Regulation of metabolite production during the encounter between the actinobacterium Frankia and its host Alnus glutinosa

Our study focuses on the metabolomic changes observed during the early stages of Alnus glutinosa and Frankia alni symbiosis. Amino acid metabolic profiling by HPLC-DAD-FLD and untargeted metabolomic analysis of secondary metabolites by UHPLC-QTOF were performed on the shoots and roots of A. glutinosa as well as on bacterial pellets of F. alni . Two culture conditions were compared: a single culture condition (where A. glutinosa or Frankia was grown alone) and a co-culture condition (where A. glutinosa and Frankia were grown together) at different culture times (D1, D2 and D3). Our results reveal a change in metabolism (primary and secondary) in both partners in the co-culture condition. For amino acids, this change was more important in the shoots than in the roots and in Frankia . A total of 16 amino acids (Asp, Asn, Ser, Gln, Gly, Cit, GABA, Ala, Arg, Tyr, Trp Val, Phe, Ile, Lys and Pro) were overproduced in the presence of Frankia in the shoots on the different sampling days. We hypothesised that the plant would modify its amino acid content in its shoots in anticipation of a transfer to Frankia for growth. At the same time, a drastic change in secondary metabolites occurs in the shoots, roots and Frankia at the three time points considered between the control condition and the co-culture condition. Statistical analyses enabled us to highlight the ions characterising the co-culture condition in the different biological compartments (i.e. shoots, roots and Frankia ). The biomarkers identified in the shoots and Frankia varied greatly depending on the sampling day (i.e. D1, D2 and D3), revealing strong dynamics. The root biomarkers appear to be more stable over time, as several of them are common to all three sampling days.