N-ACYL-HOMOSERINE LACTONE-MEDIATED REGULATION OF PHENAZINE GENE-EXPRESSION BY PSEUDOMONAS-AUREOFACIENS 30-84 IN THE WHEAT RHIZOSPHERE

Pseudomonas aureofaciens 30-84 is a soilborne bacterium that colonizes the wheat rhizosphere. This strain produces three phenazine antibioti cs which suppress take-all disease of wheat by inhibition of the causa tive agent Gaeumannomyces graminis var. tritici. Phenazines also enhan ce survival of 30-84 within the wheat rhizosphere in competition with other organisms. Expression of the phenazine biosynthetic operon is co ntrolled by the phyR/phzI N-acyl-homoserine lactone (AHL) response sys tem (L. S. Pierson III et al., J. Bacterial 176:3966-3974, 1994; D. W. Wood and L. S. Pierson III, Gene 168:49-53, 1996). By using high-pres sure liquid chromatography coupled with high resolution mass spectrome try, the AHL produced by PhzI has now been identified as N-hexanoyl-ho moserine lactone (HHL). In addition, the ability of HHL to serve as an interpopulation signal molecule in the wheat rhizosphere has been exa mined by using isogenic reporter strains. Disruption of phzI reduced e xpression of the phenazine biosynthetic operon 1,000-fold in the wheat rhizosphere. Coinoculation of an isogenic strain which produced the e ndogenous HHL signal restored phenazine gene expression in the phzI mu tant to wild-type levels in situ. These results demonstrate that HHL i s required for phenazine expression in situ and is an effective interp opulation signal molecule in the wheat rhizosphere.