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. 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.
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. 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 Allelopathy is the process by which plants produce
secondary metabolites known as
allelochemicals, which can interfere with the development of other plants or organisms. Plants release allelochemicals due to
biotic and
abiotic stresses in their environment and often release them in conjunction with defensive compounds. This increased transfer speed is hypothesized to occur if the allelochemicals move via water on hyphal surfaces or by
cytoplasmic streaming. These and other studies provide evidence that mycorrhizal networks can facilitate the effects on plant behavior caused by allelochemicals.
Defensive communication Mycorrhizal networks can connect many different plants and provide shared pathways by which plants can transfer infochemicals related to attacks by
pathogens or
herbivores, allowing receiving plants to react in the same way as the infected or infested plants. They can also manifest biochemical changes, including the production of
volatile organic compounds (VOCs) or the
upregulation of genes producing other defensive enzymes, many of which are toxic to pathogens or herbivores.
Salicylic acid (SA) and its derivatives, like
methyl salicylate, are VOCs which help plants to recognize infection or attack and to organize other plant defenses, and exposure to them in animals can cause
pathological processes.
Terpenoids are produced constituently in many plants or are produced as a response to stress and act much like methyl salicylate. AM networks can prime plant defensive reactions by causing them to increase the production of terpenoids. Many insect herbivores are drawn to their food by VOCs. When the plant is consumed, however, the composition of the VOCs change, which can then cause them to repel the herbivores and attract insect predators, such as
parasitoid wasps. The results of these studies support the conclusion that both ECM and AM networks provide pathways for defensive infochemicals from infected or infested hosts to induce defensive changes in uninfected or uninfested
conspecific and
heterospecific plants, and that some recipient species generally receive less damage from infestation or infection. In a natural ecosystem, plants simultaneously participate in symbiotic relationships with multiple fungi, and some of these relationships may be commensal or parasitic. The connectivity between plants believed to share a common mycorrhizal network is also difficult to verify in a natural ecosystem. Field observations cannot easily rule out the possibility that effects attributed to physical connection between plants via mycorrhizal networks could be happening due to other interactions. While movement of resources between plants connected to the same mycorrhizal network has been shown, it is often unclear whether the transfer is direct, as though the
mycelium is forming a literal "pipeline," or indirect, such as nutrients being released into the soil by fungi and then picked up by neighboring plants. It is furthermore unclear whether apparent nutrient transfer between plants has a significant impact on plant fitness.
Research tools and methods Isotopic labeling Carbon transfer has been demonstrated by experiments using
carbon-14 (14C)
isotopic labeling and following the pathway from ectomycorrhizal conifer seedlings to another using mycorrhizal networks. The experiment showed a bidirectional movement of the 14C within ectomycorrhizal species. Further investigation of bidirectional movement and the net transfer was analyzed using pulse labeling technique with
13C and 14C in ectomycorrhizal Douglas fir and
Betula payrifera seedlings. Results displayed an overall net balance of carbon transfer between the two, until the second year where the Douglas fir received carbon from
B. payrifera. Detection of the isotopes was found in receiver plant shoots, expressing carbon transfer from fungus to plant tissues. Plants sense carbon through a receptor in their guard cells that measure
carbon dioxide concentrations in the leaf and environment. Carbon information is integrated using proteins known as
carbonic anhydrases, in which the plant then responds by utilizing or disregarding the carbon resources from the mycorrhizal networks. One case study follows a CMN shared by a
paper birch and
Douglas fir tree. By using
carbon-13 and
carbon-14 labels, researchers found that both tree species were trading carbon–that is to say, carbon was moving from tree to tree in both directions. The rate of carbon transfer varied based on the physiological factors such as total biomass, age, nutrient status, and
photosynthetic rate. At the end of the experiment, the Douglas fir was found to have a 2% to 3% net gain in carbon. This gain may seem small, but in the past a carbon gain of less than 1% has been shown to coincide with a four-fold increase in the establishment of new seedlings. Both plants showed a threefold increase in carbon received from the CMN when compared to the soil pathway. For example, in a network that includes
Acer saccharinum (sugar maple) and
Erythronium americanum (trout lily), carbon moves to young sugar maple saplings in spring when leaves are unfurling, and shifts to move to the trout lilies in fall when the lilies are developing their roots. A further study with paper birch and Douglas fir demonstrated that the flow of carbon shifts direction more than once per season: in spring, newly budding birch receives carbon from green Douglas fir, in summer, stressed Douglas fir in the forest
