13.12 Arbutoid endomycorrhizas

Arbutoid mycorrhizas are also formed by plants in the order Ericales, but specifically by the plant genera Arbutus (Strawberry tree and madroña) and Arctostaphylos (Bearberry and manzanita) in the family Ericaceae and in Pyrola (Wintergreen) of the family Pyrolaceae (though some authors recognise this as a separate type of mycorrhiza). All of these are hardy, mainly evergreen shrubs which occur in the wild in harsh, high altitude regions, and are cultivated widely for their decorative foliage, flowers and berries.

The fungi of arbutoid mycorrhizas are basidiomycetes, often the same fungal species that form ectomycorrhizal associations with forest trees (see below).  Indeed, the structures of these two mycorrhizal types are very similar; arbutoid mycorrhizas have (Fig. 5):

  • a well-developed fungal sheath and a Hartig net restricted to the outer layers of cells of the root and the presence of the fungal sheath, as in ectomycorrhizas, insulates the host from the soil, so everything absorbed by mycorrhizal roots must pass through this sheath (all characteristics of the ectomycorrhiza);
  • but there is extensive penetration of the outer cortical cells by fungal hyphae and hyphal coils fill those cells (characteristics of the endomycorrhiza).

The arbutoid type of mycorrhiza does not fit neatly into the classification scheme that distinguished ectotrophs from endotrophs because it shares characteristics of both. The intracellular coils, along with the mantle sheath and Hartig net, and the fact that the host plant is a member of the Ericaceae are the diagnostic features of arbutoid mycorrhizas. We will see below that there is another intermediate type of mycorrhiza, called the ectendomycorrhiza, which is an ectomycorrhiza in which the hyphae penetrate the outer cortical cells, and fill them with coils, which the hyphae of  ectomycorrhizas do not normally do. In this case, and this distinguishes arbutoid from ectendomycorrhiza, the plant host is a member of the Coniferophyta (cone-bearing gymnosperm trees, including pines, yews, but also the ‘living fossil’ Ginkgo).

Updated December 17, 2016