[Show abstract][Hide abstract] ABSTRACT: Crops lack genetic resistance to most necrotrophic pathogens. To compensate for this disadvantage, plants recruit antagonistic members of the soil microbiome to defend their roots against pathogens and other pests. The best examples of this microbially based defense of roots are observed in disease-suppressive soils in which suppressiveness is induced by continuously growing crops that are susceptible to a pathogen, but the molecular basis of most is poorly understood. Here we report the microbial characterization of a Korean soil with specific suppressiveness to Fusarium wilt of strawberry. In this soil, an attack on strawberry roots by Fusarium oxysporum results in a response by microbial defenders, of which members of the Actinobacteria appear to have a key role. We also identify Streptomyces genes responsible for the ribosomal synthesis of a novel heat-stable antifungal thiopeptide antibiotic inhibitory to F. oxysporum and the antibiotic's mode of action against fungal cell wall biosynthesis. Both classical- and community-oriented approaches were required to dissect this suppressive soil from the field to the molecular level, and the results highlight the role of natural antibiotics as weapons in the microbial warfare in the rhizosphere that is integral to plant health, vigor and development.The ISME Journal advance online publication, 9 June 2015; doi:10.1038/ismej.2015.95.
The ISME Journal 06/2015; DOI:10.1038/ismej.2015.95 · 9.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas fluorescens strain 2-79, a natural isolate of the rhizosphere of wheat (Triticum aestivum L.), possesses antagonistic potential toward several fungal pathogens. We report the draft genome sequence of strain 2-79, which comprises 5,674 protein-coding sequences.
[Show abstract][Hide abstract] ABSTRACT: This publication emphasises that:
• Certain micro-organisms are currently used (or have the potential to be used) in a variety of products and applications such as biofertilisers, plant protection products, biofuel production, bioremediation, cleaners, detergents as well as in the control of disease transmission.
• To date, there have been few uses of transgenic micro-organisms in such products and applications, though that might change in the future.
• In order that products involving transgenic micro-organisms are used in a responsible way, it is important that regulatory oversight involves a rigorous risk / safety assessment.
This publication includes the proceedings of the OECD conference on the “Environmental Uses of Micro Organisms: An Overview of the State-of-the-Art and Implications for Biotechnology Risk/Safety Assessment”. It was organised under the auspices of OECD’s Working Group on the Harmonisation of Regulatory Oversight in Biotechnology (WG-HROB), in collaboration with the OECD Co-operative Research Programme under the Trade and Agriculture Directorate.
Edited by Peter William Edwards Kearns; Bertrand Dagallier; Kazuyuki Suwabe;Takahiko Nikaido;, 02/2015; OECD Publishing, Paris., ISBN: ISBN 978-92-64-20495-9 (print); ISBN 978-92-64-21356-2 (PDF)
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to inventory the types of plant growth-promoting rhizobacteria (PGPR) present in the rhizosphere of plants grown in soils contaminated with heavy metals, recalcitrant organics, petroleum sewage or salinity in China. We screened 1223 isolates for antifungal activity and about 24% inhibited Rhizoctonia solani or Sclerotinia sclerotiorum. Twenty-four strains inhibitory to R. solani, Gaeumannomyces graminis var. tritici and/or S. sclerotiorum and representing the dominant morphotypes were assayed for PGPR activity. Seven strains contained phlD, prnD, pltC or phzF genes and produced the antibiotics 2,4-diacetylphloroglucinol, pyrrolnitrin, pyoluteorin and phenazines respectively. Six strains contained acdS, which encodes 1-aminocyclopropane-1-carboxylic acid deaminase. Phylogenetic analysis of 16S rDNA and phlD, phzF and acdS genes demonstrated that some strains identified as Pseudomonas were similar to model PGPR strains Pseudomonas protegens Pf-5, Pseudomonas chlororaphis subsp. aureofaciens 30–84 and P. brassicacearum Q8r1-96. Pseudomonas protegens- and P. chlororaphis-like strains had the greatest biocontrol activity against Rhizoctonia root rot and take-all of wheat. Pseudomonas protegens and P. brassicacearum-like strains showed the greatest promotion of canola growth. Our results indicate that strains from contaminated soils are similar to well-described PGPR found in agricultural soils worldwide.
