Publications by authors named "Olga Mavrodi"

30 Publications

  • Page 1 of 1

Draft Genome Sequences of Six Strains Isolated from the Rhizosphere of Wheat Grown in Cadmium-Contaminated Soil.

Microbiol Resour Announc 2020 Aug 20;9(34). Epub 2020 Aug 20.

School of Biological, Environmental, and Earth Sciences, University of Southern Mississippi, Hattiesburg, Mississippi, USA

This study presents high-quality draft genome assemblies of six bacterial strains isolated from the roots of wheat grown in soil contaminated with cadmium. The results of this study will help to elucidate at the molecular level how heavy metals affect interactions between beneficial rhizobacteria and crop plants.
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http://dx.doi.org/10.1128/MRA.00676-20DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7441233PMC
August 2020

Draft Genome Sequences of Xylella fastidiosa subsp. Strains OK3, VB11, and NOB1, Isolated from Bunch and Muscadine Grapes Grown in Southern Mississippi.

Microbiol Resour Announc 2020 Jun 18;9(25). Epub 2020 Jun 18.

USDA-ARS Southern Horticultural Research Laboratory, Poplarville, Mississippi, USA

We report here high-quality draft whole-genome assemblies of subsp. strains OK3, VB11, and NOB1, which were isolated from symptomatic bunch and muscadine grape plants grown in southern Mississippi.
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http://dx.doi.org/10.1128/MRA.00562-20DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7303414PMC
June 2020

Comparative Analysis of Rhizosphere Microbiomes of Southern Highbush Blueberry ( L.), Darrow's Blueberry ( Camp), and Rabbiteye Blueberry ( Aiton).

Front Microbiol 2020 12;11:370. Epub 2020 Mar 12.

Department of Cell and Molecular Biology, School of Biological, Environmental, and Earth Sciences, The University of Southern Mississippi, Hattiesburg, MS, United States.

Plants are inhabited by millions of parasitic, commensal, and mutualistic microorganisms that coexist in complex ecological communities, and profoundly affect the plant's productivity, health, and capacity to cope with environmental stress. Therefore, a better understanding of the rhizosphere microbiome may open a yet untapped avenue for the rational exploitation of beneficial plant-microbe interactions in modern agriculture. Blueberries encompass several wild and cultivated species of shrubs of the genus that are native to North America. They are grown commercially for the production of fruits, which are considered a health food due to the rich content of minerals, trace elements, and phenolic compounds with antioxidant, antitumor, and anti-inflammatory properties. Despite a long history of breeding and extensive commercial use, remarkably little is known about the composition and function of the blueberry root microbiome. To address this gap, we employed molecular approaches to characterize and compare microbial communities inhabiting the roots of rabbiteye blueberry (), Darrow's blueberry (), and southern highbush blueberry (SHB; an interspecific hybrid of and ). Our results revealed that these plant species share a common core rhizobiome, but at the same time differ significantly in the diversity, relative abundance, richness, and evenness of multiple groups of prokaryotic and eukaryotic microorganisms. Although the host signature effects were especially pronounced at the plant species level, we also observed genotype-level variations in the distribution of specific microbial taxa, which suggests that the assembly of the blueberry microbiome is shaped by the plant genotype and modifications associated with the domestication and breeding of members of the genus. We also demonstrated that the studied species differ in the abundance of beneficial rhizobacteria and ericoid mycorrhizal fungi, which play a vital role in their adaptation to soils with low pH and slow turnover of organic matter.
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http://dx.doi.org/10.3389/fmicb.2020.00370DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7081068PMC
March 2020

2-79 Transformed with Pyrrolnitrin Biosynthesis Genes Has Improved Biocontrol Activity Against Soilborne Pathogens of Wheat and Canola.

Phytopathology 2020 May 24;110(5):1010-1017. Epub 2020 Mar 24.

U.S. Department of Agriculture-Agriculture Research Service, Wheat Health, Genetics and Quality Research Unit, Pullman, WA 99164-6430, U.S.A.

A four-gene operon () from Pf-5 encoding the biosynthesis of the antibiotic pyrronitrin was introduced into (formerly ) 2-79, an aggressive root colonizer of both dryland and irrigated wheat roots that naturally produces the antibiotic phenazine-1-carboxylic acid and suppresses both take-all and Rhizoctonia root rot of wheat. Recombinant strains ZHW15 and ZHW25 produced both antibiotics and maintained population sizes in the rhizosphere of wheat that were comparable to those of strain 2-79. The recombinant strains inhibited in vitro the wheat pathogens anastomosis group 8 (AG-8) and AG-2-1, var. , , , and significantly more than did strain 2-79. Both the wild-type and recombinant strains were equally inhibitory of . When applied as a seed treatment, the recombinant strains suppressed take-all, Rhizoctonia root rot of wheat, and Rhizoctonia root and stem rot of canola significantly better than did wild-type strain 2-79.
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http://dx.doi.org/10.1094/PHYTO-09-19-0367-RDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7238759PMC
May 2020

Exploring the Pathogenicity of Q8r1-96 and Other Strains of the Complex on Tomato.

Plant Dis 2020 Apr 29;104(4):1026-1031. Epub 2020 Jan 29.

