ArticleGoogle Scholar
- Goodman RN, Novacky AJ: The Hypersensitive Response in Plants to Pathogens: A Resistance Phenomenon. St. Paul, MN: APS Press. 1994Google Scholar
- Cao H, Bowling SA, Gordon AS, Dong X: Characterization of an Arabidopsis Mutant that is Nonresponsive to Inducers of Systemic Acquired Resistance. Plant Cell. 1994, 6: 1583-1592. 10.1105/tpc.6.11.1583.PubMedPubMed CentralView ArticleGoogle Scholar
- Delaney TP, Friedrich L, Ryals JA: Arabidopsis Signal Transduction Mutant Defective in Chemically and Biologically Induced Disease Resistance. Proc Natl Acad Sci USA. 1995, 92: 6602-6606.PubMedPubMed CentralView ArticleGoogle Scholar
- Shah J, Tsui F, Klessig DF: Characterization of a Salicylic Acid-Insensitive Mutant (sai1) of Arabidopsis thaliana, Identified in a Selective Screen Utilizing the SA-Inducible Expression of the tms2 Gene. Mol Plant Microb Interact. 1997, 10: 69-78.View ArticleGoogle Scholar
- Shirasu K, Nakajima H, Rajasekhar VK, Dixon RA, Lamb C: Salicylic Acid Potentiates an Agonist-Dependent Gain Control that Amplifies Pathogen Signals in the Activation of Defense Mechanisms. Plant Cell. 1997, 9: 261-270. 10.1105/tpc.9.2.261.PubMedPubMed CentralView ArticleGoogle Scholar
- Ward ER, Uknes SJ, Williams SC, Dincher SS, Wiederhold DL, Alexander DC, Ahl-Goy P, Métraux J-P, Ryals JA: Coordinate Gene Activity in Response to Agents that Induce Systemic Acquired Resistance. Plant Cell. 1991, 3: 1085-1094. 10.1105/tpc.3.10.1085.PubMedPubMed CentralView ArticleGoogle Scholar
- Uknes S, Mauch-Mani B, Moyer M, Potter S, Williams S, Dincher S, Chandler D, Slusarenko A, Ward E, Ryals J: Acquired Resistance in Arabidopsis. Plant Cell. 1992, 4: 645-656. 10.1105/tpc.4.6.645.PubMedPubMed CentralView ArticleGoogle Scholar
- Cao H, Glazebrook J, Clarke JD, Volko S, Dong X: The Arabidopsis NPR1 Gene That Controls Systemic Acquired Resistance Encodes a Novel Protein Containing Ankyrin Repeats. Cell. 1997, 88: 57-63.PubMedView ArticleGoogle Scholar
- Ryals J, Weymann K, Lawton K, Friedrich L, Ellis D, York-Steiner H, Johnson J, Delaney TP, Jesse T, Vos P, et al: The Arabidopsis NIM1 Protein Shows Homology to the Mammalian Transcription Factor Inhibitor I-kappa-B. Plant Cell. 1997, 9: 425-439. 10.1105/tpc.9.3.425.PubMedPubMed CentralGoogle Scholar
- Zhang YL, Fan WH, Kinkema M, Li X, Dong XN: Interaction of NPR1 with Basic Leucine Zipper Protein Transcription Factors that Bind Sequences Required for Salicylic Acid Induction of the PR-1 Gene. Proc Natl Acad Sci USA. 1999, 96: 6523-6528. 10.1073/pnas.96.11.6523.PubMedPubMed CentralView ArticleGoogle Scholar
- Fan W, Dong X: In Vivo Interaction between NPR1 and Transcription Factor TGA2 Leads to Salicylic Acid-Mediated Gene Activation in Arabidopsis. Plant Cell. 2002, 14: 1377-1389. 10.1105/tpc.001628.PubMedPubMed CentralView ArticleGoogle Scholar
- Glazebrook J, Rogers EE, Ausubel FM: Isolation of Arabidopsis Mutants with Enhanced Disease Susceptibility by Direct Screening. Genetics. 1996, 143: 973-982.PubMedPubMed CentralGoogle Scholar
- Nawrath C, Métraux JP: Salicylic Acid Induction-Deficient Mutants of Arabidopsis Express PR-2 and PR-5 and Accumulate High Levels of Camalexin after Pathogen Inoculation. Plant Cell. 1999, 11: 1393-1404. 10.1105/tpc.11.8.1393.PubMedPubMed CentralGoogle Scholar
