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Mycorrhizal network

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Mycorrhizal networks (also known as common mycorrhizal networks - CMN) are underground hyphal networks created by mycorrhizal fungi that connect individual plants together and transfer water, carbon, and nutrients. The formation of these networks is context dependent, and can be influenced by soil fertility, resource availability, host or myco-symbiont genotype, disturbance and seasonal variation.[1]

Substances transferred through mycorrhizal networks

Several studies have demonstrated that mycorrhizal networks can transport carbon,[2][3][4] phosphorus,[5] nitrogen,[6][7] water,[1][8] defense compounds,[9] and allelochemicals [10][11] from plant to plant. The flux of nutrients and water through hyphal networks has been proposed to be driven by a source-sink model,[1] where plants growing under conditions of relatively high resource availability (e.g., high light or high nitrogen environments) transfer carbon or nutrients to plants located in less favorable conditions. A common example is the transfer of carbon from plants with leaves located in high light conditions in the forest canopy, to plants located in the shaded understory where light availability limits photosynthesis.

Types of mycorrhizal networks

There are two main types of mycorrhizal networks: arbuscular mycorrhizal networks and ectomycorrhizal networks.

  • Arbuscular mycorrhizal networks are formed between plants that associate with Glomeromycetes. Arbuscular mycorrhizal associations (also called endomycorrhizas) predominate among land plants, and are formed with 150-200 known fungal species, although true fungal diversity may be much higher.[12] It has generally been assumed that this association has low host specificity. However, recent studies have demonstrated preferences of some host plants for some Glomeromycete species [13][14]
  • Ectomycorrhizal networks are formed between plants that associate with ectomycorrhizal fungi and proliferate by way of Ectomycorrhizal extramatrical mycelium. In contrast to Glomeromycetes, ectomycorrhizal fungal are a highly diverse and polyphyletic group consisting of 10,000 fungal species.[15] These associations tend to be more specific, and predominate in temperate and boreal forests.[12]

Benefits of mycorrhizal networks for plants

Several positive effects of mycorrhizal networks on plants have been reported. These include increase establishment success, higher growth rate and survivorship of seedlings;[16] improved inoculum availability for mycorrhizal infection;[17] transfer of water, carbon, nitrogen and other limiting resources increasing the probability for colonization in less favorable conditions.[18] These benefits have also being identified as the primary drivers of positive interactions and feedbacks between plants and mycorrhizal fungi that influence plant species abundance [19]

Mycorrhizal networks and mycoheterotrophic and mixotrophic plants

Monotropastrum humile - an example of a myco-heterotrophic plant that gains all of its energy through mycorrhizal networks

Mycoheterotrophic plants are plants that are unable to photosynthesize and instead rely on carbon transfer from mycorrhizal networks as their main source of energy. This group of plants includes about 400 species. Some families that include mycotrophic species are: Ericaceae, Orchidaceae, Monotropaceae, and Gentianaceae. In addition, partially mycoheterotrophic ('mixotrophic') plants also benefit from energy transfer via hyphal networks. These plants have fully developed leaves but usually live in very nutrient and light limited environments that restrict their ability to photosynthesize.[20]

Importance of mycorrhizal networks at the forest community level

Connection to mycorrhizal networks creates positive feedbacks between adult trees and seedlings of the same species and can disproportionally increase the abundance of a single species, potentially resulting in monodominance.[3][16] Monodominance occurs where a single tree species accounts for the majority of individuals in a forest stand.[21] McGuire (2007), working with the monodominant tree Dicymbe corymbosa in Guyana demonstrated that seedlings with access to mycorrhizal networks had higher survival, number of leaves, and height than seedlings isolated from the ectomycorrhizal networks.[16]

