Utilization of trehalose, benzoate, valerate, and seed and root exudates by genotypes of 2,4-diacetylphloroglucinol producing Pseudomonas fluorescens

Isolates of Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are effective biocontrol agents against soilborne pathogens. A previous study showed that the superior (“premier”) root colonizer P. fluorescens Q8r1-96 (genotype D) utilized trehalose, benzoate and valerate as sole carbon sources but average colonizers Q2-87 (genotype B) and 1M1-96 (genotype L) did not. We tested the utilization of these three carbon sources by a collection of 55 2,4-DAPG-producing P. fluorescens strains from 17 genotypes and found no correlation between a strain's ability to utilize these carbon sources and superior rhizosphere competence on wheat and pea. Of the strains tested, 73%, 48% and 69% were able to utilize trehalose, benzoate and valerate as sole carbon sources, respectively. With some exceptions, we found a correlation between the utilization of these compounds and previous groupings of these strains by BOX-PCR; genotype D strains utilized all three compounds. Twenty-three strains grew efficiently on root and seed exudates from wheat and pea, with doubling times between 0.9 and 1.6 h generation−1 and lag phases between 5 and 8 h, comparable to growth on glucose as a sole carbon source. Only 10 strains, including those 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 medium. Growth of four strains in media containing glucose or wheat or pea seed exudates as a sole carbon source was not influenced by whether the bacterial cells used as inoculum were harvested from wheat seeds or broth culture. We conclude that the superior ability of some strains to colonize the roots of certain crops cannot be explained by the utilization of the carbon sources tested in our study.