Nitrogen availability to Pseudomonas fluorescens DF57 is limited during decomposition of barley straw in bulk soil and in the barley rhizosphere.

The availability of nitrogen to Pseudomonas fluorescens DF57 during straw degradation in bulk soil and in barley rhizosphere was studied by introducing a bioluminescent reporter strain (DF57-N3), responding to nitrogen limitation, to model systems of varying complexity. DF57-N3 was apparently not nitrogen limited in the natural and sterilized bulk soil used for these experiments. The soil was subsequently amended with barley straw, representing a plant residue with a high carbon-to-nitrogen ratio (between 60 and 100). In these systems the DF57-N3 population gradually developed a nitrogen limitation response during the first week of straw decomposition, but exclusively in the presence of the indigenous microbial population. This probably reflects the restricted ability of DF57 to degrade plant polymers by hydrolytic enzymes. The impact of the indigenous population on nitrogen availability to DF57-N3 was mimicked by the cellulolytic organism Trichoderma harzianum Rifai strain T3 when coinoculated with DF57-N3 in sterilized, straw-amended soil. Limitation occurred concomitantly with fungal cellulase production, pointing to the significance of hydrolytic activity for the mobilization of straw carbon sources, thereby increasing the nitrogen demand. Enhanced survival of DF57-N3 in natural soil after straw amendment further indicated that DF57 was cross-fed with carbon/energy sources. The natural barley rhizosphere was experienced by DF57-N3 as an environment with restricted nitrogen availability regardless of straw amendment. In the rhizosphere of plants grown in sterilized soil, nitrogen limitation was less severe, pointing to competition with indigenous microorganisms as an important determinant of the nitrogen status for DF57-N3 in this environment. Hence, these studies have demonstrated that nitrogen availability and gene expression in Pseudomonas is intimately linked to the structure and function of the microbial community. Further, it was demonstrated that the activities of cellulolytic microorganisms may affect the availability of energy and specific nutrients to a group of organisms deficient in hydrolytic enzyme activities.     

Nitrogen Availability to Pseudomonas fluorescens DF57 Is Limited during Decomposition of Barley Straw in Bulk Soil and in the Barley Rhizosphere

The availability of nitrogen to Pseudomonas fluorescens DF57 during straw degradation in bulk soil and in barley rhizosphere was studied by introducing a bioluminescent reporter strain (DF57-N3), responding to nitrogen limitation, to model systems of varying complexity. DF57-N3 was apparently not nitrogen limited in the natural and sterilized bulk soil used for these experiments. The soil was subsequently amended with barley straw, representing a plant residue with a high carbon-to-nitrogen ratio (between 60 and 100). In these systems the DF57-N3 population gradually developed a nitrogen limitation response during the first week of straw decomposition, but exclusively in the presence of the indigenous microbial population. This probably reflects the restricted ability of DF57 to degrade plant polymers by hydrolytic enzymes. The impact of the indigenous population on nitrogen availability to DF57-N3 was mimicked by the cellulolytic organism Trichoderma harzianum Rifai strain T3 when coinoculated with DF57-N3 in sterilized, straw-amended soil. Limitation occurred concomitantly with fungal cellulase production, pointing to the significance of hydrolytic activity for the mobilization of straw carbon sources, thereby increasing the nitrogen demand. Enhanced survival of DF57-N3 in natural soil after straw amendment further indicated that DF57 was cross-fed with carbon/energy sources. The natural barley rhizosphere was experienced by DF57-N3 as an environment with restricted nitrogen availability regardless of straw amendment. In the rhizosphere of plants grown in sterilized soil, nitrogen limitation was less severe, pointing to competition with indigenous microorganisms as an important determinant of the nitrogen status for DF57-N3 in this environment. Hence, these studies have demonstrated that nitrogen availability and gene expression in Pseudomonas is intimately linked to the structure and function of the microbial community. Further, it was demonstrated that the activities of cellulolytic microorganisms may affect the availability of energy and specific nutrients to a group of organisms deficient in hydrolytic enzyme activities.     

