Mycorrhizal network
A mycorrhizal network is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individual plants together. Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times. Mycorrhizal networks were discovered in 1997 by Suzanne Simard, professor of forest ecology at the University of British Columbia in Canada. Simard grew up in Canadian forests where her family had made a living as foresters for generations. Her field studies revealed that trees are linked to neighboring trees by an underground network of fungi that resembles the neural networks in the brain. In one study, Simard watched as a Douglas fir that had been injured by insects appeared to send chemical warning signals to a ponderosa pine growing nearby. The pine tree then produced defense enzymes to protect against the insect.
The formation and nature of these networks is context-dependent, and can be influenced by factors such as soil fertility, resource availability, host or mycosymbiont genotype, disturbance and seasonal variation. Some plant species, such as buckhorn plantain, a common lawn and agricultural weed, benefit from mycorrhizal relationships in conditions of low soil fertility, but are harmed in higher soil fertility. Both plants and fungi associate with multiple symbiotic partners at once, and both plants and fungi are capable of preferentially allocating resources to one partner over another.
Mycorrhizal associations have profoundly impacted the evolution of plant life on Earth ever since the initial adaptation of plant life to land. In evolutionary biology, mycorrhizal symbiosis has prompted inquiries into the possibility that symbiosis, not competition, is the main driver of evolution.
Referencing an analogous function served by the World Wide Web in human communities, the many roles that mycorrhizal networks appear to play in woodland have earned them a colloquial nickname: the "Wood Wide Web". Many of the claims made about common mycorrhizal networks, including that they are ubiquitous in forests, that resources are transferred between plants through them, and that they are used to transfer warnings between trees, have been criticised as being not strongly supported by evidence.
Definitions and types
As a scientific term, mycorrhizal network has broad meanings and usage. Scientific understandings and thus publications utilize more specific definitions arising from the term common mycorrhizal network. The keyword "common" requires that two or more individual plants are connected by the same underground fungal network, through which matter of various types and functions may flow. The plants themselves may be individuals of the same or different species. In turn, the fungal network that is composed of threadlike hyphae may be limited to a single type or entail several. The kinds of evidence deemed necessary for supporting scientific conclusions, along with the tendency for disputes to arise, depend in part on the definitions used.There are two main types of mycorrhizal networks. These are determined by the two main categories of fungal growth forms. Arbuscular mycorrhizal networks are those in which fungal hyphae not only enter the plant's roots but also penetrate into the cells themselves. Ectomycorrhizal networks send hyphae into the roots where they thread their way between the plant cells but do not penetrate cell walls. The arbuscular type is the most common among land plants and is regarded as the ancestral type. However, tree species comprising the canopy of temperate and especially boreal forests in the Northern Hemisphere tend to associate with ectomycorrhizal fungi.
Plant and fungal partners within a network may enact a variety of symbiotic relationships. Earliest attention was given to mutualistic networks by which the plant and fungal partners both benefit. Commensal and parasitic relationships are also found in mycorrhizal networks. A single partnership may change between any of the three types at different times.
File:Indian pipe PDB.JPG|thumb|alt='Monotropa' plant unable to photosynthesis, collects food from monotropoid mycorrhiza|Monotropa plant unable to photosynthesis, collects food from monotropoid mycorrhiza. see also Myco-heterotrophy
Knowns, unknowns, and controversies
The mycorrhizal symbiosis between plants and fungi is fundamental to terrestrial ecosystems, with evolutionary origins before the colonization of land by plants. In the mycorrhizal symbiosis, a plant and a fungus become physically linked to one another and establish an exchange of resources between one another. The plant provides to the fungus up to 30% of the carbon it fixes by photosynthesis, while the fungus provides the plant with nutrients that are limiting in terrestrial environments, such as nitrogen and phosphorus.As this relationship has been better investigated and understood by science, interest has emerged in its potential influence on interactions between different plants, particularly in the possibility that connectivity through the mycorrhizal network may allow plants to positively impact the survival of other plants. Evidence and potential mechanisms for a variety of plant-plant interactions mediated by the mycorrhizal symbiosis have been presented, but their validity and significance is still controversial.
