Mycorrhizae: Definition, Types and Examples

In this article we will discuss about mycorrhizae definition, types and examples

What is Mycorrhizae?

• Vitadini (1842) was first to recognize the benefits of fungal structures (mycelia) that grow around plant roots.
• The term “mycorrhiza” was coined by Albert Bernard Frank in 1885
• Mycorrhiza is the symbiotic association of fungi with the roots of higher plants.
• In this association both the fungi & higher plants derive benefit form each other.
• The host plant provides shelter & food for the fungal symbiont & in turn plant acquire an array of benefits such as better uptake of phosphorus, salinity & drought tolerance, maintenance of water balance & overall increase in plant growth.
• About 90% of all land plants are associated with mycorrhiza.
• The mycorrhizal association is not available in Cruciferae, Chenopodiaceae and Resedaceae.

Types of Mycorrhizae

Peterson and Farquhar (1994) classified the mycorrhizae into following types. These are :

1. Ectomycorrhiza
2. Endomycorrhiza
3. Arbutoid ectendomycorrhiza Mycorrhiza
4. Monotropoid Mycorrhiza

1. Ectomycorrhiza

• Ectomycorrhiza is commonly called sheathing mycorrhiza.
• In this type of mycorrhiza, the root system is completely surrounded by a distinct mantle or sheath of fungal tissue from which the septate hyphae penetrates the outermost cell layer (epidermis & cortex) of all the seeded plants.
• No intracellular penetration into the epidermal or cortical cells occurs.
• An extensive network of hyphae called Hartig’s net formed between these cells.
• Great majority of vascular plants like ferns, Monocots, Dicots, Pinus, Cedrus etc. are formed from these associations with fungi belonging to basidiomycota, ascomycota.
• The presence of ectomycorrhiza also prevents growth in length of roots as a result of which swollen or coralloid roots are formed.
• E.g., Amanita, Tricholoma, Boletus, Russula, Lactarius (basidiomycota); Gyromitra esculenta (ascomycota)

2. Endomycorrhiza

• In endomycorrhiza, the fungal hyphae penetrates the root epidermis and enter the cortex establishing themselves intracellularly in cortical cells.
• The fungal hyphae may be septate or aseptate.
• Generally the fungal component present on the root surface externally forming a loose mass of hyphae.
• Fungal sheath is absent
• Hartig’s net is absent
• Types of Endomycorrhiza: Vesicular Arbuscular mycorrhiza, Ericoid Mycorrhiza and Orchidoid Mycorrhiza

A. Vesicular Arbuscular mycorrhiza (VAM)

• VAM are considered as the most common type of endomycorrhiza.
• These are produced by aseptate mycelia fungi which penetrates the root & forming vesicles & arbuscles in plant roots
• These are usually found in bryophytes, pteridophytes, gymnosperms (except Pinaceae) & all angiosperms
• The fungal hyphae produces very thick walled brown to black balloon like Chlamydospores.
• This fungal component is usually from endogonaceae (Zygomycetes)
• E,g., Glomus, Gyrospora, Acaulospora etc.

B. Ericoid Mycorrhiza

• Ericoid mycorrhizae are found in the different members of Ericaceae like Erica, Calluna, Vaccinum, Rhododendron etc.
• The fungi are slow-growing, septate and culturable.
• During this association the rootlets of the plants are covered by very sparse, loose, dark, septate hyphae that penetrate the cortex forming intercellular coils.
• The fungus gets the photosynthate from the host and improves the mineral uptake and nutrition of the host, especially P and N.
• E.g., Pezizella ericae (Ascomycotina) and Clavaria vermiculata (Basidiomycotina)

C. Orchidoid Mycorrhiza

• Orchids produce millions of tiny seeds per capsule, there is virtually no storage of food.
• Majority of seeds are unable to germinate without exogenous supply of carbohydrates.
• Therefore, mycorrhizal association is obligatory for the seeds to germinate.
• The Septate hyphae enters the embryo and colonises, develops into coils often called peltrons and it’s provides the nutrition to the seed.
• E.g., Rhizoctonia, Corticium, Ceratobasidium etc

3. Ectendomycorrhizae (Arbutoid)

• These mycorrhizae intermediate in form between ecto and endomycorrhizae types, called ectendomycorrhizae.
• Arbustus and Arctostaphylos of Ericaceae show this type of mycorrhizal association.
• Fungal sheath present.
• Septate hyphae penetrate the outer cortical cells & coils inside it.
• Hartig’s net is present in the intercellular spaces
• E.g., Wilcoxina, Sphaerosporella

4. Monotropoid Mycorrhiza

• Monotropa hypopitys is a non-green saprophytic herb.
• It has short fleshy roots that are invested with a hyphal sheath and often forming Hartig net in the cortical zone.
• Due to absence of chlorophyll, they are unable to synthesise and supply carbohydrate to the fungus.
• This fungus acts as a bridge between Monotropa and Pine plants.
• E.g., Boletus

Role of Mycorrhizae in Agriculture

• The mycorrhizal association helps in the formation of dichotomous branching and profuse root growth, thus enhances plant growth.
• Mycorrhiza helps in uptake of mineral ions, nutrients and also acts as reservoir.
• In nutrient deficient soil, the mycelial association helps in the absorption of N, Ca, P, Zn, Fe, Na and others.
• Mycorrhizal association is obligatory for the germination of orchid seeds.
• Inoculation of VAM as biofertiliser provides a distinct possibility for the uptake of P in phosphorus-deficient soil.