Autoinduction of 2,4-Diacetylphloroglucinol Biosynthesis in the Biocontrol Agent Pseudomonas fluorescens CHA0 and Repression by the Bacterial Metabolites Salicylate and Pyoluteorin

Abstract

The antimicrobial metabolite 2,4-diacetylphloroglucinol (2,4-DAPG) contributes to the capacity of Pseudomonas fluorescens strain CHA0 to control plant diseases caused by soilborne pathogens. A 2,4-DAPG-negative Tn 5 insertion mutant of strain CHA0 was isolated, and the nucleotide sequence of the 4-kb genomic DNA region adjacent to the Tn 5 insertion site was determined. Four open reading frames were identified, two of which were homologous to phlA , the first gene of the 2,4-DAPG biosynthetic operon, and to the phlF gene encoding a pathway-specific transcriptional repressor. The Tn 5 insertion was located in an open reading frame, tentatively named phlH , which is not related to known phl genes. In wild-type CHA0, 2,4-DAPG production paralleled expression of a phlA′-′lacZ translational fusion, reaching a maximum in the late exponential growth phase. Thereafter, the compound appeared to be degraded to monoacetylphloroglucinol by the bacterium. 2,4-DAPG was identified as the active compound in extracts from culture supernatants of strain CHA0 specifically inducing phlA′-′lacZ expression about sixfold during exponential growth. Induction by exogenous 2,4-DAPG was most conspicuous in a phlA mutant, which was unable to produce 2,4-DAPG. In a phlF mutant, 2,4-DAPG production was enhanced severalfold and phlA′-′lacZ was expressed at a level corresponding to that in the wild type with 2,4-DAPG added. The phlF mutant was insensitive to 2,4-DAPG addition. A transcriptional phlA-lacZ fusion was used to demonstrate that the repressor PhlF acts at the level of transcription. Expression of phlA′-′lacZ and 2,4-DAPG synthesis in strain CHA0 was strongly repressed by the bacterial extracellular metabolites salicylate and pyoluteorin as well as by fusaric acid, a toxin produced by the pythopathogenic fungus Fusarium . In the phlF mutant, these compounds did not affect phlA′-′lacZ expression and 2,4-DAPG production. PhlF-mediated induction by 2,4-DAPG and repression by salicylate of phlA′-′lacZ expression was confirmed by using Escherichia coli as a heterologous host. In conclusion, our results show that autoinduction of 2,4-DAPG biosynthesis can be countered by certain bacterial (and fungal) metabolites. This mechanism, which depends on phlF function, may help P. fluorescens to produce homeostatically balanced amounts of extracellular metabolites.