User:Emnoffke/Mycorrhizal Networks and Communication

Mycorrhizal networks are important symbiotic fungi that support trees and terrestrial plants in all extremes of habitats. These networks have been around for over 400 million years, with up to 90% of all land plants taking advantage of their services. Common Mycorrhizal Networks (CMNs) are branches of hyphae from mycorrhizal fungus species that connect root systems of terrestrial plants. These networks, along with soil pathways and carbon sinks, represent a complex system of carbon flux out of plants. Suzanne Simard, a forest biologist, discovered that trees used these mycorrhizal networks to communicate with each other and help with defense against insects or pathogens. In the field, Simard observed that a Douglas Fir, that was injured by insects, sent chemical warning signals through the supporting mycorrhizal network to a Ponderosa Pine nearby. Interestingly, she discovered that this Ponderosa Pine had taken these signals and released defense enzymes to protect against the insect.

= Mycorrhizal Networks = One type of mycorrhizal fungi, called ectomycorrhizae, is an external fungus that produces a sheath around the roots, which then create hyphae that grow into the soil. However, the more common mycorrhizae is endomycorrhizae, which does not produce a sheath and the hyphae within the cells and into soil. By intertwining with the root system underground, these fungi develop a symbiotic relationship that continues to survive even after the host is no longer present. The relative emphasis a plant places on carbon transfer through soil pathways vs. CMNs depend on the plants’ use of roots. A plant species with sophisticated root systems accounting for a higher proportion of the plant’s biomass might rely more heavily on soil pathways because of the high surface area of roots in contact with the soil. Whereas plants with small root systems may benefit from the CMNs to help create contact with the available carbon.

= Phosphate Communication = Mycorrhizae respond to a variety of factors. Arbuscular mycorrhizae (AM) penetrates a vascular plant and forms arbuscules. The AM hyphae develop these arbuscules after reaching the inner cortex of the vascular system, in which they help facilitate the exchange of nutrients between the host and fungi. Arbuscular mycorrhizae growth is mediated with auxin and strigolactones (SLs), as well as nutrient availability with sugars and lipids. Since plants cannot sense phosphate directly, they use an InsP8 signaling molecule and SPX proteins to integrate the level of phosphate in their system and respond to the fungi accordingly. To inhibit the growth of AM, plants with adequate phosphate will secrete strigolactones, since a high phosphate content in neighboring plants negatively impacts fungi growth. On the other hand, high carbon dioxide and auxin concentrations help promote AM development in order to aid phosphate uptake in deficient plants. Greater polar auxin transport from the shoots as a result of increased auxin signaling allowed for enhanced mycorrhizal development.

= Carbon Communication = Besides phosphate uptake and auxin signaling, carbon is also vitally shared between the mycorrhizal networks and surrounding plants. Plants sense carbon through a receptor in their guard cells that measure CO2 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 radioactively labeled C13 and C14, 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 2-3% net gain in carbon. The 2-3% may sound small but in the past <1% carbon gain 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. Bearing in mind that the paper birch and the Douglas Fir also receive carbon from soil pathways, one can imagine a substantial disadvantage to plant competitors not in the CMN.

Another substantial source of carbon flux from plants is carbon sinks. As surplus fixed carbon accumulates from photosynthesis, plants can begin to sustain photo-oxidative damage and end-product inhibition. Rather than slowing down the rate of photosynthesis, plants opt to instead redistribute the carbon to sinks surrounding the plant such as leaves, nearby bacteria, or the CMN. As the surrounding organisms consume carbon, a steep carbon concentration gradient is maintained between the plant and the surrounding environment creating the driving force for carbon flux. The magnitude of the carbon flux into the sinks is proportional to the ratio of carbon to nitrogen, phosphorus, or water. A higher accumulation of carbon will lead to a higher carbon flux.

The importance of mycorrhizal fungi also applies to plants that may be at the end of life, with just as much vitality as those with an abundance of resources. Leafless tree stumps have been shown to be kept alive by acting as a fixed photosynthetic carbon sink for nearby plants through the CMN. Through the CMN, surplus carbohydrates move from living tree to leafless stump. It does so through hydrostatic pressure gradient: stump unloads water from phloem and respiration (releasing water). This removal of water creates an area of low pressure, so water (and carbon) is carried in through hydrostatic pressure gradient to the stump. Because it is surplus carbon, this comes at no cost to the plant at the giving end.