U.S. Department of Agriculture, Agricultural Research Service, Wheat Health, Genetics and Quality Research Unit, Pullman, WA 99164-6430, U.S.A.

and related species of the complex have long been studied as biocontrol and growth-promoting rhizobacteria involved in suppression of soilborne pathogens. We report here that Q8r1-96 and other 2,4-diacetylphloroglucinol (DAPG)-producing fluorescent pseudomonads involved in take-all decline of wheat in the Pacific Northwest of the United States can also be pathogenic to other plant hosts. Strain Q8r1-96 caused necrosis when injected into tomato stems and immature tomato fruits, either attached or removed from the plant, but lesion development was dose dependent, with a minimum of 10 CFU ml required to cause visible tissue damage. We explored the relative contribution of several known plant-microbe interaction traits to the pathogenicity of strain Q8r1-96. Type III secretion system (T3SS) mutants of Q8r1-96, injected at a concentration of 10 CFU ml, were significantly less virulent, but not consistently, as compared with the wild-type strain. However, a DAPG-deficient mutant of Q8r1-96 was significantly and consistently less virulent as compared with the wild type. Strain Q8r1-96acc, engineered to over express ACC deaminase, caused a similar amount of necrosis as the wild type. Cell-free culture filtrates of strain Q8r1-96 and pure DAPG also cause necrosis in tomato fruits. Our results suggest that DAPG plays a significant role in the ability of Q8r1-96 to cause necrosis of tomato tissue, but other factors also contribute to the pathogenic properties of this organism.
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http://dx.doi.org/10.1094/PDIS-09-19-1989-REDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7163159PMC
April 2020

Antimicrobial Activity of, and Cellular Pathways Targeted by, -Anisaldehyde and Epigallocatechin Gallate in the Opportunistic Human Pathogen Pseudomonas aeruginosa.

Appl Environ Microbiol 2020 02 3;86(4). Epub 2020 Feb 3.

School of Biological, Environmental, and Earth Sciences, The University of Southern Mississippi, Hattiesburg, Mississippi, USA

Plant-derived aldehydes are constituents of essential oils that possess broad-spectrum antimicrobial activity and kill microorganisms without promoting resistance. In our previous study, we incorporated -anisaldehyde from star anise into a polymer network called proantimicrobial networks via degradable acetals (PANDAs) and used it as a novel drug delivery platform. PANDAs released -anisaldehyde upon a change in pH and humidity and controlled the growth of the multidrug-resistant pathogen PAO1. In this study, we identified the cellular pathways targeted by -anisaldehyde by generating 10,000 transposon mutants of PAO1 and screened them for hypersensitivity to -anisaldehyde. To improve the antimicrobial efficacy of -anisaldehyde, we combined it with epigallocatechin gallate (EGCG), a polyphenol from green tea, and demonstrated that it acts synergistically with -anisaldehyde in killing We then used transcriptome sequencing to profile the responses of to -anisaldehyde, EGCG, and their combination. The exposure to -anisaldehyde altered the expression of genes involved in modification of the cell envelope, membrane transport, drug efflux, energy metabolism, molybdenum cofactor biosynthesis, and the stress response. We also demonstrate that the addition of EGCG reversed many -anisaldehyde-coping effects and induced oxidative stress. Our results provide insight into the antimicrobial activity of -anisaldehyde and its interactions with EGCG and may aid in the rational identification of new synergistically acting combinations of plant metabolites. Our study also confirms the utility of the thiol-ene polymer platform for the sustained and effective delivery of hydrophobic and volatile antimicrobial compounds. Essential oils (EOs) are plant-derived products that have long been exploited for their antimicrobial activities in medicine, agriculture, and food preservation. EOs represent a promising alternative to conventional antibiotics due to their broad-range antimicrobial activity, low toxicity to human commensal bacteria, and capacity to kill microorganisms without promoting resistance. Despite the progress in the understanding of the biological activity of EOs, our understanding of many aspects of their mode of action remains inconclusive. The overarching aim of this work was to address these gaps by studying the molecular interactions between an antimicrobial plant aldehyde and the opportunistic human pathogen The results of this study identify the microbial genes and associated pathways involved in the response to antimicrobial phytoaldehydes and provide insights into the molecular mechanisms governing the synergistic effects of individual constituents within essential oils.
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http://dx.doi.org/10.1128/AEM.02482-19DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6997733PMC
February 2020

Rhizosphere Microbial Communities of and From Restored and Natural Tidal Marshes on Deer Island, Mississippi.

Front Microbiol 2018 11;9:3049. Epub 2018 Dec 11.

Department of Biological Sciences, The University of Southern Mississippi, Hattiesburg, MS, United States.

The U. S. Gulf of Mexico is experiencing a dramatic increase in tidal marsh restoration actions, which involves planting coastal areas with smooth cordgrass () and black needlerush () for erosion control and to provide habitat for fish and wildlife. It can take decades for sedimentary cycles in restored marshes to approach reference conditions, and the contribution of the sediment microbial communities to these processes is poorly elucidated. In this study, we addressed this gap by comparing rhizosphere microbiomes of and from two restored marshes and a natural reference marsh located at Deer Island, MS. Our results revealed that plants from the restored and reference areas supported similar microbial diversity indicating the rapid colonization of planted grasses with indigenous soil microbiota. Although close in composition, the microbial communities from the three studied sites differed significantly in the relative abundance of specific taxa. The observed differences are likely driven by the host plant identity and properties of sediment material used for the creation of restored marshes. Some of the differentially distributed groups of bacteria include taxa involved in the cycling of carbon, nitrogen, and sulfur, and may influence the succession of vegetation at the restored sites to climax condition. We also demonstrated that plants from the restored and reference sites vary in the frequency of culturable rhizobacteria that exhibit traits commonly associated with the promotion of plant growth and suppression of phytopathogenic fungi. Our findings will contribute to the establishment of benchmarks for the assessment of the outcome of coastal restoration projects in the Gulf of Mexico and better define factors that affect the long-term resiliency of tidal marshes and their vulnerability to climate change.
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http://dx.doi.org/10.3389/fmicb.2018.03049DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6297177PMC
December 2018

Phenazine-1-carboxylic acid and soil moisture influence biofilm development and turnover of rhizobacterial biomass on wheat root surfaces.

