13.13 Monotropoid endomycorrhizas

This is the mycorrhizal association formed by the achlorophyllous plants that used to be placed in the family Montropaceae, though they are currently placed in the family Ericaceae; it’s convenient to talk about them collectively so we will call them monotropes. The group includes the genera Monotropa, Monotropsis, Allotropa, Hemitomes, Pityopus, Pleuricospora, Pterospora, and Sarcodes.

All these monotropes are highly unusual plants in that they are all entirely lacking in chlorophyll, and so are unable to photosynthesise. Instead, they parasitise their mycorrhizas (they are called mycoheterotrophs), using the fungi not only to obtain minerals and nutrients from the soil, but also to tap into the carbohydrate supplies of nearby photosynthesising plants via their root connections to the mycorrhizal fungus. As they are native to temperate regions of the Northern Hemisphere, these other plants can include beech, oak and cedar but are more usually pine, spruce and fir, because Monotropa species are most commonly found in coniferous forests. Carbohydrates pass from conifer to Monotropa through their common mycorrhizal partner, and it is the fungus that controls the transfer of carbohydrate to Monotropa. Radiolabelled phosphorus (32P) injected into Monotropa has been recovered in neighbouring trees, so the translocation is bidirectional.

The fungi involved in monotropoid associations belong to the Basidiomycota.  Several Boletus spp., such as Boletus edulis, have been identified as forming monotropoid endomycorrhizas.  Boletus spp. also form ectomycorrhizas with a wide range of tree species, and this may explain how Monotropa becomes associated with so many different tree species. Though they are generally considered to be scarce or rare, Monotropa and other montropes such as Pterospora and Sarcodes are often the only higher plants growing beneath the trees in forests because the dense shading often excludes the chlorophyllous plants, which are so dependent on light to produce their carbohydrates.
 
The Monotropa roots are surrounded in a dense fungal sheath, from which hyphae spread into the soil. In Monotropa and Pterospora, the sheath encloses the root apex, whilst in Sarcodes the apex remains free. In all three genera, a Hartig net surrounds the outer epidermal layer, but does not penetrate into the underlying cortex. However, individual hyphae do grow from the Hartig net into the outer cortical cells, the walls of which invaginate to accommodate the growing hypha.

These intrusions from the Hartig net into the cortical cell walls are known as fungal pegs. The peg proliferates, increasing the surface area within the cell. Eventually, the tip of the peg opens out into a membranous sac extending into the plant cell cytoplasm. Contents of the fungal peg fill the membranous sac but never directly enter the host cell cytoplasm. The number of fungal pegs produced by a monotropoid mycorrhiza is seasonal: maximum period of peg formation occurring in June, coinciding with flowering (and perhaps meeting a high demand for nutrients), whilst ‘tip-bursting’ occurs from July to August as seeds are released (and perhaps providing a late surge of nutrients to boost seed production just before the flower stalk senesces).

Updated December 17, 2016