The cpr5 mutant of Arabidopsis expresses both NPR1-dependent and NPR1-independent resistance.

The cpr5 mutant was identified from a screen for constitutive expression of systemic acquired resistance (SAR). This single recessive mutation also leads to spontaneous expression of chlorotic lesions and reduced trichome development. The cpr5 plants were found to be constitutively resistant to two virulent pathogens, Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2; to have endogenous expression of the pathogenesis-related gene 1 (PR-1); and to have an elevated level of salicylic acid (SA). Lines homozygous for cpr5 and either the SA-degrading bacterial gene nahG or the SA-insensitive mutation npr1 do not express PR-1 or exhibit resistance to P. s. maculicola ES4326. Therefore, we conclude that cpr5 acts upstream of SA in inducing SAR. However, the cpr5 npr1 plants retained heightened resistance to P. parasitica Noco2 and elevated expression of the defensin gene PDF1.2, implying that NPR1-independent resistance signaling also occurs. We conclude that the cpr5 mutation leads to constitutive expression of both an NPR1-dependent and an NPR1-independent SAR pathway. Identification of this mutation indicates that these pathways are connected in early signal transduction steps and that they have overlapping functions in providing resistance.     

Constitutive salicylic acid-dependent signaling in cpr1 and cpr6 mutants requires PAD4

Salicylic acid (SA)-dependent signaling controls activation of a set of plant defense mechanisms that are important for resistance to a variety of microbial pathogens. Many Arabidopsis mutants that display altered SA-dependent signaling have been isolated. We used double mutant analysis to determine the relative positions of the pad4, cpr1, cpr5, cpr6, dnd1 and dnd2 mutations in the signal transduction network leading to SA-dependent activation of defense gene expression and disease resistance. The pad4 mutation causes failure of SA accumulation in response to infection by certain pathogens, while the other mutations cause constitutively high levels of SA, defense gene expression and resistance. The cpr1 pad4, cpr5 pad4, cpr6 pad4, dnd1 pad4 and dnd2 pad4 double mutants were constructed and assayed for stature, presence of spontaneous lesions, resistance to Pseudomonas syringae and Peronospora parasitica, SA levels, expression of PAD4, PR-1 and PDF1.2, and accumulation of camalexin. We found that the effects of the cpr1 and cpr6 mutations on SA-dependent gene expression are completely dependent on PAD4 function. In contrast, SA accumulation in the lesion-mimic mutant cpr5 is partially PAD4-independent, while in dnd1 and dnd2 mutants it is completely PAD4-independent. A model describing a possible arrangement of activities in the signal transduction network is presented.     

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

Disease resistance in Arabidopsis is regulated by multiple signal transduction pathways in which salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) function as key signaling molecules. Epistasis analyses were performed between mutants that disrupt these pathways (npr1, eds5, ein2, and jar1) and mutants that constitutively activate these pathways (cpr1, cpr5, and cpr6), allowing exploration of the relationship between the SA- and JA/ET-mediated resistance responses. Two important findings were made. First, the constitutive disease resistance exhibited by cpr1, cpr5, and cpr6 is completely suppressed by the SA-deficient eds5 mutant but is only partially affected by the SA-insensitive npr1 mutant. Moreover, eds5 suppresses the SA-accumulating phenotype of the cpr mutants, whereas npr1 enhances it. These data indicate the existence of an SA-mediated, NPR1-independent resistance response. Second, the ET-insensitive mutation ein2 and the JA-insensitive mutation jar1 suppress the NPR1-independent resistance response exhibited by cpr5 and cpr6. Furthermore, ein2 potentiates SA accumulation in cpr5 and cpr5 npr1 while dampening SA accumulation in cpr6 and cpr6 npr1. These latter results indicate that cpr5 and cpr6 regulate resistance through distinct pathways and that SA-mediated, NPR1-independent resistance works in combination with components of the JA/ET-mediated response pathways.     

Uncoupling PR gene expression from NPR1 and bacterial resistance: characterization of the dominant Arabidopsis cpr6-1 mutant.

In Arabidopsis, NPR1 mediates the salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance (SAR). Here, we report the identification of another component, CPR 6, that may function with NPR1 in regulating PR gene expression. The dominant CPR 6-1 mutant expresses the SA/NPR1-regulated PR genes (PR-1, BGL 2, and PR-5) and displays enhanced resistance to Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2 in the absence of SAR induction. cpr 6-1-induced PR gene expression is not suppressed in the cpr 6-1 npr1-1 double mutant but is suppressed when SA is removed by salicylate hydroxylase. Thus, constitutive PR gene expression in cpr 6-1 requires SA but not NPR1. In addition, resistance to P. s. maculicola ES4326 is suppressed in the cpr 6-1 npr1-1 double mutant, despite expression of PR-1, BGL 2, and PR-5. Resistance to P. s. maculicola ES4326 must therefore be accomplished through unidentified antibacterial gene products that are regulated through NPR1. These results show that CPR 6 is an important regulator of multiple signal transduction pathways involved in plant defense.     

