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‣ Increase in Alfalfa Nodulation, Nitrogen Fixation, and Plant Growth by Specific DNA Amplification in Sinorhizobium meliloti

Castillo, Marcela; Flores, Margarita; Mavingui, Patrick; Martínez-Romero, Esperanza; Palacios, Rafael; Hernández, Georgina
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /06/1999 Português
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To improve symbiotic nitrogen fixation on alfalfa plants, Sinorhizobium meliloti strains containing different average copy numbers of a symbiotic DNA region were constructed by specific DNA amplification (SDA). A DNA fragment containing a regulatory gene (nodD1), the common nodulation genes (nodABC), and an operon essential for nitrogen fixation (nifN) from the nod regulon region of the symbiotic plasmid pSyma of S. meliloti was cloned into a plasmid unable to replicate in this organism. The plasmid then was integrated into the homologous DNA region of S. meliloti strains 41 and 1021, which resulted in a duplication of the symbiotic region. Sinorhizobium derivatives carrying further amplification were selected by growing the bacteria in increased concentrations of an antibiotic marker present in the integrated vector. Derivatives of strain 41 containing averages of 3 and 6 copies and a derivative of strain 1021 containing an average of 2.5 copies of the symbiotic region were obtained. In addition, the same region was introduced into both strains as a multicopy plasmid, yielding derivatives with an average of seven copies per cell. Nodulation, nitrogenase activity, plant nitrogen content, and plant growth were analyzed in alfalfa plants inoculated with the different strains. The copy number of the symbiotic region was critical in determining the plant phenotype. In the case of the strains with a moderate increase in copy number...

‣ Permeabilization of Fungal Membranes by Plant Defensins Inhibits Fungal Growth

Thevissen, Karin; Terras, Franky R. G.; Broekaert, Willem F.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /12/1999 Português
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We used an assay based on the uptake of SYTOX Green, an organic compound that fluoresces upon interaction with nucleic acids and penetrates cells with compromised plasma membranes, to investigate membrane permeabilization in fungi. Membrane permeabilization induced by plant defensins in Neurospora crassa was biphasic, depending on the plant defensin dose. At high defensin levels (10 to 40 μM), strong permeabilization was detected that could be strongly suppressed by cations in the medium. This permeabilization appears to rely on direct peptide-phospholipid interactions. At lower defensin levels (0.1 to 1 μM), a weaker, but more cation-resistant, permeabilization occurred at concentrations that correlated with the inhibition of fungal growth. Rs-AFP2(Y38G), an inactive variant of the plant defensin Rs-AFP2 from Raphanus sativus, failed to induce cation-resistant permeabilization in N. crassa. Dm-AMP1, a plant defensin from Dahlia merckii, induced cation-resistant membrane permeabilization in yeast (Saccharomyces cerevisiae) which correlated with its antifungal activity. However, Dm-AMP1 could not induce cation-resistant permeabilization in the Dm-AMP1-resistant S. cerevisiae mutant DM1, which has a drastically reduced capacity for binding Dm-AMP1. We think that cation-resistant permeabilization is binding site mediated and linked to the primary cause of fungal growth inhibition induced by plant defensins.

‣ Frequency and Biodiversity of 2,4-Diacetylphloroglucinol-Producing Bacteria Isolated from the Maize Rhizosphere at Different Stages of Plant Growth

Picard, C.; Di Cello, F.; Ventura, M.; Fani, R.; Guckert, A.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /03/2000 Português
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A Pseudomonas 2,4-diacetylphloroglucinol (DAPG)-producing population that occurred naturally on the roots, in rhizosphere soil of Zea mays and in the nonrhizosphere soil was investigated in order to assess the microbial diversity at five stages of plant growth. A total of 1,716 isolates were obtained, and 188 of these isolates were able to produce DAPG. DAPG producers were isolated at each stage of plant growth, indicating that the maize rhizosphere is colonized by natural DAPG producers throughout development. The frequency of DAPG producers was very low in the first stage of plant growth and increased over time. An analysis of the level of biodiversity of the DAPG producers at the species level was performed by comparing the AluI restriction patterns of the 16S ribosomal DNAs (rDNAs) amplified by PCR from 167 isolates. This comparison allowed us to cluster the isolates into four amplified rDNA restriction analysis (ARDRA) groups, and the main group (ARDRA group 1) contained 89.8% of the isolates. The diversity of the 150 isolates belonging to ARDRA group 1 was analyzed by the random amplified polymorphic DNA (RAPD) technique. An analysis of RAPD patterns by a molecular variance method revealed that there was a high level of genetic diversity in this population and that the genetic diversity was related to plant age. Finally...

