Science in Society Archive

Plants Warn One Another of Pest Attack through Mycorrhizal Fungal Network

Exquisite inter-species relationships between plants and fungi threatened by industrial farming Dr Eva Sirinathsinghji

Underground intercom between plants

A network of the soil microorganism - arbuscular mycorrhizal fungi - act as an underground intercommunications system between plants to warn off aphid attacks, a recent study from the University of Aberdeen in the UK reveals [1].

Broad bean plants (Vicia faba) free of pea aphids (Acyrthosiphon pisum) were found to release aphid-deterring chemicals when neighbouring plants were under attack. Such a chemical response was previously only associated with plants directly under attack. These chemicals also attracted the aphid predator – the parasitoid wasp (Aphidius ervi Haliday) - that keeps aphid populations down. The response was dependent on the arbuscular mycorrhizal fungus (Glomus intraradices) that forms branching mycelial networks between the plants.

This remarkable phenomenon is an example of interspecific symbiotic relationships, many of which researchers are yet to fully comprehend. It is an aspect of agricultural science totally ignored by industrial farming systems. Pesticides and herbicides kill soil microorganisms and damage mycorrhizal/plant associations. This may explain the effects of Monsanto’s Roundup herbicide, which has been blamed for the epidemic spread of plant diseases associated with the planting of Roundup tolerant GM crops (see [2] Ban GMOs Now). 

Mycorrhizal fungi are among the most functionally important soil microorganisms. They form symbiotic relationships with a range of plants to increase plant mineral uptake, increase tolerance to root and shoot pathogens and redistribute water during drought stress [3]. The fungi in return, gain carbohydrates supplied by plants to enhance their mycelial networks underground. These networks are extensive, connecting plants of the same species, and can even connect to plants of different species as mycorrhizae lack host plant specificity. Common mycelial networks facilitate seedling establishment, influence plant community composition and are the primary pathways through which many species of non-photosynthetic plants acquire their energy. Furthermore, they have been shown to transport signalling molecules from plant to plant, including allelochemicals - metabolites release d by plants to positively or negatively affect neighbouring plants. Another example of such symbiosis is the recent finding that tomato plants rely on mycelial networks to increase enzyme activity and defence-related gene expression in resisting early leaf blight [4].

New tests show mycorrhizal fungi are underground agents

The new study, for the first time, tests whether these mycelial networks are able to facilitate interplant signalling during attack by herbivores. Also tested is whether this signalling induces the release of volatile organic chemicals (VOCs) used to deter or attract pests or natural enemies of pests. It is already known that plants often produce VOCs systemically, which are transmitted through the air. VOCs are also known to be produced in the root system that may be transmitted by mycelial networks.

To test these theories, the researchers grew pea plants in small growth chambers that bring a small part of the natural environment under controlled conditions. They used ‘donor’ plants that are to be infested with aphids and ‘receiver’ plants connected via mycelial networks created by inoculating the soil with mycorrhizal fungi. One receiver plant was connected to the donor without any physical barrier between donor and receiver plant allowing the intermingling of mycorrhizal fungi and roots, the other receiver plant was connected through a 40 µm mesh penetrable by mycelial networks but not by roots. These two conditions allow distinction between the possibility of communication between plant root systems and the mycelial network. Control ‘receiver’ plants were blocked from receiving information: one was surrounded by 0.5 µm mesh that prevents mycorrhizal penetration and the other was grown in a penetrable 40 µm mesh, but immediately before aphids were added to the donor plant the chamber was rotated to snap any connections that had been made. To confirm the presence of mycelial networks, roots were stained with dye and analysed under a microscope.

Five weeks into the study after mycelial networks should be well established, bags were placed over plants before aphid infestation to prevent aerial communication between plants. To measure whether plants released VOCs to repel aphids and attract to the predatory parasitoid wasp, the air or ‘headspace’ around the plant was captured and exposed to both insect species. The researchers found that headspace from donor plants both repelled aphids and attracted the wasps; the same result was obtained with headspace from receiver plants connected via mycelial networks, while both types of control plants remained attractive to aphids, as shown in Figure 1. Furthermore, there was not a significant difference in repellent properties of receiver plants connected to the donor plant via both roots and mycelial networks compared to those connected by mycelial networks alone, implying that mycorrhizal fungi and not root-to-root connectivity were responsible for the communication.

