Mycorrhiza (plural: mycorrhizae): a symbiotic association between a fungus and the roots of a plant. The term means literally “fungus-root” (Greek mykes + rhiza). In a mycorrhizal association, the fungal mycelium extends from the plant’s root system into the surrounding soil, vastly increasing the surface area available for nutrient and water absorption. The plant gains enhanced mineral nutrition — especially phosphorus, which is poorly mobile in soil — and the fungus gains photosynthetically fixed carbon from the plant.
Two major types exist. Ectomycorrhizae: the fungus sheaths the root surface in a dense hyphal mantle and grows between root cortical cells (forming a structure called the Hartig net) but does not penetrate cell walls. Ectomycorrhizal associations are common in temperate forest trees — oaks, pines, birches, beeches — and the fungi involved are often Basidiomycota that produce familiar fruiting bodies (boletes, chanterelles, fly agarics). Endomycorrhizae (arbuscular mycorrhizae, or AM): the fungus penetrates root cell walls and forms highly branched structures called arbuscules within the cells, creating an intimate interface for nutrient exchange. AM fungi belong to the Glomeromycota and associate with over 80% of land plant species. They do not produce conspicuous fruiting bodies.
Mycorrhizal associations are ancient — fossil evidence places them at over 400 million years old, coinciding with the earliest colonization of land by plants. It is likely that mycorrhizal fungi were essential to plant terrestrialization, providing the mineral nutrition that early rootless plants could not obtain alone. These associations are foundational: most land plants depend on them, and in many ecosystems, mycorrhizal networks connect multiple plants, forming the underground infrastructure described in mycelial networks. Through these shared networks, resources and chemical signals can move between plants — a phenomenon that blurs the boundary between individual organisms and suggests that the relevant ecological unit is the network, not the individual plant or fungus.
Related terms
- Mycelium — the fungal body that forms the mycorrhizal network
- Hyphae — the filaments that interface with plant root cells
- Symbiosis — the broader biological concept, of which mycorrhiza is the most ancient and widespread terrestrial instance
- Fungal Symbiosis — mycorrhiza as a central case of fungal symbiotic association
- Mycelial Networks — the network structures that mycorrhizal fungi form between plants
- Holobiont — multi-species assemblages, which mycorrhizal networks extend to the landscape scale
- Niche Construction — mycorrhizal networks as underground infrastructure constructing the nutrient environment
- Hartig Net — the intercellular hyphal network that forms the exchange interface in ectomycorrhizae
- Arbuscule — the branched nutrient-exchange structure formed inside root cells by AM fungi
- Nutrient Cycling — the biogeochemical cycles that mycorrhizal networks mediate
- Lichen — another major fungal symbiosis, with photosynthetic partners rather than plant roots