A gain-of-function mutation in a plant disease resistance gene leads to constitutive activation of downstream signal transduction pathways in suppressor of npr1-1, constitutive 1

Autoactive alleles of the flax L6 rust resistance gene induce non-race-specific rust resistance associated with the hypersensitive response

L6 is a nucleotide binding site-leucine rich repeat (NBS-LRR) gene that confers race-specific resistance in flax (Linum usitatissimum) to strains of flax rust (Melampsora lini) that carry avirulence alleles of the AvrL567 gene but not to rust strains that carry only the virulence allele. Several mutant and recombinant forms of L6 were made that altered either the methionine-histidine-aspartate (MHD) motif conserved in the NBS domain of resistance proteins or exchanged the short domain C-terminal to the LRR region that is highly variable among L allele products. In transgenic flax some of these alleles are autoactive; they cause a gene dosage-dependent dwarf phenotype and constitutive expression of genes that are markers for the plant defense response. Their effects and penetrance ranged from extreme to mild in their degree of plant stunting, survival, and reproduction. Dwarf plants were also resistant to flax rust strains virulent to wild-type L6 plants, and this nonspecific resistance was associated with a hypersensitive response (HR) at the site of rust infection. The strongest autoactive allele, expressed in Arabidopsis from an ethanol-inducible promoter, gave rise to plant death dependent on the enhanced disease susceptibility 1 (EDS1) gene, which indicates that the mutant flax (Linaceae) L6 gene can signal cell death through a defined disease-resistance pathway in a different plant family (Brassicaceae).

Salicylic acid (SA)-dependent gene activation can be uncoupled from cell death-mediated gene activation: the SA-inducible NIMIN-1 and NIMIN-2 promoters, unlike the PR-1a promoter, do not respond to cell death signals in tobacco

SUMMARY Tobacco pathogenesis-related (PR) genes of group 1 are induced during pathogen defence (hypersensitive response, HR, and systemic acquired resistance, SAR), after exogenous application of salicylic acid (SA), and by developmental cues. Likewise, SA enhances transcripts for Arabidopsis NIMIN-1 and NIMIN-2, which interact with NPR1/NIM1, a key regulator of SAR. To further illuminate gene activation during pathogen defence, reporter gene expression from the NIMIN-1 and NIMIN-2 promoters was analysed in transgenic tobacco plants in direct comparison to PR-1 gene expression. NIMIN[GUS] chimeric genes were highly sensitive to SA, whereas NIMIN[GUS], unlike PR1a[GUS], expression was only weak in necrotic tissue exhibiting HR. Furthermore, PR-1a, but not NIMIN, promoter constructs were activated systemically in response to local cell death elicited by expression of the proapoptotic Bax gene. Conversely, NIMIN-1[GUS] expression was completely suppressed during pathogen defence in plants depleted from SA, whereas PR-1 proteins still accumulated in necrotic tissue. These findings demonstrate that SA-dependent gene activation can be uncoupled from cell death-induced gene activation. Whereas PR-1a induction during the HR and SAR responses is mediated by HR-associated signals and SA, activation of the NIMIN-1 and NIMIN-2 promoters in infected tobacco relies on SA, but not on cell death signals.

The atypical resistance gene, RPW8, recruits components of basal defence for powdery mildew resistance in Arabidopsis

Genetic studies have identified a number of components of signal transduction pathways leading to plant disease resistance and the accompanying hypersensitive response (HR) following detection of pathogens by plant resistance (R) genes. In Arabidopsis, the majority of R proteins so far characterized belong to a plant superfamily that have a central nucleotide-binding site and C-terminal leucine-rich-repeats (NB-LRRs). Another much less prevalent class comprises RPW8.1 and RPW8.2, two related proteins that possess a putative N-terminal transmembrane domain and a coiled-coil motif, and confer broad-spectrum resistance to powdery mildew. Here we investigated whether RPW8.1 and RPW8.2 engage known pathway(s) for defence signalling. We show that RPW8.1 and RPW8.2 recruit, in addition to salicylic acid and EDS1, the other NB-LRR gene-signalling components PAD4, EDS5, NPR1 and SGT1b for activation of powdery mildew resistance and HR. In contrast, NDR1, RAR1 and PBS3 that are required for function of certain NB-LRR R genes, and COI1 and EIN2 that operate, respectively, in the jasmonic acid and ethylene signalling pathways, do not contribute to RPW8.1 and RPW8.2-mediated resistance. We further demonstrate that EDR1, a gene encoding a conserved MAPKK kinase, exerts negative regulation on HR cell death and powdery mildew resistance by limiting the transcriptional amplification of RPW8.1 and RPW8.2. Our results suggest that RPW8.1 and RPW8.2 stimulate a conserved basal defence pathway that is negatively regulated by EDR1.

