Augustin De Tournemire 2017 Mycorrhizal Symbiosis, Mayor Player Of Soil Ecosystem [627762]

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THE MYCORRHIZAL SYMBIOSIS
A MAYOR PLAYER OF SOIL ECOSYSTEM

Augustin
DE TOURNEMIRE
First semester 2016 -2017 ERASMUS

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TABLE

INTRODUCTION ………………………….. ………………………….. ………………………….. ……………………….. 4
I. The mycorrhizal symbiosis ………………………….. ………………………….. ………………………….. ………. 5
a. A symbiotic association plant -fungi ………………………….. ………………………….. …………………. 5
b. Endomycorrhizal symbiosis ………………………….. ………………………….. ………………………….. ..6
c. Endotrophic mycorrhizae ………………………….. ………………………….. ………………………….. …..7
d. Ectomycorrhizal symbiosis ………………………….. ………………………….. ………………………….. …8
e. Establishment of the mycorrhizal symbiosis ………………………….. ………………………….. ………. 8
II. Physiology of mycorrhizae ………………………….. ………………………….. ………………………….. ……. 10
a. An exchange of nutritional essential elements ………………………….. ………………………….. …. 10
b. A better water absorption ………………………….. ………………………….. ………………………….. . 11
c. Influence on the soil aggregat ion………………………….. ………………………….. …………………… 11
d. Protection against pathogens ………………………….. ………………………….. ………………………. 12
e. Resistance against stress of the environment ………………………….. ………………………….. ….. 12
f. Mycorrhizal symbiosis as protection against pollutants ………………………….. …………………… 13
III. Synthesis of phyto -hormones from ectomycorrhizal fungi ………………………….. …………………… 14
a. Phyto -hormones ………………………….. ………………………….. ………………………….. …………… 14
b. Synthesis of plant hormones by ectomycorrhizal fungi ………………………….. …………………… 14
c. Influence of fungal auxin on ectomycorrhizal symbiosis ………………………….. …………………. 15
d. Auxin over -producing fungal strains and mycorrhizal activity ………………………….. …………… 15
IV. Interaction between the mycorrhizal symbiosis and soil o rganisms ………………………….. ………. 17
a. Interaction between mycorrhizal system and soil bacteria ………………………….. ………………. 17
b. Interaction with protozoa ………………………….. ………………………….. ………………………….. .. 17
c. Influence of invertebrate community of the soil ………………………….. ………………………….. . 18
V. Mycorrhizal symbiosis and agriculture ………………………….. ………………………….. ………………… 20
a. Impact of intensive agricultural practices on mycorrhizae ………………………….. ……………….. 20
b. Mycorrhizal inoculants for agriculture ………………………….. ………………………….. ……………. 22
CONCLUSION ………………………….. ………………………….. ………………………….. …………………………. 23
BIBLIOGRAPHY ………………………….. ………………………….. ………………………….. ………………………. 24

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INTRODUCTION

In popular language, mushrooms are represented by what we pick up in the forest and
put in our basket. This definition is not correct because the harvest product is only the
fructification of the mushroom. In fact, the main part of mushrooms is the mycelium, a
filamentous lace w hich grows in the soil. If the condition are favourable, the mycelium can
make aerial parts.
Mycelium can reach a huge si ze and live a long time. It can be considered as the biggest and
oldest living organism on our planet. Today, about 70 000 fungal species have been identified
and the number of species is estimated at 1 million on earth. There is more fungal species
than f lowering plants and there is a lot of diversified and complex structures, from unicellular
to pluricellular.
Mushrooms constitute an autonomous reign, the fungal reign, because they don’t belong to
plant or animal kingdom. Their eating behaviour make them closer than animals than plants.
Just like animal, fungal species are heterotrophic for carbon and they need an external source
of organic carbon to feed. On the contrary, green plants can make their own organic carbon
from photosynthesis.
According to the origin of their food, mushrooms are divided in three groups:
 Saprophytic fungi : they decompose dead organic matter to feed
 Parasitic fungi : they live at the expense of living organisms
 Symbiotic fungi : They live in symbiosis with living plants
This las t group is mainly constitute by mycorrhizal fungi, mushrooms which live in symbiosis
with the trees in forest. This symbiosis is beneficial for the trees and for the mushrooms as
well and this phenomenon is playing an important role for forest ecosystem but also in
agriculture .
So we can now ask ourselves what is a mycorrhiza? How does this phenomenon work? And
how mycorrhiza influence ecosystem s? And how mycorrhizal symbiosis can be useful for
agriculture?

