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ISME J. 2015 Aug;9(8):1723-33. doi: 10.1038/ismej.2014.257. Epub 2015 Jan 20.

Alnus peptides modify membrane porosity and induce the release of nitrogen-rich metabolites from nitrogen-fixing Frankia.

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Université Lyon 1, Université de Lyon, CNRS, Ecologie Microbienne, UMR 5557, Villeurbanne, France.
Equipe Rhizogenèse, UMR DIADE (IRD, UM2), Institut de Recherche pour le Développement, Montpellier, France.
Institut des Sciences du Végétal, CNRS, UPR 2355, Gif-sur-Yvette, France.
INSA-Lyon, INRA, UMR203 BF2I, Biologie Fonctionnelle Insectes et Interactions, Villeurbanne, France.


Actinorhizal plant growth in pioneer ecosystems depends on the symbiosis with the nitrogen-fixing actinobacterium Frankia cells that are housed in special root organs called nodules. Nitrogen fixation occurs in differentiated Frankia cells known as vesicles. Vesicles lack a pathway for assimilating ammonia beyond the glutamine stage and are supposed to transfer reduced nitrogen to the plant host cells. However, a mechanism for the transfer of nitrogen-fixation products to the plant cells remains elusive. Here, new elements for this metabolic exchange are described. We show that Alnus glutinosa nodules express defensin-like peptides, and one of these, Ag5, was found to target Frankia vesicles. In vitro and in vivo analyses showed that Ag5 induces drastic physiological changes in Frankia, including an increased permeability of vesicle membranes. A significant release of nitrogen-containing metabolites, mainly glutamine and glutamate, was found in N2-fixing cultures treated with Ag5. This work demonstrates that the Ag5 peptide is central for Frankia physiology in nodules and uncovers a novel cellular function for this large and widespread defensin peptide family.

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