Trophic exchanges constitute the bases of the symbiosis between the nitrogen-fixing actinomycete Frankia and its host plant Alnus, but the identity of the compounds exchanged is still poorly known. In the current work, previously published transcriptomic studies of Alnus nodules and of symbiotic Frankia were reexamined for TCA cycle related genes. The bacterial TCA enzyme genes were all upregulated, especially the succinyl-CoA synthase and the citrate synthase while on the plant side, none was significantly modified in nodules relative to non-inoculated roots. A preliminary metabolomics approach permitted to see that citrate, 2-oxoglutarate, succinate, malate and fumarate were all more abundant (FC (Fold change) = 5–70) in mature nitrogen-fixing nodules than in roots. In the evaluation of the uptake and metabolism of these organic acids, a significant change was observed in the morphology of nitrogen fixing vesicles in vitro: the dicarboxylates malate, succinate and fumarate induced the formation of larger vesicles than was the case with propionate. Moreover, the production of spores was also modified depending on the organic acid present. The assays showed that most C4 dicarboxylates were taken up while C6 tricarboxylates were not and citrate even partially blocked catabolism of reserve carbon. Tests were performed to determine if the change in membrane permeability induced by Ag5, a peptide previously shown to modify the membranes of Frankia, increased the uptake of specific organic acids. No effect was observed with citrate while an increase in nitrogen fixation was seen with propionate.
KeywordsDicarboxylates Frankia Propionate Nitrogen fixation Respiration Vesicles
We acknowledge grants from French ANR (BugsInACell ANR-13-BSV7-0013-03), from the FR BioEnvironment and Health (Lyon) and a MEC postdoctoral fellowship from the Spanish government to LC (Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional de I-D + i 2008–2011). We thank AME, PGE and DTAMB platforms for measurements. TP and KP acknowledge a grant from the Swedish research council FORMAS (229-2005-679) and support by the Carl Tryggers Foundation.
Compliance with ethical standards
Conflict of Interest
Lorena Carro, Tomas Persson, Petar Pujic, Nicole Alloisio, Pascale Fournier, Hasna Boubakri, Katharina Pawlowski and Philippe Normand declare that they have no conflict of interest.
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