Environ Microbiol 2018 06 26;20(6):2178-2194. Epub 2018 Jul 26.

United States Department of Agriculture - Agricultural Research Service, Wheat Health, Genetics, and Quality Research Unit, Pullman, WA 99164-6430, USA.

Phenazine-1-carboxylic acid (PCA) is produced by rhizobacteria in dryland but not in irrigated wheat fields of the Pacific Northwest, USA. PCA promotes biofilm development in bacterial cultures and bacterial colonization of wheat rhizospheres. However, its impact upon biofilm development has not been demonstrated in the rhizosphere, where biofilms influence terrestrial carbon and nitrogen cycles with ramifications for crop and soil health. Furthermore, the relationships between soil moisture and the rates of PCA biosynthesis and degradation have not been established. In this study, expression of PCA biosynthesis genes was upregulated relative to background transcription, and persistence of PCA was slightly decreased in dryland relative to irrigated wheat rhizospheres. Biofilms in dryland rhizospheres inoculated with the PCA-producing (PCA ) strain Pseudomonas synxantha 2-79RN were more robust than those in rhizospheres inoculated with an isogenic PCA-deficient (PCA ) mutant strain. This trend was reversed in irrigated rhizospheres. In dryland PCA rhizospheres, the turnover of N-labelled rhizobacterial biomass was slower than in the PCA and irrigated PCA treatments, and incorporation of bacterial N into root cell walls was observed in multiple treatments. These results indicate that PCA promotes biofilm development in dryland rhizospheres, and likely influences crop nutrition and soil health in dryland wheat fields.
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http://dx.doi.org/10.1111/1462-2920.14244DOI Listing
June 2018

Long-Term Irrigation Affects the Dynamics and Activity of the Wheat Rhizosphere Microbiome.

Front Plant Sci 2018 21;9:345. Epub 2018 Mar 21.

Wheat Health, Genetics and Quality Research Unit, USDA Agricultural Research Service, Pullman, WA, United States.

The Inland Pacific Northwest (IPNW) encompasses 1. 6 million cropland hectares and is a major wheat-producing area in the western United States. The climate throughout the region is semi-arid, making the availability of water a significant challenge for IPNW agriculture. Much attention has been given to uncovering the effects of water stress on the physiology of wheat and the dynamics of its soilborne diseases. In contrast, the impact of soil moisture on the establishment and activity of microbial communities in the rhizosphere of dryland wheat remains poorly understood. We addressed this gap by conducting a three-year field study involving wheat grown in adjacent irrigated and dryland (rainfed) plots established in Lind, Washington State. We used deep amplicon sequencing of the V4 region of the 16S rRNA to characterize the responses of the wheat rhizosphere microbiome to overhead irrigation. We also characterized the population dynamics and activity of indigenous Phz rhizobacteria that produce the antibiotic phenazine-1-carboxylic acid (PCA) and contribute to the natural suppression of soilborne pathogens of wheat. Results of the study revealed that irrigation affected the Phz rhizobacteria adversely, which was evident from the significantly reduced plant colonization frequency, population size and levels of PCA in the field. The observed differences between irrigated and dryland plots were reproducible and amplified over the course of the study, thus identifying soil moisture as a critical abiotic factor that influences the dynamics, and activity of indigenous Phz communities. The three seasons of irrigation had a slight effect on the overall diversity within the rhizosphere microbiome but led to significant differences in the relative abundances of specific OTUs. In particular, irrigation differentially affected multiple groups of and , including taxa with known plant growth-promoting activity. Analysis of environmental variables revealed that the separation between irrigated and dryland treatments was due to changes in the water potential (Ψ) and pH. In contrast, the temporal changes in the composition of the rhizosphere microbiome correlated with temperature and precipitation. In summary, our long-term study provides insights into how the availability of water in a semi-arid agroecosystem shapes the belowground wheat microbiome.
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http://dx.doi.org/10.3389/fpls.2018.00345DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5871930PMC
March 2018

A bio-based pro-antimicrobial polymer network via degradable acetal linkages.

Acta Biomater 2018 02 18;67:196-205. Epub 2017 Dec 18.

School of Polymer Science and Engineering, The University of Southern Mississippi, Hattiesburg, MS 39406, United States. Electronic address:

The synthesis of a fully degradable, bio-based, sustained release, pro-antimicrobial polymer network comprised of degradable acetals (PANDA) is reported. The active antimicrobial agent - p-anisaldehyde (pA) (an extract from star anise) - was converted into a UV curable acetal containing pro-antimicrobial monomer and subsequently photopolymerized into a homogenous thiol-ene network. Under neutral to acidic conditions (pH < 8), the PANDAs undergo surface erosion and exhibit sustained release of pA over 38 days. The release of pA from PANDAs was shown to be effective against both bacterial and fungal pathogens. From a combination of confocal microscopy and transmission electron microscopy, we observed that the released pA disrupts the cell membrane. Additionally, we demonstrated that PANDAs have minimal cytotoxicity towards both epithelial cells and macrophages. Although a model platform, these results point to promising pathways for the design of fully degradable sustained-release antimicrobial systems with potential applications in agriculture, pharmaceuticals, cosmetics, household/personal care, and food industries.

