1.1. Successful Coexistence of Plants and Fungi
Mycorrhizas are symbioses between plants and fungi. These nutritional partnerships have evolved independently hundreds of times in multiple lineages in the plant and fungal kingdoms (Bidartondo et al., 2011; Tedersoo and Smith, 2013). Estimates suggest that approximately 50,000 species of fungi form mycorrhizal associations with approximately 250,000 species of plants (van der Heijden et al., 2015). Photosynthetic plants require minerals and water to synthesize organic compounds, but these essential resources are often in limited supply. Mycorrhizas greatly expand the capacity of plants to acquire nutrients and water because fungi have several traits that make them particularly well suited for mining minerals from organic and inorganic substrates and improving the hydraulic properties of soil. Fungal mycelium is composed of fine, threadlike hyphae that can access pores too small for roots to enter; consequently by associating with mycorrhizal fungi, plants substantially increase their contact with mineral particles and organic residues in the soil. Also, fungi have the capacity to synthesize organic acids and other compounds that may depolymerize organic compounds and solubilize mineral nutrients (Smith and Read, 2008). From the perspective of biological markets, plants can often increase their fitness by investing photosynthate in fungal partnerships (Werner et al., 2014). The prevalence of mycorrhizas in all types of vegetation throughout the Earth’s history suggests that this symbiosis is a mechanism for innovation in a constantly changing environment.
Although mycorrhizas have evolved independently many times, four general types are often recognized based upon the identities of the plant and fungal partners (Table 1.1). Arbuscular mycorrhizas, the most ancient and widespread type of mycorrhiza, form between fungi in the phylum Glomeromycota and the majority of plant species, ranging from nonvascular plants to angiosperms (Parniske, 2008; Brundrett, 2009). Ectomycorrhizas form between thousands of Basidiomycota or Ascomycota and many important trees and woody species that are often stand dominants in temperate and boreal forests (Tedersoo et al., 2010; Tedersoo and Smith, 2013). Ericoid mycorrhizas are associations between highly specialized Ascomycota (and a few Basidomycota) and several large families within the order Ericales and also bryophytes that occur in vast areas of tundra, boreal forests, and heathlands (Straker, 1996; van der Heijden et al., 2015). Orchid mycorrhizas are associations between Basidiomycota and all members of the Orchidaceae, a remarkably diverse family encompassing 9% of all angiosperm species (Brundrett, 2009). Orchid mycorrhizas are particularly interesting because orchids have a prolonged seedling stage, during which they are unable to photosynthesize and are entirely dependent upon mycorrhizal symbioses to provide an exogenous supply of carbohydrate (Smith and Read, 2008). In this regard, orchid mycorrhizas represent an unconventional carbon dynamic in which a heterotrophic fungus provides carbohydrate to its plant host. Although orchid mycorrhizas are fascinating, only Chapter 21 will consider orchid mycorrhizas. The other chapters will mainly focus on the arbuscular mycorrhizal (AM), ectomycorrhizal (EcM), and ericoid mycorrhizal (ErM) symbioses because of their key roles as mediators of soil fertility, structure, and carbon storage.
Table 1.1
Four Major Types of Mycorrhizas Distinguished by the Taxa of Host Plants, Fungal Symbionts, and the Biomes Where the Mycorrhizas Are Most Common
Mycorrhizal Type | Host Plants | Main Fungal Symbionts | Predominant Biomes |
Arbuscular mycorrhizal (AM) | ∼200,000 Species of angiosperms, gymnosperms, bryophytes, and pteridophytes | ∼300–1600 species of Glomeromycota | Tropical and temperate forests, grasslands, savannas, shrublands, deserts, and most agricultural crops including fruit trees |
Ectomycorrhizal (EcM) | ∼6000 Species of angiosperms and gymnosperms | ∼20,000 species of Basidiomycota and Ascomycota | Boreal (taiga), temperate, and tropical forests; tundra; and agroforestry |
Ericoid mycorrhizal (ErM) | Members of the Ericaceae, Epacridaceae, and Empetraceae families, and some bryophytes | >150 species of Ascomycota (primarily) and some Basidiomycota | Tundra, boreal, and temperate forests |
Orchid mycorrhizal | All Orchidaceae | ∼25,000 species of Basidiomycota | Tropical and temperate biomes |
Information derived from Smith, S.E., Read, D.J., 2008. Mycorrhizal Symbiosis. Academic Press, New York and van der Heijden, M.G.A., Martin, F.M., Selosse, M.-A., Sanders, I.R., 2015. Mycorrhizal ecology and evolution: the past, the present, and the future. New Phytologist 205, 1406–1423.
1.2. Mycorrhizal Research: Past, Present, and Future
It has long been recognized that fungi inhabit plant roots. Frank coined the term mycorrhiza in 1885; but, many published descriptions of fungal associations with plant roots predate this classic paper (Frank, 1885; Trappe and Berch, 1985). In a subsequent paper, Frank suggested that EcM fungi aided nitrogen uptake from humus and thus increased the growth of host plants (Frank, 1894). This insight initiated a period of experimentation on the nutritional role of EcM symbioses (Harley, 1985). Although AM fungi were described inside plant roots several decades before Frank’s publication, the identity of the fungal partner remained elusive because Glomeromycota are obligate biotrophs that cannot be cultured using traditional mycological methods (Koide and Mosse, 20...