Influence of an arbuscular mycorrhizal fungus on Pseudomonas fluorescens DF57 in rhizosphere and hyphosphere soil

The influence of Glomus intraradices (BEG87) on Pseudomonas fluorescens DF57 in hyphosphere and rhizosphere soil was examined. Cucumis sativus (Aminex, F₁ hybrid) was grown in symbiosis with the arbuscular mycorrhizal fungus G. intraradices in PVC tubes, consisting of a central root compartment and two lateral root-free compartments. Two Tn 5 - lux AB-marked strains of P. fluorescens DF57 were used. Strain DF57-P2, which has an insertion of Tn 5::lux AB in a phosphate starvation-inducible locus, was used as a phosphate starvation reporter. Another lux -tagged strain DF57-40E7, which carries a constitutively expressed lux AB fusion, was used as control for strain DF57-P2 and for measuring the metabolic activity of P. fluorescens DF57. A strain of P. fluorescens DF57, which carries a constitutively expressed gfp gene, was used in studies of attachment between the bacteria and the hyphae. G. intraradices decreased the culturability of P. fluorescens DF57 significantly, both in rhizosphere and hyphosphere soil, whereas the total number of P. fluorescens DF57 measured by immunofluorescence microscopy was decreased in hyphosphere soil only. G. intraradices did not induce a phosphorus starvation response in P. fluorescens DF57, and the metabolic activity of the bacteria was not affected by the fungus after 48 h. P. fluorescens DF57 did not attach to G. intraradices hyphae and was not able to use the hyphae as carbon substrate. The negative effect of G. intraradices on culturability and on number of P. fluorescens DF57 in hyphosphere soil is discussed.     

Interactions between proteolytic and non-proteolytic Pseudomonas fluorescens affect protein degradation in a model community

The metabolic interactions between proteinase-producing bacteria and other members of bacterial communities are poorly investigated, although they are important for the understanding of structure-function relationships in complex ecosystems. We constructed simple model communities consisting of proteolytic and non-proteolytic Pseudomonas fluorescens strains to identify relevant interactions and to assess their specific significance during the mobilization of protein for growth. The proteolytic or non-proteolytic model communities were established by co-inoculating proteolytic or proteinase-deficient Tn5-mutants of P. fluorescens strain ON2 with the non-proteolytic reporter strain DF57-N3 that expresses bioluminescence in response to nitrogen limitation. The growth medium was composed such that growth would be nitrogen limited in the absence of proteolytic activity. In the proteolytic communities data on growth and nitrogen availability showed that the protein hydrolysates were available to both the proteolytic and the non-proteolytic strain. Competition between these strains profoundly affected both growth and proteinase production. Hence, the mobilization of protein was closely coupled to the competitive success of the proteolytic strain. These findings provide new insight into the metabolic interactions that occur when protein is degraded in mixed bacterial communities.     

Distribution of metabolic activity and phosphate starvation response of lux-tagged Pseudomonas fluorescens reporter bacteria in the barley rhizosphere.

The purpose of this study was to determine the metabolic activity of Pseudomonas fluorescens DF57 in the barley rhizosphere and to assess whether sufficient phosphate was available to the bacterium. Hence, two DF57 reporter strains carrying chromosomal luxAB gene fusions were introduced into the rhizosphere. Strain DF57-40E7 expressed luxAB constitutively, making bioluminescence dependent upon the metabolic activity of the cells under defined assay conditions. The DF57-P2 reporter strain responded to phosphate limitation, and the luxAB gene fusion was controlled by a promoter containing regulatory sequences characteristic of members of the phosphate (Pho) regulon. DF57 generally had higher metabolic activity in a gnotobiotic rhizosphere than in the corresponding bulk soil. Within the rhizosphere the distribution of metabolic activity along the root differed between the rhizosphere soil and the rhizoplane, suggesting that growth conditions may differ between these two habitats. The DF57-P2 reporter strain encountered phosphate limitation in a gnotobiotic rhizosphere but not in a natural rhizosphere. This difference in phosphate availability seemed to be due to the indigenous microbial population, as DF57-P2 did not report phosphate limitation when established in the rhizosphere of plants in sterilized soil amended with indigenous microorganisms.     

