Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/31/2011
Publication Date: 1/27/2012
Citation: Fellbaum, C., Gachomo, E.W., Beesetty, Y., Choudhari, S., Strahan, G.D., Pfeffer, P., Kiers, T., Bucking, H. 2012. Carbon availability for the fungus triggers nitrogen uptake and transport in the arbuscular mycorrhizal symbiosis. Proceedings of the National Academy of Sciences. Available: http://www.pnas.org/content/early/2012/01/27/1118650109.full.pdf+html. Interpretive Summary: The arbuscular mycorrhizal (AM) symbiosis, formed by fungi plays a key role for the nutrient uptake of the majority of land plants, including important crop species such as corn, soybean and rice. The AM symbiosis can increase the uptake of phosphate (P) and nitrogen (N), but also of trace elements, such as copper and zinc. The extraradical mycelium (ERM) of the fungus extends the nutrient absorbing surface of the root and allows an uptake of nutients from the soil far beyond the root rhizosphere. The widely accepted view of this association was that the host plant controlled the symbiosis. We analyzed the nitrogen uptake and metabolism in roots colonized by AM fungi and studied how that varied when the amount of sugar available to the fungus ranged from deficient to sufficient. In this study we learned that the host is given a supply of N only after the fungus has received sufficient C from the host plant. Thus both partners reward each other according to the benefit that each receives from its partner in the symbiosis. These findings give insight into the most effective way of utilizing these symbiotic fungi to deliver nitrogen to crop plants under a low input sustainable farming regime. Thus when photosynthetic activity is high N input can be limited due to effective delivery of N from the mycorrhizal fungal partner.
Technical Abstract: The arbuscular mycorrhizal (AM) symbiosis is characterized by a transfer of nutrients in exchange for carbon. We tested the effect of the carbon availability for the AM fungus Glomus intraradices on nitrogen (N) uptake and transport in the symbiosis. We followed the uptake and transport of 15N and 14C-arginine in mycorrhizal root organ cultures, the enzymatic activities of arginase and urease, and the fungal gene expression in the extraradical and intraradical mycelium (ERM, IRM) in response to different carbon supply conditions. An increase in the carbon transport from the host led to an increase in N and arginine uptake and transport to the host, stimulated the arginase and urease activities in the IRM but not the ERM, and triggered changes in fungal gene expression. Genes that are putatively involved in N assimilation and arginine biosynthesis were upregulated in the ERM, but downregulated in the IRM. A direct supply of carbon as acetate to the ERM did not stimulate the N uptake and transport to the host plant. The results highlight the significance of the carbon flux from the root to the fungus as an important trigger for N uptake and transport in the AM symbiosis.