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MYCORRHIZAE
Mycorrhizae
The name mycorrhiza(e) from the
Greek words:
Mykes = fungus
Rhiza = root
Mycorrhizae
Mycorrhizae are modified roots consisting of
symbiotic associations of fungi and roots.
Mycorrhizae are not unusual and this plantfungus symbiosis might have been one of
the evolutionary adaptations that made it
possible for plants to colonize land in the
first place.
Symbiosis
The symbiosis of mycorrhizae is mutualistic,
meaning both parties benefit. The fungus, which
cannot photosynthesize benefits from a
hospitable environment and a steady supply of
sugar donated by the host plant.
The fungus provides increased surface area for
water and nutrient uptake. It also secretes a
growth hormone that stimulates roots to grow
and branch. The fungus also produces
antibiotics that help protect the plant from
disease.
Mycorrhizae
• Mycorrhizae have a greater surface area
than the roots, so they are able to absorb
soil nutrients more efficiently and pass
them onto the roots. Almost every plant
has some type of mycorrhizae to enhance
nutrient uptake.
• In return, the fungi get photosynthetic
products from the plants.
Fungal Basics
A fungus consists of fine, thread-like filaments
called hyphae. Most hyphae are 2-10 µm, so
they are so fine that they cannot be seen by the
naked eye. A hyphal mass is referred to as a
mycelium.
Hypha
Septate VS Nonsepate Hyphae
If the hyphae contain cross-walls, it is called
septate hyphae. Non-septate hyphae contain
no crosswalls. One of the ways fungi are
classified is according to their hyphae.
Septum
Mycorrhizae
There are seven different types of mycorrhizae, and can
be distinguished by their positions along two gradients,
from a lack of penetrations of cortical cells
(ectomycorrhizae) to penetration (vesicular-arbuscular
and arbuscular mycorrhizae) and from enclosed
(ectomycorrhizae) to open root (vesicular-arbuscular
mycorrhizae). Any natural ecosystem normally contains
a mixture of types of mycorrhizal associations.
The two main types of mycorrhizae are:
Endomycorrhizae and Ectomycorrhizae
Ectomycorrhizae
In ectomycorrhizae, the mycelium form a dense
sheath or mantle, over the surface of the root,
but does not enter the root cell.
Hyphae extend from the mantle into the soil.
Fungal hyphae grow into the cortex of the root,
making the root much thicker, shorter, and more
branched. The hyphae may aggregate to form
coarse hyphal strands or rhizomorphs.
Ectomycorrhizae
Ectomycorrhizae do not form root hairs.
Ectomycorrhizae are especially common
in woods plants, including trees of the
pine, spruce, oak, walnut, birch, willow,
and eucalyptus families.
The fungi often form mushrooms or
truffles.
Endomycorrhizae
Endomycorrhizae are much more common
than ectomycorrhizae, and are found in
over 90% of plant species.
Endomycorrhizae
Endomycorrhizae are within the root cell.
They do not have a dense mantle
ensheathing the root, although
microscopic hyphae extend into the soil.
The hyphae extend inward by digesting
small patches of the root cell wall.
Endomycorrhizae hyphae
A hypha does not actually pierce the plasma
membrane and enter the cytoplasm of the host
cell, but instead grows into a tube formed by
invagination of the root cell’s membrane.
To the unaided eye, endomycorrhizae look like
normal roots with root hairs.
Arbuscular Mycorrhiza
Once the hyphae have penetrated the cell,
some of the fungal hyphae become highly
branched to form dense knot-like
invaginations called arbuscles.
Vesicular-arbuscular
mycorrhizae
All but 2 genera of arbuscular mycorrhizae
form vesicles. Vesicles are lipid filled,
terminal swelling of hyphae. This primary
function is storage.
AM Fungi
The hyphae of AM fungi can make up to
70% of the soil’s microbial biomass.
Under the microscope, the hyphae are
usually recognizably distinct from other
soil fungi. These septate hyphae are
relatively large, with smaller side branches
that arise from bumps and angle on the
main trunk hyphae.
Other Mycorrhizae
Arbutoid mycorrhizae-look like
ectomycorrhizae and similar fungi, but are
technically endomycorrhizae because the outer
cortical cells and hyphae fill the cells
Ectendomycorrhizae-related to
ectomycorrhizae, except the fungus enters the
root cells.
Monotropoid mycorrhizae-found on certain
plants without chlorophyll, share a fungus with a
nearby tree and are parasites of the tree through
the fungus. Ex: Indian Pipe
Other Mycorrhizae
Ericoid mycorrhizae - fungus grows
loosely over lateral “hair” roots of the host
plant and the hyphae penetrate the single
layer of cortical cells. Ex: blueberries
Orchid mycorrhizae – for all or part of
their life cycle, orchids are obligately
dependent on their mycorrhizal partner.
Out of It
There are four major plant families that do
not form mycorrhizae:
Amaranthaceae (Pigweed)
Brassicaceae (Mustard)
Chemopodiaceae (Goosefoot)
Zygophyllaceae (Caltrop)
References
1. http://scitec.uwichill.edu.bb/bcs/mycorrhizas
2. Campbell, N.A. and Jane B. Reese. 2002. Biology
6th Ed., Benjamin Cummings, San Francisco, CA.
3. Madigan, M.T., Martinko, J.M., Parker, J. 2003.
Brock Biology of Microorganisms, 10th Ed.,
Pearson Education, Upper Saddle River, NJ