[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.
No preview · Article · Nov 2012 · Soil Biology and Biochemistry
[Show abstract][Hide abstract] ABSTRACT: 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 km2. 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. (r2 = 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 (r2 = 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.
Full-text · Article · Dec 2011 · Applied and Environmental Microbiology
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Production of antifungal metabolites, including the polyketide 2,4-diacetylphloroglucinol (2,4-DAPG), is one mechanism by which biocontrol strains of Pseudomonas fluorescens suppress soilborne fungal pathogens. P. fluorescens strains vary in ability to produce 2,4-DAPG, but the role of the host in modulating metabolite accumulation in the rhizosphere is not well defined. To examine 2,4-DAPG production and accumulation during early stages of rhizoplane interactions, we compared metabolite production by two P. fluorescens strains in culture and on seedling roots of three Triticum aestivum L. (wheat) cultivars, Buchanan, Finley, and Tara, in a soil-free system. P. fluorescens strain Q8r1-96, an aggressive colonizer of the wheat rhizosphere, produced 1850 μg mL−1 2,4-DAPG after 48 h of growth in King’s Medium B, significantly (P > 0.05) more than 19.4 μg mL−1 metabolite produced by the moderately aggressive strain Q2-87V1 under the same conditions. Rhizoplane levels of 2,4-DAPG after 4 d of Q8r1-96 colonization were 1946, 1650, and 2767 ng g−1 for Buchanan, Finley, and Tara, respectively. Metabolite levels obtained for Q2-87V1 colonization were 1468, 366, and 80 ng g−1 on the respective cultivars. Strain Q8r1-96 deposited significantly (P < 0.05) more 2,4-DAPG than Q2-87V1 on Tara and Finley roots, whereas both strains produced similar (P > 0.05) amounts of the metabolite on Buchanan roots. In greenhouse experiments, take-all damage was reduced only on Tara roots inoculated with Q8r1-96. To our knowledge, this is the first report to compare 2,4-DAPG accumulation in the rhizoplanes of different cultivars, and to demonstrate that rhizoplane 2,4-DAPG accumulation depends on a cultivar–bacterial strain interaction.
[Show abstract][Hide abstract] ABSTRACT: It has long been known that certain antibiotic-producing soil microorganisms are inhibitory to plant pathogens, both in the
laboratory and in the field (Stallings 1954). The exploitation of these natural antagonistic interactions has been a driving
force in research on the biological control of plant pathogens over the past century, but only in recent decades has pathogen
control by antibiotics produced at biologically relevant levels in the environment been demonstrated conclusively. This progress,
resulting from conceptual and technological advances made initially in the laboratory and then extended to the field, has
set new standards for biocontrol research involving antibiotics. More generally, the approaches used in these studies may
be useful in exploring the significance of other bioactive metabolites produced by microorganisms in their native habitats.
[Show abstract][Hide abstract] ABSTRACT: Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.
[Show abstract][Hide abstract] ABSTRACT: A seven-gene operon for the synthesis of phenazine-1-carboxylic acid was introduced into Pseudomonas fluorescens Q8r1-96, an aggressive root colonizer that produces 2,4-diacetylphloroglucinol and consistently suppresses take-all of wheat. The recombinant strains produced both antifungal metabolites and maintained population sizes comparable to those of Q8r1-96 over a seven-week period in the rhizosphere of wheat. The strains were no more suppressive of take-all or Pythium root rot than was Q8r1-96, but suppressed Rhizoctonia root rot at a dose of only 10(2) CFU per seed, one to two orders of magnitude lower than the dose of Q8r1-96 required for comparable disease control.
