Most land plants are symbiotic with arbuscular mycorrhizal fungi (AMF), which take up mineral nutrients from the soil and exchange them with plants for photosynthetically fixed carbon. This exchange is a significant factor in global nutrient cycles as well as in the ecology, evolution and physiology of plants. Despite its importance as a nutrient, very little is known about how AMF take up nitrogen and transfer it to their host plants. Here we report the results of stable isotope labelling experiments showing that inorganic nitrogen taken up by the fungus outside the roots is incorporated into amino acids, translocated from the extraradical to the intraradical mycelium as arginine, but transferred to the plant without carbon. Consistent with this mechanism, the genes of primary nitrogen assimilation are preferentially expressed in the extraradical tissues, whereas genes associated with arginine breakdown are more highly expressed in the intraradical mycelium. Strong changes in the expression of these genes in response to nitrogen availability and form also support the operation of this novel metabolic pathway in the arbuscular mycorrhizal symbiosis.

Summary Nitrogen (N) is known to be transferred from fungus to plant in the arbuscular mycorrhizal (AM) symbiosis, yet its metabolism, storage and transport are poorly understood. In vitro mycorrhizas of Glomus intraradices and Ri T‐DNA‐transformed carrot roots were grown in two‐compartment Petri dishes. 15N‐ and/or 13C‐labeled substrates were supplied to either the fungal compartment or to separate dishes containing uncolonized roots. The levels and labeling of free amino acids (AAs) in the extraradical mycelium (ERM) in mycorrhizal roots and in uncolonized roots were measured by gas chromatography/mass spectrometry (GC‐MS) and high‐performance liquid chromatography (HPLC). Arginine (Arg) was the predominant free AA in the ERM, and almost all Arg molecules became labeled within 3 wk of supplying 15NH4+ to the fungal compartment. Labeling in Arg represented > 90% of the total 15N in the free AAs of the ERM. [Guanido‐2‐15N]Arg taken up by the ERM and transported to the intraradical mycelium (IRM) gave rise to 15N‐labeled AAs. [U‐13C]Arg added to the fungal compartment did not produce any 13C labeling of other AAs in the mycorrhizal root. Arg is the major form of N synthesized and stored in the ERM and transported to the IRM. However, NH4+ is the most likely form of N transferred to host cells following its generation from Arg breakdown.

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the roots of more than 80% of land plants. Experiments on the relationship between the host plant and AM in soil or in sterile root-organ culture have provided clear evidence that the extraradical mycelia of AM fungi uptake various forms of nitrogen (N) and transport the assimilated N to the roots of the host plant. However, the uptake mechanisms of various forms of N and its translocation and transfer from the fungus to the host are virtually unknown. Therefore, there is a dearth of integrated models describing the movement of N through the AM fungal hyphae. Recent studies examined Ri T-DNA-transformed carrot roots colonized with AM fungi in 15 N tracer experiments. In these experiments, the activities of key enzymes were determined, and expressions of genes related to N assimilation and translocation pathways were quantified. This review summarizes and discusses the results of recent research on the forms of N uptake, transport, degradation, and transfer to the roots of the host plant and the underlying mechanisms, as well as research on the forms of N and carbon used by germinating spores and their effects on amino acid metabolism. Finally, a pathway model summarizing the entire mechanism of N metabolism in AM fungi is outlined.

Summary• Here, nitrogen (N) uptake and metabolism, and related gene expression, were analyzed in germinating spores of Glomus intraradices to examine the mechanisms and the regulation of N handling during presymbiotic growth.• The uptake and incorporation of organic and inorganic N sources into free amino acids were analyzed using stable and radioactive isotope labeling followed by highperformance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS) and liquid scintillation counting and the fungal gene expression was measured by quantitative polymerase chain reaction (Q-PCR).• Quiescent spores store Asp, Ala and Arg and can use these internal N resources during germination. Although not required for presymbiotic growth, exogenous N can also be utilized for the de novo biosynthesis of amino acids. Ammonium and urea are more rapidly assimilated than nitrate and amino acids. Root exudates do not stimulate the uptake and utilization of exogenous ammonium, but the expression of genes encoding a putative glutamate dehydrogenase (GDH), a urease accessory protein (UAP) and an ornithine aminotransferase (OAT) were stimulated by root exudates. The transcript levels of an ammonium transporter (AMT) and a glutamine synthetase (GS) were not affected.• Germinating spores can make effective use of different N sources and the ability to synthesize amino acids does not limit presymbiotic growth of arbuscular mycorrhizal (AM) spores.