Effect of N concentration and N source on root colonization by Pseudomonas fluorescens 2-79RLI

Wheat (Triticum aestivum L.) was grown in nutrient solution with low or high N supply (NH₄NO₃ as N source). To further evaluate the influence of N form and its interaction with the nutrient solution pH, wheat plants were grown with NH ₄ ⁺ or NO ₃ ^- either in an conventional nutrient solution or in a nutrient solution in which the pH was maintained at pH 6.5 using a pH-stat system. The nutrient solution was inoculated with Pseudomonas fluorescens 2-79RLI, a genetically modified bacterium that contains lux genes activated by a ribosomal promoter. Cell numbers and physiological status of P. fluorescens 2-79RLI (length of the lag phase of bioluminescence) in the rhizosphere were determined at the root tip and in the lateral root zone. Nitrogen deficiency decreased both plant growth and root colonization by P. fluorescens 2-79RLI at the root tip while it had no effect on root colonization in the lateral root zone. The physiological status of P. fluorescens 2-79RLI was not affected by nitrogen deficiency. Ammonium nutrition increased root colonization by P. fluorescens 2-79RLI at the root tip and in the lateral root zone when the pH of the nutrient solution was allowed to change according to the N form provided. Under these conditions, the physiological status of P. fluorescens 2-79RLI was higher in the lateral root zone than at the root tip. In contrast, N source had no effect on root colonization or physiological status of P. fluorescens 2-79RLI in the nutrient solution maintained at pH 6.5. It is concluded that the stimulation of root colonization by NH ₄ ⁺ in the nutrient solution, not maintained at a constant pH, may be due to increased leakage of solutes into the rhizosphere as a result of impaired exudate retention by high H⁺ concentration in the rhizosphere or the apoplast. This revised version was published online in June 2006 with corrections to the Cover Date.     

Iron Stress and Pyoverdin Production by a Fluorescent Pseudomonad in the Rhizosphere of White Lupine (Lupinus albus L.) and Barley (Hordeum vulgare L.)

Induction of high-affinity iron transport during root colonization by Pseudomonas fluorescens Pf-5 (pvd-inaZ) was examined in lupine and barley growing in microcosms. P. fluorescens Pf-5 (pvd-inaZ) contains a plasmid carrying pvd-inaZ; thus, in this strain, ice nucleation activity is regulated by pyoverdin production. Lupine or barley plants were grown for 18 or 8 days, respectively, in soil amended with 2% calcium carbonate and inoculated with P. fluorescens Pf-5 (pvd-inaZ) at a density of 4 x 10(sup8) CFU g (dry weight) of soil(sup-1). A filter paper blotting technique was used to sample cells from the rhizosphere in different root zones, and then the cells were resuspended for enumeration and measurement of ice nucleation activity. The population density of P. fluorescens Pf-5 (pvd-inaZ) in the rhizosphere decreased by one order of magnitude in both lupine and barley over time. The ice nucleation activity ranged from -3.4 to -3.0 log ice nuclei CFU(sup-1) for lupine and -3.0 to -2.8 log ice nuclei CFU(sup-1) for barley, was similar in all root zones, and did not change over time. An in vitro experiment was conducted to determine the relationship between ice nucleation activity and pyoverdin production in P. fluorescens Pf-5 (pvd-inaZ). An ice nucleation activity of approximately -3.0 log ice nuclei CFU(sup-1) was measured in the in vitro experiment at 25 to 50 (mu)M FeCl(inf3). By using the regression between ice nucleation activity and pyoverdin production determined in vitro and assuming a P. fluorescens Pf-5 (pvd-inaZ) population density of 10(sup8) CFU g of root(sup-1), the maximum possible pyoverdin accumulation by P. fluorescens Pf-5 (pvd-inaZ) in the rhizosphere was estimated to be 0.5 and 0.8 nmol g of root(sup-1) for lupine and barley, respectively. The low ice nucleation activity measured in the rhizosphere suggests that nutritional competition for iron in the rhizosphere may not be a major factor influencing root colonization by P. fluorescens Pf-5 (pvd-inaZ).     

Relative importance of fluorescent siderophores and other factors in biological control of Gaeumannomyces graminis var. tritici by Pseudomonas fluorescens 2-79 and M4-80R.

