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Under agricultural field conditions, crops do not have roots, they have mycorrhiza.
Mycorrhizas (sing, mycorrhiza), discovered by the German botanist, Frank in 1885, are structures formed by association of fungi and roots of plants.
Mycorrhizas are classified into three categories- ectotophic, endotrophic and ectendotrophic. In ectotrophic mycorrhiza, the fungus forms a mantle or hyphal sheath around the rootlets and also enters the roots forming an intercellular net of hyphae, called the Hartig net (named after Hartig who described it in 1851, without knowing its fungal nature). The hyphae of the Hartig net do not form haustoria.
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In the endotrophic mycorrhiza, which is the predominant type, the fungus invades and forms hyphal knots in the cells of the root cortex. A portion of the fungus lives in the soil, but no external sheath is formed. The ectendotrophic mycorrhizas share characters of both; form the hyphal mantle and, inside the root, form intercellular, as well as intracellular hyphae.
Ectotrophic Mycorrhizas:
The ectotrophic mycorrhizas occur only in about 3 per cent of plant species, majority of which are forest trees, viz., pines, spruces, larches, firs, oaks, beeches, birches, sweet chestnut, eucalyptus, etc. These are more common in temperate regions. The associated fungi usually belong to Basidiomycota which are common in the forest litter.
Morphology and Anatomy:
The ectotrophic mycorrhizas are conspicuous and easily recognizable by the fungus mantle that gives the roots a swollen and stumpy look.
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While the uninfected rootlets remain unbranched and are short-lived, the mycorrhizal roots, by repeated branching, form a crowded cluster of swollen stumps that persist for several years. The new rootlets that emerge from the mycorrhizal roots become infected during their passage through the cortex and the hyphal sheath. The two partners, the fungus and the host root, grow in an organized manner with equal pace.
However, occasionally, during spring season, some of the root apices grow out of the hyphal sheath and extend beyond it, though temporarily. In Pinus, the mycorrhizal rootlets branch dichotomously, while in Fagus and all other species, the branching is racemose.
A transverse section of an ectotrophic mycorrhiza shows that the fungus forms a pseudoparenchymatous (occasionally prosenchymatous) sheath around the root and sends branches inwards (between the cortical cells) as well as outside into the soil. The hyphae which penetrate the cortex form the Hartig net in the intercellular spaces.
The cells of the cortex, which are in close contact with the Hartig net, are larger and more radially elongated than the healthy cells. The mycorrhizal roots have a poorly developed apical meristem. The hyphae in the soil give rise to the fruit bodies above ground during fruiting season. The ectotrophic mycorrhizas are differentiated and classified on the basis of their mode of ramification, colour and the structure of their sheath.
The Fungi:
The fungi associated with ectotrophic mycorrhizas belong to Basidiomycota, especially Basidiomycetes (Agaricales and Boletaceae). Some belong to the Ascomycota e.g., Tuber and Elaphomyces.
Endotrophic Mycorrhizas:
The fungi of endotrophic mycorrhizas invade the cells of the root cortex, with a portion lying externally as a loose mass of hyphae in the soil.
The endotrophic mycorrhizas are separated into two groups on the basis of the nature of the fungi:
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(1) Those produced by septate fungi belonging to Basidiomycota, and
(2) Those produced by aseptate fungi, belonging to Zygomycota.
Endotrophic Mycorrhizas with Septate Fungi:
These mycorrhizas are found in the plants belonging to plant families Ericaceae, Orchidaceae and Gentianaceae. The orchids, which are obligately dependent on mycorrhizal association for germination and further development (either temporarily or for the entire life), exemplify the importance of symbiotic mode of life.
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Seeds of orchids are typically minute and are produced in enormous numbers (up to several millions in a single capsule). These have undifferentiated small embryo and with little or no reserve food material. They require prolonged soaking in water (2-3 months) for germination. Only seeds with infected embryos can germinate. The infection occurs through specific entrance cells in the suspensor when the imbibed seeds rupture.
Growth and cell division are dependent on external source of carbohydrate, which is supplied by the fungus. The symbiosis exemplified by the orchids is called phase separation symbiosis. As soon as the orchid becomes an independent photosynthetic unit, the carbohydrate diffusion gradient is reversed and the orchid starts supplying food, at least partly, to the fungus.
The Fungi:
The fungi that form endotrophic mycorrhizas with orchids are all members of Basidiomycota, e.g., Armillaria mellea, species of Fomes, Xerotinus, Corticium, and Marasmius.