Growth-promoting rhizobacteria in polluted soils
[Show abstract][Hide abstract] ABSTRACT: Beneficial microbes in the microbiome of plant roots improve plant health. Induced systemic resistance (ISR) emerged as an important mechanism by which selected plant growth-promoting bacteria and fungi in the rhizosphere prime the whole plant body for enhanced defense against a broad range of pathogens and insect herbivores. A wide variety of root-associated mutualists, including Pseudomonas, Bacillus, Trichoderma, and mycorrhiza species sensitize the plant immune system for enhanced defense without directly activating costly defenses. This review focuses on molecular processes at the interface between plant roots and ISR-eliciting mutualists, and on the progress in our understanding of ISR signaling and systemic defense priming. The central role of the root-specific transcription factor MYB72 in the onset of ISR and the role of phytohormones and defense regulatory proteins in the expression of ISR in aboveground plant parts are highlighted. Finally, the ecological function of ISR-inducing microbes in the root microbiome is discussed. Expected final online publication date for the Annual Review of Phytopathology Volume 52 is August 04, 2014. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT Pseudomonas fluorescens HC1-07, previously isolated from the phyllosphere of wheat grown in Hebei province, China, suppresses the soilborne disease of wheat take-all, caused by Gaeumannomyces graminis var. tritici. We report here that strain HC1-07 also suppresses Rhizoctonia root rot of wheat caused by Rhizoctonia solani AG-8. Strain HC1-07 produced a cyclic lipopeptide (CLP) with a molecular weight of 1,126.42 based on analysis by electrospray ionization mass spectrometry. Extracted CLP inhibited the growth of G. graminis var. tritici and R. solani in vitro. To determine the role of this CLP in biological control, plasposon mutagenesis was used to generate two nonproducing mutants, HC1-07viscB and HC1-07prtR2. Analysis of regions flanking plasposon insertions in HC1-07prtR2 and HC1-07viscB revealed that the inactivated genes were similar to prtR and viscB, respectively, of the well-described biocontrol strain P. fluorescens SBW25 that produces the CLP viscosin. Both genes in HC1-07 were required for the production of the viscosin-like CLP. The two mutants were less inhibitory to G. graminis var. tritici and R. solani in vitro and reduced in ability to suppress take-all. HC1-07viscB but not HC-07prtR2 was reduced in ability to suppress Rhizoctonia root rot. In addition to CLP production, prtR also played a role in protease production.
[Show abstract][Hide abstract] ABSTRACT: In agro-ecosystems worldwide, some of the most important and devastating diseases are caused by soil-borne necrotrophic fungal pathogens, against which crop plants generally lack genetic resistance. However, plants have evolved approaches to protect themselves against pathogens by stimulating and supporting specific groups of beneficial microorganisms that have the ability to protect either by direct inhibition of the pathogen or by inducing resistance mechanisms in the plant. One of the best examples of protection of plant roots by antagonistic microbes occurs in soils that are suppressive to take-all disease of wheat. Take-all, caused by Gaeumannomyces graminis var. tritici, is the most economically important root disease of wheat worldwide. Take-all decline (TAD) is the spontaneous decline in incidence and severity of disease after a severe outbreak of take-all during continuous wheat or barley monoculture. TAD occurs worldwide, and in the United States and The Netherlands it results from a build-up of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent Pseudomonas spp. during wheat monoculture. The antibiotic 2,4-DAPG has a broad spectrum of activity and is especially active against the take-all pathogen. Based on genotype analysis by repetitive sequence-based-PCR analysis and restriction fragment length polymorphism of phlD, a key 2,4-DAPG biosynthesis gene, at least 22 genotypes of 2,4-DAPG producing fluorescent Pseudomonas spp. have been described worldwide. In this review, we provide an overview of G. graminis var. tritici, the take-all disease, Pseudomonas biocontrol agents, and mechanism of disease suppression.
[Show abstract][Hide abstract] ABSTRACT: We investigated the taxonomic placement of phenazine-producing fluorescent Pseudomonas spp. in the Inland Pacific Northwest, USA. Five distinct species were identified, two of which were provisionally considered to be new. Agroclimatic zone and soil silt content affected the species diversity across the region.
