Soil Switchboard: The Dark Web Underfoot

Do you know a place where you can eavesdrop on classified defense messages? Or a place where chemicals are traded but hidden from the watchful eye of the rest of the world?

No, I’m not referring to The Dark Web on the internet – made infamous by cyber-criminals and controversial whistle-blowers alike. I’m talking about something that occurs a bit more naturally, but essentially functions much like the internet and social networking. To find it, you have to look beneath the surface. Literally, because it’s in the soil.

Underground communication networks – Photo Credit: Mabel Amber via Pexels (CC0)

Networking

We often think of trees and plants as individual organisms – responsible for their own growth and existence. This isn’t quite accurate. Thanks to mycorrhiza (see explanation below), most trees in a forest find themselves biologically connected to one another by a fungal network. Fungi are not discerning in their connectivity. They are able to create connections between trees of the same species as well as those that are not. Some researchers refer to this biological network as the ‘Wood Wide Web’. It has a couple of very interesting functions. But first, what is mycorrhiza?

The animation illustrates a root tuber (yellow) colonized by an arbuscular mycorrhizal fungus (purple). Animation Credit: by Scivit (CC BY-SA 4.0) via Wikimedia Commons

Mycorrhiza

Mycorrhiza – the symbiotic association between plants and fungi.

When you see a mushroom in the forest, this is just the ‘tip of the iceberg’ of a much larger organism. The mushroom is the reproductive organ that enables fungi to disperse its spores above ground. Below ground, you will find that the mushroom is connected to a vast mass of mycelium, which is made up of a network of hyphae. Hyphae are branching tubular filaments that can act like a biological straw that can infiltrate the walls of plant cells. This creates a biological connection between plant and fungi; a place where they can exchange biochemicals. Fungi that are able to do this are called ‘arbuscular mycorrhizal fungi’ (AMF).

AMF are interesting and important because they can increase the nutrient and water harvesting capacity of plants. The relationship is mutual, fungi rely on the plants to feed them carbon (which it creates from photosynthesis). Fungi cannot photosynthesise, they are saprotrophic – meaning they process and feed on decayed organic matter. Through the process of decomposition they are able to release nutrients such as phosphorus, nitrogen and sulfur that would otherwise be bound up in the soil and unavailable to the plants.

Eaves Dropping

When plants suffer physical damage (such as being munched on by bugs, infected by bacteria or even when we prune them) they release volatile organic compounds (VOCs) into the air (e.g. ethylene, or methyl jasmonate). Once detected by other parts of the plant, or by plants that are downwind from the signal, a defense is mounted. For example, methyl jasmonate induces plants to produce antimicrobial chemicals or chemicals that inhibit insect digestion (e.g. protease inhibitors); protecting the leaves from further infection or from further insect damage respectively.

Outnumbered by caterpillars. Photo Credit: By Olgierd Rudak from Wrocław, Poland (CC BY 2.0) via Wikimedia Commons

Whilst VOCs are great because they can travel quickly through the air to warn other plants, the downside is: it’s not a very precise signal. The VOCs are at the whim of the wind.

This is where the AMF network comes in. Plants can also eavesdrop on each other’s defense responses through the wood wide web. In addition to gas compounds, some plants produce jasmonic acid in their cells when they are under threat. Because the AMF is directly connected to the plant cell, this signal is picked up and transmitted through the hyphae network to neighbouring plants. Although the speed of the signal is slow (it can take up to 6 hours to reach the closest neighbour); it is targeted and guaranteed to reach trees that are linked into the wood wide web.

The above and below ground methods of defense signaling seem to complement each other nicely in their strengths and weaknesses.

Trading Chemicals

However, the wood wide web is not only used for good. The word allelopathy is derived from Greek words meaning ‘mutual’ and ‘harm’. It describes the process by which one plant produces and releases biochemicals into its surrounding environment to hinder the growth of other plants. The American black walnut tree (Juglans nigra) releases jugalone into the soil through its roots. Jugalone is toxic to many types of plants including common food crops such as corn, potato and tomatoes. In soil that has an AMF network, you can find up to four times more jugalone in the root-zone of neighbouring plants, than in soil that has no AMF network.The wood wide web can therefore enhance and facilitate the transport of toxic chemicals. This increased delivery of jugalone through the wood wide web can stunt the growth of neighbouring plants by up to 36%.

Ear to the Ground

Not unlike our own internet – the wood wide web is capable of transmitting messages that can help, as well as ones that can harm. Scientists are still trying to understand the biological processes at play and the extent to which this connectivity impacts on the overall health of trees and the whole forest as an ecosystem. So, keep your ear to the ground on this topic – no doubt there will be more interesting discoveries made in the next few years.