• Research in Plant Disease
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• v.24 no.3
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• pp.242-247
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• 2018

Pseudomonas chlororaphis O6 induces systemic tolerance in plants against drought stress. A volatile, 2R, 3R-butanediol, produced by the bacterium causes partial stomatal closure, thus, limiting water loss from the plant. In this study, we report that applications of extracellular polymeric substances (EPS) from P. chlororaphis O6 to epidermal peels of leaves of Arabidopsis thaliana also reduce the size of stomatal openings. Growth of A. thaliana seedlings with applications of the EPS from P. chlororaphis O6 reduced the extent of wilting when water was withheld from the plants. Fluorescence measurements showed photosystem II was protected in the A. thaliana leaves in the water stressed EPS-exposed plants. These findings indicate that P. chlororaphis O6 has redundancy in traits associated with induction of mechanisms to limit water stress in plants.

• The Plant Pathology Journal
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• v.29 no.3
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• pp.323-329
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• 2013

The stationary-phase sigma factor, RpoS, influences the expression of factors important in survival of Pseudomonas chlororaphis O6 in the rhizosphere. A partial proteomic profile of a rpoS mutant in P. chlororaphis O6 was conducted to identify proteins under RpoS regulation. Five of 14 differentially regulated proteins had unknown roles. Changes in levels of proteins in P. chlororaphis O6 rpoS mutant were associated with iron metabolism, and protection against oxidative stress. The P. chlororaphis O6 rpoS mutant showed increased production of a pyoverdine-like siderophore, indole acetic acid, and altered isozyme patterns for peroxidase, catalase and superoxide dismutase. Consequently, sensitivity to hydrogen peroxide exposure increased in the P. chlororaphis O6 rpoS mutant, compared with the wild type. Taken together, RpoS exerted regulatory control over factors important for the habitat of P. chlororaphis O6 in soil and on root surfaces. The properties of several of the proteins in the RpoS regulon are currently unknown.

• Research in Plant Disease
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• v.23 no.3
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• pp.212-227
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• 2017

Pseudomonas chlororaphis O6, isolated from the roots of dryland, field-grown commercial wheat in the USA, enhances plant health and therefore it is used in agriculture as a biofertilizer and biocontrol agent. The metabolites produced by this pseudomonad stimulate plant growth through direct antagonism of pathogens and by inducing systemic resistance in the plant. Studies upon P. chlororaphis O6 identify the pathways through which defined bacterial metabolites generate protection against pathogenic microbes, insects, and nematodes. P. chlororaphis O6 also triggers plant resistance to drought and salinity stresses. The beneficial determinants are produced from bacterial cells as they form biofilms during root colonization. Molecular control these processes in P. chlororaphis O6 involves the global regulatory Gac/Rsm signaling cascade with cross-talk between other global regulatory pathways. The Gac/Rsm regulon allows for coordinate phasing of expression of the genes that encode these beneficial traits among a community of cells. This review provides insights on the Gac/Rsm regulon in expression of beneficial traits of the P. chlororaphis O6 which can contribute to help yield enhancement and quality in agricultural production.

• The Plant Pathology Journal
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• v.27 no.3
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• pp.272-279
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• 2011

Root colonization of Arabidopsis thaliana with Pseudomonas chlororaphis O6 induces systemic tolerance against diverse pathogens, as well as drought and salt stresses. In this study, we demonstrated that 11 genes in the leaves were up-regulated, and 5 genes were down-regulated as the result of three- to five-days root colonization by P. chlororaphis O6. The identified priming genes were involved in cell signaling, transcription, protein synthesis, and degradation. In addition, expression of selected priming genes were induced in P. chlororaphis O6-colonized plants subjected to water withholding. Genes encoding defense proteins in signaling pathways regulated by jasmonic acid and ethylene, such as VSP1 and PDF1.2, were additional genes with enhanced expression in the P. chlororaphis O6-colonized plants. This study indicated that the expression of priming genes, as well as genes involved in jasmonic acid- and ethylene-regulated genes may play an important role in the systemic induction of both abiotic and biotic stress due to root colonization by P. chlororaphis O6.

• The Plant Pathology Journal
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• v.28 no.2
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• pp.202-206
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• 2012

Root colonization by the rhizobacterium Pseudomonas chlororaphis O6 in Arabidopsis thaliana Col-0 plants resulted in induced tolerance to drought and salinity caused by halide salt-generated ionic stress but not by osmotic stress caused by sorbitol. Stomatal apertures decreased following root colonization by P. chlororaphis O6 in both wild-type and ABA-insensitive Arabidopsis mutant plants. These results suggest that an ABA-independent stomatal closure mechanism in the guard cells of P. chlororaphis O6-colonized plants could be a key phenotype for induced systemic tolerance to drought and salt stress.

• The Plant Pathology Journal
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• v.34 no.1
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• pp.35-43
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• 2018

Root-knot nematodes (Meloidogyne spp.) are parasites that attack many field crops and orchard trees, and affect both the quantity and quality of the products. A root-colonizing bacterium, Pseudomonas chlororaphis O6, possesses beneficial traits including strong nematicidal activity. To determine the molecular mechanisms involved in the nematicidal activity of P. chlororaphis O6, we constructed two mutants; one lacking hydrogen cyanide production, and a second lacking an insecticidal toxin, FitD. Root drenching with wild-type P. chlororaphis O6 cells caused juvenile mortality in vitro and in planta. Efficacy was not altered in the fitD mutant compared to the wild-type but was reduced in both bioassays for the mutant lacking hydrogen cyanide production. The reduced number of galls on tomato plants caused by the wild-type strain was comparable to that of a standard chemical nematicide. These findings suggest that hydrogen cyanide-producing root colonizers, such as P. chlororaphis O6, could be formulated as "green" nematicides that are compatible with many crops and offer agricultural sustainability.

