Use of Phosphate-Reporter Bacteria to Study Phosphate Limitation in the Rhizosphere and in Bulk Soil. Letty A. de Weger. Institute of Molecular Plant Sciences, Leiden University, Clusius Laboratory, Wassenaarseweg 64, 2333AL Leiden, The Netherlands. Linda C. Dekkers, Arjan J. van der Bij, Ben J. J. Lugtenberg. Institute of Molecular Plant Sciences, Leiden University, Clusius Laboratory, Wassenaarseweg 64, 2333AL Leiden, The Netherlands. MPMI 7:32-38. Accepted 13 September 1993. Copyright 1994 The American Phytopathological Society.

Many relevant bacterial-plant interactions occur in the rhizosphere of the plant, but the conditions under which these interactions take place are largely unknown. In this study, we focused on the phosphate availability in the rhizosphere and in bulk soil. Using a promoterless TnlacZ, four phosphate-reporter strains of Pseudomonas putida strain WCS358 were constructed that respond to phosphate limitation by the production of ß-galactosidasc. These strains did not respond to other nutrient-limiting or stress conditions. In the phosphale-reporter strain LP7, showing the highest levels of ß-galactosidase activity upon phosphate-limited growth, ß-gaIactosidase was induced when the level of phosphate dropped below 30-35 (M. When cells of this strain were growing under gnotobiotic conditions in the rhizosphere of potato, tomato, or radish plants, or in bulk soil or sand, they sensed phosphate limitation as judged from the significant increase in ß-galactosidasc activity in these cells. This study showed that reporter bacteria can be used to report on the growth conditions in the rhizosphere and in bulk soil or sand. From these results, it can be predicted that the use of a combination of different reporter bacteria responding to various conditions (e.g., limitation for nitrogen, carbon, or iron) will reveal the growth conditions in ecologically relevant niches.