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Multi-faceted Mycorrhizae

Arbuscular Mycorrhizae

What are mycorrhizae and how do they work?

Mycorrhizae are structures that are known for their ability to increase nutrient uptake in plants via symbiotic relationships with fungi. They are separated into either endo- or ecto-mycorrhizal categories based on whether or not their hyphae can penetrate plant cell walls. This WikiArticle will focus primarily on the type whose hyphae can penetrate the cell wall known as endo-mycorrhizae (also known as arbuscular mycorrhizae) and all the roles that they play in aiding plant growth, development, and health.

These structural associations have been produced exclusively by fungi in the Glomeromycota division since the colonization of land plants (~400-600 mya) and have since been highly successful in plant colonization—specifically crop plants. In fact, this symbiosis is present in approximately eight-five percent of all plant species.

Hyphae

Perhaps the reason for their immense success is that arbuscular mycorrhizae express multi-nucleated cells (heterokaryon) within their hyphae. This important adaptation allows each unit to be genetically unique and maintain the rate of nutrient exchange while also providing varying resistance to plant pathogens. Fungal-mycorrhizal hyphae are the primary site of nutrient exchange. These extensions can grow to form vesicles or branching invaginations of plant cell membranes called arbuscles. With regards to efficiency, the arbuscular mechanism of these symbiotic mycorrhizae function to increase the surface area of contact between fungal hyphae and plant cell cytoplasm, thus increasing the rate of carbon and nitrogen transfer between plant and fungi.

Glomalin

The rate at which these mycorrhizae are able to induce this nutrient transfer relationship are credited to a glycoprotein complex, also referred to as glomalin-related soil proteins (GRSPs). Discovered in 1996 by Sara F. Wright, glomalin is now known to be related to soil fertility and is regularly produced on the hyphae of the arbuscular mycorrhizae and the spores of their respective fungi. These proteins associate with humic acid to bind mineral particles together within soil organic matter and function to increase soil quality despite normal wear. In other words, this glycoprotein complex increases the binding stability of soil components such that soil erosion occurs at a much slower rate. Moreover, maintenance of soil quality reduces the vulnerability of plant systems in times of environmental stressors. (source) This is significant because these proteins help to maintain the complex root network that lies beneath the earth’s surface as well as the more intricate hyphal networks that are associated with the roots.

The discovery of glomalin was especially significant because it revealed the ability of this protein complex to store carbon and nitrogen. In fact, in mutualistic relationships wherein carbon is transferred from plant to fungus, arbuscular mycorrhizae are not only found to be the primary mechanism of facilitating carbon exchange and transport, but are also found to have unique properties that allow them to store relatively large amounts of carbon. This phenomenon is referred to as carbon sequestration. Though it is not known to what extent, these arbuscles can retain carbon. It is important when considering the ever-increasing rate of carbon emissions in today’s society. In fact, if atmospheric levels continue to rise, the earth will get progressively hotter which will cause additional environmental stressors that will further limit plant growth and development. (source)

Symbiotic equalities

mycelium

Since the discovery of these microbiota, researchers have wondered about the transactional fairness and mechanisms of nutrient exchange. It is logical that plants that obtain the majority of their nutrients via photosynthetic mechanisms and are then able to obtain any minerals they may be lacking post-photosynthetic mechanisms from mycorrhizae. (clarify sentence) In a true mutualism, a plant will primarily take the nutrients that are not readily biologically available to it from photosynthesis and in return, during the right conditionsthe plant will provide the fungal mycorrhizal symbiont with carbohydrates so that it may develop more extensive mycelial networks.

Symbiotic inequalities

However, in the case of non-photosynthetic plants, incidents of cheating have been observed such that the plant gets all (or most) of its food from the fungi. These plants are known as mycoheterotrophs. This phenomenon is referred to as epiparasitism and refers to the abilty of a non-photosynthetic plant to indirectly retrieve nutrients from the photosynthetic plant via robbing symbiotic mycorrhizae of nutrients. Through a non mutualistic approach, mycoheterotrophy is thought to have evolved from photosynthetic-capable plants to be able to survive in low light environments. It is for this reason that this phenomenon occurs most often in woodland environments. (source needed)

Despite the fact that the majority of mycoheterotrophs associate with arbuscular mycorrhizae, most of the research regarding the mechanisms of this behavioral response to environmental constraints has been conducted on ecto-mycorrhizae. This is due to the extensive ecological variability of plants in regard to their environments, soil mixtures, and fungal/mycelial-hyphal networks. It is hard to measure exactly how this occurs in nature and is thus more easily studied in ecto-mycorrhizae. In other words, data collected from field research may be hard to isolate due to the unpredictability and complexity of mycelial networks in a natural setting.

Host specialization

Furthermore, despite the complexity of hyphal networks, this symbiotic relationship has revealed that host specificity exists. Mycoheterotrophy is a derived trait which suggests that it has lost the ability to form the complex networks that its photosynthetic counterparts possess. Therefore, these epiparasitic plants communicate via cell-cell networks and must be more closely associated with their fungal hosts. This phenomenon suggests a more sophisticated version of specificity. Rather than communicating via a network of mycorrhizae that are associated with both photosynthetic and non-photosynthetic plants, recent research suggests that mycoheterotrophs have developed extreme specificity with their fungal counterparts.

Currently, researchers believe that this relationship is very general and functions very similarly among arbuscular mycorrhizae. (source) This discovery is significant because it gives insight to how communication takes place among plants and their symbionts. Moreover, the same mechanisms by which arbuscular mycorrhizae are able to retain carbon for sequestering are also being studied to test the ability of these symbionts to resist epiparasitic plants. Of the four percent of the arbuscular mycorrhizal associations with mycoheterotrophs studied, it is not proven that nutrients will be withheld every time. Thus, researchers have come to the current conclusion that this phenomenon must not be exclusively parasitic and is probably context-dependent instead. (source)

Insect Resistance

In addition to facilitating nutrient exchange between plants and fungi, arbuscular mycorrhizae also have the ability to communicate via their mycelial network. In the event that an individual plant is experiencing an insect infection, a signal is delivered to plants within the network to warn of potential infection. Simultaneously, the infected host plant will be stimulated to begin producing volatile organic compounds (VOCs) Additionally, any uninfected plants in this network will also be signaled to produce these VOCs as a mechanism of preventing infection.