Mycorrhizal Fungi (part 1)

Most plants take up nutrients with the aid of symbiotic interactions. In 1885 Albert Bernhard Frank discovered mycorrhizae (which mean root fungus); he believed they took over the function of the root hairs that were not adequate. Mycorrhizae are split into two types; endomycorrhizae (inside the cell wall) and ectomycorrhizae (outside the cell wall).

Arbuscular mycorrhizal fungi are a type of endomycorrhizae and are some of the most abundant organisms on Earth; they form symbiotic relationships with almost all land plants and vegetation. The mycorrhizae are important in the uptake of nutrients from the soil, consequently playing an important role in plant health. This symbiotic relationship has been shown not only to improve plant growth but to modify community structure and efficiency because they allow a greater uptake of nitrogen and in particular phosphorus which is a common limiting nutrient in many soils.

In exchange for this support the plant in return, provides the mycorrhizae with the carbon and carbohydrates it needs for repair, growth and reproduction. The carbohydrates are usually taken out of the leaves of the plant, transported down into the roots and into the mycorrhizae. The mycorrhizae is able to take up more nutrients and water than the plants root system alone because of the sizeable surface area of the hyphae compared to the root system, increasing the mineral absorption efficiency.

The hyphae of the mycorrhizae attach to a plant root and grow along its surface which then produce swellings called appressoria between epidermal cells (outer layer of the plant); From here the hyphae penetrate and enter the root and delve into the cortex (first internal layer), spread out and form a colony. From the hyphae, arbuscules begin to form, which are the primary sites of nutrient exchange between the plant and the mycorrhizae due to the large surface area of interaction. Arbuscules grow inside individual cells but outside the cytoplasm, they start to form about two days after the hyphae have penetrated the root and may only last up to a week. In contrast hyphae may remain for years if undisturbed.

Hyphae can form networks underground that connect individual plants to make communities enabling the exchange of water, carbon and nutrients between plants. Some benefits to being a part of this network include factors such as seedlings can become a part of this it to help them grow from an early stage. The balance of competition between plants may be changed if nutrients are being exchanged and taken from a mycorrhizal network than if the plants were all acting alone, this may reduce competitive dominance. Nutrients from roots connected to a mycorrhizal network that are dying can be transferred directly to the plants without ever having to become a part of the soil itself.

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Kyle Bennett

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