Molecular Analysis of the Rhizobium Genes Involved in the Induction of Nitrogen-Fixing Nodules on Legumes [and Discussion]

Recent developments in the molecular genetics of Rhizobium spp. are presented, and the use of mutant bacterial strains to determine which properties are required for symbiotic nitrogen fixation and nodulation of legumes is described. Both the lipopolysaccharide and the exopolysaccharide of Rhizobium spp. are implicated in infection. Recent studies have identified several genes involved in the early steps of this process and in the determination of host-range specificity. Analysis of their products has given some indications of their functions. The expression of most of these nodulation (nod) genes is controlled by the regulatory gene nodD, which is itself expressed constitutively, whereas other nod genes are transcribed only when the cells are exposed to compounds present in the rhizosphere of legumes. These compounds were identified as various flavones and flavanones. Other plant-specified aromatic molecules, such as isoflavonoids, antagonize this induction.