Nitrogen transfer in the arbuscular mycorrhizal symbiosis

Indirect root for nitrogen The discovery of a previously unknown mechanism of nitrogen transfer from the arbuscular mycorrhizal fungi found on the roots of most land plants, to the host plants suggests that this symbiotic relationship may be a much more important factor in the global nitrogen cycle than was thought. The mechanism involves uptake of inorganic nitrogen by the fungus outside the roots, conversion to amino acids within the fungus, then transfer as ammonium ions from the fungal mycelium into the plant. The first event in host recognition by arbuscular mycorrhizal fungi is thought to be hyphal branching. A strigolactone, 5-deoxy-strigol, isolated from Lotus japonicus has now been identified as an inducer of branching. Strigolactones are root metabolites, previously isolated as seed germination stimulants for root parasitic weeds. This finding highlights the close relationship between plant and fungus, and may provide a new strategy for the control of both beneficial fungal symbionts and destructive parasitic weeds in agriculture and natural ecosystems.