The Mutualistic Relationship between Fungi and Plants: Mycorrhizae and their Types

Mutualism in fungi refers to a symbiotic relationship between two organisms, where both organisms benefit from each other's presence. In the case of fungi, mutualism often involves a mutualistic association with another organism, such as plants or animals, where the fungi provide essential nutrients to the host in exchange for carbohydrates produced by the host through photosynthesis. In some cases, the mutualistic association can be so strong that the host and the fungus become almost completely dependent on each other for survival.

Lichens are a symbiotic association between a fungus (mostly Ascomycetes and imperfect fungi, and a few basidiomycetes), a cyanobacterium, and a green alga. The body of the lichen is made up of three layers: (1) a thin and tough upper layer consisting of fungal hyphae, (2) a middle layer composed of fungal hyphae interwoven with photosynthetic cells, and (3) a loosely packed bottom layer consisting of fungal hyphae. Specialized fungal hyphae that penetrate or envelop the photosynthetic cells transfer nutrients directly to the rest of the fungus.

A cross section of lichen

Different types of lichen

In the past, lichens were assumed to have mutualistic relationships in which the fungus received nutrients from the algae cells, and the algae cells were protected from desiccation by the fungus. However, recent research suggests that lichens may involve a controlled form of parasitism of the algae cells by the fungus. There are three recognized types of lichens.

Lichens come in three different growth forms that are as follows:

 (1) Crustose Lichens: These are compact and can often be seen on bare rocks or tree bark.

(2) Foliose Lichens: These are leaf-like in appearance.

(3) Fruticose Lichens: These are shrub-like and branched.

 

In terms of appearance, lichens can vary in color, shape, and overall appearance. They are highly efficient at acquiring nutrients and moisture, enabling them to survive in areas with low moisture and temperature as well as in areas with poor or no soil. They also have the ability to produce and improve the soil, making it suitable for other plants to grow.

However, lichens have a downside too. They absorb pollutants from the air, and therefore, cannot survive in areas where the air is polluted. Thus, their presence serves as a bio-indicator for the quality of the air that we breathe.

 

Mycorrhizae

Mycorrhizae are beneficial associations between fungi in soil and the roots of most plants, occurring in about 95% of higher plant families. Fungal hyphae increase the surface area of the roots for absorption, aiding in the direct uptake of phosphorus, zinc, copper, and other nutrients from the soil. The presence of mycorrhizae enhances plant growth and success, and plants lacking these associations are at a disadvantage. Two main types of mycorrhizae are recognized based on the location of fungal mycelium:

(1) Endomycorrhizae: These penetrate only the outer cells of plant roots, forming coils, swellings, and small branches while also extending into the surrounding soil.

(2) Ectomycorrhizae: These form a mantle outside the root and grow between cell walls. They are often formed with pines, firs, and other tree species.

Endomycorrhizae and Ectornycorrhizae