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Legumes have the ability to develop root nodules hosting nitrogen-fixing bacteria, called rhizobia. Symbiosis gives a clear advantage for plants growing on soils with a low nitrogen content. However, it requires for the plant to host and feed a significant number of rhizobia. Recent studies suggest that innate immunity is reduced or suppressed in nodules; probably to maintain steady rhizobia populations. To study the immunity of nodules, the authors developed a tripartite system including the model legume Medicago truncatula, its nodulating symbiont of the genus Sinorhizobium, and the pathogenic bacteria Ralstonia solanacearum.

M. truncatula nodules can be infected by the pathogenic bacteria R. solanacearum

Benjamin Gourion, a LIPM researcher supported by a New Frontiers TULIP project in 2016 followed by an ANR Young Researcher grant, and last author of this article, talks about it in more details: “We show that nodules are frequently infected with a pathogen multiplication that is comparable to that observed in susceptible organs. We also demonstrated that the functionality of the symbiotic organ has no impact on the development of the pathogen. In addition, transcriptomic analyzes indicate that, despite the presence of rhizobia, the nodules are capable of developing weak defenses against the pathogen R. solanacearum."

The defense response of root nodules present a specificity for induced genes, in comparison with the ones induced in the roots. In agreement with the innate immunity of the nodules, optimal growth of R. solanacearum requires the presence of virulence factors that are essential for its pathogenicity.

The pathogenic infection is confined to the nodule of M. truncatula

Finally, the data analyzed by the authors indicate that the susceptibility of nodules to pathogen infection is compensated by the existence of a diffusion barrier. The spread of the pathogen from the nodules is therefore limited, thereby letting the other tissues of the plant healthy.