Pseudomonas fluorescens 2-79 suppresses take-all, a major root disease of wheat caused by Gaeumannomyces graminis var. tritici. The bacteria produce an antibiotic, phenazine-1-carboxylic acid (PCA), and a fluorescent pyoverdin siderophore. Previous studies have established that PCA has an important role in the biological control of take-all but that antibiotic production does not account fully for the suppressiveness of the strain. To define the role of the pyoverdin siderophore more precisely, mutants deficient in production of the antibiotic, the siderophore, or both factors were constructed and compared with the parental strain for control of take-all on wheat roots. In all cases, strains that produced PCA were more suppressive than those that did not, and pyoverdin-deficient mutant derivatives controlled take-all as effectively as their respective fluorescent parental strains. Thus, the phenazine antibiotic was the dominant factor in disease suppression and the fluorescent siderophore had little or no role. The siderophore also was of minor importance in a second strain, P. fluorescens M4-80R, that does not produce PCA. Strains 2-79 and M4-80R both produced substances distinct from the pyoverdin siderophore that were responsible for fungal inhibition in vitro under iron limitation, but these substances also had, at most, a minor role in disease suppression in situ.     

Relative importance of fluorescent siderophores and other factors in biological control of Gaeumannomyces graminis var. tritici by Pseudomonas fluorescens 2-79 and M4-80R.

Pseudomonas fluorescens 2-79 suppresses take-all, a major root disease of wheat caused by Gaeumannomyces graminis var. tritici. The bacteria produce an antibiotic, phenazine-1-carboxylic acid (PCA), and a fluorescent pyoverdin siderophore. Previous studies have established that PCA has an important role in the biological control of take-all but that antibiotic production does not account fully for the suppressiveness of the strain. To define the role of the pyoverdin siderophore more precisely, mutants deficient in production of the antibiotic, the siderophore, or both factors were constructed and compared with the parental strain for control of take-all on wheat roots. In all cases, strains that produced PCA were more suppressive than those that did not, and pyoverdin-deficient mutant derivatives controlled take-all as effectively as their respective fluorescent parental strains. Thus, the phenazine antibiotic was the dominant factor in disease suppression and the fluorescent siderophore had little or no role. The siderophore also was of minor importance in a second strain, P. fluorescens M4-80R, that does not produce PCA. Strains 2-79 and M4-80R both produced substances distinct from the pyoverdin siderophore that were responsible for fungal inhibition in vitro under iron limitation, but these substances also had, at most, a minor role in disease suppression in situ.     

The Fe Chelator Proferrorosamine A Is Essential for the Siderophore-Mediated Uptake of Iron by Pseudomonas roseus fluorescens

Pseudomonas roseus fluorescens produces, besides the Fe chelator proferrorosamine A, Fe -chelating compounds, called siderophores. The production of proferrorosamine A and siderophores by P. roseus fluorescens appears to be controlled in a similar way by the concentration of available iron and by the concentration of dissolved oxygen. The higher the concentration of iron available for the microorganism, the lower the production of both chelating compounds. However, the production of siderophores was much more sensitive to iron availability than was proferrorosamine A production. Proferrorosamine A and siderophores were only produced in minimal medium C if the concentration of dissolved oxygen ranged from 4.5 to 2.0 ppm. At higher or lower concentrations, none of the iron-chelating compounds were produced. Furthermore, it has been shown that proferrorosamine-negative Tn5 mutants of P. roseus fluorescens were able to form siderophores only under iron-limiting conditions when proferrorosamine A was added to the medium. Our data suggest that proferrorosamine A production is essential for siderophore synthesis by P. roseus fluorescens; the production of siderophores occurred only when proferrorosamine A was present.     

Siderophore production by using free and immobilized cells of two pseudomonads cultivated in a medium enriched with Fe and/or toxic metals (Cr, Hg, Pb)

Pseudomonads are serious candidates for siderophore production applied to toxic metal (TM) solubilization. The bioaugmentation of contaminated soils by these TM-solubilizing bacteria combined with phytoextraction is an emerging clean-up technology. Unfortunately, siderophore synthesis may be drastically reduced by soluble iron in soils and bacteria can suffer from TM toxicity. In this study, we compared siderophore production by Pseudomonas aeruginosa and Pseudomonas fluorescens by using free and immobilized cells in Ca-alginate beads incubated in a medium containing Fe and/or TM (mixture of Cr, Hg, and Pb in concentrations which represented the soluble fraction of a contaminated agricultural soil). Free cell growth was stimulated by Fe, whatever the microorganism, the inoculum size and the presence or not of TM might have been. P. aeruginosa was less sensitive to TM than P. fluorescens. By comparison with free cells, immobilization with the high inoculum size showed less sensitivity to TM most probably because of lower metal diffusion in beads. Indeed, a maximum of 99.1% of Cr, 57.4% of Hg, and 99.6% of Pb were adsorbed onto beads. The addition of iron in the culture medium reduced significantly siderophore production of free cells while it led only to a low decrease with their immobilized counterparts, in particular with P. aeruginosa. In culture medium enriched with Fe and/or TM, siderophore-specific production of immobilized cells was higher than for free cells.     

