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.

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The arbuscular mycorrhizal (AM) symbiosis, formed between the majority of land plants and ubiquitous soil fungi of the phylum Glomeromycota, is responsible for massive nutrient transfer and global carbon sequestration. AM fungi take up nutrients from the soil and exchange them against photosynthetically fixed carbon (C) from the host. Recent studies have demonstrated that reciprocal reward strategies by plant and fungal partners guarantee a "fair trade" of phosphorus against C between partners [Kiers ET, et al. (2011) Science 333:880-882], but whether a similar reward mechanism also controls nitrogen (N) flux in the AM symbiosis is not known. Using mycorrhizal root organ cultures, we manipulated the C supply to the host and fungus and followed the uptake and transport of N sources in the AM symbiosis, the enzymatic activities of arginase and urease, and fungal gene expression in the extraradical and intraradical mycelium. We found that the C supply of the host plant triggers the uptake and transport of N in the symbiosis, and that the increase in N transport is orchestrated by changes in fungal gene expression. N transport in the symbiosis is stimulated only when the C is delivered by the host across the mycorrhizal interface, not when C is supplied directly to the fungal extraradical mycelium in the form of acetate. These findings support the importance of C flux from the root to the fungus as a key trigger for N uptake and transport and provide insight into the N transport regulation in the AM symbiosis.arbuscular mycorrhiza | arginine catabolism | carbon transport | Glomus intraradices | urea cycle T he arbuscular mycorrhizal (AM) symbiosis plays a key role in nutrient uptake in the majority of land plants, including such important crop species as corn, soybean, and rice. The AM symbiosis can increase the uptake of phosphate (P) and nitrogen (N), as well as of trace elements such as copper and zinc, and improves the abiotic and biotic stress resistance of the host (2). Previous work has focused primarily on the transport of P in AM symbiosis, but more recent work has highlighted the potential importance of N uptake by fungal symbionts (3, 4). The extraradical mycelium (ERM) of the fungus is able to take up NH 4 + (5, 6), NO 3 − (5-7), and organic N resources (3-5) from the soil and to transfer N to the host. The high mobility of N in the soil has raised the question of whether AM fungi can contribute significantly to the N nutrition of the host (8). It has been suggested that an improved N status of mycorrhizal plants may be simply a consequence of an improved P nutrition (9); however, other studies have demonstrated that AM fungi can deliver substantial amounts of N to the host, with an estimated 21% of total N taken up by the fungal ERM in root organ cultures (10) and 74% of the total N in the leaves of Zea mays coming from the fungal ERM with access to urea (11).Current models of N transport in the AM symbiosis involve uptake of inorganic N from the soil and N assimilation via the anabolic arm of the urea...

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SummaryCommon mycorrhizal networks (CMNs) of arbuscular mycorrhizal (AM) fungi in the soil simultaneously provide multiple host plants with nutrients, but the mechanisms by which the nutrient transport to individual host plants within one CMN is controlled are unknown.Using radioactive and stable isotopes, we followed the transport of phosphorus (P) and nitrogen (N) in the CMNs of two fungal species to plants that differed in their carbon (C) source strength, and correlated the transport to the expression of mycorrhiza-inducible plant P (MtPt4) and ammonium (1723.m00046) transporters in mycorrhizal roots.AM fungi discriminated between host plants that shared a CMN and preferentially allocated nutrients to high-quality (nonshaded) hosts. However, the fungus also supplied low-quality (shaded) hosts with nutrients and maintained a high colonization rate in these plants. Fungal P transport was correlated to the expression of MtPt4. The expression of the putative ammonium transporter 1723.m00046 was dependent on the fungal nutrient supply and was induced when the CMN had access to N.Biological market theory has emerged as a tool with which the strategic investment of competing partners in trading networks can be studied. Our work demonstrates how fungal partners are able to retain bargaining power, despite being obligately dependent on their hosts.

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.

(J.W.A., Y.S.-H.) Arbuscular mycorrhizal (AM) fungi take up photosynthetically fixed carbon from plant roots and translocate it to their external mycelium. Previous experiments have shown that fungal lipid synthesized from carbohydrate in the root is one form of exported carbon. In this study, an analysis of the labeling in storage and structural carbohydrates after 13 C 1 glucose was provided to AM roots shows that this is not the only pathway for the flow of carbon from the intraradical to the extraradical mycelium (ERM). Labeling patterns in glycogen, chitin, and trehalose during the development of the symbiosis are consistent with a significant flux of exported glycogen. The identification, among expressed genes, of putative sequences for glycogen synthase, glycogen branching enzyme, chitin synthase, and for the first enzyme in chitin synthesis (glutamine fructose-6-phosphate aminotransferase) is reported. The results of quantifying glycogen synthase gene expression within mycorrhizal roots, germinating spores, and ERM are consistent with labeling observations using 13 C-labeled acetate and glycerol, both of which indicate that glycogen is synthesized by the fungus in germinating spores and during symbiosis. Implications of the labeling analyses and gene sequences for the regulation of carbohydrate metabolism are discussed, and a 4-fold role for glycogen in the AM symbiosis is proposed: sequestration of hexose taken from the host, long-term storage in spores, translocation from intraradical mycelium to ERM, and buffering of intracellular hexose levels throughout the life cycle.The arbuscular mycorrhizal (AM) symbiosis is important because it benefits most land plants. AM plants show enhanced growth, increased resistance to biotic and abiotic stresses, and greater ecological diversity (for review, see Smith and Read, 1997). AMs are also responsible for directing the movement of huge quantities of photosynthate to the soil (for review, see Douds et al., 2000; Graham, 2000). Carbon in the root flows from plant to fungus in the form of sugars (Shachar-Hill et al., 1995; Solaiman and Saito, 1997), and together with the transfer of mineral nutrients from fungus to root (Koide and Schreiner, 1992; George et al., 1995; Jakobsen, 1995), this is the nutritional mainstay of what is arguably the world's most important mutualistic symbiosis.Rather little was known about the forms and pathways through which carbon flows in the AM symbiosis until recently (Jennings, 1995; Smith and Read, 1997). The development of in vitro monoxenic AM root cultures (Mugnier and Mosse, 1987) with separate host and fungal compartments (St. Arnaud et al., 1996) has facilitated the application of stable isotope labeling (Pfeffer et al., 1999), gene expression analysis (Lammers et al., 2001; Bago et al., 2002), in vivo microscopy (Bago et al., 2002), and other methods (for review, see Fortin et al., 2002). Together with previous work on enzyme activities and analysis of metabolites, these approaches have begun to illuminate the metabo...