An Overview of Mycorrhizae - Types, Examples, and Importance

Mycorrhizae is a symbiotic association between fungi and plant roots. It forms when fungi colonize a plant's root system. The plant provides the fungus with carbohydrates, and the fungus helps the plant absorb more water and nutrients. Many mycorrhizae types come in different forms, depending on the host plant and fungal taxonomy. Mycorrhizae helps plants absorb more water and nutrients, especially phosphorus. They also make plants less susceptible to water stress.

What are Mycorrhizae?

Mycorrhizae are symbiotic relationships between plant roots and fungi. The word "mycorrhizae" meaning "fungus" and "root" come from the words "myco" and "rhiza" respectively. Mycorrhizae have been around for about 400 million years and can be found in about 90% of all land plants. The fungi infect the roots without causing disease. The plant provides the fungus with sugars and carbon and, in return, receives water and nutrients.

Mycorrhizae hyphae absorb and directly deliver nutrients to the roots. It increases the absorption area by 50 times and increases the overall root biomass. Mycorrhizal symbiosis increases the plant's effective water utilization capability and also improves the utilization of soil nutrient capability, including nitrogen, phosphorus, potassium and micronutrients.

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Structure of Mycorrhizae

Mycorrhizae are fungal roots resulting from a symbiotic relationship between fungi and plant roots. Mycorrhizae comprise a root-like structure and a network of mycelium that extends into the soil. The mycelium is made up of microscopic thread-like strands called hyphae. This network can cover up to 700 times more soil than the plant roots alone. The mycelium absorbs nutrients and transports them back to the host plant. The structure of mycorrhizae varies depending on the type of mycorrhizal association. We will briefly discuss the structures associated with the two main types: Ectomycorrhizae (ECM) and Endomycorrhizae (Arbuscular Mycorrhizal Fungi - AMF).

Ectomycorrhizae (ECM)

Ectomycorrhizas have a hyphal sheath, or mantle, that covers the root tip. It provides a protective sheath around the root that aids in nutrient exchange. Hartig Net is formed between the fungal hyphae and the root cells that facilitates the transfer of nutrients, especially minerals, from the fungus to the plant and vice versa. Mantle is a dense structure that protects the root tip and helps regulate nutrient exchange.

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Endomycorrhizae (AMF - Arbuscular Mycorrhizal Fungi)

Endomycorrhizae have Arbuscules located inside the plant root cells. These are specialized structures for nutrient exchange. These are highly branched and increase the surface area for nutrient transfer between the fungus and the plant. Within the root cells storage structure are present known as vesicles. They store lipids and glycogen that act as reservoir of nutrients for both the fungus and the plant.

Hyphal Network extends into the soil, forming a network around the plant roots. It explores the soil for nutrients, particularly phosphorus, and facilitates nutrient uptake by the plant. Fungal hyphae penetrate the outer layers of the root, but not the cells themselves. It enhances nutrient absorption and provides a pathway for the exchange of signaling molecules between the fungus and the plant.

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Diagram of Mycorrhizae

The labeled diagram of mycorrhizal is given below:

Types of Mycorrhizae

There are seven types of mycorrhizae: Endo (arbuscular), Ecto, Ectendo, Arbutoid, Monotropoid, Ericoid, and Orchidaceous. Endo mycorrhizae and ecto mycorrhizae are the most abundant and widespread. They are explained in detail as follows:

Ectomycorrhizae

Symbiotic relation is formed between basidiomycete and ascomycete fungi and the roots of trees, especially in temperate and boreal forests. These fungi are only externally associated with the plant root. Ectomycorrhizae form a dense sheath around the plant roots, called a mantle, from which the hyphae grow.

Endomycorrhizae

Most common type of mycorrhiza, formed by the Glomeromycota fungal group and the roots of a wide range of plants, including many agricultural crops such as maize, wheat, and soybeans. Fungal hyphae penetrate the root cells, forming specialized structures called arbuscules and vesicles. Arbuscules facilitate nutrient exchange between the fungus and the plant.

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Ectendo

Ectendomycorrhizae are a type of symbiotic relationship between fungi and the roots of higher plants. They are mainly found in seedlings that are under shaded conditions in natural forests. Structurally, ectendomycorrhizas are characterized by a thin mantle (sometimes absent), Hartig net, and various degrees of intracellular hyphal penetration into epidermal and cortical cells. They are restricted to the plant genera Pinus (pine), Picea (spruce) and, to a lesser extent, Larix (larch).

