MYCORHIZAL ASSOCIATION

1. Mycorrhizal associations
3rd

2. Mycorrhizal Fungi Facilitate Nutrient Uptake by Roots
Mycorrhizae (singular mycorrhiza, from the Greek words for “fungus” and
“root”) are not unusual; in fact, they are widespread under natural
conditions
83% of dicots, 79% of monocots, and all gymnosperms regularly form
mycorrhizal associations (Wilcox 1991).
families Cruciferae (cabbage), Chenopodiaceae (spinach), and
Proteaceae (macadamia nuts), as well as aquatic plants, rarely if ever
have mycorrhizae. Mycorrhizae are absent from roots in very dry,
saline, or flooded soils, or where soil fertility is extreme, either high or
low. In particular, plants grown under hydroponics and young, rapidly
growing crop plants seldom have mycorrhizae.

3. Types are arbuscular mycorrhizae, ectomycorrhizae, ericoid mycorrhizae,
arbutoid mycorrhizae and orchid mycorrhizae.
Arbuscular mycorrhizae (often called AM) are the most common and
widespread of all mycorrhizae and are found in as many as 85%-90% of the
world's plant species.
-the fungus occurs inside the cells of the plant root as a highly branched
shrubby structure called an arbuscule.
-the arbuscule fills the root cell it actually occurs between the root cell wall and
the cell membrane within, Thus the fungus never comes into direct contact with
the root cell nucleus, mitochondria or other cell structures.
Mycorrhizal associations produced by Glomeromycotan fungi are known as
arbuscular mycorrhizas, or vesicular-arbuscular mycorrhizas (formerly also
endomycorrhizas, or endotrophic mycorrhizas) and are abbreviated as VAM

4. Absorptive hyphae
Thin highly branched hyphae which are thought to absorb nutrients.
Arbuscules
intricately
branched
haustoria in
cortex cells.
Vesicles
storage
structures
formed by
many fungi.

5. AM fungi are obligate biotrophs and rely on their autotrophic host to complete their life
cycle and to produce the next generation of spores.The spores are able to germinate
without the presence of a host, but the spores respond with an increase in hyphal
branching and metabolic activity to root exudates. Plant roots release for example
strigolactones that are able to induce pre-symbiotic growth of AM fungal spores.
On the host root surface, AM fungi form a specific appressorium – the hyphopodium.
Fungal hyphae emerging from this hyphopodium penetrate into the root through the
prepenetration apparatus, which guides the fungal hyphae through the root cells toward
the cortex. In the cortex the hyphae enter the apoplast, and grow laterally along the root
axis, and penetrate into inner root cortical cells. In ‘typical’AM associations of the
‘Arum type’ enters the fungus the cell by small hyphal branches that by continuous
dichotomous branching develop into characteristic highly branched arbuscules

6. 1. one or more hyphae produce swellings called appressoria between epidermal cells
2. Aseptate hyphae spread along the cortex in both directions from the entry point to
form a colony.
3. Gallaud (1905) observed that VAM associations in different species formed two
distinctive morphology types, which he named the Arum (linear) and Paris
(coiling)series after host plants.
4. Arbuscules (little trees) are intricately branched haustoria that formed within a root
cortex cell.
5. Vesicles are hyphal swellings in the root cortex that contain lipids and cytoplasm.

7. By contrast, in ‘Paris type’ mycorrhizas spreads the fungus primarily from cell to cell
and develops extensive intracellular hyphal coils that sometimes show an arbuscular
like branching . The fungus does not enter the plant symplast and is excluded from the
host cytoplasm by the enlarged periarbuscular membrane (PAM) of the host. Some
fungi also form vesicles, fungal storage organs in the root apoplast.
Despite its coenocytic nature, the mycelium that is formed within the root, the
intraradical mycelium (IRM) differs morphologically and functionally from the
extraradical mycelium (ERM), the mycelium that grows into the soil. The ERM absorbs
nutrients from the soil and transfers these nutrients to the host root. The IRM on the
other hand releases nutrients into the interfacial apoplast and exchanges them against
carbon from the host. The fungus uses these carbon resources to maintain and to enlarge
the ERM, for cell metabolism (e.g. active uptake processes, nitrogen assimilation), and
for the development of spores, which are able to initiate the colonization of a next
generation of host plants

8. Arbuscules are intricately branched haustoria that formed within a root cortex
cell. They were named by Gallaud (1905), because they look like little trees.
Arbuscules are formed by repeated dichotomous branching and reductions in
hyphal width, starting from an initial trunk hypha (5-10 um in diameter) and
ending in a proliferation of fine branch hyphae (< 1 um diameter).
Arbuscules start to form approximately 2 days after root penetration (Brundrett
et al. 1985). They grow inside individual cells of the root cortex, but remain
outside their cytoplasm, due to invagination of the plasma membrane.
Arbuscules are considered the major site of exchange between the fungus and
host. This assumption is based on the large surface area of the arbuscular
interface, but has not been confirmed

9. Vesicles develop to accumulate storage products in many VAM associations.
Vesicles are initiated soon after the first arbuscules, but continue to develop
when the arbuscules senesce. Vesicles are hyphal swellings in the root
cortex that contain lipids and cytoplasm. These may be inter- or intracellular.