Involvement of secondary metabolites and extracellular lytic enzymes produced by Pseudomonas fluorescens in inhibition of Rhizoctonia solani, the rice sheath blight pathogen

Fourteen strains of Pseudomonas fluorescens isolated from rhizosphere soil of rice were tested for their antagonistic effect towards Rhizoctonia solani, the rice sheath blight fungus. Among them, PfMDU2 was the most effective in inhibiting mycelial growth of R. solani in vitro. Production of chitinase, beta-1,3-glucanase, siderophores, salicylic acid (SA) and hydrogen cyanide (HCN) by P. fluorescens strains was evaluated. The highest beta-1,3-glucanase activity, siderophore production, SA production and HCN production were recorded with PfMDU2. A significant relationship between the antagonistic potential of P. fluorescens against R. solani and its level of beta-1,3-glucanase, SA and HCN was observed.     

Spatial Patterns of Rhizoplane Populations of Pseudomonas fluorescens

Geostatistical analysis was used to compare rhizoplane colonization patterns of an antibiotic-producing biological control bacterium versus a non-antibiotic-producing mutant strain. Pea seeds were inoculated with Pseudomonas fluorescens 2-79RN(inf10) or P. fluorescens 2-79-B46 (a phenazine-deficient Tn5 mutant of P. fluorescens 2-79RN(inf10)) (10(sup8) CFU/pea), planted in sterile sand, and incubated at 20(deg)C. After 3 days, seedlings were prepared for scanning electron microscopy. Photomicrographs (x1,000) of the root surface were taken at the seed-root junction and at 0.5-cm intervals to the root tip. Bacterial counts on the root surface were made in 5- by 5-(mu)m sample units over an area which was 105 by 80 (mu)m. Coordinates and number of bacteria were recorded for each sample unit. Spatial statistics were calculated by covariance for the following directions: omnidirectional, 0, 45, 90, and 135(deg). The ranges of spatial influence and nugget (estimator of spatially dependent variation) were determined. For both P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, spatial structure was evident along the entire root, particularly in the 0(deg) direction (along the root length) (e.g., range = 24 (mu)m, nugget = 0.52). The degree of spatial dependence observed indicated aggregation of bacterial cells. No differences were detected in the spatial patterns of colonies of P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, indicating that the lack of phenazine production did not influence spatial patterns on the rhizoplane.     

Colonizing ability of Pseudomonas fluorescens 2112, among collections of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens spp. in pea rhizosphere

Pseudomonas fluorescens 2112, isolated in Korea as an indigenous antagonistic bacteria, can produce 2,4- diacetylphloroglucinol (2,4-DAPG) and the siderophore pyoveridin2112 for the control of phytophthora blight of red-pepper. P. fluorescens 2112 was classified into a new genotype C among the 17 genotypes of 2,4-DAPG producers, by phlD restriction fragment length polymorphism (RFLP). The colonizing ability of P. fluorescens 2112 in pea rhizosphere was equal to the well-known pea colonizers, P. fluorescens Q8r1 (genotype D) and MVP1-4 (genotype P), after 6 cycling cultivations for 18 weeks. Four tested 2,4- DAPG-producing Pseudomonas spp. could colonize with about a 96% dominance ratio against total bacteria in pea rhizosphere. The strain P. fluorescens 2112 was as good a colonizer as other Pseudomonas spp. genotypes in pea plant growth-promoting rhizobacteria.     

Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.)

Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO₃, pH(H₂O)7.5) which had been sterilized prior to planting and fertilized with nitrate as source of nitrogen. In response to P deficiency, proteoid roots developed which accounted for about 50% of the root dry weight. In the rhizosphere soil of the proteoid root zones, the pH dropped to 4.8 and abundant white precipitates became visible. X-ray spectroscopy and chemical analysis showed that these precipitates consisted of calcium citrate. The amount of citrate released as root exudate by 13-week-old plants was about 1 g plant⁻¹, representing about 23% of the total plant dry weight at harvest. In the rhizosphere soil of the proteoid root zones the concentrations of available P decreased and of available Fe, Mn and Zn increased. The strong acidification of the rhizosphere and the cation/anion uptake ratio of the plants strongly suggests that proteoid roots of white lupin excrete citric acid, rather than citrate, into the rhizosphere leading to intensive chemical extraction of a limited soil volume. In a calcareous soil, citric acid excretion leads to dissolution of CaCO₃ and precipitation of calcium citrate in the zone of proteoid roots.     

Distribution of Carboxylates and Acid Phosphatase and Depletion of Different Phosphorus Fractions in the Rhizosphere of a Cereal and Three Grain Legumes

This study investigates the distribution of carboxylates and acid phosphatases as well as the depletion of different phosphorus (P) fractions in the rhizosphere of three legume crop species and a cereal, grown in a soil with two different levels of residual P. White lupin (Lupinus albus L.), field pea (Pisum sativum L.), faba bean (Vicia faba L.) and spring wheat (Triticum aestivum L.) were grown in small sand-filled PVC tubes to create a dense root mat against a 38-μm mesh nylon cloth at the bottom, where it was in contact with the soil of interest contained in another tube. The soil had either not been fertilised (P0) or fertilised with 15 (P15) kg P ha⁻¹ in previous years. The mesh size did not allow roots to grow into the soil, but penetration of root hairs and diffusion of nutrients and root exudates was possible, and a rhizosphere was established. At harvest, thin (1 mm) slices of this rhizosphere soil were cut, down to a 10-mm distance from the mesh surface. The rhizosphere of white lupin, particularly in the P0 treatment, contained citrate, mostly in the first 3 mm, with concentrations decreasing with distance from the root. Acid phosphatase activity was enhanced in the rhizosphere of all species, as compared with bulk soil, up to a distance of 4 mm. Phosphatase activity was highest in the rhizosphere of white lupin, followed by faba bean, field pea and wheat. Both citrate concentrations and phosphatase activities were higher in P0 compared with P15. The depletion of both inorganic (P_i) and organic (P_o) phosphorus fractions was greatest at the root surface, and decreased gradually with distance from the root. The soil P fractions that were most depleted as a result of root activity were the bicarbonate-extractable (0.5 M) and sodium hydroxide-extractable (0.1 M) pools, irrespective of plant species. This study suggests that differences among the studied species in use of different P pools and in the width of the rhizosphere are relatively small.     

Effect of Ferric Iron on Siderophore Production and Pyrene Degradation by Pseudomonas fluorescens 29L

The effect of ferric iron [Fe(III)] on pyrene degradation and siderophore production was studied in Pseudomonas fluorescens 29L. In the presence of 0.5 muM of Fe(III) and 50 mg of pyrene per liter of medium as a carbon source, 2.2 mg of pyrene was degraded per liter of medium per day and 25.3 muM of 2,3-DHBA (2,3-dihydroxybenzoic acid) equivalent of siderophores was produced per day. However, the pyrene degradation rate was 1.3 times higher and no siderophores were produced with the addition of 1 muM of Fe(III). Similar trends were seen with 50 mg of succinate per liter of medium as a carbon source, although the growth of strain 29L and the succinate degradation rate were higher. In the absence of siderophore production, pyrene and succinate continued to be biodegraded. This indicates that Fe(III) and not siderophore production affects the hydrocarbon degradation rate. Only 18% of strain 29L mutants capable of growth on pyrene produced siderophores, while among the mutants capable of growth on succinate, only 10% produced siderophores. This indicates that siderophores are not required for pyrene biodegradation. Fe(III) enhances pyrene degradation in Pseudomonas fluorescens 29L but it may be utilized by mechanisms other than siderophores.     

