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Enzymatic Evidence for the Key Role of Arginine in Nitrogen Translocation by Arbuscular Mycorrhizal Fungi^1,[OA]

Universidade de Lisboa, Faculdade de Ciências, Departamento de Biologia Vegetal, Centro de Ecologia e Biologia Vegetal, 1749–016 Lisboa, Portugal (C.C., M.A.M.-L.); Technical University of Denmark, Risø National Laboratory, Biosystems Department, DK–4000 Roskilde, Denmark (H.E., C.T., P.A., I.J.); and Universität Karlsruhe, Institut für Angewandte Biowissenschaften, Fungal-Plant Interactions Group, D–76187 Karlsruhe, Germany (N.R.)

Key enzymes of the urea cycle and ¹⁵N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. ¹⁵NH₄⁺ was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of L-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. ¹⁵N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 µg Arg mg^–1 fresh weight h^–1. The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Iver Jakobsen (iver.jakobsen{at}risoe.dk).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.090522

^* Corresponding author; e-mail iver.jakobsen{at}risoe.dk; fax 45–4677–4109.

Received September 28, 2006; accepted November 26, 2006; published December 1, 2006.

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