Arbuscular Mycorrhizal Fungi

AM Fungi 1

Derivation of scientific name: Endomycorrhiza, (AMF)

Introduction:

Fungus is a microscopic organism of the group eukaryotes that includes yeasts, moulds and mushrooms. These organisms are so small they require a microscope for thorough observation. They are globally abundant, present in a wide range of habitats.

To many, the word “fungus” often brings up images of athletes foot or spoiled green oranges and bread. We are quick to forget the numerous beneficial fungal species that helped shape civilisation. For example, ever since humans discovered fermentation of barley to produce beer, or the baking of bread, fungi have had a significant impact on human history.

The only other organisms known to have a longer relationship with these little microscopic organisms are plants. When plants first started establishing across terrestrial environments more than 430 million years ago, they were accompanied by a symbiotic fungus, arbuscular mycorrhizal fungi (AMF).

This fungus (Endomycorrhiza, AMF), from the fungal phylum, Glomeromycota- which contains all known AMF- has coevolved with the majority of plants ever since. Considering the long evolutionary history of AMF, it is not surprising that the mycorrhizal association is present in 95% of all vascular plant families, as AMF seemingly lacks host specificity.

How to recognise arbuscular mycorrhizae fungi:

As arbuscular mycorrhizae fungi are microscopic organisms, therefore microscopes and staining procedures of plant root samples will need to be done to be able to visualise the fungi that colonised the root samples. This staining procedure is known as Trypan-blue staining, which is used to selectively colour dead cells or tissues blue to allow easy observation of fungal structures under a microscope. The stain highlights fungal structures such as the hyphae, arbuscules, and vacuoles, as shown in figure 1. The characteristic feature of this fungus is the arbuscules, these are high branched structures that colonise the root cells, which allows for the carbon transfer from plant to fungi (see figure 2).

Distribution and habitat:

Arbuscular mycorrhizae fungi are present in all soils as fungal spores, and lack any intrinsic movement capabilities. Spores are commonly present in the rhizosphere soil zone which is generally the “middle zone” of soil, in the immediate area of a plant root system and soil associated microorganisms. However, once in the vicinity of a nearby host plant’s root system and under the correct conditions spore germination can commence. This results in the growth of hyphae (branchlike structures) that branches out to nearby host plant root systems. Whereby, a mutual symbiotic relationship establishes between the two organisms. 

Food:

The relationship between AMF and plants is purely symbiotic in nature. These associations are initiated by spore germination. The AMF acts as extension of the root system of plants, to assist in absorbing water and nutrients, whilst the plant provides the fungus with a source of carbohydrates. This relationship is essential for AMF as they are considered obligate biotrophs, in that they are unable to grow and complete their life cycle without the association with a living root.

The symbiotic relationship is initiated when a potential host plant, releases strigolactones, a phytohormone (i.e. plant hormones), that promotes seed germination and root and root-hair elongation.  The phytohormone and low-phosphorous concentrations, ensures the growth of branch-like structures called hyphae, from AMF spores in the soil, and initiate symbiosis between AMF and the host plant. The fungi then colonises the plant root network, when the AMF hyphae penetrates the plant’s epidermal cell wall, the fungus forms “arbuscules” which are highly branched structures for nutrient exchange with the plant. These are the defining characteristic of AMF.

These arbuscules are the sites where; phosphorous, carbon, water and other nutrient exchanges take place. This process is also beneficial towards the plant as the hyphal strands of the AMF have access to phosphorous that plant roots cannot reach. The figure below illustrates the relationship between the plant root and AMF. It indicates the characteristic structures of AMF when symbiosis is established with a host plant.

Arbuscular Mycorrhizal Fungi

Click here for source

Reproduction and life cycle:

Reproduction in fungi occurs through spores, which are produced on the hyphae. These thick-walled spores remain in the soil for extended periods of time. Some of the spores are large enough that allows recovery through sieving the soil.

In laboratory tests, AMF spores have been able to germinate and produce hyphae, but as of yet no one has been able to grow the fungus independently of a root, thereby emphasising the requirement of plant roots to ensure germination of the spores.

