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A molecular phylogeny of Phytophthora and related oomycetes.

Phylogenetic relationships among 50 Phytophthora species and between Phytophthora and other oomycetes were examined on the basis of the ITS sequences of genomic rDNA. Phytophthora grouped with Pythium, Peronospora, and Halophytophthora, distant from genera in the Saprolegniales. Albugo was intermediate between these two groups. Unlike Pythium, Phytophthora was essentially monophyletic, all but three species forming a cluster of eight clades. Two clades contained only species with nonpapillate sporangia. The other six clades included either papillate and semipapillate, or semipapillate and nonpapillate types, transcending traditional morphological groupings, which are evidently not natural assemblages. Peronospora was related to P. megakarya and P. palmivora and appears to be derived from a Phytophthora that has both lost the ability to produce zoospores and become an obligate biotroph. Three other Phytophthoras located some distance from the main Phytophthora-Peronospora cluster probably represent one or more additional genera.

DNA, Fungal↗

Are elicitins cryptograms in plant-Oomycete communications?

Stimulation of plant natural defenses is an important challenge in phytoprotection prospects. In that context, elicitins, which are small proteins secreted by Phytophthora and Pythium species, have been shown to induce a hypersensitive-like reaction in tobacco plants. Moreover, these plants become resistant to their pathogens, and thus this interaction constitutes an excellent model to investigate the signaling pathways leading to plant resistance. However, most plants are not reactive to elicitins, although they possess the functional signaling pathways involved in tobacco responses to elicitin. The understanding of factors involved in this reactivity is needed to develop agronomic applications. In this review, it is proposed that elicitins could interact with regulating cell wall proteins before they reach the plasma membrane. Consequently, the plant reactivity or nonreactivity status could result from the equilibrium reached during this interaction. The possibility of overexpressing the elicitins directly from genomic DNA in Pichia pastoris allows site-directed mutagenesis experiments and structure/function studies. The recent discovery of the sterol carrier activity of elicitins brings a new insight on their molecular activity. This constitutes a crucial property, since the formation of a sterol-elicitin complex is required to trigger the biological responses of tobacco cells and plants. Only the elicitins loaded with a sterol are able to bind to their plasmalemma receptor, which is assumed to be an allosteric calcium channel. Moreover, Phytophthora and Pythium do not synthesize the sterols required for their growth and their fructification, and elicitins may act as shuttles trapping the sterols from the host plants. Sequence analysis of elicitin genes from several Phytophthora species sheds unexpected light on the phylogenetic relationships among the genus, and suggests that the expression of elicitins is under tight regulatory control. Finally, general involvement of these lipid transfer proteins in the biology of Pythiaceae, and in plant defense responses, is discussed. A possible scheme for the coevolution between Phytophthora and tobacco plants is approached.

Algal Proteins↗

Ecophysiological Characterization of Rhizosphere Bacterial Communities at Different Root Locations and Plant Developmental Stages of Cucumber Grown on Rockwool.

Bacterial communities from the rhizosphere of cucumber were characterized with respect to growth rates and carbon source utilization, in order to develop a selection strategy for biocontrol agents against Pythium aphanidermatum. Rhizosphere samples were collected from different root regions (root tips, the root base, and the intermediate region where lateral roots emerge) and developmental stages (the seedling, vegetative, and generative stage) from plants cultivated on reused rockwool. By colony counts on 1/10 strength TSA on subsequent days after plating, percentages of fast- and slow-growing isolates (i.e., forming visible colonies within 2 days, or after 3 or more days, respectively) were determined for each rhizosphere sample. At all plant developmental stages, root tips had the highest percentages of fast growing isolates, and root bases the lowest. During plant growth, the relative amounts of slowly growing bacteria increased. Community-level carbon source utilization was determined for the different rhizosphere samples with Biolog GN plates. Principal component analysis showed that rhizosphere samples from different developmental stages and root locations had distinct carbon source utilization patterns. Communities from root tips of seedlings showed the highest utilization of several monosaccharides. Communities from tips and intermediate regions of plants in the vegetative stage utilized relatively many amino acids and several organic acids, and in the generative stage, more di- and polysaccharides were used. Root base samples scored low with respect to carbon source utilization, except for some disaccharides. From the different rhizosphere samples, 826 bacteria, randomly collected from 1/10 strength TSA plates, were screened on the utilization of 9 carbon sources. The 9 selected carbon sources were chosen because they are reported to occur in the rhizosphere, to be used by the zoospores of Pythium in the infection process, or appeared to be discriminant in the analysis of community-level carbon source utilization performed in this study. It appeared that monosaccharides (glucose and fucose), amino acids (alanine and asparagine), and organic acids (galacturonic, succinic, and linoleic acid) were used for growth mainly by bacteria from the root tips, and to a lesser extent from the intermediate region, of young plants. Disaccharides were predominantly utilized by isolates from plants in the vegetative stage. Overall, the results indicated that growth rates and carbon source utilization reflect the adaptation of bacteria to the rhizosphere environment. The possibility of using these characteristics to screen for rhizosphere competent biocontrol agents that compete for substrates with P. aphanidermatum is discussed.

