Secondary Metabolites of Pseudomonas Fluorescens Strain CHA0 Involved in the Suppression of Root Diseases

Pseudomonas fluorescens strain CHA0 protects plants from diseases caused by various soil-borne pathogenic fungi. Strain CHA0 produces and releases several metabolites: pyoverdine (pseudobactin), indoleacetate, HCN, pyoluteorin (Plt), and 2,4-diacetylphloroglucinol (Phl). We have begun to assess the importance of these compounds in disease suppression by testing non-producing CHA mutants in gnotobiotic systems and by cloning relevant genomic fragments from strain CHA0. No major role was found for pyoverdine and indoleacetate. HCN helped suppress black root rot caused by Thielaviopsis basicola (Tb) on tobacco. In contrast, an Hcn- mutant did not differ significantly from the wildtype in the suppression of wheat disease due to Gaeumannomyces graminis var. tritici (Ggt). The antifungal and herbicidal compound Phl was found to be produced in the rhizosphere of wheat by the wildtype CHA0. A Phi- mutant gave reduced suppression of diseases caused by Tb and Ggt; a recombinant cosmid restored both Phl production in the rhizosphere and disease suppression. Another cosmid carrying a 22 kb fragment from the genome of strain CHA0 enhanced the production of Phl and Pit in vitro and improved the protection of cucumber from Pythium ultimum, a Plt-sensitive fungus. Interestingly, the same 22 kb amplification rendered strain CHA0 deleterious to cress, presumably because of a herbicidal effect of Phl and Plt on cress. Thus, the production of antifungal compounds contributes importantly to the suppressive properties of strain CHA0, but overproduction of such compounds may have deleterious effects on plants.