In this article we will discuss about the definition of mycorrhiza.
The term ‘Mycorrhiza’ is a composite word made up of two words mycos meaning fungi, rhizos meaning roots. Fungi in association of roots of flowering parts form the non pathogenic or feebly pathogenic association which is generally mutually symbiotic.
In this association both the fungi and higher plants derive benefit from each other. When the other component is weak, the fungus becomes pathogenic. Both the components are associated in a relationships that is nutrition and often termed as mycotrophic.
Generally, this word should be applied to association of fungi with thalli of lower plants, rhizomes and roots of higher plants. The association of fungi with roots of higher plants should be termed as Mycorrhiza and that of fungi with thalli as mycothalli.
It is believed that roots provide an excellent habitat for the growth of these fungi. While the fungal presence might be essential for the growth of higher plants assisting in the mineral up take by the roots, the host plant supplies the nutritional requirements of the fungi.
In this way, this association helps both the partners. The important feature of mycorrhizal organs is that the plants which possess them do not suffer any serious disease, may be because of the presence of fungal hyphae.
Mycorrhiza appears to have been known to the mankind since fourth century B.C. Fossils of mycorrhizal association in Lepidodendron sp. (Clubmosses) appear to have occurred but no clear picture emerged except the presence of intracellular hyphae and a few infection points.
It was Frank (1885) who gave definite information about the composite fungus root organs of plants Cupuliferae. Frank also introduced the term mycorrhiza for this association.
He also showed that seeds of certain orchids germinated only in the presence of specific type of fungi. It was also observed that in most of the forest trees, mycorrhizas may be formed with a number of different fungi but in others, the association is specific.
However, in 1936, Burges established that mycorrhizal association is feebly parasitic and has no symbiotic value.
The physiological and biological significance of mycorrhizal association has been a matter of debate for a long time. Some botanists regarded fungal components as parasities upon the roots.
Others believed the relationship between the fungi and roots in this association is more complex than fungal parasitism and that both the roots and fungi benefit from each other.
Experimentally also it has been shown that mycorrhizal roots have greater absorption rate and disease resistance than the non-mycorrhizal ones.
Mycorrhizas could be:
(a) Ectomycorrhiza, and
(b) Endomycorrhiza.
(a) Ectomycorrhiza (Ectotrophic mycorrhiza):
In this type of mycorrhiza, the root system is completely surrounded by a distinct mantle or sheath of fungal tissue from which hyphae penetrate the outermost cell layer (a) of the root of all seeded plants.
Sometimes the light pseudoparenchymatous fungal tissues send branches between the cortical, cells of the root and outward in the soil thus isolating the root from the soil and comprising about half of the total weight and volume of the organ.
The hyphae penetrate the cells of the cortex to form a network of hyphae Hartignet. The hyphae which move into the soil ramify in the soil particles. In some species, the Hartignet is formed only internally while in majority of the species, an external sheath is present.
New lateral foots formed by the mother root also get the mycorrhiza as they pass through the cortex. The presence of mycorrhiza also prevents growth in length of the roots as a result of which swollen or coralloid roots are formed.
The ectomycorrhizas are also called sheathing mycorrhizas. It is estimated that about 3% of all seeded plants are have ectomycorrhizas being common in northern temperate forest trees especially in Fagaceae and Pinaceae but also in Betulaceae and Tilliaceae.
In tropics, the ectomycorrhizas are well represented in Myrtacceae and Dipterocarpacae. The trees in which ectomycorrhizas are common include Pinus, Picea, Cedrus. Abies, Fagus and Quercus.
Majority of fungi forming ectomycorrhizas are from Basidomycotina especially Hymenomycetes. Over 100 species are known to form mycorrhizas.
Majority comes from the general Amanita, Boletus and Tricholoma. Many species in the genera Cortinarius, Lactarius and Russia also form Ectomycorrhizas.
Some of the fungi like Amanita muscaria and Bletus edulis have wide host range while other show host specificity. Boletus elegans is one such example which forms mycorrhiza only with larch.
The fungus depends on the host for carbon source. The mycorrhizas are responsible for enhanced mineral uptake and hydrolysis of organic phosphorus’ in soil. The mycorrhizae sheath serves as a reservoir of mineral ions.
Mycorrhizal trees have greater vigour than the non-mycorrhizal trees. In some cases the fungus produces extra hyphal enzymes which break down lignin and hydrolyze cellulose of humus and plant debris making suitable for the plants to utilise.
