A mycorrhiza (typically seen in the plural forms mycorrhizae or mycorrhizas, Greek for fungus roots) is the result of a symbiotic association between a fungus and a plant. This symbiosis takes place at the root level, where individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant.
This symbiotic association provides the fungus with a renewable source of food through access to fixed carbon (sugars) from plant photosynthate. These are translocated to the root tissues from their source location (usually leaves), and then to the fungal partners. In return, the plant gains the use of the mycelium's tremendous surface area to absorb mineral nutrients from the soil. It is believed that the mycelia of mycorrhizal networks have better mineral absorption capabilities compared to plant roots. An example of this is the manner in which phosphate ions are tightly bound to iron oxides in many soils. Plant roots are generally incapable of accessing these phosphorus sources (which can be large and are termed sinks), yet mycorrhizal mycelia can access these forms of phosphorus. The mechanisms of increased absorption are both physical—mycorrhizal mycelia are much smaller in diameter than the smallest root hair and thus are able to explore a greater volume of soil and have a much larger surface area for absorption—and chemical—the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient poor soils.
Furthermore, mycorrhizal plants are generally more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought.
The cytoplasmic streaming of the mycorrhizal hypha is a mechanism that facilitates the transfer of nutrients from the soil, at relatively remote distances from the root, to the root at rates far exceeding those that would be possible by osmotic flow alone. This has an energy cost to the fungus. In return, the mycorrhizal fungus is rewarded by the "payment" of nutrients in the form of sugars, starches, proteins and lipids from the plant roots. These nutrients, in turn, flow to the whole mycelial network through cytoplasmic streaming.
Mycorrhizas form a mutualistic symbiotic relationship with the roots of most plant species (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 "infect" 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.
Early evidence of mycorrhizal associationsEdit
Some of the earliest fossil plants show evidence of mycorrhizas associated with them. Their structure has been higly conserved since they first colonize the soil about 400 million years ago. This date is important since it corresponds to the transition from aquatic to terrestrial life forms. It is then believed that the fungal partner played a key role in helping plants to colonize the land.
Types of mycorrhizaeEdit
The two most common types of mycorrhizas are the ectomycorrhizas and the endomycorrhizas (also known as arbuscular mycorrhizas). The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate the cell wall of the plant's root cells, while the hyphae of arbuscular mycorrhizal fungi penetrate the cell wall.
Arbuscular mycorrhizas (formerly known as vesicular-arbuscular mycorrhizas) are an example of a mycorrhiza that involves entry of the hyphae into the plant cell walls to produce structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules). The fungal hyphae does not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginates 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 mycorrhizal fungi are quite extraordinary organisms. First they have been asexual for many million years and secondly, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis) .
Other forms of mycorrhizaeEdit
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