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Arbuscular Mycorrhizal Fungi

A mycorrhiza is a mutually beneficial relationship between a plant root and a fungus. The fungal partners of this symbiosis colonize plant roots and extend far into the soil. Mycorrhizal fungal filaments in the soil act as extensions of the root system and are more effective at nutrient and water absorption than the roots themselves.
These fungi increase the absorbing surface area of roots by up to one hundred times, compared to roots alone, thereby greatly enhancing the ability of the plants to utilize the soil's resources. Mycorrhizal fungi also increase nutrient uptake by releasing organic acids into the soil, which mobilize nutrients such as phosphorus. This extraction process is particularly important in plant nutrition and explains why non-mycorrhizal plants require high levels of soil fertility to maintain their health. There are many types of mycorrhizae, the most abundant and widespread of which are arbuscular mycorrhizae.

 

Arbuscular Mycorrhizal Fungi (AMF) are the fungal partners of these types of mycorrhizae and are root-inhabiting organisms. They are among the most common soil fungi. AMF form symbioses with the roots of a majority of plants on earth. It is believed that approximately 80% of all vascular plant species form symbioses with AMF (Smith and Read, 2008) including the most important crops such as wheat, rice, potato, tomato, onion, soybean, etc. and plant legume models such as Medicago truncatula. Although, there are exceptions. Plants that are not able to form arbuscular mycorrhizae include members of the Brassicaceae (e.g. Arabidopsis, canola, cauliflower, etc.). AMF are grouped in a fungal the phylum Glomeromycota that was scribed in 2001 by Schüßler et al. and now the term ‘glomeromycetes’ is commonly used for AMF. The glomeromycetes offer a variety of services to their host plants, in particular, improved capacity for phosphorus (P) uptake, as well as drought, and pathogen tolerance.

 

Earliest fossils morphologically resembling glomeromycetes date back as far as the Ordovician, at least 455 million years ago (Redecker et al. 2000) and they probably evolved alongside the first land plants (Brundrett 2002).

 

One of the most challenging aspects of glomeromycete biology is that they are obligate biotrophs; that is, they cannot survive without a suitable host plant. Therefore obtaining sufficient fungal material for any study is complicated and requires a lot of effort and time.

 

The mycelium of glomeromycetes consists of hyphae that lack septa, thus nuclei, and cell organelles share the same cytoplasm and can freely move along the hyphae. This structure is known as coenocyte. Some hyphae colonize the cortex of plant roots where they form arbuscules and vesicles, depending on the taxa. Other hyphae are located outside of the roots while remaining connected with intraradical hyphae. Hyphae are classified as running or branching hyphae, according to their morphology. The later form spores that are believed to be asexual. The morphology of these spores is used to describe the different species of AMF: size, colour, shape, layers of cell wall, hyphal attachment, etc. The glomeromycetes spores are multinucleated and one spore can contain up to many thousands of nuclei sharing a common cytoplasm. Another feature of glomeromycetes is that they maintain the multinucleate state throughout their life cycle; i.e., they completely lack a structure that contains a single nucleus (Marleau et al. 2011 [Open access]).

 

The symbiosis between plants and glomeromycetes is complex and far from being a straightforward one-to-one relationship. On the one hand, an individual plant may be mycorrhized by several glomeromyces belonging to different species; and on the other hand, one individual glomeromycete may colonize many different plant species. Some authors describe this complex relationship as an interaction between communities.

 

From a genetic point of view, glomeromycetes are far more complex than other fungal groups and other eukaryotes. There is much evidence that the thousands of nuclei that they contain are genetically divergent, and individual AMF isolates show extensive intracellular genetic polymorphism.

 

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References:

Brundrett MC (2002) Coevolution of roots and mycorrhizas of land plants. New Phytologist 154, 275-304.

Redecker D, Kodner R, Graham LE (2000) Glomalean fungi from the Ordovician. Science 289, 1920-1921.

Schüßler, A, Schwarzott, D, Walker, C, (2001) A new fungal phylum, the Glomeromycota: phylogeny and evolution. Mycological Research 105, 1413-1421.

Smith SE, Read DJ (2008) Mycorrhizal Symbiosis, Third Edition edn. Academic Press, London.

 

 

Stained Mycorrhized roots

Typical mycorrhized plant roots stained with trypan-blue showing intraradical hyphae and vesicules. The growth of the fungus is limited to the root cortex.

Arbuscules in root cells

Typical arbuscules of Glomus species inside the cells of the root cortex. Courtesy of Adrian Navarro-Borrell.

Coenocytic hypha of Rhizophagus

Example of AMF hypha lacking septa showing nuclei (green spots) and mitochondria (red) sharing a common cytoplasm.

R. irregularis spores observed by scanning electron microscopy.

Spores and hyphae of the model species Rhizophagus irregularis isolate DAOM-197198 observed with a scanning electron microscope. The picture was taken by Julie Marleau during the Université de Montréal microscopy class (Bio-6020).

Multinucleated spore of R. irregularis

Typical juvenile (three weeks old) multinucleated spore of R. irregularis showing nuclei (green spots) stained with Syto Green, a live fluorescent dye. 

Gigaspora rosea spore

This is not a planet, but a spore the glomeromycote Gigaspora rosea harvested fron a pot culture and observed by scanning electron miscrospy. The spore surface is populated by a emultitude of soil microbes (bacteria and fungi). The picture was taken by Bachir Iffis during th emicroscopy class (Bio-6020).

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