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, which are the commonest mycorrhizas, and  first to evolve; the fungi are members of the Glomeromycota, 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 of the fungi form vesicles (see Table 1 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 even is 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 (Fig. 5). 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’. 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 1 summarises the main characteristics of these seven types of mycorrhiza.

Table 1. Summary of the characteristics of the seven types of mycorrhiza.
Feature
Mycorrhizal type
Endomycorrhizas
Ectomycorrhizas
AM
Ericoid
Arbutoid
Mono-tropoid
Orchid
Ecto-
Ectendo-
Fungi septate
no
yes
yes
yes
yes
yes
yes
Fungi aseptate
yes
no
no
no
no
no
no
Intracellular colonisation
yes
yes
yes
yes
yes
no
yes
Fungal sheath
no
no
yes or no
yes
no
yes
yes or no
Hartig net
no
no
yes
yes
no
yes
yes
Vesicles
yes or no
no
no
no
no
no
no
Plant host chlorophyllous*
yes
 (? no)
yes
yes
no
no*
yes
yes
Fungal taxa
Glomero-mycota
Asco-mycota
Basidio-mycota
Basidio-mycota
Basidio-mycota
Basidio-mycota
Asco-mycota
Basidio-
Asco-
(Glomero-mycota)
Plant taxa†
Bryo
Pterido
Gymno
Angio
Ericales
Bryo
Ericales
Monotrop-aceae
Orchid-
aceae
Gymno
Angio
Gymno
Angio
*All orchids are achlorophyllous in the early seedling stages, but usually chlorophyllous as adults. Table based on Table 1 in Smith & Read (1997) and Harley (1991). †Bryo = Bryophyta, Pterido = Pteridophyta, Gymno = Gymnospermae, Angio = Angiospermae.

Updated December 17, 2016