Pseudomonas fluorescens General Description

Pseudomonas fluorescens is a common Gram-negative, rod-shaped bacterium. It is found in many soils throughout the globe but in small numbers. The species name ‘fluorescens’ was coined because of its ability to secrete a soluble, green colored fluorescent pigment called pyoverdin. It is well known that Pseudomonas fluorescens, in association with the plant rhizosphere, is able to exert a beneficial effect upon plant growth. It’s use as a bio-fertilizer as well as a pathogen control agent for microbial-agriculture. This beneficial microbe is a commonly used strain of bacteria primarily because of it’s ability to liberate phosphorus for plant uptake. However it also promotes plant growth by suppressing pathogens in root zones. Pseudomonas fluorescens secretes antibiotics and hydrogen cyanide that are lethal to plant pathogens. So you can see why this bacterial species is a topic of common interest for microbial-horticulturalist all over the world.

 Pseudomonas Mechanism for Phosphate Solubilization

The following is a summery of the research review paper “Phosphate solubilizing bacteria and their role in plant growth promotion” by Hilda Rodríguez, of the Department of Microbiology, Cuban Research Institute.

The principal mechanism for mineral phosphate solubilization of Pseudomonas is its production of organic acids and acid phosphatases which play a major role in the mineralization of organic phosphorous. Although several phosphate solubilizing bacteria occur in soil, usually their numbers are not high enough to compete with other bacteria commonly established in the rhizosphere. Thus, the amount of P liberated by them is generally not sufficient for a substantial increase in plant growth. Therefore, inoculation of plants by a target microorganism at a much higher concentration than that normally found in soil is necessary to take advantage of the property of phosphate solubilization for plant yield enhancement.

It has been shown how phosphate solubilizing bacteria assists mycorrhizal fungus to further help plants [1,2]. Several studies have shown that P solubilizing bacteria interact with vesicular arbuscular mycorrhizae by liberating phosphate ions in the substrate. This causes a synergistic interaction that allows for better (326 H. Rodríguez, R. Fraga/Biotechnology Advances 17 (1999) 319–339) use of insoluble phosphate sources [3-5]. The P solubilized by Pseudomonas fluorescens is more easily taken up by the plants through a mycorrhizae mediated channel between roots and surrounding soil.  This would allow nutrient transfer from soil to plants [6]. In fact, Toro et al. [7], using radioactive 32P labeling, demonstrated that phosphate- solubilizing bacteria associated with mycorrhizae improved mineral accumulation of phosphorus and nitrogen in plants. These authors suggested that the inoculated rhizobacteria could have released phosphate ions from insoluble rock phosphate and/or other P sources, which were then taken up by the external mycorrhizal mycelium.

It is generally accepted that the major mechanism of mineral phosphate solubilization is the action of organic acids synthesized by soil microorganisms [8,9-14 ]. Production of organic acids results in acidification of the microbial cell and its surroundings. Consequently, Pi may be released from a mineral phosphate by proton substitution for Ca21 [15]. The production of organic acids by phosphate solubilizing bacteria has been well documented. Among them, gluconic acid seems to be the most frequent agent of mineral phosphate solubilization. It is reported as the principal organic acid produced by phosphate solubilizing bacteria such as Pseudomonas sp.


[1] Chabot R, Antoun H, Cescas MP. Stimulation de la croissance du mais et de la laitue romaine par desmicroorganismes dissolvant le phosphore inorganique. Can J Microbiol 1993;39:941–7.
[2] Chabot R, Antoun H, Kloepper JW, Beauchamp CJ. Root colonization of maize and lettuce by bioluminiscent Rhizobium leguminosarum biovar. phaseoli. Appl Environ Microbiol 1996a;62:2767–72[3] Ray J, Bagyaraj DJ, Manjunath A. Influence of soil inoculation with versicular arbuscular mycorrhizal (VAM) and a phosphate dissolving bacteria on plant growth and 32P uptake. Soil Biol Biochem 1981;13:105–8.
[4] Azcón-Aguilar C, Gianinazzi-Pearson V, Fardeau JC, Gianinazzi S. Effect of vesicular-arbuscular mycorrhizal fungi and phosphate-solubilizing bacteria on growth and nutrition of soybean in a neutral-calcareus soil amended with 32P-45Ca-tricalcium phosphate. Plant Soil 1986;96:3–15.
[5] Piccini D, Azcón R. Effect of phosphate-solubilizing bacteria and versicular arbuscular mycorrhizal (VAM) on the utilization of bayoran rock phosphate by alfalfa plants using a Sand-vermiculite medium. Plant Soil 1987;101:45–50.
[6] Jeffries P, Barea JM. Bioeochemical cycling and arbuscular mycorrhizas in the sustainability of plant-soil system. In: Gianinazzi S, Schüepp H, editors. Impact of Arbuscular Mycorrhizas on Sustainable Agriculture and Natural Ecosystems. Basel, Switzerland: Birkhäuser Verlag, 1994. pp. 101–15.
[7] Toro M, Azcón R, Barea JM. Improvement of arbuscular mycorrhiza development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (32P) and nutrient cycling. Appl Environ Microbiol 1997;63:4408–12.
[8] Halder AK, Mishra AK, Bhattacharyya P, Chakrabartty PK. Solubilization of rock phosphate by Rhizobium and Bradyrhizobium. J Gen Appl Microbiol 1990;36:81–92.
[9] Duff RB, Webley DM. 2-Ketogluconic acid as a natural chelator produced by soil bacteria. Chem Ind 1959;1376–77.
[10] Sundara Rao WVB, Sinha MK. Phosphate dissolving micro-organisms in the soil and rhizosphere. Indian J Agric Sci 1963;33:272–8.
[11] Banik S, Dey BK. Available phosphate content of an alluvial soil is influenced by inoculation of some isolated phosphate-solubilizing microorganisms. Plant Soil 1982;69:353–64. H. Rodríguez, R. Fraga/Biotechnology Advances 17 (1999) 319–339 337
[12] Craven PA, Hayasaka SS. Inorganic phosphate solubilization by rhizosphere bacteria in a Zostera marina community. Can J Microbiol 1982;28:605–10.
[13] Leyval C, Berthelin J. Interaction between Laccaria laccata, Agrobacterium radiobacter and beech roots: influence on P, K, Mg and Fe movilization from minerals and plant growth. Plant Soil 1989;117:103–10.
[14] Salih HM, Yahya AY, Abdul-Rahem AM, Munam BH. Availability of phosphorus in a calcareus soil treated with rock phosphate or superphosphate as affected by phosphate dissolving fungi. Plant Soil 1989;120:181–5.
[15] Goldstein AH. Involvement of the quinoprotein glucose dehydrogenase in the solubilization of exogenous phosphates by gram-negative bacteria. In: Torriani-Gorini A, Yagil E, Silver, S, editors. Phosphate in Microorganisms: Cellular and Molecular Biology. Washington, DC: ASM Press, 1994. pp. 197–203. 334 H. Rodríguez, R. Fraga/Biotechnology Advances 17 (1999) 319–339