What Is a Mycorrhiza? The Partnership Under Almost Every Plant

A plant can photosynthesise and cannot search soil efficiently, since roots are thick relative to soil pores and grow slowly. A fungus cannot photosynthesise and is extremely good at exploring soil, since its filaments are a fraction of the diameter of a root and extend enormously in total length for very little material. The partnership exchanges what each has, with the plant supplying sugars produced by photosynthesis and the fungus supplying phosphorus, nitrogen and water gathered from a volume of soil the roots could never reach. The exchange is substantial, with estimates that a large share of the carbon a plant fixes goes to its fungal partners, and the benefit to the plant is correspondingly large, with phosphorus uptake in particular depending almost entirely on the arrangement in many species.

Two arrangements dominate and differ in structure and in partners:

• •Arbuscular mycorrhizas, where the fungus penetrates root cells and forms branching structures inside them, found in the large majority of plant species
• •Ectomycorrhizas, where the fungus sheathes the root without entering cells, found in many trees including pines, oaks, birches and beeches
• •The fungi involved differ completely, with the first group being an ancient specialised lineage and the second including many familiar mushroom-forming species
• •Orchid mycorrhizas, which supply the seedling entirely, since orchid seeds carry no reserves and cannot germinate without a fungal partner
• •Ericoid mycorrhizas, associated with heathers and adapted to acidic peaty soils
• •Non-mycorrhizal plants, a minority including many members of the cabbage family, which use other strategies

Fossil evidence places the partnership at the very beginning of plant life on land, with structures resembling arbuscular mycorrhizas preserved in the earliest well-preserved land plant fossils, over four hundred million years old, in plants that had no true roots at all. That timing supports the argument that the partnership was what made colonisation of land possible, since the barren mineral substrate available then would have been extremely difficult for a plant to extract nutrients from without fungal assistance. Genetic evidence supports an ancient single origin for the arbuscular arrangement. The ectomycorrhizal partnership arose much later and independently many times in different fungal lineages, which is why it involves such a variety of unrelated fungi and why it is associated with particular tree families rather than being universal.

The practical consequences for agriculture and restoration are substantial and are not fully exploited. Heavy phosphorus fertilisation suppresses the partnership, since a plant with abundant phosphorus reduces its investment in fungal partners, which means the fertiliser both costs money and removes the free service it replaces. Tillage disrupts the fungal network physically, which is one of the arguments for reduced tillage systems. Fallow periods without host plants reduce fungal populations, since the fungi cannot survive long without a partner, and a crop following a long bare fallow establishes worse. Several crop species depend on the partnership heavily and others barely at all, which means rotation design affects it. Commercial inoculants are sold and their effectiveness is mixed, since the fungi are frequently already present and adding more achieves nothing, while in disturbed or sterilised soils inoculation genuinely helps.

Fungal networks connecting several plants have attracted enormous popular attention, with claims that trees share resources, warn each other of attack and preferentially support their own offspring through a connected underground system. Connections between plants through shared fungi are real and well documented, and transfer of carbon between plants through them has been measured. The stronger claims have been challenged sharply in recent reviews, which found that the evidence for resources flowing in ecologically meaningful quantities between mature trees is weaker than the popular accounts suggest, that the experiments frequently cannot distinguish transfer through fungi from transfer through soil, and that the framing of cooperation understates the extent to which fungi act in their own interest and may take more than they give. The subject is genuinely unsettled and is worth following rather than summarising confidently.

Plants supply sugar and fungi supply phosphorus, nitrogen and water gathered from soil roots cannot reach, with a large share of fixed carbon going to the partner. Arbuscular fungi penetrate root cells and serve most plant species, while sheathing fungi serve many trees and arose independently many times. The partnership dates to the earliest land plants, and the popular network claims have been seriously challenged.