PHYTOSIDEROPHORES DECREASE IRON STRESS AND PYOVERDINE PRODUCTION OF PSEUDOMONAS-FLUORESCENS PF-5 (PVD-INAZ)

Experiments were conducted to study the relationship between iron stre ss and pyoverdine production during growth of Pseudomonas fluorescens Pf-5 (pvd-inaZ) in vitro, and the Fe stress status of this bacterium i n the rhizosphere of barley and rice. The bacterial strain used for th ese experiments has been engineered with a plasmid containing an iron- regulated ice nucleation reporter that enables monitoring of the relat ive Fe stress status of the cells. In vitro, Fe stress and pyoverdine production of P. fluorescens Pf-5 (pvd-inaZ) decreased in the followin g order: 0 Fe > 50 mu M FeCl3 > 50 mu M Fe-phytosiderophore or 50 mu M Fe-citrate, demonstrating that P. fluorescens Pf-5 (pvd-inaZ) could u tilize both Fe phytosiderophore and Fe-citrate as Fe sources. To study the relative Fe stress status of P. fluorescens Pf-5 (pvd-inaZ) in th e 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-ef ficiency and phytosiderophore release rates. Control treatments employ ed microcosms in which the plants received a foliar Fe treatment. Ice nucleation activity and population densities of P. fluorescens Pf-5 (p vd-inaZ) were determined periodically over 18 d using a non-destructiv e filter paper technique to lift bacteria from the root surface in dif ferent root zones. Results showed that cells of this bacterium sampled from both barley and rice were relatively non iron-stressed as compar ed 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, whic h produces high quantities of phytosiderophore, as compared to iron-in efficient rice. In barley, foliar Fe treatments, which alleviated plan t Fe stress and concomitant phytosiderophore release, resulted in incr eased Fe stress for the bacterium in the rhizosphere. These data stron gly suggest that P. fluorescens Pf-5 (pvd-inaZ) utilizes phytosideroph ores for Fe acquisition: The relatively low iron-stress status of this bacterium in the rhizosphere indicates that only very low concentrati ons of pyoverdine might be expected to accumulate in the rhizosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.