Mycorrhizae

How the symbiosis works

Roots are surprisingly bad at exploring soil. A root tip is hundreds of micrometers thick — too fat to enter the smallest pores where most of the soil's water and ions sit. A fungal hypha is one to two orders of magnitude thinner. Plug a fungus into the root and the plant suddenly has access to a network of microscopic plumbing reaching tens of centimeters past its own root surface, with total absorptive area sometimes 100 times larger.

The bargain is simple. The fungus is heterotrophic — it cannot make its own carbon. The plant is photoautotrophic — it makes more carbon than it knows what to do with, but is starved of phosphorus, which is locked up in soil minerals and replenished slowly. So the two partners run a barter: sugars out, phosphorus in, across a thin double membrane at the contact zone.

The exchange interface depends on the type of mycorrhiza. In arbuscular associations the fungus pushes hyphae through root cell walls and inflates them inside the cell into a tree-shaped arbuscule, the cell's own membrane wrapping every twig of the fungal tree. The result is a dense, high-surface-area trading floor inside a single cortical cell. In ectomycorrhizal associations the fungus stays outside the cells but wraps the root tip in a dense sheath (mantle) and slips between cortical cells in a structure called the Hartig net.

Anatomy of an arbuscular mycorrhiza

The arbuscule is short-lived; individual structures last only a few days before they collapse and are digested by the plant cell, then rebuilt nearby. Continuous turnover keeps the trade fresh.

Types of mycorrhizae

"Mycorrhiza" describes a relationship, not a clade. Several distantly-related fungal lineages have evolved root partnerships independently, and the four major types differ in fungal partner, host plant family and physical structure.

	Arbuscular (AM)	Ectomycorrhizal (ECM)	Ericoid	Orchid
Fungal phylum	Glomeromycota	Basidiomycota, Ascomycota	Ascomycota	Basidiomycota
Hyphae enter cells?	Yes — arbuscules	No — only between cells	Yes — coiled	Yes — pelotons
Forms a mantle?	No	Yes (visible sheath on root tip)	No	No
Typical hosts	Most herbs, grasses, crops, tropical trees	Pines, oaks, beeches, birches	Heather, blueberry, rhododendron family	All orchids (≈ 28 000 species)
Soil habitat	Most agricultural and tropical soils	Cool temperate and boreal forests	Acidic, peaty heath soils	Forest understorey, tropics
Plant pays carbon?	Yes	Yes	Yes	Reversed in seedling — fungus pays plant
Visible to naked eye?	No	Yes (truffles, boletes are ECM fruiting bodies)	No	Sometimes

The orchid case is the strangest. Orchid seeds are dust-fine and contain no stored food; they cannot germinate without a fungal partner that initially supplies both sugar and minerals. The seedling spends years as an underground "myco-heterotroph," only later turning the relationship around once it grows leaves.

Why phosphorus is the prize

Soil phosphorus exists mostly as phosphate ions tightly bound to iron, aluminum and calcium minerals. Free phosphate diffuses through soil at roughly 10⁻⁹ cm² s⁻¹ — so slowly that a depletion zone forms within millimeters of an actively-feeding root in days. Hyphae bridge that zone: they grow past it into fresh soil and ferry phosphate back through cytoplasmic streaming, which is much faster than diffusion.

For nitrogen the story is less clean. AM fungi take up some ammonium and a little nitrate, but most plants get their nitrogen from microbial decomposition or from rhizobial nodules, not from mycorrhizae. Ectomycorrhizal fungi, however, secrete proteases that crack open nitrogen-rich proteins in forest litter — a major reason temperate forests stay productive on apparently nitrogen-poor soils.

The common mycorrhizal network

A single fungal individual can colonize many plants. Hyphae that connect two plants form a common mycorrhizal network (CMN), and isotope-tracer experiments have shown small but measurable transfers of carbon and nutrients between plants of the same and even different species. The popular "wood-wide web" framing — old "mother trees" deliberately nursing their seedlings — overstates what the data actually show. Most working ecologists treat the CMN as shared plumbing: solutes flow down concentration gradients between plants, the fungus skims its cut, and the network's "altruism" is a description of physics, not intent.

Mycorrhizae and agriculture

Modern farming systems typically run with depressed mycorrhizal populations. Three reasons:

• Phosphate fertilizer. When soil P is high the plant down-regulates the symbiosis to save sugar — colonization can drop by 50% within a season of heavy fertilization.
• Tillage. Plowing shreds the extraradical hyphal network. AM hyphae are physically delicate and reform slowly.
• Bare fallow. AM fungi are obligate biotrophs: without a living host, the inoculum dies down. Long fallow periods or non-host crops (brassicas, beets) crash the population.

Conservation tillage, cover-cropping with mycorrhizal hosts, and lower phosphate inputs each measurably restore colonization. Commercial AM inocula exist for crops but are mainly useful in disturbed soils (mine reclamation, container nurseries) where the natural propagule bank has been wiped out.

Common misconceptions and pitfalls

• "Mycorrhizae fix nitrogen." No. Rhizobia bacteria in legume nodules fix N₂. Mycorrhizal fungi only scavenge soil nitrogen — they do not have the nitrogenase enzyme.
• "More fungus is always better." Mycorrhizae cost the plant 10–30% of fixed carbon. In rich, well-watered soils heavy colonization can be a net drag on growth.
• "Soaking seeds in inocula always works." Inocula must contain the right fungus for the host and survive transport; many commercial products in field trials show no detectable effect.
• "Trees feed each other through the network." Carbon does cross networks, but in tiny amounts and usually down concentration gradients. The directional, deliberate "mother tree" narrative is not well supported by source-and-sink labelled-carbon data.
• "Ectomycorrhizal mushrooms are the whole fungus." The visible mushroom is the fruiting body. The fungus is the unseen mycelium permeating the soil and root tips, often weighing many kilograms.

An ancient relationship

The earliest unambiguous fossils of arbuscular mycorrhizae are in the Rhynie chert of Scotland, dated to roughly 410 million years ago, in some of the first vascular plants. Molecular clocks push the origin further back, to about 460 Mya — close to the moment plants began colonizing land. Land soils of that era had almost no organic matter and were poor in available phosphorus; the symbiosis is one of the strongest candidates for the innovation that made plant life on land possible at all. Roughly 80% of plant species today still rely on it.