13.16 Ectendomycorrhizas

Rather than being a definitive subgroup, ectendomycorrhiza is a 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. Formation of ectendomycorrhizas begins with formation of a Hartig net, which grows behind the apical meristem of the growing root.

The Hartig net hyphae grow between the epidermal and outer cortical cells and later extend to the inner cortex. Growing up the older parts of the root, intracellular penetration increases, once inside a cell, the hyphae branch repeatedly. The oldest plant cells become almost filled with coils of septate hyphae. The ectendomycorrhizal association can persist for a full year, and there is 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; the coarse sheath develops between root hairs and eventually covers the entire root.

Assessment of the influence of inorganic and organic phosphorus and the lack of phosphorus in the culture medium on the type of mycorrhizal colonisation formed (ecto-, ectendo-, or endomycorrhiza) during Helianthemum almeriense × Terfezia claveryi symbiosis in vitro revealed that the mycorrhizal types formed depended on the phosphorus source in the medium, suggesting that it is the organic phosphorus that induces the formation of intracellular colonisation (Navarro-Ródenas et al., 2012).

The fungi involved in ectendomycorrhizal symbioses are Ascomycota; most belong to the genus Wilcoxina and the majority of isolates can be assigned to two species, Wilcoxina mikolae and W. rehmii. These two taxa occupy distinctive habitats: W. mikolae is a chlamydospore-producing fungus, found predominantly in disturbed mineral soils, whilst W. rehmii thrives in peaty soils, but does not produce chlamydospores.

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Updated July, 2019