[Show abstract][Hide abstract] ABSTRACT: Background
Yeast deletion-mutant collections have been successfully used to infer the mode-of-action of drugs especially by profiling chemical-genetic and genetic-genetic interactions on a genome-wide scale. Although tens of thousands of those profiles are publicly available, a lack of an accurate method for mining such data has been a major bottleneck for more widespread use of these useful resources.
For general usage of those public resources, we designed FitRankDB as a general repository of fitness profiles, and developed a new search algorithm, FitSearch, for identifying the profiles that have a high similarity score with statistical significance for a given fitness profile. We demonstrated that our new repository and algorithm are highly beneficial to researchers who attempting to make hypotheses based on unknown modes-of-action of bioactive compounds, regardless of the types of experiments that have been performed using yeast deletion-mutant collection in various types of different measurement platforms, especially non-chip-based platforms.
We showed that our new database and algorithm are useful when attempting to construct a hypothesis regarding the unknown function of a bioactive compound through small-scale experiments with a yeast deletion collection in a platform independent manner. The FitRankDB and FitSearch enhance the ease of searching public yeast fitness profiles and obtaining insights into unknown mechanisms of action of drugs. FitSearch is freely available at http://fitsearch.kaist.ac.kr.
[Show abstract][Hide abstract] ABSTRACT: Take-all, caused by Gaeumannomyces graminis var. tritici, is an important soilborne disease of wheat worldwide. Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents of take-all and provide natural suppression of the disease during wheat monoculture known as take-all decline. To identify factors that could contribute to the effectiveness of 2,4-DAPG producers in take-all suppression, P. fluorescens strains Q8r1-96 (genotype D) and Q2-87V1 (genotype B; reduced antibiotic production) were tested against three pathogen isolates differing in sensitivity to 2,4-DAPG (LD5, ARS-A1 and R3-111a-1) and two wheat cultivars (Tara and Buchanan). The antibiotic sensitivity of the pathogen and cultivar significantly affected the level of take-all suppression by Q8r1-96 and Q2-87V1; suppression was greatest with LD5 and Tara. Q8r1-96 suppressed ARS-A1 and R3-111a-1 on Tara but not Buchanan, and Q2-87V1 failed to suppress either pathogen isolate on either cultivar. Q8r1-96 colonized the rhizosphere of Tara and Buchanan grown in soil similarly, but 2,4-DAPG accumulation was higher on the roots of Buchanan than Tara. 2,4-DAPG at 7.5 μg mL−1 reduced the growth of roots of both cultivars, and 10 μg mL−1 caused brown necrosis and tissue collapse of seedling roots and reduced root hair development. The half-life of 2,4-DAPG in the rhizosphere was estimated to be 0.25 days. These results suggest that several interconnected factors including sensitivity of G. graminis var. tritici to 2,4-DAPG, wheat cultivar, fluctuations in populations of 2,4-DAPG producers, and antibiotics accumulation in the rhizosphere will impact the robustness of take-all suppression by P. fluorescens in the field.
[Show abstract][Hide abstract] ABSTRACT: Phenazine compounds represent a large class of bacterial metabolites that are produced by some fluorescent Pseudomonas spp. and a few other bacterial genera. Phenazines were first noted in the scientific literature over 100 years ago, but for a long time were considered to be pigments of uncertain function. Following evidence that phenazines act as virulence factors in the opportunistic human and animal pathogen Pseudomonas aeruginosa and are actively involved in the suppression of plant pathogens, interest in these compounds has broadened to include investigations of their genetics, biosynthesis, activity as electron shuttles, and contribution to the ecology and physiology of the cells that produce them. This minireview highlights some recent and exciting insights into the diversity, frequency and ecological roles of phenazines produced by fluorescent Pseudomonas spp.
