Regulation of the Nitrogen Transfer Pathway in the Arbuscular Mycorrhizal Symbiosis: Gene Characterization and the Coordination of Expression with Nitrogen Flux    

Abstract

The arbuscular mycorrhiza (AM) brings together the roots of over 80% of land plant species and fungi of the phylum Glomeromycota and greatly benefits plants through improved uptake of mineral nutrients. AM fungi can take up both nitrate and ammonium from the soil and transfer nitrogen (N) to host roots in nutritionally substantial quantities. The current model of N handling in the AM symbiosis includes the synthesis of arginine in the extraradical mycelium and the transfer of arginine to the intraradical mycelium, where it is broken down to release N for transfer to the host plant. To understand the mechanisms and regulation of N transfer from the fungus to the plant, 11 fungal genes putatively involved in the pathway were identified from Glomus intraradices, and for six of them the full-length coding sequence was functionally characterized by yeast complementation. Two glutamine synthetase isoforms were found to have different substrate affinities and expression patterns, suggesting different roles in N assimilation. The spatial and temporal expression of plant and fungal N metabolism genes were followed after nitrate was added to the extraradical mycelium under N-limited growth conditions using hairy root cultures. In parallel experiments with 15 N, the levels and labeling of free amino acids were measured to follow transport and metabolism. The gene expression pattern and profiling of metabolites involved in the N pathway support the idea that the rapid uptake, translocation, and transfer of N by the fungus successively trigger metabolic gene expression responses in the extraradical mycelium, intraradical mycelium, and host plant.

The Role of the Mycorrhizal Symbiosis in Nutrient Uptake of Plants and the Regulatory Mechanisms Underlying These Transport Processes

“…AM fungi assimilate N into Arg via the anabolic arm of the urea cycle and its key enzymes: carbamoyl phosphate synthase, argininosuccinate synthase, and argininosuccinate lyase 2 [112,115]. The transcript levels in the ERM of these genes respond within hours to an exogenous supply of NO3 - [112], what supports the view that Arg is rapidly assimilated in the fungal ERM of the AM symbiosis.…”

“…GintAMT1, an NH4 + transporter of the AM fungus Glomus intraradices seems to be mainly involved in the uptake of NH4 + by the ERM under low NH4 + availabilities [111]. An exogenous supply of NO3 -stimulates the expression of a fungal NO3 -transporter in the ERM of G. intraradices [112]. Similar to the N repression observed for the NH4 + transporters in ECM fungi, the expression of this transporter is repressed by an increase in the internal levels of NH4 + or a downstream metabolite, such as glutamine [113].…”

“…In AM fungi and most ECM fungi, NH4 + is assimilated via the glutamine synthase/glutamine oxoglutarate aminotransferase (GS/GOGAT) pathway into amino acids [103,112].…”

The expression of GintPT, the phosphate transporter of Rhizophagus irregularis, depends on the symbiotic status and phosphate availability

“…Another widespread and evolutionary ancient strategy is the establishment of arbuscular mycorrhizal (AM) symbiosis which involves the majority of land plants and fungi belonging to the Glomeromycota phylum (Parniske 2008;Bonfante and Genre 2010). AM fungi ensure that their host plants have an efficient supply of mineral nutrients, particularly P and nitrogen (Allen and Shachar-Hill 2009;Tian et al 2010;. Thanks to the hyphal network they develop in the soil (up to 100 fold more extensive than root hairs), AM fungi acquire nutrients not only for their own needs, but also for delivering them to the root cortical cells via a fast but not fully understood mechanism Fellbaum et al 2012).…”

Phylogenetic, structural, and functional characterization of AMT3;1, an ammonium transporter induced by mycorrhization among model grasses

“…Once taken up, N is assimilated and translocated to the intraradical hyphae, mainly in the form of arginine (Govindarajulu et al 2005), converted to NH 4 + in the arbuscule, and finally transferred to the plant (Tian et al 2010). This transfer is assumed to proceed via export of NH 4 + by transporters present in the fungal plasma membrane into the periarbuscular space, followed by uptake through plant ammonium transporters (AMT) in the periarbuscular membrane .…”