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Nitrogen Transfer And Assimilation Between The Arbuscular Mycorrhizal Fungus Glomus Intraradices Schenck & Smith And Ri T-DNA Roots Of Daucus Carota L. In An In Vitro Compartmented System

Jean-Patrick Toussaint, Marc St-Arnaud, Christiane Charest

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Nitrogen metabolism was examined in monoxenic cultures of carrot roots (Daucus carota L.) colonized with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith. Glutamine synthetase and glutamate dehydrogenase activities were significantly increased in mycorrhizal roots for which only the extraradical mycelium had exclusive access to NH4NO3 in a distinct hyphal compartment inaccessible to the roots. This was in comparison with the water controls but was similar to the enzyme activities of non-arbuscular-mycorrhizal (non-AM) roots that had direct access to NH4NO3. In addition, glutamate dehydrogenase activity was significantly enhanced in AM roots compared with non-AM roots. Carrot roots took up 15NH4+ more efficiently than 15NO3, and the extraradical hyphae transfered 15NH4+ to host roots from the hyphal compartment but did not transfer 15NO3. The extraradical mycelium was shown, for the first time, to have a different glutamine synthetase monomer than roots. Our overall results highlight the active role of AM fungi in nitrogen uptake, transfer, and assimilation in their symbiotic root association.Key words: arbuscular mycorrhizal fungus, Ri T-DNA carrot roots, in vitro root-organ culture, nitrogen metabolism.