Dark Septate Endophytes (DSE) in Polluted Areas 135The term mycorrhiza was coined by the German botanist Albert Berhhard
Frank in 1885, who defined it as a symbiotic association between the roots
(rhizos) of a plant and certain groups of soil fungi (mycos), where both
members of the association benefit and actively involved in the transport and
absorption of nutrients (Mykorhizen). Brundrett (2004) redefined the term
mycorrhizas as symbiotic associations between a fungus (specialised for life in
soils and plants) and a root (or other substrate-contacting organ) of a living
plant that is primarily responsible for nutrient transfer, and that could be
essential for one or both partners. Mycorrhizas take place in a specialised plant
organ where intimate contact results from synchronised plant-fungus
development. This last definition is based on developmental and functional
characteristics that can summarized on 5 criteria (Brundrett, 2009):
- The structure and development of mycorrhizal fungus hyphae is
substantially altered in the presence of roots of host plants. These
root-borne hyphae are distinct from hyphae, which are specialised for
growth in soil. - All mycorrhizas have intimate contact between hyphae and plant cells
in an interface where nutrient exchange occurs. - The primary role of mycorrhizas is the transfer of mineral nutrients
from fungus to plant. In most cases, there is also substantial
metabolite transfer from plant to fungus. - Mycorrhizas require synchronised plant-fungus development, since
hyphae only colonise young roots. - Plants control the intensity of mycorrhizas by root growth, digestion
of old interface hyphae in plant cells, or altered root system form.
It is clear that DSE establish an intimate contact with the host plant and
could generate modified root systems, but the crucial point of nutrient and
metabolite exchange is until unclear. For example, Peterson et al., (2008)
described that both DSE’s hyphae and microsclerotia in root cells lack a host-
derived perifungal membrane and interfacial matrix material, and hence cannot
be regarded as specialized interfaces for nutrient transfer between plant and
fungus. On the other hand, however, it is been widely reported that
Rhizoscyphus ericae (Current Name Pezoloma ericae), a member of the order
Helotiales and the typical mycorrhizal associate of ericaceous plant species,
enhances the uptake of organic N from the acidic heathland soils that its hosts
inhabit (Smith & Read, 2008). DSE’s effect on host plants is controversial,
with plant responses to experimental inoculation ranging from negative