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I. THE MYCORRHIZAL SYMBIOSIS
a. A symbiotic associat ion plant -fungi
About a third of all the fungal macromycetes species we can find in forests are
mycorrhizal species. There is about 2000 mycorrhizal species and lots of them are eatable
mushrooms and really appreciate like truffle ( tuber melanosporum ), Boletus edulis, Russula
cyanoxantha, Lactarius deliciosus, or Cantharellus cibarius. We can also find venomous
mycorrhizal species like Amanita muscaria. A lot of these mycorrhiza l mushrooms have
specific hosts so they can make a symbiosis only with some typ ical trees. For example, Suillu
visicus can only be in symbiosis with Larches. Others can be find only in softwood o r hardwood
forests. Fungi like hebeloma sp. are doing mycorrhiza with seedlings and young trees whereas
others like Boletus sp . and Russula sp. can only be found in timberlands, where big and o ld
trees are present . So there is a quite big diversity among the different mycorrhizal species i f
we talk about physiological and morphological aspect.
A mycorrhiza e is a symbiotic association between a fungal specie and the root of a plant. In
our regions, all the forest trees are mycorrhized. The roots are col onised by a mycorrhizal fungi
which modify its structure. The mushroom is developing around the extremity of rootlets by
forming a thick filamentous material called mycelium. The two pictures below show us the
difference between a colonised rootlet and a non -colonised one (image 1).
These are photographs of spruce roots in different
conditions. In fact, we can see that the picture above is
a spruce root without mycelium, so without mycorrhiza.
On the contrary, the picture below is a spruce root
colonised by Hebeloma mycelium. On the f irst picture
we can see the presence of absorbing hairs. These root
hairs are tubular outgrowth of a trichoblast. The
function of root hairs is to collect water and mineral
nutrients present in the soil and take this solution up
through the roots to the re st of the plant. They have a
large surface area which makes absorbing water during
osmosis and minerals during active uptake.
As we can see on the second picture of figure 1, there is the presence of a coat of mycelium
around the rootlets. We can also not ice the absence of root hairs. The absence of such an
important component of the roots reveal that the plant doesn’t need it. In fact, the mycelium
Figure 1 : photography of spruce root with and without
mycorrhiza

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play the r ole of absorbent hairs by providing water and minerals nutrient to the plant. In
exchange of that, the mushroom benefit for a source of organic carbon.
b. Endomycorrhizae
This kind of symbiosis is the most expanded on ear th. More than 8 0 % of the plant on our
planet are able to host a n endo mycorrhiza e. We can find associations with bryophytes,
vascular plants, herbaceous and ligneous plants. The figure below shows us examples of
compatible species with an endomicorrhizae.

The family of endomy corrhi zae can also be divided in three groups:
 Arbuscular endomycorrhizae

This kind of mycorrhizae are associate with
herbaceous and ligneous plants. They make
arbuscular structures inside the cells of the plant
and even if they can’t enter the plasma, they
cross the cell wall. The goal is to increase the
contact area between hy phae and cell plant in
order to favour metabolites exchange between
the partners. Hyphae grow in the cortical
parenchyma of the roots and form arbuscular structure which contain stocks for the host
Figure 2 : compatible species with endomicorrhizae
Figure 3 : photography of arbuscular mycorrhizae

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plant. Arbuscular endomycorrhizae are only formed by funga l species from the division of
glomeromycetes, who lost the sexual reproduction.
 Endomycorrh izae with intracellular pelotons
This type of mycorrhizae involves basidiomycetes as fungal species and orchidaceae as vegetal
species. Hyphae form heap in cortica l cells. As we can see on the picture 4, h yphae penetrate
the cell wall inside roots cortex cells, pushing the plasma membrane but without crossing it.
The contact area between hyphae wall and plasma membrane can be increased by the
formation of ramificati ons and the roots morphology is not distorted.
 Ericoid endomycorrhizae

