Mycorrhizal Symbioses: Mechanisms, Ecological Function and Responses to Global Change

Abstract

Most land plants acquire mineral nutrients through fungal partners rather than through roots alone. This review examines the cellular basis of that partnership, the differences between the principal mycorrhizal types, and the consequences for plant communities and soil processes. Fossil evidence places arbuscular associations in the earliest land floras, and roughly seventy percent of vascular plant species retain them today. Recognition begins with a reciprocal exchange of diffusible signals, proceeds through a conserved symbiosis signalling pathway shared with nitrogen-fixing associations, and ends in a highly branched fungal structure enclosed by a plant-derived membrane across which phosphate moves inward and carbon moves outward. Work with isotope tracers has shown that the fungus receives both sugars and fatty acids, which it cannot make for itself, and that plants preferentially reward the more productive fungal partners. At the ecosystem scale the type of association predicts how nitrogen is cycled, how much carbon accumulates in soil, and how strongly plant growth responds to rising carbon dioxide. Responses to warming, nitrogen deposition and land conversion differ between types, so shifts in the composition of these associations are likely to alter nutrient cycling in ways that current models capture only in outline.