[Show abstract][Hide abstract] ABSTRACT: We provide here a comparative genome analysis of ten strains within the Pseudomonas fluorescens group including seven new genomic sequences. These strains exhibit a diverse spectrum of traits involved in biological control and other multitrophic interactions with plants, microbes, and insects. Multilocus sequence analysis placed the strains in three sub-clades, which was reinforced by high levels of synteny, size of core genomes, and relatedness of orthologous genes between strains within a sub-clade. The heterogeneity of the P. fluorescens group was reflected in the large size of its pan-genome, which makes up approximately 54% of the pan-genome of the genus as a whole, and a core genome representing only 45-52% of the genome of any individual strain. We discovered genes for traits that were not known previously in the strains, including genes for the biosynthesis of the siderophores achromobactin and pseudomonine and the antibiotic 2-hexyl-5-propyl-alkylresorcinol; novel bacteriocins; type II, III, and VI secretion systems; and insect toxins. Certain gene clusters, such as those for two type III secretion systems, are present only in specific sub-clades, suggesting vertical inheritance. Almost all of the genes associated with multitrophic interactions map to genomic regions present in only a subset of the strains or unique to a specific strain. To explore the evolutionary origin of these genes, we mapped their distributions relative to the locations of mobile genetic elements and repetitive extragenic palindromic (REP) elements in each genome. The mobile genetic elements and many strain-specific genes fall into regions devoid of REP elements (i.e., REP deserts) and regions displaying atypical tri-nucleotide composition, possibly indicating relatively recent acquisition of these loci. Collectively, the results of this study highlight the enormous heterogeneity of the P. fluorescens group and the importance of the variable genome in tailoring individual strains to their specific lifestyles and functional repertoire.
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas fluorescens 2112, isolated in Korea as an indigenous antagonistic bacteria, can produce 2,4- diacetylphloroglucinol (2,4-DAPG) and the siderophore pyoveridin2112 for the control of phytophthora blight of red-pepper. P. fluorescens 2112 was classified into a new genotype C among the 17 genotypes of 2,4-DAPG producers, by phlD restriction fragment length polymorphism (RFLP). The colonizing ability of P. fluorescens 2112 in pea rhizosphere was equal to the well-known pea colonizers, P. fluorescens Q8r1 (genotype D) and MVP1-4 (genotype P), after 6 cycling cultivations for 18 weeks. Four tested 2,4- DAPG-producing Pseudomonas spp. could colonize with about a 96% dominance ratio against total bacteria in pea rhizosphere. The strain P. fluorescens 2112 was as good a colonizer as other Pseudomonas spp. genotypes in pea plant growth-promoting rhizobacteria.
Journal of Microbiology and Biotechnology 06/2012; 22(6):763-70. · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas fluorescens strains that produce the polyketide antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are among the most effective rhizobacteria that suppress root and crown rots, wilts, and damping-off diseases of a variety of crops, and they play a key role in the natural suppressiveness of some soils to certain soilborne pathogens. Root colonization by 2,4-DAPG-producing P. fluorescens strains Pf-5 (genotype A), Q2-87 (genotype B), Q8r1-96 (genotype D), and HT5-1 (genotype N) produced induced systemic resistance (ISR) in Arabidopsis thaliana accession Col-0 against bacterial speck caused by P. syringae pv. tomato. The ISR-eliciting activity of the four bacterial genotypes was similar, and all genotypes were equivalent in activity to the well-characterized strain P. fluorescens WCS417r. The 2,4-DAPG biosynthetic locus consists of the genes phlHGF and phlACBDE. phlD or phlBC mutants of Q2-87 (2,4-DAPG minus) were significantly reduced in ISR activity, and genetic complementation of the mutants restored ISR activity back to wild-type levels. A phlF regulatory mutant (overproducer of 2,4-DAPG) had ISR activity equivalent to the wild-type Q2-87. Introduction of DAPG into soil at concentrations of 10 to 250 μM 4 days before challenge inoculation induced resistance equivalent to or better than the bacteria. Strain Q2-87 induced resistance on transgenic NahG plants but not on npr1-1, jar1, and etr1 Arabidopsis mutants. These results indicate that the antibiotic 2,4-DAPG is a major determinant of ISR in 2,4-DAPG-producing P. fluorescens, that the genotype of the strain does not affect its ISR activity, and that the activity induced by these bacteria operates through the ethylene- and jasmonic acid-dependent signal transduction pathway.
