ROOT COLONIZATION AND IRON NUTRITIONAL-STATUS OF A PSEUDOMONAS-FLUORESCENS IN DIFFERENT PLANT-SPECIES

Root colonization and induction of an iron stress regulated promoter f or siderophore production by Pseudomonas fluorescens 2-79RLI was studi ed in vitro and in the rhizosphere of different plant species. P. fluo rescens 2-79RLI was previously genetically modified with an iron regul ated ice nucleation reporter, which allowed calibration of ice nucleat ion activity with siderophore production. Initial experiments examined ice nucleation activity and siderophore production under different gr owth conditions in vitro. These studies demonstrated that P. fluoresce ns 2-79RLI could utilize both Fe-citrate and Fe-phytosiderophore as ir on sources, suggesting that production of these compounds by plants wo uld increase iron availability for P. fluorescens 2-79RLI in the rhizo sphere. Fe demand and Fe stress were further shown to be a function of nutrient availability and were reduced when carbon was limiting for g rowth. Subsequent experiments extended these observations to rhizosphe re cells. Cells were sampled from the rhizosphere and the rhizoplane. Results of a soil microcosm experiment showed that Fe stress was reduc ed for P. fluorescens 2-79RLI in the barley rhizosphere as compared to the cells in the rhizosphere of lupin. In lupin, relative Fe stress o f P. fluorescens 2-79RLI was greater at the root tip than in the later al root zone. In a second experiment comparing zucchini and bean, iron stress was greater for P. fluorescens 2-79RLI associated with zucchin i than with bean. In a third experiment with rape plants under P defic ient conditions, addition of soluble P was shown to increase Fe stress for P. fluorescens 2-79RLI located at the root tip, but not in the la teral root zone. This study showed that Fe stress of P. fluorescens 2- 79RLI in the rhizosphere may be influenced by plant species, P source, root zone and localization of the cells within the rhizosphere.