ROOT EXUDATE-INDUCED PROMOTER ACTIVITY IN PSEUDOMONAS-FLUORESCENS MUTANTS IN THE WHEAT RHIZOSPHERE

Tn5-B20 (lacZ as reporter gene) transcriptional fusion mutants of Pseu domonas fluorescens R2f were screened for their response to wheat root exudate, Several mutants showed beta-galactosidase activity under the influence of wheat root exudate, In one such mutant, RIWE8, gene expr ession was specifically induced by proline but not by 125 other substr ates, This mutant also showed reporter gene induction, albeit to a les ser extent, by exudate of maize and grass roots but not by that of clo ver roots, In situ promoter activity of RIWE8 was found in Flevo silt loam soil amended with proline but not in water-, arginine-, glutamic acid-, or malic acid-amended soils. Reporter gene expression in RIWE8 was triggered in a model rhizosphere-soil system in the presence of wh eat, maize, and grass roots but not in the presence of clover roots or in root-free (bulk) soil. The induction of expression of the reporter gene in soil, using this approach, is suggestive that promoter activi ty in RIWE8 may be useful for the construction of organisms with rhizo sphere-controlled beneficial genes.