Mycorrhiza

A mycorrhiza is an intimate, biological relationship in which fungal hyphae integrate closely with the root tissues of a vascular plant. Plant roots that have a mycorrhizal fungus tend not to develop root hairs, relying heavily on the fungus to absorb nutrients and moisture from the soil. The fungal hyphae (root-like projections) are associated with the cortical cells of the root, and not with vascular tissues or the actively growing cells of the meristematic tissue at the tip of the root.

There are two basic types of mycorrhizae—endomycorrhizae and ectomycorrhizae. Endomycorrhizae are more common than ectomycorrhizae, although the latter type is more frequent in the economically important species of trees of temperate forests.

In ectomycorrhizae, the fungal hyphae form a diffuse veil or mantle around the outside of the plant root. The fungal biomass of a ectomycorrhiza typically comprises about 40% of the dry weight of the structure. The fungal hyphae penetrate the root to some degree, but they only occur in the spaces between the cortical and epidermal cells. Ectomycorrhizae occur in association with the roots of such tree species as pines (Pinus spp.), spruces (Picea spp.), willows (Salix spp.), oaks (Quercus spp.), and birches (Betula spp.). Under the influence of hormones secreted by the fungus, the plant roots tend to grow in a stubby, thickened, and much-branched fashion. Most of the species of fungi that are involved in ectomycorrhizae are Basidiomycetes, and a smaller number are Ascomycetes. Most forest mushrooms are the fruiting bodies of Basidiomycete fungi that are mycorrhizal with tree species.

In endomycorrhizae, the fungal hyphae mostly grow inside the plant root, and they penetrate and grow inside the cortical cells of the root. Many economically important species of plants depend on endomycorrhizal fungi, including apples (Pyrus malus), strawberries (Fragaria vesca), tomatoes (Lycopersicon esculentum), orchids (family Orchidaceae), and grasses (Poaceae). In fact, most of the important agricultural plants used by humans for food are endomycorrhizal. Although most conifers are ectomycorrhizal, a few species are endomycorrhizal, notably the redwoods (Sequoia sempervirens) and junipers (Juniperus spp.).