Statement Of Significance: With the increasing number of patients prescribed immunosuppressants coupled with the rise in antibiotic resistance - life-threatening microbial infections are a looming global threat. With limited success within the antibiotic pipeline, nature-based essential oils (EOs) are being investigated for their multimodal effectiveness against microbes. Despite the promising potential of EOs, difficulties in their encapsulation, limited water solubility, and high volatility limit their use. Various studies have shown that covalent attachment of these EO derivatives to polymers can mitigate these limitations. The current study presents the synthesis of a fully-degradable, sustained release, cytocompatible, pro-antimicrobial acetal network derived from p-anisaldehyde. This polymer network design provides a pathway toward application-specific EO releasing materials with quantitative encapsulation efficiencies, sustained release, and broad-spectrum antimicrobial activity.
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http://dx.doi.org/10.1016/j.actbio.2017.12.016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6064185PMC
February 2018

Draft genome sequences of strains SMB35, sp. MH3R3-1 and sp. SMB17 from the Verkhnekamsk potash mining region of Russia.

Stand Genomic Sci 2017 19;12:39. Epub 2017 Jul 19.

Department of Biological Sciences, The University of Southern Mississippi, Hattiesburg, MS 39406 USA.

Halomonads are moderately halophilic bacteria that are studied as models of prokaryotic osmoadaptation and sources of enzymes and chemicals for biotechnological applications. Despite the progress in understanding the diversity of these organisms, our ability to explain ecological, metabolic, and biochemical traits of halomonads at the genomic sequence level remains limited. This study addresses this gap by presenting draft genomes of SMB35, sp. MH3R3-1 and sp. SMB17, which were isolated from potash mine tailings in the Verkhnekamsk salt deposit area of Russia. The analysis of these genomes confirmed the importance of ectoines and quaternary amines to the capacity of halomonads to tolerate osmotic stress and adapt to hypersaline environments. The study also revealed that and share 75-90% of the predicted proteome, but also harbor a set of genus-specific genes, which in amounted to approximately 0.5 Mbp. These genus-specific genome segments may contribute to the phenotypic diversity of the and the ability of these organisms to adapt to changing environmental conditions and colonize new ecological niches.
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http://dx.doi.org/10.1186/s40793-017-0251-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5518125PMC
July 2017

Relationships between Root Pathogen Resistance, Abundance and Expression of Antimicrobial Genes, and Soil Properties in Representative Swiss Agricultural Soils.

Front Plant Sci 2017 29;8:427. Epub 2017 Mar 29.

Department of Fundamental Microbiology, University of LausanneLausanne, Switzerland.

Strains of that produce antimicrobial metabolites and control soilborne plant diseases have often been isolated from soils defined as disease-suppressive, i.e., soils, in which specific plant pathogens are present, but plants show no or reduced disease symptoms. Moreover, it is assumed that pseudomonads producing antimicrobial compounds such as 2,4-diacetylphloroglucinol (DAPG) or phenazines (PHZ) contribute to the specific disease resistance of suppressive soils. However, pseudomonads producing antimicrobial metabolites are also present in soils that are conducive to disease. Currently, it is still unknown whether and to which extent the abundance of antimicrobials-producing pseudomonads is related to the general disease resistance of common agricultural soils. Moreover, virtually nothing is known about the conditions under which pseudomonads express antimicrobial genes in agricultural field soils. We present here results of the first side-by-side comparison of 10 representative Swiss agricultural soils with a cereal-oriented cropping history for (i) the resistance against two soilborne pathogens, (ii) the abundance of bacteria harboring genes involved in the biosynthesis of the antimicrobials DAPG, PHZ, and pyrrolnitrin on roots of wheat, and (iii) the ability to support the expression of these genes on the roots. Our study revealed that the level of soil disease resistance strongly depends on the type of pathogen, e.g., soils that are highly resistant to often are highly susceptible to and vice versa. There was no significant correlation between the disease resistance of the soils, the abundance of bacteria carrying DAPG, PHZ, and pyrrolnitrin biosynthetic genes, and the ability of the soils to support the expression of the antimicrobial genes. Correlation analyses indicated that certain soil factors such as silt, clay, and some macro- and micronutrients influence both the abundance and the expression of the antimicrobial genes. Taken together, the results of this study suggests that pseudomonads producing DAPG, PHZ, or pyrrolnitrin are present and abundant in Swiss agricultural soils and that the soils support the expression of the respective biosynthetic genes in these bacteria to various degrees. The precise role that these pseudomonads play in the general disease resistance of the investigated agricultural soils remains elusive.
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http://dx.doi.org/10.3389/fpls.2017.00427DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372754PMC
March 2017

Destruction of Opportunistic Pathogens via Polymer Nanoparticle-Mediated Release of Plant-Based Antimicrobial Payloads.

Adv Healthc Mater 2016 05 4;5(9):1094-103. Epub 2016 Mar 4.

School of Polymers and High Performance Materials, University of Southern Mississippi, Hattiesburg, MS, 39406, USA.