This kind of mycorrhizae involves
ascomycetes and basidiomycetes in
symbiosis with plants from the order
of Ericales.
As we can see on the picture 4,
ericoid endomycorrhizae are
characterised by the formation of
pelotons in transitory roots of little
diameters.
The picture 4 also represents the
ectomycorrhizae and
ectendomycorrizae that we are going
to explain further.
c. Endotrophic mycorrhizae
Also called ectendomycorrhizae or arbutoid mycorrhizae, this type of symbiosis is a
combination of ecto and endomycorrhizae . In fact, this mycorrhizae develops a hyphae coat
around the roots but also an intracellular network in the cortical parenchyma. This
phenomenon involves also Ericales.
Figure 4: Diagram representing all types of mycorrhizae

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d. Ectomycorrhizae
Ectomycorrhizae are rarer and about only 10% of the plants are concerned. This kind of
mycorrhizae are essentially developing with ligneous species such as oak, Burch or spruce .
Mushrooms involved in ectomycorrhizae are superior mushrooms, those who can be picked
up in our forests. Hyphae are developing at the surface of radicular system and between the
cortical cells of young roots. In our forests, all tree roots are mycorrhize d and most of the time
it’s an ectomycorrhizae which can be recognized by the presence of a thick fungal coat around
the roots.

First, the fungal specie is associated
with little determined growth roots
without absorbent hairs. Then the
roots are enveloped by a hyphae coat
(called “manchon mycelien” on the
picture 5) . Hyphae are growing
between the cells in the external part
of external parenchyma, forming a
symbiotic interface called “Hartig
Network” (picture 5) .
This symbiosis modify the
physiognomy of the roots. In fact,
they swell and don’t stop growing. The roots can ramified a lot and so the cap and the apical
meristem can be reduced.
e. Establishment of the mycorrhizal symbiosis
The establishmen t of the mycorrhizal symbiosis has to get through several steps in order to
create an interdependence between the plant and the fungi.
 Initiation process : The beginning of the mycorrhiza formation starts from an exchange
of specific signals between the two partners. A high concentration of monosaccharides
at the root -soil interface activate physiological modification of the fungal specie. In
parallel, the presence of nitrogen compounds from the fungi leads to modification in
the radicular metabolism.
 Spec ific recognition phase : It is a bidirectional process where the both partners are
secreting phytohormones. M orphological modifications of root cells are interfering.
These modifications are controlled through gene activity regulation by fungal or plant
Picture 5 : representation of Hartig Net

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phytohormones (auxines, ethylene, abscisic acid). The root exudates secreted during
fungal inoculation, induce defenc e mechanisms against the pathogens .
 Hyphal growth and inhibition of root hairs elongation : This process create an
interdependence between the two partners. F lavonoidic compounds from the root
exudates are increasing the spore germination and t he hyphal growth . Also, during the
formation of the mycorrhiza, the fungi are influencing the expression of genes involved
in phenylpropanoids, flavonoids and isoflavonoids radicular metabolism. The rutin
induces hyphal growth, and hypaphorine, an auxine analogous indolic compound,
inhibits the root hairs elongation .
 Hyphal adhesion : The hyphal adhesion is influenced by some hyphal wall compounds,
as hydrophobines, cysteine -reached proteins, α -tubulin and actin .
 Penetration process : Low defensive responses are activated in plant organism, such
as peroxidases production and proteins phosphorylation modifications.
 Infective phase : There is a modification of the synthesis of proteins from the fungi and
the plant. Some polyamines produced by fungal mycelium, have roles in the plants
germination processe s. During a root infection made by a pathogenic fungal species,
the plant reaction is strong and invariabl e, by contrary, the root infection made by a
mycorrhizal species is permitted by the plant .

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II. PHYSIOLOGY OF THE MYCORRHIZAE
a. An exchange of nutritional essential elements
Mycorrhiza is a mecha nism where the tree and the mycorrhizal mushroom are exchanging
matter. The tree provides to the mushroom organic substances (sugars) elaborate d during the
photosynthesis. In exchange of that, the mushroom provides to the tree elements as nitrogen
and phosphor. The mushroom has the capacity to collect these components in microscopic
porous areas of the soil thanks to its hyphae. Giving the fact that mushroom hyphae are widely
expanding in the soil, the absorption area is a lot larger than the one occupied by non
mycorrhized trees.
In fact, the tissues of mycorrhized pl ants contain a higher level of nitrogen and phosphor than
trees who don’t benefit the symbiosis. A compa rison has been realised to show the difference
of level of nitrogen and phosphor between a Picea abies in symbiosis with Laccaria laccata and
the same t ree without the mycorrhizae.

