Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2005
Publication Date: 6/9/2005
Citation: Jin, H., Pfeffer, P.E., Douds, D.D., Piotrowski, E.G., Lammers, P.J., Shachar-Hill, Y. 2005. The form of nitrogen stored and transported by the extraradical hyphae of an arbuscular mycorrhizal symbiosis. New Phytologist. 435. pp. 819-823. Interpretive Summary: Arbuscular mycorrhizal (AM) fungi can help deliver nitrogen to crop plants, thus lowering the need for excessive amounts of synthetic fertilizers. Presently little is known about the preferred source of nitrogen taken up by these fungi nor is there any knowledge of how it is transported to the host plant. We have pursued the process of how ammonium is used in the delivery of nitrogen in the symbiotic and germination phases of the AM life cycle. The fungus readily takes up ammonium in the symbiotic state and quickly converts it to arginine, which is transported to the host root. The amino acid, arginine acts as a store of nitrogen and moves it in both directions between the fungus and plant. It remains intact within the hyphae until the fungus becomes nitrogen limited. Arginine is easily taken up by the roots but uptake is limited in the associated hyphae and spores. Arginine is principally a 'mover' of nitrogen from fungus to the host. These findings give us insight into how AM fungi take up and transport nitrogen to their host crop plants. This information will allow us to determine the most efficient methods for utilizing AM fungi for low input nitrogen utilization.
Technical Abstract: The form of nitrogen stored and transported by the extraradical hyphae of arbuscular mycorrhizal fungi in symbiosis with Ri T-DNA roots was studied. The AM fungus Glomus intraradices Schenck Smith was grown in monoxenic culture with Ri T-DNA transformed roots of Daucus carota L. in two compartment Petri dishes. 15NH4Cl, [guanido-15N2]Arg, and 13C6-arginine, were introduced into the root-free compartment. Free amino acids of the extraradical mycelium and root compartment tissue were then analyzed with GC-MS after 1, 3 and 6 weeks of exposure. Results of exposure to 15NH4 showed that the main free amino acid in the extraradical mycelium (ERM) was arginine, which had 99% enrichment in 15N. The concentration of arginine was 227.93 nmol /g d.wt of hyphae, representing more than 90% of the total nitrogen of free amino acids in the ERM. 15NH4Cl, [guanido-N2]Arg, and 13C6-arginine, added to the fungal compartment, were found isotopically intact in the free amino acids of the root compartment tissue indicating that they were transported to the root compartment tissue with minimum metabolic breakdown. Thus, arginine is a major form of nitrogen stored and transported to the root host by the extraradical hyphae of AM fungi.