|De La Fuente, Leonardo|
Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2005
Publication Date: 7/20/2006
Citation: De La Fuente, L., Landa, B.B., Weller, D.M. Host crop affects rhizosphere colonization and competitiveness of 2,4-diacetylphloroglucinol-producing pseudomonas fluorescens. Journal of Phytopathology. 96:751-762. 2006. Interpretive Summary: Some fluorescent 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, 22 distinct genotypes (A-T, PfY and PfZ) are known. Genotypes differ significantly in ability to colonize the roots of wheat and pea. Root colonizing ability is directly related to biocontrol activity against root diseases such as take-all. Isolates of multiple genotypes occur in a suppressive soil but no information has been available about what biotic and abiotic factors influence the interactions among genotypes. We introduced into the soil isolates of three genotypes, D, K and P, which are aggressive colonists of the rhizospheres of wheat and pea. Although all of the genotypes are equivalent in colonizing wheat and pea when introduced alone, the host crop species dramatically affects the interactions among genotypes when isolates of two or more genotypes present together. For example, the K-genotype isolate out-competes the P-genotype isolate in the wheat rhizosphere when both are present, but the opposite occurs in the pea rhizosphere. This study reveals why the composition of the genotypes found in a soil differ so dramatically when two different crops are grown in the same soil. This is important because the relative suppressiveness of the soil is in large part determined by the dominant genotype that has been selected by the plant from the pool of genotypes in the soil.
Technical Abstract: Strains of Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents, which play a key role in the suppressiveness of some soils against soilborne pathogens. We evaluated the effect of the host plant genotype on rhizosphere colonization by both indigenous and introduced 2,4-DAPG-producing P. fluorescens. First, population densities of 2,4-DAPG producers in the rhizospheres of alfalfa, barley, bean, flax, lentil, lupine, oat, pea, and wheat grown in a Fusarium wilt suppressive Puget silt loam soil were determined. Population densities differed among the various crops and among pea cultivars, with lentil and oat supporting the highest and lowest densities of 2,4-DAPG producers, respectively. To determine the interactions among 2,4-DAPG producers in the rhizosphere, a Shano sandy loam was inoculated individually and with all possible combinations of P. fluorescens Q8r1 96 (genotype D), F113 (genotype K) and MVP1-4 (genotype P), sown to wheat or pea, and the rhizosphere population dynamics of each strain was monitored.. All three strains were similar in ability to colonize the rhizosphere of wheat and pea when introduced alone into the soil, but when introduced together in equal densities, the outcome of the interactions differed according to the host crop. In the wheat rhizosphere, the population density of strain F113 was significantly greater than that of Q8r1-96 in the mixed inoculation studies, but no significant differences were observed on pea. The population density of strain Q8r1-96 was greater than that of MVP1-4 in the mixed inoculation on wheat, but the opposite occurred on pea. In the wheat rhizosphere, the population of MVP1-4 dropped below the detection limit (log 3.26 CFU g-1 root) in the presence of F113, but on pea the population densitity of MVP1-4 was higher than that of F113. When all three strains were present together, F113 had the greatest density in the wheat rhizosphere, but MVP1-4 was dominant in the pea rhizosphere. Finally, eight pea cultivars were grown in soil inoculated with either MVP1-4 or Q8r1-96. The effect of the pea cultivar on rhizosphere colonization was dependent on the bacterial strain inoculated. Rhizosphere population densities of MVP1-4 did not differ significantly among pea cultivars, whereas population densities of Q8r1-96 did. We conclude from these studies that the host crop plays a key role in modulating both rhizosphere colonization by 2,4-DAPG-producing P. fluorescens and in the interactions among different genotypes present in the same rhizosphere.