Download mycorrhizae-study material-2012

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MYCORRHIZA
MYCORRHIZA
Mycorrhiza (plural : Mycorrhizae) refers to an association or symbiosis between plants and fungi that colonize
the cortical tissue of roots during periods of active plant growth. The association is characterized by the
movement of: i) plant produced carbon to the fungus and ii) fungal acquired nutrients to the plants.
The term Mycorrhiza means fungus root was first applied to fungus tree associations described in 1885 by the
German forest pathologist A.B Frank. Now, it is known that vast majority of land plants form symbiotic
associations with fungi. The mycorrhizal conditions are the rule among the plants, not the exception.
The benefits to the plants from mycorrhizal symbiosis can be :i) agronomically increase in growth and yield and ii)
ecologically improved fitness. Mycorrhizal fungi usually proliferate both in root and in the soil. The soil borne or
extra metrical hyphae take up nutrients from the soil and transport them to the root. By absorptive mechanism
mycorrhizae increases the effective absorptive surface area of the plant. In nutrient poor or moisture deficient
soils, nutrients taken up by the extrametrical hyphae can lea to improved plant growth and reproduction. As a
result mycorrhizal plants are often more competitive and better able to tolerate environmental stress than non
mycorrhizal plants.
TYPES OF MYCORRHIZAL ASSOCIATIONS:
1. ECTOMYCORRHIZAE (EM): The feature of this is the presence of hyphae between root cortical cells,
producing a net like
structure called the
Hartig net, after Robert
Hartig who is considered
the father of forest
biology. Many EM also
have a sheath or mantle
of fungal tissue that may
completely cover the
absorbing root (usually
fine feeder roots). The
mantle can vary widely
in thickness, colour and
texture depending on
particular plant fungus
combinations. The roots
often affect fine root
morphology resulting in
root bifurcation and
clustering. Contiguous
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MYCORRHIZA
with mantle are hyphal strands that extend into the soil. Often hyphal starnds will aggregate to
rhizomorph that may be visible to naked eye. The internal portion of rhizomorph can differentiate into
tube like structures specialized for long distance transport of nutrients and water.
Ectomycorrhizae are found on woody plants ranging from scrubs to forest trees. Many of host plants
belong to the families Pinaceae, Fagaceae, Betulaceae and Myrtaceae. Over 4000 species belonging
primarily to the Basidomycotina and fewer to the Ascomycotina are known to form ectomycorrihae.
Many of these produce mushrooms and puff balls on the forest floor.
2. Arbuscular Mycorrhizae(AM ): The diagnostic feature of arbuscular mycorrhizae (AM) are the
development of a highly branched arbuscles within root cortical cells. The fungus initially grows between
cortical cells but soon
penetrate the root cells.
The general term for all
mycorrhizal types where
fungus grows within
cortical
cells
is
ENDOMYCORRHIZA. In
this association neither
the fungus cell wall nor
the host cell membrane
are breached. As the
fungus grows, the host
cell
membrane
invaginates and envelops
the fungus, creating a
compartment
where
material
of
high
molecular complexity is
deposited. This space
presents direct contact
between
plant
and
fungus cytoplasm and allows for efficient transfer of nutrients between the symbionts. The arbuscles are
relatively short –lived, less than 15 days and are often difficult to see in field collected samples.
Other structure produced by the AM includes vesicles, auxillary cells and asexual spores.
Vesicles: These are thin walled, lipid filled structures that usually form in intracellular spaces. Their primary
function is thought to be for storage; however, vesicles can also serve as reproduced propagules for the fungus.
Auxillary Cells: These are formed in the soil and can be coiled or knobby. The function of this not known.
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MYCORRHIZA
Asexual spores: These are formed either in the root or more commonly in the soil. Spores produced by the fungi
forming AM associations are asexual forming by the differentiation of vegetative hyphae. For some fungi a
vesicle in the root undergo secondary thickening and septum is laid down across the hyphal attachment leading
to spore formation , but more often spore develop in the soil from hyphal swelling.
