Mycorrhiza
A fungus-root symbiosis that improves the uptake of water and nutrients, present in 85 to 90% of land plants.
Explanation
Mycorrhizae greatly extend the root surface and allow the plant to reach phosphorus and water in areas inaccessible to roots alone. A symbiosis present in the great majority of land plants, about 85 to 90% according to Smith & Read (Mycorrhizal Symbiosis, 2008). The main exceptions are the Brassicaceae (cabbage, turnip, radish, mustard) and the Amaranthaceae, formerly Chenopodiaceae (beetroot, chard, spinach).
Refined definition
A mycorrhiza (from the Greek mukês “fungus” and rhiza “root”) is the symbiotic association between a fungus and a plant's roots. The fungus colonises the roots (sometimes inside the cells, endomycorrhizae; sometimes between the cells, ectomycorrhizae) and extends into the soil a network of hyphae 100 to 1000 times finer and longer than the roots. It supplies the plant with water, phosphorus and trace elements; in exchange it receives up to 20% of the sugars from photosynthesis.
Key points
Two main types
ENDOMYCORRHIZAE (arbuscular, AM): the majority (80% of land plants), invisible to the naked eye, the main suppliers of phosphorus. Present in tomato, apple tree, vine, wheat, legumes, aromatics. ECTOMYCORRHIZAE (ECM): form a visible sheath around the roots, present mainly in forest trees (oak, beech, pine, birch). It is the ectomycorrhizae that produce the edible mushrooms we harvest (ceps, chanterelles, truffles). Source: Marc-André Selosse, Jamais seul, Actes Sud, 2017.
How to preserve and multiply them
Do NOT dig deeply (tilling destroys the extended hyphae). Prefer no-dig, the broadfork, light hoeing. Keep the soil covered permanently (mulch). Limit soluble phosphate fertilisers: at high doses they make the symbiosis useless to the plant, which “lays off” its fungus. Avoid synthetic fungicides, they also destroy mycorrhizae.
The “Wood Wide Web”
A single mycorrhizal fungus can link the roots of several plants. The idea of a vast network through which a “mother tree” feeds its seedlings or sends alarm signals, popularised by Suzanne Simard's work in British Columbia and by the expression “Wood Wide Web” (the journal Nature, 1997), is now considered poorly supported in the field (Karst et al., 2023; Henriksson et al., 2023).
Watch out for
- • Commercial mycorrhizal inoculants are very uneven: in an evaluation of 28 products, most improved neither root colonisation nor plant growth in the greenhouse (Salomon et al., 2022), which a meta-analysis confirms (Koziol et al., 2025).
- • On Brassicaceae and Amaranthaceae, formerly Chenopodiaceae (not mycorrhizable), plant clovers or Apiaceae as intercrops to maintain the network in the soil.
- • Phosphate fertilisers reduce mycorrhizal fungi: by about a third on average in field studies (Treseder, 2004).
Examples
Sources
- Smith, S.E. & Read, D.J. (2008). Mycorrhizal Symbiosis (3e éd.). Londres : Academic Press.
- Selosse, M.-A. (2017). Jamais seul : ces microbes qui construisent les plantes, les animaux et les civilisations. Arles : Actes Sud.
- Simard, S.W. et al. (1997). Net transfer of carbon between ectomycorrhizal tree species in the field. Nature, 388, 579-582.
- Karst, J., Jones, M.D. & Hoeksema, J.D. (2023). Positive citation bias and overinterpreted results lead to misinformation on common mycorrhizal networks in forests. Nature Ecology & Evolution, 7(4), 501-511.
- Henriksson, N. et al. (2023). Re-examining the evidence for the mother tree hypothesis: resource sharing among trees via ectomycorrhizal networks. New Phytologist, 239(1), 19-28.
- Salomon, M.J. et al. (2022). Global evaluation of commercial arbuscular mycorrhizal inoculants under greenhouse and field conditions. Applied Soil Ecology, 169, 104225.
- Koziol, L., McKenna, T.P. & Bever, J.D. (2025). Meta-analysis reveals globally sourced commercial mycorrhizal inoculants fall short. New Phytologist, 246(3), 821-827.
- Treseder, K.K. (2004). A meta-analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies. New Phytologist, 164(2), 347-355.