EDS1 in tomato is required for resistance mediated by TIR-class R genes and the receptor-like R gene Ve

In tobacco and other Solanaceae species, the tobacco N gene confers resistance to tobacco mosaic virus (TMV), and leads to induction of standard defense and resistance responses. Here, we report the use of N-transgenic tomato to identify a fast-neutron mutant, sun1-1 (suppressor of N), that is defective in N-mediated resistance. Induction of salicylic acid (SA) and expression of pathogenesis-related (PR) genes, each signatures of systemic acquired resistance, are both dramatically suppressed in sun1-1 plants after TMV treatment compared to wild-type plants. Application of exogenous SA restores PR gene expression, indicating that SUN1 acts upstream of SA. Upon challenge with additional pathogens, we found that the sun1-1 mutation impairs resistance mediated by certain resistance (R) genes, (Bs4, I, and Ve), but not others (Mi-1). In addition, sun1-1 plants exhibit enhanced susceptibility to TMV, as well as to virulent pathogens. sun1-1 has been identified as an EDS1 homolog present on chromosome 6 of tomato. The discovery of enhanced susceptibility in the sun1-1 (Le_eds1-1) mutant plant, which contrasts to reports in Nicotiana benthamiana using virus-induced gene silencing, provides evidence that the intersection of R gene-mediated pathways with general resistance pathways is conserved in a Solanaceous species. In tomato, EDS1 is important for mediating resistance to a broad range of pathogens (viral, bacterial, and fungal pathogens), yet shows specificity in the class of R genes that it affects (TIR-NBS-LRR as opposed to CC-NBS-LRR). In addition, a requirement for EDS1 for Ve-mediated resistance in tomato exposes that the receptor-like R gene class may also require EDS1.     

Environmentally sensitive, SA-dependent defense responses in the cpr22 mutant of Arabidopsis

To investigate the signaling pathways through which defense responses are activated following pathogen infection, we have isolated and characterized the cpr22 mutant. This plant carries a semidominant, conditional lethal mutation that confers constitutive expression of the pathogenesis-related (PR) genes PR-1, PR-2, PR-5 and the defensin gene PDF1.2. cpr22 plants also display spontaneous lesion formation, elevated levels of salicylic acid (SA) and heightened resistance to Peronospora parasitica Emco5. The cpr22 locus was mapped to chromosome 2, approximately 2 cM telomeric to the AthB102 marker. By analyzing the progeny of crosses between cpr22 plants and either NahG transgenic plants or npr1 mutants, all of the cpr22-associated phenotypes except PDF1.2 expression were found to be SA dependent. However, the SA signal transducer NPR1 was required only for constitutive PR-1 expression. A cross between cpr22 and ndr1-1 mutants revealed that enhanced resistance to P. parasitica is mediated by an NDR1-dependent pathway, while the other cpr22-induced defenses are not. Crosses between either coi1-1 or etr1-1 mutants further demonstrated that constitutive PDF1.2 expression is mediated by a JA- and ethylene-dependent pathway. Based on these results, the cpr22 mutation appears to induce its associated phenotypes by activating NPR1-dependent and NPR1-independent branches of the SA pathway, as well as an ethylene/JA signaling pathway. Interestingly, the SA-dependent phenotypes, but not the SA-independent phenotypes, are suppressed when cpr22 mutants are grown under high humidity.     

The role of EDS1 (enhanced disease susceptibility) during singlet oxygen-mediated stress responses of Arabidopsis