Culturability and Expression of Outer Membrane Proteins during Carbon, Nitrogen, or Phosphorus Starvation of Pseudomonas fluorescens DF57 and Pseudomonas putida DF14

Changes in culturability and outer membrane protein profiles were investigated in Pseudomonas fluorescens DF57 and Pseudomonas putida DF14 during starvation for carbon, nitrogen, and phosphorus. P. fluorescens DF57 remained fully culturable for 4 days in all starvation regimes. The cell mass increased during starvation for nitrogen and phosphorus, indicating the accumulation of storage compounds, whereas it decreased slightly in carbon-starved cells. P. putida DF14 lost culturability during phosphorus starvation, and the mass of phosphate-starved cells did not increase. Analysis of additional P. fluorescens and P. putida strains, however, showed that the ability to preserve culturability during phosphorus starvation was not species but strain dependent. In DF57, an outer membrane protein of 55 kDa appeared during starvation for phosphorus, while another protein of 63 kDa was seen during all starvation conditions. DF14 induced two outer membrane proteins of 28 and 29 kDa during starvation for carbon and nitrogen, but no phosphorus-specific starvation protein could be detected. Therefore, starvation-induced outer membrane proteins do not seem to be conserved among the fluorescent pseudomonads and a unique starvation response might be found in individual strains.     

Effective dose of a microbial inoculant is one to four cells in the rhizosphere

A single-cell approach for studying the growth potential and the establishment of bacteria in the barley phytosphere is presented, using Pseudomonas fluorescens strain with the capability for biological control. The incidence of growth of one to four bacterial cells dispersed to the young rhizosphere approximated to 100%, and specific growth rate averaged 0.05. Net growth occurred for cells added to the rhizosphere at densities between 1 and 100,000 cells, while at higher densities population sizes declined, but always approached 10(5)-10(6) cells per rhizosphere. No net growth was observed in bulk soil, and cells died in the phyllosphere. Our results showed that bacterial establishment was more related to the availability of microhabitats supporting growth, than related to the number of bacteria released.     

Dynamic responses of Pseudomonas fluorescens DF57 to nitrogen or carbon source addition

Cultures of Pseudomonas fluorescens DF57 were grown on different carbon and nitrogen sources. Glucose, succinate and acetate were used as carbon source and pulsed to an aerobic steady-state cultivation of P. fluorescens DF57 at D = 0.1 h(-1) with citrate as limiting carbon source. Glucose was utilised with the fastest uptake rate (19.4 C mmol l(-1) h(-1)) compared to succinate (8.8 C mmol l(-1) h(-1)) and acetate (4.3 C mmol l(-1) h(-1)). Acetate triggered an inhibition of cellular metabolism, which resulted in 2-h long growth arrest after its addition to the steady-state culture. The influence of the nitrogen source was investigated in an aerobic cultivation on a mixture of ammonium and nitrate as limiting nitrogen sources and citrate as non-limiting carbon source. When ammonia and nitrate were pulsed to the steady-state culture, they were mainly assimilated into biomass with a maximum uptake rate of 111 and 33 mg N l(-1) h(-1), respectively. Nitrate uptake was never complete as the residual concentration in the chemostat cultivation was 30 mg N l(-1) nitrate. A pulse of nitrite in the cultivation broth resulted in an inhibition of the growth but not of the primary metabolism, as nitrite was taken up at 38 mg N l(-1) h(-1), citrate was consumed and cofactors were produced continuously. In all experiments, oxygen was used as electron acceptor.     

荧光假单胞菌2P24中gidA基因对PcoI/PcoR群体感应系统的影响

【目的】荧光假单胞菌(Pseudomonas fluorescens)2P24中PcoI/PcoR群体感应系统是调控生物膜形成与植物根部定殖能力的重要元件,同时该系统也受到多种上游因子的调控。利用遗传学方法研究P.fluorescens 2P24中gidA基因对群体感应系统的调控作用。【方法】将群体感应信号合成基因pcoI的转录报告质粒p970km-pcoIp转入菌株2P24和gidA基因突变体中以检测gidA对pcoI基因表达的影响,并利用报告菌Agrobacterium tumefaciens NTL4(pZLR4)测定菌株2P24及其衍生菌的信号N-乙酰高丝氨酸内酯(AHL)产量。【结果】gidA基因突变后pcoI基因的转录表达和AHL产量与野生型2P24相比显著降低。gidA基因突变体的游动能力没有受到明显的影响,但生物膜的形成显著低于野生型和互补菌株。小麦根部定殖实验表明,温室条件下gidA突变菌株在灭菌土和自然土中对小麦根尖和根围的定殖量较野生型和互补菌显著减少。此外,突变gidA基因并不影响菌株在LB培养基中的生长,但以葡萄糖、蔗糖、果糖、甘油、半乳糖、阿拉伯糖、甘露糖、木糖或山梨醇为唯一碳源时,gidA突变体的生长受到明显的抑制。【结论】GidA对假单胞菌2P24中的PcoI/PcoR群体感应系统、生物膜形成、定殖和碳源利用具有显著的正调控作用。     