‣ Effect of Field Inoculation with Sinorhizobium meliloti L33 on the Composition of Bacterial Communities in Rhizospheres of a Target Plant (Medicago sativa) and a Non-Target Plant (Chenopodium album)—Linking of 16S rRNA Gene-Based Single-Strand Conformation Polymorphism Community Profiles to the Diversity of Cultivated Bacteria

Schwieger, Frank; Tebbe, Christoph C.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /08/2000 Português
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Fourteen weeks after field release of luciferase gene-tagged Sinorhizobium meliloti L33 in field plots seeded with Medicago sativa, we found that the inoculant also occurred in bulk soil from noninoculated control plots. In rhizospheres of M. sativa plants, S. meliloti L33 could be detected in noninoculated plots 12 weeks after inoculation, indicating that growth in the rhizosphere preceded spread into bulk soil. To determine whether inoculation affected bacterial diversity, 1,119 bacteria were isolated from the rhizospheres of M. sativa and Chenopodium album, which was the dominant weed in the field plots. Amplified ribosomal DNA restriction analysis (ARDRA) revealed plant-specific fragment size frequencies. Dominant ARDRA groups were identified by 16S rRNA gene nucleotide sequencing. Database comparisons indicated that the rhizospheres contained members of the Proteobacteria (α, β, and γ subgroups), members of the Cytophaga-Flavobacterium group, and gram-positive bacteria with high G+C DNA contents. The levels of many groups were affected by the plant species and, in the case of M. sativa, by inoculation. The most abundant isolates were related to Variovorax sp., Arthrobacter ramosus, and Acinetobacter calcoaceticus. In the rhizosphere of M. sativa...

‣ Plant Genome Complexity May Be a Factor Limiting In Situ the Transfer of Transgenic Plant Genes to the Phytopathogen Ralstonia solanacearum

Bertolla, Franck; Pepin, Regis; Passelegue-Robe, Eugenie; Paget, Eric; Simkin, Andrew; Nesme, Xavier; Simonet, Pascal
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /09/2000 Português
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The development of natural competence by bacteria in situ is considered one of the main factors limiting transformation-mediated gene exchanges in the environment. Ralstonia solanacearum is a plant pathogen that is also a naturally transformable bacterium that can develop the competence state during infection of its host. We have attempted to determine whether this bacterium could become the recipient of plant genes. We initially demonstrated that plant DNA was released close to the infecting bacteria. We constructed and tested various combinations of transgenic plants and recipient bacteria to show that the effectiveness of such transfers was directly related to the ratio of the complexity of the plant genome to the number of copies of the transgene.

‣ Management of Indigenous Plant-Microbe Symbioses Aids Restoration of Desertified Ecosystems

Requena, Natalia; Perez-Solis, Estefania; Azcón-Aguilar, Concepción; Jeffries, Peter; Barea, José-Miguel
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /02/2001 Português
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Disturbance of natural plant communities is the first visible indication of a desertification process, but damage to physical, chemical, and biological soil properties is known to occur simultaneously. Such soil degradation limits reestablishment of the natural plant cover. In particular, desertification causes disturbance of plant-microbe symbioses which are a critical ecological factor in helping further plant growth in degraded ecosystems. Here we demonstrate, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality. The dual symbiosis increased the soil nitrogen (N) content, organic matter, and hydrostable soil aggregates and enhanced N transfer from N-fixing to nonfixing species associated within the natural succession. We conclude that the introduction of target indigenous species of plants associated with a managed community of microbial symbionts is a successful biotechnological tool to aid the recovery of desertified ecosystems.

