Mycorrhizae:
The roots of most species of seed plants have a symbiotic relationship with soil fungi in which both organisms benefit. The associations are known as mycorrhizae. Part of the fungal mycelium is in the host tissue, and part is in contact with the surrounding soil and usually spreads in it. Mycorrhizal associations increase the area of contact of the roots with the soil. The extension of hyphae into the soil also makes the soil more stable. It is likely that the great majority of land plants enter into this kind of relation with soil fungi and sometimes the host cannot thrive without the benefits derived from the fungus. They are of great significance because they are probably the major route of entry of water and nutrients into roots especially when the plant is growing in nutrient-limiting soils.
A very diverse array of mutualistic associations exist between the fungi and the roots of plants. The fungus receives organic nutrients, mainly carbohydrates and vitamins from the plant and in return absorbs mineral salts and water which can pass to the plant root. Generally only young roots are infected. Root hair production either ceases or is greatly reduces in infection. Host growth is almost always enhanced compared to uninfected plants of the same species.
On morphological and anatomical features such mycorrhizas can be divided into four principle types.
i) Vesicular-Arbuscular Mycorrhizas(VAM)
They are by far the commonest of all mycorrhizas and are found in Bryophytes, Pteridophytes, Gymnosperms and virtually in all families of angiosperms.
The name vesicular arbuscular derives from structures formed by the infecting fungus in host tissue. Fungus hyphae invade the cell but do not break the host plasma membrane. Inside the cell they branch repeatedly forming dichotomously branched projections called an arbuscle. Vesicle are terminal or sometimes intercalary swellings which may either be between host cells or within them. Vesicles have been ascribed a storage and reproductive function. Within the plant, the mycelium develops only in primary cortex and epithelium of the root and does not penetrate the endodermis into the conducting tissues within the stele. Hyphae within the root are connected to an external soil mycelium. The infection does not alter the morphology of the root or its external appearance and root hairs may be formed.
The group of fungi forming VAMs belong to a single family, the Endogonaceae of Zygomycotina.
ii) Ectomycorrhizas:
Ectomycorrhizas are believed to be more common in temperate zones of the world, where there are seasonal climatic changes, than in the tropics; these mycorrhizas are mainly formed on roots of woody plants. In such mycorrhizas, the ultimate absorbing rootlets of the root system are completely surrounded by a distinct mantle or sheath of fungal tissue which also encloses the root apex. Fungal hyphae extend inwards from the sheath into the outer two three cell layers of the cortex forming an intercellular network of hyphae known as the " Hartig net".
iii) Ericaceous Mycorrhizas:
The order Ericales are mostly woody shrubs or trees and form ecologically important plant communities particularly on moors, swamps and on peat.
Three different types of mycorrhizas are formed by the Ericales.
a) Ericoid mycorrhizas:
These occur on plants which have very fine, hair like roots and cortex is composed of only a few cells. In the mature phase of the infection, every cell of the cortex is filled with extensive coils of hyphae. There are numerous hyphal connections between the root and soil surface.
b) Arbutoid mycorrhizas:
These occur on the members of the Arbutoideae and Pyrolaceae. The roots are infected in a manner resembling ectomycorrhizas in that they are enclosed in a fungal sheath. A hartig net is also formed, but this differs from that of ectomycorrhizas by its restrictions to the outer epidermal layer and by abudant penetration into cells by coils of hyphae.
c) Monotropoid mycorrhizas:
Monotropa plant is usually herbaceous and lacks chlorophyll. They have mycorrhizas which facilitate transfer of carbon compounds from the roots of photosynthetic host plants.
Monotropoid mycorrhizas resemble arbutoid in having a sheath and a Hartig net limited to the epidermal layer, but differ in that the penetration into host cells are in the form of simple pegs.