Journal Article↗

Survival and ecological fitness of Pseudomonas fluorescens genetically engineered with dual biocontrol mechanisms.

The antibiotic 2,4-diacetylphloroglucinol (Phl) is produced by a range of naturally occurring fluorescent pseudomonads. One isolate, Pseudomonas fluorescens F113, protects pea plants from the pathogenic fungus Pythium ultimum by reducing the number of pathogenic lesions on plant roots, but with a concurrent reduction in the emergence of plants such as pea. The genes responsible for Phl production have been shown to be functionally conserved between the wild-type (wt) P. fluorescens strains F113 and Q2-87. In this study the genes from F113 were isolated using an optimized long PCR method and a 6.7-kb gene cluster inserted into the chromosome of the non-Phl-producing P. fluorescens strain SBW25 EeZY6KX. This strain is a lacZY, km(R) marked derivative of the wt SBW25 which effects biological control against the plant pathogen Pythium ultimum by competitive exclusion as a result of its strong rhizosphere-colonizing ability. We describe here the integration of the Phl antifungal and competitive exclusion mechanisms into a single strain, and the impact this has on survival and plant emergence in microcosms. The insertion of the Phl biosynthetic genes from the F113 into the SBW25 chromosome gave a Phl-producing transformant (strain Pa21) able to suppress P. ultimum through antibiotic production. The growth of Pa21 was not reduced in flask culture at 20 degrees C compared with its parent strain. When inoculated on pea seedlings, the strain containing the Phl operon behaved similarly to the SBW25 EeZY6KX parent but did not show the tendency of the wt Phl producer F113 to cause lower pea seed emergence. Pea roots inoculated with SBW25 EeZY6KX have significantly lower indigenous populations than with F113 and the control. This is indicative of this strain's strong colonising presence. Pa21, the Phl-modified strain, is able to exclude the resident population from roots to the same degree as the SBW25 EeZY6KX from which it is derived. This suggests that it has maintained its competitiveness around the root systems of plants even with the introduction of the Phl locus. Thus, strain Pa21 possesses the qualities necessary to provide effective integrated biocontrol, through maintaining both its wt trait of competitive exclusion on the plant roots, while also expressing the genes from the F113 biocontrol strain for Phl production. Interestingly, however, an additional beneficial trait appears to emerge with the strain Pa21's lowered survival competence compared with SBW25 EeZY6KX in the rhizosphere soil. With fears of the spread of genetically modified organisms and persistence in the soil, this trait may be of some ecological and commercial benefit and becomes a candidate for further investigation and possible exploitation.

Biodiversity↗

Ginsenosides stimulate the growth of soilborne pathogens of American ginseng.