‣ The Viable But Nonculturable State of Ralstonia solanacearum May Be Involved in Long-Term Survival and Plant Infection

Grey, Brian E.; Steck, Todd R.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /09/2001 Português
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The role of the dormant-like viable but nonculturable (VBNC) condition in the etiology of bacterial infection was examined using a plant system. The plant-pathogenic bacterium Ralstonia solanacearum was first shown to enter into the VBNC state both in response to cupric sulfate when in a saline solution and when placed in autoclaved soil. To determine if the VBNC condition is related to pathogenesis, the physiological status of bacteria recovered from different regions of inoculated tomato plants was determined at different stages of infection. The fraction of in planta bacteria that were VBNC increased during infection and became greater than 99% by the late stage of disease. The possibility that soil-dwelling VBNC bacteria may resuscitate and infect plants was also examined. When tomato seeds were germinated in sterile soil that contained VBNC but no detectable culturable forms of R. solanacearum cells, resuscitation was observed to occur in soil adjacent to plant roots; these resuscitated bacteria were able to infect plants. This is the first report of R. solanacearum entering the VBNC state and of resuscitation of any VBNC plant-pathogenic bacteria and provides evidence that the VBNC state may be involved in explaining the persistent nature of some infections.

‣ Bacterial Conjugation Protein MobA Mediates Integration of Complex DNA Structures into Plant Cells

Bravo-Angel, Ana María; Gloeckler, Véronique; Hohn, Barbara; Tinland, Bruno
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /09/1999 Português
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Agrobacterium tumefaciens transfers T-DNA to plant cells, where it integrates into the genome, a property that is ensured by bacterial proteins VirD2 and VirE2. Under natural conditions, the protein MobA mobilizes its encoding plasmid, RSF1010, between different bacteria. A detailed analysis of MobA-mediated DNA mobilization by Agrobacterium to plants was performed. We compared the ability of MobA to transfer DNA and integrate it into the plant genome to that of pilot protein VirD2. MobA was found to be about 100-fold less efficient than VirD2 in conducting the DNA from the pTi plasmid to the plant cell nucleus. However, interestingly, DNAs transferred by the two proteins were integrated into the plant cell genome with similar efficiencies. In contrast, most of the integrated DNA copies transferred from a MobA-containing strain were truncated at the 5′ end. Isolation and analysis of the most conserved 5′ ends revealed patterns which resulted from the illegitimate integration of one transferred DNA within another. These complex integration patterns indicate a specific deficiency in MobA. The data conform to a model according to which efficiency of T-DNA integration is determined by plant enzymes and integrity is determined by bacterial proteins.

‣ Comparison of Soil Bacterial Communities in Rhizospheres of Three Plant Species and the Interspaces in an Arid Grassland

Kuske, Cheryl R.; Ticknor, Lawrence O.; Miller, Mark E.; Dunbar, John M.; Davis, Jody A.; Barns, Susan M.; Belnap, Jayne
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /04/2002 Português
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Soil bacteria are important contributors to primary productivity and nutrient cycling in arid land ecosystems, and their populations may be greatly affected by changes in environmental conditions. In parallel studies, the composition of the total bacterial community and of members of the Acidobacterium division were assessed in arid grassland soils using terminal restriction fragment length polymorphism (TRF, also known as T-RFLP) analysis of 16S rRNA genes amplified from soil DNA. Bacterial communities associated with the rhizospheres of the native bunchgrasses Stipa hymenoides and Hilaria jamesii, the invading annual grass Bromus tectorum, and the interspaces colonized by cyanobacterial soil crusts were compared at three depths. When used in a replicated field-scale study, TRF analysis was useful for identifying broad-scale, consistent differences in the bacterial communities in different soil locations, over the natural microscale heterogeneity of the soil. The compositions of the total bacterial community and Acidobacterium division in the soil crust interspaces were significantly different from those of the plant rhizospheres. Major differences were also observed in the rhizospheres of the three plant species and were most apparent with analysis of the Acidobacterium division. The total bacterial community and the Acidobacterium division bacteria were affected by soil depth in both the interspaces and plant rhizospheres. This study provides a baseline for monitoring bacterial community structure and dynamics with changes in plant cover and environmental conditions in the arid grasslands.

