Imhof, S. (2003):
Odd mycorrhizas: evolution shapes VAM colonization pattern.
International Conference on Mycorrhizae - ICOM4 - Montreal, August
10-15, 2003; #14.
- 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.
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