The exchange of those substances between the fungal specie and the tree pass by this
specifically area called Hartig Net. As we said earlier, the Hart ig net is composed from a thick 024681012141618
Nitrogen PhosphorPicture 6 : comparison nitrogen & phosphor level with and
without mycorrhizae
With myc Without myc

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fungal coat which install itself across the roots cell and rootlets to ensure a contact between
the two partners.
When we watch the transversal cut of an
ectomycorrhizae, we can see that hyphae are forming a
net, hence the name of Hartig net.
The fungal coat and the Hartig net have the capacity t o
store and to stock phosphor and nitrogen. Phosphor is
accumulated under the shape of long chain of
polyphosphate or polyphosphate grain which are stored
in fungal cells at a solid state.

b. A better water absorption
The mycorrhizal system has a great influence on the absorption of water by the host
plant. In fact, the presence of the hyphae network around the roots is favourable to the water
absorption. Fungi mycelium is able to take water fro m little micro hollows, usually not
reachable by the root system, and to provide it to the host plant. Consequently, micorrhized
plants show a better development in a dry and arid environment than non -micorrhized
species.
c. Influence on soil aggregation
Mycorrhizal fungi have a good influence on the structure and stability of the substrate. The
mycelium is a dynamic structure that physically contribute to the elaboration of aggregates of
the soil, due to the fast growth and constant turnover of the hyphae network.
Dead Mycelium also takes part in the preservation of soil structure. In fact, when a site is
lacking of nutrients, the fungi make the hyphae cytoplasm move to an other mycelium
network where the soil is richer. So the fungi let the external wall s of the old mycelium that
contribute to maintain the structure and stability of aggregates until their full decomposition.
The fungal mycelium also have a biochemical influence on the soil by the production of fungal
substances like glomaline. This a glyc oprotein is hardly degradable and accumulate in the soil.
The protein plays a binder role and contributes to the formation of macro -aggregates. Macro –
aggregates maintains nutritive elements and are favourable to gas exchanges.
Picture 7 : transversal cut of an ectomycorrhizae

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Finally, the presence of myco rrhizal fungi reduce the risk of compaction and erosion of the
soil, and are beneficial to the soil fertility.
d. Protection against pathogens
The mycorrhizae plays a defensive role against pathogens, and mainly against fungi and
nematodes that attack the roots of host plants.
Several mechanisms can explain this phenomenon. First of all, the nutritional advantages that
mycorrhizae offers permit to host pants to be stronger and to have a better resistance against
pathogenic organisms. Secondly, it has been demonstrated that infection of the root system
of the plant by mycorrhizal species prepare the plant to prejudicial organisms. The
mycorrhizati on let the plant to be in an active state of immunisation.
We also know that some fungal species like Glomus irregular can control the development of
pathogenic fungi and reduce their production of mycotoxins.
The protection of the plant by mycorrhizal fu ngi is also due to the competition that takes place
between the fungi and others pathogenic organisms. Hyphae can reach 80% of the biomass
of the soil so there is a few potential infection sites left.
Finally, it’s important to say that the protection abil ity of mycorrhizae has boundaries and
depends on factors like the plant sand fungi specie, the virulence of pathogens and
environmental conditions.
e. Resistance against stress of the environment
We can notice that the presence of mycorrhizae modify some aspects of the host plant and of
the environment and confers to the plant a better resistance against environmental threats.
Like we said before, mycorrhizal symbiosis allows the plant to have a better access to
nutritional elements and water from substrat e. This favoura ble to the growth and strength of
the plant, it confers to the vegetal specie a better resistance to phenomenon like drought, and
infections by pathogenic organisms .
By their structuration power of soil aggregates, mycorrhizal symbiosis ensu re to the host plant
a more fertile substrate that has a better resistance to bad weather.
We can finally say that this symbiosis plant -fungi has a lot of benefits to bring to the plants
and it’s hopeful when we think about the applicability of mycorrhizae in agriculture.
Unfortunately, intensive methods in modern agriculture usually damage the fungal population
of the soil.