‣ Protection of Tomato Seedlings against Infection by Pseudomonas syringae pv. Tomato by Using the Plant Growth-Promoting Bacterium Azospirillum brasilense†

Bashan, Yoav; de-Bashan, Luz E.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /06/2002 Português
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Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (107 versus 105 CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (107 versus 106 CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (105 to 106 CFU/g [dry weight] of root). However...

‣ Role of Pseudomonas putida Indoleacetic Acid in Development of the Host Plant Root System

Patten, Cheryl L.; Glick, Bernard R.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /08/2002 Português
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Many plant-associated bacteria synthesize the phytohormone indoleacetic acid (IAA). While IAA produced by phytopathogenic bacteria, mainly by the indoleacetamide pathway, has been implicated in the induction of plant tumors, it is not clear whether IAA synthesized by beneficial bacteria, usually via the indolepyruvic acid pathway, is involved in plant growth promotion. To determine whether bacterial IAA enhances root development in host plants, the ipdc gene that encodes indolepyruvate decarboxylase, a key enzyme in the indolepyruvic acid pathway, was isolated from the plant growth-promoting bacterium Pseudomonas putida GR12-2 and an IAA-deficient mutant constructed by insertional mutagenesis. The canola seedling primary roots from seeds treated with wild-type P. putida GR12-2 were on average 35 to 50% longer than the roots from seeds treated with the IAA-deficient mutant and the roots from uninoculated seeds. In addition, exposing mung bean cuttings to high levels of IAA by soaking them in a suspension of the wild-type strain stimulated the formation of many, very small, adventitious roots. Formation of fewer roots was stimulated by treatment with the IAA-deficient mutant. These results suggest that bacterial IAA plays a major role in the development of the host plant root system.

‣ The Pseudomonas syringae HopPtoV Protein Is Secreted in Culture and Translocated into Plant Cells via the Type III Protein Secretion System in a Manner Dependent on the ShcV Type III Chaperone

Wehling, Misty D.; Guo, Ming; Fu, Zheng Qing; Alfano, James R.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /06/2004 Português
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The bacterial plant pathogen Pseudomonas syringae depends on a type III protein secretion system and the effector proteins that it translocates into plant cells to cause disease and to elicit the defense-associated hypersensitive response on resistant plants. The availability of the P. syringae pv. tomato DC3000 genome sequence has resulted in the identification of many novel effectors. We identified the hopPtoV effector gene on the basis of its location next to a candidate type III chaperone (TTC) gene, shcV, and within a pathogenicity island in the DC3000 chromosome. A DC3000 mutant lacking ShcV was unable to secrete detectable amounts of HopPtoV into culture supernatants or translocate HopPtoV into plant cells, based on an assay that tested whether HopPtoV-AvrRpt2 fusions were delivered into plant cells. Coimmunoprecipitation and Saccharomyces cerevisiae two-hybrid experiments showed that ShcV and HopPtoV interact directly with each other. The ShcV binding site was delimited to an N-terminal region of HopPtoV between amino acids 76 and 125 of the 391-residue full-length protein. Our results demonstrate that ShcV is a TTC for the HopPtoV effector. DC3000 overexpressing ShcV and HopPtoV and DC3000 mutants lacking either HopPtoV or both ShcV and HopPtoV were not significantly impaired in disease symptoms or bacterial multiplication in planta...

‣ Impact of Plant Species and Site on Rhizosphere-Associated Fungi Antagonistic to Verticillium dahliae Kleb.