The synthesis of antimicrobial thymol/carvacrol-loaded polythioether nanoparticles (NPs) via a one-pot, solvent-free miniemulsion thiol-ene photopolymerization process is reported. The active antimicrobial agents, thymol and carvacrol, are employed as "solvents" for the thiol-ene monomer phase in the miniemulsion to enable facile high capacity loading (≈50% w/w), excellent encapsulation efficiencies (>95%), and elimination of all postpolymerization purification processes. The NPs serve as high capacity reservoirs for slow-release and delivery of thymol/carvacrol-combination payloads that exhibit inhibitory and bactericidal activity (>99.9% kill efficiency at 24 h) against gram-positive and gram-negative bacteria, including both saprophytic (Bacillus subtilis ATCC 6633 and Escherichia coli ATCC 25922) and pathogenic species (E. coli ATCC 43895, Staphylococcus aureus RN6390, and Burkholderia cenocepacia K56-2). This report is among the first to demonstrate antimicrobial efficacy of essential oil-loaded nanoparticles against B. cenocepacia - an innately resistant opportunistic pathogen commonly associated with debilitating respiratory infections in cystic fibrosis. Although a model platform, these results point to promising pathways to particle-based delivery of plant-derived extracts for a range of antimicrobial applications, including active packaging materials, topical antiseptics, and innovative therapeutics.
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http://dx.doi.org/10.1002/adhm.201500974DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5474087PMC
May 2016

Biocontrol and plant growth-promoting activity of rhizobacteria from Chinese fields with contaminated soils.

Microb Biotechnol 2015 May 15;8(3):404-18. Epub 2014 Sep 15.

State Key Laboratory of Agricultural Microbiology, College of Life Science and Technology, Huazhong Agricultural University, Wuhan, Hubei, 430070, China; Department of Plant Pathology, Washington State University, Pullman, WA, 99164-6430, USA.

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.
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http://dx.doi.org/10.1111/1751-7915.12158DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4408174PMC
May 2015

Biological control of wheat root diseases by the CLP-producing strain Pseudomonas fluorescens HC1-07.

Phytopathology 2014 Mar;104(3):248-56

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.
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http://dx.doi.org/10.1094/PHYTO-05-13-0142-RDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5523110PMC
March 2014

Role of bacterial communities in the natural suppression of Rhizoctonia solani bare patch disease of wheat (Triticum aestivum L.).

Appl Environ Microbiol 2013 Dec 20;79(23):7428-38. Epub 2013 Sep 20.

Department of Plant Pathology, Washington State University, Pullman, Washington, USA.

Rhizoctonia bare patch and root rot disease of wheat, caused by Rhizoctonia solani AG-8, develops as distinct patches of stunted plants and limits the yield of direct-seeded (no-till) wheat in the Pacific Northwest of the United States. At the site of a long-term cropping systems study near Ritzville, WA, a decline in Rhizoctonia patch disease was observed over an 11-year period. Bacterial communities from bulk and rhizosphere soil of plants from inside the patches, outside the patches, and recovered patches were analyzed by using pyrosequencing with primers designed for 16S rRNA. Taxa in the class Acidobacteria and the genus Gemmatimonas were found at higher frequencies in the rhizosphere of healthy plants outside the patches than in that of diseased plants from inside the patches. Dyella and Acidobacteria subgroup Gp7 were found at higher frequencies in recovered patches. Chitinophaga, Pedobacter, Oxalobacteriaceae (Duganella and Massilia), and Chyseobacterium were found at higher frequencies in the rhizosphere of diseased plants from inside the patches. For selected taxa, trends were validated by quantitative PCR (qPCR), and observed shifts of frequencies in the rhizosphere over time were duplicated in cycling experiments in the greenhouse that involved successive plantings of wheat in Rhizoctonia-inoculated soil. Chryseobacterium soldanellicola was isolated from the rhizosphere inside the patches and exhibited significant antagonism against R. solani AG-8 in vitro and in greenhouse tests. In conclusion, we identified novel bacterial taxa that respond to conditions affecting bare patch disease symptoms and that may be involved in suppression of Rhizoctonia root rot and bare batch disease.
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http://dx.doi.org/10.1128/AEM.01610-13DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3837727PMC
December 2013

Taxonomy and distribution of phenazine-producing Pseudomonas spp. in the dryland agroecosystem of the Inland Pacific Northwest, United States.

Appl Environ Microbiol 2013 Jun 12;79(12):3887-91. Epub 2013 Apr 12.

School of Molecular Biosciences, Washington State University, Pullman, WA, USA.

We investigated the taxonomic placement of phenazine-producing fluorescent Pseudomonas spp. in the Inland Pacific Northwest region of the United States. 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.
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http://dx.doi.org/10.1128/AEM.03945-12DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3675913PMC
June 2013

Recent insights into the diversity, frequency and ecological roles of phenazines in fluorescent Pseudomonas spp.

Environ Microbiol 2013 Mar 13;15(3):675-86. Epub 2012 Aug 13.

Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430, USA.

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.
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http://dx.doi.org/10.1111/j.1462-2920.2012.02846.xDOI Listing
March 2013

Irrigation differentially impacts populations of indigenous antibiotic-producing pseudomonas spp. in the rhizosphere of wheat.

Appl Environ Microbiol 2012 May 2;78(9):3214-20. Epub 2012 Mar 2.

Department of Plant Pathology, Washington State University, Pullman, Washington, USA.

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.
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http://dx.doi.org/10.1128/AEM.07968-11DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3346461PMC
May 2012

Population structure and diversity of phenazine-1-carboxylic acid producing fluorescent Pseudomonas spp. from dryland cereal fields of central Washington State (USA).

Microb Ecol 2012 Jul 2;64(1):226-41. Epub 2012 Mar 2.

School of Molecular Biosciences, Washington State University, Pullman, WA 99164-4234, USA.

