A mycorrhizal-specific ammonium transporter from Lotus japonicus acquires nitrogen released by arbuscular mycorrhizal fungi.

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 are 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 the first evidence of a plant ammonium transporter which is involved in N uptake during mycorrhiza symbiosis. The gene LjAMT2;2, which has been shown to be the highest upregulated gene in a transcriptomic analysis of Lotus japonicus roots upon colonization with Gi. margarita, has been characterized as a high affinity ammonium transporter 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 mutant complementation has confirmed its functionality and revealed its dependency on acidic pH. The transport experiments using Xenopus 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 P uptake.