THE BIG PICTURE:

Friends and foes:

Arbuscular mycorrhizal fungi are extremely beneficial to the environment, as they dramatically assist in improving plant health and maximise the absorption of nutrients. This symbiotic relationship between fungus and plant spanning millions of years and have allowed for plants to colonise areas that would have been too harsh to withstand. However, their presence in soil makes them vulnerable to erosion, and tilling. Tilling reduces the inoculation of the soil and the efficacy of the fungi by disrupting the hyphal network.

Smart Strategies:

The symbiotic relationship between the fungi and plant can assist in restoring areas affected by desertification or degradation. This is achieved by inoculating the soil with AMF when reintroducing vegetation in ecological restoration projects (phytoremediation) that has been affected by industrial activities such as mining.

The benefits of AMF are greatest in areas with low phosphorous concentration, as soil’s phosphorous levels available for the plants increase, the amount of phosphorous also increases in the plant’s tissues.

Additionally, in the agricultural processes AMF act as natural biofertilizers, since they are capable of providing the host with water, nutrients, and pathogen protection, in exchange for photosynthetic products. Therefore, as with the phytoremediation strategy, the direct re-introduction of AMF into the target soil can be utilised for farming purposes. 

Poorer world without me:

The important and beneficial symbiotic relationship between plants and AMF cannot be taken for granted. Life absent AMF would be drastically different, as the helpful relationship between fungi and plant are of utter importance for their survival. Their role in the ecosystem as well as agriculture would be impossible to replace.

People & I:

The full potential that AMF can play in agriculture, phytoremediation habitat loss and even as supplement to achieve healthier lawns is presently being investigated. The potential for restoring regions affected with industrial processes could improve the productive value of the area by promoting the revegetation of the area and rebuild the soil of the disturbed area. Similarly, the potential to improve agricultural yields, if not improve plant survival, by inoculating the soil with AMF could prove to be a useful strategy in the future to ensure food security.

Threats to AMF:

Climate change poses a great threat to AMF, due to irreparable damage to various ecosystems, as well as the increasing rate of habitat loss due to human activities. Therefore, successive steps should be implemented to mitigate subsequent disasters that may arise from these concerns.

References and further reading:

Marleau, J., Dalpé, Y., St-Arnaud, M. and Hijri, M. (2011). Spore development and nuclear inheritance in arbuscular mycorrhizal fungi. BMC Evolutionary Biology, 11(1).

Mycorrhizas.info. (2017). Mycorrhizal Associations: Arbuscular Mycorrhizas. [online] Available at: https://mycorrhizas.info/vam.html [Accessed 11 May 2017].

Rossouw, M. (2016). Application of plant growth promoting substances and arbuscular mycorrhizal fungi for phytostabilisation of mine tailings. MSc. University of Stellenbosch.

Soilquality.org.au. (2017). Soil Biological Fertility | Fact Sheets | soilquality.org.au. [online] Available at: http://www.soilquality.org.au/factsheets/soil-biological-fertility [Accessed 11 May 2017].

Stürmer, S. (2012). A history of the taxonomy and systematics of arbuscular mycorrhizal fungi belonging to the phylum Glomeromycota. Mycorrhiza, 22(4), pp.247-258.

Website.nbm-mnb.ca. (2017). Mycorrhizae. [online] Available at: http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Mycorrhizae.html [Accessed 11 May 2017].

 Author: Marthinus Jacob Rossouw

GMO Programme: SANBI/NRF Intern,

Kirstenbosch

Official Common Name: Arbuscular mycorrhizal fungi

 

Scientific Name and Classification:

Kingdom: Fungi

Phylum/Division: Glomeromycota

Class: Glomeromycetes

Order: Glomerales

Family: Glomeraceae

Genera: Funneliformis, Glomus, Rhizophagus, Sclerocystis, Septoglomus and over 200 described species.  

Share Share this article

Comments

    Add a comment

    By submitting this form, you accept the Mollom privacy policy.
    Future update to occur on: 
    8 September 2017
    Copyright 2017 © SANBI | All Rights Reserved | Terms & Conditions