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.
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http://dx.doi.org/10.1007/s00248-012-0015-0DOI Listing
July 2012

Accumulation of the antibiotic phenazine-1-carboxylic acid in the rhizosphere of dryland cereals.

Appl Environ Microbiol 2012 Feb 2;78(3):804-12. Epub 2011 Dec 2.

Department of Plant Pathology, Washington State University, Pullman, Washington, USA.

Natural antibiotics are thought to function in the defense, fitness, competitiveness, biocontrol activity, communication, and gene regulation of microorganisms. However, the scale and quantitative aspects of antibiotic production in natural settings are poorly understood. We addressed these fundamental questions by assessing the geographic distribution of indigenous phenazine-producing (Phz(+)) Pseudomonas spp. and the accumulation of the broad-spectrum antibiotic phenazine-1-carboxylic acid (PCA) in the rhizosphere of wheat grown in the low-precipitation zone (<350 mm) of the Columbia Plateau and in adjacent, higher-precipitation areas. Plants were collected from 61 commercial wheat fields located within an area of about 22,000 km(2). Phz(+) Pseudomonas spp. were detected in all sampled fields, with mean population sizes ranging from log 3.2 to log 7.1 g(-1) (fresh weight) of roots. Linear regression analysis demonstrated a significant inverse relationship between annual precipitation and the proportion of plants colonized by Phz(+) Pseudomonas spp. (r(2) = 0.36, P = 0.0001). PCA was detected at up to nanomolar concentrations in the rhizosphere of plants from 26 of 29 fields that were selected for antibiotic quantitation. There was a direct relationship between the amount of PCA extracted from the rhizosphere and the population density of Phz(+) pseudomonads (r(2) = 0.46, P = 0.0006). This is the first demonstration of accumulation of significant quantities of a natural antibiotic across a terrestrial ecosystem. Our results strongly suggest that natural antibiotics can transiently accumulate in the plant rhizosphere in amounts sufficient not only for inter- and intraspecies signaling but also for the direct inhibition of sensitive organisms.
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http://dx.doi.org/10.1128/AEM.06784-11DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3264129PMC
February 2012

Biological control of take-all by fluorescent Pseudomonas spp. from Chinese wheat fields.

Phytopathology 2011 Dec;101(12):1481-91

Department of Plant Pathology, Nanjing Agricultural University, Jiangsu Province, China.

Take-all disease of wheat caused by the soilborne fungus Gaeumannomyces graminis var. tritici is one of the most important root diseases of wheat worldwide. Bacteria were isolated from winter wheat from irrigated and rainfed fields in Hebei and Jiangsu provinces in China, respectively. Samples from rhizosphere soil, roots, stems, and leaves were plated onto King's medium B agar and 553 isolates were selected. On the basis of in vitro tests, 105 isolates (19% of the total) inhibited G. graminis var. tritici and all were identified as Pseudomonas spp. by amplified ribosomal DNA restriction analysis. Based on biocontrol assays, 13 strains were selected for further analysis. All of them aggressively colonized the rhizosphere of wheat and suppressed take-all. Of the 13 strains, 3 (HC9-07, HC13-07, and JC14-07, all stem endophytes) had genes for the biosynthesis of phenazine-1-carboxylic acid (PCA) but none had genes for the production of 2,4-diacetylphloroglucinol, pyoluteorin, or pyrrolnitrin. High-pressure liquid chromatography (HPLC) analysis of 2-day-old cultures confirmed that HC9-07, HC13-07, and JC14-07 produced PCA but no other phenazines were detected. HPLC quantitative time-of-flight 2 mass-spectrometry analysis of extracts from roots of spring wheat colonized by HC9-07, HC13-07, or Pseudomonas fluorescens 2-79 demonstrated that all three strains produced PCA in the rhizosphere. Loss of PCA production by strain HC9-07 resulted in a loss of biocontrol activity. Analysis of DNA sequences within the key phenazine biosynthesis gene phzF and of 16S rDNA indicated that strains HC9-07, HC13-07, and JC14-07 were similar to the well-described PCA producer P. fluorescens 2-79. This is the first report of 2-79-like bacteria being isolated from Asia.
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http://dx.doi.org/10.1094/PHYTO-04-11-0096DOI Listing
December 2011

Structural and functional analysis of the type III secretion system from Pseudomonas fluorescens Q8r1-96.

J Bacteriol 2011 Jan 22;193(1):177-89. Epub 2010 Oct 22.

Department of Plant Pathology, Washington State University, Pullman, Washington 99164-6430, USA.

Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a type III protein secretion system (T3SS) gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1-1 and hopM1, as well as a novel type III effector gene designated ropB. These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere, but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.
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http://dx.doi.org/10.1128/JB.00895-10DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3019950PMC
January 2011

Diversity and evolution of the phenazine biosynthesis pathway.

Appl Environ Microbiol 2010 Feb 11;76(3):866-79. Epub 2009 Dec 11.

Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430, USA.