Upon a dark/light shift the conditional flu mutant of Arabidopsis starts to generate singlet oxygen (1O2) that is restricted to the plastid compartment. Distinct sets of genes are activated that are different from those induced by hydrogen peroxide/superoxide. One of the genes that is rapidly upregulated is EDS1 (enhanced disease susceptibility). The EDS1 protein has been shown to be required for the resistance to biotrophic pathogens and the accumulation of salicylic acid (SA) that enhances the defenses of a plant by inducing the synthesis of pathogen-related (PR) proteins. Because of the similarity of its N-terminal portion to the catalytic site of lipases, EDS1 has also been implicated with the release of polyunsaturated fatty acids and the subsequent formation of various oxylipins. The release of singlet oxygen in the flu mutant triggers a drastic increase in the concentration of free SA and activates the expression of PR1 and PR5 genes. These changes depend on the activity of EDS1 and are suppressed in flu/eds1 double mutants. Soon after the beginning of singlet oxygen production, the synthesis of oxylipins such as jasmonic acid (JA) and 12-oxophytodienoic acid (OPDA) also start and plants stop growing and induce a cell-death response. The inactivation of EDS1 does not affect oxylipin synthesis, growth inhibition and the initiation of cell death, but it does allow plants to recover much faster from singlet oxygen-mediated growth inhibition and it also suppresses the spread of necrotic lesions in leaves. Hence, singlet oxygen activates a complex stress-response program with EDS1 playing a key role in initiating and modulating several steps of it. This program includes not only responses to oxidative stress, but also responses known to be activated during plant-pathogen interactions and wounding.     

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

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

Drought tolerance established by enhanced expression of the CC-NBS-LRR gene, ADR1, requires salicylic acid, EDS1 and ABI1

An activation-tagged allele of activated disease resistance 1 (ADR1) has previously been shown to convey broad spectrum disease resistance. ADR1 was found to encode a coiled-coil (CC)-nucleotide-binding site (NBS)-leucine-rich repeat (LRR) protein, which possessed domains of homology with serine/threonine protein kinases. Here, we show that either constitutive or conditional enhanced expression of ADR1 conferred significant drought tolerance. This was not a general feature of defence-related mutants because cir (constitutive induced resistance)1, cir2 and cpr (constitutive expressor of PR genes)1, which constitutively express systemic acquired resistance (SAR), failed to exhibit this phenotype. Cross-tolerance was not a characteristic of adr1 plants, rather they showed increased sensitivity to thermal and salinity stress. Hence, adr1-activated signalling may antagonise some stress responses. Northern analysis of abiotic marker genes revealed that dehydration-responsive element (DRE)B2A but not DREB1A, RD (response to dehydration)29A or RD22 was expressed in adr1 plant lines. Furthermore, DREB2A expression was salicylic acid (SA) dependent but NPR (non-expressor of PR genes)1 independent. In adr1/ADR1 nahG (naphthalene hydroxylase G), adr1/ADR1 eds (enhanced disease susceptibility)1 and adr1/ADR1 abi1 double mutants, drought tolerance was significantly reduced. Microarray analyses of plants containing a conditional adr1 allele demonstrated that a significant number of the upregulated genes had been previously implicated in responses to dehydration. Therefore, biotic and abiotic signalling pathways may share multiple nodes and their outputs may have significant functional overlap.     

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.     

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

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

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.     

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

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

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

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

Components of Arabidopsis defense- and ethylene-signaling pathways regulate susceptibility to Cauliflower mosaic virus by restricting long-distance movement

We analyzed the susceptibility of Arabidopsis mutants with defects in salicylic acid (SA) and jasmonic acid (JA)/ethylene (ET) signaling to infection by Cauliflower mosaic virus (CaMV). Mutants cpr1-1 and cpr5-2, in which SA-dependent defense signaling is activated constitutively, were substantially more resistant than the wild type to systemic infection, implicating SA signaling in defense against CaMV. However, SA-deficient NahG, sid2-2, eds5-1, and pad4-1 did not show enhanced susceptibility. A cpr5 eds5 double mutant also was resistant, suggesting that resistance in cpr5 may function partially independently of SA. Treatment of cpr5 and cpr5 eds5, but not cpr1, with salicyl-hydroxamic acid, an inhibitor of alternative oxidase, partially restored susceptibility to wild-type levels. Mutants etr1-1, etr1-3, and ein2-1, and two mutants with lesions in ET/JA-mediated defense, eds4 and eds8, also showed reduced virus susceptibility, demonstrating that ET-dependent responses also play a role in susceptibility. We used a green fluorescent protein (GFP)-expressing CaMV recombinant to monitor virus movement. In mutants with reduced susceptibility, cpr1-1, cpr5-2, and etr1-1, CaMV-GFP formed local lesions similar to the wild type, but systemic spread was almost completely absent in cpr1 and cpr5 and was substantially reduced in etr1-1. Thus, mutations with enhanced systemic acquired resistance or compromised ET signaling show diminished long-distance virus movement.     