This has been shown in Boletus subtomentosus with Pinus montana as also in Lactarius delicious with Poxius sylvesrius. Mycelium of the fungus in the soil attached to mycorhiza is capable of tapping greater volume of soil than the lost root. The nutrients absorbed by the host pass through the fungal mycelia enabling these to grow.
Basically ectomycorrhizas are either coralloid or ball types. These have also been differentiated on the basis of external forms into three types (Melin and others):
(i) Long thin unbranched structures which grow upon roots,
(ii) Short dichotomously branched coloured roots the colour depending upon the fungus and
(iii) Knots or tuber like growths developed by fusion of flaps produced by merger of clusters of forked roots.
(b) Endomycorrhiza (Endotrophic mycorrhiza):
In endomycorrhiza, the fungal hyphae penetrate the root epidermis and enter the cortex establishing themselves in the cortical cells.
The hyphae may be present on the root surface externally as well. The fungal hyphae may be septate and aseptate and grow intra-or-intercellularly. These do not invade green tissues and they also do not enter the storage tissues.
The endomycorrhizas are not harmful to the plants but these are associated with the process of root development. Roots becomes dichotomously branched increasing the surface area of absorption.
In saprophytic angiosperm Monotropa, the endomycorrhiza are shown to be associated with the process of absorption of nutrients. The germination of seeds and spores and early development of young and adults are also influenced by endomycorrhizal fungi as these provide food materials and enzymes essential for metabolism.
In addition, these fungi break down the cellulose and lignin in the soil and supply carbon compound to their hosts.
The endomycorrhizas are ubiquitous present in arctic, temperate and tropical regions. These are found in almost all flowering plants including gymnosperms as also ferns and liverworts.
This type of mycorrhiza has also been found to be present in plants of economic importance such as grasses, cereals and legumes. In these plants, the external mycelium is absent but present in Monotropa and members of family Ericaceae.
In orchids, the external mycelium is scanty. Generally the fungal component is present on the root surface externally forming a loose mass of hyphae without entering it. The branches of these hyphae enter the root cortex and establish themselves in cortical cells.
There are two main types of endomycorrhizas:
(a) Endomycorrhiza:
Endomycorrhiza due to aseptate hyphae belonging to Phycomycetes. The fungal component belong to the genera Pythium and Endogone and Rhizofagus – Endogone complex and the non-fungal component comes from the families Graminae, Palmae, Leguminosae, and gymnosperms excluding Pinaceae, pteridophytes and liverworts.
Vascular arbuscular mycorrhizas (VAM) are considered as the most important type of endomycorrhiza. These are produced by aspetate myceliae fungi which penetrate the root and forming vesicles and arbuscles in plants roots after infection.
These are found in bryophytes and pteridophytes and gymnospersms except Pinaceae and in virtually all families of angiosperms. VAM are of general occurrence in Gramineae, Palmae, Rosaceae and Leguminosae but are absent in Chenopodiaceae, Cruciferae, Cyperaceae and Reseadaceae.
The fungal hyphae produce very thick walled brown to black balloon like Chlamydospores which range between 10-250 µ in diameter. These are produced singly or in clusters as vesicles at the loose end of hyphae.
Loose aggregations of spores may also be formed within a poorly developed reproductive structure called spororcarp. Chalmydospores are most common in soil though Zygospores are sometimes also formed.
These are produced by the intercellular hyphae, form vesicles, intercalary or apical in position and produced as large thick walled swellings. The vesicles are known to function as storage organs or as resting spores.
They remain in soil when the roots decay and infect the new root when produced. The vesicle containing mycorrhizas are known as Vesciular mycorrhiza.
Arbuscules (Haustoria) are formed by branches of smaller diameter which pass into cortical cells. These arbuscules are often dichotomously branched and they function as complex haustoria.
Arbuscules become swollen at their apices and form sporangioles which become free within the cells of the host. The sporangioles disintegrate later. Such structures are known as arbuscule-sporangiole complex and are of common occurrence and in Papilionaceae and Graminae families.
The intercellular hyphae may produce a combination of vesicles and arbuscules. Such mycorrhizas are known as Vesicular-Arbuscular mycorrhiza (VAM) and are common in plant kingdom.
VAM are also known as phycomycetous mycorrhiza and has also been reported from carboniferous and Devonian periods.
In roots of orchids, form species of Rhizoctonia form intracellular hyphae which become coiled and digested in the host cell. These coiled intracellular hyphae are known as pelotons.
Mostly VAM fungi comes from Endogonaceae family belonging to order Mucorales of class Zygomycetes. Generally the genera belong to Gloius, Glomerulus, Gigaspora and Sclerocystis. Spores of these isolated from soil have been used to infect healthy plants.
