Underground networks

Nearly all plants on Earth form a symbiosis with mycorrhizal fungi. These fungi have altered the evolutionary history of the earth.

V. Caldas

The world under our feet

What are mycorrhizal fungi?

Mycorrhizal fungi are a group of network-forming soil fungi that form symbiotic associations with plants. Nearly all plants form symbiotic associations with mycorrhizal fungi. These associations have shaped life on earth for more than 475 million years.

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Symbiotic relationships between plants and mycorrhizal fungi are around 475 million years old and play fundamental roles in the Earth’s biosphere. The rise of plant-fungal partnerships corresponds with a 90% reduction in atmospheric CO2 levels. Today, between 80-90% of all plant species form a trade symbiosis with mycorrhizal fungi. Mycorrhizal fungi grow into large networks of tubular cells, known as mycelium (individual cells are called hyphae), which forage for nutrients in the soil and exchange them with their plant partners. A single gram of soil can contain up to 90 meters of mycelium. The total length of mycorrhizal mycelium in the top ten centimeters of soil is around 450 quadrillion kilometers: around half the width of our galaxy. Mycelium forms a sticky living mesh that provides soil with much of its structure. Mycorrhizal fungi do more than provide plants with nutrients. Mycorrhizal fungi are also important in pathogen protection, heavy metal tolerance, and water uptake.

Sources
  1. Tedersoo, L. et al. “How mycorrhizal associations drive plant population and community biology” Science 36 (2020)
  2. Frew, A. et al. “Plant herbivore protection by arbuscular mycorrhizas: A role for fungal diversity?” New Phytol. (2021)
  3. Rimington, W.R. et al. “The distribution and evolution of fungal symbioses in ancient lineages of land plants.” Mycorrhiza 30, 23–49 (2020)
  4. Strullu-Derrien, C. et al. “The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics.” New Phytol. 220, 1012–1030 (2018)

L. Oyarte Galvez, AMOLF

Niels Hoebers

Underground economics

Plants and fungi exchange resources with one another and are able to strike compromises, resolve trade-offs and deploy sophisticated trading strategies.

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Soils are some of the most complex ecosystems on Earth, and fungi must trade to survive. Animals rely on a central nervous system to make trade decisions, but fungal networks must evaluate trade environments without a brain. First, they must forage for nutrients in the soil (like phosphorus and nitrogen). Second, they must exchange these nutrients for carbon compounds (like sugars and fats). To do this, mycorrhizal fungi have evolved sophisticated trading strategies, and can discriminate between plant partners, exchanging more resources to plants that provide them with more carbon. Fungi can capitalize on value differences across complex trade networks by moving resources to where they gain a better price from plant ‘buyers’. In one study, when faced with an unequal supply of nutrients across their networks, mycorrhizal fungi moved phosphorus to areas of scarcity, where it was in higher demand and therefore fetched a higher ‘price’. By doing so, the fungus was able to receive larger quantities of carbon in return. Fungi can even hoard resources until they retain a higher ‘price’. Researchers are using new tools to tag nutrients inside fungal networks and track fungal trade decisions.

Sources
  1. Kiers, E.T. et al. “Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis.” Science 33, 880-882 (2011)
  2. Noë, R, & Kiers, E.T. “Mycorrhizal Markets, Firms, and Co-ops.” Trends Ecol. Evol. 33, 777-789 (2018)
  3. Oskar, F. et al. “Forests trapped in nitrogen limitation – an ecological market perspective on ectomycorrhizal symbiosis” New Phytol. 203(2): 657–666 (2014)
  4. van’t Padje, A. et al. “Temporal tracking of quantum-dot apatite across in vitro mycorrhizal networks shows how host demand can influence fungal nutrient transfer strategies.” ISME J. 1-15 (2020)
  5. Whiteside, M.D. et al. “Mycorrhizal fungi respond to resource inequality by moving phosphorus from rich to poor patches across networks.” Curr. Biol. 29, 2043–2050 (2019)

Underground Connections

Mycorrhizal fungi form networks that connect plants underground. These networks help distribute nutrients across ecosystems. Organisms, like bacteria, also use these fungal ‘super-highways’ for transport, allowing them to travel between between different roots.

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Underground, mycorrhizal fungi form networks of hyphae connecting roots of diverse host plants. These networks, also called common mycorrhizal networks (CMNs), facilitate the flow of nutrients and carbon, mediate both cooperative and competitive relationships between plants, and help protect plants from pests and pathogens. Once a plant has ‘plugged into’ a CMN, the network can serve as a physical conduit for the movement of nutrients, chemicals and even electrical impulses. Common mycorrhizal networks potentially allow resources to move preferentially between parent trees and their offspring, although we still don’t understand how kin recognition occurs. Bacteria can also use fungal networks as highways to travel efficiently between different roots.

Sources
  1. Barto, E.K. et al. “Fungal superhighways: Do common mycorrhizal networks enhance below ground communication?” Trends Plant Sci. 17, 633–637 (2012)
  2. File, A. et al. “Plant Kin Recognition Enhances Abundance of Symbiotic Microbial Partner.” PLoS One 7(9): e45648 (2012)
  3. Johnson, D. & Gilbert, L. “Interplant signalling through hyphal networks.” New Phytol. 205, 1448–1453 (2015)
  4. Birch, J.D. “Beyond seedlings: Ectomycorrhizal fungal networks and growth of mature Pseudotsuga menziesii. J. of Ecol. 109, 806-818 (2021)
  5. Klein, T. “Belowground carbon trade among tall trees in a temperate forest.” Science 352, 342–344 (2016)

Underground Communication

Plants are able to pick up cues from neighboring plants via shared mycorrhizal networks, potentially allowing them to prepare for insect attacks.