Berg, Gabriele; Zachow, Christin; Lottmann, Jana; Götz, Monika; Costa, Rodrigo; Smalla, Kornelia
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /08/2005 Português
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Fungi with antagonistic activity toward plant pathogens play an essential role in plant growth and health. To analyze the effects of the plant species and the site on the abundance and composition of fungi with antagonistic activity toward Verticillium dahliae, fungi were isolated from oilseed rape and strawberry rhizosphere and bulk soil from three different locations in Germany over two growing seasons. A total of 4,320 microfungi screened for in vitro antagonism toward Verticillium resulted in 911 active isolates. This high proportion of fungi antagonistic toward the pathogen V. dahliae was found for bulk and rhizosphere soil at all sites. A plant- and site-dependent specificity of the composition of antagonistic morphotypes and their genotypic diversity was found. The strawberry rhizosphere was characterized by preferential occurrence of Penicillium and Paecilomyces isolates and low numbers of morphotypes (n = 31) and species (n = 13), while Monographella isolates were most frequently obtained from the rhizosphere of oilseed rape, for which higher numbers of morphotypes (n = 41) and species (n = 17) were found. Trichoderma strains displayed high diversity in all soils, but a high degree of plant specificity was shown by BOX-PCR fingerprints. The diversity of rhizosphere-associated antagonists was lower than that of antagonists in bulk soil...

‣ Paenibacillus polymyxa Invades Plant Roots and Forms Biofilms

Timmusk, Salme; Grantcharova, Nina; Wagner, E. Gerhart H.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /11/2005 Português
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Paenibacillus polymyxa is a plant growth-promoting rhizobacterium with a broad host range, but so far the use of this organism as a biocontrol agent has not been very efficient. In previous work we showed that this bacterium protects Arabidopsis thaliana against pathogens and abiotic stress (S. Timmusk and E. G. H. Wagner, Mol. Plant-Microbe Interact. 12:951-959, 1999; S. Timmusk, P. van West, N. A. R. Gow, and E. G. H. Wagner, p. 1-28, in Mechanism of action of the plant growth promoting bacterium Paenibacillus polymyxa, 2003). Here, we studied colonization of plant roots by a natural isolate of P. polymyxa which had been tagged with a plasmid-borne gfp gene. Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms. Accumulation of bacteria was observed in the intercellular spaces outside the vascular cylinder. Systemic spreading did not occur, as indicated by the absence of bacteria in aerial tissues. Studies were performed in both a gnotobiotic system and a soil system. The fact that similar observations were made in both systems suggests that colonization by this bacterium can be studied in a more defined system. Problems associated with green fluorescent protein tagging of natural isolates and deleterious effects of the plant growth-promoting bacteria are discussed.

‣ Detection of Plant-Modulated Alterations in Antifungal Gene Expression in Pseudomonas fluorescens CHA0 on Roots by Flow Cytometry▿

de Werra, Patrice; Baehler, Eric; Huser, Aurélie; Keel, Christoph; Maurhofer, Monika
Fonte: American Society for Microbiology (ASM) Publicador: American Society for Microbiology (ASM)
Tipo: Artigo de Revista Científica
Português
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The biocontrol activity of the root-colonizing Pseudomonas fluorescens strain CHA0 is largely determined by the production of antifungal metabolites, especially 2,4-diacetylphloroglucinol. The expression of these metabolites depends on abiotic and biotic environmental factors, in particular, elements present in the rhizosphere. In this study, we have developed a new method for the in situ analysis of antifungal gene expression using flow cytometry combined with green fluorescent protein (GFP)-based reporter fusions to the phlA and prnA genes essential for the production of the antifungal compounds 2,4-diacetylphloroglucinol and pyrrolnitrin, respectively, in strain CHA0. Expression of phlA-gfp and prnA-gfp in CHA0 cells harvested from the rhizosphere of a set of plant species as well as from the roots of healthy, leaf pathogen-attacked, and physically stressed plants were analyzed using a FACSCalibur. After subtraction of background fluorescence emitted by plant-derived particles and CHA0 cells not carrying the gfp reporters, the average gene expression per bacterial cell could be calculated. Levels of phlA and prnA expression varied significantly in the rhizospheres of different plant species. Physical stress and leaf pathogen infection lowered phlA expression levels in the rhizosphere of cucumber. Our results demonstrate that the newly developed approach is suitable to monitor differences in levels of antifungal gene expression in response to various plant-derived factors. An advantage of the method is that it allows quantification of bacterial gene expression in rhizosphere populations at a single-cell level. To our best knowledge...