The fungi that form AM are currently classified in the order Glomales. The taxonomy is further divided
into suborders based on the presence of: i) vesicles in the root and formation of Chlamydospores (thick walled,
asexual spore) borne from subtending hyphae-Suborder, Glominea.
ii) absence of vesicles in the root and the formation of auxillary cells and azygospores (spores resembling a
zygospore but developing asexually from subtending hypha resulting in a bulbuous attachment) in the soil.
Suborder-Gigasporinea.
The term vesicular Arbuscular mycorrhizae (VAM) was originally applied to symbiotic associations formed to all
fungi Glomales, but because a major suborder lacks the ability to form vesicle in roots, AM is now preferred
acronym. The Glomales is further divided into families and genera according to the method of spore formation.
The spores of AM fungi are very distinctive. They range from 10µm to 100 µm. They vary in colour.
AM type of symbiosis is very common as the fungi involved can colonize a vast taxonomic range.AM fungi differ
widely in the level of colonization they produce in a root system and in their impact on nutrient uptake and
plant growth.
3).Ericaceous Mycorhizae: The term EC is applied to mycorrhizal associations found on plants in the order
Ericales.The hyphae in the root can penetrate cortical cells (endomycorrhizal habit); however, no arbuscles
are found. Three major forms of Ericaceous mycorrhizae have been described.
a) Ericoid Type: Cells of inner cortex become packed fungal hyphae. A loose welt of hyphae grows over the root
surface, but a true mantle is not formed. The Ericoid mycorrhizae are found on plants such as Calluna (
heather),Rhododendon.
b) Arbutoid Type: Characteristic of both ecto and endomycorrhizae are found. Intracellular penetration can
occur, a mantle forms and a Hartig net is present. These associations are found in Artutans and several species
of pyrolaceae. The fungi involved in the associations are basidomycetes and may be identical to the fungi that
colonize Ectomycorrhizae.
c) Monotrophoid Type: The fungi colonize achlorophyllous plants in Monotrohaceae,producing the Hartig net
and mantle. The same fungi also form Ectomycorrhizal associations with trees and therby form a link through
which carbon and other nutrients can flow from the autotrophic host plant to the heterotrophic parasitic plant.
4). Orchidaceous Mycorrhizae: Mycorrhizae fungi have a unique role in the life cycle of plants in the
Orchidaceae. Orchids typically have very small seed with little nutrient reserve. The plants becomes colonized
shortly after germination and the mycorrhizal fungus supplies carbon and vitamins to developing embryo. For
achlorophyllous species , the plants depends on the fungal partner to supply carbon throughout life. The fungus
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grows into the plant cell, invinating the cell membrane and forming hyphal coils with in the cell. These coils are
active for only a few days, after which they lose turgor and degenerate and the nutrient contents are absorbed
by the developing Orchid. The fungi participating in the symbiosis are basidomycetes. In mature Orchids,
mycorrhizae also have roles in nutrient uptake and translocation.
5).Mixed Infections: Several fungi can colonize roots of a single plant , but the type of a mycorrhiza formed is
usually depends on the host. E.g.Alnus(Alder),Salix(Willow).
Benefits to Plants:
1 Mycorrhiza improves overall plant growth by improving Phosphorus and Zinc nutrients.
2 They allow more e efficient use of Phosphorus and Zinc in fertilizer.
3 They also stimulate nitrogen fixation in nodulated plants by increasing Phosphorus flow through plants
roots.
4 They increase disease tolerance in plants by improving the plant nutrition and by competing with
pathogenic microorganisms for the space on the plant root.
