Roles of Arbuscular Mycorrhizas in Plant Nutrition and Growth: New Paradigms from Cellular to Ecosystem Scales

Root systems of most land plants form arbuscular mycorrhizal (AM) symbioses in the field, and these contribute to nutrient uptake. AM roots have two pathways for nutrient absorption, directly through the root: epidermis and root hairs and via AM fungal hyphae into root cortical cells, where arbuscules or hyphal coils provide symbiotic interfaces. New physiological and molecular evidence shows that for phosphorus the mycorrhizal pathway (MP) is operational regardless of plant growth responses (positive or negative). Amounts delivered cannot be determined from plant nutrient contents because when responses are negative the contribution of the direct pathway (DP) is reduced. Nitrogen (N) is also delivered to roots via an MP, but the contribution to total N requirement and the costs to the plant are not clear. The functional interplay between activities of the DP and MP has important implications for consideration of AM symbioses in ecological, agronomic, and evolutionary contexts.