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f. The myc orrhizae as a protection against pollutants
Since the beginning of industrialisation, pollutants emissions contain, among others
substances, heavy metals w hich can then b e find in forests. Some of those substances like
iron, zinc and copper are es sential for the plants. O thers elements like lead, cadmium, mercury
and chrome are very toxic for the trees. Those components are n ot degradable, so they
accumulates in the biosphere and constitute a real danger for living organisms.
On the contrary, some mycorrhizian
mushrooms have a good resistance
to those elements. As we can see on
the picture 8, aluminium (like others
toxic sub stances) are fixed in the
mycelium and can be found in
polyphosphates grains, inside the
cells, on walls and cell nucleus, and
in specific proteins. The toxic
aluminium has been colored in blue
on the phot ography and we can
notice that i t is maintain in th e
fungal coat and can only reach the
roots in little quantity. So the
mycorrhizae plays the role of a filter
for pollutants . Nevertheless, as the
toxic substances are accumulated in
hyphae, we can easily find thoses
elements in eatable mushrooms.
For radioactive substances, we can
observe the same behaviour.
Elements like caesium we can find in
the soil is fixed by fungal and
bacteria species. This the reason
why we can only find little quantities in the p lants, but we can find mycorrhizal mushrooms
fructifications with a high rate of radioactive elements.

Pictur e 8 : lengthwise cut of Picea mycorrhizae

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III. SYNTHESIS OF PHYTO -HORMONES FROM ECTOMYCORRHIZAL FUNGI
a. Phyto -hormones
The word of hormones refers to actives organic substances, produced in very low
concentrations in a tissue, and transported to another tissue where they set off a physiological
response.
Vegetal hormones take part in regulation of growth and development o f the plants, according
to environmental factors. Five families of v egetal hormones are known today : Auxins,
cytokines, gibberellins, abscisic acids, and ethylene.
Auxins are the first plant hormones discovered and the most
studied. The main active auxin in plants is the indole -3-acetic
acid (IAA), this molecule has been characterised in plants
tissues by Hagen -Smith in 1942 (picture 9) .
The IAA molecule is involved in many physiological
mechanisms:
 Responsible for phototropism
 Involved in gravitropism
 Takes part in growth regulation of fruits
 Regulation of apical dominance
 Control of elongation of stems
 Lots of process of cellular division and differentiation
A hig h number of micro -organisms, bacteria, mushrooms have the ability to produce plant
hormones and can influence plant development. Ectomycorrhizal fungal species have this
ability.
b. Synthesis of plant hormones by ectomycorrhizal fungi
Some of ectomycorrhizal fungi release cytokines, gibberellins, but the production of these
hormones is rare compared to the synthesis of auxins (IAA) by fungal species. Many studies
show that ectomycorrhizal fungi could produce IAA when there is the presence of tryptophan
as a p recursor of the synthesis. The production of IAA was huge when there was a precursor
(like tryptophan) in the culture medium, but any studies until 1983 could show the ability of
auxin synthesis by fungal species without precursor. Then , with the developme nt of analysis
technologies like high performance liquid chromatography, we managed to highlight very low
auxins quantity (of the order of picomole) in fungal tissues, which has been synthetized
Picture 9 : Molecule of IAA

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without any precursor. These quantities, even very low, can a ffect the physiology of the roots
system.
c. Influence of fungal auxin on ectomycorrhizal sym biosis
Auxin modify the morphology of roots system. In fact, roots system are hyper -ramified when
they are colonised by ectomycorrhizal fungi. Slankis (1950, 1973) made an experience where
he shows the influence of IAA on roots of a Pinus sylvestrus . He submit ted excised Pinus roots
to auxin from ectomycorrhizal mushroom like Seuillus luteus . This treatment induce the
formation of short roots without absorbent hairs, just like ectomycorrhizae. Regular inputs of
auxins are needed to stabilise those morphology varia tions. In conclusion, this experience
shows that fungal auxin is necessary to maintain the ectomycorrhizae. So, according to Slankis
“hormonal” theory, the specific morphology of roots systems in the case of ectomycorrhizae
would come from fungal species.
On the other hand, it is important to say that the mechanisms of hormones and association
between plants and mushrooms are poorly known and the theory of Slankis has been
discussed a lot.

d. Auxin over -producing fungal strains increase mycorrhizal activity
In 1992, Durand & al. selected auxin over -producing fungi ( Hebeloma cylindrosporum ) in order
to constitute a biologic model to determine the role of IAA in establishment of
ectomycorrhizae. The fungal mutant specie has been associate with the specific plan t host
(Pinus pinaster ). Then a confrontation has been done between excised roots with muntant
fungi and excised roots with a non -mutant strains of Hebeloma.
The picture 10 shows the confrontation between the two roots system. The photographies a,
d, e an d f are realised on the symbiosis between the mutant fungal specie and the roots of
Pinus. We can clearly see the difference between the two roots system and we can notice that
the mutagenic symbiosis has a strong influence on the development of the roots. So the
presence of IAA is determining for the formation of mycorrhizae and has a strong influence on
the root system of the tree.

