GintAMT1 encodes a functional high-affinity ammonium transporter that is expressed in the extraradical mycelium of Glomus intraradices

We report the cloning and characterization of the first NH4+ transporter gene (GintAMT1) in an arbuscular mycorrhizal fungus. GintAMT1 encodes a polypeptide of 479 amino acids sharing high sequence similarity with previously characterized NH4+ transporters from other fungi. Heterologous expression of GintAMT1 in the yeast triple mep mutant complemented the defect of this strain to grow in the presence of less than 1 mM NH4+. As revealed by [14C]methylammonium uptake experiments carried out in yeast, GintAMT1 encodes a high-affinity NH4+ transporter. In mycelia developed in the presence of 0.9 mM NO3−, GintAMT1 transcription was increased after the addition of 30 μM NH4+ but decreased after the addition of 3 mM NH4+. However, in mycelia grown in the presence of higher N concentrations, GintAMT1 transcripts decreased after the addition of NH4+, irrespective of the concentration used. These data suggest that GintAMT1 is involved in NH4+ uptake by the extraradical mycelia from the surrounding media when it is present at micromolar concentrations.