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

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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.

Le métabolisme de l'azote a été étudié dans des cultures monoxéniques de racines de carottes (Daucus carota L.) colonisées par le champignon mycorhizien arbusculaire (AM) Glomus intraradices Schenck & Smith. Les activités glutamine synthétase et glutamate déshydrogénase ont été significativement augmentées dans des racines mycorhizées chez lesquelles le mycélium extra-racinaire avait un accès exclusif à du NH4NO3 disponible dans un compartiment hyphal inaccessible aux racines, comparativement aux témoins consistant en de l'eau, et étaient proches de celles mesurées chez des racines sans AM qui avaient un accès direct au NH4NO3. De plus, l'activité glutamate déshydrogénase fut augmentée dans des racines avec AM par rapport à des racines sans AM. Les racines de carottes ont absorbé le 15NH4+ plus efficacement que le 15NO3, et les hyphes extra-racinaires ont transféré le 15NH4+, mais non le 15NO3, du compartiment hyphal aux racines de l'hôte. Il fut démontré pour la première fois que le mycélium extra-racinaire avait un monomère glutamine synthétase différent des racines. Nos résultats pris dans leur ensemble soulignent le rôle actif des champignons AM dans l'absorption, le transfert et l'assimilation de N, dans le contexte de leur association symbiotique avec les racines.Mots clés : champignons mycorhiziens arbusculaires, racines de carottes Ri T-DNA, culture d'organes racinaires in vitro, métabolisme de l'azote.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: April 1, 2004

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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