Phylogenetically, this symbiosis occurs among eight plant families and three orders that have diverse global distributions ( Benson and Silvester, 1993). N 2-fixing root nodule symbioses by actinorhizal plants are important contributors to ecosystems worldwide. Legumes and actinorhizal plants have acquired the ability to establish a root nodule symbiosis with nitrogen-fixing bacteria known as rhizobia and frankiae, respectively. Most plants rely for their nutrition on inorganic nitrogen (ammonium and nitrate), a macronutrient found to be limiting in most terrestrial ecosystems ( Chapin et al., 2002). 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. A significant release of nitrogen-containing metabolites, mainly glutamine and glutamate, was found in N 2-fixing cultures treated with Ag5. In vitro and in vivo analyses showed that Ag5 induces drastic physiological changes in Frankia, including an increased permeability of vesicle membranes. We show that Alnus glutinosa nodules express defensin-like peptides, and one of these, Ag5, was found to target Frankia vesicles. Here, new elements for this metabolic exchange are described. However, a mechanism for the transfer of nitrogen-fixation products to the plant cells remains elusive. Vesicles lack a pathway for assimilating ammonia beyond the glutamine stage and are supposed to transfer reduced nitrogen to the plant host cells. Nitrogen fixation occurs in differentiated Frankia cells known as vesicles. 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.
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