Plant Communication: How Plants Talk to Each Other and to the World

For most of human history, plants were considered passive, silent, and essentially inert compared to the dynamic world of animal behaviour. The scientific revolution in our understanding of plant signalling over the past three decades has fundamentally changed this picture. Plants communicate with each other through complex chemical signals released into the air and soil. They signal to pollinators and seed dispersers through precisely calibrated combinations of colour, scent, and reward chemistry. They communicate distress to neighbouring plants when attacked by herbivores. They transmit electrical signals through their tissues in response to wounding and stress. And they are connected in extensive underground networks — the "wood wide web" — that may facilitate the sharing of resources between trees.

Herbivore Defence Signals

When a plant is attacked by a herbivore — a caterpillar chewing its leaves, an aphid piercing its stem — it releases a complex cocktail of volatile organic compounds (VOCs) into the air. These signals serve multiple functions. Some are direct defences: terpenes and other compounds that are toxic, repellent, or digestibility-reducing to the attacking herbivore. Others are indirect defences: specific blends of volatiles that attract the natural enemies of the herbivore — the parasitic wasps and predatory insects that kill caterpillars. And some appear to function as signals to neighbouring plants — both other individuals of the same species and individuals of different species — that "prime" them to mount faster and stronger defences when they are subsequently attacked.

The Wood Wide Web

Approximately 90% of all terrestrial plant species form mycorrhizal associations — partnerships with fungi in which the plant provides sugars to the fungus and the fungus extends the plant's effective root system by orders of magnitude, providing access to water and nutrients beyond the reach of the roots alone. In forest ecosystems, the mycorrhizal networks connecting different trees can form a continuous underground network — the "wood wide web" — through which carbon, water, and nutrients have been shown to move between individual trees. The extent to which this network constitutes genuine communication — rather than simply physical transfer of resources — remains scientifically contested, but the evidence that established trees preferentially allocate carbon to seedlings of the same species through mycorrhizal networks is substantial.

Volatile Signalling — The Language of Leaves

Plants communicate through the release of volatile organic compounds (VOCs) — airborne chemical signals that carry information about the plant's physiological state, attack status, and identity to both neighbouring plants and the insects that interact with them. When a plant is attacked by a herbivore, it releases a complex blend of herbivory-induced plant volatiles (HIPVs) that simultaneously attracts the natural enemies of the attacking herbivore (parasitoid wasps homing in on the blend from several metres away), warns neighbouring plants to upregulate their own defences pre-emptively, and may even communicate the identity of the attacking herbivore (plants appear to produce different volatile blends in response to different herbivore species, and these differences can be detected by parasitoid wasps that specialise on specific herbivores). This chemical communication occurs without any specialised sensory or nervous system — plants detect neighbouring plants' volatile emissions through receptor proteins in their own tissues that trigger hormonal defence responses when herbivory-associated volatiles are detected in the air.

Volatile Signalling — Plants Talking to Animals

Plants emit extraordinarily diverse mixtures of volatile organic compounds (VOCs) — small molecules that evaporate from leaf surfaces and carry information through the air. The most familiar plant volatiles are the scents of flowers — evolved to attract pollinators — and the aromas of ripe fruit — evolved to attract seed dispersers. But plant volatile signalling extends far beyond these obvious examples. When a plant is attacked by herbivores, it releases a specific blend of "herbivory-induced plant volatiles" (HIPVs) that function as distress signals attracting the natural enemies (parasitoid wasps, predatory insects) of the attacking herbivores — a strategy of indirect defence that recruits the herbivore's own predators to the plant's defence. These HIPVs can also prime neighbouring plants to upregulate their own defences before they are attacked — a phenomenon called "plant communication" that was initially dismissed as impossible but is now supported by hundreds of peer-reviewed studies. The ecological significance of these airborne signals extends to the shaping of entire arthropod communities: the volatile blends emitted by undamaged, herbivore-damaged, and pathogen-infected plants are so distinctive that specialist insects use them to locate their preferred host plant condition from distances of many metres.