References

  1. ^ a b c Simard, S.W. (2012). "Mycorrhizal networks: Mechanisms, ecology and modeling". Fungal Biology Review. 26: 39–60.
  2. ^ Selosse M.A., Richard F., He X., Simard S.W. 2006. "Mycorrhizal networks: des liaisons dangereuses?". Trends in ecology and evolution 21: 621–628.
  3. ^ a b Teste F.P., Simard S.W., Durall D.M. 2009. "Role of mycorrhizal networks and tree proximity in ectomycorrhizal colonization of planted seedlings". "Fungal Ecology" 2: 21-33.
  4. ^ Hynson N.A., Mambelli S., Amend A.S., Dawson T.E. 2012. "Measuring carbon gains from fungal networks in understory plants from the tribe Pyroleae (Ericaceae): a field manipulation and stable isotope approach". "Oecologia" 169: 307–317.
  5. ^ Eason W.R., Newman E.I., Chuba P.N. 1991. "Specificity of interplant cycling of phosphorus: the role of mycorrhizas". "Plant Soil" 137: 267-274.
  6. ^ He X.H., Critchley C., Ng H. 2004. "Reciprocal N (15NH4+or 15NO3) transfer between non-N2-fixing Eucalyptus maculata and N2-fixing Casuarina cunninghamiana linked by the ectomycorrhizal fungus Pisolithus sp". "New Phytologist" 163: 629–40.
  7. ^ He X., Xu M., Qui G.Y., Zhou J. 2009. "Use of 15N stable isotope to quantify nitrogen transfer between mycorrhizal plants". "Journal of Plant Ecology" 2(3):107–118.
  8. ^ Bingham M.A., Simard S.W. 2011."Do mycorrhizal network benefits to survival and growth of interior Douglas-fir seed- lings increase with soil moisture stress?". "Ecol. Evol." 1: 306-316.
  9. ^ Song Y.Y., Zeng R.S., Xu J.F., Li J., Shen X., Yihdego W.G. 2010. "Interplant communication of tomato plants through underground common mycorrhizal networks". "PLoS ONE" 5: e13324.
  10. ^ Barto E.K., Hilker M., Muller F., Mohney B.K., Weidenhamer J.D., Rillig M.C., 2011. "The fungal fast land: common mycorrhizal networks extend bioactive zones of al- lelochemicals in soils". "PLoS ONE" 6: e27195.
  11. ^ Barto E.K., Weidenhamer J.D., Cipollini D., Rillig M.C. 2012. "Fungal superhighways: do common mycorrhizal networks enhance below ground communication?". "Trends in Plant Science" 17(11): 633-637.
  12. ^ a b Finlay, R.G. 2008. "Ecological aspects of mycorrhizal symbiosis: with special emphasis on the functional diversity of interactions involving the extraradical mycelium". "Journal of Experimental Botany" 59(5): 1115–1126.
  13. ^ Vandenkoornhuyse P. et al. 2003. Co-existing grass species have distinctive arbuscular mycorrhizal communities. Molecular Ecology 12:3085-3095.
  14. ^ Schechter S.P., Bruns T.D. 2013. "A Common Garden Test of Host-Symbiont Specificity Supports a Dominant Role for Soil Type in Determining AMF Assemblage Structure in Collinsia sparsiflora". "PloSONE" 8(2): e55507.
  15. ^ Taylor A.F.S., Alexander I. 2005. "The ectomycorrhizal symbiosis: life in the real world". "Mycologist" 19: 102–112.
  16. ^ a b c McGuire, K. L. 2007. Common ectomycorrhizal networks may maintain monodominance in a tropical rain forest. Ecology 88(3): 567–574.
  17. ^ Dickie, I.A., Reich, P.B. 2005. Ectomycorrhizal fungal communities at forest edges. Journal of Ecology 93: 244–255.
  18. ^ van der Heijden M.G.A and Horton T.R. 2009. "Socialism in soil? The importance of mycorrhizal fungal networks for facilitation in natural ecosystems". "Journal of Ecology" 97: 1139–1150.
  19. ^ Bever, J.D.; Dickie, I.A.; Facelli, E.; Facelli, J.M.; Klironomos, J.; Moora, M.; Rillig, M.C.; Stock, W.D.; Tibbett, M.; Zobel, M. 2010. Rooting Theories of Plant Community Ecology in Microbial Interactions. Trends Ecol Evol 25(8): 468–478.
  20. ^ Selosse, M.A., Roy, M. 2009. Green plants that feed on fungi: facts and questions about mixotrophy. Trends Plant Sci. 14: 64-70.
  21. ^ Peh, K.S.H.; Lewis, S.L. and Lloyd, J. 2011. Mechanisms of monodominance in diverse tropical tree-dominated systems. Journal of Ecology: 891–898.
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