21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 12: Development and morphogenesis
2. Chapter 13: Ecosystem mycology: saprotrophs, and mutualisms between plants and fungi
   1. Ecosystem mycology
   2. Fungi as recyclers and saprotrophs
   3. Make the earth move
   4. Fungal toxins: food contamination and deterioration
   5. Decay of structural timber in dwellings
   6. Using fungi to remediate toxic and recalcitrant wastes
   7. Release of chlorohydrocarbons to the atmosphere by wood decay fungi
   8. Introduction to mycorrhizas
   9. Types of mycorrhiza
   10. Arbuscular (AM) endomycorrhizas
   11. Ericoid endomycorrhizas
   12. Arbutoid endomycorrhizas
   13. Monotropoid endomycorrhizas
   14. Orchidaceous endomycorrhizas
   15. Ectomycorrhizas
   16. Ectendomycorrhizas
   17. The effects of mycorrhizas and their commercial applications, and the impact of environmental and climate changes
   18. Introduction to lichens
   19. Introduction to endophytes
   20. Epiphytes
   21. Chapter 13 References and further reading
   22. Buy a PDF of Chapter 13
3. Chapter 14: Fungi as pathogens of plants

13.9 Types of mycorrhiza

Mycorrhizas were traditionally classified into the two types: ectotrophic and endotrophic, a classification based on the location of the fungal hyphae in relation to the root tissues of the plant; ecto means outside the root, endo means inside. This classification is now regarded as too simplistic, and there is now a nomenclature identifying seven mycorrhizal types; however we will telescope this into 4 major types with 3 additional subclasses as follows:

Endomycorrhizas; in which the fungal structure is almost entirely within the host root, comprising three major and two minor groupings:

• Arbuscular (AM) endomycorrhizas, are the commonest mycorrhizas, and were the first to evolve; the fungi are members of the Glomeromycotina, they are obligate biotrophs, and they are associated with roots of about 80% of plant species, including many crop plants. The AM association is endotrophic, and has previously been referred to as Vesicular-Arbuscular Mycorrhiza (VAM).  This name has since been dropped in favour of AM, since not all the fungi form vesicles (see Table 2 and Selosse & Le Tacon, 1998) but you may still find that other textbooks refer to ‘VAM’ or ‘VA’ mycorrhizas.

• Ericoid endomycorrhizas are mycorrhizas of Erica (heather), Calluna (ling) and Vaccinium (bilberry), that is, plants that endure moorlands and similar challenging environments. Fungi are members of the Ascomycota (an example is Hymenoscyphus ericae. The plant’s rootlets are covered with a sparse network of hyphae; the fungus digests polypeptides saprotrophically and passes absorbed nitrogen to the plant host; in extremely harsh conditions the mycorrhiza may even provide the host with carbon sources (by metabolising polysaccharides and proteins for their carbon content). Two specialised sub-groups may be separated out of the ericoid endomycorrhizal group

  • Arbutoid endomycorrhizas , and
  • Monotropoid endomycorrhizas (the mycorrhizal association formed by the achlorophyllous plants of the Montropaceae).
• Orchidaceous endomycorrhizas are similar to ericoid mycorrhizas but their carbon nutrition is even more dedicated to supporting the host plant, as the young orchid seedling is non-photosynthetic and depends on the fungus partner utilising complex carbon sources in the soil, and making carbohydrates available to the young orchid. All orchids are achlorophyllous in the early seedling stages, but usually chlorophyllous as adults, so in this case the seedling stage orchid can be interpreted as parasitising the fungus. A characteristic fungus example is the basidiomycete genus Rhizoctonia (although this is a complex genus which can be divided into several new genera).

Ectomycorrhizas are the most advanced symbiotic association between higher plants and fungi, involving about 3% of seed plants including the majority of forest trees. In this association the plant root system is completely surrounded by a sheath of fungal tissue which can be more than 100 µm thick, though it is usually up to 50 µm thick. The hyphae penetrate between the outermost cell layers forming what is called the Hartig net (shown in Fig. 11 in Section 13.15). From this a network of hyphal elements (hyphae, strands and rhizomorphs) extend out to explore the soil domain and interface with the fungal tissue of the root. Ectomycorrhizal fungi are mainly Basidiomycota and include common woodland mushrooms, such as Amanita spp., Boletus spp., Tricholoma spp. Ectomycorrhizas can be highly specific (for example Boletus elegans with larch) and non-specific (for example Amanita muscaria with 20 or more tree species. In the other specificity direction, forty fungal species are capable of forming mycorrhizas with pine.

Ectomycorrhizas can link together groups of trees (the submerged mycelium acting as what has been described as a ‘wood wide web’ (Wohlleben, 2017) or, more formally, ‘Common Mycorrhizal Networks’ (Gilbert & Johnson, 2017) (see Section 13.15). Ectomycorrhizal fungi depend on the plant host for carbon sources, most being uncompetitive as saprotrophs. With few exceptions (Tricholoma fumosum being one), the fungi are unable to utilise cellulose and lignin; but the fungus provides greatly enhanced mineral ion uptake for the plant and the fungus is able to capture nutrients, particularly phosphate and ammonium ions, which the root cannot access. Host plants grow poorly when they lack ectomycorrhizas. This ectomycorrhizal group is reasonably homogenous, but a subgroup, ectendomycorrhizas, has been appended.

• Ectendomycorrhiza is a purely descriptive name for mycorrhizal roots that exhibit characteristics of both ectomycorrhizas and endomycorrhizas. Ectendomycorrhizas are essentially restricted to the plant genera Pinus (pine), Picea (spruce) and, to a lesser extent, Larix (Larch). Ectendomycorrhizas have the same characteristics as endomycorrhizas but also show extensive intracellular penetration of the fungal hyphae into living cells of the host root.

Table 2 summarises the main characteristics of these seven types of mycorrhiza.

Mycorrhizal associations are being studied on larger and larger spatial scales as more extensive data sets and the computational aids to study them become available. For example, van der Linde et al. (2018) investigated the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity in forests across the continent of Europe. They concluded that environmental and host factors account for most of the variation in ectomycorrhizal diversity, but that the importance of the belowground ecosystem has been underappreciated.

On an even more expansive measure, Steidinger et al. (2019) mapped the ‘wood wide web’ on a global scale for the first time, using a database of more than 28,000 tree species living in over 1.1 million forest plots in more than 70 countries. These analyses show that climate variables, especially those that control the rate of decomposition, are the primary drivers of the global distribution of these crucial mutualisms. The authors estimated that ectomycorrhizal trees, which represent only 2% of all plant species, nevertheless make up about 60% of the tree stems on Earth. The ectomycorrhizal symbiosis is the predominant form of symbiosis at high latitudes and elevations; dominating in forests where seasonally cold and dry climates inhibit decomposition. In contrast, arbuscular mycorrhizal trees dominate in non-seasonal, warm tropical forests that feature rapid decomposition. Arbuscular mycorrhizal and ectomycorrhizal symbioses occur together in temperate biomes in which seasonally warm-and-wet climates enhance decomposition. Symbiotic nitrogen fixers, which are insensitive (as compared with mycorrhizal fungi) to climatic control of decomposition are most abundant in arid biomes with alkaline soils and high maximum temperatures. The authors point out that the climatically driven global symbiosis gradient that they document provides a quantitative spatial understanding of microbial symbioses at the global scale and demonstrates the critical role of microbial mutualisms in shaping the distribution of plant species.

Updated July, 2019