Rhizophagus intraradices: A Jagged Past and a Hopeful Future

Hanna Jackson

*This report is a class project NOT peer-reviewed science

Taxonomy and Features:

The fungus Rhizophagus intraradices has had some major taxonomic twists and turns as a result of the taxonomic confusion surrounding Glomeromycota (Redecker et al, 2013). Fungi in the Glomeromycota (such as R. intraradices) were initially considered to be under the phyla Zygomycota however this was a polyphyletic group and could not be considered a true phylogenetic classification (Schüßler et al, 2001). It was found that putting them in a clade alongside the Ascomycota and Basidiomycota made them a monophyletic group (Schüßler et al, 2001).

As to the genus of this species of fungi, it was placed in the genus Glomus, and at the time it was said that the genera Rhizophagus and Glomus were synonymous (Gerdemann & Trappe, 1974), but more recent work has shown that they are distinct from each other. Rhizophagus is a fungus that forms many spores within the roots of vascular plants, which is distinct from Glomus (Schüßler & Walker, 2010). Redecker et al (2013) re-analysed the classification of the Glomeromycota and found that the group had been irresponsibly categorized and added to without sufficient scientific rigor, only leading to more confusion surrounding this group of fungi. Up to date classification specifics can be found in their study. From this reclassification, this species that is now officially known as Rhizophagus intraradices has, and is, also known as Glomus intraradices and Rhizoglomus intraradices. Overall, its classification lies within the kingdom Fungi, phylum Glomeromycota, class Glomeromycetes, order Glomerales and family Glomeraceae ("Catalogue of Life : Glomus intraradices N.C. Schenck & G.S. Sm. 1982", 2018).

Morphology and Reproduction:

Rhizophagus intraradices is an arbuscular mycorrhizal fungi (Tisserant et al, 2013) of the phyla Glomeromycota, consisting of mycorrhiza (fungus-root) forming fungi ("Catalogue of Life : Glomus intraradices N.C. Schenck & G.S. Sm. 1982", 2018).

R. intraradices is believed to be an asexual fungus, a puzzling phenomenon of evolution (Sanders, 1999). It has coenocytic hyphae (Tisserant et al, 2013), meaning that it has many nuclei in one cell (Sanders, 1999). These many nuclei reproduce clonally with large multinucleated spores (Halary et al, 2011). They have survived this way for 500 myr reproducing clonally, a form of asexuality, and meiosis has never been observed. Without recombination, the evolution of genes isn’t well understood (Sanders, 1999). However, there is some evidence the genus is capable of it (Halary et al, 2011). Indirect genetic evidence of recombination has been found (Halary et al, 2011), and they have maintained a high genetic diversity (Sander, 1999), which also suggest some form of recombination is occurring.

In development, the first spores are formed seven days after germination on hyphae that developed from germ tubes (Chabot et al, 1992). The spores are produced at the terminal ends of the short-branched hyphae (Chabot et al, 1992). Arbuscule-like branches are at the base of the hyphae (Chabot et al, 1992). Mature spores are yellow in colour (Chabot et al, 1992).

Identification Methods:

Spores are produced singly and in aggregates of differing numbers (Fungi.invam.wvu.edu, 2018). They are globular, ovoid, oblong or irregular in shape and also quite variable in colour, but are usually somewhat yellow-brown (Fungi.invam.wvu.edu, 2018). When juvenile, these spores have one layer, and when mature, they have three layers (Fungi.invam.wvu.edu, 2018). Hyphae have a similar yellow-brown colour to the outermost layer of the spore wall (Fungi.invam.wvu.edu, 2018). Hyphal structure also has three layers, and they are continuous with the spore wall layers. When juvenile, the two outermost layers are present exclusively, and as they age, they degrade and thin (Fungi.invam.wvu.edu, 2018).