10. Mycorrhizas produced by Glomus
species Mycorrhizas produced by Scutellospora
and Gigaspora species

11. Spores form as swellings on one or more subtending hypha in the soil or in
roots. These structures contain lipids, cytoplasm and many nuclei. Spores
usually develop thick walls with more than one layer and can function as
propagules. Spores may be aggregated into groups called sporocarps.
Sporocarps may contain specialised hyphae and can be encased in an outer
layer (peridium). Spores apparently form when nutrients are remobilised from
roots where associations are senescing. They function as storage structures,
resting stages and propagules. Spores may form specialised germination
structures, or hyphae may emerge through the subtending hyphae or grow
directly through the wall

14. Carbon is transferred from the plant to the fungus inside the roots as hexose
and this is made into short term fungal storage carbohydrates (glycogen and
trehalose). The fungus makes triacylglycerol (TAG) in the root which is exported
within the fungus to the external mycelium and to fungal mycelium in other roots
(but carbon is not transferred to roots). TAG is broken down in the external
mycelium and the glyoxylate cycle is used to produce storage and structural
carbohydrates (cell walls) as well as energy via the TCA cycle and other
products including amino acids (AA).

15. Inorganic N (NO3
– and NH4
+) is taken up by the external mycelium, assimilated and
converted to arginine, which is transported (probably in association with Polyphosphate)
within the fungus to the fungal mycelium inside plant roots. There the arginine is broken
down to release ammonium which is transferred to the plant without carbon. More
recently we have identified most of the fungal metabolic genes involved in N movement
and begun to characterize the regulation of the pathway.

16. Mycorrhizal fungi are composed of fine, tubular filaments called hyphae
(singular hypha). The mass of hyphae that forms the body of the fungus is
called the mycelium (plural mycelia). There are two major classes of
mycorrhizal fungi: ectotrophic mycorrhizae and vesicular-arbuscular
mycorrhizae . Ectotrophic mycorrhizal fungi typically show a thick
sheath, or “mantle,” of fungal mycelium around the roots, and some of the
mycelium penetrates between the cortical Cells. The cortical cells
themselves are not penetrated by the fungal hyphae but instead are
surrounded by a network of hyphae called the Hartig net.

17. Often the amount of fungal
mycelium is so extensive that its
total. Fungal mycelium also
extends into the soil, away from
this compact mantle, where it
forms individual hyphae. The
capacity of the root system to
absorb nutrients is improved by
the presence of external fungal
hyphae that are much finer than
plant roots and can reach
beyond the areas of nutrient-
depleted soil near the roots

19. Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations
differ in their structural characteristics and in the plant and fungal species
that they involve. In AM roots the fungus penetrates intercellularly and
intracellularly into the root cortex, whereas in ECM roots the fungus
only penetrates intercellularly into the root cortex.
AM fungi are coenocytic and hyphae and spores contain hundreds of
nuclei.

21. Nutrient uptake pathways in arbuscular mycorrhizal or ectomycorrhizal roots
Mycorrhizal plants can take up nutrients from the soil via two pathways: the ‘plant
pathway’ that involves the direct uptake of nutrients from the soil by the root epidermis
and its root hairs or the ‘mycorrhizal pathway’ that involves the uptake of nutrients
via the ERM of the fungus and the transport to the Hartig net in ECM interactions or to
the IRM in AM interactions, and the uptake by the plant from the interfacial apoplast.
The uptake of nutrients from the soil via the plant pathway, however, is often limited by
the low mobility of nutrients in the soil. The mobility of for example phosphate (P) is so
low that its uptake leads rapidly to the development of depletion zones around the roots
and limits the further P uptake via the plant pathway to the low rate of diffusion

22. AM roots do not form a fungal
sheath and can theoretically
use both pathways for
nutrient uptake

23. Characteristic ECM symbiosis AM symbiosis
Fungal life style Facultative saprotroph Obligate biotroph
Structural components
Mantle, Hartig net, and ERM
with or without rhizomorphs
Arbuscules or intercellular
hyphal coils, ERM, vesicles in
some types
Penetration Exclusively intercellularly
Intercellularly and
intracellularly
Nutrient uptake pathway
ECM roots represent a
significant proportion of the
nutrient absorbing surface and
nutrient uptake predominately
via the mycorrhizal pathway
Theoretically plant and
mycorrhizal pathway, but
mycorrhizal pathway can
dominate nutrient uptake in
mycorrhizal roots
Contribution to plant nutrition
Particularly important for N
nutrition, but also significant
contributions to P nutrition
Particularly important for P
nutrition; contributions to N
nutrition still under debate
Fungal nutrient resources
Efficient uptake of inorganic
and organic nutrient resources
Uptake predominately of
inorganic nutrient resources,
utilization of organic nutrient
resources considered to be small

24. Uptake of phosphate from the soil
•The exploration of large soil volumes by the ERM in which orthophosphate
(Pi) is scavenged and delivered to plant cortical cells, bypassing the plant
pathway for P uptake.
•The small hyphal diameter that allows the fungus to penetrate into small soil
cores in search for P, and higher P influx rates per surface unit
•The capability of mycorrhizal fungi to store P in form of polyphosphates,
which allows the fungus to keep the internal Pi concentration relatively low,
and allows an efficient transfer of P from the ERM to the IRM and
•The production and secretion of acid phosphatases and organic acids that
facilitate the release of P from organic complexes