Plant Communication via VOCs: How Wounded Plants Warn Their Neighbors

Plants Emit Over 1,700 Different Volatile Compounds — Many as Distress Signals

When a caterpillar begins feeding on a corn plant, the plant responds within minutes by synthesizing and releasing a blend of volatile organic compounds (VOCs) that simultaneously repels the caterpillar, attracts parasitic wasps that prey on caterpillars, and chemically primes neighboring corn plants to upregulate their own defenses before they are attacked. This communication cascade happens without nervous systems, without electrical signals traveling through the air, and without any of the biological machinery we associate with directed communication — yet it is functionally specific, context-sensitive, and demonstrably effective at increasing the survival of both the emitting plant and its neighbors.

The idea that plants actively communicate was dismissed as anthropomorphism when first proposed in the early 1980s. Decades of biochemical research have made it one of the most fascinating fields in ecology.

What Are Herbivore-Induced Plant Volatiles?

Plants constitutively emit low levels of VOCs as byproducts of normal metabolism. Under herbivore attack, the profile and quantity change dramatically. Herbivore-Induced Plant Volatiles (HIPVs) are synthesized de novo in response to damage and differ from constitutive emissions in their chemical composition, temporal dynamics, and function. The most studied HIPV blends include green leaf volatiles (GLVs), terpenoids, and aromatic compounds.

The synthesis begins with damage recognition. Caterpillar saliva contains bioactive elicitors — fatty acid-amino acid conjugates (FACs) — that bind to plant cell receptors and trigger the jasmonic acid signaling cascade. Jasmonic acid (JA) is the plant hormone primarily responsible for coordinating defense responses, analogous in some functional ways to the role of adrenaline in vertebrate stress responses. JA signaling activates transcription of defense genes and biosynthetic pathways that produce both direct defenses (toxins, protease inhibitors) and indirect defenses (the volatiles that attract predators of herbivores).

Recruiting Allies: Tritrophic Interactions

One of the most remarkable functions of HIPVs is their role in tritrophic interactions — recruiting the natural enemies of herbivores to do the plant's defense work. When corn plants are attacked by fall armyworm (Spodoptera frugiperda) caterpillars, they emit a specific blend that attracts Cotesia marginiventris, a parasitic wasp that lays eggs inside caterpillars. The wasp larvae consume the caterpillar from the inside. The plant, in effect, sends a chemical distress call that attracts a predator specialized for its attacker.

This relationship is specific: the volatile blend produced in response to caterpillar feeding differs from the blend produced in response to aphid feeding, and different parasitoids respond to these different blends. The plant's chemical signal is not a generic alarm but a contextually appropriate call for specific allies. Research on Lima beans and spider mites documented that plants release specific terpene blends that attract predatory mites — the natural enemy of spider mites — but not generalist predators that would also consume beneficial insects.

Neighbor Priming: Listening Plants

Neighboring plants can detect and respond to VOCs released by damaged individuals. This "priming" effect does not turn on full defenses in undamaged plants — that would waste metabolic resources on threats that haven't arrived. Instead, primed plants are prepared to respond faster and more strongly when they themselves are attacked. The molecular mechanism involves GLVs (particularly (Z)-3-hexenyl acetate) binding to receptors on neighboring plants and activating "priming" epigenetic marks — chromatin modifications that keep defense genes in a ready-to-activate state.

The ecological significance was demonstrated in field studies where plants surrounded by artificially damaged neighbors (emitting HIPVs) showed 50–80% higher survival rates when subsequently exposed to herbivore attack compared to plants in undamaged environments. This inter-plant communication provides genuine fitness benefits measurable under natural conditions.

Belowground Volatile Signaling

VOC communication is not limited to airborne signals. Root-to-root chemical communication via soil has been documented in multiple species. Maize roots attacked by western corn rootworm larvae release (E)-β-caryophyllene into the soil, attracting entomopathogenic nematodes (Heterorhabditis megidis) that parasitize the rootworm larvae. European maize varieties domesticated for high yield have largely lost this trait through selective breeding — a finding that has generated interest in restoring the mechanism for more sustainable pest management.

• Within-plant signaling: Volatile signals can also travel internally — damage at one leaf primes adjacent leaves on the same plant, faster than transport through the phloem would allow
• Species specificity: Plants distinguish between their own VOCs and those of other species; some only respond to volatiles from conspecifics
• Diurnal variation: HIPV emissions often peak during daylight hours when pollinating and parasitoid insects are active, not randomly throughout the day
• Agricultural applications: "Push-pull" cropping strategies deliberately use VOC-emitting companion plants to repel pests (push) and VOC-emitting border plants to attract parasitoids (pull), reducing pesticide use in East African cereal farming