Information Super Highways Of The Forest
When you wander in a forest, you feel its ambience: birds and insects chirping, animals being in their natural habitat, etc. Who can forget the lovely trees of the forest who always calm our minds? They all give us an idea of the interactions in the world above. How busy and pleasantly noisy our natural world is. What about the world below? Beneath the soil? Is it all dark and lonely? It may seem at first but the underground is as active as the aboveground, if not more.
Forest image from https://pixabay.com/photos/forest-trees-sun-rays-sunlight-fog-1072828/
Fungi image from https://www.pexels.com/photo/assorted-color-of-mushrooms-surrounded-by-trees-797946/
Dr Suzanne Simard from the Department of Forest and Conservation Sciences at the University of British Columbia, lets us peek into her life’s work, "Trees are the foundation of forests. But a forest is much more than what you see. Underground, there is this other world, a world of infinite biological pathways that connect trees and allow them to communicate and allow the forest to behave as if it’s a single organism." [4]
Image credits: https://suzannesimard.com/about/
This is the trees “talking to each other” in the forest. Yes, you heard it right!! Talking by the exchange of phytochemicals between each other using fungal hyphae intertwined with their large roots. Does it remind you of something? Why, yes, it resembles the Internet or the World Wide Web. Scientists have smartly dubbed this network as the “Wood Wide Web.”
Schematic of Wood Wide Web. Image credits: How trees secretly talk to each other - BBC News - https://www.youtube.com/watch?v=yWOqeyPIVRo
These filamentous fungi present underground is in a symbiotic relationship called mycorrhizae (Greek, myco= fungus; rhizae = root) with the higher and complex plant species in almost all the ecosystems present worldwide. The symbiosis is very old: 400-450 million years. The fungal species involved are Ascomycota, Basidiomycota and Glomeromycota. They form a mutualistic symbiotic relationship with these plants. This means that they are not saprophytic, making them different from the rest of the fungi. Instead of degrading the plant roots and absorbing them, these fungi instead accept nourishment from their plant partners. They accept carbohydrates from these plants produced after photosynthesis which helps in their multiplication and growth. In turn, the plants are benefited, by nutrient transfer uptake of nitrogen and phosphorus from the soil, plant growth, absorption of water from the surrounding soil and resistance to plant diseases. They form a relationship with around 80-90% of plant species. These spread as far as the plant’s roots and this makes them appear as a network. This makes the Wood Wide Web possible. [1, 5]
The extent of the underground fungal network. Image credits: How trees secretly talk to each other - BBC News - https://www.youtube.com/watch?v=yWOqeyPIVRo
They are majorly categorized into two types: endomycorrhizzae or arbuscular mycorrhizae (AM) and ectomycorrhizae (ECM). The former colonizes the roots of the plant by penetrating the cells and intracellular spaces of the root. The latter just forms an outer sheath of fungal hyphae outside the root. Let’s talk about ECM first. ECM is formed by Ascomycota, Basidiomycota. The fungal species involved are both mutualistic and saprophytic in nature. Mutualism exists when they form a symbiosis with the roots of the plant and in the soil, these species are facultatively saprophytic in the soil. Here, facultative means that, them being saprophytic depends on the favourability of the environment they thrive in. Plantlife from temperate regions is colonized and usually belong to the families ofCaesalpinoidaceae, Fagaceae, Pinaceae and Dipterocarpaceae. The lateral roots that have divided from the central root are colonized. It develops as a mycelium that penetrates intercellular spaces and also extends and deepens as a thick mycelial sheath that covers the extent of the whole root. Basically, if you uproot some of these roots, you will find them covered with a thick mycelial sheath and underneath the blunt root tip will be visible. This could be because these fungal hyphae ensure the nutrient and water uptake hence the root tips need not be sharp and thin to do the task. Hence, ECM ensures that the root tips stay blunt. The plant is saved from doing all the extra hard work. Also, this network of dense filaments is known as a ‘rhizomorph.’ Enough of the outside, let's speak about the inside. Around the cortical cells and outer epidermal cells of the root, hyphal filaments penetrate and this gives rise to the structure of the ‘Hartig net.’ So now we can see and decipher the path of the two-way exchange between the plant and the fungus. [1, 5]
Comparison of colonization of AM and ECM [2]
Moving on to the second type, the endomycorrhizzae (AM) which are made up of Glomeromycota. It is even more prevalent than ECM and colonizes almost 80% of the terrestrial plants. Now, what does an arbuscule mean? It is defined as a transient and branched structure produced by the fungal species that merges into the folds of the plasma membrane of the plant cell without any penetration. These resemble tree-like branches and hence their name. These are considered to be the key element of the symbiotic nutrient exchanges between the plant and the fungus. They only colonize the middle shaft of the root and not the tip. The nearby colonized fungal hyphae, release some spores and they germinate into a hyphopodium/appressorium (root base). Metabolites released by the plant leads to the branching and penetration of the hyphae in between and into the cells without the lysis of the plasma membrane. The inner cortical cells show the presence of these arbuscles. These occupy the maximum cell volume to enable the optimum exchange of nutrients. Hence, the penetration is deeper than ECM. [1, 5]
So, for the Wood Wide Web, let’s take an example of a forest, individual mycorrhizal network (let’s call them MN) from every tree intermingles to form a complete network consisting of all the trees. According to plant and fungal biologist, Merlin Sheldrake, “All of these trees will have mycorrhizal fungi growing into their roots. You could imagine the fungi themselves as forming a massive underground tree, or as a cobweb of fine filaments, acting as a sort of prosthesis to the trees, a further root system, extending outwards into the soil, acquiring nutrients and floating them back to the plants, as the plants fix carbon in their leaves and send sugar to their roots, and out into the fungi. And this is all happening right under our feet.” [6]
This figure [3] gives us an idea about the network and the exchange of substances.