Ecological Similarity and Coexistence of Epiphytic Ice-Nucleating (Ice) Pseudomonas syringae Strains and a Non-Ice-Nucleating (Ice) Biological Control Agent

De Wit replacement series were used to study competitive interactions between epiphytic IcePseudomonas syringae strains and the biological frost control agents IceP. syringae TLP2del1 and Pseudomonas fluorescens A506. Mixtures containing two strains in different proportions but at a constant total population size were inoculated onto potato leaves. The population sizes of each strain and the total population size were determined when the community had reached equilibrium. A near-isogenic P. syringae strain pair exhibited an interaction similar to that expected for strains competing equally for limiting environmental resources. Replacement series with nonisogenic Ice and IceP. syringae strain pairs suggested that these strains competed for limiting resources according to their relative competitive abilities. There was no evidence of any niche differentiation between the IceP. syringae strains and the IceP. syringae strain. The growth responses of epiphytes following addition of nutrients to the phyllosphere indicated that the epiphytic P. syringae populations were nutrient limited and that, under growth chamber conditions, the populations were more limited by the availability of carbon than by the availability of nitrogen. Determination of in vitro carbon source utilization profiles provided further evidence for the lack of niche differentiation between the Ice and the IceP. syringae strains. Niche overlap indices calculated for the IceP. syringae strains with respect to IceP. syringae TLP2del1 were uniformly high, indicating ecological similarity, and were consistent with the observed low level of coexistence. The biological frost control agent P. fluorescens A506 replaced P. syringae. This was correlated with a high degree of niche overlap between these species.     

A tripartite microbial reporter gene system for real-time assays of soil nutrient status

Plant-derived carbon is the substrate which drives the rate of microbial assimilation and turnover of nutrients, in particular N and P, within the rhizosphere. To develop a better understanding of rhizosphere dynamics, a tripartite reporter gene system has been developed. We used three lux-marked Pseudomonas fluorescens strains to report on soil (1) assimilable carbon, (2) N-status, and (3) P-status. In vivo studies using soil water, spiked with C, N and P to simulate rhizosphere conditions, showed that the tripartite reporter system can provide real-time assessment of carbon and nutrient status. Good quantitative agreement for bioluminescence output between reference material and soil water samples was found for the C and P reporters. With regard to soil nitrate, the minimum bioavailable concentration was found to be greater than that analytically detectable in soil water. This is the first time that bioavailable soil C, N and P have been quantified using a tripartite reporter gene system.     

Production of cyclic lipopeptides by Pseudomonas fluorescens strains in bulk soil and in the sugar beet rhizosphere

The production of cyclic lipopeptides (CLPs) with antifungal and biosurfactant properties by Pseudomonas fluorescens strains was investigated in bulk soil and in the sugar beet rhizosphere. Purified CLPs (viscosinamide, tensin, and amphisin) were first shown to remain highly stable and extractable (90%) when applied (ca. 5 microg g(-1)) to sterile soil, whereas all three compounds were degraded over 1 to 3 weeks in nonsterile soil. When a whole-cell inoculum of P. fluorescens strain DR54 containing a cell-bound pool of viscosinamide was added to the nonsterile soil, declining CLP concentrations were observed over a week. By comparison, addition of the strains 96.578 and DSS73 without cell-bound CLP pools did not result in detectable tensin or amphisin in the soil. In contrast, when sugar beet seeds were coated with the CLP-producing strains and subsequently germinated in nonsterile soil, strain DR54 maintained a high and constant viscosinamide level in the young rhizosphere for approximately 2 days while strains 96.578 and DSS73 exhibited significant production (net accumulation) of tensin or amphisin, reaching a maximum level after 2 days. All three CLPs remained detectable for several days in the rhizosphere. Subsequent tests of five other CLP-producing P. fluorescens strains also demonstrated significant production in the young rhizosphere. The results thus provide evidence that production of different CLPs is a common trait among many P. fluorescens strains in the soil environment, and further, that the production is taking place only in specific habitats like the rhizosphere of germinating sugar beet seeds rather than in the bulk soil.     

