Monotropoid Mycorrhizas

Monotropaceae is an achlorophyllous family within the Ericales. Plants of this family are, therefore, even more dependent upon their mycorrhizal partners than their autotrophic relatives. The monotropoid mycorrhizas have a thick compact mycelial sheath, a Hartig net one cell deep in the root cortex, and, thus, limited intracellular penetration. The penetration of epidermal and outer cortical cells is achieved by the development of fungal pegs that become extensively invaginated. The final structure resembles the specialized transfer cells which are formed in a variety of plant species when there is a need for rapid, short-distance transport of solutes.

The fungus forming the monotropoid mycorrhiza also forms an ectomycorrhizal relationship with a tree. In this tripartite relationship, species of Monotropa, Pterospora and Sarcodes obtain some, possibly most, of their carbon and energy needs from the tree via the shared mycorrhizal fungus. Several different species from different major fungal groups have been found forming this type of mycorrhiza in North America and Britain.

The montropoid plants may not be totally parasitic on their fungus' tree host. Preliminary studies of freshly isolated endophytes from Monotropa hypopitys L. have shown them to have considerable proteolytic capability. Access to protein could, in the organic rich substrates in which these plants normally grow, provide the carbon and nitrogen required for growth.

Both hosts of the fungus probably obtain their mineral nutrition via the fungus.