Figure 1   Mycorrizhal fungi provide hyphal connections from aphid infested ‘donor’ plants to aphid-free ‘receiver’ plants to repel aphids and attract their predator, the parasitoid wasp (A. ervi) (see text for further details)

Analysis of the compounds released by the donor plant found that the principle compound both repelling aphids and attracting wasps was methyl salicylate. This compound was also present at high levels in the headspace of donor and connected receiver plants. Addition of this chemical to the headspace surrounding plants that were attractive to aphids reversed this response and resulted in repelling the aphids. Other compounds present in the headspace of aphid repelling plants were naphthalene, (E)-β-farnesene and 6-methyl-5-hepten-2-one.

Damage to soil fertility by industrial farming may explain the spread of crop diseases

Herbicides such as glyphosate had been found to damage soil fertility, including the destruction of mycorrhizal fungi, with a reduction of root colonisation by as much as 33 % in chilli peppers [5]. Another study published in 2013 shows that even recommended low doses of glyphosate significantly reduce root colonisation as well as spore viability of the mycorrhizal fungus G. intraradices in Argentinian fields [6]. This contradicts the claim that glyphosate is immobilised too quickly to affect soil fertility. This loss of mycorrhizal association may also contribute to the many existing problems associated with GM crop cultivation, including the rise in plant diseases in glyphosate-tolerant crops due to glyphosate’s strong metal-chelating properties that sequester micronutrients including manganese that are critical for the plant immune systems (see [7] Glyphosate Tolerant Crops Bring Diseases and Death, SiS 47).

A recent study in Kenya has also exposed the inability of commercial hybrid maize varieties to produce VOCs to attract parasitoid insects. This was done in comparison with landraces from Cuba, Brazil and Haiti which were successful in producing this natural plant defence mechanism [8]. Whether or not these commercial varieties lost this ability through domestication and breeding processes or through chemical treatment of seeds are not clear.

The biotech industry has greatly underestimated the complexity of plant and soil systems. Monsanto may have recognized the problem as it has recently bought a part of Craig Venter’s company Aradis, whose focus is “discovering and developing microorganisms that improve plant growth and provide protection from plant pests” [9]. It is likely that they will sell back to farmers a patented version of nature that they first destroyed with their GM chemical farming system. Focusing on agroecological methods that promote soil fertility, on the other hand, will allow crops to use their natural defence mechanisms to full potential, avoiding the excessive use of chemical pesticides that are both harmful to health and the environment.  

Article first published 28/10/13



References

  1. Babikova Z, Gilbert L, Bruce TJ, Birkett M, Caulfield JC, Woodcock C, Pickett JA, Johnson D. Underground signals carried through common mycelial networks warn neighbouring plants of aphid attack. Ecology Letters 2013 ;16, 835-43
  2. Ho MW & Sirinathsinghji E. Ban GMOs Now.Health and Environmental Hazards Especially in Light of the New Genetics. ISIS Special Report, 2013. http://www.i-sis.org.uk/Ban_GMOs_Now.php
  3. Smith, S.E. & Read, D.J. (2008). Mycorrhizal Symbiosis, 3rd edn. Academic Press, London (Elsevier Ltd.).
  4. Song YY, Zeng RS, Xu JF, Li J, Shen X, Yihdego WG. Interplant communication of tomato plants through underground common mycorrhizal networks. PLoS One 2010, 5, e13324
  5. Ronco MG, Beltrano J, Ruscitti M, Arango MC. Glyphosate and mycorrhization induce changes in plant growth and in root morphology and architecture in pepper plants (Capsicum annuum L.). The Journal of Horticultural Science and Biotechnology 2008, 83, 497-505.
  6. Druille M, Cabello MN, Omacinia M, Golluscioa RA. Glyphosate reduces spore viability and root olonisation of rbuscular mycorrhizal fungi. Applied Soil Ecology 2013, 64, 99-103.
  7. Ho MW. Glyphosate tolerant crops bring diseases and death.Science in Society 47, 12-15, 2010.
  8. Tamiru A, Bruce TJ, Woodcock CM, Caulfield JC, Midega CA, Ogol CK, Mayon P, Birkett MA, Pickett JA, Khan ZR. Maize landraces recruit egg and larval parasitoids in response to egg deposition by a herbivore. Ecology Letters 2011, 14, 1075-83.
  9. Agradis Homepage, Agradis.com, accessed 14th October 2013 http://www.agradis.com/

