Endomycorrhizal fungi
Introduction
Endomycorrhizal fungi (more commonly referred to as endomycorrhizae) is one of the major types of known mycorrhizae which differs from the another type of mycorrhizae, ectomycorrhizae, in structure. Unlike ectomycorrhizae which form a system of hyphae that grow around the cells of the root, the hyphae of the endomycorrhizae not only grow inside the root of the plant but penetrate the root cell walls and become enclosed in the cell membrane as well (1). This makes for a more invasive symbiotic relationship between the fungi and the plant. The penetrating hyphae create a greater contact surface area between the hyphae of the fungi and the plant. This heightened contact facilitates a greater transfer of nutrients between the two. Endomycorrhizae have further been classified into five major groups: arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizae (2).
Arbuscular Mycorrhizae
Endomycorrhizae have several functions, the major one being nutrient acquisition. Endomycorrhizae facilitate the exchange of nutrients between the host plant and the soil. Mycorrhizae aid in the uptake of water, inorganic phosphorus, mineral or organic nitrogen, and amino acids. In exchange for the mycorrhizae providing all of these nutrients, the plant in turn provides the mycorrhizae with carbon (1). This relationship benefits both organisms immensely. The mycorrhizae greatly increase the surface area of the plant’s root system which is hugely beneficial in areas where drought is common. This is also beneficial in areas where the soil is nutrient-poor. The larger surface area gives those plants an advantage over plants lacking this symbiotic relationship allowing plants with mycorrhizal relationships to out-compete for nutrients. Mycorrhizae also offer the roots of the plant a little more protection (3).
Ericoid Mycorrhizae
Ericoid mycorrhizae are found in inhospitable environments, particularly acidic environments (5). The fungi involved in this symbiotic relationship are Ascomycota. Despite the harsh conditions the mycorrhizae are still able to take up nitrogen and phosphorous for the host plant. The mycorrhizae also helps to regulate the acquisition of iron, manganese and aluminium ions which are often present in highly available forms in acidic soils (6). Ericoid mycorrhizae differ not only in where they are found but also in structure. Instead of forming arbuscules the fungi forms hyphal coils in the exterior cells of the fine root hairs of plants in the family Ericaceae (4). Root volume can be up to eighty percent fungal tissue and it is through these coils that nutrient exchange occurs. These fungi can actually be found free-living in the soil but the symbiotic relationship between the fungi and plant is thought to be more beneficial to both species (6).
Arbutoid Mycorrhizae
Arbutoid Mycorrhizae are found in the plant genera Arctostaphylos and Arbutus. The fungi that form arbutoid mycorrhizal relationships are basidiomycetes. Most fungal species that form ectomycorrhizal associations are also basidiomycetes. In arbutoid mycorrhizal associations a fungal sheath, or mantle covers the roots of the host plant, similar to the structure of ectomycorrhizal associations. The fungal hyphae also form a structure known as a Hartig Net into the outer cortical cells. Nutrients acquired from the soil pass through the mantle into the roots of the host plant. Because the sheath can also function as a place to store excess nutrients, at times when nutrient levels are running low the fungi can release the stored nutrients into the plant. The major difference between ectomycorrhizal fungi and arbutoid fungi is that the hyphae of the arbutoid fungi do in fact penetrate the outer cortical cells of the plant root. The hyphae form coils within the cells which allow for the transfer of nutrients from the fungi to the plant and vice versa (7).
Monotropoid Mycorrhizae
Until recently monotropoid mycorrhizae were thought to be part of the arbutoid mycorrhizae group. While arbutoid mycorrhizal hyphae penetrate and from extensive structures within the cells of the host plant, monotropoid mycorrhizae do not penetrate the cell walls of the host plant. Monotropoid fungi produce no chlorophyll. Because of this they are unable to perform photosynthesis or produce their own carbohydrates. Due to their inability to perform either of these functions the monotropoid mycorrhizal fungi use their hyphae to acquire not only minerals and nutrients, but also to obtain carbon from their host plant. Monotropoid mycorrhizae are most commonly found in coniferous forests and tend to form their symbiotic relationships with pine, spruce and fir trees but they are known to form monotropoid mycorrhizal associations with other trees such as beech, oak and cedar. Like arbutoid mycorrhizae, monotropoid mycorrhizae form a dense sheath around the roots of the host plant and a Hartig net which surrounds the outer epithelial layer of the root cells. Individual hyphae may then grow out of the Hartig net into the outer cortical cells. The walls of these cells are not penetrated by the hyphae but bend to accommodate the growing hyphae. This type of growth is referred to as a fungal peg. These growths greatly increase the surface area of the cell which allows for an easier transfer of nutrients between the fungi and the host plant. At some point the fungal peg will rupture allowing a membranous sac to extend from the peg into the cytoplasm of the cell. The sac is filled with the contents of the fungal peg, but the contents never directly enter the cytoplasm of the cell. It is thought that the bursting of the fungal peg provides a surge of nutrients to help seed production (8).
Orchid Mycorrhizae
At some point in their life cycle all orchids go through a period of time where they are not photosynthetic. During this time the orchid cannot perform photosynthesis or manufacture its own carbohydrates so it must rely upon mycorrhizal fungi to provide it with nutrients. Generally orchids are not photosynthetic when the orchid is in the seedling stage of its life. During this time the orchid relies on the mycorrhizal fungi to provide the nutrients, particularly carbohydrates, needed for the growth of the seedling. In many cases orchid seeds will not begin to germinate until they have formed an association with a mycorrhizal fungus. The mycorrhizal fungus can form an association with the seedling once the seed coat has ruptured from water absorption and root hairs emerge. The hyphae penetrate the cells of the embryo form hyphal coils called pelotons within the cells. As the orchid matures it tends to rely less on the mycorrhizae for nutrients like carbohydrates, but still acquires phosphorous and nitrogen through the association (9).
Function of Endomycorrhizal fungi
Endomycorrhizae have several functions, the major one being nutrient acquisition. Endomycorrhizae facilitate the exchange of nutrients between the host plant and the soil. Mycorrhizae aid in the uptake of water, inorganic phosphorus, mineral or organic nitrogen, and amino acids. In exchange for the mycorrhizae providing all of these nutrients, the plant in turn provides the mycorrhizae with carbon. This relationship benefits both organisms immensely. The mycorrhizae greatly increase the surface area of the plant’s root system which is hugely beneficial in areas where drought is common. This is also beneficial in areas where the soil is nutrient-poor. The larger surface area gives those plants an advantage over plants lacking this symbiotic relationship allowing plants with mycorrhizal relationships to out-compete for nutrients. Mycorrhizae also offer the roots of the plant a little more protection (1).
Conclusion
Endomycorrhizal fungi are an incredibly diverse group of organisms that are highly adapted their environments. They were probably crucial in the colonization of land by plants. They perform so many functions for their host plant and it is undebatable that the variety and success of vascular plants would be greatly diminished without mycorrhizal fungi. These symbiotic relationships have evolved over many millions of years into highly functioning associations that benefit both species.
References
(2) [Peterson, R. L.; Massicotte, H. B. & Melville, L. H. (2004). Mycorrhizas: anatomy and cell biology. National Research Council Research Press. ISBN 978-0-660-19087-7.]
(4) Davies, F. "Dr. Davies Research Page." Dr. Davies Research Page. Texas A&M University, n.d.
(5) [Smith SE & Read DJ (2008) Mycorrhizal Symbiosis. Academic press, London.]
Edited by Alexandra Schaal, student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2013, Kenyon College.
