The relentless upwards growth of plants is driven by the need for light. A plant’s mission is simple: outgrow the nearby competitors and win the light, sometimes quite literally leaving challengers in the dark. Water is important too of course, so an extensive root network is required, threading through soil in order to secure sufficient resources. Soil is much more than a source of water and nutrients, however. It is teeming with life: insects, bacteria and fungi all thrive here. Some of this life is beneficial to plants, while some is lethal. The underworld is a battleground, as highlighted by a recent review exploring relationships in the soil beneath our feet.

This subterranean war is no clean competition: chemical warfare changes the soil environment dramatically. So much so that our soil is no longer called soil, this underground warzone is named the rhizosphere. This is the thin layer of soil that is influenced by the root excretions of plants, within which plants emit various compounds that either aid their allies or inhibit their enemies.

In some cases, if a plant is grazed upon by an insect herbivore it is able to produce certain chemicals that can ‘warn’ neighbouring plants that they are under attack. In response to this message, the neighbouring plants are able to bolster their own chemical defences in order to prevent damage. Even more impressively, some plants
are able to respond to this ‘warning’ by excreting compounds that attract predators of the insect that is gorging on them, such in the case of spider mites and lima beans. This is effectively a call for help that is communicated between members of two different biological kingdoms – animals and plants.

Whilst the above is an example of a defensive use of root-emitted chemicals, many other chemical excretions have been seen to be offensive. For example, the release of certain chemical compounds by one species can prevent germination and growth in another. Thus, the plant is able to prevent the presence of a nearby competitor at its earliest life stage. This inhibition of the growth of another species is known as allelopathy, and has been widely documented across the plant kingdom.

A final use of root-excreted chemical compounds is the recruitment of bacteria to the plant’s cause. These bacteria colonise the immediate area around the roots and often the outer root cells. Once colonised, these bacteria are useful to the plant through transforming nitrogen from the soil, an element that is crucial to the plants, and the production of other compounds and nutrients.

Below-ground ecology has seen a boom in research recently as scientists discover the myriad relationships and interactions occurring in the darkness underfoot. The underground war in the rhizosphere is a complex series of biological interactions that help to shape the terrestrial ecosystem around us, and, for now, much of it remains a mystery waiting to be uncovered.

Reference
Rasmann, S., & Turlings, T. C. (2016). Root signals that mediate mutualistic interactions in the rhizosphere. Current Opinion in Plant Biology, 32, 62-68. DOI 10.1016/j.pbi.2016.06.017
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