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Phylum Glomeromycota - Mycorrhizae
Phylum Glomeromycota - Mycorrhizae
In the previous stations you examined fungi that grow independently on organic material such as bread, plant debris, or animal dung. However, many fungi spend their entire lives living in close partnership with plants.
More than 400 million years ago, when the first plants began colonizing land, they faced a major challenge: obtaining nutrients from soil. Early plants had very simple root systems and were not efficient at absorbing minerals from their environment. Evidence from fossils and molecular studies suggests that early plants formed partnerships with fungi that helped them obtain nutrients from the soil.
Fungi in the phylum Glomeromycota represent one of the oldest and most widespread examples of this partnership. These fungi live in association with plant roots and form structures known as mycorrhizae, a term that literally means fungus-root. Through this relationship both the plant and the fungus benefit.
Mycorrhizae: The Plant–Fungus Partnership
Mycorrhizae: The Plant–Fungus Partnership
A mycorrhiza is a symbiotic relationship between a fungus and the roots of a plant. In this relationship the plant provides the fungus with carbohydrates produced during photosynthesis, which serve as an energy source for the fungus.
In return, the fungal hyphae absorb water and mineral nutrients from the soil, particularly phosphorus and nitrogen, and transport these nutrients back to the plant roots.
Because fungal hyphae are extremely thin compared to plant roots, they can explore much smaller spaces between soil particles. This greatly increases the surface area from which the plant can absorb nutrients and water. In effect, the fungal network functions as an extension of the plant’s root system.
The association between fungi in the soil and plant roots can be extensive. In many ways the fungus acts almost like an external digestive system for the plant, breaking down materials in the soil and exchanging those nutrients for sugars produced by the plant.
The association between fungi in the soil and the roots of a plant can be very extensive. In many ways the fungi are acting as the digestive system for the plant digesting organic matter in the soil and exchanging the organic molecules for photosynthetic products.
Ectomycorrhizae and Endomycorrhizae
Ectomycorrhizae and Endomycorrhizae
Mycorrhizal fungi interact with plant roots in two primary ways.
In ectomycorrhizae, fungal hyphae form a dense sheath around the outside of the root. The hyphae grow between the root cells but do not penetrate the cells themselves. These associations are common in many forest trees and are often associated with fungi that produce mushrooms.
Fungi in the phylum Glomeromycota, however, form endomycorrhizae, also known as arbuscular mycorrhizae. In this type of relationship the fungal hyphae penetrate the root cells of the plant.
This diagram illustrates the difference between ectomycorrhizae and endomycorrhizae. Fungi from the phylum Glomeromycota form endomycorrhizae and represent the majority of mycorrhizal associations (approximately 70%). Ectomycorrhizae are typically formed by fungi in the phyla Basidiomycota and Ascomycota, and in these associations the hyphae do not penetrate the cell walls of root cells. This type of relationship is common in trees such as beech, oak, and pine.
This diagram illustrates the difference between endo- and ectomycorrizae. Fungi from the phylum Glomeromycota for endomycorrizae and they represent the vast majority of mycorrizal associations (~70%). Ectomycorriza are formed from fungi from the phyla Basidiomycota and Ascomycota and these hypha do not penetrate the cell walls of root cells. This association is typical of beech, oak and pine trees.
Arbuscules
Arbuscules
The defining structure of Glomeromycota fungi is the arbuscule.
When a fungal hypha enters a root cell it branches repeatedly, forming a highly complex structure that resembles a tiny tree. The name arbuscule comes from the Latin word for “little tree.”
These branching structures dramatically increase the surface area where the plant and fungus exchange nutrients. Through the arbuscule the plant supplies sugars to the fungus, while the fungus provides mineral nutrients and water to the plant.
Because this exchange occurs within the root cells themselves, arbuscular mycorrhizae are classified as endomycorrhizal associations.
In these relationships the fungus grows between root cells and then enters the cell wall of an individual cell but does not penetrate the plasma membrane. This creates a very close interaction between the plant cell and the fungal hyphae.
In endomycorrhizae the fungi penetrate into the root between cells and then will enter the cell wall of an individual cell but not the plasma membrane. This creates a very intimate connection between the cell and the hyphae. On the right you can see the dark purple strands that are the hyphae inside of the cell wall of the root cell.
Lab Activity: Endomycorrhizae (Arbuscular Mycorrhizae)
Lab Activity: Endomycorrhizae (Arbuscular Mycorrhizae)
Materials
Materials
Prepared slide of endomycorrhizae (plant root with fungal association)
Compound microscope
Notes: Handle microscope slides carefully to avoid breakage.
Instructions
Instructions
Place the prepared slide of endomycorrhizae on the microscope stage and examine the root tissue.
Locate the plant root cells and the fungal hyphae associated with them.
Observe how the fungal hyphae penetrate the root cells rather than forming a layer on the outside of the root.
Look for the highly branched structures formed inside the root cells. These structures are called arbuscules and are the main sites where nutrients are exchanged between the plant and the fungus.
In some areas you may also observe vesicles, which are rounded storage structures produced by the fungus.
Compare this arrangement with ectomycorrhizae discussed earlier. Consider how the location of the fungal hyphae differs between ectomycorrhizae and endomycorrhizae.
Identify the following features on the slide and use them to answer the questions on your worksheet:
Root cells
Fungal hyphae inside the root
Arbuscules (branching structures inside cells)
Vesicles (if visible)
Clean Up
Clean Up
Return the prepared slide to its storage container.
Turn off the microscope and leave the workstation ready for the next group.
The Underground Fungal Network
The Underground Fungal Network
Although part of the fungus grows inside the plant root, much of the fungal body extends outward through the surrounding soil.
These hyphae form an extensive network that continually explores the soil for nutrients and water. Nutrients absorbed by the fungal hyphae are transported back to the plant through the mycorrhizal connection.
This underground fungal network greatly increases the plant’s ability to obtain nutrients from the soil and allows plants to grow successfully in environments where nutrients might otherwise be limited.
These images show spores and hyphae of an arbuscular mycorrhizal fungus associated with plant roots.
These are spores and hyphae of an arbuscular mycorrizae and a plant root.
Ecological Importance
Ecological Importance
Fungi in the phylum Glomeromycota play a major role in terrestrial ecosystems. By improving nutrient uptake, these fungi can significantly enhance plant growth and survival.
Mycorrhizal fungi also influence the structure of plant communities by helping certain plants establish and compete successfully in nutrient-poor soils. In many ecosystems the majority of plants depend on mycorrhizal relationships to obtain sufficient nutrients for growth.
These fungi also contribute to soil health. Networks of fungal hyphae help stabilize soil structure, improve soil aggregation, and influence nutrient cycling within ecosystems.
Because of these benefits, Glomeromycota fungi are increasingly studied in agriculture. Researchers are exploring ways to encourage mycorrhizal fungi in crop systems in order to improve nutrient uptake and reduce reliance on chemical fertilizers.
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