The Versatile Roles of Type III Secretion Systems in Rhizobium-Legume Symbioses

To suppress plant immunity and promote the intracellular infection required for fixing nitrogen for the benefit of their legume hosts, many rhizobia use type III secretion systems (T3SSs) that deliver effector proteins (T3Es) inside host cells. As reported for interactions between pathogens and host plants, the immune system of legume hosts and the cocktail of T3Es secreted by rhizobia determine the symbiotic outcome. If they remain undetected, T3Es may reduce plant immunity and thus promote infection of legumes by rhizobia. If one or more of the secreted T3Es are recognized by the cognate plant receptors, defense responses are triggered and rhizobial infection may abort. However, some rhizobial T3Es can also circumvent the need for nodulation (Nod) factors to trigger nodule formation. Here we review the multifaceted roles played by rhizobial T3Es during symbiotic interactions with legumes.

Automated summary

The type III secretion systems (T3SSs) used by rhizobia deliver effector proteins (T3Es) inside host cells to suppress plant immunity and promote infection for nitrogen fixation. The immune system of legume hosts and the cocktail of T3Es secreted by rhizobia determine the outcome of their symbiotic interaction. Rhizobial T3Es can either reduce plant immunity and promote infection or trigger defense responses if recognized by plant receptors. Some rhizobial T3Es can also bypass nodulation factors to induce nodule formation.