Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/24/2007
Publication Date: 4/24/2007
Citation: De La Fuente, L., Mavrodi, D.V., Thomashow, L.S., Weller, D.M. 2007. Utilization of Trehalose, Benzoate, Valerate, and Seed and Root Exudates as Sole Carbon Sources is Not Correlated With Superior Rhizosphere Colonization by 2,4-Diacetylphloroglucinol Producing Pseudomonas spp.. Soil Biology and Biochemistry. 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 suppressiveness of certain soils to diseases such as take-all of wheat. 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: some are superior and some are average. Understanding root colonizing ability is important because it directly relates to biocontrol activity against root diseases such as take-all. The purpose of this study was to determine if the ability of a genotype to utilize certain sole carbon sources for growth (trehalose, benzoate or valerate) or seed or root exudates was related to superior root colonization of wheat or pea. Although superior colonizing D-genotype isolates utilized trehalose, benzoate and valerate as sole carbon sources, average colonizing isolates of other genotypes also utilized one or more of these compounds. In addition, superior and average colonizers did not differ in ability to grow on seed and root exudates. Although this study did not reveal the physiological basis of superior root colonization, it is important because it narrowed the field of possible traits that could account for superior colonization. Understanding the traits involved in root colonization will greatly enhance the use of pseudomonads for biocontrol, which is a sustainable and ecologically-friendly approach to controlling soilborne pathogens.
Technical Abstract: Fluorescent Pseudomonas spp. producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are effective biological control agents against several soilborne pathogens. A previous study showed that the superior (“premier”) root colonizer P. fluorescens Q8r1-96 differed from two average colonizers in the utilization of trehalose, benzoate and valerate as sole carbon sources. We tested the utilization of trehalose, benzoate and valerate by a collection of 55 2,4-DAPG-producing fluorescent Pseudomonas spp. strains, and we found no correlation between the ability to utilize these carbon sources and rhizosphere competence. 73 %, 48 %, and 69 % of strains were able to utilize trehalose, benzoate and valerate as sole carbon sources, respectively. We found a correlation between the utilization of these compounds and previous groupings of these strains by BOX-PCR. Twenty three strains grew efficiently on wheat and pea root and seed exudates, with doubling times between 0.9 and 1.6 h generation-1 and lag phases between 5 and 8 h, comparable to growth in glucose as a sole carbon source. Only 10 strains including strains with premier (Q8r1-96) and average (Q2-87) rhizosphere competence showed slower growth in wheat root exudates, with lag phases between 16 and 22 h. Results were the same when soil was added to the culture media. Growth of four strains in minimal medium containing glucose, wheat seed exudate or pea seed exudates was not influenced by whether culture media were obtained from wheat seeds or rich medium. We conclude that the superior ability of some strains to colonize the roots of certain crops can not be explained by the utilization of the carbon sources tested in our study.