Summary To circumvent the paucity of nitrogen sources in the soil legume plants establish a symbiotic interaction with nitrogen‐fixing soil bacteria called rhizobia. During symbiosis, the plants form root organs called nodules, where bacteria are housed intracellularly and become active nitrogen fixers known as bacteroids. Depending on their host plant, bacteroids can adopt different morphotypes, being either unmodified (U), e...

Summary In rhizobial species that nodulate inverted repeat‐lacking clade ( IRLC ) legumes, such as the interaction between S inorhizobium meliloti and M edicago , bacteroid differentiation is driven by an endoreduplication event that is induced by host nodule‐specific cysteine rich ( NCR ) antimicrobial peptides and requires the participation of the bacterial protein BacA . We have studied bacteroid differentiation of S inorhi...