Submitted to: Annual BIORHIZ Symposium
Publication Type: Abstract Only
Publication Acceptance Date: October 22, 2006
Publication Date: N/A
Technical Abstract: Plants lack genetic resistance to some of the most common and widespread soilborne pathogens. Therefore, they have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, U.S.A. and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of at least 105 CFU/g of root, the threshold required to suppress the take-all pathogen Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the natural defense of plant roots in many different agroecosystems. Twenty-two distinct genotypes of 2,4-DAPG producers (designated A – T, PfY and PfZ) currently have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of phlD, and phylogenetic analysis of phlD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil but usually only one or two genotypes are dominant. The crop species modulates the outcome of the competition among genotypes in the rhizosphere. Some genotypes and crop species have a mutual a preference for each other; for example genotype-D isolates are very aggressive colonists of the rhizosphere of wheat and pea but genotype A-, B-, E-, L-, O- and Q-isolates are not. 2,4- DAPG producers are highly effective biocontrol agents against a variety of plant pathogens and are ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.