Pseudomonas fluorescens Pf-5, a rhizosphereinhabiting bacterium that suppresses several soilborne pathogens of plants, produces the antibiotics pyrrolnitrin, pyoluteorin, and 2,4-diacetylphloroglucinol. A gene necessary for pyrrolnitrin production by Pf-5 was identified as rpoS, which encodes the stationary-phase sigma factor as. Several pleiotropic effects of an rpoS mutation in Escherichia coli also were observed in an RpoS-mutant of Pf-5. These included sensitivities of stationary-phase cells to stresses imposed by hydrogen peroxide or high salt concentration. A plasmid containing the cloned wild-type rpoS gene restored pyrrolnitrin production and stress tolerance to the RpoS-mutant of Pf-5. The RpoS-mutant overproduced pyoluteorin and 2,4-diacetylphloroglucinol, two antibiotics that inhibit growth of the phytopathogenic fungus Pythium ultimum, and was superior to the wild type in suppression of seedling damping-off of cucumber caused by Pythium ultimum. When inoculated onto cucumber seed at high cell densities, the RpoS-mutant did not survive as well as the wild-type strain on surfaces of developing seedlings. Other stationary-phase-specific phenotypes of Pf-5, such as the production of cyanide and extracellular protease(s) were expressed by the RpoS-mutant, suggesting that a-s is only one of the sigma factors required for the transcription of genes in stationary-phase cells of P. fluorescens. These results indicate that a sigma factor encoded by rpoS influences antibiotic production, biological control activity, and survival of P. fluorescens on plant surfaces.

2,4-Diacetylphloroglucinol was detected in and isolated from culture extracts of the biological control bacterium Pseudomonas fluorescens Pf-5. Its structure was identified using a combination of chromatographic techniques and NMR spectroscopic methods. Carbon source influenced 2,4-diacetylphloroglucinol production by Pf-5 in culture media. 2,4-Diacetylphloroglucinol inhibited growth of the plant pathogenic fungi Pythium ultimum and Rhizoctonia solani, and the plant pathogenic bacterium Erwinia carotovora subsp. atroceptica, which cause diseases that are suppressed by strain Pf-5. The results of this study provide further evidence for the prevalence of 2,4-diacetylphloroglucinol production among strains of Pseudomonas fluorescens that suppress plant diseases.Key words: 2,4-diacetylphloroglucinol, Pseudomonas fluorescens, biocontrol.

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