So far these have not been grown on artificial culture media. Although spores or chlamydospores do germinate on nutrient media, the hyphae grow till food supply is available in the nutrient agar and these hyphae cannot be sub-cultured.
Spores from infected maize plants have been successfully used to produce typical VAM in soyabean, onion, redclover and strawberry.
(b) Endomycorrhiza:
Endomycorrhiza caused by fungi with septate hyphae belong to mainly Basidiomycetes or Deuteromycetes having affinities with Basidiomycetes.
The fungal component comes from Armillaria mellea and species of Rhizoctonia while the non-fungal component comes from Ericaceae, Orchidaceae and Gentiamaceae and a few bryophytes and pteridophytes.
Other types of Mycorrhizas:
(i) Ericoid mycorrhiza:
It is another type of endomycorrhiza in which different kind of fungi is involved. These mycorrhizas are found in plants of family Ericaceae where the rootlets are covered with a very sparse loose mass or weft.
Some branches penetrate the cortical cells forming intercellular hyphae coils enclosed by host plasmalemma. After sometime, the host cells disintegrate allowing these parts to collapse, such mycorrhizas are called Ericoid mycorrhizas which help in phosphorus and nitrogen uptake by host plants.
(ii) Ectendomycorrhiza:
This type of mycorrhiza is a little different from the above and has been reported in a number of plants of family Ericaceae and plants of other families of Ericales.
The short roots of the host are swollen and are invested with hyphal sheath. Although Harting net is absent in the hyphae sheath but Intercellular hyphal corls develop in outer cortical cells which ultimately get lysed.
Since the mycorrhiza fungus is present both externally and internally, this type of mycorrhiza is known as ectendomycorrhiza. Arbutus and Arctostaphylos are the best examples of this type of mycorrhiza.
(iii) Pseudomycorrhiza:
This term was introduced by Melin and others to include short roots attacked by certain root parasites in the absence of any other fungi. This resulted in slow growth and branching was rare or absent. Mostly the fungus involved was Rhizoctonia sylvestris which is a weak pathogen.
Nature and Imporance of Mycorrhiza:
There has been conflicting views about the nature of mycorrhiza.
These views can be grouped in two categories:
(a) Mycorrhizas are pathogenic in nature and cause harm to the host.
(b) Mycorrhizas are symbiotic in nature benefitting both the partners.
Most of the scientists are inclined to veer around the view that the mycorrhizas are symbiotic in nature as evidenced by the following:
(i) Mycorrhizas induced increasing in branching of roots followed by increasing surface of absorption of roots. Mycorrhizal roots receive better nutrition than the non-mycorrhizal ones.
(ii) Mycorrhizas are equally important in plant nutrition in deficient soils. The mycorrhiza may increase the availability of phosphorus and several other nutrients. Reports have been published to show that mycorrhiza enhance the uptake of N2, P, Ca, Na, Fe, Cu, Bo, Zn, A1 and strontium. According to Brown (1968) Sanders and others (1975), mycorrhizal infection may be a precondition for nodule formation in phosphorus deficient soil.
(iii) Mycorrhizal plants show greater dry-weight than the non-mycorrhizal plants growing on nutrient deficient soils because the former absorb nutrients more and disproportionately than the latter.
(iv) Mycorrhizal trees have greater vigour because the infection release several inhibitory compounds to establish barriers against pathogenic infection.
(v) The fungal partner gets suitable habitat for the growth on the root surface where nutrients required for the fungal growth are available in plenty. Roots also excrete certain substances which promote spore germination and stimulate root infection.
(vi) Mycorrhizas are universally present in orchids because the fungal mycorrhiza provides sugars and vitamins essential for germination of seeds and their early development.
(vii) In forest plants like Pinus montara and Pinus sylvestris, the interdependence of both the partners is well understood. In these plants, the fungal partner produces extra hyphae enzymes which break down lignin and hydrolyse cellylose of plant debris and humus making these available for use by the host.
(viii) Mycorrhizal infection has morphogenetic effect on the host leading to prolonged life growth and branching of booklets. It has been shown that mycorrhizal infection enchanced auxin production due to which dichotomous branching increased.
(ix) Mycorrhizas play an important role in Afforestation. It has been experimentally shown that spruce and pine seedlings the due to starvation if they are not invaded with mycorrhizal fungi Boletus viscidus – Of all the mycorrhizal fungi tested, this fungus produced the best results. Seedlings with mycorrhizal infection are immune to infection.
(x) VAM fungi are obligate biotrophs but are not host specific. Therefore they can be used as biological fertilisers.