A putative nucleoporin 96 Is required for both basal defense and constitutive resistance responses mediated by suppressor of npr1-1,constitutive 1

The Arabidopsis thaliana suppressor of npr1-1, constitutive 1 (snc1) mutant contains a gain-of-function mutation in a Toll Interleukin1 receptor-nucleotide binding-Leu-rich repeat-type resistance gene (R-gene), which leads to constitutive activation of disease resistance response against pathogens. In a screen for suppressors of snc1, a recessive mutation, designated mos3 (for modifier of snc1,3), was found to suppress the constitutive pathogenesis-related gene expression and resistance to virulent Pseudomonas syringae maculicola ES4326 and Peronospora parasitica Noco2 in snc1. In addition, mos3 is also compromised in resistance mediated by Resistance to Peronospora parasitica 4 (RPP4), Resistance to Pseudomonas syringae pv maculicola (RPM1), and Resistance to Pseudomonas syringae 4 (RPS4). Single mutant mos3 plants exhibited enhanced disease susceptibility to P. s. pv maculicola ES4326, suggesting that MOS3 is required for basal resistance to pathogens as well. mos3-1 was identified by map-based cloning, and it encodes a protein with high sequence similarity to human nucleoporin 96. Localization of the MOS3-green fluorescent protein fusion to the nuclear envelope further indicates that MOS3 may encode a nucleoporin, suggesting that nuclear and cytoplasmic trafficking plays an important role in both R-gene-mediated and basal disease resistance.

Arabidopsis RIN4 negatively regulates disease resistance mediated by RPS2 and RPM1 downstream or independent of the NDR1 signal modulator and is not required for the virulence functions of bacterial type III effectors AvrRpt2 or AvrRpm1

Bacterial pathogens deliver type III effector proteins into the plant cell during infection. On susceptible (r) hosts, type III effectors can contribute to virulence. Some trigger the action of specific disease resistance (R) gene products. The activation of R proteins can occur indirectly via modification of a host target. Thus, at least some type III effectors are recognized at site(s) where they may act as virulence factors. These data indicate that a type III effector's host target might be required for both initiation of R function in resistant plants and pathogen virulence in susceptible plants. In Arabidopsis thaliana, RPM1-interacting protein 4 (RIN4) associates with both the Resistance to Pseudomonas syringae pv maculicola 1 (RPM1) and Resistance to P. syringae 2 (RPS2) disease resistance proteins. RIN4 is posttranslationally modified after delivery of the P. syringae type III effectors AvrRpm1, AvrB, or AvrRpt2 to plant cells. Thus, RIN4 may be a target for virulence functions of these type III effectors. We demonstrate that RIN4 is not the only host target for AvrRpm1 and AvrRpt2 in susceptible plants because its elimination does not diminish their virulence functions. In fact, RIN4 negatively regulates AvrRpt2 virulence function. RIN4 also negatively regulates inappropriate activation of both RPM1 and RPS2. Inappropriate activation of RPS2 is nonspecific disease resistance 1 (NDR1) independent, in contrast with the established requirement for NDR1 during AvrRpt2-dependent RPS2 activation. Thus, RIN4 acts either cooperatively, downstream, or independently of NDR1 to negatively regulate RPS2 in the absence of pathogen. We propose that many P. syringae type III effectors have more than one target in the host cell. We suggest that a limited set of these targets, perhaps only one, are associated with R proteins. Thus, whereas any pathogen virulence factor may have multiple targets, the perturbation of only one is necessary and sufficient for R activation.

NPR1, all things considered

Recent work has shown that the Arabidopsis NPR1 protein not only plays an essential role in salicylic acid (SA)-mediated systemic acquired resistance and rhizobacterium-triggered induced systemic resistance, but also is involved in crosstalk inhibition of jasmonic acid (JA)-mediated defense responses. Molecular characterization has revealed that activation of NPR1 and certain TGA transcription factors occurs under the reducing conditions that follow an initial oxidative burst after the induction of defense responses. In addition to NPR1 and TGA, the single-stranded DNA-binding transcription factor AtWhy1 and the WRKY70 transcription factor were recently found to be involved in SA-mediated defense and SA-JA crosstalk, respectively.

Plant disease resistance protein signaling: NBS-LRR proteins and their partners

Most plant disease resistance (R) proteins contain a series of leucine-rich repeats (LRRs), a nucleotide-binding site (NBS), and a putative amino-terminal signaling domain. They are termed NBS-LRR proteins. The LRRs of a wide variety of proteins from many organisms serve as protein interaction platforms, and as regulatory modules of protein activation. Genetically, the LRRs of plant R proteins are determinants of response specificity, and their action can lead to plant cell death in the form of the familiar hypersensitive response (HR). A total of 149 R genes are potentially expressed in the Arabidopsis genome, and plant cells must deal with the difficult task of assembling many of the proteins encoded by these genes into functional signaling complexes. Eukaryotic cells utilize several strategies to deal with this problem. First, proteins are spatially restricted to their sub-cellular site of function, thus improving the probability that they will interact with their proper partners. Second, these interactions are architecturally organized to avoid inappropriate signaling events and to maintain the fidelity and efficiency of the response when it is initiated. Recent results provide new insights into how the signaling potential of R proteins might be created, managed and held in check until specific stimulation following infection. Nevertheless, the roles of the R protein partners in these regulatory events that have been defined to date are unclear.