Ginseng saponins (ginsenosides) were isolated from soil associated with the roots of commercially grown American ginseng (Panax quinquefolius L.), identified via LC-MS and quantified via analytical HPLC. The ginsenosides, including F(11), Rb(1), Rb(2), Rc, Rd, Re and Rg(1), represented between 0.02 and 0.098% (average 0.06%) of the mass of the soil collected from roots annually between 1999 and 2002. The same ginsenosides were also isolated from run-off of undisturbed plants grown in pots in a greenhouse using a root exudate trapping system. To investigate (1) whether these saponins could influence the growth of pythiaceous fungi pathogenic to ginseng, and (2) whether soil levels of ginsenosides were sufficient to account for any effects, bioassays were completed using a crude saponin extract and an ecologically relevant concentration of purified ginsenosides. Thus, when cultured on media containing crude saponins, the colony weight of both Phytophthora cactorum and Pythium irregulare was significantly greater than that of control, indicating a strong growth stimulation by ginsenosides. The growth of Pythium irregulare was also significantly stimulated after addition of an ecologically relevant, low concentration (i.e. 0.06%) of purified ginsenosides to culture medium. By contrast, growth of the saprotrophic fungus Trichoderma hamatum was slightly (but not significantly) inhibited under the same conditions. These results imply that ginsenosides can act as allelopathic stimulators of the growth of pythiaceous fungi in the rhizosphere, and this may contribute to the disease(s) of this crop.

Dose-Response Relationship, Drug↗

Use of non-pathogenic or hypovirulent fungal strains to protect plants against closely related fungal pathogens.

Nonpathogenic (avirulent), or low virulent (hypovirulent) strains are capable of colonizing infection site niches on the plants' surfaces and protecting susceptible plants against their respective pathogens. Such phenomena have been demonstrated for a considerable number of plant pathogens. The modes of protection differ among the nonpathogenic strains, and one strain can protect by more than one mechanism. Competition for infection sites, or for nutrients (such as carbon, iron) as well as induction of the host plant resistance, have been demonstrated for several pathogens such as Rhizoctonia spp., Fusarium spp. and Pythium spp. Mycoparasitism was shown for Pythium spp. Transmission of double stranded RNA mycoviruses from hypovirulent strains to virulent strains renders the virulent strains hypovirulent. Chestnut trees infected with the chestnut blight pathogen, Cryphonectria (Endothia) parasitica, recovered after inoculation with transmissible hypovirulent strains. Nonpathogenic strains of various fungi are potential candidates for development of biocontrol preparations. Some strains are already used in Agriculture.

Journal Article↗

Antifungal activity of 4-methyl-6-alkyl-2H-pyran-2-ones.

A number of 4-methyl-6-alkyl-alpha-pyrones were synthesized and characterized on the basis of 1H NMR and mass spectroscopy. These compounds were tested in vitro against pathogenic fungi, namely, Sclerotium rolfsii Saccardo, Rhizoctonia bataticola (Taub.) Butler, Pythium aphanidermatum (Edson) Fitz., Macrophomina phaseolina (Tassi), Pythium debaryanum (Hesse), and Rhizoctonia solani Nees. Lower homologues were less effective, whereas compounds such as 4-methyl-6-butyl-alpha-pyrone, 4-methyl-6-pentyl-alpha-pyrone, 4-methyl-6-hexyl-alpha-pyrone, and 4-methyl-6-heptyl-alpha-pyrone were found effective against all of the test fungi. They inhibited mycelial growth by approximately 50% (ED50) at 15-50 microg/mL. 4-Methyl-6-hexyl-alpha-pyrone, which was found most effective, was tested against S. rolfsii in a greenhouse at 1, 5, and 10% concentrations. The 10% aqueous emulsion of 4-methyl-6-hexyl-alpha-pyrone suppressed disease development in tomato by 90-93% as compared with the untreated infested soil in the greenhouse after 35 days of treatment.

Alkylation↗

Ecological basis for biocontrol of damping-off disease by pseudomonas fluorescens 54/96

Pseudomonas fluorescens 54/96, originally isolated from the rhizosphere of sugar beet, has been shown to be commercially effective in field trials for the suppression of a number of fungal diseases of seedlings. In vitro and microcosm-based assays revealed that both the timing and method of application of bacteria were important for effective control of Pythium ultimum, the causative agent of damping-off disease. Following transposon mutagenesis (Tn5lac), mutants deficient for the suppression of Pythium ultimum infections of peas were isolated. Three major classes of insertional mutants of Ps. fluorescens 54/96 were identified which either inhibited sporulation, reduced mycelial growth or affected the regulation of bacterial metabolic activity. Evaluation of the metabolic capability of pathogen and antagonist revealed evidence for direct competition, as both the fungus and bacterium had similar sole carbon source nutrient utilization profiles. Further comparisons of the activity of the transposon mutants indicated that although the mechanisms of disease control were multifactorial, the most significant factor was the prevention of rapid spore germination in the presence of pea seeds.

