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The Whispering World of Plants: "The Wood Wide Web"

Beech is ectomycorrhizal. "Grib skov" by Malene Thyssen - Own work. Licensed under CC BY-SA 3.0 via Commons

The notion that plants can "talk" to one another was, until relatively recently, dismissed as fantasy, but the reality of inter-plant communication is now becoming an accepted part of mainstream science. Although plants, by definition, being "planted" in the ground, cannot move per se, they are able to send signals to one another, by means of volatile organic compounds, and it is thought that not only can a plant communicate with other plants, it may also engage in a "soliloquy" by communicating between different parts of itself. Although this mere fact is fascinating enough, it appears that plants may both send out chemical messengers as airborne species that other plants can receive, and send messages to one another via a network of connections within the soil, provided by the fungal networks known as the "mycorrhizal mycelium". This has been described as a kind of below ground "internet" which is appropriately termed "The Wood Wide Web".

It is quite well documented that if a plant is attacked by an insect or fungal pest, it can signal to its neighbours, so that they release compounds that repel the pests directly, or attract other organisms that are antagonistic to them. While the airborne action of such messenger compounds is established, the idea that one plant can warn another that it is in danger via a common (shared) mycelial network (CMN) is rather more novel. According to a recent study: “Key roles in facilitating nutrient transport and redistribution” are played by the CMN, but they can also “facilitate defense against insect herbivores and foliar necrotophic fungi by acting as conduits for interplant signaling.”

This is a beautiful illustration of the interconnectedness of natural systems, and it has been proposed that there are dominant "Mother Trees", which act as hubs for the mutual connection of all the trees that grow in a forest. This connection is thought to function to some degree through the mycorrhizal mycelium, which lives in and around the tree roots, and serves as a conduit for the transport of carbohydrates, nutrients and water between the trees, via the fungal hyphae. The mother trees serve to "feed" the younger ones, and without them, most of the seedlings would not survive. Recent research has shown that without mother trees, attempts to regenerate forests often fail, and when a mother tree is felled, the survival rate of seedlings tends to be dramatically reduced. As a tree dies, it may also deliver resources to neighbours of different species, feeding them, and contributing to the overall biodiversity and resilience of the forest ecosystem.

The implications of inter-plant communication via fungal networks are potentially far reaching. For example, forest management (harvesting) practices should involve preserving the Mother Trees to nurture new growth. In agriculture, too, practices that leave the mycorrhizal (mycelial) network intact, are thought to aid the absorption of water and nutrients from the soil, and to improve the ability of plants to resist pathogens. Hence the practices of heavy and deep ploughing, which breaks-up the mycelial networks, have been called into question.

Not everything that is transmitted between plants is beneficial to an individual plant, since toxins may also be transported via mycelial networks. The influence of one plant to influence (usually restrict) the growth of another is termed allelopathy, and functions via chemical messengers, e.g. the production of juglone by walnut trees, which was found to reduce the weight of tomato seedlings by about one third. The whole system is integrated, holistic and complex, and a new area of research has emerged which aims to understand inter-plant communication at the molecular level. It appears that plants may use a form of "language", in which different molecules act as "words", although the precise nature of the dialogue has yet to be deciphered.

In a study of the action of a parasitic plant (dodder) with two host plants, Arabidopsis and tomatoes, it has been found that messenger RNA (mRNA) is transferred between the two species on a massive scale. Since RNA acts as a translator of information from the DNA of an organism, it is possible that the parasitic plant may be giving orders to its "victim", to weaken its defenses, BUT the host might also be delivering a masochistic response. The question arises of whether information can be similarly transmitted between other organisms within the soil food web, i.e. earthworms, bacteria, nematodes and other microbes, along with the plant roots, and their associated mycorrhizal fungi. These organisms are, collectively, the vanguard for the recycling of nutrients in the soil, which enhances the growth of plants.

To seek an understanding of the complex communications and interactions that take place between the soil food web organisms, at the molecular level, may be a worthy aim, since the soil contains perhaps one quarter of all the biodiversity on Earth. Until this is achieved, however, and in any case, it is clear that by supporting this hidden biodiversity below the ground, the more visible biodiversity above the ground is further buttressed. Practical actions to preserve and build the soil are paramount to a viable future, and their implementation should not be delayed.

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