Regulation of the nitrogen transfer pathway in the arbuscular mycorrhizal symbiosis: Gene characterization and the coordination of expression with nitrogen flux

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 identifiedfrom Glomus intraradices, and for six of them the full-length coding sequence was functionally characterized by yeastcomplementation. Two glutamine synthetase isoforms were found to have different substrate affinities and expressionpatterns, suggesting different roles in N assimilation. The spatial and temporal expression of plant and fungal N metabolismgenes were followed after nitrate was added to the extraradical mycelium under N-limited growth conditions using hairy rootcultures. In parallel experiments with ¹⁵N, the levels and labeling of free amino acids were measured to follow transport andmetabolism. The gene expression pattern and profiling of metabolites involved in the N pathway support the idea that therapid uptake, translocation, and transfer of N by the fungus successively trigger metabolic gene expression responses in theextraradical mycelium, intraradical mycelium, and host plant.