Symbiotic Fungi: Principles and Practice (Soil Biology)

1 Symbiosis: The Art of Living 21
penetration of cortical cells takes place, and thus they are similar to the arbutoid
type. Ect-endomycorrhizae in Pinaceae seem to be limited to forest nurseries, and
are formed by a group of fungi called E-strain. These fungi are most likely to be the
imperfect stage of ascomycetes; they may cause ect-endomycorrhizae in some tree
species, and ECM in other tree species.
Orchidaceous Mycorrhiza
The fungal association is of the endomycorrhizal type, where the fungus penetrates
the cell wall and invaginates the plasmalemma and forms hyphal coils within the
cell. Once the plant is invaded, spread of the fungus may occur from cell to cell
internally. The internal hyphae eventually collapse and are digested by the host cell.
Since the symbiosis forms an external network of hyphae, it would seem probable
that the fungal hyphae function in nutrient uptake as with other mycorrhizae, and
that the coarse root system of orchids would be supplemented by the increased
absorbing surface area of the hyphae (Smith and Read 1997). A number of
basidiomycete genera have been shown to be involved in the symbiosis, although
many reports on isolation of the symbiotic fungus from the roots of orchids have
placed the symbionts in the form genus Rhizoctonia when the perfect stage was not
known or the isolate was not induced to fruit in culture. Orchid seed germinates
only in the presence of suitable fungus.
The closest relatives of Piriformospora indica are members of Rhizoctina group
(Ceratobasidales). Preliminary studies indicated that P. indica was able to interact
with orchids (Dactylorhiza purpurella and D. majalis) as a mycorrhizal partner. In
this respect, more scientific studies are needed to be performed to quantify the
promoting effect, and to study the interacting structures formed during the early
stages of plant development.
1.10.1.2 How Do Mycorrhizae Work?
Mycorrhizal root systems increase the absorptive area of roots 10–1,000 times,
thereby greatly improving the ability of the plants to utilize the soil resource.
Mycorrhizal fungi are able to absorb and transfer all of the 15 major macro- and
micronutrients necessary for plant growth. Mycorrhizal fungi release powerful
chemicals into the soil that dissolve hard to capture nutrients such as phosphorous,
iron, and other ‘‘tightly bound’’ soil nutrients (Fig. 1.7). This extraction process is
particularly important in plant nutrition, and explains why non-mycorrhizal plants
require high levels of fertility to maintain their health. Mycorrhizal fungi form a
complex web that captures and assimilates nutrients, conserving the nutrient capital
in soils. In non-mycorrhizal conditions, much of this fertility is wasted or lost from
the system.