Published online by Cambridge University Press: 12 August 2009
Introduction
A wide variety of fungi form an intimate association with the roots of plants, and the word “mycorrhiza” is used to describe the overall structure formed by the union of these partners. There are seven different types of mycorrhiza, but the two that are most abundant in nature and of most importance ecologically, are the arbuscular and ectomycorrhiza. An excellent review of all aspects of mycorrhizal biology can be found in Smith and Read (1997). Arbuscular mycorrhizae are formed by about 150 different species of fungi within the Glomeromycota and are mostly associated with herbaceous plants. The fungus enters the roots of plants where hyphae grow intercellularly. In addition, unique highly branched structures are formed within the cells of plant roots, called arbuscules. These are thought to be sites of nutrient exchange. Arbuscular mycorrhizal (AM) fungi occur in all ecosystems of the world and associate with the roots of about 70% of all vascular plants (Hodge 2000). Ectomycorrhizal (ECM) fungi generally associate with woody plants. They are formed by about 5000 species of fungi, mainly from the Basidiomycotina, with some representatives from the Ascomycotina also. The fungus forms a sheath over the root tips and there is some intercellular, but no intracellular, growth of the hyphae. Ectomycorrhizae also have worldwide distributions, and although they associate with only about 3% of seed plants, their global importance is huge, as they associate with important timber and natural forest trees.
Essentially, arbuscular and ectomycorrhizae function in the same way.
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