Symbiotic Fungi: Principles and Practice (Soil Biology)

18 A. Das and A. Varma
Mycorrhizae can also be defined as ‘‘a mutualistic symbiosis between plant and
fungus, localised in a root or root-like structure in which energy moves primarily
from plant to fungus and inorganic resources move from fungus to plant’’.
These symbioses are characterized by bi-directional movement of nutrients,
where carbon flows to the fungus and inorganic nutrients move to the plant, thereby
providing a critical linkage between the plant root and soil. In infertile soils,
nutrients taken up by the mycorrhizal fungi can lead to improved plant growth
and reproduction. As a result, mycorrhizal plants are often more competitive
and better able to tolerate environmental stresses than are nonmycorrhizal plants
(Fig. 1.5).
The term mycorrhiza, which literally means ‘‘fungus–root’’, was first applied to
fungus–tree associations described in 1885 by the German forest pathologist A.B.
Frank. The vast majority of land plants form symbiotic associations with fungi: an
estimated 95% of all plant species belong to diverse genera that characteristically
form mycorrhizae.
Depending on the environment in which they are growing, plants may divert up
to 80% or more of the net energy fixed as sunlight to below-ground processes. Some
of this energy goes into root growth; but a high proportion may be used to feed
mycorrhizal fungi and other soil organisms. This is not energy that is lost to the
plant. On the contrary, soil organisms living in the root zone greatly influence the
ability of plants to establish themselves, through effects on nutrient cycling, pathogens,
soil aeration, and soil water uptake. Of the various soil organisms that benefit
plant establishment, the most is known about mycorrhizal fungi (http://www.fungi.
com/mycogrow/amaranthus.html).
Fig. 1.5 Mycorrhiza are considered to play vital role in overcoming drought stress