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Mycorrhiza

Introduction

The word Mycorrhiza comes from the Greek words mykes meaning fungi and rhiza meaning root (Bonfante and Genre, 2010). Mycorrhiza hyphae are finer than plant roots but equally as important, there may be up to 20m of fungal hyphae in a single gram of soil. This enables hyphae to be able to access nutrients and water that normal plant roots cannot, it also means they can continue to grow even in highly compacted soils.

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They`re able to access specific nutrients that plants often struggle to access, such as phosphorus although there are others, they also aid in accessing water reducing the risk of mortality during droughts, an ever more important step with climate change causing more extreme weather events putting strain on agriculture (Lange et al., 2012). In return the mycorrhiza absorb carbon, this is important as mycorrhiza can’t photosynthesize, thus it is estimated that between 10-30% of the carbon and sugars gained by plants are shared with mycorrhiza. Moreover, over 90% of all plants rely on mycorrhizal relationships to survive (Bonfante and Genre, 2010).

 

There are 2 main types of Mycorrhizal fungi:

  • Ectomycorrhiza

  • Endomycorrhiza

Arbuscular Mycorrhizal spores

Image credit: Michael Allsop

Endomycorrhizas

There are several sub categories of endo mycorrhizas and so the main focus will be defining them, the most important characteristic to know about endomycorrhizas is that almost their entire fungal structure occurs within the plant roots.

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Arbuscular Mycorriza

Arbuscular mycorrhizas are the most common type of mycorrhiza and associate with around 80% of plant species (Akiyama, 2005), most namely the crop species such as wheat and maize. The main features of arbuscular mycorrhiza are as the image to the right shows, free living spores that exist in the soils germinate, where hyphae enter the plant root and grow between the cortical cells, penetrating some cells to form hyphal coils (Bago, Pfeffer and Shachar-hill, 2000). They then continue to enter more cells forming arbuscules, which are finely branched clusters of hyphae, this is where nutrient exchange between the plant and fungi occurs. Lastly there are also some hyphae that grow between the cortical cells, forming vesicles that usually contain lipids, nucleic acids, and enzymes to name a few (Brown et al., 2016).

 

This relationship between plants and Arbuscular mycorrhiza are important especially in food production and agriculture, plants can absorb more nutrients namely phosphate (Bouwmeester et al., 2007) and become more draught resilient. However, there is a wide range of functional diversity in arbuscular mycorrhizas, for instance in gigasporaceae they invest in biomass of the extra radical hyphae and contain a dramatically increased colonisation where phosphate availability was reduced (Chagnon, 2015).

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Ericoid mycorrhizas

Ericoid mycorrhizas unlike arbuscular mycorrhizas only associate with one particular family of plants, the Ericaceae family (Lehmann et al., 2015). Ericaceae are best known as heathland plants such as heather (Calluna), heath (Erika), and bilberry (vaccinium). Heathlands typically have unfertile, free draining, sandy soils which are highly acidic and low in nutrients. Ericoid mycorrhizas are likely the reason that plants can live in such a harsh environment. The ericoid fungal hyphae attach and form a loose network over the root surface, it then penetrates the epidermal cells (mature cells), and fills them with coiled hyphae (Read, 1996). 80% of the root can be filled with fungal tissue which carry out the nutrient exchange between fungus and plant.

There are 2 sub categories of ericoid mycorrhiza:

  • Arbutoid mycorrhiza

  • Monotropoid mycorrhiza

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Arbutoid mycorrhizas

Arbutoid like ericoid mycorrhizas are only associated with the family of plants Ericaceae, specifically they are commonly found in the vegetation of forest understories (Kühdorf et al., 2014). Like many other mycorrhizas, arbutoids form a sheath like layer over the surface of the roots, it then penetrates the outer cortical cells forming coils of hyphae.

Arbuscular mycorrhizal roots

Mycorrhizal colonisation (white) of the root system (yellow)

Image credit: Scivit [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

Ectomycorrhiza

Ectomycorrhizas are the most advanced mycorrhiza, even though they only associate with 3-5% of seed plants, mostly trees in boreal, temperate, and tropical ecosystems. Ectomycorrhizal interactions involve covering plant roots completely with a film like structure typically around 40-50um thick (Pena et al., 2014), these are then connected to a hyphal network that forms connections between cortical and epidermal root cells known as the hartig net (Blasius et al,. 1986).

 

These hyphal networks can link trees together in what has been named the "wood wide web". These mycorrhizal networks between the trees not only mutually benefit the original source (mother tree) and itself, but also every other tree that it connects too, by doing so trees can begin to share their carbon amongst all the trees of the forest. Those with ectomycorrhizal colonisation typically contain 70% more carbon per unit of nitrogen, then those associated with arbuscular mycorrhizae. More over in return for only 15% of the host carbon ectomycorrhizas provide up to 80% of the plant’s nitrogen (Willis, 2018).

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Ectendomycorrhizas

Ectendomycorrhizas are regarded as being a sub group of ectomycorrhizas, although they exhibit characteristics of both ecto and endomycorrhizas. The thing that makes ectendomycorrhizas unique is that they only generate relationships between pine (Pinus), spruce (Picea), and some but not all larch (Larix). Ectendomycorrhizas have thin sometimes absent mantle's (Yu, Egger and Peterson, 2001), with hyphal penetration into cortical and epidermal cells forming hyphal coils and branching.

Mycorrhizal root infection

Arbuscular Mycorrhiza establishment in a plant root

Image credit: M. Piepenbring [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Monotropoid mycorrhizas

Monotropoid mycorrhiza associate with plants in the monotropaceae family, Monotropoid mycorrhiza are one of the most recently named categories of mycorrhiza, they used to be categorised as arbutoid mycorrhiza until subtle differences were noted and so a new category of mycorrhiza was founded. The difference between arbutoid and montropoid is that monotropoid never penetrate the cells and instead just form a dense sheath of hyphal fibres over the roots. This is especially important as monotropaceae are achlorophyllous (Grünig et al., 2009), so contain no chlorophyll making it impossible to photosynthesize, they gain all their nutrients from mycorrhiza. The mycorrhiza as part of the wood wide web tap carbon sources from nearby plants, so both the monotropaceae plant, and the monotropoid mycorrhizas can feed off the carbon sources of other plants.

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Orchidaceous mycorrhizas

Orchidaceous mycorrhizas associate with orchidaceae one of the largest and most diverse of all the plant families. This is especially surprising since orchidaceae cannot live without mycorrhiza at some point in their lifetime they will be unable to photosynthesize, and so rely on the mycorrhiza for their energy. The orchidaceous mycorrhizae rapidly colonise the roots of orchids almost as soon as they break out from the seed coat, and in some cases,  it’s now thought that some orchid seeds won’t germinate unless they have already been infected by mycorrhiza. The mycorrhiza act by hyphae infecting the cells of the orchid seed forming coils called pelotons (Sathiyadash et al., 2012), these pelotons have a very short life cycle only lasting a couple of days before it gets digested by the cell. Mature mycorrhiza establishes vacuoles ensuring surviving hyphae which go on to establish more orchidaceous mycorrhiza, ensuring constant fungal infection.

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