Mycorrhiza (fungus root)

·      Symbiotic association between fungi and plant roots-  Rhizosphere interaction

·      The term Mycorrhiza introduced by A B Frank

·      Mutualistic association where both partners depend on each other and have coevolved

Important component of soil life and soil chemistry

Mycorrhizae form a mutualistic relationship with the roots of most plant species (80% of plant families are predominantly mycorrhizal; (with arbuscular mycorrhizas being the predominant form). Mycorrhizal fungi has a unique ecological position of being partly inside and partly outside the host

Mycorrhizal fungi use photosynthetically derived organic C from their host and unlike most fungi, are not saprophytic

Mycorrhizae differ from other rhizosphere interactions due to

·      Greater specifity & organization of plant fungus relationship           

·         Association for prolonged periods

 

Types of mycorrhiza

Mycorrhizas are broadly divided into ectomycorrhizas and endomycorrhizas, though six types are recognized based on the characteristic features.

·         Ectomycorrhizae:  Extracellular - Fungi do not penetrate individual cells within the root, instead penetrate the intercellular spaces of epidermis and of the cortical region of the root, forming a sheath of interconnecting hyphae around the roots called Hartig net and mantle.

·         Endomycorrhizae: Fungi penetrate the cell wall and invaginate the cell membrane. Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza.

·         Ectendomycorrhizae: Shares the features of both ecto- and endomycorrhiza.- Hartig net and mantle present but occasional penetration of hyphae into root cells.  Arbutoid mycorrhizas can be classified as ectoendomycorrhizas. 

                Monotropoid mycorrhizas form a special category.

Ectomycorrhiza

·         Ectomycorrhizas, or ECM

·      Predominately seen in  mostly woody plants in cooler climates, like the birch, eucalyptus, oak, pine, and rose families

·         Fungi belonging to the Basidiomycota and some Ascomycota.

·         Strict host specifity is rare - An individual tree may have 15 or more different fungal ECM partners at one time.  Some ECM fungi, such as many Leccinum and Suillus, are symbiotic with only one particular genus of plant, while other fungi, such as the Amanita, form mycorrhizas with many different plants

·         Unlike Endomycorrhizae, ectomycorrhizal fungi do not penetrate their host’s cell walls.

·        ECM fungi secrete signalling molecules that limit the growth of root hairs – ECM colonized roots often appear blunt and covered in fungi    

·         From the root surface, fungi extend hyphae into the soil, which aggregate to form rhizomorphs- dense mats of hyphae often visible to the naked eye

·         Hyphae form characteristic structures like the hyphal sheath, or mantle, covering the root tip and a Hartig net of hyphae surrounding the plant cells within the root cortex.

·         Mantle is a dense, and multi-layered covering of fungal mycelia surrounding the root surface. Thickness of mantle varies from 20-40 mm depending on mycorrhizal fungi, temperature, nutritional factors etc. The hyphae extend up to several centimetres into the surrounding soil. 

    `    In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza.

·         Hartig net is made of highly branched hyphae forming a mycelial network between epidermal and cortical root cells.

·         Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter- efficient two way transfer of soil nutrients to the plant and carbohydrates to the fungus

·         This hyphal network aids in water and nutrient uptake and help the host plant to survive adverse conditions and in exchange, the fungal symbiont is provided with access to carbohydrates. Nutrients such as Carbon, Nitrogen and Phosporous can move between different plants through the fungal network (sometimes called the wood wide web).

·         Many ECM fungal fruiting bodies are well known including the economically important and edible truffle (Tuber), deadly death caps and destroying angels (Amanita).

 

Endomycorrhiza

An Endomycorrhiza is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plants including bryophytes, pteridophytes, gymnosperms and angiosperms. Fungi are predominately from Zygomycotina (eg; Glomus, Gigaspora etc).  Endomycorrhizas have been further classified as arbuscular, ericoid and orchid mycorrhizas.

1.      Arbuscular mycorrhiza

Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, between the plant cell wall and invaginations in the plasma membrane, producing unique highly branched structures that are either balloon-like (vesicles) or branching invaginations (arbuscules).  

 The fungal hyphae enter root cells The structure of the arbuscules increases the contact surface area between the hypha and the cell cytoplasm and act as the functional site of nutrient exchange.

Arbuscular mycorrhizas are formed only by fungi in the division Glomeromycota. They are found in 85% of all plant families, and occur in many crop species (eg, wheat).

 They provide the host plant with mineral nutrients (phosphorus and micronutrients) and water, in exchange for photosynthetic products. The AMF mycelium that emerges from the root system can acquire nutrients from soil volumes that are inaccessible to roots.

