Plant roots and fungi: Mycorrhizae

Blue-stained serpentine Neotyphodium coenophialum mycelia inhabiting the intercellular spaces of tall fescue leaf sheath tissue. Magnified 400x.

Mutualistic associations between plant roots and fungi are called mycorrhizae (sing. mycorrhiza), which means "fungus roots." 95 percent of  plants that have been studied  have mycorrhizal associations.

Fungi form associations with plant roots by means of specialized hyphae called haustoria (sing. haustorium). The plant-haustoria complex benefits both organisms. It allows plants to take up water and minerals, particularly phosphorus, more efficiently from the soil.
  

Haustoria 

Through the haustorium, the plant also sends organic molecules (primarily glucose and sucrose) to the fungus, apparently under control of signals sent by the fungus. 


The haustoria do not penetrate the cell membranes of the plant roots. Instead, they increase the amount of fungal surface area in contact with plant cell membranes. They release enzymes that break down cell walls and allow more efficient movement of materials between fungus and plant. 

The haustorium is more metabolically active than the area outside it.
These associations are extremely important in the fields of agriculture, forestry, and ecosystem restoration. For example, foresters often inoculate the roots of pine seedlings with mycorrhizal fungi to enhance pine growth. 

Mycorrhizae are better able to absorb nutrients than plant roots alone for two reasons. First, they are physically smaller in diameter than plant roots and thus can penetrate a larger volume of soil, increasing the surface area for absorption. Second, the chemistry of their cell membranes is different than that of plant roots, making them particularly valuable to plants in nutrient-poor soils.  
  

There are two types of mycorrhizal fungi -- ectomycorrhizal and arbuscular mycorrhizal. 


 

Ectomycorrhizal beech

In ectomycorrhizal fungi, the hyphae do not penetrate the individual root cells. They form sheaths around the root tips and a network surrounding the cells of the root cortex.

They also form an extensive network in the surrounding soil and leaf litter. 


   

Ectomycorrhizal fungi exist in about 10 percent of plant families, mostly trees such as birch, eucalyptus, oak, pine, and rose. Fungal families that produce ectomycorrhizal fungi include Basidiomycota, Ascomycota, and Zygomycota.



Some genera of fungi (such as Leccinum and Suillus) are specialists; that is, they form associations with only one plant genus. 

  

Leccinum aurantiacum

Suillus americanus

Others (such as Amanita) are generalists, forming associations with many plant genera. A given tree may have as many as fifteen fungal partners at once. 

  

Arbuscular mycorrhizal wheat

In arbuscular mycorrhizal fungi, the hyphae are branched (tree-like). They reach through the cell wall and cause the cell membrane to invaginate, forming tubes. 

These associations are formed by members of the group Glomeromycota. They are the most prevalent form of plant symbiosis known, occurring in about 85 percent of vascular plant families, including many crop species.
  

Glomeromycota

Mycorrhizal fungi reproduce and disseminate by means of spore formation.