‣ Genome of the Actinomycete Plant Pathogen Clavibacter michiganensis subsp. sepedonicus Suggests Recent Niche Adaptation▿ †

Bentley, Stephen D.; Corton, Craig; Brown, Susan E.; Barron, Andrew; Clark, Louise; Doggett, Jon; Harris, Barbara; Ormond, Doug; Quail, Michael A.; May, Georgiana; Francis, David; Knudson, Dennis; Parkhill, Julian; Ishimaru, Carol A.
Fonte: American Society for Microbiology (ASM) Publicador: American Society for Microbiology (ASM)
Tipo: Artigo de Revista Científica
Português
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Clavibacter michiganensis subsp. sepedonicus is a plant-pathogenic bacterium and the causative agent of bacterial ring rot, a devastating agricultural disease under strict quarantine control and zero tolerance in the seed potato industry. This organism appears to be largely restricted to an endophytic lifestyle, proliferating within plant tissues and unable to persist in the absence of plant material. Analysis of the genome sequence of C. michiganensis subsp. sepedonicus and comparison with the genome sequences of related plant pathogens revealed a dramatic recent evolutionary history. The genome contains 106 insertion sequence elements, which appear to have been active in extensive rearrangement of the chromosome compared to that of Clavibacter michiganensis subsp. michiganensis. There are 110 pseudogenes with overrepresentation in functions associated with carbohydrate metabolism, transcriptional regulation, and pathogenicity. Genome comparisons also indicated that there is substantial gene content diversity within the species, probably due to differential gene acquisition and loss. These genomic features and evolutionary dating suggest that there was recent adaptation for life in a restricted niche where nutrient diversity and perhaps competition are low...

‣ Naturally Occurring Nonpathogenic Isolates of the Plant Pathogen Pseudomonas syringae Lack a Type III Secretion System and Effector Gene Orthologues▿ †

Mohr, Toni J.; Liu, Haijie; Yan, Shuangchun; Morris, Cindy E.; Castillo, José A.; Jelenska, Joanna; Vinatzer, Boris A.
Fonte: American Society for Microbiology (ASM) Publicador: American Society for Microbiology (ASM)
Tipo: Artigo de Revista Científica
Português
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Pseudomonas syringae causes plant diseases, and the main virulence mechanism is a type III secretion system (T3SS) that translocates dozens of effector proteins into plant cells. Here we report the existence of a subgroup of P. syringae isolates that do not cause disease on any plant species tested. This group is monophyletic and most likely evolved from a pathogenic P. syringae ancestor through loss of the T3SS. In the nonpathogenic isolate P. syringae 508 the genomic region that in pathogenic P. syringae strains contains the hrp-hrc cluster coding for the T3SS and flanking effector genes is absent. P. syringae 508 was also surveyed for the presence of effector orthologues from the closely related pathogenic strain P. syringae pv. syringae B728a, but none were detected. The absence of the hrp-hrc cluster and effector orthologues was confirmed for other nonpathogenic isolates. Using the AvrRpt2 effector as reporter revealed the inability of P. syringae 508 to translocate effectors into plant cells. Adding a plasmid-encoded T3SS and the P. syringae pv. syringae 61 effector gene hopA1 increased in planta growth almost 10-fold. This suggests that P. syringae 508 supplemented with a T3SS could be used to determine functions of individual effectors in the context of a plant infection...