Journal Article↗

RpoN (sigma54) controls production of antifungal compounds and biocontrol activity in Pseudomonas fluorescens CHA0.

Pseudomonas fluorescens CHA0 is an effective biocontrol agent of root diseases caused by fungal pathogens. The strain produces the antibiotics 2,4-diacetylphloroglucinol (DAPG) and pyoluteorin (PLT) that make essential contributions to pathogen suppression. This study focused on the role of the sigma factor RpoN (sigma54) in regulation of antibiotic production and biocontrol activity in P. fluorescens. An rpoN in-frame-deletion mutant of CHAO had a delayed growth, was impaired in the utilization of several carbon and nitrogen sources, and was more sensitive to salt stress. The rpoN mutant was defective for flagella and displayed drastically reduced swimming and swarming motilities. Interestingly, the rpoN mutant showed a severalfold enhanced production of DAPG and expression of the biosynthetic gene phlA compared with the wild type and the mutant complemented with monocopy rpoN+. By contrast, loss of RpoN function resulted in markedly lowered PLT production and plt gene expression, suggesting that RpoN controls the balance of the two antibiotics in strain CHA0. In natural soil microcosms, the rpoN mutant was less effective in protecting cucumber from a root rot caused by Pythium ultimum. Remarkably, the mutant was not significantly impaired in its root colonization capacity, even at early stages of root infection by Pythium spp. Taken together, our results establish RpoN for the first time as a major regulator of biocontrol activity in Pseudomonas fluorescens.

Amino Acid Sequence↗

Cloning and Characterization of Aerobactin Biosynthesis Genes of the Biological Control Agent Enterobacter cloacae.

Five strains of Enterobacter cloacae that are biological control agents of Pythium damping-off diseases produced the hydroxamate siderophore aerobactin under iron-limiting conditions. Genes determining aerobactin biosynthesis of the biocontrol strain E. cloacae EcCT-501 were localized to a 12.3-kb region, which conferred aerobactin production to Escherichia coli DH5alpha. The aerobactin biosynthesis genes of E. cloacae hybridized to those of the pColV-K30 plasmid of E. coli, but restriction patterns of the aerobactin regions of pColV-K30 and E. cloacae differed. A derivative strain with a deletion in the aerobactin biosynthesis locus was as effective as strain EcCT-501 in biological control of Pythium damping-off of cucumber. Thus, aerobactin production did not contribute significantly to the biological control activity of EcCT-501 under the conditions of this study.

Journal Article↗

Secondary metabolite- and endochitinase-dependent antagonism toward plant-pathogenic microfungi of pseudomonas fluorescens isolates from sugar beet rhizosphere

Forty-seven isolates representing all biovars of Pseudomonas fluorescens (biovars I to VI) were collected from the rhizosphere of field-grown sugar beet plants to select candidate strains for biological control of preemergence damping-off disease. The isolates were tested for in vitro antagonism toward the plant-pathogenic microfungi Pythium ultimum and Rhizoctonia solani in three different plate test media. Mechanisms of fungal inhibition were elucidated by tracing secondary-metabolite production and cell wall-degrading enzyme activity in the same media. Most biovars expressed a specific mechanism of antagonism, as represented by a unique antibiotic or enzyme production in the media. A lipopeptide antibiotic, viscosinamide, was produced independently of medium composition by P. fluorescens bv. I, whereas the antibiotic 2, 4-diacetylphloroglucinol was observed only in glucose-rich medium and only in P. fluorescens bv. II/IV. Both pathogens were inhibited by the two antibiotics. Finally, in low-glucose medium, a cell wall-degrading endochitinase activity in P. fluorescens bv. I, III, and VI was the apparent mechanism of antagonism toward R. solani. The viscosinamide-producing DR54 isolate (bv. I) was shown to be an effective candidate for biological control, as tested in a pot experiment with sugar beet seedlings infested with Pythium ultimum. The assignment of different patterns of fungal antagonism to the biovars of P. fluorescens is discussed in relation to an improved selection protocol for candidate strains to be used in biological control.

Journal Article↗

Human vascular pythiosis: pathogenesis, diagnosis, and evidence-informed management.

