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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

A Mycorrhizal-Specific Ammonium Transporter from Lotus japonicus Acquires Nitrogen Released by Arbuscular Mycorrhizal Fungi¹

Department of Plant Biology, University of Torino and Istituto per la Protezione delle Piante/Consiglio Nazionale delle Ricerche, 10125 Torino, Italy (M.G., R.B., P.B.); and Center for Plant Molecular Biology, Plant Physiology, University of Tuebingen, D–72076 Tuebingen, Germany (B.N., M.D., U.L.)

In mycorrhizal associations, the fungal partner assists its plant host by providing nitrogen (N) in addition to phosphate. Arbuscular mycorrhizal (AM) fungi have access to inorganic or organic forms of N and translocate them via arginine from the extra- to the intraradical mycelium, where the N is transferred to the plant without any carbon skeleton. However, the molecular form in which N is transferred, as well as the involved mechanisms, is still under debate. NH₄⁺ seems to be the preferential transferred molecule, but no plant ammonium transporter (AMT) has been identified so far. Here, we offer evidence of a plant AMT that is involved in N uptake during mycorrhiza symbiosis. The gene LjAMT2;2, which has been shown to be the highest up-regulated gene in a transcriptomic analysis of Lotus japonicus roots upon colonization with Gigaspora margarita, has been characterized as a high-affinity AMT belonging to the AMT2 subfamily. It is exclusively expressed in the mycorrhizal roots, but not in the nodules, and transcripts have preferentially been located in the arbusculated cells. Yeast (Saccharomyces cerevisiae) mutant complementation has confirmed its functionality and revealed its dependency on acidic pH. The transport experiments using Xenopus laevis oocytes indicated that, unlike other plant AMTs, LjAMT2;2 transports NH₃ instead of NH₄⁺. Our results suggest that the transporter binds charged ammonium in the apoplastic interfacial compartment and releases the uncharged NH₃ into the plant cytoplasm. The implications of such a finding are discussed in the context of AM functioning and plant phosphorus uptake.

The author responsible for the 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: Paola Bonfante (p.bonfante{at}ipp.cnr.it).

www.plantphysiol.org/cgi/doi/10.1104/pp.109.136390

^* Corresponding author; e-mail p.bonfante{at}ipp.cnr.it.

Received January 31, 2009; accepted March 24, 2009; published March 27, 2009.