• Journal of Microbiology and Biotechnology
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• v.18 no.10
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• pp.1659-1662
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• 2008

Pseudomonas chlororaphis O6 exhibits induced systemic resistance (ISR) against P. syringae pv. tabaci in tobacco. To identify one of the ISR metabolites, O6 cultures were extracted with organic solvents, and the organic extracts were subjected to column chromatography followed by spectroscopy analyses. The ISR bioassay-guided fractionation was carried out for isolation of the metabolite. High-resolution mass spectrometric analysis of the metabolite found $C_{9}H_{9}O_{3}N$ with an exact mass of 179.0582. LC/MS analysis in positive mode showed an $(M+H)^{+}$ peak at m/z 180. Nuclear magnetic resonance ($^{1}H,\;^{13}C$) analyses identified all protons and carbons of the metabolite. Based on the spectroscopy data, the metabolite was identified as 4-(aminocarbonyl) phenylacetate (4-ACPA). 4-ACPA applied at 68.0 mM exhibited ISR activity at a level similar to 1.0 mM salicylic acid. This is the first report to identify an ISR metabolite produced by P. chlororaphis O6 against the wildfire pathogen P. syringae pv. tabaci in tobacco.

• Research in Plant Disease
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• v.24 no.2
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• pp.162-169
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• 2018

The GacS/GacA system in the root colonizer Pseudomonas chlororaphis O6 is a key regulatory system of many traits relevant to the plant probiotic nature of this bacterium. The work in this paper elucidates proteins using proteomics approach in P. chlororaphis O6 under the control of the cytoplasmic regulatory protein, GacA. A gacA mutant of P. chlororaphis O6 showed loss in production of phenazines, acyl homoserine lactones, hydrogen cyanide, and protease, changes that were associated with reduced in vitro antifungal activity against plant fungal pathogens. Production of iron-chelating siderophore was significantly enhanced in the gacA mutant, also paralleling changes in a gacS mutant. However, proteomic analysis revealed proteins (13 downregulated and 7 upregulated proteins in the mutant compared to parental strain) under GacA control that were not apparent by a proteomic study of a gacS mutant. The putative identity of the downregulated proteins suggested that a gacA mutant would have altered transport potentials. Notable would be a predicted loss of type-VI secretion and PEP-dependent transport. Study of mutants of these GacA-regulated proteins will indicate further the features required for probiotic potential in this rhizobacterium.

• The Plant Pathology Journal
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• v.29 no.2
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• pp.209-220
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• 2013

Root colonization by Pseudomonas chlororaphis O6 induces systemic drought tolerance in Arabidopsis thaliana. Microarray analysis was performed using the 22,800-gene Affymetrix GeneChips to identify differentially-expressed genes from plants colonized with or without P. chlororaphis O6 under drought stressed conditions or normal growth conditions. Root colonization in plants grown under regular irrigation condition increased transcript accumulation from genes associated with defense, response to reactive oxygen species, and auxin- and jasmonic acid-responsive genes, but decreased transcription factors associated with ethylene and abscisic acid signaling. The cluster of genes involved in plant disease resistance were up-regulated, but the set of drought signaling response genes were down-regulated in the P. chlororaphis O6-colonized under drought stress plants compared to those of the drought stressed plants without bacterial treatment. Transcripts of the jasmonic acid-marker genes, VSP1 and pdf-1.2, the salicylic acid regulated gene, PR-1, and the ethylene-response gene, HEL, also were up-regulated in plants colonized by P. chlororaphis O6, but differed in their responsiveness to drought stress. These data show how gene expression in plants lacking adequate water can be remarkably influenced by microbial colonization leading to plant protection, and the activation of the plant defense signal pathway induced by root colonization of P. chlororaphis O6 might be a key element for induced systemic tolerance by microbes.

• The Plant Pathology Journal
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• v.34 no.3
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• pp.241-249
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• 2018

Commercial biocontrol of microbial plant diseases and plant pests, such as nematodes, requires field-effective formulations. The isolate Pseudomonas chlororaphis O6 is a Gram-negative bacterium that controls microbial plant pathogens both directly and indirectly. This bacterium also has nematocidal activity. In this study, we report on the efficacy of a wettable powder-type formulation of P. chlororaphis O6. Culturable bacteria in the formulated product were retained at above $1{\times}10^8$ colony forming units/g after storage of the powder at $25^{\circ}C$ for six months. Foliar application of the diluted formulated product controlled leaf blight and gray mold in tomato. The product also displayed preventative and curative controls for root-knot nematode (Meloidogyne spp.) in tomato. Under laboratory conditions and for commercially grown melon, the control was at levels comparable to that of a standard commercial chemical nematicide. The results indicated that the wettable powder formulation product of P. chlororaphis O6 can be used for control of plant microbial pathogens and root-knot nematodes.