Mycorrhiza: Meaning And Types
Definition
A mycorrhiza is a symbiotic association between a green plant and a fungus. The
plant makes organic molecules such as sugars by photosynthesis and supplies
them to the fungus, and the fungus supplies to the plant water and mineral
nutrients, such as phosphorus, taken from the soil. Thus, mycorrhizae are the
symbiotic associations between plant root and fungi, with bidirectional nutrient
exchange between the partners.
Mycorrhizas are located in the roots of vascular plants, but mycorrhiza-like
associations also occur in bryophytes and there is fossil evidence that early land
plants that lacked roots formed arbuscular mycorrhizal associations. Most plant
species form mycorrhizal association, though some families
like Brassicaceae and Chenopodiaceae cannot. Different forms for the
association are detailed in the next section. The autotrophic host plant acts as the
carbon source for the fungus, while the fungus supplies mineral nutrients to the
plant. About 90% of all land plants are associated with mycorrhiza. The most
common is the arbuscular type that is present in 70% of plant species, including
many crop plants such as wheat and rice.
Vitadini (1842) was the first to recognise the possible beneficial role of fungal
mycelia which mantle the root of higher plants, and this association is named as
mycorrhiza (pl. mycorrhizae) i.e., the fungal root, by Frank (1885).
Types
Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas.
The two types are differentiated by the fact that the hyphae of ectomycorrhizal
fungi do not penetrate individual cells within the root, while the hyphae of
endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane.
Endomycorrhiza includes arbuscular, ericoid, and orchidoid mycorrhiza, while
arbutoid mycorrhizas can be classified as ectendomycorrhizas.
Monotropoid mycorrhizas form a special category.
Types of Mycorrhiza:
(1) Ectomycorrhizae:
Ectomycorrhiza is commonly called “sheathing mycorrhiza”. Ectomycorrhizas
consist of a hyphal sheath, or mantle, covering the root tip and a Hartig net of
hyphae surrounding the plant cells within the root cortex. Outside the
root, ectomycorrhizal extramatrical mycelium forms an extensive network within
the soil and leaf litter. They occur in 3% of all seed plants in forests of temperate
regions, especially on pine, beech, spruce, birch etc.
Generally, they cause extensive branching and growth of roots and modification
of branching pattern, such as racemose type in dicots (Fagus) and dichotomous
in gymnosperms (Pinus). In beech (Fagus) the ultimate lateral rootlets are
differentiated into ‘long’ and ‘short’ roots.
The long roots show indefinite growth and their branches are the short roots that
are thickened, forked and mycorrhizal. They appear in various colours like white,
brown, yellow, black etc., depending on the colour of the fungus. The fungus
enters the cortex forming ‘Hartig net’, but never goes inside the endodermis or
stele. They form a mantle of varying thickness (Fig. 4.103A, B).
Majority of the fungi belongs to Agaricales of Hymenomycetes under
Basidiomycotina. More than 100 species of toadstools are reported to form
mycorrhiza. Most of the members are belonging to the genera Amanita,
Tricholoma, Boletus, Russula, Lactarius etc.
Members of Gasteromycetes under Basidiomycotina like Rhizopogon and
Scleroderma are involved in this process. Some members of Ascomycotina like
Gyromitra esculenta, all species of Tuber (T. melanospora) form mycorrhizae.
(2) Vesicular-arbuscular mycorrhizae (VAM) or Arbuscular mycorrhizas
(AM):
It is a type of endomycorrhizal association, where both vesicles and arbuscles are
developed together. VAM is by far the commonest of all mycorrhizae and has
been reported in more than 90% of land plants. Arbuscular mycorrhizas, or AM
(formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas
whose hyphae penetrate plant cells, producing structures that are either balloon-
like (vesicles) or dichotomously branching invaginations (arbuscules) as a means
of nutrient exchange. The fungal hyphae do not in fact penetrate
the protoplast (i.e. the interior of the cell), but invaginate the cell membrane. The
structure of the arbuscules greatly increases the contact surface area between the
hypha and the cell cytoplasm to facilitate the transfer of nutrients between them.
They are found in bryophytes, pteridophytes, gymnosperm (except Pinaceae) and
most of angiosperms, commonly in Leguminosae (Fabaceae), Rosaceae,
Gramineae (Poaceae) and Palmae (Arecaceae). VAM is not found in Ericaceae
and Orchidaceae, where other type of association is available. VAM has even
been reported in Lower Devonian plant, Rhynia.
Arbuscular mycorrhizas are formed only by fungi in
the division Glomeromycota. The important genera involved in VAM are
Glomus, Gyrospora, Acaulospora etc. Most of the members are not culturable.
The VAM is so named because of the presence of two characteristic structures
i.e., vesicles and arbuscles:
(i) The vesicles are thin or thickwalled vesicular structures produced intra-
cellularly and stored materials like polyphosphate and other minerals (Fig. 4.104).
(ii) The arbuscles are repeated dichotomously branched haustoria which grow
intracellularly (Fig. 4.104). The arbuscles live for four days and then get lysed
releasing the stored food as oil droplets, mostly polyphosphate.
