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A Rapid Polymerase Chain Reaction-Based Assay Characterizing Rhizosphere Populations of 2,4-Diacetylphloroglucinol-Producing Bacteria

First, third, and fourth authors: U.S. Department of Agriculture, Agricultural Research Service, Root Disease and Biological Control Research Unit, Washington State University, Pullman 99164; and second author; Washington State University, Department of Plant Pathology, Pullman 99164

Pseudomonas species that produce 2,4-diacetylphloroglucinol (2,4-DAPG) play a significant role in the suppression of fungal root pathogens in the rhizosphere of crop plants. To characterize the abundance and diversity of these functionally important bacterial populations, we developed a rapid polymerase chain reaction (PCR)-based assay targeting phlD, an essential gene in the phloroglucinol biosynthetic pathway. The phlDgene is predicted to encode a polyketide synthase that synthesizes mono-acetylphloroglucinol, the immediate precursor to 2,4-DAPG. A major portion of the phlD open reading frame was cloned and sequenced from five genotypically distinct strains, and the sequences were screened for conserved regions that could be used as gene-specific priming sites for PCR amplification. Several new phlD-specific primers were designed and evaluated. Using the primers B2BF and BPR4, we developed a PCR-based assay that was robust enough to amplify the target gene from a diverse set of 2,4-DAPG producers and sensitive enough to detect as few as log 2.4 cells per sample when combined with enrichment from a selective medium. Restriction fragment length polymorphism analysis of the amplified phlD sequence allows for the direct determination of the genotype of the most abundant 2,4-DAPG producers in a sample. The method described was useful for characterizing both inoculant and indigenous phlD⁺ pseudomonads inhabiting the rhizosphere of crop plants. The ability to rapidly characterize populations of 2,4-DAPG-producers will greatly enhance our understanding of their role in the suppression of root diseases.