Mycorrhizae are fungi that form symbiotic relationships with plants. In exchange for carbon fixed by a plant in photosynthesis, mycorrhizae give such benefits as enhanced nutrient uptake (especially phosphorus), drought tolerance, and pathongen resistance.
The two most commonly found classes of mycorrhiza in nature are Vesicular Arbuscular Mycorrhiza (VAM) and Ectomycorrhiza.
VAM fungi are so named because of the balloon-shaped vesicles they form within or between a plant's cortical cells, and the shrub-shaped abuscules they form inside the cortical cells.
Some species of arbuscular forming mycorrhizae, like those in the family Gigasporaceae, do not form vesicles. They are therefore referred to as simply arbuscular mycorrhizae (AM), rather than vesicular arbuscular mycorrhizae.
VAM fungi are obligate symbionts. This means they cannot survive in nature apart from their plant hosts. In order to propagate individual VAM species for scientific purposes, isolated spores are mixed with sterile growing media and seeded with a host plant of choice (eg. Sorghum bicolor). Joe Morton's INVAM site provides good instructions on how to establish VA mycorrhizal cultures.
Though our lab's research interests are centered around VAM fungal research, it is worth providing a brief introduction to ectomycorrhizae as well.
Ectomycorrhizal fungi, as the name implies, do not penetrate the cells of plant roots. Rather, they are characterized by a 'Hartig Net', which is a labyrinth-like network of fungal hyphae around and between the cortical and epidermal cells in a root. A mantel of fungal tissue completely encloses the root while hyphae extend into the soil.
Ectomycorrhizae are most commonly associated with woody rather than herbacious plants. Examples of genera which form ectomycorrhizae include Pinus, Acer, and Eucalyptus.
Unlike VAM fungi, ectomycorrhiza are not obligate symbionts and can be cultured in petri plates independent of plant hosts.
Colonization by both VAM and Ectomycorrhizal fungi:
Some plants can be colonized by both VA and ectomycorrhizae. Examples include plants in the Populus genera (eg. eastern cottonwood) and the Salix genera (eg. various willows).