Novel screen for investigating in situ rhizosphere production of the antibiotic 2,4-diacetylphloroglucinol by bacterial inocula

The rhizosphere is a complex zone of multitrophic interactions comprising plant roots, associated bacteria, fungi, and micro-, meso-, and macro-fauna. Of considerable importance in this system is the production of antibiotics by root-associated or "rhizo" bacteria. This is a widespread phenomenon of which a much-studied exemplar is the production by pseudomonads of 2,4-diacetylphloroglucinol (DAPG), known to be effective in the suppression of soil-borne fungal pathogens. Rapid advances in understanding the molecular and biochemical bases of antibiotic (particularly DAPG) production have been made. However, our understanding of in situ antibiotic production currently lags behind this. There is therefore a need for a rapid soil-based screen with which to identify antibiotic producers under rhizosphere C-flow conditions. Here, a novel "rhizocosm," comprising porous pipe and Rhizon samplers, was superior to a simple, nonperfusing incubation-type microcosm with respect to supporting a rhizobacterial inoculum. Its use as a screening tool is illustrated by screening known DAPG-producing mocula in soil continuously supplied with simulated rhizosphere carbon flow. The DAPG was then extracted from soil using acetone and quantified by high-performance liquid chromatography (HPLC). Simple sugars (glucose and fructose) stimulated the greatest DAPG production, while glucose with an additional organic or amino acid, or with a signal molecule, resulted in strongly variable DAPG expression.