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Mycorrhizae

Also known as
mycorrhiza, mycorrhizal fungi, AMF, arbuscular mycorrhizal fungi

Mycorrhizae are symbiotic associations between certain soil fungi and plant roots. The fungi extend a network of fine threads into the soil that gathers water and nutrients, especially phosphorus, for the plant, and the plant supplies the fungi with sugars in return.

What are mycorrhizae?

The word mycorrhiza means “fungus root.” Most land plants form these partnerships. The fungus colonizes the root and sends out hyphae, fine threads much thinner than roots, into the surrounding soil. The hyphae reach water and nutrients beyond the zone the roots themselves have depleted and pass them back to the plant. In exchange, the plant supplies the fungus with carbon from photosynthesis.

Main types

  • Arbuscular mycorrhizal fungi (AMF) grow partly inside root cells, where they form branched exchange structures called arbuscules. They associate with most crop plants, including corn, soybeans, wheat and many vegetables and fruit crops.
  • Ectomycorrhizal fungi form a sheath around root tips and grow between root cells. They are typical of many trees, such as pines and oaks.

Some crops, notably members of the cabbage family such as canola, and sugar beet, do not form mycorrhizal associations.

How mycorrhizae help crops

  • Phosphorus uptake. Phosphorus moves very little in soil, so extending the reach of roots through hyphae is especially valuable; see phosphorus solubilization for the related chemistry.
  • Uptake of zinc and other nutrients.
  • Water access and drought tolerance.
  • Soil structure. Hyphae and the substances they produce help bind soil particles into stable aggregates, a component of soil health.

What affects mycorrhizae in fields

  • Tillage, which breaks hyphal networks.
  • Fallow periods and non-host crops, which leave the fungi without a partner.
  • High soil phosphorus, which reduces how much the plant invests in the partnership.
  • Some fungicides.

Practices such as reduced tillage, cover crops and continuous living roots tend to support mycorrhizal populations. Commercial mycorrhizal inoculants are available, with results that depend on existing populations and conditions.

Frequently asked questions

Mycorrhizae are a specific kind of fungus-root partnership that extends into the soil to gather nutrients. Endophyte is a broader term for any microorganism living inside plant tissue without causing disease.

Most do, but not all. Canola and other members of the cabbage family, and sugar beet, are common exceptions, which matters when planning rotations.

  • Endophyte

    An endophyte is a bacterium or fungus that lives inside a plant's tissues for at least part of its life without causing disease. Some endophytes benefit their host, for example by improving nutrient uptake, growth or tolerance to stress.

  • Rhizosphere

    The rhizosphere is the thin zone of soil immediately surrounding plant roots that is directly influenced by them. Roots release sugars, amino acids and other compounds there, which feed a dense, active community of microorganisms that affects nutrient availability and plant health.

  • Phosphorus solubilization

    Phosphorus solubilization is the release of phosphorus that is bound in soil minerals and organic matter into soluble forms that plant roots can absorb. Certain soil bacteria and fungi do this by producing organic acids and enzymes.

  • Soil microbiome

    The soil microbiome is the community of microorganisms living in soil, including bacteria, archaea, fungi, protists and viruses. These organisms decompose organic matter, cycle nutrients, build soil structure and influence plant health.

Sources

  1. Mycorrhizal fungi and soil health, USDA Natural Resources Conservation Service
  2. Mycorrhizal Symbiosis, 3rd edition, S. E. Smith and D. J. Read, Academic Press, 2008