Conditions for Growth:

Achieving in vitro cultures of arbuscular mycorrhizal fungi is difficult, as they rely on the plant host (St-Arnaud et al, 1996). Difficulties in producing as inoculum is a hinderance to the implementation of mycorrhizal symbiosis in agriculture (St-Arnaud et al, 1996). In a methods paper by St-Arnaud et al (1996), they determined that hyphal densities were increased up to six times in an area of petri dishes that were not colonized by roots, and spore density was up to 17 times higher. They used Ri T-DNA transformed carrot roots on minimal growth media that were then inoculated with the spores that had been processed and washed. Ten to fifteen spores were placed near the apex of a section of carrot root. The petri dishes were incubated in the dark, kept at 27 degrees Celsius.

Past papers had been published that set out the growth behaviour, including the one done by Chabot et al (1992). They found that spores could be produced with low contamination that successfully germinated and produced spores, hyphae and arbuscule-like structures (Chabot et al, 1992). They also showed that minimal (M) medium was sufficient to germinate the spores, and pregermination was not required (Chabot et al, 1992).


The success of the arbuscular mycorrhizal fungi as a group relies on well-tuned interaction between the fungus and its host plant, communication and metabolic regulation are key (Lanfranco et al, 2018)

Arbuscular mycorrhizal interactions are formed between a wide variety of plants and themycorrhizal fungi of the glomeromycetes (Bücking & Shacher-Hill, 2004), in which R. intraradices is included. They have been shown to increase the growth of many land plants via a multitude of mechanisms. They (1) enhance soil nutrient uptake (Bucking & Shacher-Hall, 2005), (2) influence soil aggregation (Rillig & Mummey, 2006), and (3) increase resistance to stressors (Bücking & Shacher-Hill, 2004) such as influencing metal toxicity (Leyval et al, 1997), alleviating salt stress (Evelin et al, 2009) and providing enhanced drought tolerance (Marulanda et al, 2007).

Arbuscular mycorrhizal fungi have been shown to stimulate house plant growth through the enhancement of soil nutrient uptake (Bücking & Shacher-Hill, 2004). Uptake, allocation, and transfer of phosphorous to the host plant is stimulated by the presence of carbon from the host plant (Bücking & Shacher-Hill, 2004), clearly showing the mutualistic relationship. It is believed that the fungus is able to access higher amounts of phosphorous due to the large soil volume it can explore, the high surface area due to the small diameter of fungal hyphae, the formation of polyphosphates, and the production of organic acids and phosphates that stimulate the release of phosphorous from organic compounds (Marschner & Dell, 1994). It has also been found that the fungus stores phosphorous in times where it is abundant, which allows it to provide a continuous supply to the plant (Harley & Smith, 1983- ADD). This mechanism of enhancing plant growth is thought to be the primary one (Marschner & Dell, 1994), however the benefits do not always outweigh the costs; if there is exceptionally low phosphorous in the environment the fungus may hinder the growth of the plant (Amijee et al, 1993-ADD).

Arbuscular mycorrhizal fungi are also shown to increase the resistance of the host to biotic and abiotic stressors (Bücking & Shacher-Hill, 2004). They ameliorate salt stresses resulting in benefits in growth and productivity (Evelin et al, 2009). They are important to heavy metal toxicity as well, as they are the direct route from the soil to the roots of the host. Arbuscular mycorrhizal fungi accumulate heavy metals in their thallus, therefore keeping it out of and protecting the host from metal toxicity (Gonzalez-Guerrero et al, 2008). Metal tolerant fungi grow readily in polluted soil (Leyval et al, 1997), and can pass this on to their host. It was found that these heavy metals accumulate in fungal cell walls and vacuoles when exposed, but there is little change to the cytoplasm content of heavy metals (Gonzalez-Guerrero et al, 2008). This may be the mechanism by which R. intraradices and other arbuscular mycorrhizal protect their host plants (Gonzalez-Guerrero et al, 2008). R. intraradices inoculation was proven as a method of attaining the greatest tolerance to water stresses in lavender (Marulanda et al, 2007). It was found that R. intraradices increased root growth by 35% under these conditions, and overall plant biomass, nitrogen, potassium, and water content also increased (Marulanda et al, 2007). Glomeromycota are thought to have been an integral part of the evolution of land plants during the Middle Paleozoic Era (Tisserant et al, 2013) due to their ability to aid the plant in growth.