模拟氮沉降对华西雨屏区苦竹林土壤有机碳和养分的影响

从2007年11月至2009年10月, 对华西雨屏区苦竹(Pleioblastus amarus)人工林进行了模拟氮(N)沉降试验, N沉降水平分别为对照(CK, 0 g N·m^-2·a^-1)、低N (5 g N·m^-2·a^-1)、中N (15 g N·m^-2·a^-1)和高N (30 g N·m^-2·a^-1)。在N沉降进行1年后, 每月采集各样方0-20 cm的土壤样品, 连续采集12个月, 测定其土壤总有机C、微生物生物量C、浸提性溶解有机C、活性C、全N、微生物生物量N、NH₄⁺-N、NO₃^--N、有效P和速效K。结果表明: N沉降显著增加了土壤总有机C、微生物生物量C、全N、微生物生物量N、NH₄⁺-N和有效P含量, 对其余几个指标无显著影响。土壤微生物生物量C和微生物生物量N的季节变化明显, 并与气温极显著正相关。土壤有效P、速效K与微生物生物量C、微生物生物量N呈极显著负相关关系。N沉降提高了土壤中C、N、P元素的活性, 并通过微生物的转化固定作用使得C、N、P元素在土壤中的含量增加。苦竹林生态系统处于N限制状态, 土壤有机C和养分对N沉降呈正响应, N沉降的增加可能会提高土壤肥力并促进植被的生长, 进而促进生态系统对C的固定。     

Trichoderma harzianum enhances the production of nematicidal compounds in vitro and improves biocontrol of Meloidogyne javanica by Pseudomonas fluorescens in tomato

AIMS: To determine the influence of soil-borne fungus Trichoderma harzianum on the biocontrol performance of Pseudomonas fluorescens strain CHA0 and its 2,4-diacetylphloroglucinol (DAPG) overproducing derivative CHA0/pME3424 against Meloidogyne javanica. METHODS AND RESULTS: Amendment of the culture filtrate (CF) or methanol extract of the CF of a T. harzianum strain Th6 to P. fluorescens growth medium enhanced the production of nematicidal compound(s) by bacterial inoculants in vitro. In addition, bacteria overwhelmingly expressed phl'-'lacZ reporter gene when the medium was amended with CF of T. harzianum. Pseudomonas fluorescens and T. harzianum applied together in unsterilized sandy loam soil caused greater reduction in nematode population densities in tomato roots. CONCLUSIONS: Trichoderma harzianum improves root-knot nematode biocontrol by the antagonistic rhizobacterium P. fluorescens both in vitro and under glasshouse conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: The synergistic effect of T. harzianum on the production of nematicidal compound(s) critical in biocontrol may improve the efficacy of biocontrol bacteria against plant-parasitic nematodes. Considering the inconsistent performance of the biocontrol agents under field conditions, application of a mixture of compatible T. harzianum and P. fluorescens would more closely mimic the natural situation and might broaden the spectrum of biocontrol activity with enhanced efficacy and reliability of control.     

The sigma factor AlgU (AlgT) controls exopolysaccharide production and tolerance towards desiccation and osmotic stress in the biocontrol agent Pseudomonas fluorescens CHA0.

A variety of stress situations may affect the activity and survival of plant-beneficial pseudomonads added to soil to control root diseases. This study focused on the roles of the sigma factor AlgU (synonyms, AlgT, RpoE, and sigma(22)) and the anti-sigma factor MucA in stress adaptation of the biocontrol agent Pseudomonas fluorescens CHA0. The algU-mucA-mucB gene cluster of strain CHA0 was similar to that of the pathogens Pseudomonas aeruginosa and Pseudomonas syringae. Strain CHA0 is naturally nonmucoid, whereas a mucA deletion mutant or algU-overexpressing strains were highly mucoid due to exopolysaccharide overproduction. Mucoidy strictly depended on the global regulator GacA. An algU deletion mutant was significantly more sensitive to osmotic stress than the wild-type CHA0 strain and the mucA mutant were. Expression of an algU'-'lacZ reporter fusion was induced severalfold in the wild type and in the mucA mutant upon exposure to osmotic stress, whereas a lower, noninducible level of expression was observed in the algU mutant. Overexpression of algU did not enhance tolerance towards osmotic stress. AlgU was found to be essential for tolerance of P. fluorescens towards desiccation stress in a sterile vermiculite-sand mixture and in a natural sandy loam soil. The size of the population of the algU mutant declined much more rapidly than the size of the wild-type population at soil water contents below 5%. In contrast to its role in pathogenic pseudomonads, AlgU did not contribute to tolerance of P. fluorescens towards oxidative and heat stress. In conclusion, AlgU is a crucial determinant in the adaptation of P. fluorescens to dry conditions and hyperosmolarity, two major stress factors that limit bacterial survival in the environment.     