Iron uptake by plants from microbial siderophores : a study with 7-nitrobenz-2 oxa-1,3-diazole-desferrioxamine as fluorescent ferrioxamine B analog

The synthetically produced fluorescent siderophore NBD-desferrioxamine B (NBD-DFO), an analog of the natural siderophore ferrioxamine B, was used to study iron uptake by plants. Short-term (10-hour) ⁵⁵Fe uptake rates by cotton (Gossypium spp.) and maize (Zea mays L.) plants from the modified siderophore were similar to those of the natural one. In longer-term uptake experiments (3 weeks), both siderophore treatments resulted in similar leaf chlorophyll concentration and dry matter yield. These results suggest that the synthetic derivative acts similarly to the natural siderophore. The NBD-DFO is fluorescent only when unferrated and can thus be used as a probe to follow iron removal from the siderophore. Monitoring of the fluorescence increase in a nutrient solution containing Fe³⁺-NBD-DFO showed that iron uptake by plants occurs at the cell membrane. The rate of iron uptake was significantly lower in both plant species in the presence of antibiotic agent, thus providing evidence for iron uptake by rhizosphere microbes that otherwise could have been attributed to plant uptake. Confocal fluorescence microscopy revealed that iron was taken up from the complex by cotton plants, and to a much lesser extent by maize plants. The active cotton root sites were located at the main and lateral root tips. Significant variations in the location and the intensity of the uptake were noticed under nonaxenic conditions, which suggested that rhizosphere microorganisms play an important role in NBD-DFO-mediated iron uptake.     

Combat of iron-deprivation through a plant growth promoting fluorescent Pseudomonas strain GRP3A in mung bean (Vigna radiata L. Wilzeck)

Microorganisms and plants sustain themselves under iron-deprived conditions by releasing siderophores. Among others, fluorescent pseudomonads are known to exert extensive biocontrol action against soil and root borne phytopathogens through release of antimicrobials and siderophores. In this study, production and regulation of siderophores by fluorescent Pseudomonas strain GRP3A was studied. Among various media tested, standard succinate medium (SSM) promoted maximum siderophore production of 56.59 mg l(-1). There were low levels of siderophore in complex media like King's B medium, trypticase soya medium and nutrient medium (41.27, 29.86 and 27.63 mg l(-1)), respectively. In defferrated SSM, siderophore level was quantified to be 68.74 mg l(-1). Supplementation with iron (FeCl3) resulted in decreased siderophore levels depending on concentration. Siderophore production was promoted by Zn2+ (78.94 mg l(-1)), Cu2+ (68.80 mg l(-1)) whereas Co2+ (57.33 mg l(-1)) and Fe3+ reduced siderophore production (37.44 mg l(-1) as compared to control (55.97 mg l(-1)). Strain GRP3A showed plant growth promotion under iron limited conditions.     

Rhizosphere interactions and siderophores

Certain plant growth-promoting pseudomonads inhibit deleterious and pathogenic rhizosphere bacteria and fungi by producing siderophores. Properties of a siderophore transport system which might provide a competitive advantage under iron stress conditions include ability to utilize other organisms' siderophores, higher Fe(III) stability constant, faster kinetics of dissolution of Fe(III) minerals, more efficient transport system, and resistance to degradation. In order to determine the concentration and localization of siderophores in the rhizosphere monoclonal antibodies (Mabs) to ferric pseudobactin, the siderophore of Pseudomonas putida B10, have been developed. Several Mabs cross reacted differently with various pseudobactins. A growth medium has been developed for the study for siderophore-mediated rhizosphere interactions in the laboratory.     

Phosphorus benefits of different legume crops to subsequent wheat grown in different soils of Western Australia

Certain legume crops, including white lupin (Lupinus albus L.), mobilise soil-bound phosphorus (P) through root exudates. The changes in the rhizosphere enhance P availability to these crops, and possibly to subsequent crops growing in the same soil. We conducted a pot experiment to compare phosphorus acquisition of three legume species with that of wheat, and to determine whether the legume crops influence growth and P uptake of a subsequent wheat crop. Field pea (Pisum sativum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in three different soils to which we added no or 20 mg P kg^–1 soil (P0, P20). Growth, P content and rhizosphere carboxylates varied significantly amongst crops, soils and P levels. Total P content of the plants was increased with applied phosphorus. Phosphorus content of faba bean was 3.9 and 8.8 mg/pot, at P0 and P20, respectively, which was about double that of all other species at the respective P levels. Field pea and white lupin had large amounts of rhizosphere carboxylates, whereas wheat and faba bean had negligible amounts in all three soils at both P levels. Wheat grew better after legumes than after wheat in all three soils. The effect of the previous plant species was greater when these previous species had received P fertiliser. All the legumes increased plant biomass of subsequent wheat significantly over the unplanted pots in all the soils. Faba bean was unparalleled in promoting subsequent wheat growth on all fertilised soils. This experiment clearly demonstrated a residual benefit of the legume crops on the growth of the subsequent wheat crop due to enhanced P uptake. Faba bean appeared to be a suitable P-mobilising legume crop plant for use in rotations with wheat.     

