Ectendomycorrhizas show many of the same characteristics of endomycorrhizas (EM), but also show extensive intracellular penetration.  The EM association sometimes has hyphae entering a few dead cells, but in ectendomycorrhizas infection can occur in living plant cells.

Ectendomycorrhizas are restricted mostly to the plant families Pinus (pine), Picea (spruce) and to a lesser extent Larix (larch). Identification of the fungi involved in ectendomycorrhizal symbioses was initially prevented as no fruiting structures were seen.  However, fungi isolated from Pinus seedlings in nurseries produce fruiting bodies in culture which are suitable for identification and place the fungi within the phylum Ascomycota, Order Pezizales.  Analysis of nuclear and mitochondrial genomes of the fungi confirmed that most belong to the Wilcoxina genus and the majority of strains can be assigned to two taxa - Wilcoxina mikaloe and W. rehmii.

These two taxa occupy distinctive habitats; W. mikaloe is a chlamydospore-producing fungus, found predominantly in disturbed mineral soils, whilst W. rehmii thrives in peaty soils, but does not produce chlamydospores.

The formation of ectendomycorrhizas begins with formation of a Hartig net, which grows behind the apical meristem of the growing root. Growing up the older parts of the root, intracellular penetration increases, with the oldest cells being almost filled with coils of septate hyphae, up to 15 mm wide.

The ectendomycorrhizal association is stable for at least a year, with no evidence of hyphal degeneration or lysis. Ectoendomycorrhizal formation induces the growth of short roots, similar to the ectomycorrhizal association. Emergent roots become covered in a matrix of highly branched hyphae. This coarse sheath develops behind the apex, between root hairs, and will eventually cover the entire root, except when the lateral roots grow very rapidly.

The Hartig net penetrates between the epidermal and outer cortical cells and later extends to the inner cortex. Intracellular penetration occurs a few cells away from those in which the earliest hartig net formation is seen. Once inside a cell, the hyphae branch repeatedly.

With the exception of the intracellular hyphal penetration, the structure and development of ectendomycorrhizas are very similar to that seen in the ectomycorrhizal association. Comparing early mycorrhizal colonisation in Pinus sylvestris and Picea abies seedlings, Picea developed normal ectomycorrhizas, but on Pinus seedlings, ectendomycorrhizas were initially very abundant. This indicates a strong influence of the plant on the mycorrhizal structure formed, and this plant effect is also seen in Arbustus and Monotropa species, where certain fungi will from arbutoid or monotropoid mycorrhizas with the aforementioned species, but form typical ectomycorrhizas with conifers such as Pinus and Fagus.

The term ectendomycorrhiza should be used as a purely descriptive name for mycorrhizal roots which exhibit the characteristics of both ectomycorrhizas and endomycorrhizas and no functional significance is implied.


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