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Genes coding for proteins involved in nitrogen and phosphate metabolism (ammonium transporter, alkaline phosphatase) in Glomus intraradices

Arbuscular mycorrhizal (AM) fungi are obligate biotrophs, which need to colonize appropriate host roots to complete their life cycle. In their symbiotic status, the fungi explore the soil surrounding the plant and help it in the absorption of relatively immobile nutrients such as P and N. Genes involved in phosphate metabolism and ammonium transport were isolated from the AM fungus Glomus intraradices.

This fungus was monoxenically cultured, using compartmented dishes, to avoid contamination from any other soil organism. Both ammonium transporter (GintAMT1) and alkaline phosphatase (GintALP1) genes were isolated from the extraradical mycelium of the fungus by a combination of RT-PCR using degenerated primers and RACE techniques. GintAMT1 codes for a 479 amino acid protein with high homology to ammonium transporters of other fungi.

The predicted topology is similar to that of ammonium transporters in eukaryotes: 11 transmembrane domains with an N terminusOUT/C terminusIN orientation. Heterologous expression of GintAMT1 in a yeast mutant defective in the three plasma membrane NH4+ transporters complemented the defect of this strain to grow in the presence of less than 1mM ammonium. The phenotipically restaured mutant yeast was used to study the kinetics of ammonium uptake by GintAMT1 through (14C)-methylammonium uptake experiments.

These studies indicated GintAMT1 is a high affinity ammonium transporter with an approximate Km of 30microM. Expression studies using real time RT-PCR indicate that GintAMT1 is induced by low ammonium concentrations and inhibited by high concentrations. GintALP1 codes for a 525amino acid protein with high homology to yeast alkaline phosphatase genes. Expression studies of the gene by Real Time RT-PCR showed that GintALP1 expression in the extraradical mycelium of G. intraradices is inhibited by pulses with 350microM P.

More information on the Genomyca -project can be found at:

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Consejo Superior de Investigaciones Cientificas
Profesor Albareda 1
18008 Granada
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