Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 66

3.3.2 Mycorrhizae

plants, which results in the formation of a microanaerobic
zone inside the nodule where N 2 fixation occurs after
the oxygen is consumed (Hardy and Havelka 1975). In
return the plant supplies the bacteria with organic
compounds which the bacteria need for energy and cell
growth.
Let us look more closely at the bacteria that live inside
these plant-root nodules. Rhizobia are free-living bacteria in
the soil and must gain entry into plant roots before nitrogen
fixation occurs. The evolution of plants permitted infection
avoidance by disease-causing bacteria and viruses, so
entrance of any particular bacteria, even potentially benefi-
cial ones, is highly regulated. The root hairs of plants, such
as legumes, release lectin, a protein that binds with a sugar
released by the Rhizobia . Upon such surficial linkage, the
root hair curls around the bacterium and the bacterium then
purchases entry into the root hair cell by the insertion of a
tube, called the infection thread, which causes the cell to
divide and forms a nodule around the site of bacterial entry.
This inversion and convolution produces the ideal structure
for subsequent nitrogen fixation: an anoxic central core
where the bacteria reduce nitrogen into ammonia, and an
oxic outer rind where nitrification can occur and also supply
the root cells with oxygen.
Another problem quickly arises, however. Both bacterial
and plant cells require nitrogen so how is nitrogen rationed
among these competing needs? It turns out that the bacteria
fix more nitrogen than they require for themselves, so some
becomes available to the plants. In turn, in order to ensure a
supply that is greater than the nitrogen needs of the plant,
the plant provides excess glucose to the bacteria. Part of the
rationale behind this production of excess nitrogen by the
bacteria may be found in the fact that plants that have root
nodules carry the gene for part of a compound called
leghemoglobin, which is needed by the Rhizobia to utilize
oxygen for respiration, and the Rhizobia contain the other
gene (Hardy and Havelka 1975). This is an important pro-
cess for plant nutrition and survival, because elements that a
plant needs other than nitrogen are derived from the dissolu-
tion of the soil matrix. Moreover, the bacteria recycle some
of the nitrogen back into the atmosphere during denitrifica-
tion. The nitrogen cycle is discussed in greater detail in
Chap. 11.
What about plants that do not have an association with
nitrogen-fixing bacteria? How do these plants meet their
nitrogen needs? One example of a non-leguminous nitrogen-
fixing plant is the alder tree ( Alnus spp. ). Alder roots have
filamentous bacteria, such as Frankia , that perform nitrogen
fixation. But not all non-leguminous plants have Frankia .In
Chap. 11, the relation between such plants and the use of
groundwater as a source of nitrogen is discussed; it will be
shown that most of these plants possess the phreatophytic
habit.

The presence of plant roots increases not only the numbers
of bacteria in the soil but also the number of fungi. In
general, fungi are primarily responsible for the decomposi-
tion of dead plant matter. The fungi of many plant roots in
the rhizosphere are called mycorrhizae, from the Greek,
mykes , meaning fungus, and rhiza , meaning root. More
than 80% of all vascular plants have mycorrhizae. This is
another example of a symbiotic relation that benefits both
the fungi and the plant. Plants can grow in the absence of
mycorrhizae, but enhanced growth always is observed where
fungi are present. The fungi gain entry into plant tissue
through insertion of hyphae into open stomata or wounds
on the bark or suberized roots. The linkage between tree
wounds and potential fungal entrance has implications for
sample-collection methods pertaining to tree-tissue monitoring
during phytoremediation projects, which is discussed in
Chaps. 9 and 14.
The association between plants and fungi is not a recent
phenomenon. The fossil record contains evidence of root and
mycorrhizal connections. The role that this relation played in
the transition of plants to land is unclear, but the association
with fungi undoubtedly would have been an advantage. Not
all fungal and root interactions are beneficial to both
organisms, however. In fact, one of the largest impediments
to a stable food supply is the attack of various fungi on
food crops. Moreover, some of the commonly used trees
and planting methods, such as monoculture, used for
phytoremediation purposes are vulnerable to widespread
fungal attacks.
If most plant diseases and infections are caused by fungal
entry and growth, how did the beneficial symbiotic relation
between plants and mycorrhizae develop? At first, bacteria
probably were associated with plant infection, until the
growth and reproduction of each was enhanced by their
mutual interaction. Keep in mind that fungi are heterotrophs
and require reduced organic carbon as a food and energy
source. Plant roots while alive provide a carbon source much
like after death. Perhaps plants shed organic matter into the
rhizosphere while alive to satisfy the organic carbon needs
of the fungi in order to remain negatively unaffected by
fungal root colonization. Roots secrete the organic substance
mucigel, as well as organic chemicals that could ward off
potential threats, as is discussed in Chap. 11. Plant roots also
can contribute to feeding these rhizospheric bacteria and
fungi as a consequence of root turnover, the annual shedding
of dead root matter.
Many different kinds of mycorrhizae are associated with
plant roots depending on the site of colonization.
Mycorrhizae that exist on the inside of the roots are called
endotrophic mycorrhizae or vesicular arbuscular mycorrhizae
(VAM) (Safir 1987) and are found within the cells of the root