A massive horizontal transfer of water, carbon, allelochemicals, nutrients and defence signals usually takes place. Allelochemicals are those secondary metabolites that are spread by plants to either positively influence or deteriorate the growth of other plants. This is a competition mediated tactic for the optimal use of available resources. These are of particular importance, as we will find out. Now, the transfer takes place according to the source-sink model in terms of resources i.e. from a tree having an abundance of a particular resource to another tree having scarcity of that resource. This is particularly noteworthy for a long-distance exchange. Also, at the growing points of MN, we see nutrient exchange either in the form of active transport or facilitated or simple diffusion. This is true also at the sites of interaction with the respective host plants. [2]
An illustration showing how mother trees support younger trees beside them. Image credits: https://blog.byjus.com/the-learning-tree/science-feed/talks-mother-tree-baby-trees/
The source where it all starts is known as the ‘mother’ or ‘Hub’ tree. Such trees are super senior in terms of their heights and age. Courtesy to their height, they get better sunlight, so more photosynthesis. Hence, these trees have an abundance of extra glucose in themselves which they then spread to those fungal partners underground, and then the remaining nutrients are supplied to nearby trees and saplings. For saplings, MNs ensure their survival and nurture them. These hub trees have proven to be the most important for connections since if they die or are cut down, many networks can potentially get lost. [7]
MNs have also been discovered to significantly affect plant behaviour which usually changes as per the cues from the environment. Examples of plant behaviour are competing for sunlight by growing taller, extending the roots deeper for the getter nutrition, etc. Using allelochemicals for communication these are changed for the better or worse: photosynthetic rate, foliar nutrition, foliar defence chemistry and defence response, etc. Defence signals sent to recipient plants lead to sudden changes in their defence chemistry and eventually lead to pest resistance. For example, the response of Vicia faba to aphids infesting on it by transfer of defence signals using MN to nearby plants. They, in exchange, provide chemicals that attract the respective predators or repel these insects. [3]
All is not roses or ethical in this network. Unfair advantages have been taken by orchids by hijacking the MN to nourish themselves. Also, black walnut trees spread toxins in order to kill their neighbouring saplings or trees. [8]
With all this, it is evident that MNs play an important role in ecosystems functioning as complex entities of the biosphere. Understanding MNs for their function and structure especially for an ecosystem can help us fuel our curiosity and fulfil our quest to understand ecological stability and evolution. This will provide us with new insights on how to manage ecosystems by improving our current conservation practices. [2]
References
[1] P. Bonfante and A. Genre, “Mechanisms underlying beneficial plant–fungus interactions in mycorrhizal symbiosis,” Nature Communications, vol. 1, no. 1, Jul. 2010, DOI: 10.1038/ncomms1046.
[2] S. W. Simard, K. J. Beiler, M. A. Bingham, J. R. Deslippe, L. J. Philip, and F. P. Teste, “Mycorrhizal networks: Mechanisms, ecology and modelling,” Fungal Biology Reviews, vol. 26, no. 1, pp. 39–60, Apr. 2012, DOI: 10.1016/j.fbr.2012.01.001.
[3] M. A. Gorzelak, A. K. Asay, B. J. Pickles, and S. W. Simard, “Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities,” AoB Plants, vol. 7, p. plv050, 2015, DOI: 10.1093/aobpla/plv050.
[4] TED, “How trees talk to each other | Suzanne Simard,” YouTube. Aug. 30, 2016, Accessed: Nov. 05, 2020. [Online]. Available: https://www.youtube.com/watch?v=Un2yBgIAxYs.
[5] J. M. Willey, L. Sherwood, C. J. Woolverton, and L. M. Prescott, Prescott’s Microbiology, 10th ed. New York, NY: Mcgraw-Hill Education, 2017.
[6] R. Macfarlane, “The Secrets of the Wood Wide Web,” The New Yorker, Aug. 07, 2016. https://www.newyorker.com/tech/annals-of-technology/the-secrets-of-the-wood-wide-web
[7] National Geographic, “How Trees Secretly Talk to Each Other in the Forest | Decoder,” YouTube. Sep. 11, 2018, [Online]. Available: https://www.youtube.com/watch?v=7kHZ0a_6TxY.
[8] BBC News, “How trees secretly talk to each other,” YouTube. Jun. 29, 2018, [Online]. Available: https://www.youtube.com/watch?v=yWOqeyPIVRo.
Comments