Imhof, S. (2006):

An amazing mycorrhizal complexity in Afrothismia gesnerioides (Burmanniaceae) but still not complex enough for evolution

17th International Symposium Biodiversity and Evolutionary Biology, Bonn, Sept. 24 - 28, p. 154.

Abstract:

Afrothismia gesnerioides from Central Africa was described by H. Maas-van de Kamer in 2003. It is an achlorophyllous herb up to 6 cm tall showing peculiar subterranean organs made of a short rhizome beset with numerous lemon-shaped root tubercles, which can be elongated by short filiform extensions. The whole subglobose subterranean complex is not bigger than 1.5 cm in diameter. Since A. gesnerioides lacks chlorophyll, it essentially depends on its root fungus for carbon supply.
The rhizome cortex is divided into an outer and an inner part. The inner is packed with starch grains whereas the outer is free of starch but colonized by longitudinally growing mycorrhizal hyphae. These straight hyphae can be traced throughout the outer rhizome cortex, the root tubercle epidermis as well as in the short filiform extensions. Branches of these hyphae penetrate the heteromorphic hypodermal root layer exclusively at the base of the root tubercles in order to enter the root cortex. As soon as situated the root cortex the hyphae grow in a coiled manner. Once passed the hypodermis they initially spread within the third root layer only, in doing so encompassing the inner cortex like a collar. From there the inner parenchyma of the cortex is colonized, but in contrast to the coils in the third root layer the hyphae in the inner cortex cells become digested within a few cell passages and only amorphous clumps of fungal material remain. External penetrations by hyphae can occur at any part of the subterranean organs, the rhizome, the root epidermis and the filiform extension. They represent the connection to the rhizosphere, linking external hyphae with the straight hyphae in the outer rhizome cortex and root epidermis.
If a myco-heterotrophic plant simply digests the fungal hyphae that happen to penetrate its subterranean organs, it would lack continuity of carbon supply not unlike a parasite that kills its host. However, A. gesnerioides developed a mycorrhizal compartmentation system to attain and stabilize a sustained use of the root fungus: (1) Only those hyphae become digested which colonize the inner root cortex. (2) The collar-like colonization of the third root layer around the inner tubercle is important for undigested, functional hyphae to reach the distal part of the tubercle. (3) For the required connection between all root tubercles and the rhizosphere, the straight growing hyphae are the fastest solution. This colonization pattern is amazingly complex, compared to the rather undifferentiated Arum- and Paris-type VAM in other plants. However, it is not the end of the evolutionary trend to optimise mycorrhizal benefit for the plant. A sister species has even refined this strategy .