The Mycorrhizal Network

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By Freya Beinart


Introduction

Fungal colonization of a plant's rhizosphere representing a mycorrhizal symbiosis. [1].

Mycorrhizae is the symbiotic relationship between fungi and plants. Mycelium, the vegetative part of a fungus, extends in a branching network of hyphae that secrete enzymes, breaking down organic material into nutrients. The vast mycelial branches of the fungi are made of branching networks of hyphae which, in a mycorrhizal association, colonize the root systems of many plant species, greatly increasing the surface area of a plant’s rhizosphere in which it collects water and essential nutrients. In exchange for these nutrients, the plants provide the fungi with carbohydrates that the fungi cannot produce on its own, being heterotrophic. The hyphae of soil fungi colonize the root system of plants, providing the photosynthetic organisms with nutrients and water in exchange for carbohydrates that the heterotrophic fungi cannot produce on its own. This colonization may be intracellular as arbuscular mycorrhizal fungi (AMF) which trade nutrients via a signal transduction pathway, or extracellular as ectomycorrhizal fungi that envelop plant roots.

The fungal symbionts assist host plants to be less susceptible to pathogens and environmental stresses such as lack of nutrient density, drought, and high salinity. The vast underground networks of hyphae increase nutrient cycling, erosion resistance, air permeability, and water permeability within the soil itself. Including and maintaining these associations between fungi and plants has proven to be highly beneficial for agricultural health and biogeochemical cycling.

Types of Mycorrhizal Associations

Figure 1. Diagram of different types of mycorrhizal associations on the cellular level. [2].

Characterization of mycorrhizal association is dependent upon the location of fungal colonization and is specific to both the plant species and their fungal symbionts.

Endomycorrhizae

Endomycorrhizal symbioses occur when the mycorrhizal fungi colonizes a host plant intracellularly.

Arbuscular mycorrhizal fungi
Fungi of the phylum Glomeromycota that forms mutualistic symbioses, called arbuscular mycorrhizae, with vascular plants are referred to as arbuscular mycorrhizal fungi, or AMF. These fungi are associated with 90% of vascular plant species. AM fungi are unable to live without the presence of the host plants, as they depend on the transfer of carbohydrates from the plant tissue, making the fungi obligate biotrophs [1]. Plant taxa that are observed to exhibit this relationship are bryophyta, pteridophyta, gymnosperms, and angiosperms [2]

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Hyphae from a germinating spore infect the host root, passing the epidermis and penetrating cortical cells which form arbuscules - the structure from which the name of these fungi originated [3]. The arbuscules are networks of extremely fine clustered hyphae within the host cells where nutrient transfer is centralized [4]. The hyphae may also form vesicles between or within the root cells, acting as an organ for storage with a thickened cell wall which aids in the establishment of new colonies [5] [6]. The mycelium which extends from the plant root is called the extraradical mycelium, which connects from plant to plant, forming a continuum of nutrient and water exchange [7].

Ectomycorrhizal fungi

Figure 2. Light micrograph of ectomycorrhizal roots with penetrating fungal hyphae forming the Hartig net structure. [3].

Fungi that form ectomycorrhizae are majorly Basidiomycota, as well as some Azcomycota and Zygomycetes of the genus Endogone. These associations differ from AM symbioses in the way that the fungi does not penetrate the root cells, instead it grows intercellularly. Hyphae grow between epidermal and cortical cells of the plant, forming a structure called the Hartig net where nutrient transfer is localized [8].

Ericoid mycorrhizae




Evolution of Mycorrhizae

Fossilized hyphae and spores from the Ordovician period (A, B, C, E, F, G) and spores formed by extant glomalean fungi (D and H). [4].
Fossil ectomycorrhizae associated with Pinus roots [5].

Fossil records indicate that mycorrhizal fungi predates the evolution of vascular plants, about 460 million years ago in the Ordovican period [9]. The most ancestral mycorrhizal fungi has been identified as arbuscular, penetrating the root cortical cells in most extant plant taxa. The phylum of fungi that associates with land plants as arbuscular mycorrhizal fungi (AMF) is that of Glomeromycota [10] The presence of these fungi were involved in the development of soil and the evolution and colonization of vascular land plants, specifically by aiding the non-vascular plants with acquisition of nutrients through fungal hyphae, as the “soil” lacked a significant amount of nutrients for terrestrial plants to survive without their fungal symbionts [11]. As the fungi cycles nutrients such as nitrogen, phosphorus, and sulfur to the plants, the fungus receives and fixes carbon into the ground, assisting in the lowering of atmospheric CO2 leading to the oxygenation of the atmosphere during the development of terrestrial plants [12] [13].

In a study by Krings et al (2007) fossil evidence from the Rhynie chert sediment of the Early Devonian period suggests that fungal endophytes, specifically those which colonize the rhizoids of Nothia aphylla, actively influenced the evolution of these plants due to observed host responses. The responses to fungal infection in N. aphylla - rhizoid bulging, separation of infected cells via thickening of cell walls, and the motile inhibition of hypha - suggests its susceptibility to colonization by fungi at least 400 million years ago, advancing selection between plant species with the increasing complexity of interspecies interactions [14].

