Nitrogen-fixing symbiosis induces differential accumulation of Medicago truncatula leaf defence metabolites in response to pea aphid infestation

Goodluck Benjamin^1*Marie Pacoud¹Stéphanie Boutet²Gilles Clement²Renaud Brouquisse¹Jean-Luc Gatti¹Marylène Poirié¹Pierre Frendo^1*

• ¹Institut Sophia Agrobiotech, Sophia Antipolis, France
• ²Université Paris-Saclay, Institut Jean-Pierre Bourgin, Versailles, France

• Legume symbiosis with rhizobial nitrogen-fixing bacteria enable them to grow in nitrate-depleted soils. Rhizobial symbioses also induces systemic plant defence against bioagressors. • We investigated how nitrogen-fixing symbiosis (NFS) in the legume Medicago truncatula can prime plant defence against the pea aphid Acyrthosiphon pisum. We analysed metabolite modification both by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) and defence pathway gene expression by qPCR in leaves of both NFS and nitrate-fed (non-inoculated; NI) plants after aphid infestation (Amp). • The accumulation of primary and secondary metabolites was modulated by both NFS and aphid infestation. Sixty two defense-related metabolites such as salicylate, pipecolate, gentisic acid and several soluble sugars were differentially regulated by aphid infestation in both NFS and NI conditions. Nineteen metabolites, including triterpenoid saponins, accumulated specifically in NFS_Amp conditions. Gene expression analysis showed that aphid-infested plants exhibited significantly higher expression of Chalcone isomerase, flavonol synthase, hydroxyisoflavone-O-methyl transferase and Pterocarpan synthase, while D-pinitol dehydrogenase was only significantly induced in NI-infested leaves. • Our data suggest that NFS, in addition to being a plant nitrogen provider, stimulates specific legume defenses upon pest attack and should also be considered as a potential tool in Integrated Pest Management strategies.