MYCORRHIZA

DISCOVERY:
Associations of fungi with the roots of plants have been known since at least the mid-19th century. However early observers simply recorded the fact without investigating the relationships between the two organisms. This symbiosis was studied and described by Franciszek Kamieoski in 1879–1882. Further research was
carried out by Albert Bernhard Frank, who introduced the term mycorrhiza in 1885.
 DEFINITION:
Mycorrhizas are symbiotic relationships between fungi and plant roots (the term means literally 'fungus
root'). Perhaps more than 80% of the species of higher plants have these relationships, and so do many
pteridophytes (ferns and their allies) and some mosses (especially liverworts). They are as common on crop plants (cereals, peas, tomatoes, onions, apples, strawberry, etc) as in wild plant communities, and in several cases they have been shown to be important or even essential for plant performance.


TYPES OF MYCORRHIZA
ARBUSCULAR MYCORRHIZA is formed only by fungi in the division Glomeromycota. The hyphae of
arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of
carbon in the soil.
ECTOMYCORRHIZA or EcM, are typically formed between the roots of around 10% of plant families, mostly
woody plants including the birch, dipterocarp, eucalyptus, oak, pine, and rose families, orchids, and fungi
belonging to the Basidiomycota, Ascomycota, and Zygomycota. 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.

ERICOID MYCORRHIZA is the third of the three more ecologically important types. They have a simple
intraradical (grow in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells.
Ericoid mycorrhizas have also been shown to have considerable saprotrophic capabilities, which would
enable plants to receive nutrients from not-yet-decomposed materials via the decomposing actions of their ericoid partners.

ARBUTOID MYCORRHIZA involves plants in the subfamily Arbutoideae. It is however different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved.

MONOTROPOID MYCORHIZA occurs in the subfamily Monotropoideae . These plants are heterotrophic or
mixotrophic and derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis.

ORCHID MYCORRHIZA All orchids are myco-heterotrophic at some stage during their lifecycle and form
orchid mycorrhizas with a range of basidiomycete fungi. Their hyphae penetrate into the root cells and form typical coils.

FUNCTIONS OF MYCORRHIZA
 
INCREASED ABSORPTION
The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much
smaller in diameter than the smallest root, and thus can explore a greater volume of soil, providing a larger surface area for absorption.

DISEASE AND DROUGHT RESISTANCE
Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne
pathogens.

COLONIZATION OF BARREN SOIL
Plants grown in sterile soils and growth media often perform poorly without the addition of spores or
hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients.
 
RESISTANCE TO TOXICITY
Fungi have been found to have a protective role for plants rooted in soils with high metal concentrations,
such as acidic and contaminated soils.