USE OF PHOSPHATE-REPORTER BACTERIA TO STUDY PHOSPHATE LIMITATION IN THE RHIZOSPHERE AND IN BULK SOIL

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 beta-galactosidase. These strains did not respond to other nutrient-limiting or stress conditions. In the phosphate-reporter strain LP7, showing the highest levels of beta-galactosidase activity upon phosphate-limited growth, beta-galactosidase was induced when the level of phosphate dropped below 30-35 mu 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 beta-galactosidase 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.