Signals and Responses Choreographing the Complex Interaction between Legumes and alpha- and beta-Rhizobia

The nitrogen-fixing symbiosis between bacteria in the family Rhizobiaceae and members of the legume family (Fabaceae) has been well studied, particularly from the perspective of the early signaling and recognition events. Recent studies of nonnodulating legume mutants have resulted in the identification of a number of genes that are responsive to signal molecules from the bacteria. However, a second group of nodule-forming bacteria, completely unrelated to the Rhizobiaceae, which are alpha-Proteobacteria, has been discovered. These bacteria belong to the beta-Proteobacteria and have been designated beta-rhizobia to distinguish them from the better-known alpha-rhizobia. Here, we review what is known in this economically important symbiosis about the interaction between legumes and alpha-rhizobia, and we incorporate information, where known, about the beta-rhizobia.