[Show abstract][Hide abstract] ABSTRACT: Two seven-gene phenazine biosynthetic loci were cloned fromPseudomonas aeruginosa PAO1. The operons, designatedphzA1B1C1D1E1F1G1 and phzA2B2C2D2E2F2G2, are homologous to previously studied phenazine biosynthetic operons from Pseudomonas fluorescens and Pseudomonas aureofaciens. Functional studies of phenazine-nonproducing strains of fluorescent pseudomonads indicated that each of the biosynthetic
operons from P. aeruginosa is sufficient for production of a single compound, phenazine-1-carboxylic acid (PCA). Subsequent conversion of PCA to pyocyanin
is mediated in P. aeruginosa by two novel phenazine-modifying genes,phzM and phzS, which encode putative phenazine-specific methyltransferase and flavin-containing monooxygenase, respectively. Expression
of phzS alone inEscherichia coli or in enzymes, pyocyanin-nonproducingP. fluorescens resulted in conversion of PCA to 1-hydroxyphenazine. P. aeruginosa with insertionally inactivated phzM or phzS developed pyocyanin-deficient phenotypes. A third phenazine-modifying gene,phzH, which has a homologue in Pseudomonas chlororaphis, also was identified and was shown to control synthesis of phenazine-1-carboxamide from PCA in P. aeruginosa PAO1. Our results suggest that there is a complex pyocyanin biosynthetic pathway in P. aeruginosaconsisting of two core loci responsible for synthesis of PCA and three additional genes encoding unique enzymes involved in
the conversion of PCA to pyocyanin, 1-hydroxyphenazine, and phenazine-1-carboxamide.
Full-text · Article · Dec 2001 · Journal of Bacteriology
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT Fluorescent Pseudomonas spp. that produce 2,4-diacetylphloroglucinol (2,4-DAPG) have biocontrol activity against damping-off, root rot, and wilt diseases caused by soilborne fungal pathogens, and play a key role in the natural suppression of Gaeumannomyces graminis var. tritici, known as take-all decline. Diversity within phlD, an essential gene in the biosynthesis of 2,4-DAPG, was studied by restriction fragment length polymorphism (RFLP) analysis of 123 2,4-DAPG-producing isolates from six states in the United States and six other locations worldwide. Clusters defined by RFLP analysis of phlD correlated closely with clusters defined previously by BOX-polymerase chain reaction (PCR) genomic fingerprinting, indicating the usefulness of phlD as a marker of genetic diversity and population structure among 2,4-DAPG producers. Genotypes defined by RFLP analysis of phlD were conserved among isolates from the same site and cropping history. Random amplified polymorphic DNA analyses of genomic DNA revealed a higher degree of polymorphism than RFLP and BOX-PCR analyses. Genotypic diversity in a subset of 30 strains representing all the phlD RFLP groups did not correlate with production in vitro of monoacetylphloroglucinol, 2,4-DAPG, or total phloroglucinol compounds. Twenty-seven of the 30 representative strains lacked pyrrolnitrin and pyoluteorin biosynthetic genes as determined by the use of specific primers and probes.
[Show abstract][Hide abstract] ABSTRACT: Certain strains of root-colonizing fluorescentPseudomonas spp. produce phenazines, a class of antifungal metabolites that can provide protection against various soilborne root pathogens.
Despite the fact that the phenazine biosynthetic locus is highly conserved among fluorescent Pseudomonas spp., individual strains differ in the range of phenazine compounds they produce. This study focuses on the ability of Pseudomonas aureofaciens 30-84 to produce 2-hydroxyphenazine-1-carboxylic acid (2-OH-PCA) and 2-hydroxyphenazine from the common phenazine metabolite
phenazine-1-carboxylic acid (PCA). P. aureofaciens 30-84 contains a novel gene located downstream from the core phenazine operon that encodes a 55-kDa aromatic monooxygenase
responsible for the hydroxylation of PCA to produce 2-OH-PCA. Knowledge of the genes responsible for phenazine product specificity
could ultimately reveal ways to manipulate organisms to produce multiple phenazines or novel phenazines not previously described.
