Symbiosis is the ecological interaction between two organisms that live together. However, the definition does not describe the quality of the interaction. When both members of the association benefit, the symbiotic relationship is called mutualistic. Fungi form mutualistic associations with many types of organisms, including cyanobacteria, plants, and animals. Lichens and mycorrhizae are two well-known examples and are both important components of most terrestrial ecosystems. Lichens contribute to soil production by breaking down rock, and they are early colonizers in soilless environments such as lava flows. The cyanobacteria in some lichens can fix nitrogen and act as a nitrogen source in some environments. Lichens are also important soil stabilizers in some desert environments and they are an important winter food source for caribou and reindeer. Finally, lichens produce compounds that have antibacterial effects, and further research may discover compounds that are medically useful to humans. Mycorrhiza, which comes from the Greek words “myco” meaning fungus and “rhizo” meaning root, refers to the association between vascular plant roots and their symbiotic fungi. About 80 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant, thereby increasing a plant’s nutrient uptake. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus.

Characteristics of Mycorrhizae

There are a number of types of mycorrhizae, however the two most important are the ectomycorrhizae and the endomycorrhizae. Ectomycorrhizae (“outside” mycorrhizae) are characterized by fungal growth that envelops, but does not penetrate, the root cells. The fungal growth forms a sheath (called a mantle) and a Hartig net of hyphae extends into the intercellular spaces between root cells, establishing extensive contact with the root cells (Figure 10.10). The fungal partner can belong to the Ascomycota, Basidiomycota, or Zygomycota. In this mutualistic relationship, the fungi are not saprophytic – the plant provides the fungi with organic carbon, and the fungal partner provides nutrients, in particular, phosphorus and nitrogen, as well as water. The network of fungal mycelia may extend well beyond the plant’s rhizosphere, effectively expanding the root’s ability to absorb nutrients and water from surrounding soils.

In a second type, the Glomeromycete fungi form vesicular–arbuscular  mycorrhizae or endomycorrhizae. In these mycorrhizae, the fungal hyphae penetrate the walls of the root cells, and cause invagination of the plasma membrane (Figure 10.10). They do not, however, breach the plasma membrane of the root cells. The arbuscules (from the Latin for “little trees”) have a shrub-like appearance. This is also a mutualistic relationship, with the same benefits derived by both partners as in the ectomycorrhizae. In addition, however, the Glomeromycete partner may provide protection against nematodes and other pests.

The orchidaceous mycorrhizae form between orchids and a fungal partner, usually a Basidiomycete. Orchids are epiphytes that form small seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner. After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing the necessary carbohydrates and minerals. Some orchids produce chloroplasts only after passing the seedling stage. Others never produce chloroplasts and continue to be mycorrhizal throughout their lifecycle. In contrast to the ectomycorrhizae and arbuscular mycorrhizae, this is not a mutualistic relationship but a parasitic one. The fungal partner is saprophytic and as such, can grow on its own. The orchid is therefore a parasite.

Edible Mycorrhizae

The most expensive mushrooms in the world are the product of mycorrhizal fungi: truffles (Figure 10.11), chanterelles, porcini mushrooms and morels. The cost for these delicacies is related to their highly complex life cycles, which have proven difficult, and in some cases, impossible to replicate. Thus, for some species, commercial production is not possible and trying to overcome this problem is an active area of research.

 

• How do plants benefit from an association with mycorrhizal fungi?
• Given the fact they penetrate plant root cell walls, why are endomycorrhizae considered mutualists, rather than parasites?
• What are some of the edible fruiting bodies of mycorrhizal fungi and why are some so expensive?