understory receives carbon from birch in full leaf, and in fall, birch again receives carbon from Douglas fir as birch trees shed their leaves and evergreen Douglas firs continue photosynthesizing. When the ectomycorrhizal fungus-receiving end of the plant has limited sunlight availability, there was an increase in carbon transfer, indicating a source–sink gradient of carbon among plants and shade surface area regulates carbon transfer. It has been demonstrated that mechanisms exist by which mycorrhizal fungi can preferentially allocate nutrients to certain plants without a source–sink relationship. Studies have also detailed bidirectional transfer of nutrients between plants connected by a network, and evidence indicates that carbon can be shared between plants unequally, sometimes to the benefit of one species over another. Evidence is also mounting that
micronutrients transferred via mycorrhizal networks can communicate relatedness between plants. Carbon transfer between Douglas fir seedlings led workers to hypothesize that micronutrient transfer via the network may have increased carbon transfer between related plants. Photosynthesis was also shown to be increased in Douglas fir seedlings by the transport of carbon, nitrogen and water from an older tree connected by a mycorrhizal network. Furthermore, nutrient transfer from older to younger trees on a network can dramatically increase growth rates of the younger receivers. Physiological changes due to environmental stress have also initiated nutrient transfer by causing the movement of carbon from the roots of the stressed plant to the roots of a conspecific plant over a mycorrhizal network. These include increased establishment success, higher growth rate and survivorship of seedlings; improved
inoculum availability for mycorrhizal infection; transfer of water, carbon, nitrogen and other limiting resources increasing the probability for colonization in less favorable conditions. These benefits have also been identified as the primary drivers of positive interactions and feedbacks between plants and mycorrhizal fungi that influence plant
species abundance. 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. It is hypothesized that
fitness is improved by the transfer of infochemicals through common mycorrhizal networks, as these signals and cues can induce responses which can help the receiver survive in its environment. Furthermore, changes in behavior of one partner in a mycorrhizal network can affect others in the network; thus, the mycorrhizal network can provide
selective pressure to increase the fitness of its members. Receipt of defensive signals or cues from an infested plant would be adaptive, as the receiving plant would be able to prime its own defenses in advance of an attack by herbivores. Many believe the process of new seedlings becoming infected with existing mycorrhizae expedite their establishment within the community. The seedling inherit tremendous benefits from their new formed symbiotic relation with the fungi. Mycorrhizal networks aid in regeneration of seedlings when
secondary succession occurs, seen in temperate and boreal forests. The Douglas fir was the focus of another study to understand its preference for establishing in an ecosystem. Two shrub species,
Arctostaphylos and
Adenostoma both had the opportunity to colonize the seedlings with their ectomycorrhizae fungi.
Arctostaphylos shrubs colonized Douglas fir seedlings who also had higher survival rates. The mycorrhizae joining the pair had greater net carbon transfer toward the seedling. The researchers were able to minimize environmental factors they encountered in order to avoid swaying readers in opposite directions. In burned and salvaged forest,
Quercus rubrum establishment was facilitated when acorns were planted near
Q. montana but did not grow when near arbuscular mycorrhizae
Acer rubrum Seedlings deposited near
Q. montana had a greater diversity of ectomycorrhizal fungi, and a more significant net transfer of nitrogen and phosphorus content, demonstrating that ectomycorrhizal fungi formation with the seedling helped with their establishment. Results demonstrated with increasing density; mycorrhizal benefits decrease due to an abundance of resources that overwhelmed their system resulting in little growth as seen in
Q. rubrum. Mycorrhizal networks decline with increasing distance from parents, but the rate of survival was unaffected. This indicated that seedling survival has a positive relation with decreasing competition as networks move out farther. One study displayed the effects of ectomycorrhizal networks in plants which face
primary succession. In an experiment, Nara (2006) transplanted
Salix reinii seedlings inoculated with different ectomycorrhizal species. It was found that mycorrhizal networks are the connection of ectomycorrhizal fungi colonization and plant establishment. Results showed increased biomass and survival of germinates near the inoculated seedlings compared to inoculated seedlings. Studies have found that association with mature plants correlates with higher survival of the plant and greater diversity and species richness of the mycorrhizal fungi.
Mycorrhizal mapping Using more than 2.8 billion fungal sequences sampled from 130 countries, in 2025 the
Society for the Protection of Underground Networks (SPUN) released the world's first high-resolution, predictive biodiversity maps of Earth's underground mycorrhizal fungal communities. The mycorrhizal biodiversity map is called the Underground Atlas. The data reveal that "over 90% of Earth's most diverse underground mycorrhizal fungal ecosystems remain unprotected, threatening carbon drawdown, crop productivity, and ecosystem resilience to climate extremes." == See also ==