Submitted to: Federation of European Microbiological Societies Microbiology Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2005
Publication Date: 1/20/2006
Citation: Landa, B.B., Mavrodi, O.V., Schroeder, K.L., Allende-Molar, R., Weller, D.M. 2005. Enrichment and genotypic diversity of phld-containing fluorescent pseudomonas spp. in two soils after a century of wheat and flax monoculture. Federation of European Microbiological Societies Microbiology Ecology. FEMS Microbiol Ecol 55 (2006) 351-368. Interpretive Summary: Pseudomonas bacteria producing the antifungal, biocontrol metabolite 2,4-diacetylphloroglucinol (DAPG) suppress a wide spectrum of soilborne plant pathogens that cause wilts, damping-off and root diseases of food, fiber and ornamental crops. From a worldwide collection of DAPG producers, 18 distinct genotypes have been identified by molecular fingerprinting techniques. The build up of DAPG-producing Pseudomonas bacteria during wheat and barley monoculture is responsible for the natural suppressiveness of certain soils to take-all of wheat. The results of this research demonstrated that monoculture of flax also enriches for DAPG producers, indicating that these bacteria play a much broader role in plant defense against soilborne pathogens than previously thought.
Technical Abstract: Isolates of fluorescent Pseudomonas spp. producing the antibiotic 2,4-diacetylphloroglucinol (DAPG) play a key role in the natural suppression of take-all decline, which develops when a field is continuously cropped to wheat or barley after a take-all outbreak, and also contribute to the disease suppressiveness of other soils. In this study, soils from a unique site on the campus of North Dakota State University, Fargo, North Dakota, USA, where side-by-side fields have undergone continuous wheat, continuous flax or crop rotation for over 100 years were assayed for the presence of DAPG producers. Flax and wheat monoculture, but not crop rotation, enriched for DAPG producers, and populations sizes of log 5.0 CFU/g of root or higher were detected in the rhizosphere of wheat and flax grown in the two monoculture soils. The composition of the genotypes enriched by the two crops differed. Four known BOX-PCR genotypes (D, F, G, and J) and a new genotype (T) of DAPG producers were detected in the continuous flax soil, with F- and J-genotype isolates dominating (41.0 and 39.0% of the total, respectively). In contrast, two genotypes (D and I) were detected in the soil with continuous wheat, with D-genotype isolates comprising 76.5% of the total. In the crop rotation soils, populations of DAPG producers generally were below the detection limit and only one genotype (J) was detected. Under growth chamber and field conditions, D and I genotypes (enriched by wheat monoculture) colonized the wheat rhizosphere significantly better than isolates of other genotypes, while a J-genotype isolate colonized wheat and flax rhizospheres to the same extent. Results from this study, suggests that over many years of monoculture, the crop species grown in a field enriches for genotypes of DAPG producers from the reservoir of genotypes naturally present in the soil that are especially adapted to colonizing the rhizosphere of the crop grown.