Full-text · Article · Feb 2001 · Journal of Bacteriology
[Show abstract][Hide abstract] ABSTRACT: 2,4-Diacetylphloroglucinol (DAPG) is an antibiotic with broad-spectrum antibacterial and antifungal activities. It is a major determinant in the biological control of several plant diseases. DAPG is produced by Pseudomonas fluorescens both in vitro and in the rhizosphere of wheat. It is involved in the natural suppression of take-all disease known as take-all decline, which develops in soils following extended monoculture of wheat or barley. A one-step synthesis of DAPG from the commercially available 2-acetylphloroglucinol is described. This reaction involves the direct alkylation of 2-acetylphloroglucinol using acetic anhydride as the acetylation reagent, with boron trifluoride-etherate as the catalyst. This synthesis is simple and produces higher yields of DAPG (90%) as compared with previously described procedures. As ecological concerns are gaining equal status with agricultural concerns, the demand for natural biocontrol measures is increasing. There is tremendous pressure from society on agriculture to reduce the use of pesticides. A discussion is given on the agricultural and ecological importance of this natural antibiotic and its application as an alternative to reduce the use of synthetic pesticides.
Full-text · Article · Jun 2000 · Journal of Agricultural and Food Chemistry
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT The role of antibiotics in biological control of soilborne pathogens, and more generally in microbial antagonism in natural disease-suppressive soils, often has been questioned because of the indirect nature of the supporting evidence. In this study, a protocol for high pressure liquid chromatography/mass spectrometry is described that allowed specific identification and quantitation of the antibiotic 2,4-diacetylphloroglucinol (Phl) produced by naturally occurring fluorescent Pseudomonas spp. on roots of wheat grown in a soil suppressive to take-all of wheat. These results provide, for the first time, biochemical support for the conclusion of previous work that Phl-producing fluorescent Pseudomonas spp. are key components of the natural biological control that operates in take-all-suppressive soils in Washington State. This study also demonstrates that the total amount of Phl produced on roots of wheat by P. fluorescens strain Q2-87, at densities ranging from approximately 10(5) to 10(7) CFU/g of root, is proportional to its rhizosphere population density and that Phl production per population unit is a constant (0.62 ng/10(5) CFU). Thus, Phl production in the rhizosphere of wheat is strongly related to the ability of the introduced strain to colonize the roots.
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas fluorescens 2-79 produces the broad-spectrum antibiotic phenazine-1-carboxylic acid (PCA), which is active against a variety of fungal root pathogens. In this study, seven genes designated phzABCDEFG that are sufficient for synthesis of PCA were localized within a 6.8-kb BglII-XbaI fragment from the phenazine biosynthesis locus of strain 2-79. Polypeptides corresponding to all phz genes were identified by analysis of recombinant plasmids in a T7 promoter/polymerase expression system. Products of the phzC, phzD, and phzE genes have similarities to enzymes of shikimic acid and chorismic acid metabolism and, together with PhzF, are absolutely necessary for PCA production. PhzG is similar to pyridoxamine-5'-phosphate oxidases and probably is a source of cofactor for the PCA-synthesizing enzyme(s). Products of the phzA and phzB genes are highly homologous to each other and may be involved in stabilization of a putative PCA-synthesizing multienzyme complex. Two new genes, phzX and phzY, that are homologous to phzA and phzB, respectively, were cloned and sequenced from P. aureofaciens 30-84, which produces PCA, 2-hydroxyphenazine-1-carboxylic acid, and 2-hydroxyphenazine. Based on functional analysis of the phz genes from strains 2-79 and 30-84, we postulate that different species of fluorescent pseudomonads have similar genetic systems that confer the ability to synthesize PCA.