Organophosphonate utilization by the wild-type strain of Pseudomonas fluorescens.

The wild-type strain of Pseudomonas fluorescens was found to utilize a range of structurally diverse organophosphonates as its sole carbon or nitrogen sources. Representative compounds included aminoalkylphosphonates, hydroxyalkylphosphonates, oxoalkylphosphonates, and phosphono dipeptides. Among them, amino(phenyl)methylphosphonate,2-aminoethylphosphonate, aminomethylphosphonate, diisopropyl 9-aminofluoren-9-ylphosphonate, and 2-oxoalkylphosphonates were used by P. fluorescens as its sole sources of phosphorus. Only slight growth was observed on the herbicide glyphosate (N-phosphonomethylglycine), which was metabolized to aminomethylphosphonate. Neither phosphinothricin nor its dialanyl tripeptide, bialaphos, supported growth of P. fluorescens. The possible mechanisms of organophosphonate degradation by this strain are discussed.     

Organophosphonate utilization by the wild-type strain of Pseudomonas fluorescens.

The wild-type strain of Pseudomonas fluorescens was found to utilize a range of structurally diverse organophosphonates as its sole carbon or nitrogen sources. Representative compounds included aminoalkylphosphonates, hydroxyalkylphosphonates, oxoalkylphosphonates, and phosphono dipeptides. Among them, amino(phenyl)methylphosphonate,2-aminoethylphosphonate, aminomethylphosphonate, diisopropyl 9-aminofluoren-9-ylphosphonate, and 2-oxoalkylphosphonates were used by P. fluorescens as its sole sources of phosphorus. Only slight growth was observed on the herbicide glyphosate (N-phosphonomethylglycine), which was metabolized to aminomethylphosphonate. Neither phosphinothricin nor its dialanyl tripeptide, bialaphos, supported growth of P. fluorescens. The possible mechanisms of organophosphonate degradation by this strain are discussed.     

Root colonization and iron nutritional status of a Pseudomonas fluorescens in different plant species

Root colonization and induction of an iron stress regulated promoter for siderophore production by Pseudomonas fluorescens 2-79RLI was studied in vitro and in the rhizosphere of different plant species. P. fluorescens 2-79RLI was previously genetically modified with an iron regulated ice nucleation reporter, which allowed calibration of ice nucleation activity with siderophore production. Initial experiments examined ice nucleation activity and siderophore production under different growth conditions in vitro. These studies demonstrated that P. fluorescens 2-79RLI could utilize both Fe-citrate and Fe-phytosiderophore as iron sources, suggesting that production of these compounds by plants would increase iron availability for P. fluorescens 2-79RLI in the rhizosphere. Fe demand and Fe stress were further shown to be a function of nutrient availability and were reduced when carbon was limiting for growth. Subsequent experiments extended these observations to rhizosphere cells. Cells were sampled from the rhizosphere and the rhizoplane. Results of a soil microcosm experiment showed that Fe stress was reduced for P. fluorescens 2-79RLI in the barley rhizosphere as compared to the cells in the rhizosphere.of lupin. In lupin, relative Fe stress of P. fluorescens 2-79RLI was greater at the root tip than in the lateral root zone. In a second experiment comparing zucchini and bean, iron stress was greater for P. fluorescens 2-79RLI associated with zucchini than with bean. In a third experiment with rape plants under P deficient conditions, addition of soluble P was shown to increase Fe stress for P. fluorescens 2-79RLI located at the root tip, but not in the lateral root zone. This study showed that Fe stress of P. fluorescens 2-79RLI in the rhizosphere may be influenced by plant species, P source, root zone and localization of the cells within the rhizosphere.