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Abstract

In natural conditions, legumes interact with diverse soil Rhizobia to form symbiotic root nodules. When young nodules are formed, bacteroids begin to fix atmospheric N₂ and provide ammonium to the plant which in return provides phytosynthates to the bacteria. This symbiotic interaction occurs under plant nitrogen deficit and only between compatible partners. In soil there is frequently many possible compatible bacteria and nodulation of competitive but inefficient bacteria often limits the agro-ecological services of symbiosis. Little is known about mechanisms involved in legume-rhizobium partner choice despite the relevance of this complex trait to design new strategies to improve legume cropping systems.

I a first part of my presentation,  will summarize our recent findings obtained in pea/Rhizobium leguminosarum biovar viciae symbiosis illustrating that host specific Competitiveness to Form root Nodules (CFN) has a strong impact on early partner choice within natural populations of soil bacteria. I will further present association genetic studies combined with DNA metabarcoding strategies to identify bacterial and plant genes controlling pea/fababean/lentil CFN.

In the second part of my presentation, I will present recent studies concerning Post-Infection Partner Preference (PIPP) in the model legume Medicago truncatula/Sinorhizobium. PIPP allows the plant to favor efficient symbiotic structure to the expense of the inefficient ones. Using transcriptomic and metabolomic approaches on plants cultivated in split-root systems, we demonstrated that the systemic signaling of the whole plant nitrogen demand pilots various aspects of the development of the symbiotic organs. Interestingly, this trait varies among symbiotic  partners is also involved in the whole plant compensation of local water stress.