Characterization of a new Arabidopsis mutant exhibiting enhanced disease resistance

In many plant-pathogen interactions, resistance is associated with the synthesis and accumulation of salicylic acid (SA) and pathogenesis-related (PR) proteins. At least two general classes of mutants with altered resistance to pathogen attack have been identified in Arabidopsis. One class exhibits increased susceptibility to pathogen infection; the other class exhibits enhanced resistance to pathogens. In an attempt to identify mutations in resistance-associated loci, we screened a population of T-DNA tagged Arabidopsis thaliana ecotype Wassilewskija (Ws) for mutants showing constitutive expression of the PR-1 gene (cep). A mutant was isolated and shown to constitutively express PR-1, PR-2, and PR-5 genes. This constitutive phenotype segregated as a single recessive trait in the Ws genetic background. The mutant also had elevated levels of SA, which are responsible for the cep phenotype. The cep mutant spontaneously formed hypersensitive response (HR)-like lesions on the leaves and cotyledons and also exhibited enhanced resistance to virulent bacterial and fungal pathogens. Genetic analyses of segregating progeny from outcrosses to other ecotypes unexpectedly revealed that alterations in more than one gene condition the constitutive expression of PR genes in the original mutant. One of the mutations, designated cpr20, maps to the lower arm of chromosome 4 and is required for the cep phenotype. Another mutation, which has been termed cpr21, maps to chromosome 1 and is often, but not always, associated with this phenotype. The recessive nature of the cep trait suggests that the CPR20 and CPR21 proteins may act as negative regulators in the disease resistance signal transduction pathway.     

Arabidopsis ssi2-conferred susceptibility to Botrytis cinerea is dependent on EDS5 and PAD4

Loss of a stearoyl-ACP desaturase activity in the Arabidopsis thaliana ssi2 mutant confers susceptibility to the necrotroph, Botrytis cinerea. In contrast, the ssi2 mutant exhibits enhanced resistance to Pseudomonas syringae, Peronospora parasitica, and Cucumber mosaic virus. The altered basal resistance to these pathogens in the ssi2 mutant plant is accompanied by the constitutive accumulation of elevated salicylic acid (SA) level and expression of the pathogenesis-related 1 (PR1) gene, the inability of jasmonic acid (JA) to activate expression of the defensin gene, PDF1.2, and the spontaneous death of cells. Here, we show that presence of the eds5 and pad4 mutant alleles compromises the ssi2-conferred resistance to Pseudomonas syringae pv. maculicola. In contrast, resistance to B. cinerea was restored in the ssi2 eds5 and ssi2 pad4 double-mutant plants. However, resistance to B. cinerea was not accompanied by the restoration of JA responsiveness in the ssi2 eds5 and ssi2 pad4 plants. The ssi2 eds5 and ssi2 pad4 plants retain the ssi2-conferred spontaneous cell death phenotype, suggesting that cell death is not a major factor that predisposes the ssi2 mutant to infection by B. cinerea. Furthermore, the high SA content of the ssi2 pad4 plant, combined with our previous observation that the SA-deficient ssi2 nahG plant succumbs to infection by B. cinerea, suggests that elevated SA level does not have a causal role in the ssi2-conferred susceptibility to B. cinerea. Our results suggest that interaction between an SSI2-dependent factor or factors and an EDS5- and PAD4-dependent mechanism or mechanisms modulates defense to B. cinerea.     

The F-box protein CPR1/CPR30 negatively regulates R protein SNC1 accumulation

Disease resistance (R) proteins, as central regulators of plant immunity, are tightly regulated for effective defense responses and to prevent constitutive defense activation under non-pathogenic conditions. Here we report the identification of an F-box protein CPR1/CPR30 as a negative regulator of an R protein SNC1 likely through SCF (Skp1-cullin-F-box) mediated protein degradation. The cpr1-2 (cpr30-1) loss-of-function mutant has constitutive defense responses, and it interacts synergistically with a gain-of function mutant snc1-1 and a bon1-1 mutant where SNC1 is upregulated. The loss of SNC1 function suppresses the mutant phenotypes of cpr1-2 and cpr1-2 bon1-1, while overexpression of CPR1 rescues mutant phenotypes of both bon1-1 and snc1-1. Furthermore, the amount of SNC1 protein is upregulated in the cpr1-2 mutant and down-regulated when CPR1 is overexpressed. The regulation of SNC1 by CPR1 is dependent on the 26S proteosome as a protease inhibitor MG132 stabilizes SNC1 and reverses the effect of CPR1 on SNC1. Interestingly, CPR1 is induced after infection of both virulent and avirulent pathogens similarly to the other negative defense regulator BON1. Thus, this study reveals a new mechanism in R protein regulation likely through protein degradation and suggests negative regulation as a critical component in fine control of plant immunity.     

Fitness costs of mutations affecting the systemic acquired resistance pathway in Arabidopsis thaliana

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.