Diversity and classification of mycorrhizal associations

Abstract

Most mycorrhizas are ' balanced ' mutualistic associations in which the fungus and plant exchange commodities required for their growth and survival. Myco-heterotrophic plants have 'exploitative ' mycorrhizas where transfer processes apparently benefit only plants. Exploitative associations are symbiotic (in the broad sense), but are 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 at an interface resulting from synchronised 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. Arbutoid and monotropoid associations should be considered subcategories of epidermal ECM and ectendomycorrhizas should be relegated to an ECM morphotype. Both arbuscules and vesicles define mycorrhizas formed by glomeromycotan fungi. A new classification scheme for categories, subcategories and morphotypes of mycorrhizal associations is provided.

The extent of mycorrhizal colonization of roots and its influence on plant growth and phosphorus content

“…Nevertheless, arbuscules are more ephemeral than other intraradical AMF structures, and they can be difficult to observe, so this metric is rarely reported (Allen 1983;Brundrett 2009). In addition, the role of arbuscules have not been fully confirmed, and other exchange sites-including coiling AMF-are possible (Brundrett 2004;Brundrett 2009). Total root length colonized by AMF, either on a per-plant or per-ground area basis, has also been used (e.g., Nadian et al 1997;Allen 2001).…”

“…This potential mechanism is consistent with the characterization of the generally mutualistic relationship between plants and AMF (Mosse 1957(Mosse , 1973Smith and Read 2008;Smith 2011b, 2012). Indeed, a fair amount of evidence has accumulated indicating that AMF are balanced mutualisms that function by a form of regulated exchange, so that plants should support high levels of PRLC only if the AMF provide benefits (reviewed in Brundrett 2004). Nevertheless, Smith and Smith (2012) reported physiological evidence for large differences in P transfer by different AM fungi that would be expected to influence the size (and possible direction) of AM-mediated growth responses and total P uptake.…”

Genetic diversity of the endemic gourmet mushroom Thelephora ganbajun from south-western China

“…Many of these fungi form symbiotic associations with plant roots, establishing structures called mycorrhizae. In such associations, the fungal mycelia around the roots help plants obtain essential minerals and water from the soil and can contribute to the plants' disease resistance and drought tolerance (Brundrett, 2004). Their importance is reflected by the fact that over 80 % of land plants form mycorrhizal associations with fungi.…”

Characterization of Ectomycorrhizal species through molecular biology tools and morphotyping

“…Fungal symbiosis is defined as any association where the fungi have contact with the host plant from which they obtain a variety of metabolites and nutrients. The term mutualism implies association with mutual benefits involving two or more different organisms (Boucher et al, 1990;Brundrett, 2004).…”