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Published on December 1, 2006; 10.1104/pp.106.090522

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Received September 28, 2006
Accepted November 26, 2006

Enzymatic Evidence for the Key Role of Arginine in Nitrogen Translocation by Arbuscular Mycorrhiza Fungi

Cristina Cruz , Helge Egsgaard , Carmen Trujillo , Per Ambus , Natalia Requena , Maria Amélia Martins-Loução , and Iver Jakobsen *

Universidade de Lisboa, Faculdade de Ciências, Departamento de Biologia Vegetal, Centro de Ecologia e Biologia Vegetal, Campo Grande, C-4, Piso 1, 1749-016 Lisboa, Portugal
Risø National Laboratory, Biosystems Department, DK-4000 Roskilde, Denmark
Universität Karlsruhe, Institut für Angewandte Biowissenschaften, Fungal-Plant Interactions Group, D-76187 Karlsruhe, Germany

* Corresponding author; email: iver.jakobsen{at}risoe.dk.

Key enzymes of the urea cycle and 15N labelling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen (N) translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota L.) roots and Glomus intraradices in two-compartment Petri dishes and three ammonium levels were supplied to the compartment containing extra-radical mycelium (ERM), but no roots. Time courses of specific enzyme activities were obtained for glutamine synthetase, argininosuccinate synthetase, arginase and urease in ERM and in AM roots. 15NH4+ was used to follow the dynamics of N incorporation into and turnover of Arg. Both the absence of external N 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 ERM. The 15N labelling of Arg in the ERM was very fast and the analysis of its time course and isotopomer pattern allowed the estimation of the translocation rate of Arg along the mycelium as 0.13 µg Arg mg-1 FW 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 N translocation in the AM mycelium.




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