Nitrogen metabolism of external hyphae of the arbuscular mycorrhizal fungus Glomus intraradices

The arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck and Smith was grown in symbiosis with Cucumis sativus L. cv. Aminex (F1 hybrid) in mesh bags surrounded by a sand-filled hyphal compartment (HC), allowing only the fungal hyphae to protrude into the HC. The hyphae in the HC were supplied with N-15-labelled NH4+ or NO3- after 60 d (expt 1). Following a 48 h labelling period, the sand was removed from the HC and the hyphae extracted. In another experiment (expt 2), the hyphae were extracted from the sand before being incubated in vitro in a nutrient solution containing N-15-labelled NH4+ for 15 h. The hyphal material was incubated in a 0 or 2.5 mM solution of the GOGAT-inhibitor albizzine prior to labelling. In both experiments the hyphal content of free amino acids and fatty acids were measured as well as the amino acid N-15 enrichment. Asparagine was the hyphal amino acid measured in highest concentration followed by glutamine, glutamate, aspartate and alanine. Ornithine, serine and glycine were detected in lower concentrations, but gamma-aminobutyric acid and citrulline were not detectable. This pattern was the same in both experiments and was unaffected by the type of N applied to the hyphae or by preincubation with albizzine, although the amino-acid concentration decreased considerably in expt 2 compared with expt 1. Both NH4+-N and NO3--N were assimilated into amino acids but the levels of N-15 enrichment following application of NO3- were much lower than those following application of NH4+ indicating that the latter was more readily assimilated. Albizzine decreased the hyphal amino acid concentration by c. 30% (without affecting the N-15 enrichment of the individual amino acids) indicating that the AM-fungal hyphae might possess a GS-GOGAT enzyme system for assimilation of inorganic N. The fatty-acid profiles (especially phospholipid fatty acids 16:1 omega 5 and 20:5) obtained from the hyphae of G. intraradices showed that contamination of the samples by fungi other than C. intraradices and bacteria was insignificant, and confirmed the usefulness of specific fatty-acid measurement to estimate soil AM-fungal content.