Endomycorrhizae

Far more common are the endomycorrhizae, which have a zygomycete as the fungal component and which actually penetrate the cell walls of the root cortex. Although the hyphae do not enter the cytoplasm of the cortical cells, in most cases they cause the plasma membrane to bulge inward, forming highly branched structures called arbuscules and terminal swellings called vesicles. Thus, this type of endomycorrhizae is referred to as vesicular-arbuscular mycorrhizae, or VAM. The arbuscules are in intimate contact with the cortical cells and provide an increased surface area over which carbohydrates can pass from the plant to the fungus and mineral elements from the fungus to the plant. The vesicles are thought to function as storage compartments for the fungus. As with the ectomycorrhizae, the fungal hyphae extend from the root into the soil and increase the surface area for absorption, but there is no mantle or Hartig net, and root hairs are often present. The VAM are found on almost all herbaceous angiosperms, some gymnosperms, and many ferns and mosses. Endomycorrhizae are particularly important in the tropics where the soils are typically poor in phosphates. Studies have indicated that roots associated with

Fungal sheath

100 iim

Fungal sheath

100 iim

Ectotrophic mycorrhizal fungi infecting a root. Redrawn from Taiz and Zeiger, 1998, Figure 5.10.

pathogen disease-causing organism filament a threadlike extension hyphae the threadlike body mass of a fungus cortical relating to the cortex of a plant

External Cortex mycelium > Y

Root

Association of vesicular-arbuscular mycorrhizal fungi with a plant root section. Redrawn from Taiz and Zeiger, 1998, Figure 5.11.

External Cortex mycelium > Y

Root

Association of vesicular-arbuscular mycorrhizal fungi with a plant root section. Redrawn from Taiz and Zeiger, 1998, Figure 5.11.

enzyme a protein that controls a reaction in a cell compound a substance formed from two or more elements ecosystem an ecological community together with its environment mycorrhizal fungi can take up phosphate four times faster than roots without such fungi. Mycorrhizal fungi are particularly effective in utilizing highly insoluble rock phosphorus, Ca3(PO4)2, that cannot be used by plants. The fungal hyphae make phosphates available to the plant by converting them to a soluble form.

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