Phytosiderophores decrease iron stress and pyoverdine production of Pseudomonas fluorescens PF-5 (pvd-inaZ)

Experiments were conducted to study the relationship between iron stress and pyoverdine production during growth of Pseudomonas fluorescens Pf-5 (pvd-inaZ) in vitro, and the Fe stress status of this bacterium in the rhizosphere of barley and rice. The bacterial strain used for these experiments has been engineered with a plasmid containing an iron-regulated ice nucleation reporter that enables monitoring of the relative Fe stress status of the cells. In vitro, Fe stress and pyoverdine production of P. fluorescens Pf-5 (pvd-inaZ) decreased in the following order: 0 Fe>50 μm FeCl3>50 μm Fe-phytosiderophore or 50 μm Fe-citrate, demonstrating that P. fluorescens Pf-5 (pvd-inaZ) could utilize both Fe-phytosiderophore and Fe-citrate as Fe sources. To study the relative Fe stress status of P. fluorescens Pf-5 (pvd-inaZ) in the rhizosphere, and the influence of plant phytosiderophore release on Fe stress of the bacterium, a microcosm experiment was conducted in Fe-deficient soil planted with two grass species which differ in iron-efficiency and phytosiderophore release rates. Control treatments employed microcosms in which the plants received a foliar Fe treatment. Ice nucleation activity and population densities of P. fluorescens Pf-5 (pvd-inaZ) were determined periodically over 18 d using a non-destructive filter paper technique to lift bacteria from the root surface in different root zones. Results showed that cells of this bacterium sampled from both barley and rice were relatively non iron-stressed as compared to cells grown in liquid media with low Fe concentrations, but were nonetheless significantly influenced by plant species. The bacterium was less Fe stressed in the rhizosphere of iron-efficient barley, which produces high quantities of phytosiderophore, as compared to iron-inefficient rice. In barley, foliar Fe treatments, which alleviated plant Fe stress and concomitant phytosiderophore release, resulted in increased Fe stress for the bacterium in the rhizosphere. These data strongly suggest that P. fluorescens Pf-5 (pvd-inaZ) utilizes phytosiderophores for Fe acquisition. The relatively low iron-stress status of this bacterium in the rhizosphere indicates that only very low concentrations of pyoverdine might be expected to accumulate in the rhizosphere.