MYCORRHIZAE AND PLANT GROWTH
Dr. Michael Pfeiffer
During the latter half of the 19th century, several
individuals noted the presence of fungi growing on and
within the roots of plants with no apparent ill effects to
the plants. In 1885, Frank coined the term mycorrhizae
(myco=fungus & rhizae=root) to describe these fungusroot associations. Mycorrhizae are symbiotic relationships
between certain fungi and roots of plants. In symbiotic
relationships, all members of the union benefit. With
mycorrhizae, the host supplies the fungi with organic
nutrients such as carbohydrates. In exchange, the fungi
facilitate increased water and nutrient uptake. Many
beneficial effects of mycorrhizae have been
demonstrated. Growth increases of 2-20 times or more
have been demonstrated with mycorrhizal compared to
non-mycorrhizal plants when grown in soils low in
available nutrients. In some cases the fungi have been
shown to protect roots from pathogens. Mycorrhizal
fungi have also been shown to increase plant survival in
hostile environments such as mine tailings. Mycorrhizal
fungi occur in most if not all "natural soils". The
exceptions are severely disturbed soils such as mine
tailings. The vast majority of all plant species including
agronomically important crops form mycorrhizae. The
most prevalent types of mycorrhizae are termed
ectomycorrhizae (ecto=external) and endomycorrhizae
(endo=internal) depending on how fungi colonize
roots.
Ectomycorrhizae occur primarily on forest trees
such as alder, beech, junipers, oaks, pines, popular,
spruce and willow. The benefits of ectomycorrhizae on
the growth of many species of forest trees has been
demonstrated. Typically seedlings inoculated with
ectomycorrhizal fungi have a more rapid rate of growth,
do not suffer from nutritional deficiencies and have
increased survival (Fig 1A). Many species of fungi
which form ectomycorrhizae have been shown to
produce antimicrobial compounds which may offer
protection from root pathogens. Ectomycorrhizae are
characterized by fungal growth primarily on the exterior
of roots. Roots become encased with a covering of
fungal tissue known as a hyphal mantle (Fig 1C & 1D).
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The fungi forming ectomycorrhizae also grow between
cells in root tissue to a depth of 2 to 3 cells (Fig 1D). A
network of connected fungal cells within root tissues are
formed which is known as the Hartig Net. Root systems
of plants colonized by ectomycorrhizae are essentially
turned into absorptive organs. Strands of mycelium
(fungal filaments) extend into soil from the hyphal
mantle and effectively act as an extension of the root
system increasing uptake of mineral nutrients and water.
There are in excess of 5,000 different species of fungi
which will form ectomycorrhizae with tree counterparts.
The fungi which establish these mycorrhizae are
abundant in most forest soils as evidenced by the
diversity of mushrooms seen. Mushrooms are the
fruiting bodies of the fungi which form ectomycorrhizae.
Although trees will form endomycorrhizae with many
different fungi, certain fungus-plant combinations appear
better at stimulating plant growth and increasing
survivability than other combinations. Catastrophic wild
fires have been shown to significantly reduce the
population of fungi which form ectomycorrhizal
relationships.
Endomycorrhizae are formed on 80% or more of
all plant species. The beneficial effects of endomycorrhizae on plant growth have been well
documented. Increased plant growth and increased
nutrient uptake of elements such as phosphorus, zinc
and copper has been demonstrated with mycorrhizal
versus nonmycorrhizal plants (Fig 2A). In addition to
increased plant growth, endomycorrhizal plants have
been shown in some cases to be more drought tolerant
and more resistant to certain root diseases. Fungi
forming endomycorrhizae grow between and within
cells of root tissue. Endomycorrhizae are generally
termed arbuscular mycorrhizae (AM) or by the older
terminology, vesicular-arbuscular mycorrhizae (VAM).
Arbuscules are fungal structures which develop inside
individual plant cells (Figs 2D & 2E) . It is thought that
arbuscules are the site of exchange between the fungus
and the plant. Arbuscules have a very large surface area
to volume ratio which likely facilitates exchange between
FIGURE 1. ECTOMYCORRHIZAE
A. Effect of ectomycorrhizae on the growth of pine; Plants are the same age.
B. Nonmycorrhizal pine root.
C. Mycorrhizal pine root: note the increased root mass and root branching compared with B.
D. Root tip of Pinus taeda cut in cross section and stained to reveal the mycorrhizae: The tissue outside the white line is
hyphal mantle (hm) and the tissue inside the white line is pine root tissue (prt).
the partners. Vesicles are thought to be storage organs
for the fungus. Some species of fungi that initiate
endomycorrhizae form only "arbuscular" types of
mycorrhizae while others form arbuscules and vesicles
within root tissues (Figs 2B & 2E). Growth of the fungi
on the exterior of roots extends into soil a greater
distance than do root hairs. This extension of the root
system allows plants access to water and nutrients in a
greater volume of soil. Fungi that form endomycorrhizae are found in all soils. The exceptions being
"disturbed" sites which have been devoid of host plants
for an extended period of time. These AM relationships
like the ectomycorrhizae are symbiotic relationships.
