Not All Allies Are Welcome: Partner Discrimination in Legume–Rhizobium Symbiosis

The nitrogen-fixing symbiosis between leguminous plants and soil bacteria, collectively termed rhizobia, is a major contributor of fixed nitrogen to the biosphere. The ability of legumes to secure nitrogen from the atmosphere underlies their ecological success and has made them important crops in both traditional and modern sustainable agriculture. Many genes directing the establishment and functioning of this beneficial interaction have been identified under laboratory conditions using a limited number of bacterial strains and plant species in pairwise combinations. Under natural and field conditions, however, plants encounter numerous potential partners, as soil microbiomes contain diverse bacteria equipped with the necessary toolkit for symbiosis. Consequently, legumes must possess mechanisms to select for or against specific partners. This review highlights how legumes employ elements of their immune system for the negative selection of rhizobia via processes resembling the gene-for-gene model of effector-triggered immunity in plant–pathogen interactions. [Figure: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.