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Plants can prepare themselves for insect attack by picking up cues that travel through common mycorrhizal networks. In this sense, plants use the fungal network to ‘eavesdrop’ on their neighbors. The ’social networking’ of plants via fungal connections may be used in the future to help improve forestry and crop management in agricultural systems.

Sources
  1. Barto, E.K. et al. “Fungal superhighways: Do common mycorrhizal networks enhance below ground communication?” Trends Plant Sci. 17, 633–637 (2012)
  2. Johnson, D. & Gilbert, L. “Interplant signalling through hyphal networks.” New Phytol. 205, 1448–1453 (2015)
  3. Tedersoo, L. et al. “How mycorrhizal associations drive plant population and community biology.” Science 367, 6480 (2020)
  4. Sharifi, R. & Ryu, C. “Social networking in crop plants: Wired and wireless cross-plant communications.” Plant. Cell Environ. 44, 1095–1110 (2021)
  5. Song, Y.Y. et al. “Defoliation of interior Douglas-fir elicits carbon transfer and stress signalling to ponderosa pine neighbors through ectomycorrhizal networks.” Sci. Reports 5, 8495 (2015)
  6. Simard S. “Mycorrhizal networks facilitate tree communication, learning, and memory.”
  7. Memory and Learning in Plants, 191-21 (2018)

Underground flows

Phosphorus, nitrogen, carbon and other nutrients flow through mycorrhizal networks in complex patterns. To understand how fungi coordinate flows, researchers are simultaneously monitoring the architecture of networks and the direction and speed of flows within the networks.

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How do fungi control the flows of nutrients across their large and complex networks? Fungal networks are bathed in a rich field of sensory information and must integrate a complex array of chemical, physical, and environmental stimuli. Fungal networks must constantly remodel themselves, sending carbon to growing tips to build new trade routes, and collect nutrients, like phosphorus and nitrogen, to send to plants roots. Scientists are studying the complex flow patterns inside fungal networks to find out how we can better harness the power of mycorrhizal associations to store carbon and support the health of ecosystems.

L. Oyarte Galvez, AMOLF

What are the four types of mycorrhizal fungi?

There are four main types of mycorrhizal association. Each type interacts with plants in different ways and differ in their ability to store carbon and forage for nutrients.

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Arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi are the ancient, ancestral form of mycorrhizal symbiosis. These fungi played a key part in the movement of plants’ ancestors onto dry land. By the time the first roots evolved, the mycorrhizal association was already some 50 million years old.

Plants that associate with arbuscular mycorrhizal fungi make up around 70% of global plant biomass

Physiologically, arbuscular mycorrhizal fungi form intracellular structures in roots called arbuscules. Arbuscule means ‘a branched treelike organ’. This is because arbuscules look like mini-trees inside the roots of plants. Arbuscules are the main sites of nutrient exchange between plants and fungi. Plants associated with arbuscular mycorrhizal fungi represent around 70% of global plant biomass, including all major crops, making it one of the most important symbiotic relationships on Earth.

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Ectomycorrhizal fungi

Trees in most boreal and temperate forests depend on ectomycorrhizal associations. Ectomycorrhizal associations have evolved more than seventy times since the first movement of plants onto land. These fungi form a mycelial sleeve around plant root tips, called a ‘Hartig net’, where exchange of nutrients and carbon takes place. Unlike arbuscular mycorrhizal fungi, ectomycorrhizal fungi do not grow into plant cells (‘ecto’ means outside).

The ectomycorrhizal lifestyle has evolved independently more than 70 different times

Ectomycorrhizal fungi evolved from free-living decomposer fungi and retain a wide range of enzymes that allow them to degrade complex substances in soils. Ectomycorrhizal fungi are excellent foragers and are able to obtain nutrients unavailable to arbuscular mycorrhizal fungi. They require more energy from their plant partners than arbuscular mycorrhizal fungi, and tend to form relationships with shrubs and trees.

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Orchid mycorrhizal fungi

Most plants supply their mycorrhizal partners with carbon in exchange for mineral nutrients. Orchids are able to do something different, and obtain both carbon and mineral nutrients from their fungal partners for at least some of their lives.

~17,000 species of orchids rely on very specialized fungal partnerships for nutrients

Of all the plant families, orchids are the most diverse, and orchid-specific mycorrhizal fungi may have played a role in their evolutionary success. There are 250 species of orchid that have lost the ability to photosynthesize entirely, and acquire all the carbon and nutrients they need to survive from their fungal partners.

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Ericoid mycorrhizal fungi

Ericoid mycorrhizal fungi form relationships with plants in the family Ericaceae, which includes heather, blueberries and cranberries. They are most commonly found in acidic and infertile soils, including bogs, heathlands, and boreal forests. Ericoid mycorrhiza are formed between several lineages of mycorrhizal fungi and one plant family: the Ericaceae.

Ericoid mycorrhizal fungi produce enzymes which allow them to break down complex organic molecules

Ericoid mycorrhizal fungi form coils in the root cells of their plant partners rather than arbuscules, and produce enzymes that allow them to break down complex organic molecules.