How do entities in a mutualistic association benefit from each other as seen in mycorrhiza ? Class XI
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Mutualistic Relationships
Symbiosis is the ecological interaction between two organisms that live together. The definition does not describe the quality of the interaction. When both members of the association benefit, the symbiotic relationship is called mutualistic. Fungi form mutualistic associations with many types of organisms, including cyanobacteria, algae, plants, and animals.
Fungus-Plant Mutualism
One of the most remarkable associations between fungi and plants is the establishment of mycorrhizae. Mycorrhiza, which comes from the Greek words myco meaning fungus and rhizo meaning root, refers to the association between vascular plant roots and their symbiotic fungi. Somewhere between 80 and 90 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus.
There are a number of types of mycorrhizae. Ectomycorrhizae (“outside” mycorrhiza) depend on fungi enveloping the roots in a sheath (called a mantle) and a Hartig net of hyphae that extends into the roots between cells (Figure 1).
Part A compares two types of mycorrhizae are shown: ectomycorrhiza and arbuscular mycorrhiza. In ectomycorrhiza, fungal hyphae form a structure called a Hartig net inside the root. The Hartig net forms rows of cells that extend straight down, and branch toward the outside of the root. A fungal mantle surrounds the root. Mycelia extend from the fungal mantle. In arbuscular mycorrhiza, the fungi form finger-like clusters that are connected to mycelia that extend from the root into the soil. Part B is a micrograph of arbuscular mycorrhiza, which appear as grape-like clusters in a root tip.
Figure 1. (a) Ectomycorrhiza and (b) arbuscular mycorrhiza have different mechanisms for interacting with the roots of plants. (credit b: MS Turmel, University of Manitoba, Plant Science Department)
The fungal partner can belong to the Ascomycota, Basidiomycota or Zygomycota. In a second type, the Glomeromycete fungi form vesicular–arbuscular interactions with arbuscular mycorrhiza (sometimes called endomycorrhizae). In these mycorrhiza, the fungi form arbuscules that penetrate root cells and are the site of the metabolic exchanges between the fungus and the host plant (Figure 1 and Figure 2). The arbuscules (from the Latin for little trees) have a shrub-like appearance. Orchids rely on a third type of mycorrhiza. Orchids are epiphytes that form small seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner (usually a Basidiomycete). After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their lifecycle.
A mycorrhiza is a symbiotic association between the roots of vascular plants and a fungus. This association provides the fungus with a constant supply of carbohydrates. Plants in return procure the benefit of the mycelium of the fungus which promotes its absorptive capacity of minerals and water provided the large surface area of mycelium.