Molecular basis of symbiotic promiscuity

Eukaryotes often form symbioses with microorganisms. Among these, associati ons between plants and nitrogen-fixing bacteria are responsible for the nit rogen input into various ecological niches. Plants of many different famili es have evolved the capacity to develop root or stem nodules with diverse g enera of soil bacteria. Of these, symbioses between legumes and rhizobia (A zorhizobium, Bradyrhizobium, Mesorhizobium, and Rhizobium) air the most imp ortant from an agricultural perspective. Nitrogen-fixing nodules arise when symbiotic rhizobia penetrate their hosts in a strictly controlled and coor dinated manner. Molecular codes are exchanged between the symbionts in the rhizosphere to select compatible rhizobia from pathogens. Entry into the pl ant is restricted to bacteria that have the "keys" to a succession of legum e "doors". Some symbionts intimately associate with many different partners (and are thus promiscuous), while others are more selective and have a nan ow host range. For historical reasons, narrow host range has been more int ensively investigated than promiscuity. In our view, this has given a false impression of specificity in legume-Rhizobium associations. Rather, we sug gest that restricted host ranges are limited to specific niches and represe nt specialization Of widespread and more ancestral promiscuous symbioses. H ere we analyze the molecular mechanisms governing symbiotic promiscuity in rhizobia and show that it is controlled by a number of molecular keys.