Forms of nitrogen uptake,translocation,and transfer via arbuscular mycorrhizal fungi:A review

Arbuscular mycorrhizal(AM) fungi are obligate symbionts that colonize the roots of more than 80% of land plants.Experiments on the relationship between the host plant and AM in soil or in sterile root-organ culture have provided clear evidence that the extraradical mycelia of AM fungi uptake various forms of nitrogen(N) and transport the assimilated N to the roots of the host plant.However,the uptake mechanisms of various forms of N and its translocation and transfer from the fungus to the host are virtually unknown.Therefore,there is a dearth of integrated models describing the movement of N through the AM fungal hyphae.Recent studies examined Ri T-DNA-transformed carrot roots colonized with AM fungi in 15 N tracer experiments.In these experiments,the activities of key enzymes were determined,and expressions of genes related to N assimilation and translocation pathways were quantified.This review summarizes and discusses the results of recent research on the forms of N uptake,transport,degradation,and transfer to the roots of the host plant and the underlying mechanisms,as well as research on the forms of N and carbon used by germinating spores and their effects on amino acid metabolism.Finally,a pathway model summarizing the entire mechanism of N metabolism in AM fungi is outlined.