A mycorrhiza (Greek for fungus roots coined by Frank, 1885; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant. In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.
This mutualistic association provides the fungus with relatively constant and direct access to mono- or dimeric carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis. The carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the mycelium's very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots. Plant roots alone may be incapable of taking up phosphate ions that are immobilized, 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. The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.
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.
Mycorrhizae form a mutualistic relationship with the roots of most plant species (although only a small proportion of all species have been examined, 95% of all plant families are predominantly mycorrhizal).
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.
Occurrence of mycorrhizal associations
At around 400 million years old, the Rhynie chert contains the earliest fossil assemblage yielding plants preserved in sufficient detail to detect mycorrhizae - and they are indeed observed in the stems of Aglaophyton major.
Mycorrhizae are present in 92% of plant families (80% of species), with arbuscular mycorrhizae being the ancestral and predominant form, and indeed the most prevalent symbiotic association found in plants at all. The structure of arbuscular mycorrhizae has been highly conserved since their first appearance in the fossil record, with both the development of ectomycorrhizae, and the loss of mycorrhizae, evolving convergently on multiple occasions.
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.
Endomycorrhiza are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizae . Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, producing structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules). The fungal hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane. The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm to facilitate the transfer of nutrients between them.
Arbuscular mycorrhizae are formed only by fungi in the division Glomeromycota. Fossil evidence and DNA sequence analysis suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species. The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil. Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis).
Many plants in the order Ericales form ericoid mycorrhizas, while some members of the Ericales form arbutoid and monotropoid mycorrhizas. All orchids are mycoheterotrophic at some stage during their lifecycle and form orchid mycorrhiza with a range of basidiomycete fungi.
Ectomycorrhizas, or EcM, are typically formed between the roots of woody plants and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota. Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a hartig net of hyphae surrounding the plant cells within the root cortex. In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza. Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter.
The ectomycorrhizal fungus Laccaria bicolor has been found to lure and kill springtails to obtain nitrogen, some of which may then be transferred to the mycorrhizal host plant. In a study by Klironomos and Hart, Eastern White Pine inoculated with L. bicolor was able to derive up to 25% of its nitrogen from springtails.
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