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

Abstract

We report the cloning and characterization of the first NH(4)(+) transporter gene (GintAMT1) in an arbuscular mycorrhizal fungus. GintAMT1 encodes a polypeptide of 479 amino acids sharing high sequence similarity with previously characterized NH(4)(+) 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 1mM NH(4)(+). As revealed by [(14)C]methylammonium uptake experiments carried out in yeast, GintAMT1 encodes a high-affinity NH(4)(+) transporter. In mycelia developed in the presence of 0.9 m M NO(3)(-), GintAMT1 transcription was increased after the addition of 30 microM NH(4)(+) but decreased after the addition of 3 mM NH(4)(+). However, in mycelia grown in the presence of higher N concentrations, GintAMT1 transcripts decreased after the addition of NH(4)(+), irrespective of the concentration used. These data suggest that GintAMT1 is involved in NH(4)(+) uptake by the extraradical mycelia from the surrounding media when it is present at micromolar concentrations.