Imhof, S. (2003):

Odd mycorrhizas: evolution shapes VAM colonization pattern.

International Conference on Mycorrhizae - ICOM4 - Montreal, August 10-15, 2003; #14.

Abstract:

The existence of aberrant colonization pattern of vesicular-arbuscular mycorrhizal fungi (VAMF), particularly found in achlorophyllous plants, is little known, and the structural plasticity of VAM is widely underestimated. This presentation gives an introduction to several of those odd mycorrhizas, and explains them as advanced stages of an evolutionary progression.
Mycoheterotrophic plants essentially depend on their root fungus. Therefore the evolutionary pressure on mycorrhizal efficiency should have been hardest here. In fact, each of the three plant families considered here show specific mycorrhizal adaptations, including morphology of the root/rhizome system as well as the fungal colonization pattern. The most progressive stages are represented by 1. the Gentianaceae Voyria tenella, where fungal colonization proceed from the inner toward the outer cortex (!), 2. the Triuridaceae Sciaphila polygyna, having a dorsiventral mycorrhizal root, and 3. the Burmanniaceae Afrothismia winkleri, likely to have the most complex mycorrhiza known so far. Although differently accomplished, they all show a clump-like root/rhizome system, combined with a high degree of mycorrhizal complexity. Structurally and functionally distinct tissue compartments, where the fungus attains different appearances, allow a sustained benefit from the fungus, by keeping it alive in one compartment, while digesting it in another. Beyond that, Afrothismia winkleri even employs the fungus for several other purposes, like storage, transportation and distribution of matter.
Considering all VAM colonization pattern known so far, we may arrange them according to evolutionary progression from the intercellular Arum-type, over intermediate types to the intracellular Paris-type, and from there to the various forms of more complicated mycorrhizas in achlorophyllous species, differently accomplished in Gentianaceae, Burmanniaceae and Triuridaceae.