- Jakobek JL, Lindgren PB: Generalized Induction of Defense Responses in Bean is Not Correlated with the Induction of the Hypersensitive Reaction. Plant Cell. 1993, 5: 49-56. 10.1105/tpc.5.1.49.PubMedPubMed CentralView ArticleGoogle Scholar
- Asai T, Tena G, Plotnikova J, Willmann MR, Chiu W-L, Gomez-Gomez L, Boller T, Ausubel FM, Sheen J: MAP Kinase Signaling Cascade in Arabidopsis Innate Immunity. Nature. 2002, 415: 977-983. 10.1038/415977a.PubMedView ArticleGoogle Scholar
- Chen Z, Kloek AP, Boch J, Katagiri F, Kunkel BN: The Pseudomonas syringae avrRpt2 Gene Product Promotes Pathogen Virulence from Inside Plant Cells. Mol Plant-Microbe Interact. 2000, 13: 1312-1321.PubMedView ArticleGoogle Scholar
- Genoud T, Trevino Santa Cruz MB, Métraux J-P: Numeric Simulation of Plant Signaling Networks. Plant Physiol. 2001, 126: 1430-1437. 10.1104/pp.126.4.1430.PubMedPubMed CentralView ArticleGoogle Scholar
- Ritter C, Dangl JL: Interference between Two Specific Pathogen Recognition Events Mediated by Distinct Plant Disease Resistance Genes. Plant Cell. 1996, 8: 251-257. 10.1105/tpc.8.2.251.PubMedPubMed CentralView ArticleGoogle Scholar
- Grant M, Brown I, Adams S, Knight M, Ainslie A, Mansfield J: The RPM1 Plant Disease Resistance Gene Facilitates a Rapid and Sustained Increase in Cytosolic Calcium that is Necessary for the Oxidative Burst and Hypersensitive Cell Death. Plant J. 2000, 23: 441-450. 10.1046/j.1365-313x.2000.00804.x.PubMedView ArticleGoogle Scholar
- Reuber TL, Ausubel FM: Isolation of Arabidopsis Genes that Differentiate Between Resistance Responses Mediated by the RPS2 and RPM1 Disease Resistance Genes. Plant Cell. 1996, 8: 241-249. 10.1105/tpc.8.2.241.PubMedPubMed CentralView ArticleGoogle Scholar
- Asthagiri AR, Lauffenburger DA: Bioengineering Models of Cell Signaling. Annu Rev Biomed Eng. 2000, 2: 31-53. 10.1146/annurev.bioeng.2.1.31.PubMedView ArticleGoogle Scholar
- Chen KC, Csikasz-Nagy A, Gyorffy B, Val J, Novak B, Tyson JJ: Kinetic Analysis of a Molecular Model of the Budding Yeast Cell Cycle. Mol Biol Cell. 2000, 11: 369-391.PubMedPubMed CentralView ArticleGoogle Scholar
- Kitano H: Systems Biology: A Brief Overview. Science. 2002, 295: 1662-1664. 10.1126/science.1069492.PubMedView ArticleGoogle Scholar
- Whalen M, Innes R, Bent A, Staskawicz B: Identification of Pseudomonas syringae Pathogens of Arabidopsis thaliana and a Bacterial Gene Determining Avirulence on both Arabidopsis and Soybean. Plant Cell. 1991, 3: 49-59. 10.1105/tpc.3.1.49.PubMedPubMed CentralView ArticleGoogle Scholar
- Konieczny A, Ausubel FM: A Procedure for Mapping Arabidopsis Mutations Using Co-Dominant Ecotype-Specific PCR-Based Markers. Plant J. 1993, 4: 403-410. 10.1046/j.1365-313X.1993.04020403.x.PubMedView ArticleGoogle Scholar
- Tornero P, Dangl JL: A High-Throughput Method for Quantifying Growth of Phytopathogenic Bacteria in Arabidopsis thaliana. Plant J. 2001, 28: 475-481. 10.1046/j.1365-313X.2001.01136.x.PubMedView ArticleGoogle Scholar
- Chou IT, Gasser CS: Characterization of the Cyclophilin Gene Family of Arabidopsis thaliana and Phylogenetic Analysis of Known Cyclophilin Proteins. Plant Mol Biol. 1997, 35: 873-892. 10.1023/A:1005930024796.PubMedView ArticleGoogle Scholar
- Sambrook J, Russell DW: Molecular Cloning: A Laboratory Manual,. Cold Spring Harbor. NY: Cold Spring Harbor Laboratory Press, Third 2001.Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.