Review. Rhizobia: From saprophytes to endosymbionts (Nat. Rev. Microbiol.) ($)

One of the best characterized plant-bacteria interactions is that between legumes and rhizobia. This review by Poole et al. explores rhizobia in their non-plant associated state (as saprophytes that derive energy and nutrients from organic matter in the soil), through the complex signals that lead to their attraction and attachment to the root, and finally through their developmental transition to endosymbiotic bacteroids. The authors observe that many studies of rhizobia focus on the bacteria in pure culture conditions, making it difficult to assess how their interactions with other species in the rhizosphere affects their transition from saprophyte to endosymbiont; these interactions are highlighted in this review. (Summary by Mary Williams) Nat. Rev. Microbiol. 10.1038/nrmicro.2017.171