Understanding the frontiers between pathogenicity and legume-rhizobia symbiosis – PATHOSYM

Legume plants can grow without nitrogen fertilizer because they establish symbiosis with Rhizobia. This interaction results in the formation of root nodules where rhizobia fix atmospheric nitrogen for the benefit of the plant. In addition to this well studied interaction, legume nodules are also triggered by non-fixing rhizobia and can host non-rhizobial bacteria. The molecular determinants governing the output of these interactions are not fully understood. From nodules, we have isolated an atypical bacterium (Ensifer adhaerens T4) that behaves as a pathogen or as a nodulating rhizobium on the same plant species: Medicago truncatula. The output of the interaction depends on the plant age. T4 can also co-colonize M. truncatula nodules induced by a classical rhizobium.
The PATHOSYM project will allow to better describe the pathogen and symbiotic-like interactions of this system, and then by integrating genomic (the T4 genome sequence is known) and physiological data of the strain, to build a functional model of the T4 genome. This model will be confronted with the omics data produced during the project to predict the key genes of the T4-Medicago interaction. We will build mutants of these genes to validate the model. We will also study the response of the plant during symbiotic-like and pathogenic interactions with the T4 strain. Altogether, the PATHOSYM project will allow describing in details the T4-M. truncatula interactions in order to understand the molecular basis of T4 contrasted behaviors and how the plant can differentiate pathogens from symbionts.