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

Abstract

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.

A Mycorrhizal-Specific Ammonium Transporter from Lotus japonicus Acquires Nitrogen Released by Arbuscular Mycorrhizal Fungi

“…AM fungi have access to inorganic or organic forms of N (Lopez-Pedrosa et al, 2006;Cappellazzo et al, 2008) and translocate them via Arg from the extra-to the intraradical mycelium, where N is transferred to the plant in inorganic form (Govindarajulu et al, 2005;Jin et al, 2005). However, the molecular form in which N is transferred from AM fungi to their hosts is still under debate: while Govindarajulu et al (2005) considered NH 4 + , Chalot et al (2006) added NH 3 as a potential candidate.…”

Arbuscular mycorrhizal networks: process and functions. A review

“…N uptake and incorporation into amino acids via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle has been found in AM fungi (Smith et al, 1985). Stable isotope labeling has now suggested that inorganic nitrogen is taken up by the extraradical mycelium, incorporated into amino acids, translocated from extra-to intraradical fungal structures as arginine and then transported as ammonium to the plant (Govindarajulu et al, 2005;Jin et al, 2005). Further support for this uptake route comes from the finding that transcript abundance of key enzymes of nitrogen assimilation and arginine breakdown preferentially accumulate in extra-and intraradical mycelia, respectively (Govindarajulu et al, 2005).…”

“…Various models have been proposed by Jin et al (2005), Govindarajulu et al (2005) and Chalot et al (2006) which involve a number of steps for direct transfer of ammonia from fungal to plant cells (i) AM fungi take up inorganic N (NH + 4 , NO − 3 ); (ii) absorbed N is mostly incorporated and stored in arginine; (iii) AM fungi assimilate the N through GS/GOGAT, asparagine synthase and the urea cycle; (iv) stored arginine can be co-transported with PolyP intact to the intraradical mycelium form the extraradical mycelium of AM fungi, and arginine is also bi-directionally transported within the extraradical mycelium; and (v) N released from transported arginine is transferred to the host as NH + 4 and can be incorporated into other free amino acids in mycorrhizal roots, while carbon (C) not transferred to the host is recycled back to the extraradical mycelium. In analogy to the path of symbiotic P i uptake, the arbuscule may be the site of symbiotic nitrogen uptake involving plant encoded nitrogen transporters located within the periarbuscular plant membrane (Paszkowski, 2006).…”

Chromatographic Analysis of Nitrogen Utilization and Transport in Arbuscular Mycorrhizal Fungal Symbiosis

“…To follow the uptake, assimilation and transfer of nitrogen in the arbuscular mycorrhizal symbiosis, we added isotopically labeled substrates to in vitro arbuscular mycorrhizal cultures of carrot (Daucus carota L.) roots colonized by G. intraradices. [28][29][30] When grown in divided Petri plates, this model mycorrhiza excludes other microorganisms and prevents diffusion of nonvolatile solutes between the compartments. This model system shows normal life cycle and development of fungal morphology.…”