The uptake, metabolism, transport and transfer of nitrogen in an arbuscular mycorrhizal symbiosis

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. N-15- and/or C-13-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 (NH4+)-N-15 to the fungal compartment. Labeling in Arg represented > 90% of the total N-15 in the free AAs of the ERM. [Guanido-2-N-15]Arg taken up by the ERM and transported to the intraradical mycelium (IRM) gave rise to N-15-labeled AAs. [U-C-13]Arg added to the fungal compartment did not produce any C-13 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.