Terminal bacteroid differentiation in the legume−rhizobium symbiosis: nodule‐specific cysteine‐rich peptides and beyond

Summary Terminal bacteroid differentiation (TBD) is a remarkable case of bacterial cell differentiation that occurs after rhizobia are released intracellularly within plant cells of symbiotic legume organs called nodules. The hallmarks of TBD are cell enlargement, genome amplification and membrane permeabilization. This plant‐driven process is governed by a large family of bacteroid‐targeted nodule‐specific cysteine‐rich (NCR) peptides that were until recently thought to be restricted to a specific lineage of the legume family, including the model plant Medicago truncatula. Recently, new plant and bacterial factors involved in TBD have been identified, challenging our view of this phenomenon at mechanistic and evolutionary levels. Here, we review the recent literature and discuss emerging questions about the mechanisms and the role(s) of TBD. Contents Summary 411 I. Terminal bacteroid differentiation: a tool to optimize nitrogen fixation? 411 II. Terminal bacteroid differentiation in non‐IRLC legume plants: a case of convergent coevolution? 412 III. NCR peptide intracellular targets: from a blurry landscape to the line of sight 412 IV. The Janus face of NCR peptides: two NCR peptides prevent bacteroid death 413 V. A general fitness defect as a major cause of why rhizobial mutants cannot undergo terminal differentiation? 414 VI. Bacterial strategies to survive the NCR peptide challenge and/or to escape TBD 414 VII. Beyond NCR peptides, other plant proteins are required for bacteroid differentiation 415 VIII.