The other form of mycorrhiza associated with the intercellular colonization of fungal symbionts within plants, ectomycorrhiza, evolved much later than its ancestor AMF. Fossil records suggest that ectomycorrhizal fungi may have evolved at least 156 million years ago, as the oldest known extant plant family associated with ectomycorrhizal fungi, Pinaceae, appear to have evolved in that time period ((Tedersoo, May, & Smith, 2010). Another study shows the morphological identification of ectomycorrhizal fungi based on apparent Hartig net, mantle, and hyphal structures on fossils dating back 50 million years, clearly representing the established EMF associations (Lepage, Currah, Stockey, & Rothwell, 1997).

Biogeochemical Cycles

Nitrogen Fixation

Nitrogen is an extremely important nutrient for all living organisms as it is essential for amino acid synthesis and therefore the formation of proteins. Despite its importance, nitrogen cannot be used by many organisms until it has been heavily reduced into ammonia [15]. This anabolic process is very energy-intensive and therefore many organisms do not possess the ability to fix nitrogen (https://www.semanticscholar.org/paper/The-microbial-nitrogen-cycling-network-Kuypers-Marchant/ec5210fe0ef4ec95865bbd684127bf77abd782f1) (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC98982/). Organisms that can fix atmospheric nitrogen are known as diazotrophs - bacteria and archaea that do not require fixed nitrogen for their growth or survival (Takai 2019).

Root nodule symbiosis between plants and nitrogen-fixing bacteria is apparent in plants of the Fabaceae, also known as the legume family [16]. Legumes have a symbiotic relationship with rhizobia, single-celled, Gram-negative bacteria [17] [18]. The mutualism between the legumes and rhizobia allows the bacteria to take up atmospheric nitrogen, feeding it to the plant. In return, the plant provides carbohydrates to the bacteria as it is essential for their metabolic processes and growth (Zahran 1999). Although mycorrhizal symbionts do not possess the metabolism required for nitrogen fixation, studies have shown that mycorrhizae may indirectly affect nitrogen fixation as they increase root nodulation [19] [20] [21].

The legume symbiotic signaling pathway (SYM) has been studied in relation to arbuscular mycorrhizal symbiosis [22] [23]. The SYM pathway controls both nodulation of legumes and the mycorrhization of land plants, and is suggested to evolve first in legumes and adapted for arbuscular mycorrhizal symbiosis (https://gtr.ukri.org/projects?ref=BB%2FR017859%2F1#:~:text=A%20common%20SYM%20pathway%20controls,symbiosis%20(~400%20MYA)%20pathway.). This pathway is characterized by its use of calcium as a second messenger, regulating both AM and rhizobial symbiosis.

Unlike legumes which maintain root nodule symbiosis, AMF have a limited accessibility to acquire atmospheric nitrogen. A study has shown that the arbuscular mycorrhizal association between the fungus Rhizophagus irregularis and rhizobia communities increase nitrogen acquisition in the mycorrhizal grass species Brachypodium distachyon [24]. Growth of Rhizophagus irregularis fertilized with Rhizophagus irregularis spores acquired two to threefold more nitrogen from organic matter than from the Rhizophagus irregularis control without fertilization with AMF or soil microbial communities [25]. Rhizophagus irregularis fertilized with both Rhizophagus irregularis and soil microbial communities acquired ten to twelvefold more nitrogen from organic matter than the control [26]. The increased uptake of organic nitrogen in host plants with AM symbiosis represents the extreme degree of importance that AM symbiosis has on nutrient cycles within global ecosystems.

Applications

Conclusion

References

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  2. Barman, J., Samanta, A., Saha, B., & Datta, S. (2016). Mycorrhiza. Resonance, 21(12), 1093-1104. doi:10.1007/s12045-016-0421-6
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  4. Moore, D., Robson, G., & Trinci, A. (2011, July 14). 21St century guidebook TO Fungi: Plant science. Retrieved April 05, 2021, from https://www.cambridge.org/gb/academic/subjects/life-sciences/plant-science/21st-century-guidebook-fungi?format=WW&isbn=9780521186957
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  13. Strullu-Derrien, C., Selosse, M.-A., Kenrick, P., & Martin, F. M. (2018). The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics. The New Phytologist, 220(4), 1012–1030. doi: 10.1111/nph.15076
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  18. Hayman, D. S. (1986). Mycorrhizae of nitrogen-fixing legumes. MIRCEN Journal of Applied Microbiology and Biotechnology, 2(1), 121–145. doi: 10.1007/BF00937189
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  20. Hayman, D. S. (1986). Mycorrhizae of nitrogen-fixing legumes. MIRCEN Journal of Applied Microbiology and Biotechnology, 2(1), 121–145. doi: 10.1007/BF00937189
  21. Mus, F., Crook, M. B., Garcia, K., Garcia Costas, A., Geddes, B. A., Kouri, E. D., … Peters, J. W. (2016). Symbiotic nitrogen fixation and the challenges to its extension to nonlegumes. Applied and Environmental Microbiology, 82(13), 3698–3710. doi: 10.1128/AEM.01055-16
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  24. Averill, C., Turner, B. L., & Finzi, A. C. (2014). Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature, 505(7484), 543–545. doi: 10.1038/nature12901
  25. Averill, C., Turner, B. L., & Finzi, A. C. (2014). Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature, 505(7484), 543–545. doi: 10.1038/nature12901
  26. Averill, C., Turner, B. L., & Finzi, A. C. (2014). Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature, 505(7484), 543–545. doi: 10.1038/nature12901



Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2021, Kenyon College.

Applications

Conclusion

References



Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2021, Kenyon College.