Phytohormone production and colonization of canola (Brassica napus L.) roots by Pseudomonas fluorescens 6-8 under gnotobiotic conditions

Pseudomonas fluorescens 6-8, a rhizosphere isolate previously shown to enhance root elongation of canola ( Brassica napus L.), was characterized for its ability to produce indole-3-acetic acid and cytokinins in pure culture and in the rhizosphere of canola under gnotobiotic conditions in comparison with the cytokinin-producing strain P. fluorescens G20-18 and its mutant CNT2. Strain 6-8 produced isopentenyl adenosine, zeatin riboside, and dihydroxyzeatin riboside at levels similar to those of G20-18, but only very low concentrations of indole-3-acetic acid. In a gnotobiotic assay canola inoculated with 6-8 and G20-18 had higher concentrations of isopentenyl adenosine and zeatin riboside in the rhizosphere and greater root length than the noninoculated control. The ability of strain 6-8 to colonize canola roots was assessed following transformation with the green fluorescent protein and inoculation onto canola seed in a gnotobiotic assay. Higher populations of strain 6-8 were observed on the proximal region of the root closest to the seed than on the mid and distal portions 9?days after seed inoculation. The ability of P. fluorescens 6-8 to produce cytokinins, colonize the roots of canola seedlings, and enhance root elongation may contribute to its ability to survive in the rhizosphere and may benefit seedling growth.     

烟草根际铁载体产生菌G-229-21T的筛选、鉴定及拮抗机理

[目的]从烟草根际筛选烟草疫霉[Phytophthora parasitica var.nicotianae(Breda de Hann)Tucker]拮抗菌,探索其拮抗机理.[方法]限铁(2.0 μmol/L FeCl3)蔗糖-天冬酰胺平板对峙法筛选烟草疫霉拮抗菌;刃天青(CAS)法检测其铁载体的产生及其对铁离子的亲和能力.结合形态、生理生化、16s rRNA序列同源性和系统发育分析及种特异性分子法对其进行鉴定.XAD-2吸附层析法提取其铁载体,分光光度法检测其铁载体类型.不同铁离子浓度下,比较其铁载体对烟草疫霉的抑制作用.[结果]我们筛选到一株限铁条件下烟草疫霉拮抗菌G-229-21T,该菌产生高亲和力铁载体,被初步鉴定为Pseudomonas mediterranea.该菌产生的羧酸型铁载体,在低铁条件下(0.16μmol/L～10μmol/L,FeCl3)对烟草疫霉的抑制率达92.3%以上,而在富铁条件下(100 μmol/L FeCl3)抑制率仅为2.0%.[结论]首次报道P. mediterranea G-229-21T产生高亲和力羧酸型铁载体,该铁载体在低铁条件下对烟草疫霉有显著的抑制作用.     

Diversity of root-associated fluorescent pseudomonads as affected by ferritin overexpression in tobacco

A transgenic tobacco overexpressing ferritin (P6) was recently shown to accumulate more iron than the wild type (WT), leading to a reduced availability of iron in the rhizosphere and shifts in the pseudomonad community. The impact of the transgenic line on the community of fluorescent pseudomonads was assessed. The diversity of 635 isolates from rhizosphere soils, rhizoplane + root tissues, and root tissues of WT and P6, and that of 98 isolates from uncultivated soil was characterized. Their ability to grow under iron stress conditions was assessed by identifying their minimal inhibitory concentrations of 8-hydroxyquinoline for each isolate, pyoverdine diversity by isoelectrofocusing and genotypic diversity by random amplified polymorphism DNA. The antagonistic activity of representative isolates and of some purified pyoverdines against a plant pathogen (Pythium aphanidermatum Op4) was tested in vitro. In overall, isolates taken from P6 tobacco showed a greater ability to grow in iron stress conditions than WT isolates. The antagonism by some of the representative isolates was only expressed under iron stress conditions promoting siderophore synthesis and their pyoverdines appeared to have a specific structure as assessed by mass spectrometry. For other isolates, antagonism was still expressed in the presence of iron, suggesting the involvement of metabolites other than siderophores. Altogether, these data indicate that the transgenic tobacco that over-accumulates iron selected fluorescent pseudomonads, less susceptible to iron depletion and more antagonistic to the tested plant pathogen than those selected by the tobacco WT.