Differential induction of Medicago truncatula defence metabolites in response to rhizobial symbiosis and pea aphid infestation

Summary Legumes symbiosis with rhizobial nitrogen-fixing bacteria enable them to grow in nitrate-depleted soils. Rhizobial symbioses also induces systemic plant defence against bioagressors. We investigate 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 LC-MS and GC-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. 62 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. 19 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 strategy.