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The Hartig net play an
essential role for the
development of the
symbiosis. This net is
the interface where all
the exchanges between
the fungi and the plant
are done. The
mycorrhizae formed by
savages strains of fungi
are “normal”, hyphae
usually don’t re ach the endoderm. On the contrary, the Hartig net is hypertrophic when the
ectomycorrhizae is induced by an IAA over -producing fungal specie. Hyphae can reach
endoderm and even penetrate the living cells of cortical parenchyma, which can’t happen in
normal ectomycorrhizae. Those results s how that fungal IAA is involved in the setting of Hartig
net. We know that IAA induce the acidification of the periplasmic area and causes the
softening of the wall. This mechanism would favour the penetration of hyphae in the cells.
It can also be pointed out that auxin is a messenger which can affect the physiology of the
tissues away from the production site of the hormone. So fungal auxin could affect the
physiology of aerial parts of the plants and particularly the phot osynthetic activity.
Picture 10 :
Pinus pinaster roots
mycorrhized in vitro
with Hebeloma
cylindrosporum
a, d, e, f :
mycorrhizae with
mutant auxin over –
producing fungi
b, c: mycorrhizae
with normal fungi

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IV. INTERACTION S BETWEEN THE MYCORRHIZAL SYSTEMS AND SOIL ORGANISMS
A lot of soil organisms are c onsidered as neutral because the y don’t interact directly with the
host plants or the mycorrhizal fungal species. However, these organisms h ave a great
influence on the soil activity and properties and can be involved in many physico -chemical
processes like the mineralisation of organic matter. So indirectly, we will see that many
categories of soil organisms contribute to the mycorrhizal syst em.
a. Interaction between mycorrhi zal system and soil bacteria
Many categories of rhizospheric bacteria have the capacity of stimulating the plant
development beyond the presence of the mycorrhizal species. For these species, it was used
the acronym PGPR (Plant Growth Promoting Rhizobact eria) , including both free and
nodulating nitrogen fixing bacteria, soil phosphate solubilising bacteria, as well the bacteria
that produce plant growth stimulators or pathogens inhibitors. But when they are both
present, t he mycorrhizal species and PGPR pres ent complementary functions.
On the other hand, non -favourable microorganisms can directly inhibit the fungi by releasing
the antifungal toxins, or indirectly by releasing the phytotoxines, bactericidal or bacteriostati c
substances and by modifying the soil properties (pH modification or ratio between different
substances in the soil)
 Interaction with nitrogen fixing bacteria

Nodulating nitrogen fixing bacteria like Rhizobium are aerobic soil
bacteria from the family of Rhizobiaceae. These bacteria can get in
symbiosis with plants from the family of Fabaceae and give them
the ability to fix atmospheric nitrogen. The organs of this symbiosis
is located in the nodules. Nodules are protuberances (Picture 11)
where a mutual exchange of ammonium and organic compounds is
done between bacteria and plant. The bacteria convert nitrogen in
ammonium, which one is directly assimilable by the host plant.
Many studies about the int eraction between fungal specie, bacteria
and host plant have been realised due to its interest in agriculture.
For example, SIVERIO & al. realised double inoculation of a nodule forming plant with a fungal
mycorrhizal specie and Rhizobium bacteria. It has been described that the colonization from
the bacteria is beneficial for the host plant but there is a lower level of saccharides available
for fungi. This is not favourable to a mycorrhizae but on the other hand, the good development
of the plant goes wit h an increasing quantity of roots exudates which have positive effects on
the mycorrhizal units number.
Picture 11 : Leguminous roots with nodules

