1. Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, USA
2. USDA-ARS Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, USA
3. Plant Biology Department, Michigan State University, East Lansing, Michigan 48824-1312, USA

Correspondence to: Philip E. Pfeffer²Peter J. Lammers¹ Correspondence and requests for materials should be addressed to P.J.L. (Email: plammers@nmsu.edu) or P.E.P. (Email: ppfeffer@arserrc.gov).
Accession numbers (GenBank) for gene sequences used to design the TaqMan assays are as follows: GS (DQ063587), GDH (AY745984), UAP (CV186300), OAT (BI452207) and S4 RP (BI452093).

Most land plants are symbiotic with arbuscular mycorrhizal fungi (AMF), which take up mineral nutrients from the soil and exchange them with plants for photosynthetically fixed carbon. This exchange is a significant factor in global nutrient cycles¹ as well as in the ecology², evolution³ and physiology⁴ of plants. Despite its importance as a nutrient, very little is known about how AMF take up nitrogen and transfer it to their host plants⁵. Here we report the results of stable isotope labelling experiments showing that inorganic nitrogen taken up by the fungus outside the roots is incorporated into amino acids, translocated from the extraradical to the intraradical mycelium as arginine, but transferred to the plant without carbon. Consistent with this mechanism, the genes of primary nitrogen assimilation are preferentially expressed in the extraradical tissues, whereas genes associated with arginine breakdown are more highly expressed in the intraradical mycelium. Strong changes in the expression of these genes in response to nitrogen availability and form also support the operation of this novel metabolic pathway in the arbuscular mycorrhizal symbiosis.

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