Importance to People (Unique Features)

Arbuscular mycorrhizal fungi can be used as biological indicators of ecosystem functioning (Chen et al, 2018). Sites that had been ecologically damaged and restored had more phylogenetically heterogeneous fungal communities than pristine areas, indicating that the current methods of ecological restoration could be significantly improved with more focus on fungal communities (Chen et al, 2018).

They also have great usage in the agricultural field in multiple ways. Arbuscular mycorrhizal fungi increase the growth of the hosts they associate with by increasing the uptake of minerals (Casellanos-Morales et al, 2010). They make nitrogen, an essential growth factor for plants, more abundant and accessible (Casellanos-Morales et al, 2010). It has been found that adding Rhizophagus intraradices to strawberry plant cultivations increases the quality of the fruit, much like adding nitrogen alone does (Casellanos-Morales et al, 2010). Adding the fungus affected the colour of the fruit as well as increased the levels of some phenolic compounds, an indicator of fruit quality (Casellanos-Morales et al, 2010). Another study recommended the inoculation of nursery olive plants with Rhizophaus intraradices to increase growth (Jiménez-Moreno et al, 2018)

Surprisingly, R. intraradices can also be used as a biological control method. It decreased the efficacy of broomrape as a plant pathogen of tomato plants, as evidenced by the decrease in seed germination, number of nodules and dry weight in comparison to controls that did not contain the fungus (Tadayyon et al, 2018). Further, it benefited the tomato plant itself, increasing root area and dry weight (Tadayyon et al, 2018). It has also been shown to supress the biological control agent Trichoderma harcianum, while casing no harm to R. intraradices in terms of growth or phosphorus uptake.

What I chose to write about this fungus

I chose this fungus without knowing much about it other than it had in it, the key descriptor I was looking for, mycorrhizal fungus.

My introduction to post-secondary level biology was Bisc 102 taught by Joan Sharp, which was an experience that most certainly changed my life in many more ways than those that are within the scope of an essay about Rhizophagus intraradices. Specifically, the evolution of algae into land plants and the interactions between trees and fungi in the ground astonished me and made me realise how much bigger and more interesting biology was than we’d been taught in high school. I saw nature with new, hopeful eyes, ones that saw that with time, I too could look at a forest and see all the minute complexities and interactions that the biology could provide to my curiosity. It inspired a drive, determination, and a fire that to this day hasn’t calmed.

To this extent, I do owe mycorrhizal fungi a lot more thanks than just for helping grow the food I eat, and trees my home is made of. I also owe it a significant amount of credit for determining my career path and shaping a large section of my values and interests. Unfortunately, as far as I’ve dug into the research, Science has yet to find a way to say thank you to fungi, but when they do, I’ll be the first one on it.

Cited References

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Bücking, H., & Shachar-Hill, Y. (2005). Phosphate uptake, transport and transfer by the arbuscular mycorrhizal fungus Glomus intraradices is stimulated by increased carbohydrate availability. New Phytologist, 165(3), 899–911. https://doi.org/10.1111/j.1469-8137.2004.01274.x

Castellanos-Morales, V., Villegas, J., Wendelin, S., Vierheilig, H., Eder, R., & Cárdenas-Navarro, R. (2010). Root colonisation by the arbuscular mycorrhizal fungus Glomus intraradices alters the quality of strawberry fruits (Fragaria ?? ananassa Duch.) at different nitrogen levels. Journal of the Science of Food and Agriculture, 90(11), 1774–1782. https://doi.org/10.1002/jsfa.3998

Catalogue of Life : Glomus intraradices N.C. Schenck & G.S. Sm. 1982. (2018). Retrieved from http://www.catalogueoflife.org/col/details/species/id/504756b67ddbe2955ceecfbc061a88 ad/synonym/842a060b1e35101a0f775f300ceee6f9

Chen, X. W., Wong, J. T. F., Chen, Z. T., Leung, A. O. W., Ng, C. W. W., & Wong, M. H. (2018). Arbuscular mycorrhizal fungal community in the topsoil of a subtropical landfill restored after 18 years. Journal of Environmental Management, 225(April), 17–24. https://doi.org/10.1016/j.jenvman.2018.07.068

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