Furthermore, fungal hyphae are much thinner than roots and are therefore able to penetrate smaller pores. Carbohydrates and mineral nutrients are then exchanged inside the roots across the interface between the plant and the fungus.

The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil. Thus, AMF can alleviate the limitation in plant growth caused by an inadequate nutrient supply.

 In addition to an improved nutritional supply, AM interactions provide other benefits to plants, such as improved drought and salinity tolerance and disease resistance.

AM fungi are known to reduce heavy metal toxicity in the host plants and to tolerate high metal concentrations in the soil.

AM fungi can also have a direct effect on the ecosystem, as they improve the soil structure and aggregation and productivity AM fungi contribute to reducing emissions of N₂O, which is an important greenhouse gas, thus suggesting that they could play a role in controlling climate change.

It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonization of land by plants and in the evolution of the vascular plants.

AMF or AM fungi represent a key link between plants and soil mineral nutrients, and are of great interest as natural fertilizers.

2.      Ericoid mycorrhiza

Ericoid mycorrhizas have a simple intraradical (grow in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. Plants are woody shrubs or small trees of the Ericacea subfamily Ericoidae, found in open or acid peaty soil. They have usually fine roots on which fungus attached. Fungi is usually Ascomycetes (Eg. Clavaria, Pezizella spp). Ericoid mycorrhizas have been shown to have saprotrophic capabilities, which would enable plants to receive nutrients from by the decomposing actions of their ericoid partners. 

3.      4.  Orchid mycorrhiza

All orchids, in nature germinate only when infected with endomycorrrhizal fungi which subsequently colonize the host plants. fungi. The fungi are mostly from Rhizoctonia genus of Basidiomycetes and Ascomycetes. Their hyphae penetrate into the root cells and form typical coils are similar to ericoid mycorrhizas. Their carbon nutrition is exclusively to supporting the host plant as the young orchid seedling is non-photosynthetic and depends on the fungus partner. The fungus utilises complex carbon sources in the soil, and make carbohydrates available to the young orchid. All orchids are achlorophyllous in the early seedling stages, but usually chlorophyllous as adults, so in this case the seedling stage orchid can be interpreted as parasitising the fungus.

Ectendomycorrhizae-   Arbutoid mycorrhiza

This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae. It is different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved. The difference to ectomycorrhiza is that some hyphae actually penetrate into the root cells, making this type of mycorrhiza a form of transition between ecto- and endo mycorrhizae and hence classified as ectendomycorrhiza. Mycosymbionts are of Basidiomycetes. The host plants are mostly woody shrubs and trees. The roots are covered with mycorrhiza with a sheath and a Hartig net, but the fungus penetrates cortical cells to form extensive coils of hyphae.

Monotropoid mycorrhiza

This type of mycorrhiza occurs in the subfamily Monotropoideae of the Ericaceae. These plants are achlorophyllous and heterotrophic or mixotrophic, hence derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis. Roots form ball throughout which fungal mycelium grows enclosing mycorrhizal roots of surrounding green plants. Root ball is the survival organ in winter for Monotropa and when the favourable conditions come, they form flowering shoots. A Hartig net and sheath (mantle) is also seen occasionally. The structure and function of monotropoid mycorrhiza change with seasonal development of host plants.

Benefits

·         Mycorrhiza has a broad ecological adaptability and is known to occur in deserts as well as arctic, temperate, tropical and other inhospitable habitats.

·         Healthy physiological interaction between plant and fungus- Nutrient exchange between both partners, Plants occupy habitats they otherwise cannot.

·         Mycorrhiza increases the rate of photosynthesis and hence improves plant growth, productivity and yield.

·         Mycorrhiza facilitates better uptake of nutrients like phosphorus and immobile trace elements like zinc, cobalt, magnesium, iron, copper, molybdenum, etc, leading to better nutrition for plants.

·         Mycorrhiza offers tolerance against a range of soil stresses like heavy metal toxicity, salinity, drought, and high soil temperatures. This enhances the chances of plant survival immensely.

·         Mycorrhiza with hyphae that extend much beyond a few meters away the plant root zone, can acquire nutrients from a much wider soil area and is categorized as a biofertilizer. Mycorrhiza offers up to 50% reduction in chemical fertilizer application.

·         Mycorrhiza offers higher resistance to various soil and root-borne pathogens, thus becoming a potential disease control agent.

·         Mycorrhiza helps in soil conservation and soil structure stabilization, thus restoring land productivity.