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

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].

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 Ventor’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.

ISIS Report 28/10/13
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