Role of Antibiotic Biosynthesis in the Inhibition of Pythium ultimum in the Cotton Spermosphere and Rhizosphere by Pseudomonas fluorescens. William J. Howie. DNA Plant Technology Corporation, Oakland, CA 94608 U.S.A. Trevor V. Suslow. DNA Plant Technology Corporation, Oakland, CA 94608 U.S.A. MPMI 4:393-399. Accepted 17 April 1991. Copyright 1991 The American Phytopathological Society.

Pseudomonas fluorescens strain Hv37aR2 and its isogenic Afu‾ mutants deficient in biosynthesis of an antifungal compound were used to examine the role of an antibiotic in the suppression in vivo of Pythium ultimum infection on cotton. The results demonstrated that suppression of disease development is dominated by the biosynthesis of an antibiotic(s). An average of 70% of the reduction in root infection and an average of 50% of the increase in emergence caused strain by Hv37aR2 was attributed to the biosynthesis of the antibiotic, oomycin A. Afu‾ strains colonized the spermosphere and rhizosphere of cotton as well as their parental strain, indicating that the Afu‾ phenotype did not compromise their colonization ability. Seed treatment with strain Hv37aR2 resulted in the formation of significantly fewer Pythium propagules in the soil after root infection as compared to Afu‾ mutant strain WH103 and the nontreated control. A population level of strain Hv37aR2 of 10⁶ cfu per seed at planting was necessary to ensure protection from disease development caused by Pythium. Expression in vivo of an operon required for antibiotic biosynthesis was determined using a lacZ gene (Tn3HoHo-1) transcriptional fusion. Gene expression was monitored by measuring Β-galactosidase activity in strains growing on cotton seeds. Enzyme activity was readily detectable in the parental strain at 24 h, yet barely detectable in the Afu‾ mutant strain. These data demonstrate that the production of an antibiotic(s) in vitro was indirectly correlated with biosynthesis of the compound and biological control in vivo.