Use of the Luciferase (TN5-LUX) Reporter System to Study Rhizobium Meliloti Genes Responding to N/C/O2 Limitation or Plant Factors and Their Role in Rhizosphere Competition

The induction of nitrogen-fixing root and stem nodules on leguminous plants by soil bacteria belonging to the Rhizobiaceae is a highly evolved, complex process requiring a fine-tuned interaction between the bacteria and their host. Recently, considerable progress has been made in elucidating the signal exchange between rhizobia and their host plant, leading up to the activation of the microbial nodulation (nod/nol) genes, Nod factor biosynthesis and subsequent Nod factor mediated initiation of nodule ontogeny. Moreover, the molecular basis of rhizobial nitrogen fixation (nif/fix) gene induction in response to physiological cues such as oxygen (and nitrogen) availability has been studied in detail. However, much less is known about the influence of the persistence and competitive ability of rhizobia in the soil and the rhizosphere of plants on the very early stages of rhizobial infection. This is particularly relevant when one is interested in developing improved strains of nitrogen fixing rhizobia, since they tend not to be competitive when reintroduced into the soil or rhizosphere of plants (see Dowling Broughton, 1986).