Identification of genes involved in rhizobacteria-mediated induced systemic resistance in Arabidopsis

Different forms of biologically induced disease resistance have been identified in plants. Following attack by a necrotizing pathogen systemic acquired resistance (SAR) is induced, leading to a broad-spectrum disease resistance that is associated with an increase in salicylic acid (SA) levels and the accumulation of pathogenesis-related (PR) proteins. Selected strains of non-pathogenic, rootcolonizing fluorescent Pseudomonas spp. can induce systemic resistance as well, without provoking any symptoms themselves. This rhizobacteria-mediated induced systemic resistance (ISR) is phenotypically similar to pathogen-induced SAR in that it is effective against various pathogens. In Arabidopsis thaliana, the ISR signaling pathway triggered by Pseudomonas fluorescens WCS417r requires responsiveness to both jasmonate (JA) and ethylene (ET) and is independent of SA and the accumulation of PR proteins. The state of pathogen-induced SAR is characterised by the concomitant activation of a set of PR genes. Of many defence-related genes tested in Arabidopsis, none were upregulated prior to challenge in plants expressing WCS417r-mediated ISR. In an attempt to isolate ISRrelated genes, we screened a large collection of Arabidopsis lines containing enhancer-trap Ds transposons and the β-glucuronidase (GUS) reporter gene with minimal promoter. One enhancer-trap line showed local GUS activity in the roots upon colonization with WCS417r. This local GUS expression was not observed after treatment of the roots with Escherichia coli, indicating that the induction was Pseudomonas specific. Interestingly, a similar expression pattern was observed after treatment of the roots with the ET precursor ACC, indicating that this line contains a transposon insertion in the vicinity of an ET-inducible gene that is up-regulated upon colonization with WCS417r. There are several candidate genes in the vicinity of the enhancer-trap Ds transposon, one of which encodes a thaumatin-like protein. Gene expression analyses confirmed that this thaumatin-like gene, designated THL1, is up-regulated in response to treatment of the roots with WCS417r or ACC. Analysis of the role of THLI in ISR might provide more insight into the molecular mechanisms involved in rhizobacteria-mediated ISR

Regulation of Arabidopsis thaliana Em genes : role of AB15

In order to identify new factors involved in Em (a class I Late Embryogenesis Abundant protein) gene expression, Arabidopsis mutants with an altered expression of an Em promoter GUS fusion construct and a modified accumulation of Em transcripts and proteins were isolated. Germination tests on ABA showed that the most affected mutant had a weak abi phenotype. Complementation tests further revealed this mutant to be a new abi5 allele, consequently named abi5-5. In addition to reducing the final level of Em transcripts in the dry seed, the abi5-5 mutation causes a delay in the accumulation of AtEm1 during seed development. An additional characteristic of the abi5-5 mutant, is the ability of its seeds to germinate at high concentrations of salt and mannitol. The abi5-5 mutation was characterized at the molecular level and was shown to result from a two base pair deletion in the coding sequence of the ABI 5 gene. The wild type and mutant recombinant proteins were produced in E. coli and were assayed for DNA-binding activity on their target promoters by electrophoretic mobility shift assay (EMSA). The ABI5 recombinant protein binds the ABRE sequence in the AtEm6 promoter as shown by Dnase footprinting. Among the ABRE-type sequences selected on both Em promoters, the G-box type AGACACGTGGCATGT element of the AtEm6 promoter shows the strongest binding by EMSA quantificatio

Colonization of the Arabidopsis rhizosphere by fluorescent Pseudomonas spp. activates a root-specific, ethylene-responsive PR-5 gene in the vascular bundle

Plants of which the roots are colonized by selected strains of non-pathogenic, fluorescent Pseudomonas spp. develop an enhanced defensive capacity against a broad spectrum of foliar pathogens. In Arabidopsis thaliana, this rhizobacteria-induced systemic resistance (ISR) functions independently of salicylic acid but requires responsiveness to jasmonic acid and ethylene. In contrast to pathogen-induced systemic acquired resistance (SAR), ISR is not associated with systemic changes in the expression of genes encoding pathogenesis-related (PR) proteins. To identify genes that are specifically expressed in response to colonization of the roots by ISR-inducing Pseudomonas fluorescens WCS417r bacteria, we screened a collection of Arabidopsis enhancer trap and gene trap lines containing a transposable element of the Ac/Ds system and the GUS reporter gene. We identified an enhancer trap line (WET121) that specifically showed GUS activity in the root vascular bundle upon colonization of the roots by WCS417r. Fluorescent Pseudomonas spp. strains P. fluorescens WCS374r and P. putida WCS358r triggered a similar expression pattern, whereas ISR-non-inducing Escherichia coli bacteria did not. Exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) mimicked the rhizobacteria-induced GUS expression pattern in the root vascular bundle, whereas methyl jasmonic acid and salicylic acid did not, indicating that the Ds element in WET121 is inserted in the vicinity of an ethylene-responsive gene. Analysis of the expression of the genes in the close vicinity of the Ds element revealed AtTLP1 as the gene responsible for the in cis activation of the GUS reporter gene in the root vascular bundle. AtTLP1 encodes a thaumatin-like protein that belongs to the PR-5 family of PR proteins, some of which possess antimicrobial properties. AtTLP1 knockout mutant plants showed normal levels of WCS417r-mediated ISR against the bacterial leaf pathogen Pseudomonas syringae pv. tomato DC3000, suggesting that expression of AtTLP1 in the roots is not required for systemic expression of ISR in the leaves. Together, these results indicate that induction of AtTLP1 is a local response of Arabidopsis roots to colonization by non-pathogenic fluorescent Pseudomonas spp. and is unlikely to play a role in systemic resistance

