Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/14/2003
Publication Date: 6/14/2003
Citation: LANDA, B.B., MAVRODI, D.M., THOMASHOW, L.S., WELLER, D.M. 2003. INTERACTIONS BETWEEN STRAINS OF 2,4-DIACETYLPHLOROGLUCINOL-PRODUCING PSEUDOMONAS FLUORESCENS IN THE RHIZOSPHERE OF WHEAT. PHYTOPATHOLOGY 93:982-994. Interpretive Summary: Some Pseudomonas bacteria 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 bacteria also are responsible for the natural suppressiveness of certain soils to diseases such as take-all of wheat and black root rot. From a worldwide collection of DAPG producers, 17 distinct genotypes (A-Q) were identified by molecular fingerprinting techniques. Genotypes differed significantly in ability to colonize the roots of wheat. Strains belonging to genotypes D and K aggressively colonized the rhizosphere of wheat substantially better than other genotypes (A, B, and L). Root colonizing ability is directly related to biocontrol activity against root diseases such as take-all. This research demonstrates that by matching a DAPG producer genotype with a crop that it aggressively colonizes, biocontrol can be significantly improved.
Technical Abstract: Strains of fluorescent Pseudomonas spp. that produce the antibiotic 2,4-diacetylphoroglucinol (2,4-DAPG) are among the most effective rhizobacteria controlling diseases caused by soilborne pathogens. The genotypic diversity that exists among 2,4-DAPG producers can be exploited to improve rhizosphere competence and biocontrol activity. Knowing that D-genotype 2,4-DAPG-producing strains are enriched in some take-all decline soils and that P. fluorescens Q8r1-96, a representative D-genotype strain, as defined by whole-cell repetitive sequence-based polymerase chain reaction (rep-PCR) with the BOXA1R primer, is a superior colonizer of wheat roots, we analyzed whether the exceptional rhizosphere competence of strain Q8r1-96 on wheat is characteristic of other isolates. The rhizosphere population densities of four D-genotype strains and a K-genotype strain introduced individually into the soil were significantly greater than the densities of four strains belonging to other genotypes (A,B, and L) and remained above log 6.8 CFU/g of root over a 30-week cycling experiment in which wheat was grown for 10 successive cycles of 3 weeks each. We also explored the competitive interactions between strains of different genotypes inhabiting the same soil or rhizosphere when coinoculated into the soil. Strain Q8r1-96 became dominant in the rhizosphere and in nonrhizosphere soil during a 15-week cycling experiment when mixed in a 1:1 ratio with either strain Pf-5 (A genotype), Q2-87 (B genotype), or 1M1-96 (L genotype). Furthermore, the use of the de Wit replacement series demonstrated a competitive disadvantage for strain Q2-87 or strong antagonism by strain Q8r1-96 against Q2-87 in the wheat rhizosphere. Amplified rDNA restriction analysis and sequence analysis of 16S rDNA showed that species of Arthrobacter, Chryseobacterium, Flavobacterium, Massilia, Microbacterium, and Ralstonia also were enriched in culturable populations from the rhizosphere of wheat at the end of a 30-week cycling experiment in the presence of 2,4-DAPG producers. Identifying the interactions among 2,4-DAPG producers and with other indigenous bacteria in the wheat rhizosphere will help to elucidate the variability in biocontrol efficacy of introduced 2,4-DAPG producers and fluctuations in the robustness of take-all suppressive soils.