Submitted to: Nature Magazine
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/6/2005
Publication Date: 6/9/2005
Citation: Govindarajulu, M., Pfeffer, P.E., Jin, H., Abubaker, J., Douds, D.D., Allen, J.W., Bucking, H., Lammers, P.J., Shachar-Hill, Y. 2005. Nitrogen transfer in arbuscular mycorrhizal symbiosis. Nature Magazine. 435. pp. 819-823. Interpretive Summary: The uptake by plants of mineral nutrients from the soil is greatly aided by associations with mycorrhizal fungi which grow into and extend out of their roots. As well as benefiting plants by aiding phosphorus uptake from the soil, AM fungi can take up and transfer substantial amounts of N (the availability of which frequently limits plant growth) to their host plants. However despite the identification in AM fungi of enzyme activities and genes identities of primary nitrogen assimilation and catabolism we know very little about how nitrogen is moved from fungus to plant. In particular we do not know the form in which nitrogen is translocated within the fungus from the hyphae in the soil (extraradical mycelium) to the fungal structures within roots (intraradical mycelium) or the form transferred to the plant. This ignorance limits our understanding both of underground nitrogen movement worldwide and of nutrient exchange in what is arguably the world's most important symbiosis. In this study we have established that nitrogen is transported from the external fungus to the internal fungus located in the roots of the plant as the amino acid arginine. Final transfer of nitrogen to the plant cells is made in the form of ammonium. This information is critical to our understanding of how and in what form to supply nitrogen to mycorrhizal plants to optimize nitrogen utilization and plant production.
Technical Abstract: The majority of land plants are symbiotic with arbuscular mycorrhizal fungi which take up mineral nutrients from the soil and exchange them with host plants for photosynthetically fixed carbon. This exchange is a significant factor in global nutrient cycles as well as in the ecology , evolution and physiology of plants. While substantial progress has been made in the molecular dissection of phosphorus movement to the plant and of carbon movement to the fungus, much less is known about how nitrogen is translocated and transferred. Here we report the results of experiments to track nitrogen movement in the symbiosis from the uptake of different substrates by the fungus through to the incorporation into plant protein using stable isotopic labeling and measurements of the expression of N metabolic genes. From our findings we propose that nitrogen taken up by the fungus is incorporated into amino acids, is translocated from the extraradical to the intraradical mycelium as arginine, and is transferred to the plant without any accompanying carbon.