SUMMARYHuman vascular pythiosis is a rare, life-threatening angioinvasive infection caused predominantly by the oomycete Pythium insidiosum. Unlike ocular or cutaneous pythiosis, the vascular form is defined by arterial wall invasion, long-segment thrombosis, limb ischemia, aneurysmal complications, and high risks of amputation and death. Although P. insidiosum is broadly distributed in freshwater environments, reported human vascular disease remains concentrated in Thailand, indicating that exposure alone is insufficient for disease development. Available evidence supports a multifactorial model centered on repeated freshwater exposure and host susceptibility, particularly hemoglobinopathies and iron dysregulation, while the rarity of comparable vascular disease elsewhere remains unexplained. Because vascular-specific mechanistic data are limited, evidence from animal pythiosis, nonvascular human disease, environmental studies, genomic analyses, and experimental models must be interpreted selectively and linked explicitly to vascular pathogenesis, diagnosis, or management. Infection usually begins at sites of skin disruption after freshwater exposure and progresses through soft tissue and perivascular planes into the arterial wall. Longitudinal intramural extension, rather than typical hematogenous dissemination, explains long-segment arterial occlusion, proximal progression, and relapse after incomplete source control. Diagnosis requires early suspicion, serology, vascular imaging, and tissue-based confirmation. Management depends on prompt and complete surgical removal of infected arterial tissue, supported by adjunctive protein-synthesis-inhibiting antibacterial therapy, particularly when residual disease is suspected. Conventional antifungals have limited activity, and Pythium antigen immunotherapy remains historically important but of uncertain independent benefit. Earlier recognition, better surgical-margin assessment, standardized diagnosis, and prospective treatment data are needed.

P. insidiosum↗

Chemical detection of microbial prey by bacterial predators.

A motile, predacious bacterium which degraded Pythium debaryanum was strongly attracted to substances released into the medium by the fungus. A nonpredacious bacterium was not attracted to these substances. The predator bacterium was specifically attracted to cellulose and its oligomers which are known to be components of the cell wall of Pythium. Ethanol inhibited chemotaxis of the bacterium without affecting either its motility or its ability to degrade cellulose. A second predacious bacterium was isolated for the alga, Skeletonema costatum. The role of chemoreception in the detection of microbial prey by bacterial predators in natural habitats is discussed.

Agar↗

Biological control in greenhouse systems.

The controlled environment of greenhouses, the high value of the crops, and the limited number of registered fungicides offer a unique niche for the biological control of plant diseases. During the past ten years, over 80 biocontrol products have been marketed worldwide. A large percentage of these have been developed for greenhouse crops. Products to control soilborne pathogens such as Sclerotinia, Pythium, Rhizoctonia and Fusarium include Coniothyrium minitans, species of Gliocladium, Trichoderma, Streptomyces, and Bacillus, and nonpathogenic Fusarium. Products containing Trichoderma, Ampelomyces quisqualis, Bacillus, and Ulocladium are being developed to control the primary foliar diseases, Botrytis and powdery mildew. The development of Pseudomonas for the control of Pythium diseases in hydroponics and Pseudozyma flocculosa for the control of powdery mildew by two Canadian research programs is presented. In the future, biological control of diseases in greenhouses could predominate over chemical pesticides, in the same way that biological control of greenhouse insects predominates in the United Kingdom. The limitations in formulation, registration, and commercialization are discussed, along with suggested future research priorities.

Canada↗

Identification of an elicitin gene cluster in Phytophthora cinnamomi.

Elicitins are a group of highly conserved proteins secreted by species of Phytophthora and a species of the related genus Pythium, Pythium vexans. Some of these proteins act as inducers of the necrotic hypersensitive-like response and the associated systemic acquired resistance phenomenon, in some species. We cloned and characterised the cinnamomin-beta and -alpha genes and two related elicitin genes from Phytophthora cinnamomi. These four open reading frames (ORFs) are clustered in tandem pairs. Two out of these four genes present homologies with the basic and acidic elicitin groups; but the two others encode, if expressed, elicitin isoforms exhibiting homologies with the class II of highly acidic elicitins.

Algal Proteins↗

Novel butyrolactones with antifungal activity produced by Pseudomonas aureofaciens strain 63-28.

