Mycorrhiza

A mycorrhiza is a partnership between a fungus and the roots of a plant, in which the fungus gets sugars from the plant and the plant gets water and minerals, especially phosphorus and nitrogen, through the fungal threads. About 80 to 90 percent of land plant species form such partnerships.

In arbuscular mycorrhiza, formed by fungi such as Gigaspora margarita and other members of the group called arbuscular mycorrhizal fungi, the fungus enters root cells and forms tree-like structures. In ectomycorrhiza, which is typical of trees such as Scots pine, oak and birch, the fungus forms a sheath around the root tips without entering the cells. Orchids and heathers have other types. Fossils from the Rhynie chert, about 400 million years old, show that the association is ancient.

Fungal threads, or hyphae, are much thinner than roots and reach a far larger volume of soil, so they take up water and nutrients such as phosphorus that are poorly mobile. In return the plant may give up as much as a fifth of the carbon it fixes in photosynthesis. Some fungi also help plants to take up nitrogen and to resist drought and disease.

Mycorrhizal networks affect soil structure and store carbon, linking them to the carbon cycle. Fungal links between neighboring plants, sometimes called the "wood wide web," may move resources and signals between plants, but how important this is remains debated. Crops benefit from the partnership, which may reduce the need for fertilizer, and the study of soil life is part of ecosystem science.