Induced systemic resistance by plant growth-promoting rhizobacteria

Rhizobacteria are present in large numbers on the root surface, where plant exudates and lysates provide nutrients. Selected strains of beneficial, plant growth-promoting rhizobacteria (PGPR) trigger a plant-mediated induced systemic resistance (ISR) response that is effective against a broad spectrum of plant pathogens. To study the molecular basis of ISR, an Arabidopsis thaliana-based model was developed, using PGPR strain Pseudomonas fluorescens WCS417r as the inducing agent. Genetic dissection of the ISR signalling pathway revealed that ISR is regulated by a defence pathway in which the phytohormones jasmonic acid and ethylene play key roles. Interestingly, the state of ISR is not associated with major changes in gene expression. Instead, ISR-expressing plants are primed to activate specific sets of defense-related genes faster or to a higher level upon pathogen attack. Here we review the current state of knowledge of the signal transduction steps involved in the ISR pathway in Arabidopsis that leads from recognition of the rhizobacteria by the roots to systemic expression of broad-spectrum disease resistance in above-ground foliar tissues

Rhizobacteria-mediated induced systemic resistance (ISR) in Arabidopsis: involvement of jasmonate and ethylene

The capacity of a plant to express a broad-spectrum systemic acquired resistance (SAR) after primary infection is well known and extensively studied. A relatively unknown form of induced disease resistance is triggered by nonpathogenic, root-colonizing rhizobacteria and is commonly referred to as rhizobacteria-mediated induced systemic resistance (ISR). Rhizosphere bacteria are present in large numbers on the root. Certain strains stimulate plant growth and are therefore called plant growth-promoting rhizobacteria (PGPR). Selected strains with biological control activity, mainly fluorescent Pseudomonas spp., reduce plant diseases by suppressing soil-borne pathogens through competition for nutrients, siderophore-mediated competition for iron or antibiosis

Colonization of the Arabidopsis rhizosphere by fluorescent Pseudomonas spp. activates a root-specific, ethylene-responsive PR-5 gene in the vascular bundle

Plants of which the roots are colonized by selected strains of non-pathogenic, fluorescent Pseudomonas spp. develop an enhanced defensive capacity against a broad spectrum of foliar pathogens. In Arabidopsis thaliana, this rhizobacteria-induced systemic resistance (ISR) functions independently of salicylic acid but requires responsiveness to jasmonic acid and ethylene. In contrast to pathogen-induced systemic acquired resistance (SAR), ISR is not associated with systemic changes in the expression of genes encoding pathogenesis-related (PR) proteins. To identify genes that are specifically expressed in response to colonization of the roots by ISR-inducing Pseudomonas fluorescens WCS417r bacteria, we screened a collection of Arabidopsis enhancer trap and gene trap lines containing a transposable element of the Ac/Ds system and the GUS reporter gene. We identified an enhancer trap line (WET121) that specifically showed GUS activity in the root vascular bundle upon colonization of the roots by WCS417r. Fluorescent Pseudomonas spp. strains P. fluorescens WCS374r and P. putida WCS358r triggered a similar expression pattern, whereas ISR-non-inducing Escherichia coli bacteria did not. Exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) mimicked the rhizobacteria-induced GUS expression pattern in the root vascular bundle, whereas methyl jasmonic acid and salicylic acid did not, indicating that the Ds element in WET121 is inserted in the vicinity of an ethylene-responsive gene. Analysis of the expression of the genes in the close vicinity of the Ds element revealed AtTLP1 as the gene responsible for the in cis activation of the GUS reporter gene in the root vascular bundle. AtTLP1 encodes a thaumatin-like protein that belongs to the PR-5 family of PR proteins, some of which possess antimicrobial properties. AtTLP1 knockout mutant plants showed normal levels of WCS417r-mediated ISR against the bacterial leaf pathogen Pseudomonas syringae pv. tomato DC3000, suggesting that expression of AtTLP1 in the roots is not required for systemic expression of ISR in the leaves. Together, these results indicate that induction of AtTLP1 is a local response of Arabidopsis roots to colonization by non-pathogenic fluorescent Pseudomonas spp. and is unlikely to play a role in systemic resistance