Fusaric acid-producing strains of Fusarium oxysporum alter 2,4-diacetylphloroglucinol biosynthetic gene expression in Pseudomonas fluorescens CHA0 in vitro and in the rhizosphere of wheat

The phytotoxic pathogenicity factor fusaric acid (FA) represses the production of 2,4-diacetylphloroglucinol (DAPG), a key factor in the antimicrobial activity of the biocontrol strain Pseudomonas fluorescens CHA0. FA production by 12 Fusarium oxysporum strains varied substantially. We measured the effect of FA production on expression of the phLACBDE biosynthetic operon of strain CHA0 in culture media and in the wheat rhizosphere by using a translational phL4'-'lacZ fusion. Only FA-producing F. oxysporum strains could suppress DAPG production in strain CHA0, and the FA concentration was strongly correlated with the degree of phL4 repression. The repressing effect of FA on phLA'-'lacZ expression was abolished in a mutant that lacked the DAPG pathway-specific repressor Ph1F. One FA-producing strain (798) and one nonproducing strain (242) of F. oxysporum were tested for their influence on phL4 expression in CHA0 in the rhizosphere of wheat in a gnotobiotic system containing a sand and clay mineral-based artificial soil. F. oxysporum strain 798 (FA(+)) repressed phL4 expression in CHA0 significantly, whereas strain 242 (FA(-)) did not. In the phlF mutant CHA638, phLA expression was not altered by the presence of either F. oxysporum strain 242 or 798. phL4 expression levels were seven to eight times higher in strain CHA638 than in the,wild-type CHA0, indicating that Ph1F limits phL4 expression in the wheat rhizosphere.