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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.

Arbuscular mycorrhizal (AM) fungi are vital components of nearly all terrestrial ecosystems, forming mutually beneficial (mutualistic) symbioses with the roots of around 80% of vascular plants and often increasing phosphate (P) uptake and growth. We present novel data showing that AM fungi can provide the dominant route for plant P supply, even when overall growth or P uptake remains unaffected. The results will change our understanding of the roles of AM fungi in agricultural and natural ecosystems; they also predict that mycorrhiza-specific plant P transporters must play a major role in plant P uptake regardless of whether the plants respond to AM colonization by taking up more P per plant or by increased dry weight, compared with nonmycorrhizal (NM) control plants.

In this Update, we review new findings about the roles of the arbuscular mycorrhizas (mycorrhiza = fungus plus root) in plant growth and phosphorus (P) nutrition. We focus particularly on the function of arbuscular mycorrhizal (AM) symbioses with different outcomes for plant growth (from positive to negative) and especially on the interplay between direct P uptake via root epidermis (including root hairs when present) and uptake via the AM fungal pathway. The results are highly relevant to many aspects of AM symbiosis, ranging from signaling involved in the development of colonization and the regulation of P acquisition to the roles of AM fungi in determining the composition of natural plant assemblages in ecological settings and their changes with time.