The metabolism of iron by nitrogen-fixing rhizospheric bacteria

Abstract

Nitrogen-fixing bacteria require iron for rhizospheric growth as well as for plant-Associated symbiotic activities. One of the iron uptake systems involves siderophores and bacteria produce a great diversity of ferric ion chelators in iron-limited environments. This review examines some of the free-living nitrogen-fixing bacteria, including Azotobacter vinelandii which produces three different examples of iron-binding catecholates. Frankia and bacteria of the rhizobial group also produce siderophores and many of the rhizobia are capable of using different forms of chelated iron. When grown in pure culture, Frankia and rhizobia produce siderophores; however, when grown symbiotically the production of siderophores is not fully resolved. The growth of symbiotic bacteria in planta is highly complex and this review examines some of the metabolic activities of iron in this environment. While the hemoglobin in the nodules of plants promotes bacterial metabolism, there are numerous details that need clarification with respect to cellular regulation in a low oxygen environment. Additionally, this review calls attention to the oxygen stress response that may be attributed to oxidation of non-chelated ferric ions which could be a consequence of siderophores with a low iron affinity. © 2006 Springer.