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.

References

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[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