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We can also find in the soil free nitrogen fixing bacteria. This kind of bacteria like Azospi rillum
or Azotobacteria, are non -symbiotic species but can promote the plant growth. Some similar
experiences as nodulating bacteria have been realised and showed that these bacteria help
the mycelium growth and the mycorrhization rate. These bacteria have the ability to fix
nitrogen away from the mycorrhizosphère and some can even contribute to the resources in
minerals.
 Interaction with mycorrhiza helper bacteria (MHB)
Mycorrhiza promoting bacteria have a direct influence on the morphology and physiology of
the mycorrhizal system. In fact, these bacteria have ma ny ways to promote the symbiosis and
consequently the plant development:
 Release of phytohormornes such as auxins, cytokines, gibberellins
 Production of vitamins and organic acids that help the spore germination
 Production of phenolic substances that incre ase fungal agressivity
 Promote interradicular development
 Inhibition of pathogens by releasing toxic compounds

b. Interaction with protozoa
Protozoa can interact with plants by releasing nutrients and compounds like phosphor. The
mycorrhizal system can nega tively influence the level of protozoa in the soil by releasing
inhibitors. At the opposite, the protozoa community can stimulate the root ramification and
so decrease the dependence between the roots and the fungal specie. Even if both are
beneficial for the host plant due to the production of nitrogen and phosphor, the competition
between mycorrhizal and protozoa would lead to the reduction of those populations.
c. Influence of invertebrates
There is a d irect link between soil macro -fauna, mycorrhizal specie and the host plant. A lot of
organisms like insects (mostly larva) or nematodes, attack the roots of the plant and the
mycelium of the fungal specie and have a negative influence on the mycorrhizal symbiosis.
On the contrary, it is well known that e arthworms play an essential role about the health state
of the soil by many ways :
 Splitting and burying of organic compounds
 Production of phyto -hormones
 Production of Humus
 Favour the soil porosity and aggregation

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Also, some species of colenbolas decrease the competition by feeding with saprophytic fungi.
Colenbolas can also feed with mycorrhizal fungal species when a level of insects in the soil is
high but they preferentially eat saprophytic species.
To conclude about the interactions between mycorrhizal symbiosis and soil organisms, we can
say that the phenomenon apply a strong selection pressure on rizhospheric organisms in order
to encourage the plant development.

Picture 12 : Summary diagram of Rhizospheric interrelations

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V. MYCORRHIZAL SYMBIOSIS AND AGRICULTURE
a. Impact of intensive agricultural methods on mycorrhizae
Utilisation of intensive methods in modern agriculture usually leads to a decreasing rate of
mycorrhization. In this part, we will see how m odern methods in agriculture affects the
mycorrhizal symbiosis and so the soil biodiversity. We will also focus on the solutions to
restore mycorrhizal fungal species.
 Impact of fertilisation
To favour the growth of yields, farmers who are practising conventional methods of
agriculture often use Phosphate mineral fertilizers. The addition of that kind of fertilizer
directly increase the phosphate offer in the soil. Moreover, when the quantity of available
phosphate is higher than the needs of the plant, the relation of dependence between fungi
and root system of the plant is strongly reduced. The mycorrhizal dependence of the plant
and mycorrhizal potential are inversely correlated to the additional concentration of mi neral
fertilisers in fields.
The colonisation of the roots by the fungi can even be inhibited. Fertilization with phosphate
mineral products also leads to the reduction of the community of endomycorrhizal fungi
because there is a selection among the popula tion. Only species that can resist to high
concentrations of phosphor are able to stay alive.
Fertilizers like manure and compost are releasing nutritional elements in a more progressive
way than phosphate mineral fertilizers. These fertilizers don’t seem to have the negative effect
on mycorrhizae. In fact, when fertilizers from the farm like manure and compost are not too
much used, they have the ability to stimulate the community of endomycorrhizal fungal
species.
 Impact of the soil working
Classical practices of agriculture required the utilisation of the plough to realise the soil
working. Tillage has got several goals like, among others, ground levelling, control of
adventitious plants, or the incorporation of fertilizers. The tillage has a radical effect on the
mycorrhizal potential of the soil by destruction of mycelium net of fungal species. This kind of
mechanical perturbation reduce the abundance of mycorrhizal fungi in the superior part of
the soil, where the most part of host plant roots are s ituated. The renewal of spore population
and a new development of mycelium in that part of the soil ask for a lot of energy from the
fungi. So , a great quantity of energy is spent for recolonization instead of being available for
the host plants.