The bacterium Pseudomonas aureofaciens 63-28 is antagonistic to several plant pathogenic fungi, including Pythium spp. The bacterium produced at least four antifungal metabolites active against Pythium ultimum and Phytophthora cryptogea when tested in culture for antifungal activity. Two of these compounds were identified as the novel butyrolactones (Z)-4-hydroxy-4-methyl-2-(1-hexenyl)-2-butenolide and (Z)-4-hydroxymethyl-2-(1-hexenyl)-2-butenolide, by using NMR and GC-MS. All compounds were different from other antibiotics produced by Pseudomonas spp., including pyoluteorin, pyrrolnitrin, and 2,4-diacetylphloroglucinol, as determined by HPLC. This is the first report of butyrolactones with antifungal activity produced by a saprophytic Pseudomonas spp.

4-Butyrolactone↗

Microbial contamination of advanced life support (ALS) systems poses a moderate threat to the long-term stability of space-based bioregenerative systems.

Microbial contamination of terrestrial hydroponic plant-growing systems provides an effective analogue for studying microbial contamination issues for space-based advanced life support (ALS) systems. If aggressive root or foliar pathogens are introduced into ALS hydroponic systems, severe epidemics are probable. Greater than 80% of the reported outbreaks of root pathogens in terrestrial hydroponic systems describe epidemics caused by fungal species of Fusarium, Phytophthora, and Pythium. However, it is likely that a comprehensive sanitation and quarantine program for space-based ALS modules will prevent contamination by Phytophthora and Pythium spp. because these pathogens are general soilborne. However, Fusarium spp. are typically airborne, can grow saprophytically on diverse substrates, and have been common contaminants of American spacecraft. If comprehensive sanitation and quarantine programs are established for space-based ALS modules, then the threat of pathogen introductions into these systems will be significantly mitigated. Microbial contamination studies in spacecraft over the last 30 years indicate that a high diversity of bacteria, fungi, and actinomycetes are commonly carried on board probably via clothing, equipment, air currents during spacecraft handling and loading, food, and the astronauts themselves. Species of Alternaria, Aspergillus, Botrytis, Candida, Cephalosporium, Cladosporium, Fusarium, Mucor, Penicillium, Phoma, and Trichoderma were the most prevalent fungi recovered, and species of Bacillus, Escherichia, Klebsiella, Micrococcus, Pseudomonas, Staphylococcus, and Streptococcus were the most prevalent bacteria recovered from spacecraft. Most of these genera contain species that have been reported as plant pathogens. Strict quarantine procedures were not effective in preventing contamination of spacecraft during these missions. Research must be initiated to better understand how microorganisms interact with plants and animals in microgravity environments because microbial contamination of spacecraft and ALS modules cannot be avoided. An integrated pest management (IPM) program likely will be developed for managing plant disease outbreaks in space-based ALS systems.

Ecological Systems, Closed↗

[Isolation and purification of ribosome-inactivating proteins from bitter melon seeds by ion exchange chromatographic columns in series].

An anion exchange chromatographic column (DEAE-650C) and a cation exchange chromatographic column (CM-650C) were connected in series on a perfusion chromatography workstation. The crude extract of bitter melon seeds flowed through the two columns and the unadsorbed fraction on the DEAE-650C column was then directly readsorbed on the CM-650C column. Two protein components with antifungal activity were eluted from the cation exchange chromatographic column by linear salt gradient. Both of them were found to be homogeneous by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and their relative molecular masses were estimated to be about 30 000. Their N-terminal amino acid sequences are DVSFRLSGADPRSYGMFI and DVNFDLSTATAK. All of the above suggested that they are alpha-momorcharin (alpha-MMC) and beta-momorcharin (beta-MMC), respectively, two type I ribosome-inactivating proteins (Rips) of bitter melon seeds. a-MMC shows antifungal activity against Fusarium oxysporum and Pythium aphanidermatum, while beta-MMC shows antifungal activity against Pythium aphanidermatum. But they are not against Sclerotium rolfsii. In the study, quantitative recoveries of alpha-MMC and beta-MMC were 13.8% and 8.0%, respectively, from decorticated seeds by ion exchange chromatographic columns in series.

Chromatography, Ion Exchange↗