Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 9/30/2007
Publication Date: N/A
Interpretive Summary: Some strains of the bacterium Pseudomonas fluorescens produce the antifungal, biocontrol metabolite 2,4-diacetylphloroglucinol (DAPG). These bacteria suppress a wide spectrum of soilborne plant pathogens that cause wilts, damping-off and root diseases of food, fiber and ornamental crops. DAPG-producing Pseudomonas fluorescens also are responsible for the natural suppression of take-all disease of wheat known as take-all decline (TAD). TAD develops during continuous monoculture of wheat or barley and is due to the buildup of DAPG producers in the rhizosphere. Currently, 22 distinct genotypes (A-T, PfY and PfZ) of DAPG producers are known. Genotypes differ significantly in ability to colonize the roots of wheat and pea: some are superior and some are average. Root colonizing ability is important because it directly relates to biocontrol activity against root diseases such as take-all. Superior root colonizing genotypes can be engineered to produce additional biocontrol traits, thus enhance the biocontrol activity of a strain. This review focuses on the selection, monitoring, and enhancement of bacterial biocontrol agents for the control of soilborne pathogens. It also discusses the enhancement of biocontrol strains by genetic engineering. The use of environmentally-friendly natural and engineered biocontrol bacteria with superior root colonizing ability allows the management of root diseases and crown rots without the application of chemical pesticides and fumigants.
Technical Abstract: Genetic resistance to root diseases of plants is rare, and these diseases are most commonly controlled through the use of cultural practices and synthetic fungicides. Plants also defend themselves by supporting rhizosphere microorganisms antagonistic to soilborne pathogens. Antibiotic production is an important mechanism of plant defense by many of these rhizobacteria, and much is known about the genetics, biochemistry, and regulation of synthesis of some of the most commonly-produced antibiotics. Similarly, many genes that contribute to the ability of rhizobacteria to colonize roots have been identified. Studies of naturally suppressive soils have provided evidence of preferential interactions between plant hosts and protective populations of Pseudomonas fluorescens, revealing the existence of functional diversity among rhizosphere isolates that produce the antibiotic 2,4-diacetylphloroglucinol (DAPG) and are distinguishable on the basis of their genetic fingerprints. Our research has focused on P. fluorescens Q8r1-96, characteristic of the D-genotype of P. fluorescens strains that are exceptionally aggressive colonizers of the wheat rhizosphere. We have investigated the molecular basis for the unique competitiveness of Q8r1-96 on wheat, have used the strain to construct recombinant derivatives expressing genes for the synthesis of phenazine antibiotics, and have monitored the stability, persistence, spread, and nontarget effects of the wild-type and recombinant strains in the growth chamber and the field. Our results have revealed metabolic costs to competitiveness associated with pyramiding pathways, unexpected interactions affecting antibioticproduction, and host effects on the relative competitiveness of wild-type and recombinant strains.