5 They immobilize some heavy metals such as Zinc, Cadmium and Manganese.
6 They improve water use and drought tolerance.
7 They improve soil structures by helping to bind soil aggregates together.
8 Maintain a healthy physiological interaction between plants and the fungus
9 Increase longevity of feeder roots.
10 Increase rates of nutrient absorption from soil.
11 Help in selective absorption of certain ions from soil.
12 Increase tolerance to toxins.
13 Increase tolerance to environmental conditions such as pH temperature.
Benefits to Fungi: Mycorrhizal fungi derive plant carbon.
Cultural Characteristics of Ectomycorrhizal Fungi
The ectomycorrhizal fungi can easily be isolated in the vegetative form although the identification of such fungi
becomes difficult since reproductive bodies are not readily formed in culture media. Fruiting bodies can,
however, be seen on the soil surface near the trees, from which fungal cultures can easily be isolated. These
fungi grow slowly in culture and require special nutrients such as thiamine, simple amino acids and other
undefined constituents (collectively known as the M-factor) of root exudates. Melin and his associates in sweden
have shown that the M-factor is exuded by roots of plants susceptible to mycorrhizal infection and not by plants
resistant to it. Experiments with Boletus variegatus have shown that low doses of M-factor are stimulatory to
growth of the fungus whereas high doses become inhibitory.
The inhibitory portions of M-factor appear to be excessive in old secondarily thickened axils or roots while the
stimulatory portions appear to be so in the primary rootlets. Ectomycorrhizal fungi are generally not cellulolytic
or lignolytic and therefore have to depend on carbohydrates from their host plants. Experiments with 14C
labelled sucrose, glucose, fructose and 14CO2 have shown that most of the carbon requirements of the fungus
come from the host plant by way of root excretions. Metabolites produced by the fungus influence the structure
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and morphology of the root system. These substances include auxins such as indole acetic acid and other
unknown growth substances. They are partly responsible for the dichotomy of pine rootlets. In this way, the
fungus and the host plant mutually control the morphological and physiological activities of the symbiotic
system. The carbon nutrition of the fungus is dependent on the photosynthetic activity of the host which is
balanced by the greater efficiency of absorption and storage of nutrients afforded by the fungal partner
Resistance to Plant Diseases by Ectomycorrhiza
Feeder root pathogens such as Phytophthora, Pythium, Rhizoctonia and Fusarium infect immature and
meristematic cortical tissues of roots and cause necrosis. However, one of the physiological benefits of
ectomycorrhizae is the protection afforded by the fungal mantle against such root pathogens. Well formed
mycorrhizal roots are resistant to infection and non-mycorrhizal feeder roots are prone to fungal necrosis even
when adjacent roots have become mycorrhizal. The resistance is purely due to the mechanical barrier afforded
by the mycorrhizal fungal mantle.
However, species of certain fungal genera causing ectomycorrhizae such as Lactarius, Cortinarius and
Hygrophorus produce antibiotic substances while species of Russula produce none at all. Some of these
antibiotics are antifungal on Rhizoctonia salani, Pythium debaryanum and Fusarium oxyporum. Nevertheless, it
remains to be seen whether antibiotics are elaborated by ectomycorrhizal fungi in vivo in association with the
higher symbionts.
Boletus variegatus is known to produce volatile fungistatic compounds in pure culture. They have been identified
as isobutanol and isobutyric acid. Infection of roots of Pinus sylvestris with B. variegatus resulted in the
production and accumulation of volatile and fungistatic terpenes and sesquiterpenes to the extent of eight times
the concentrations of such compounds in non-mycorrhizal roots.
In this connection, it is relevant to point out that tubers of several species of orchids produce orchinol, coumarin,
hircinol and other phenolic compounds as a defense reaction to the presence of fungi such as Rhizoctonia. These
substances were not detected in tubers of orchids free of fungal symbionts. Very likely, substances such as
orchinol act as deterrents to pathogenic fungi by restricting the activity of certain fungi such as Rhizoctonia more
to a symbiotic state than to a parasitic state.
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Pass word: mycorrhiza