The influence of external nitrogen on carbon allocation to Glomus intraradices in monoxenic arbuscular mycorrhiza

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Abstract
• The influence of external nitrogen (N) on carbon (C) allocation and processes related to phosphorus (P) metabolism were studied in monoxenic arbuscular mycorrhiza (AM) cultures of Daucus carota. • Fungal hyphae of Glomus intraradices proliferated from colonized roots growing on solid medium into C-free liquid minimal medium with two different N and P levels. Furthermore, we exposed the colonized roots to high or low N availability and then studied the mycelial development. Roots were provided with 13C-glucose in order to follow the C allocation. The mycelium was analysed for phosphatase activity and transcription levels of two nutrient regulated genes. • High N availability to the monoxenic AM root reduced the C allocation to the AM fungus while N availability to the mycelium was important for the upregulation of the fungal inorganic phosphorus (Pi)-transporter GiPT. • We found that N availability can regulate nutritional processes in arbuscular mycorrhiza. We conclude that negative impacts of N on AM abundance are caused by reduced C allocation from the plant. Upregulation of the fungal Pi-transporter GiPT indicated that increased N availability might induce P limitation in the mycelium.