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 Impact of crop rotation or monoculture
It is today well -known that monoculture has a bad influence on the soil and so the practices
of crop rotation is well encouraged. By repeating the same culture each year, a selection of
mycorrhizal species is done and a decreasing number of mycorrhizal units. Crop rotation is
important to maintain a high biodiversity level among the endomycorrhizal fungi population.
Nevertheless, all crops don’t have the same dependence for mycorrhizae and they also
influence the mycorrhi zal potential of the soil. The introduction of crop without great
dependence for mycorrhizae will reduce the mycorrhizal potential of the soil. On the other
hand, the mycorrhizal potential will rise if there is a strongly dependent specie like carrot in
the rotation.
The picture below show the rate of mycorrhizal dependence for some species.

 Impact of pests
Intensification of agricultural practices w ere accompanied by the rising of utilisation of pests.
The impact of these products is often difficult to measure, effects depend on the the quantity
applied, the time of action of the product. Some product, and obviously fungicidal products
can have a dir ect impact on the colonisation and sporulation of mycorrhizal species and can
even lead to their disparition. Some fungicids are used as a coating for seed, they are probably
the most toxic products for mycorrhizal fungi. However, some pests don’t have any negative
effects on mycorrhizal symbiosis.

Picture 13 : well used
plant in agriculture and
their rate of
dependence for
mycorrhizal symbiosis

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 Impact of genetic selection of new crops
The genetic selection of new cultivars has also an influence on mycorrhizal symbiosis we can
usually find in fields. It has been observed that a lot of modern varieties has been selected to
have a weak dependence on mycorrhizal species. The selection is mainly based on criteria like
growth uniformity, fast development, high yields and resistance to pathogenic organisms. The
criteria of mycorrhizal potential is often not r etained for the selection.
With the rising of organic agriculture, we can find more selected species where the
mycorrhizal potential is taken in consideration. In fact, the absence of chemical additional
products and the long crops rotation applied with th ese this kind of practices permit the
development of mycorrhizae.
To avoid the lack of mycorrhizal symbiosis, some commercial mycorrhizal inoculants are
produced and seem to be a great solution to favour the development of the symbiosis.
b. Mycorrhizal inoculants
The goal of mycorrhizal inoculants and the applicability I agriculture is to promote
mycorrhization of crops bu the introduction of strains of endomycorrhizal fungi that have been
specific ability to induce beneficial effects to the plants. Spec ies that are used a lot as
mycorrhizal inoculants are from the Glomus . gender.

These species are naturally well expanded in
ecosystems and have the advantage to be
able to colonize a lot of host plants varieties.
The selected varieties have the ability to a
reproductive behaviour. That means a fast
growth and a high production of spo res. This
development strategy allows the strains to get in symbiosis with host plant really quickly after
the inoculation. So, the crops can quickly benefit from the positive effects of the symbiosis.
Mycorrhizal inoculants can be applied directly on the field at the moment of the seeding or at
the transplantation of plants on the field.
Due to the lack of chemical inputs in developing countries, there is a quite important
utilisation of mycorrhizal inoculants in agriculture. Since several years, those cou ntries
showed the huge potential of inoculation of crops with fungal strains to rise the yields and
reduce the consumption of chemical fertilizers.
Picture 14 : photopraphy of
Glomeromyceta

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CONCLUSION

To conclude, we saw that mycorrhizae is a symbiotic association between a fungal
specie and a host plant. The mycorrhizal symbiosis has a huge influence on the plant
development by the creation of a relation of dependence between both partners. Fungi and
plant are taking part in a mutual exchange of essential nutrients that helps the growth of the
organisms involved. It is important to say that this kind of symbiosis is interacting in the ground
with a great part of soil organisms, from bacteria to macro -fauna organisms.
Mycorrhizal system enhance the absorption of water, minerals and oligo -elements. It also has
a positive effect on the resistance to hydric and thermic stress, on the root system and
radicular ramification, but also on the soil structure.
The beneficial effects of the mycorrhizal symbiosis on the host plant development l eads to the
applicability of mycorrhizae in agriculture. In fact, classical practices in agriculture are
discussed a lot because we assist to the destruction of soil structure and soil biodiversity due
to those methods from a passed time. Studies and appli cations on the field that we can find
in some developing countries like India are very encouraging about the utilisation the fungi –
plant symbiosis to increase yields and preserve the environment.

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