The fungi shunt in more water and nutrients into the
plant. In exchange, carbon based nutrients from
photosynthesis are provided to the fungus by the plant.
Just as with ectomycorrhizae, there appears to be elite
relationships between certain species of AM fungi and
individual plant species. It is interesting to note that
members of the plant families Chenopodiaceae
(goosefoots), Cruciferaceae (mustards), Cyperaceae
(sedges), Juncaceae (rushes) Polygonaceae (buckwheats),
Portulacaceae (purslanes) and Urticaceae (nettles)
typically do not form AM relationships. There are many
members of these plant families that are considered
weeds. It is thought that non-reliance on endomycorrhizae allows these weeds to rapidly colonize
disturbed areas. Endomycorrhizae would appear to have
utility in stimulating growth of “non-forest” type plants
in soils low in available nutrients such as phosphorus,
FIGURE 2: ENDOMYCORRHIZAE
A. Effect of endomycorrhizae on the growth of guayule (Parthenium argentatum). Two plants on the left are mycorrhizal;
two plants on right (one unseen due to very small size) are nonmycorrhizal. Plants are the same age.
B. Stained root of endomycorrhizal plant illustrating vesicles (v) of the fungus in root tissue.
C. Stained root of non-mycorrhizal plant. Note absence of fungal structures within root tissue.
D. Stained root of endomycorrhizal plant in cross section. Note arbuscules (a) in plant cells .
E. Stylized diagram of fungal structures contained within root tissues of an endomycorrhizal plant: Arbuscules (a) within
plant root cells, vesicles (v) fungal hyphae (fh) between and within cells of the host and root hairs (rh) of the plant.
iron, zinc and copper. These fungi have been used with
varying degrees of success in revegetation of mine
tailings, right of ways and soils with high levels of toxic
minerals or salts.
There are many factors which can affect
establishment of mycorrhizae. Populations of fungi
which can form mycorrhizae are usually depressed in
soils where severe disturbance has occurred such as
mine tailings, after catastrophic fire or flooding events.
Any factor which prohibits the growth of host plants for
an extended period of time will reduce populations of
mycorrhizal fungi. High levels of nutrients particularly
phosphorus and nitrogen in fertilization programs
inhibit the formation of mycorrhizae. Fumigation of
soils with methyl bromide, chloropicrin or similar
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fumigants often devastate populations of resident
mycorrhizal fungi. Treatment of soil with heat as well
as application of certain fungicides can severely reduce
numbers and types of mycorrhizal fungi in soil.
The most efficient time to establish mycorrhizae is
in the greenhouse where large amounts of plants can be
made mycorrhizal with a minimum of inoculum. If nonmycorrhizal plants are transplanted to most natural
soils, they will establish mycorrhizae with resident
fungi. There has been little if any benefit shown
from inoculation of plants with mycorrhizal fungi in
the field at the time of transplanting. Currently there
is inoculum of both ecto and endomycorrhizae available
from many sources. The quality and or efficacy of this
inoculum varies greatly.
Suggested Reading
Brundrett, Mark. Mycorrhizal Associations. 2008. Web 26 Feb. 2013.<http://mycorrhizas.info/>.
Cripps, Cathy L. Mycorrhiza. In: PNW Plant Disease Management Handbook. 2002. Web 26 Feb. 2013.
<http://pnwhandbooks.org/plantdisease/node/1805/print/>.
Johri, B.N. & A.K. Sharma (Eds.) 2002. Arbuscular Mycorrhizae; Interactions in Plants, Rhizosphere and Soils.
Science Publishers, Inc. Enfield, New Hampshire, U.S.A.
Mycorrhizae. Wikipedia. 21 Feb. 2013. web 26 Feb. 2013. <http://en.wikipedia.org/wiki/Mycorrhiza/>.
Mosse, B. 1973. Advances in the Study of Vesicular-Arbuscular Mycorrhizae. Annual Review of Phytopathology
11: 171-196.
Mycorrhizae. Wikipedia. 21 Feb. 2013. Web 26 Feb. 2013. <http://en.wikipedia.org/wiki/Mycorrhiza/>.
Pfeiffer, C.M. & H.E. Bloss. 1988. Growth and Nutrition of Mycorrhizal Guayule (Parthenium argentatum) in a
saline soil as Influenced by Vesicular-Arbuscular Mycorrhizae and Phosphorus Fertilization. New Phytologist
108: 315-321.
Pleger, F.L. & R.G. Linderman (Eds.) 1994. Mycorrhizae and Plant Health. American Phytopathological Society,
St. Paul, Minnesota, U.S.A.
Safir, G.R. (Ed.) 1987. Ecophysiology of VA Mycorrhizal Plants. CRC Press, Boca Raton, Florida, U.S.A.
Schenck, N.C. (Ed.) 1984. Methods and Principle of Mycorrhizal Research. The American Phytopathological
Society, St. Paul, Minnesota, U.S.A.