| Rhizosphere isolates of Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are effective biocontrol agents, and are responsible for the suppressiveness of some soils to soilborne pathogens. The diversity within a worldwide collection of 2,4-DAPG-producers was assessed by sequencing phlD, a key gene in the 2,4-DAPG biosynthetic locus. Phylogenetic analyses resulted in groupings similar to those previously identified by BOX-PCR (genotypes A-Q and T). Exploiting polymorphisms in the sequence of phlD, allele-specific PCR primers were designed for the detection of strains of six genotypes. The primers allowed quantification of populations of strains competing in the rhizosphere. Soil was inoculated with all possible combinations of P. fluorescens Q8r1-96 (genotype D), F113 (genotype K) and MVP1-4 (genotype P) to study competitive interactions among the strains. The three strains were equally effective at colonizing the wheat and pea rhizospheres when inoculated alone into the soil, but when introduced together the outcome of the competition varied depending on the host crop. Strains F113 and Q8r1-96 were more competitive than MVP1-4 on wheat; but MVP1-4 was the most aggressive competitor on pea. Alfalfa, bean, barley, flax, lentil, lupine, oat, pea and wheat differed quantitatively in the selection of indigenous populations of 2,4-DAPG-producers from the soil. Population densities of the inoculated strain Q8r1-96 differed significantly in the rhizosphere of eight pea cultivars; whereas MVP1-4 populations did not. Host crop plays a key role in modulation of the rhizosphere colonization by 2,4-DAPG-producers, especially in the competitiveness among strains occurring in the same soil. No correlation was found between the ability to utilize trehalose, benzoate or valerate as sole carbon sources and the rhizosphere competence of 2,4-DAPG-producers. A correlation was found between the use of these compounds by 2,4-DAPG-producers and the previous groupings identified by BOX-PCR. All strains grew similarly on seed or root exudates from wheat and pea. Differences in growth were detected only in wheat root exudates, where slow-growing strains included "premier" (Q8r1-96) and "average" (Q2-87) colonizer strains. The superior ability to colonize the rhizosphere of certain crops was not explained by the utilization of the carbon sources tested in our study. |