Proposed effects and functions of the mycorrhizal network
Potential nutrient and photosynthate transfer between plants
Since multiple plants can be simultaneously colonized by the same fungus, there has been interest in the possibility that inter-plant transfer of nutrients may occur via mycorrhizal networks, with photosynthates moving from a 'donor' plant to a 'recipient' plant. Numerous studies have reported that carbon, nitrogen and phosphorus are transferred between conspecific and heterospecific plants via AM and ECM networks. Other nutrients may also be transferred, as strontium and rubidium, which are calcium and potassium analogs respectively, have also been reported to move via an AM network between conspecific plants. It is possible that in this way, mycorrhizal networks could alter the behavior of receiving plants by inducing physiological or biochemical changes, and there is evidence that these changes have improved nutrition, growth and survival of receiving plants.Potential signaling and communication between plants
Reports discuss the ongoing debate within the scientific community regarding what constitutes communication, but the extent of communication influences how a biologist perceives behaviors. Communication is commonly defined as imparting or exchanging information. Biological communication, however, is often defined by how fitness in an organism is affected by the transfer of information in both the sender and the receiver. Signals are the result of evolved behavior in the sender and effect a change in the receiver by imparting information about the sender's environment. Cues are similar in origin but only effect the fitness of the receiver. Both signals and cues are important elements of communication, but workers maintain caution as to when it can be determined that transfer of information benefits both senders and receivers. Thus, the extent of biological communication can be in question without rigorous experimentation. It has, therefore, been suggested that the term infochemical be used for chemical substances which can travel from one organism to another and elicit changes. This is important to understanding biological communication where it is not clearly delineated that communication involves a signal that can be adaptive to both sender and receiver.Behavior and information transfer
A morphological or physiological change in a plant due to a signal or cue from its environment constitutes behavior in plants, and plants connected by a mycorrhizal network have the ability to alter their behavior based on the signals or cues they receive from other plants. These signals or cues can be biochemical, electrical, or can involve nutrient transfer. Plants release chemicals both above and below the ground to communicate with their neighbors to reduce damage from their environment. Changes in plant behavior invoked by the transfer of infochemicals vary depending on environmental factors, the types of plants involved and the type of mycorrhizal network. In a study of trifoliate orange seedlings, mycorrhizal networks acted to transfer infochemicals, and the presence of a mycorrhizal network affected the growth of plants and enhanced production of signaling molecules. One argument in support of the claim mycorrhizal can transfer various infochemicals is that they have been shown to transfer molecules such as lipids, carbohydrates and amino acids. Thus, transfer of infochemicals via mycorrhizal networks can act to influence plant behavior.There are three main types of infochemicals shown to act as response inducing signals or cues by plants in mycorrhizal networks, as evidenced by increased effects on plant behavior: allelochemicals, defensive chemicals and nutrients.
Allelopathic communication
is the process by which plants produce secondary metabolites known as allelochemicals, which can interfere with the development of other plants or organisms. Allelochemicals can affect nutrient uptake, photosynthesis and growth; furthermore, they can down regulate defense genes, affect mitochondrial function, and disrupt membrane permeability leading to issues with respiration.Plants produce many types of allelochemicals, such as thiophenes and juglone, which can be volatilized or exuded by the roots into the rhizosphere. Plants release allelochemicals due to biotic and abiotic stresses in their environment and often release them in conjunction with defensive compounds. In order for allelochemicals to have a detrimental effect on a target plant, they must exist in high enough concentrations to be toxic, but, much like animal pheromones, allelochemicals are released in very small amounts and rely on the reaction of the target plant to amplify their effects. Due to their lower concentrations and the ease in which they are degraded in the environment, the toxicity of allelochemicals is limited by soil moisture, soil structure, and organic matter types and microbes present in soils. The effectiveness of allelopathic interactions has been called into question in native habitats due to the effects of them passing through soils, but studies have shown that mycorrhizal networks make their transfer more efficient. These infochemicals are hypothesized to be able to travel faster via mycorrhizal networks, because the networks protect them from some hazards of being transmitted through the soil, such as leaching and degradation. This increased transfer speed is hypothesized to occur if the allelochemicals move via water on hyphal surfaces or by cytoplasmic streaming. Studies have reported concentrations of allelochemicals two to four times higher in plants connected by mycorrhizal networks. Thus, mycorrhizal networks can facilitate the transfer of these infochemicals.
Studies have demonstrated correlations between increased levels of allelochemicals in target plants and the presence of mycorrhizal networks. These studies strongly suggest that mycorrhizal networks increase the transfer of allelopathic chemicals and expand the range, called the bioactive zone, in which they can disperse and maintain their function. Furthermore, studies indicate increased bioactive zones aid in the effectiveness of the allelochemicals because these infochemicals cannot travel very far without a mycorrhizal network. There was greater accumulation of allelochemicals, such as thiopenes and the herbicide imazamox, in target plants connected to a supplier plant via a mycorrhizal network than without that connection, supporting the conclusion that the mycorrhizal network increased the bioactive zone of the allelochemical. Allelopathic chemicals have also been demonstrated to inhibit target plant growth when target and supplier are connected via AM networks. The black walnut is one of the earliest studied examples of allelopathy and produces juglone, which inhibits growth and water uptake in neighboring plants. In studies of juglone in black walnuts and their target species, the presence of mycorrhizal networks caused target plants to exhibit reduced growth by increasing the transfer of the infochemical. Spotted knapweed, an allelopathic invasive species, provides further evidence of the ability of mycorrhizal networks to contribute to the transfer of allelochemicals. Spotted knapweed can alter which plant species a certain AM fungus prefers to connect to, changing the structure of the network so that the invasive plant shares a network with its target. These and other studies provide evidence that mycorrhizal networks can facilitate the effects on plant behavior caused by allelochemicals.