From Wikipedia,
the free encyclopedia.
A mycorrhiza (typically
seen in the plural forms
mycorrhizae or mycorrhizas
meaning "fungus roots") is a
distinct type of root
symbiosis in which individual
hyphae extending from the
mycelium of a
fungus colonize the
roots of a host
plant.
The mycorrhizal symbiosis is
divided into two main groups:
ectomycorrhizae and
arbuscular mycorrhizae (also
known as vesicular-arbuscular
mycorrhizae or endomycorrhizae).
The two groups are differentiated
by the fact that the hyphae of
ectomycorrhizal fungi do not
penetrate the
cell wall of the plant's root
cells, while the hyphae of
arbuscular mycorrhizal fungi
penetrate the cell wall.
Ectomycorrhizae typically form
between the roots of woody plants
and
Basidiomycetes or
Ascomycetes. Arbuscular
mycorrhizae are formed only by
fungi in the
division
Glomeromycota, which are
typically associated with the
roots of herbaceous plants, but
may also be associated with woody
plants. Members of the
Ericales and
Orchidaceae also form
mycorrhizae that are distinct from
these two more common types.
Mycorrhizae form a mutualistic
symbiotic relationship with the
roots of 95% of all
vascular plant species. Plant
roots in nature are predominantly
mycorrhizal. It is the non-mycorrhizal
root that is the exception.
Sterile
soils and growth media often
perform poorly without the
addition of organic supplements
that provide the
spores and hyphae of
mycorrhizal fungi to "infect"
the plant roots to restore the
natural state of affairs. The same
organic supplements also populate
the rhizosphere (plant-soil
interface) with beneficial
bacteria such as
Rhizobia (nitrogen-fixing
bacteria) and other
nitrogen or
sulfur-releasing bacteria that
make nutrients in the soil
available for absorption by
plants. The addition of biotic
supplements (such as Rhizobia-treated
legume seeds) is commonplace
in modern agricultural practice to
increase yields.
This symbiotic association
provides the fungus with a
renewable source of food by
continuous access to plant
assimilates that are mobilized
from
leaves to root tissues and
then to the fungal partners, and
allows the plant to make use of
the mycelium's tremendous surface
area to absorb nutrients from the
soil. It is also believed that the
mycelia of mycorrhizal networks
have chemical absorption
capabilities that are able to
access some plant nutrients at
lower concentrations than plant
roots are capable of absorbing. An
example of this is the manner in
which
phosphorus
ions are tightly bound to
iron oxides in many soils.
Plant roots are generally
incapable of accessing these
phosphorus sources (which can be
large and are termed sinks), yet
mycorrhizal mycelia can access
these forms of phosphorus. The
mechanisms of increased absorption
are both physical--mycorrhizal
mycelia are much smaller in
diameter than the smallest root
hair and thus have a much larger
surface area for absorption--and
chemical--the cell membrane
chemistry of fungi is different to
that of plants.
Mycorrhizal fungi explore a
larger volume of soil than root
systems at a lower cost to the
plant. Some of the earliest fossil
plants show evidence of
mycorrhizae associated with them.
Mycorrhizal plants are generally
more resistant to diseases, such
as those caused by microbial
soil-borne
pathogens, and are also more
resistant to the effects of
drought.
Some mycorrhizal
hyphae enter within the plant
cell's cell wall and grow to
envelop the cell. However, most
mycorrhizae have a more advanced
structure, in which the hypha
lives inside an extensive
invagination (inpocketing) of
the
cell membrane.
Arbuscular mycorrhizae
(formerly known as vesicular-arbuscular
mycorrhizae) are an example of
mycorrhizae that enter within
plant cell walls to produce
structures that are either
balloon-like (vesicles) or
dichotomously-branching
invaginations (arbuscules). The
structure of the arbuscules
greatly increases the contact
surface area between the hyphae
and the cell
cytoplasm to facilitate the
transfer of nutrients between
them.
The
cytoplasmic streaming of the
mycorrhizal hyphae is a mechanism
that facilitates the transfer of
nutrients from the soil at
relatively remote distances from
the root to the root at rates far
exceeding those that would be
possible by
osmotic flow alone. This has
an energy cost to the fungus. In
return, the mycorrhizal fungus is
rewarded by the "payment" of
nutrients in the form of
sugars,
starches,
proteins and
lipids from the plant roots.
These nutrients in turn flow to
the whole mycelial network through
cytoplasmic streaming.
See also