Phenazines are versatile secondary metabolites of bacterial origin that function in biological control of plant pathogens and contribute to the ecological fitness and pathogenicity of the producing strains. In this study, we employed a collection of 94 strains having various geographic, environmental, and clinical origins to study the distribution and evolution of phenazine genes in members of the genera Pseudomonas, Burkholderia, Pectobacterium, Brevibacterium, and Streptomyces. Our results confirmed the diversity of phenazine producers and revealed that most of them appear to be soil-dwelling and/or plant-associated species. Genome analyses and comparisons of phylogenies inferred from sequences of the key phenazine biosynthesis (phzF) and housekeeping (rrs, recA, rpoB, atpD, and gyrB) genes revealed that the evolution and dispersal of phenazine genes are driven by mechanisms ranging from conservation in Pseudomonas spp. to horizontal gene transfer in Burkholderia spp. and Pectobacterium spp. DNA extracted from cereal crop rhizospheres and screened for the presence of phzF contained sequences consistent with the presence of a diverse population of phenazine producers in commercial farm fields located in central Washington state, which provided the first evidence of United States soils enriched in indigenous phenazine-producing bacteria.
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http://dx.doi.org/10.1128/AEM.02009-09DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2813009PMC
February 2010

Quantification of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens strains in the plant rhizosphere by real-time PCR.

Appl Environ Microbiol 2007 Sep 13;73(17):5531-8. Epub 2007 Jul 13.

Department of Plant Pathology, Washington State University, Pullman, Washington 99164-6430, USA.

A real-time PCR SYBR green assay was developed to quantify populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing (phlD+) strains of Pseudomonas fluorescens in soil and the rhizosphere. Primers were designed and PCR conditions were optimized to specifically amplify the phlD gene from four different genotypes of phlD+ P. fluorescens. Using purified genomic DNA and genomic DNA extracted from washes of wheat roots spiked with bacteria, standard curves relating the threshold cycles (C(T)s) and copies of the phlD gene were generated for P. fluorescens strains belonging to genotypes A (Pf-5), B (Q2-87), D (Q8r1-96 and FTAD1R34), and I (FTAD1R36). The detection limits of the optimized real-time PCR assay were 60 to 600 fg (8 to 80 CFU) for genomic DNA isolated from pure cultures of P. fluorescens and 600 fg to 6.0 pg (80 to 800 CFU, corresponding to log 4 to 5 phlD+ strain CFU/rhizosphere) for bacterial DNA extracted from plant root washes. The real-time PCR assay was utilized to quantify phlD+ pseudomonads in the wheat rhizosphere. Regression analysis of population densities detected by real-time PCR and by a previously described phlD-specific PCR-based dilution endpoint assay indicated a significant linear relationship (P = 0.0016, r2 = 0.2). Validation of real-time PCR assays with environmental samples was performed with two different soils and demonstrated the detection of more than one genotype in Quincy take-all decline soil. The greatest advantage of the developed real-time PCR is culture independence, which allows determination of population densities and the genotype composition of 2,4-DAPG producers directly from the plant rhizospheres and soil.
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http://dx.doi.org/10.1128/AEM.00925-07DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2042083PMC
September 2007

Role of ptsP, orfT, and sss recombinase genes in root colonization by Pseudomonas fluorescens Q8r1-96.

Appl Environ Microbiol 2006 Nov 25;72(11):7111-22. Epub 2006 Aug 25.

Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430, USA.

Pseudomonas fluorescens Q8r1-96 produces 2,4-diacetylphloroglucinol (2,4-DAPG), a polyketide antibiotic that suppresses a wide variety of soilborne fungal pathogens, including Gaeumannomyces graminis var. tritici, which causes take-all disease of wheat. Strain Q8r1-96 is representative of the D-genotype of 2,4-DAPG producers, which are exceptional because of their ability to aggressively colonize and maintain large populations on the roots of host plants, including wheat, pea, and sugar beet. In this study, three genes, an sss recombinase gene, ptsP, and orfT, which are important in the interaction of Pseudomonas spp. with various hosts, were investigated to determine their contributions to the unusual colonization properties of strain Q8r1-96. The sss recombinase and ptsP genes influence global processes, including phenotypic plasticity and organic nitrogen utilization, respectively. The orfT gene contributes to the pathogenicity of Pseudomonas aeruginosa in plants and animals and is conserved among saprophytic rhizosphere pseudomonads, but its function is unknown. Clones containing these genes were identified in a Q8r1-96 genomic library, sequenced, and used to construct gene replacement mutants of Q8r1-96. Mutants were characterized to determine their 2,4-DAPG production, motility, fluorescence, colony morphology, exoprotease and hydrogen cyanide (HCN) production, carbon and nitrogen utilization, and ability to colonize the rhizosphere of wheat grown in natural soil. The ptsP mutant was impaired in wheat root colonization, whereas mutants with mutations in the sss recombinase gene and orfT were not. However, all three mutants were less competitive than wild-type P. fluorescens Q8r1-96 in the wheat rhizosphere when they were introduced into the soil by paired inoculation with the parental strain.
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http://dx.doi.org/10.1128/AEM.01215-06DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1636191PMC
November 2006

The role of dsbA in colonization of the wheat rhizosphere by Pseudomonas fluorescens Q8r1-96.

Microbiology (Reading) 2006 Mar;152(Pt 3):863-872

USDA-ARS, Root Disease and Biological Control Research Unit, Washington State University, Pullman, WA, USA.

