The tactics for nitrogen acquisition between plants and VAM fungi

It is unusual that roots of terrestrial plants are in bare, but they normaly form symbiotic associations with mycorrhizal fungi. The most common association is vesicular-arbuscular (VA) mycorrhiza, and it is well known that forming VA mycorrhiza can improve phosphorus (P) acquisition by the host plants. However, as for nitrogen (N) nutrition, it is still debatable whether the mycorrhizal fungi are involved or not, althhough the availability of N rather than P limits plant growth in many ecosystems. Recently, we have found that N delivery via the mycorrhizal hyphae depends on the form of N supplied; while the fungus fed with ammonium can rapidly deliver N to the host plants, the fungus fed with nitrate scarcely delivers N despite of that nitrate is metabolized by the fungus. On the basis of this finding, I discuss the involvement of the mycorrhizal symbiosis in plant N nutrition with critical review for the previous studies.