COS1: an Arabidopsis coronatine insensitive1 suppressor essential for regulation of jasmonate-mediated plant defense and senescence

The Arabidopsis thaliana CORONATINE INSENSITIVE1 (COI1) gene encodes an F-box protein to assemble SCF(COI1) complexes essential for response to jasmonates (JAs), which are a family of plant signaling molecules required for many essential functions, including plant defense and reproduction. To better understand the molecular basis of JA action, we screened for suppressors of coi1 and isolated a coi1 suppressor1 (cos1) mutant. The cos1 mutation restores the coi1-related phenotypes, including defects in JA sensitivity, senescence, and plant defense responses. The COS1 gene was cloned through a map-based approach and found to encode lumazine synthase, a key component in the riboflavin pathway that is essential for diverse yet critical cellular processes. We demonstrated a novel function for the riboflavin pathway that acts downstream of COI1 in the JA signaling pathway and is required for suppression of the COI1-mediated root growth, senescence, and plant defense.

A haplotype-specific Resistance gene regulated by BONZAI1 mediates temperature-dependent growth control in Arabidopsis

Plant growth homeostasis and defense responses are regulated by BONZAI1 (BON1), an evolutionarily conserved gene. Here, we show that growth regulation by BON1 is mediated through defense responses. BON1 is a negative regulator of a haplotype-specific Resistance (R) gene SNC1. The bon1-1 loss-of-function mutation activates SNC1, leading to constitutive defense responses and, consequently, reduced cell growth. In addition, a feedback amplification of the SNC1 gene involving salicylic acid is subject to temperature control, accounting for the regulation of growth and defense by temperature in bon1-1 and many other mutants. Thus, plant growth homeostasis involves the regulation of an R gene by BON1 and the intricate interplay between defense responses and temperature responses.

Regulatory regions and critical residues of NOD2 involved in muramyl dipeptide recognition

Multiple genetic variants of CARD15/NOD2 have been associated with susceptibility to Crohn's disease and Blau syndrome. NOD2 recognizes muramyl dipeptide (MDP) derived from bacterial peptidoglycan (PGN), but the molecular basis of recognition remains elusive. We performed systematic mutational analysis to gain insights into the function of NOD2 and molecular mechanisms of disease susceptibility. Using an archive of 519 mutations covering approximately 50% of the amino-acid residues of NOD2, the essential regulatory domains and specific residues of NOD2 involved in recognition of MDP were identified. The analysis revealed distinct roles for N-terminal and C-terminal leucine-rich repeats (LRRs) in the modulation of NOD2 activation and bacterial recognition. Within the C-terminal LRRs, variable residues predicted to form the beta-strand/betaturn structure were found to be essential for the response to MDP. In addition, we analyzed NOD1, a NOD2-related protein, revealing conserved and nonconserved amino-acid residues involved in PGN recognition. These results provide new insights into the molecular function and regulation of NOD2 and related NOD family proteins.

Systemic acquired resistance

Systemic acquired resistance (SAR) is a mechanism of induced defense that confers long-lasting protection against a broad spectrum of microorganisms. SAR requires the signal molecule salicylic acid (SA) and is associated with accumulation of pathogenesis-related proteins, which are thought to contribute to resistance. Much progress has been made recently in elucidating the mechanism of SAR. Using the model plant Arabidopsis, it was discovered that the isochorismate pathway is the major source of SA during SAR. In response to SA, the positive regulator protein NPR1 moves to the nucleus where it interacts with TGA transcription factors to induce defense gene expression, thus activating SAR. Exciting new data suggest that the mobile signal for SAR might be a lipid molecule. We discuss the molecular and genetic data that have contributed to our understanding of SAR and present a model describing the sequence of events leading from initial infection to the induction of defense genes.

The chequered history of the development and use of simultaneous equations for the accurate determination of chlorophylls a and b

Over the last half century, the most frequently used assay for chlorophylls in higher plants and green algae, the Arnon assay [Arnon DI (1949) Plant Physiol 24: 1-15], employed simultaneous equations for determining the concentrations of chlorophylls a and b in aqueous 80% acetone extracts of chlorophyllous plant and algal materials. These equations, however, were developed using extinction coefficients for chlorophylls a and b derived from early inaccurate spectrophotometric data. Thus, Arnon's equations give inaccurate chlorophyll a and b determinations and, therefore, inaccurate chlorophyll a/b ratios, which are always low. This paper describes how the ratios are increasingly and alarmingly low as the proportion of chlorophyll a increases. Accurate extinction coefficients for chlorophylls a and b, and the more reliable simultaneous equations derived from them, have been published subsequently by many research groups; these new post-Arnon equations, however, have been ignored by many researchers. This Minireview records the history of the development of accurate simultaneous equations and some difficulties and anomalies arising from the retention of Arnon's seriously flawed equations.