Suppression of Root Diseases by Pseudomonas fluorescens CHA0: Importance of the Bacterial Secondary Metabolite 2,4-Diacetylphloroglucinol. Christoph Keel. Departments of Plant Sciences/Phytomedicine Swiss Federal Institute of Technology (ETH) of Zurich. Ursula Schnider(2), Monika Maurhofer(1), Christophè Voisard(2), Jacques Laville(2), Ulrich Burger(3), Philippe Wirthner(3), Dieter Haas(2), and Geneviève Défago(1). Departments of (1)Plant Sciences/Phytomedicine and (2)Microbiology, Swiss Federal Institute of Technology (ETH) of Zurich, and (3)Department of Organic Chemistry, University of Geneva, Switzerland.. MPMI 5:4-13. Accepted 1 August 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

Pseudomonas fluorescens strain CHA0 suppresses Thielaviopsis basicola-induced black root rot of tobacco and Gaeumannomyces graminis var. tritici-induced take-all of wheat. Strain CHA0 produces 2,4-diacetylphloroglucinol, a metabolite with antifungal, antibacterial, and phytotoxic activity. The role of this compound in disease suppression was tested under gnotobiotic conditions. A P. fluorescens mutant, obtained by Tn5 insertion, did not produce 2,4-diacetylphloroglucinol, showed diminished inhibition of T. basicola and of G. g. var. tritici in vitro, and had a reduced suppressive effect on tobacco black root rot and on take-all of wheat, compared with wild-type CHA0. Complementation of the mutant with an 11-kb DNA fragment from a genomic library of wild-type CHA0 largely restored production of the metabolite, inhibition of the fungal pathogens in vitro and disease suppression. The Tn5 insertion was physically mapped using a 5.8-kb complementing fragment as a probe. 2,4-Diacetylphloroglucinol was shown to be produced in the rhizosphere of wheat by strain CHA0 and by the complemented mutant, but not by the mutant defective in 2,4-diacetylphloroglucinol synthesis. These results support the importance of 2,4-diacetylphloroglucinol production by strain CHA0 in the suppression of soilborne plant pathogens in the rhizosphere.