Definition
A mycorrhiza is a symbiotic association between a fungus and the roots of a plant.
Basics
A mycorrhiza (Greek for fungus roots) is a symbiotic association between a fungus and the roots of a plant. In a mycorrhiza, the fungus lives inside the roots. Mycorrhizas are important for plant growth in many ecosystems. Over 90% of all plant species have mycorrhiza: they depend on this for survival. They are the most common symbionts in the plant kingdom.
Mycorrhizas are divided onto two types: ectomycorrhizas and endomycorrhizas. The hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane.
The mycorrhizal symbiosis is ancient, dating to at least 400 million years ago.
How it works: Diagram of mycorrhiza with terms in Spanish. It is an endomycorrhiza: the arbuscules or vesicles are inside the plant cell wall, and attached to the cell membrane.This mutualism gives the fungus sugars, such as glucose and sucrose produced by the plant in photosynthesis. The carbohydrates move from their source (usually leaves) to the root and then to the fungal partner. In return, the plant gains the use of the mycelium's very large surface area to absorb water and mineral nutrients from the soil, especially phosphorus.
The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root. They can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane of fungi is different from that of plants. Mycorrhizae are especially helpful for the plant partner in nutrient-poor soils.
Advantages Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants. Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in harsh areas.
The fungal partners may also help plant-to-plant transfer of sugars and other nutrients. Such mycorrhizal communities are called common mycorrhizal networks. Some species inhabit the tissues inside roots, stems, and leaves, in which case they are called endophytes. Similar to mycorrhiza, endophytic colonization by fungi may benefit both partners. Endophytes of grasses give their host more resistance to grazers and get food and shelter from the plant in return.
Topics of Interest
A mycorrhiza (pl mycorrhizae, mycorrhizas) is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a plant.
In a mycorrhizal association, the fungus colonizes the host plants' roots, either intracellularly as in arbuscular mycorrhizal fungi (AMF), or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.
Mutualist dynamics Mycorrhizae form a mutualistic relationship with the roots of most plant species (and while only a small proportion of all species has been examined, 95% of these plant families are predominantly mycorrhizal).
Sugar-Water/Mineral exchange: This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant. The carbohydrates are translocated from their source (usually leaves) to root tissue and on to fungal partners. In return, the plant gains the benefits of the mycelium's higher absorbtive capacity for water and mineral nutrients (due to comparatively large surface area of mycelium:root ratio), thus improving the plant's mineral absorption capabilities. Plant roots alone may be incapable of taking up phosphate ions that are demineralized, for example, in soils with a basic pH. The mycelium of the mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonize.
Disease resistance Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drough. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.
Colonization of barren soil Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.
Types of mycorrhiza: Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas. The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane. A third group known as Ericoid mycorrhizae is also ecologically significant.
Ericoid mycorrhizas are the third of the three more ecologically important types, They have a simple intraradical (grow in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps (probably in the form of small cups), but their reproductive biology is little understood.
Rhizobia are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). Rhizobia require a plant host; they cannot independently fix nitrogen. Morphologically, they are generally gram negative, motile, non-sporulating rods.
An arbuscular mycorrhiza (plural mycorrhizae or mycorrhizas) is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant.
Orchid mycorrhiza are a symbiotic relationship between the roots of plants of the family Orchidaceae and a variety of fungi. All orchids are mycoheterotrophic at some point in their life cycle. Orchid mycorrhiza are critically important during orchid germination, as orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont. Many adult orchids retain their fungal symbionts, although the benefits to the adult photosynthetic orchid and the fungus remain largely unexplored.
Ericoid mycorrhiza are a symbiotic relationship between fungi and the roots of plants from the order Ericales. Ericoid mycorrhiza are considered crucial for the success of the family Ericaceae in variety of edaphically stressful environments worldwide. Ericaceous plants commonly co-occur in soils with leguminous or carnivorous plants, further highlighting the low nutrient status of these soils. Ericoid mycorrhizal fungi, which are predominately ascomycetes, enable their host plant to obtain nutrients in these depauperate soils.
Myco-heterotrophy is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food from parasitism upon fungi rather than from photosynthesis. A myco-heterotroph is the parasitic plant partner in this relationship. Myco-heterotrophy is considered a kind of cheating relationship and myco-heterotrophs are sometimes informally referred to as "mycorrhizal cheaters". This relationship is sometimes referred to as mycotrophy, though this term is also used for plants that engage in mutualistic mycorrhizal relationships.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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