MUTATIONAL DISRUPTION OF THE BIOSYNTHESIS GENES-CODING FOR THE ANTIFUNGAL METABOLITE 2,4-DIACETYLPHLOROGLUCINOL DOES NOT INFLUENCE THE ECOLOGICAL FITNESS OF PSEUDOMONAS-FLUORESCENS F113 IN THE RHIZOSPHERE OF SUGAR-BEETS

The ability of Pseudomonas fluorescens F113 to produce the antibiotic 2,4-diacetylphloroglucinol (DAPG) is a key factor in the biocontrol of the phytopathogenic fungus Pythium ultimum by this strain, In this st udy, a DAPG-producing strain (rifampin-resistant mutant F113Rif) was c ompared with a nearly isogenic DAPG-negative biosynthesis mutant (Tn5: :lacZY derivative F113G22) in terms of the ability to colonize and per sist in the rhizosphere of sugarbeets in soil microcosms during 10 pla nt growth-harvest cycles totaling 270 days. Both strains persisted sim ilarly in the rhizosphere for 27 days, regardless of whether they had been inoculated singly onto seeds or coinoculated in a 1:1 ratio, In o rder to simulate harvest and resolving, the roots were removed from th e soil and the pots were resown with uninoculated sugarbeet seeds for nine successive 27-day growth-harvest cycles, Strains F113Rif and F113 G22 performed similarly with respect to colonizing the rhizosphere of sugarbeet, even after nine cycles without reinoculation. The introduce d strains had a transient effect on the size of the total culturable a erobic bacterial population. The results indicate that under these exp erimental conditions, the inability to produce DAPG did not reduce the ecological fitness of strain f113 in the rhizosphere of sugarbeets.