2. INTRODUCTION :
Mycorrhizae are a symbiotic association between plant roots
and fungi.
Supporting plants under biotic or abiotic stress.
Mycorrhiza including vesicular Arbuscular Mycorrhiza and
arbuscular Mycorrhiza, ectomycorrhizal, ericoid mycorrhiza and
orchid mycorrhiza.
A coarse, intercellular, aseptate, coenocytic mycelium within the
root tissues may develop large balloon-shaped intercalary or
terminal thick walled vesicles which are multinucleate and
contain large amount of lipids.
Vesicles of Glomus species
3. CONTINUE…
Hphae penetrating host cells fork repeatedly form richly branched
arbuscules(Roots of Paris).
Janse (1897) called the intramatric spores “vesicules” and Gallaud
(1905)determined that other structures, named “arbuscules”. Thus
the name “vesicular-arbuscular mycorrhiza”.
The recognition that not all fungi formed vesicles led to the
proposal that this symbiosis should be renamed arbuscular
mycorrhiza.
Members of the Glomineae (such as Glomus, Acaulospora) form
intraradical vesicles (VAM type), whilst members of the
Gigasporineae have no intraradical vesicle (AM type).
Mature Arum-type arbuscule of
Glomus mosseae
Paris-type intracellular coils of
Glomus intraradices
4. PHYSIOLOGICAL STUDIES
(A)Colonization of Roots:
Glomus and Acaulospora colonized root from 3 inoculum sources
(spores, fresh root fragments with adhering hyphae and hyphal
fragments) Scutellospora and Gigaspora appear to depend entirely on
spores.
Spores (large thick resistant wall and numerous nuclei and are long
term survival structure) with some capacity to disperal to wind and
water and also animals.
Germination of spores :
Presence of root excudates ( Sesquiterepene, flavonoids) stimulate the
growth and branching of the mycelium and apparently converting it
into “infection ready” state.
Germination of Glomus clarum
5. CONTINUE….
Root fragments can be an important source of inoculum in
many soils. Reports suggest they can survive in dry soil for 6
months.
While infectivity is not related to presence of Vesicles. But
Vesicles itself act as propagules or supports the regrowth of
intercellular hyphae.
Hyphal networks in soil linked to established plants are of
Key importance in the colonization of seedlings in perennial
vegetation systems,
Hyphal
networks
6. ESTABLISHMENT OF COLONIZATION:
(A) Precolonization events.
AM colonization of roots can be initiated from hyphae growing
from any of the three sources of inoculum.
The main hyphae approaching a root branches profusely and give
to a characteristic fan-shaped complex of lateral branches
Development of a densely branched hyphal network on the
surface of the membrane immediately over the roots of host, but
not non-host plants.
(B) Contact and Penetration:
Hyphal contact with the root is usually followed by adhesion of the hypha
to the root surface and, after about 2–3 days, the formation of swollen
appressoria, followed by root penetration and formation of arbuscular
around 2 days later.
Appressoria of Glomus species
External hyphae and
appressoria of Glomus
species
7. CONTINUE…
(C) Development of Infection units.
Once inside the parenchyma, the fungus forms highly
branched structures for nutrient exchange with the plant called
arbuscules.
There are two forms: Paris type is characterized by the growth
of hyphae from one cell to the next; and Arum type is
characterized by the growth of hyphae in the space between
plant cells.
Formation of Vesicles: As individual infection units of all
types age, thick-walled vesicles may be formed, depending in
part on the identity of the fungus; members of the
Gigasporaceae (Scutellospora and Gigaspora) never develop
vesicles, but instead produce auxiliary Cells on the extraradical
mycelium.
Young arbuscule (red)
Cell wall (grey)
Plant cytoplasm (orange)
periarbuscular membrane (light grey)
Plant cell nucleus (blue)
8. (2) TRANSFER OF NUTRITION BETWEEN PLANT AND FUNGUS
1. Membrane transport of most metabolites can be
expected to be pH-dependent and to be powered by
the activity of plant (1) and fungal (2) H+ -ATPases.
2. Sucrose from the phloem is either cleaved by
apoplastic invertases and taken up by the plant (3) or
fungal hexose transporters or imported into root
cortical cells and cleaved there by a cytoplasmic
sucrose synthase (4).
3. The fungus transforms hexoses rapidly into trehalose,
which is either metabolized by the pentose phosphate
pathway, or used for the biosynthesis of glycogen and
lipids.
9. CONTINUE…
Fungal cell
Plant cell
Apoplast
4. These compounds are then exported to fungal
vesicles or to the external mycelium. The plant cell
takes up phosphate from the periarbuscular space
using specific, H+ -dependent plant phosphate
transporters (5).
5. Regarding nitrogen supply, AM-induced plant
nitrate transporters (6) have been found, suggesting a
similar transport mechanism as referring to
phosphate. On the other hand, the observation of
increased transcript levels of a fungal nitrate
reductase (7) suggests the transfer of nitrogen in a
reduced form (as ammonium or in an
organic form).
10. ECOLOGICAL STUDIES:
1. The supply of phosphate ( H2PO4 depending on soil pH) is often a limiting factor to plants growing in
natural soils. It is usually present in low concentrations and diffuses through soil very slowly. Its influx
may increase 3 to 4-fold in infected plant roots but here are also significant increases in other minerals
such as Zn, Cu, and ammonium.
2. The water relations and resistance of infected plants to infections by pathogens may also be improved.
3. Increased uptake of minerals is largely due to the exploration of larger volumes of soil by the
extramatrical hyphae which can extend beyond the depletion zone surrounding plant roots.
4. In nutrient-deficient soils such as sand dunes, recently disturbed soil, spoil heaps, areas covered by
volcanic ash, etc., successful colonization by plants appears to be correlated with root infection by
Glomales (Allen, 1991).
11. CONTINUE . ….
The roots of different plant species making up the
community are in close contact and may also be
connected by a hyphal network (Newman,1988).
There is experimental evidence using isotopically labelled
15N, 32P, and 14C that there may be an interchange of
mineral nutrients and carbon between unrelated plant
species mediated by VAM mycelia, but Newman (1988)
has cautioned against the conclusion that any increases
in labelled materials necessarily imply net gains to
receiver plants at the expense of donors.
Plants forming AM network
12. REFERENCE:
1. Smith S.E. and Read D.J. (1983) Mycorrhizal Symbiosis. Academic Press, London
2. Van der Heijden M. G. et al (2015) Mycorrhizal Ecology and Evolution: The past, the present and the future , New
Phytologist.
3. Koide R.T. , Mosse. B. (2004) A history of research on arbuscular mycorrhiza. Springer
4. Webster . J , Weber . R. W. (2007) Introduction to fungi . Cambridge 218-20
5. Hause B. , Fester T. (2004) Molecular and cell biology of arbuscular mycorrhizal symbiosis. Springer
6. https://en.m.wikipedia.org/wiki/Arbuscular_mycorrhiza
