How Plants Communicate: VOCs, Mycorrhizal Networks & Signals

A Damaged Plant Can Warn Its Neighbors in Under 3 Minutes

When a caterpillar begins feeding on a sagebrush (Artemisia tridentata) leaf, the plant does something unexpected: it releases a blend of volatile organic compounds (VOCs) into the air. Neighboring sagebrush plants detect these compounds and upregulate production of defensive chemicals before any caterpillar reaches them — a warning system operating faster than many human communication networks. This phenomenon was documented in careful field experiments by ecologist Richard Karban (UC Davis) and colleagues, published in Ecology Letters in 2006 and refined in subsequent work. Plants lack brains, nerves, and mouths. Yet they communicate — through chemistry, fungal intermediaries, and possibly electrical impulses — in ways that affect the survival of entire plant communities.

Volatile Organic Compounds: Airborne Warnings

Plants produce hundreds of volatile organic compounds, many synthesized specifically in response to herbivore damage or pathogen attack. These volatiles fall into several chemical classes:

Karban's research on sagebrush is notable for methodological rigor: to separate airborne signaling from root-mediated signaling, he used potted plants with no soil contact and experimentally damaged one plant while monitoring neighbors in both air-connected and air-excluded conditions. Neighbors connected by air showed significantly higher resistance to grasshopper damage than air-isolated controls — confirming airborne VOC-mediated communication. The signal molecules activate the jasmonic acid (JA) pathway in receiving plants, triggering production of defensive compounds including protease inhibitors, alkaloids, and terpenes that reduce herbivore digestive efficiency.

Mycorrhizal Networks: The Wood Wide Web

Approximately 90% of plant species form symbiotic associations with mycorrhizal fungi — fungi whose hyphae colonize plant roots and extend through surrounding soil. Two main types exist: ectomycorrhizal fungi (form a sheath around root tips; associate with conifers, oaks, beeches) and arbuscular mycorrhizal fungi (penetrate root cells; associate with most herbaceous plants and tropical trees). In exchange for plant photosynthate (carbohydrates), the fungi dramatically expand the plant's effective absorptive surface area and provide phosphorus, nitrogen, water, and minerals from soil.

When multiple plants are colonized by the same fungal network, their root systems become interconnected — creating what Suzanne Simard (University of British Columbia) has called the "wood wide web." Simard's foundational 1997 Nature paper used carbon-13 and carbon-14 isotope tracing to show that carbon fixed by birch trees was transferred via the mycorrhizal network to Douglas fir trees growing in shade, and vice versa — a net transfer from the more productive tree to the resource-limited neighbor. This was the first direct experimental demonstration of plant-to-plant carbon transfer via mycorrhizal networks in a natural forest setting.

Subsequent research expanded the picture:

• Simard's 2010 research showed that Douglas fir "mother trees" — the largest, most fungally connected trees in a stand — transferred more carbon to seedlings than to unrelated neighbors, and preferentially to their own genetic offspring (detected via marker analysis)
• A 2016 study (Song et al., Journal of Ecology) found that mycorrhizal networks also transmitted chemical defense signals between tomato plants — a tomato damaged by aphids transmitted signals through the network that induced aphid resistance in connected undamaged plants
• Resource transfer through networks is bidirectional and dynamic: plants transfer carbon to the network during photosynthetic surplus and receive minerals when mineral availability is low

Below-Ground Chemical Signals

Plants release chemical signals into the soil that neighboring plant roots detect. Root exudates — a complex mixture of sugars, amino acids, organic acids, secondary metabolites, and signaling compounds — form a chemical environment (the rhizosphere) that differs dramatically from bulk soil. Root-to-root communication via exudates includes:

• Allelopathy — some plants release chemicals that inhibit germination or growth of neighboring competitors. Black walnut (Juglans nigra) exudes juglone, which suppresses many plants within its root zone. Some cereal crops (sorghum, rye) release benzoxazinoids that inhibit weed germination — a property being developed for low-herbicide agriculture.
• Strigolactones — signaling molecules released by plant roots under phosphorus deficiency that attract mycorrhizal fungi toward the root. Parasitic plants (Striga witchweed) "eavesdrop" on these signals to detect and target host roots, making strigolactones a double-edged signaling system.
• Nod factors — signal exchange between legume roots and Rhizobium bacteria that initiates nitrogen-fixing nodule formation; a highly specific chemical dialogue that determines which bacterial strains can colonize which plant species.

Electric Signals in Plants

Joachim Fromm (University of Hamburg) and colleagues have documented propagating electrical signals in plants since the 1990s. When a plant tissue is damaged — by wounding, temperature extremes, or touch — a change in membrane potential propagates through the vasculature at speeds of 0.5–40 mm/second (far slower than animal nerve impulses but in a fundamentally similar electrical pattern).

Two types of plant electrical signals are documented:

• Action potentials — all-or-nothing depolarization events that propagate at constant amplitude along phloem or xylem parenchyma. Triggered by cold, touch, or rapid changes in turgor. Documented in numerous species including peas, wheat, barley, maize, and the carnivorous Venus flytrap (Dionaea muscipula), where action potentials from trigger hair bending are integrated to control trap closure.
• Variation potentials (slow wave potentials) — longer-duration, graded electrical events triggered by wounding or hydraulic pressure changes in the xylem. Can travel the full length of a plant and trigger systemic gene expression changes, including jasmonic acid pathway activation that induces herbivore defenses system-wide.

A 2018 study in Science (Hedrich and colleagues) used calcium imaging to show that wounding one leaf of Arabidopsis thaliana triggered a calcium wave that traveled the entire plant within minutes — a signal propagated through the vasculature and triggering defensive gene expression in unwounded leaves. The precise relationship between these electrical signals and the better-studied VOC and hormonal (jasmonate, salicylate) communication systems remains an active area of investigation. Plants are not passive. They sense, signal, and respond to their environment through a complex multimodal communication system that challenges the assumption that communication requires a nervous system.