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There are 6 comments on this article so far. Add your comment above.

Jerold Hubbard Comment left 28th October 2013 20:08:25
What a valuable piece of INFORMATION! I am a grass roots farmer, (not under the brain washing of BIG AG) from the USA. Recently I have been calling myself a PEST CREATOR...alias Grass Roots Farmer! Why? Because of the fact that over 90 percent of our food production is ANNUALIZED! What does this mean? In annualization of farming you creat an environment that SELECTS FOR PESTS! Organisms that EXPLOIT everything for their own DNA dominance! like Cancer is upon the body. How? By using mechanical plows or chemcial plows to break the synergistic bonds between all the players that may have took thousands of years to establish! This annualization of food production does not change the agents immediately, but it does break these most valuable bonds between all the players!! I end up creating pests all the way from pest corporations to the smallest microbes that exploit everything for their personal gain!!!! It also produces flooding, dust stroms, and lower recharge of the underlying water aquifers. Like humans do to the Earth when we annualize our food production, or other things! This information shows how integrated or synchronized the Earth is! It is a VOICE FOR CHANGING OUR WAY OF PRODUCING FOOD! Jerold Hubbard, Pest Creator. ..alias Grass Roots Farmer from the USA

James Cooley Comment left 28th October 2013 16:04:51
Thank you for calling attention to the importance of microorganisms in promoting plant growth and health. This is one of the many factors that mitigate against the modern chemical dependent agribusiness model, but one that has received far too little attention to date. It reinforces the concept that human health depends on plant health which depends on soil health; a concept that is intuitive to organic farmers, but totally eludes chemical dependent farmers.

Greg Fox Comment left 28th October 2013 16:04:50
Back in the eighties, I expanded my garden by first spraying the grass with glyphosate (Roundup) and for the next two years that dirt smelled different. Apparently it was the lack of fungus in the soil.

Rory Short Comment left 29th October 2013 20:08:29
I agree with Jerrold Hubbard. The reality is that the biosphere took 3.8 billion years to evolve as a set of mutually interdependent life forms. We mess with it at our peril and we actually do not need to mess with it. We can just continue with Nature's tried and, 3.8 billion year, tested approach and ourselves use our consciousnesses to work symbiotic-ally with other life forms. Industrial farming is biosphere threatening madness driven by people who only want to make money.

Ken Conrad Comment left 29th October 2013 20:08:21
Limiting the presence of microbes in our environment via the direct or indirect invasive and toxic manipulation of plants, animals, fish, soil, and water on the pretense of health, safety and productivity is a gross deception perpetrated by human beings obsessed with control for the sake of greed and self-righteous arrogance. It’s a slippery slope we find ourselves on and the slope is getting slipperier and steeper. Some look to science, or perhaps religion, and politics as a means to alleviate the problems of this world, and they can indeed without a question constructively do just that. What we seem to fail to grasp however is the phenomenon that truly motivates our actions. Have mankind’s actions to date demonstrated ample respect for the natural order of God’s complex creation and the fact that human beings have been created with a free will, or have we chosen to usurp that natural order and treat human freewill with contempt?

Nadia de Souza Pietramale Comment left 10th December 2013 12:12:50
I would like to say thank your for Dr Eva Sirinathsinghji for her research. Where I leave glyphosate is commonly used and pieces of work like yours are such important tool to educate the community.

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