ASSIMILATION OF ¹⁵NH₄⁺BY BEECH (FAGUS SYLVATICA L.) ECTOMYCORRHIZAS

Abstract

SUMMARY Ammonia assimilation has been followed in ectomycorrhizal roots of Fagus sylvatica. The absorption of ammonium ions was associated with a rapid synthesis of free amino acids in mycorrhizal tissues, glutamine being the most prominent. In the presence of [^^Njammonium, glutamate, glutamine and alanine became the most strongly labelled metabolites of ectomycorrhizas. Nitrogen-15 nuclear magnetic resonance spectroscopy demonstrated that the glutamine amide-N was the most highly enriched component of the extracts. Methionine sulphoximine and albizine, inhibitors of glutamine synthetase and glutamate synthase, almost completely blocked the incorporation of ^^N label into amino acids and induced an accumulation of NH4 . These observations suggest that in the ammonia-fed beech ectomycorrhizas, ammonia assimilation occurs mainly via the glutamine synthetase/glutamate synthase pathway, and that glutamate dehydrogenase plays little, if any, part in this process. Alternative models for the nitrogen assimilation pathways in fungal and host tissues are presented.

Nitrogen source regulates the biosynthesis of NADP‐glutamate dehydrogenase in the ectomycorrhizal basidiomycete Laccaria bicolor

“…Assuming that NH^"" concentration in the ectomycorrhiza! sheath is in the range of 5-lOmM (Harley, 1978;Martin et al, 1986), the rate of biosynthesis of the enzyme is presumably kept at a low level in symbiotic tissues. In contrast, extramatrical mycelium is growing through soil environments characterized by low NH^"' " and NO., concentrations (1-100//M; Marschner, Haussling & George, 1991).…”

A 14N and 15N Nuclear Magnetic Resonance Study of Nitrogen Metabolism in Shoot-Forming Cultures of White Spruce (Picea glauca) Buds

“…Moreover, it has the advantage that potentially one can follow metabolism in the same specimen and can be in a better position to correlate physiological findings with subsequent behavior (14,22,27). It is possible to use both '4N and '5N in noninvasive high resolution NMR studies (1,(11)(12)(13)(14)(15)(16)(17). Although solid state 'sN NMR has been used quite extensively (see [3] and references therein), only few high resolution studies utilizing '4N (1, 13) and '5N NMR (17,19) probing metabolism in plants have been reported.…”

“…Nevertheless, alternative metabolic routes for the incorporation ofammonia exist (8), necessitating the study of each individual case. To date, studies of inorganic nitrogen metabolism in plants have largely relied on the determination of enzyme activities in cell extracts and on the determination of the isotopic incorporation of '5N (10,16,20,21) or the radioactive '3N (24) in amino acids which had been purified from plant cell extracts. Obviously, it would be preferable ifsuch determinations could be made in vivo.…”

NH4+ assimilation in the ectomycorrhizal basidiomycete Laccaria bicolor (Maire) Orton, a 15N‐NMR study

“…There is now clear via two major alternative pathways, the glutamate evidence that the presence of mycorrhiza may bring dehydrogenase (GDH) pathway and the glutamine about considerable modification of N metabolism of synthetase (GS)/glutamate synthase (GOGAT) paththe symbionts and, depending on the species of way. In the ascomycetes, Candida utilis and Cenofungus and host-plant forming the symbiosis, the coccum geophilum, NADP-dependent GDH and characteristics of N acquisition and assimilation may GS are key enzymes involved in the assimilation of differ (Martin et al, 1986; NH/which is derived from both external N sources 1991; Martin et al, 1992;Martin & Botton, 1993; and internal metabolic processes such as amino acid Botton & Dell, 1994). Although it is generally catabolism (Ferguson & Sims, 1974;Genetet, Mar-accepted that tbe ectomycorrhizal fungus assimilates tin & Stewart, 1984;Martin, 1985;, absorbed N before translocation to the root cells 1988).…”