[Show abstract][Hide abstract] ABSTRACT: Certain strains of the rhizosphere bacterium Pseudomonas fluorescens contain the phenazine biosynthesis operon (phzABCDEFG) and produce redox-active phenazine antibiotics that suppress a wide variety of soilborne plant pathogens. In 2007 and 2008, we isolated 412 phenazine-producing (Phz(+)) fluorescent Pseudomonas strains from roots of dryland wheat and barley grown in the low-precipitation region (<350 mm annual precipitation) of central Washington State. Based on results of BOX-PCR genomic fingerprinting analysis, these isolates, as well as the model biocontrol Phz(+) strain P. fluorescens 2-79, were assigned to 31 distinct genotypes separated into four clusters. All of the isolates exhibited high 16S rDNA sequence similarity to members of the P. fluorescens species complex including Pseudomonas orientalis, Pseudomonas gessardii, Pseudomonas libanensis, and Pseudomonas synxantha. Further recA-based sequence analyses revealed that the majority of new Phz(+) isolates (386 of 413) form a clade distinctly separated from P. fluorescens 2-79. Analysis of phzF alleles, however, revealed that the majority of those isolates (280 of 386) carried phenazine biosynthesis genes similar to those of P. fluorescens 2-79. phzF-based analyses also revealed that phenazine genes were under purifying selection and showed evidence of intracluster recombination. Phenotypic analyses using Biolog substrate utilization and observations of phenazine-1-carboxylic acid production showed considerable variability amongst members of all four clusters. Biodiversity indices indicated significant differences in diversity and evenness between the sampled sites. In summary, this study revealed a genotypically and phenotypically diverse group of phenazine producers with a population structure not seen before in indigenous rhizosphere-inhabiting Phz(+) Pseudomonas spp.
[Show abstract][Hide abstract] ABSTRACT: This work determined the impact of irrigation on the seasonal dynamics of populations of Pseudomonas spp. producing the antibiotics phenazine-1-carboxylic acid (Phz+) and 2,4-diacetylphloroglucinol (Phl+) in the rhizosphere of wheat grown in the low-precipitation zone (150 to 300 mm annually) of the Columbia Plateau of the
Inland Pacific Northwest. Population sizes and plant colonization frequencies of Phz+ and Phl+ Pseudomonas spp. were determined in winter and spring wheat collected during the growing seasons from 2008 to 2009 from selected commercial
dryland and irrigated fields in central Washington State. Only Phz+ bacteria were detected on dryland winter wheat, with populations ranging from 4.8 to 6.3 log CFU g−1 of root and rhizosphere colonization frequencies of 67 to 100%. The ranges of population densities of Phl+ and Phz+ Pseudomonas spp. recovered from wheat grown under irrigation were similar, but 58 to 100% of root systems were colonized by Phl+ bacteria whereas only 8 to 50% of plants harbored Phz+ bacteria. In addition, Phz+ Pseudomonas spp. were abundant in the rhizosphere of native plant species growing in nonirrigated areas adjacent to the sampled dryland
wheat fields. This is the first report that documents the impact of irrigation on indigenous populations of two closely related
groups of antibiotic-producing pseudomonads that coinhabit the rhizosphere of an economically important cereal crop. These
results demonstrate how crop management practices can influence indigenous populations of antibiotic-producing pseudomonads
with the capacity to suppress soilborne diseases of wheat.
[Show abstract][Hide abstract] ABSTRACT: Fluorescent Pseudomonas isolated from the rhizosphere of diverse plants have been studied as biocontrol agents of soilborne pathogens worldwide.
Certain strains of these bacteria are capable of exerting a variety of mechanisms of plant growth promotion and protection,
including the production of the natural antibiotics 2,4-diacetylphloroglucinol (2,4-DAPG) and derivatives of phenazine (Phz).
These compounds are broadly active against plant pathogens and are produced by widely distributed taxonomically diverse Pseudomonas spp. that inhabit the rhizosphere of cereal crops and render certain soils naturally suppressive to soilborne plant diseases.
There is mounting evidence that the role of 2,4-DAPG and phenazines in the rhizosphere is not limited to antibiosis but also
involves regulatory and signaling functions, induction of systemic resistance, and reduction of minerals in soil.
Bacteria in Agrobiology: Plant Nutrient Management, Edited by DK Maheshwari, 01/2012: pages 267-283; Springer.