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Endotrophic Mycorrhizas with Aseptate Fungi (Vesicular-Arbuscular Mycorrhizas):
The Vesicular-Arbuscular type of mycorrhiza (VAM) is most common among all mycorrhizas. These are ubiquitous and occur in about 70 per cent of plant species, belonging to all groups of plants-bryophytes (especially Hepaticae), pteridophytes, gymnosperms (except Pinaceae), and angiosperms.
Large number of agricultural crops form VA-type mycorrhiza : e.g., maize, forage-legumes, soyabean, cotton, tobacco, potato, sugarcane, tomato, peas, apples, strawberry, avocado, citrus, coffee, cacao, tea, coconut, rubber tree, sugar, maple and so on. The economic importance can, therefore, never be overemphasized.
An important feature of VA mycorrhizas is the formation by the fungus of two characteristic organs in the root cortex: arbuscules (dichotomously branched, bush-like haustoria), and vesicles, formed intra, and intercellularly.
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Some external hyphae extend into the soil up to a distance of about 1cm around the roots. Sometimes, these external hyphae grow closely appressed to the root surface but they never form a mantle. The roots do not show any morphological changes.
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The External Hyphae:
These are characteristically dimorphic and are composed of 1) ‘thick-walled’ or ‘permanent hyphae’ (20-30 µm wide), and 2) ‘thin walled’ or ‘short-lived’ hyphae’ (2-7 µm wide), which arise as lateral branches of the former. The external hyphae bear various kinds of spores, auxiliary cells and sporocarps in the soil. The hyphae form appresoria at the site of penetration of the root (i.e., on root hair or epiblema cells). The fungus invades the cortex but never encroaches upon the endodermis, stele or meristem.
Arbuscules:
Shortly after infection, the hyphae form arbuscules, the dichotomously branched, bush-like haustoria in the cortical cells. These are later digested by the host cells. First, the tips are eroded and soon the entire arbuscule is degraded and digested. The host nucleus responds to the formation of arbuscules by swelling in size with the degradation of the arbuscules.
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Vesicles:
These are spherical or oval, thick-walled structures formed terminally in the intercellular spaces or occasionally in the cortical cells. These contain fat granules and serve as storage organs. In old mycorrhizal roots; the vesicles are produced in enormous numbers which completely obliterate the cortex cells. The roots, in extreme cases, may also get deformed.
The Fungi:
There are six genera that form the VA mycorrhiza, and all belong to the order Glomales (formerly Endogonales) of class Zygomycetes. The six genera fall in 3 families (2 in each).
Family Glomaceae – Glomus, Sclerocystis
Family Acaulosporaceae – Acaulospora, Entrophospora
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Family Gigasporaceae – Gigaspora, Scutellospora
While the four genera belonging to Glomaceae and Acaulosporaceae form both arbuscules and vesicles, the two genera under Gigasporaceae (Gigaspora and Scutellospora) form only arbuscules (and not vesicles). The genera are identified on the basis of formation of chlamydospores, auxiliary cells, spores and sporocarps.
Ectendotrophic Mycorrhizas:
Mycorrhizas sharing characteristics of both ecto-and endotrophic mycorrhizas are called ectendotrophic. The fungus forms a hyphal mantle and Hartig net, as do the ectotrophic mycorrhiza, and also establish haustoria and hyphal coils in the epidermal and cortical cells, like the endotrophic mycorrhizas. The external hyphae deliver organic compounds absorbed from the humus to the root cells.
One of the best studied examples of ectendotrophic mycorrhizas is the mycorrhiza of Monotropa indica, the Indian pipe. It is a non-chlorophyllous plant which grows on forest floors under Fagus, Pinus, Quercus, Carpinus, Salex, and other trees. Earlier M. indica was thought to be a root parasite of these trees but now, it is known for sure that Monotropa and the associated tree are connected by a common mycorrhizal fungus, Boletus.
The fungus forms ectendotrophic mycorrhiza with Monotropa and ectotropic mycorrhiza with the trees. Bjorkman (1960) by using tracer carbon technique proved that the common mycorrhizal fungus conducts nutrients from forest trees to Monotropa. If labelled sugars or 32 P-phosphate are introduced into the phloem of the trees, some of them find their way into the fungus, and from there into Monotropa. The Monotropa symbiosis is, thus, a three- tier system, involving chlorophyllous and non-chlorophyllous symbionts and a fungal bridge connecting the two.