There is no fungus mantle, but only a loose and very sparse network of septate
hyphae spread into the soil. These hyphae bear different types of spores,
chlamydospores, or aggregation of spores in sporocarp or zygospores. The super-
ficial hyphae bear branches that penetrate the epidermis and then grow
intercellularly only in cortex.
Intercellular hyphae form arbuscles inside the parenchyma of cortex by repeated
dichotomous branching of the penetrating hyphae. The cell membrane of the
penetrated cell is invaginated and covers the arbuscles.
The hyphae also develop both inter- and intracellular thick-walled vesicles. The
chlamydospores may germinate on nutrient agar, but the hyphae stop growing
when food inside the spore is used up, thus they cannot be subcultured.
This type of association was present very early in the evolution land plants.
Kidston and Lang (1921) reported the VAM-like organism with Rhynia and
Asteroxylon. Later, Pyrozinski and Mallock (1975) proposed the
mycorrhization/lichenisation association as a prelude to land plant evolution.
(3) Ericoid mycorrhizae:
This is actually a type of endomycorrhiza. Ericoid mycorrhizae are found in the
different members of Ericaceae like Erica, Calluna, Vaccinum, Rhododendron
etc. The fungi are slow-growing, septate and mostly sterile. They are mostly
culturable. Both Pezizella ericae (Ascomycotina) and Clavaria vermiculata
(Basidiomycotina) have been isolated from Rhododendrons.
During this association the rootlets of the plants are covered by very sparse, loose,
dark, septate hyphae that penetrate the cortex forming intercellular coils (Fig.
4.105). After 3-4 weeks the coils degenerate like arbuscles of vesicular-
asbuscular mycorrhiza (VAM).
Most of the members of Ericaceae grow in acid soil with less amount of P and N
nutrition. The fungus gets the photosynthate from the host and improves the
mineral uptake and nutrition of the host, especially P and N. Many mycotrophs
of Ericaceae show high resistance to metals like Zn and Cu. The mycorrhizal
plants also show high tolerance to these metals, which is totally absent in non-
infected plants.
(4) Ectendomycorrhizae (Arbutoid):
Some members of the family Ericaceae and members of other families of Ericales
have mycorrhizae intermediate in form between ecto- and endomycorrhizae
types, called ectendomycorrhizae. Arbustus and Arctostaphylos of Ericaceae
show this type of mycorrhizal association.
In Arbustus, the root system is differentiated into long and short roots. The short
roots are swollen and covered by hyphal mantle. Hartig net is absent in this
association, but intercellular coils develop in the outer cortical cells. Nothing is
known about the fungi involved in this association.
(5) Gentianoid mycorrhizae:
Seedlings of some members of Gentianaceae (Biackstonia perfoliata, Gentianella
amarella, etc.) get infected within 2 weeks of germination. In root, the cortical
cells become full of irregular coils of aseptate hyphae. With time the hyphae
become lysed. Vesicles are occasionally seen attached to these coils.
(6) Orchidoid mycorrhizae:
Orchids produce millions of tiny seeds per capsule, weighing about 0.3-14µg.
The embryo of seeds contains 10-100 cells and there is virtually no storage of
food. The embryo is encircled in a thin-walled net-like testa that helps in their
dispersal.
Thus, majority of seeds are unable to germinate without exogenous supply of
carbohydrates. Therefore, mycorrhizal association is obligatory for the seeds to
germinate. The fungus provides C-nutrition to the seeds.
Initially the fungus enters the embryo and colonises, being restricted to the
cortical cells and provides the nutrition (Fig. 4.106). For non-green orchids, this
is obligatory throughout their lives. Apparently, it is a case of parasitism by
orchids on the mycorrhizal fungi.
Fungi like Rhizoctonia (Basidiomycotina), are recognised by hyphal
characteristics. Corticium, Ceratobasidium etc., of Aphylloporales are associated
in this type of mycorrhiza.
(7) Monotropoid mycorrhizae:
Monotropa hypopitys is a non-green saprophytic herb that is entirely dependant
on ectomycorrhizal fungi linked to nearby trees It has short fleshy roots that are
invested with a hyphal sheath and often forming Hartig net in the cortical zone.
Due to absence of chlorophyll, they are unable to synthesise and supply
carbohydrate to the fungus. Boletus is a mycorrhizal fungus associated with roots
of both pine and Monotropa.
When 14C glucose was injected into the phloem of Pinus trees, significant amount
of radioactive glucose (14C) was recorded in Monotropa (but not in other herbs)
after 5 days. This indicates that the fungus Boletus acts as a bridge between
Monotropa and Pine plants.
Similarly, 32P injected into Monotropa was also detected in Pine roots within 2
hours. The above facts indicate a bidirectional flow of nutrients between the
plants through the fungus Boletus.
References:
https://en.wikipedia.org/wiki/Mycorrhiza
https://mycorrhizas.info/
https://www.biologydiscussion.com/fungi/mycorrhiza-meaning-types-and-role/46596
