Diversity and classification of mycorrhizal associations

Most mycorrhizas are 'balance I' mutualistic associations in which the fungus and plant exchange commodities required for their growth and so rvival. Myco-heterotrophic plants have 'exploitative' mycorrhizas where transfer processes apparently benefit on y plants. Exploitative associations are symbiotic (in the broad sense), but arc not mutualistic. A new definition of mycorrhizas that encompasses all types of these associations while excluding other plant-fungus interactions is provided. This definition recognises the importance of nutrient transfer;it an interface resulting, From synchromsed plant-fungus development. The diversity of interactions between mycorrhizal fungi and plants is considered. Mycorrhizal fungi also function as endophytes, necrotrophs and antagonists of host or non-host plants, with roles that vary during the lifespan of their associations. It is recommended that mycorrhizal associations are defined and classified primarily by anatomical criteria regulated by the host plant. A revised classification scheme for types and categories of mycorrhizal associations defined by these criteria is proposed. The main categories of vesicular-arbuscular mycorrhizal associations (VAM) are 'linear' or 'coiling', and of ectomycorrhizal associations (ECM) are 'epidermal' or 'cortical'. Subcategories of coiling VAM and epidermal ECM occur in certain host plants. Fungus-controlled features result in 'morphotypes' within categories of VAM and ECM. Arbutold and monotropold associations should be considered subcategories of epidermal EGM and ectendo mycorrhizas should be relegated to an ECM morphotype. Both arbuscules and vesicles define mycorrhiza, formed by glomeromycotan fungi. A new classification scheme for categories, subcategories and morphotype of mycorrhizal associations is provided.