Full-text · Article · May 1998 · Journal of Bacteriology
[Show abstract][Hide abstract] ABSTRACT: The broad-spectrum antibiotic 2,4-diacetylphloroglucinol (Phl) is a major determinant in the biological control of a wide range of plant diseases by fluorescent Pseudomonas spp. A protocol was developed to readily isolate and quantify Phl from broth and agar cultures and from the rhizosphere environment of plants. Extraction with ethyl acetate at an acidic pH was suitable for both in vitro and in situ sources of Phl. For soil samples, the addition of an initial extraction step with 80% acetone at an acidic pH was highly effective in eliminating polar organic soil components, such as humic and fulvic acids, which can interfere with Phl detection by high-performance liquid chromotography. The efficiency of Phl recovery from soil by a single extraction averaged 54.6%, and a second extraction added another 6.1%. These yields were substantially greater than those achieved by several standard protocols commonly used to extract polar phenolic compounds from soil. For the first time Phl was isolated from the rhizosphere environment in raw soil. Following application of Pseudomonas fluorescens Q2-87 and the Phl-overproducing strain Q2-87(pPHL5122) to the seeds of wheat, 2.1 and 2.4 (mu)g of Phl/g of root plus rhizosphere soil, respectively, were isolated from wheat grown in a Ritzville silt loam; 0.47 and 1.3 (mu)g of Phl/g of root plus rhizosphere soil, respectively, were isolated from wheat grown in a Shano silt loam. However, when the amount of Phl was calculated on the basis of cell density, Q2-87(pPHL5122) produced seven and six times more antibiotic than Q2-87 in Ritzville silt loam, and Shano silt loam, respectively.
[Show abstract][Hide abstract] ABSTRACT: An important aspect of the interaction of Pseudomonas syringae pv syringae with plant hosts is the perception of plant signal molecules that regulate expression of genes, such as syrB, required for synthesis of the phytotoxin, syringomycin. In this study, the leaves of sweet cherry (Prunus avium L.) were analyzed to determine the nature of the syrB-inducing activity associated with tissues of a susceptible host. Crude leaf extracts yielded high amounts of total signal activity of more than 12,000 units g-1 (fresh weight) based on activation of a syrB-lacZ fusion in strain B3AR132. The signal activity was fractionated by C18 reversed-phase high-performance liquid chromatography and found to be composed of phenolic glycosides, which were resolved in three regions of the high-performance liquid chromatography profile, and sugars, which eluted with the void volume. Two flavonol glycosides, quercetin 3-rutinosyl-4[prime]-glucoside and kaempferol 3-rutinosyl-4[prime]-glucoside, and a flavanone glucoside, dihydrowogonin 7-glucoside, were identified. The flavonoid glycosides displayed similar specific signal activities and were comparable in signal activity to arbutin, a phenyl [beta]-glucoside, giving rise to between 120 and 160 units of [beta]-galactosidase activity at 10 [mu]M. Although D-fructose exhibits intrinsic low level syrB-inducing signal activity, D-fructose enhanced by about 10-fold the signal activities of the flavonoid glycosides at low concentrations (e.g. 10 [mu]M). This demonstrates that flavonoid glycosides, which represent a new class of phenolic plant signals sensed by P. s. syringae, are in sufficient quantities in the leaves of P. avium to activate phytotoxin synthesis.
[Show abstract][Hide abstract] ABSTRACT: Pseudomonas fluorescens 2-79 and P. aureofaciens 30-84 produce the antibiotic phenazine-1-carboxylic acid and suppress take-all, an important root disease of wheat caused by Gaeumannomyces graminis var. tritici. To determine whether the antibiotic is produced in situ, wheat seeds were treated with strain 2-79 or 30-84 or with phenazine-nonproducing mutants or were left untreated and then were sown in natural or steamed soil in the field or growth chamber. The antibiotic was isolated only from roots of wheat colonized by strain 2-79 or 30-84 in both growth chamber and field studies. No antibiotic was recovered from the roots of seedlings grown from seeds treated with phenazine-nonproducing mutants or left untreated. In natural soils, comparable amounts of antibiotic (27 to 43 ng/g of root with adhering soil) were recovered from roots colonized by strain 2-79 whether or not the pathogen was present. Roots of plants grown in steamed soil yielded larger bacterial populations and more antibiotic than roots from natural soils. In steamed and natural soils, roots from which the antibiotic was recovered had significantly less disease than roots with no antibiotic, indicating that suppression of take-all is related directly to the presence of the antibiotic in the rhizosphere.
Full-text · Article · May 1990 · Applied and Environmental Microbiology