Rhizobium-legume symbiosis has been considered as a mutually favorable relationship for both partners. However, in certain phylogenetic groups of legumes, the plant directs the bacterial symbiont into an irreversible terminal differentiation. This is mediated by the actions of hundreds of symbiosis-specific plant peptides resembling antimicrobial peptides, the effectors of innate immunity. The bacterial BacA protein, associated in animal pathogenic bacteria with the maintenance of chronic intracellular infections, is also required for terminal differentiation of rhizobia. Thus, a virulence factor of pathogenesis and effectors of the innate immunity were adapted in symbiosis for the benefit of the plant partner.