Certain well-conserved genes in fluorescent Pseudomonas spp. are involved in pathogenic interactions between the bacteria and evolutionarily diverse hosts including plants, insects and vertebrate animals. One such gene, dsbA, encodes a periplasmic disulfide-bond-forming enzyme implicated in the biogenesis of exported proteins and cell surface structures. This study focused on the role of dsbA in Pseudomonas fluorescens Q8r1-96, a biological control strain that produces the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) and is known for its exceptional ability to colonize the roots of wheat and pea. The deduced DsbA protein from Q8r1-96 is similar to other predicted thiol : disulfide interchange proteins and contains a conserved DsbA catalytic site, a pattern associated with the thioredoxin family active site, and a signal peptide and cleavage site. A dsbA mutant of Q8r1-96 exhibited decreased motility and fluorescence, and altered colony morphology; however, it produced more 2,4-DAPG and total phloroglucinol-related compounds and was more inhibitory in vitro to the fungal root pathogen Gaeumannomyces graminis var. tritici than was the parental strain. When introduced separately into a natural soil, Q8r1-96 and the dsbA mutant did not differ in their ability to colonize the rhizosphere of wheat in greenhouse experiments lasting 12 weeks. However, when the two strains were co-inoculated, the parental strain consistently out-competed the dsbA mutant. It was concluded that dsbA does not contribute to the exceptional rhizosphere competence of Q8r1-96, although the dsbA mutation reduces competitiveness when the mutant competes with the parental strain in the same niche in the rhizosphere. The results also suggest that exoenzymes and multimeric cell surface structures are unlikely to have a critical role in root colonization by this strain.
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http://dx.doi.org/10.1099/mic.0.28545-0DOI Listing
March 2006

Enrichment and genotypic diversity of phlD-containing fluorescent Pseudomonas spp. in two soils after a century of wheat and flax monoculture.

FEMS Microbiol Ecol 2006 Mar;55(3):351-68

Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430, USA.

Fluorescent Pseudomonas spp. producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) play a key role in the suppressiveness of some soils to take-all of wheat and other diseases caused by soilborne pathogens. Soils from side-by-side fields on the campus of North Dakota State University, Fargo, USA, which have undergone continuous wheat, continuous flax or crop rotation for over 100 years, were assayed for the presence of 2,4-DAPG producers. Flax and wheat monoculture, but not crop rotation, enriched for 2,4-DAPG producers, and population sizes of log 5.0 CFU g root(-1) or higher were detected in the rhizospheres of wheat and flax grown in the two monoculture soils. The composition of the genotypes enriched by the two crops differed. Four BOX-PCR genotypes (D, F, G, and J) and a new genotype (T) were detected among the 2,4-DAPG producers in the continuous flax soil, with F- and J-genotype isolates dominating (41 and 39% of the total, respectively). In contrast, two genotypes (D and I) were detected in the soil with continuous wheat, with D-genotype isolates comprising 77% of the total. In the crop-rotation soil, populations of 2,4-DAPG producers generally were below the detection limit, and only one genotype (J) was detected. Under growth-chamber and field conditions, D and I genotypes (enriched by wheat monoculture) colonized the wheat rhizosphere significantly better than isolates of other genotypes, while a J-genotype isolate colonized wheat and flax rhizospheres to the same extent. This study suggests that, over many years of monoculture, the crop species grown in a field enriches for genotypes of 2,4-DAPG producers from the reservoir of genotypes naturally present in the soil that are especially adapted to colonizing the rhizosphere of the crop grown.
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http://dx.doi.org/10.1111/j.1574-6941.2005.00038.xDOI Listing
March 2006

Antagonistic activity among 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp.

FEMS Microbiol Lett 2005 Jan;242(2):249-56

Skryabin Institute of Biochemistry and Physiology of Microorganisms of the Russian Academy of Sciences, Pushchino, Russia.

Strains of fluorescent Pseudomonas spp. that produce 2,4-diacetylphloroglucinol (2,4-DAPG) differ in their ability to colonize roots. In this study, we screened 47 2,4-DAPG-producing strains representing17 distinct genotypes for antagonistic activity associated with the production of bacteriocins. Upon induction, over 70% of the strains inhibited the growth of other isolates in vitro. Greenhouse assays indicated that populations of sensitive strains in wheat rhizosphere soil declined more rapidly in the presence of antagonists than when introduced alone. Antagonism can influence the ability of biocontrol agents to establish and maintain effective population densities in situ.
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http://dx.doi.org/10.1016/j.femsle.2004.11.013DOI Listing
January 2005

Differential ability of genotypes of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens strains to colonize the roots of pea plants.

Appl Environ Microbiol 2002 Jul;68(7):3226-37

Root Disease and Biological Control Research Unit, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164-6430, USA.

Indigenous populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent Pseudomonas spp. that occur naturally in suppressive soils are an enormous resource for improving biological control of plant diseases. Over 300 isolates of 2,4-DAPG-producing fluorescent Pseudomonas spp. were isolated from the rhizosphere of pea plants grown in soils that had undergone pea or wheat monoculture and were suppressive to Fusarium wilt or take-all, respectively. Representatives of seven genotypes, A, D, E, L, O, P, and Q, were isolated from both soils and identified by whole-cell repetitive sequence-based PCR (rep-PCR) with the BOXA1R primer, increasing by three (O, P, and Q) the number of genotypes identified previously among a worldwide collection of 2,4-DAPG producers. Fourteen isolates representing eight different genotypes were tested for their ability to colonize the rhizosphere of pea plants. Population densities of strains belonging to genotypes D and P were significantly greater than the densities of other genotypes and remained above log 6.0 CFU (g of root)(-1) over the entire 15-week experiment. Genetic profiles generated by rep-PCR or restriction fragment length polymorphism analysis of the 2,4-DAPG biosynthetic gene phlD were predictive of the rhizosphere competence of the introduced 2,4-DAPG-producing strains.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC126803PMC
http://dx.doi.org/10.1128/aem.68.7.3226-3237.2002DOI Listing
July 2002