Pseudomonas Fluorescens - Biocontrol Properties

Biocontrol Properties

Some P. fluorescens strains (CHA0 or Pf-5, for example) present biocontrol properties, protecting the roots of some plant species against parasitic fungi such as Fusarium or Pythium, as well as some phytophagous nematodes.

It is not clear exactly how the plant growth-promoting properties of P. fluorescens are achieved; theories include:

• the bacteria might induce systemic resistance in the host plant, so it can better resist attack by a true pathogen
• the bacteria might outcompete other (pathogenic) soil microbes, e.g., by siderophores, giving a competitive advantage at scavenging for iron
• the bacteria might produce compounds antagonistic to other soil microbes, such as phenazine-type antibiotics or hydrogen cyanide

To be specific, certain P. fluorescens isolates produce the secondary metabolite 2,4-diacetylphloroglucinol (2,4-DAPG), the compound found to be responsible for antiphytopathogenic and biocontrol properties in these strains. The phl gene cluster encodes factors for 2,4-DAPG biosynthesis, regulation, export, and degradation. Eight genes, phlHGFACBDE, are annotated in this cluster and conserved organizationally in 2,4-DAPG-producing strains of P. fluorescens. Of these genes, phlD encodes a type III polyketide synthase, representing the key biosynthetic factor for 2,4-DAPG production. PhlD shows similarity to plant chalcone synthases and has been theorized to originate from horizontal gene transfer. But phylogenetic and genomic analysis has revealed that the entire phl gene cluster is ancestral to P. fluorescens, many strains have lost the capacity, and it exists on different genomic regions among strains.

There is experimental evidence to support all of these theories, in certain conditions; a good review of the topic is written by Haas and Defago.

The strain referred to as Pf-CL145A has proved itself a promising solution to the invasive Dreissena( zebra and quagga) mussels. The strain of the bacteria produces toxins that destroy the digestive system of the mussels and produces a >90% kill rate.

Several strains of "P. Fluorescens", such as Pf-5 and JL3985, have developed a natural resistance to ampicillin and streptomycin. These antibiotics are regularly used in biological research as a selective pressure tool to promote plasmid expression.