‣ Role of Plant Residues in Determining Temporal Patterns of the Activity, Size, and Structure of Nitrate Reducer Communities in Soil ▿ †

Chèneby, D.; Bru, D.; Pascault, N.; Maron, P. A.; Ranjard, L.; Philippot, L.
Fonte: American Society for Microbiology (ASM) Publicador: American Society for Microbiology (ASM)
Tipo: Artigo de Revista Científica
Português
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The incorporation of plant residues into soil not only represents an opportunity to limit soil organic matter depletion resulting from cultivation but also provides a valuable source of nutrients such as nitrogen. However, the consequences of plant residue addition on soil microbial communities involved in biochemical cycles other than the carbon cycle are poorly understood. In this study, we investigated the responses of one N-cycling microbial community, the nitrate reducers, to wheat, rape, and alfalfa residues for 11 months after incorporation into soil in a field experiment. A 20- to 27-fold increase in potential nitrate reduction activity was observed for residue-amended plots compared to the nonamended plots during the first week. This stimulating effect of residues on the activity of the nitrate-reducing community rapidly decreased but remained significant over 11 months. During this period, our results suggest that the potential nitrate reduction activity was regulated by both carbon availability and temperature. The presence of residues also had a significant effect on the abundance of nitrate reducers estimated by quantitative PCR of the narG and napA genes, encoding the membrane-bound and periplasmic nitrate reductases...

‣ Bacterial Subfamily of LuxR Regulators That Respond to Plant Compounds▿†

Subramoni, Sujatha; Gonzalez, Juan F.; Johnson, Aaron; Péchy-Tarr, Maria; Rochat, Laurène; Paulsen, Ian; Loper, Joyce E.; Keel, Christoph; Venturi, Vittorio
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /07/2011 Português
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Pseudomonas fluorescens are rhizobacteria known for their biocontrol properties. Several antimicrobial functions are crucial for this process, and the experiments described here investigate the modulation of their expression during the plant-bacterium interaction. The role of a LuxR family regulator in interkingdom signaling has been investigated using genome-scale transcriptome analysis, gene promoter studies in vivo and in vitro, biocontrol assays, and response to plant compounds. PsoR, a LuxR solo or orphan regulator of P. fluorescens, was identified. PsoR is solubilized and activates a lux-box-containing promoter only in the presence of macerated plants, suggesting the presence of a plant molecule(s) that most likely binds to PsoR. Gene expression profiles revealed that genes involved in the inhibition of plant pathogens were affected by PsoR, including a chitinase gene, iron metabolism genes, and biosynthetic genes of antifungal compounds. 2,4-Diacetylphloroglucinol production is PsoR dependent both in vitro and in vivo. psoR mutants were significantly reduced for their ability to protect wheat plants from root rot, and damping-off caused by Pythium ultimum infection. PsoR most likely senses a molecule(s) in the plant and modulates expression of genes that have a role in biocontrol. PsoR and related proteins form a subfamily of LuxR family regulators in plant-associated bacteria.

‣ Forward Genetic In Planta Screen for Identification of Plant-Protective Traits of Sphingomonas sp. Strain Fr1 against Pseudomonas syringae DC3000

Vogel, Christine; Innerebner, Gerd; Zingg, Judith; Guder, Jan; Vorholt, Julia A.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /08/2012 Português
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Sphingomonas sp. strain Fr1 has recently been shown to protect Arabidopsis thaliana against the bacterial leaf pathogen Pseudomonas syringae DC3000. Here, we describe a forward genetic in planta screen to identify genes in Sphingomonas sp. Fr1 necessary for this effect. About 5,000 Sphingomonas sp. Fr1 mini-Tn5 mutants were assayed for a defect in plant protection against a luxCDABE-tagged P. syringae DC3000 derivative in a space-saving 24-well plate system. The bioluminescence of the pathogen was used as the indicator of pathogen proliferation and allowed for the identification of Sphingomonas sp. Fr1 mutants that had lost the ability to restrict pathogen growth before disease symptoms were visible. Potential candidates were validated using the same miniaturized experimental system. Of these mutants, 10 were confirmed as plant protection defective yet colonization competent. The mutants were subsequently evaluated in a previously described standard microbox system, and plants showed enhanced disease phenotypes after pathogen infection relative to those inoculated with the parental strain as a control. However, the disease severities were lower than those observed for control plants that were grown axenically prior to pathogen challenge...