Some microorganisms form MUTUALISTIC associations
with plants. A mutualistic association is one where both parties
benefit. The two most important mutualistic associations between
microorganisms and plants are mycorrhizae and Rhizobium/legume
associations.
Mycorrhizal fungi
Mycorrhizae are an association between the roots of plants and a
fungus. Mycorrhizal associations have been classified into four
major grouping; Vesicular arbuscular (VA) mycorrhiza, ectomycorrhiza,
orchid mycorrhiza and ericoid mycorrhiza. Mycorrhizal fungi and
plants exchange materials that one of them has too much of and the
other needs.
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| VA mycorrhizae form when a
fungus invades the cells of a plant root and forms special
structures within the roots call vesicles and arbuscules.
VA mycorrhizae increase the uptake of the nutrients phosphorus
and zinc to plants and the plant lets the fungus have energy
in the form of sugar. The plant's leaves act as a factory
for turning sunlight into energy for both the plant and the
fungus. The fungus explores the soil in tiny spaces the plant
roots cannot get to find phosphorus for both the plant and
the fungus. |
Mycorrhizal fungal
spores
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Even though mycorrhizal fungi
produce spores like the ones in the photo, it cannot survive
without the plant, and in some cases the plant would not survive,
or would survive very poorly without the fungus |
Hyphae of an ectomycorrhizal
fungus form a netting over these tree roots
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Ectomycorrhiza are similar to VA mycorrhiza
except the fungus forms a sheath or glove around the roots of
the trees rather than making arbuscules and vesicles inside
the plant. The ectomycorrhiza are also a mutualistic association,
with the plants benefiting from more nutrients and the fungus
from extra energy. |
Ectomycorrhizal fungi
growing out from the tree roots
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Most of these associations are formed between
fungi which form mushrooms and large trees. Most of the mushrooms
found in forests are therefore formed by ectomycorrhizal fungi.
You can see in this photo how the fungi grow out from the roots
and explore more of the soil |
 Orchid
mycorrhizae form between an orchid and a fungus. The plant cannot
survive without the fungus. An orchid plant is very tiny and has almost
no energy reserves to get it off to a good start in life. It requires
a fungus to provide it with all of its energy requirements until it
is large enough to make its own energy. The orchid does not give the
fungus anything in return but the fungus gets its sugars from nearby
trees.
Photo courtesy of
MYCORRHIZAL ORCHIDS
Friedberg, Germany, 49 821 604860
 Legumes
Most plants have to search through the soil
with their roots to find nitrogen which is a critical nutrient required
for growth. Legumes on the other hand form symbiotic relationships
with Rhizobium bacteria. The Rhizobium live in little
nodules in the roots of the legumes and fix atmospheric nitrogen into
ammonium or nitrate, forms of nitrogen that can be used by the plant.
In other words, the Rhizobium turn air into fertiliser!
The plant benefits
because it gains nitrogen. The bacteria benefit because they get
sugars and nutrients to survive.
Photographs
on this page courtesy of
Prof. Sally Smith and Dr. Peter Murphy, University of Adelaide
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