Orchid mycorrhizae

Orchid mycorrhizae are fungi that form symbiotic relationships with the roots and seeds of orchid plants. These fungi belong to the Basidiomycetes, most often to the Rhizoctonia genus. The fungus is essential for orchid seed germination, forming a mycorrhizal peloton within the seed. In natural environments, orchids are exclusively reliant on mycorrhizal fungi for seed germination, establishment, growth, and development. As the orchid grows, it continues to rely on the fungus for nutrient uptake.

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Ericoid Mycorrhizae

Ericoid mycorrhizae are a type of fungi that colonize the roots of ericaceous plants. They are a well-defined endomycorrhizal type that can be distinguished from other mycorrhizae by the intracellular hyphal coils they form in the rhizodermal cells of healthy ericoid hair roots. Ericoid mycorrhizal fungi are relatively diverse and belong to both Ascomycota and Basidiomycota. Fungal hyphae penetrate the cells of the root's outer layer and may also form structures called microsclerotia. This type is common in plants like heathers and blueberries.

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Arbutoid Mycorrhizae

Arbutoid mycorrhizae are a type of endomycorrhizal fungi that form a fungal sheath around plant roots. They are similar to ectomycorrhizal fungi, but differ in that the hyphae of arbutoid mycorrhizae penetrate the cortical cells of plant roots. Arbutoid mycorrhizae are found in the plant order Ericales, and are formed in the genera Arctostaphylos and Arbutus. They are different from ericoid mycorrhizae, but resemble ectomycorrhizae in terms of function and the fungi involved.

Monotropoid Mycorrhizae

Monotropoid mycorrhizae are a type of mycorrhiza that occurs in the Ericaceae subfamily Monotropoideae and in several genera in the Orchidaceae. Similar to ectomycorrhizae it forms a sheath around the root tips. Monotropoid mycorrhizae are parasitic and non-mutualistic. The plants are heterotrophic or mixotrophic and derive their carbon from the fungus partner. Monotropoid mycorrhizae are associated with mycoheterotrophic plants like Indian pipes (Monotropa uniflora).

Vesicular Arbuscular Mycorrhizae

Vesicular arbuscular mycorrhizae (VAM) are symbiotic relationships between plant roots and fungi, beneficial for nutrient uptake in plants. In this association, fungi form structures called vesicles and arbuscules within the plant root cells. The vesicles serve as storage organs for nutrients, while arbuscules facilitate nutrient exchange between the fungus and the plant. VAM significantly enhance plant growth by improving nutrient absorption, particularly phosphorus, and aiding in water uptake, thereby playing a crucial role in the ecosystem's nutrient cycling and plant health.

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Mycorrhizae Examples

There are several types of mycorrhizae, but the two most common are ectomycorrhizae and arbuscular mycorrhizae.Some example of Mycorrhizae are:

• Ectomycorrhizae Examples:
  • Pines: Many species of pine trees form ectomycorrhizal associations, such as the Scots pine.
  • Oak Trees: Oaks, including species like the northern red oak , often engage in ectomycorrhizal symbiosis.
  • Birch Trees: Birch trees, such as the silver birch, commonly form ectomycorrhizal relationships.
• Arbuscular Mycorrhizae Examples:
  • Grasses: Many grass species, including wheat and maize, form arbuscular mycorrhizal associations.
  • Legumes: Plants like soybeans and peas often establish arbuscular mycorrhizal relationships.
  • Sunflowers: Sunflowers, such as the common sunflower, can form arbuscular mycorrhizal associations.

Importance of Mycorrhizae

Mycorrhizae are networks of fungi that live in the soil and help plants grow and survive. They can improve a plant's ability to absorb nutrients and protect plants from diseases and droughts. Some benefits of mycorrhizae are:

• Mycorrhizae can increase the surface area of roots, which improves a plant's ability to uptake water and nutrients from the soil.
• It reduces the irrigation and fertilizer requirements.
• It makes the plant drought and pest resistant.
• Mycorrhizae can increase plant tolerance to different environmental stresses.
• Mycorrhizae can help build and maintain soil quality and structure. They can also create humic compounds and organic glues that improve soil porosity and aggregate formation.
• It can help plants establish and survive at seeding or transplanting.
• The associations contribute to carbon sequestration in soils.
• Mycorrhizae contribute to the diversity and stability of plant communities.

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Conclusion - Mycorrhizae

Mycorrhizae are a symbiotic relationship between fungi and plant roots. The fungi colonize the plant's root system, increasing the plant's ability to absorb water and nutrients. In return, the plant provides the fungus with carbohydrates from photosynthesis. Mycorrhizae can come in many forms, depending on the host plant and the fungal taxonomy. There are seven types of mycorrhizae, including Endo (arbuscular), Ecto, Ectendo, Arbutoid, Monotropoid, Ericoid, Orchidaceous. Mycorrhizae play a key role in nutrient cycling in ecosystems and protect host plants from environmental stress.

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