The chemical repertoire of roots is astonishingly broad. More than 30,000 distinct volatile substances have been described across the plant kingdom, spanning alkanes, alcohols, aldehydes, esters, terpenoids, and sulfur- and nitrogen-containing compounds. Roots manufacture these molecules through several well-characterized biochemical routes. Terpenoids arise from isopentenyl diphosphate and dimethylallyl diphosphate via the mevalonate and methylerythritol phosphate pathways, with terpene synthases and cytochrome P450 enzymes shaping the final products. Arabidopsis roots, for instance, produce the monoterpene 1,8-cineole through a dedicated root-expressed terpene synthase gene, while maize roots generate the sesquiterpene (E)-beta-caryophyllene via the TPS23 gene. Fatty acid derivatives follow the lipoxygenase pathway, in which linoleic and alpha-linolenic acids are oxidized and cleaved into aldehydes that can be reduced to alcohols, esterified, or oxidized further. Glucosinolate metabolism supplies volatile isothiocyanates and nitriles upon tissue disruption, and roots also emit inorganic gases such as carbon dioxide and nitric oxide alongside volatile phytohormones like ethylene, methyl salicylate, and methyl jasmonate.

What makes these emissions ecologically meaningful is their specificity. Carbon dioxide from root respiration acts as a universal beacon for soil-dwelling herbivores: larvae of the western corn rootworm orient along minute gradients of CO2 toward respiring roots. But CO2 alone cannot tell a foraging insect which plant it is approaching. Specific volatile blends fill that gap. Cockchafer larvae detect complex mixtures through dedicated sensory appendages, wireworms are strongly drawn to simple aldehydes such as hexanal, and root-knot nematodes distinguish tomato roots from spinach by their methyl salicylate emissions. Responses are also concentration-dependent: larvae of the click beetle Agriotes sordidus are attracted to low doses of hexanal and (E)-2-hexenal yet avoid the same compounds at higher concentrations, suggesting that volatile profiles help insects judge host identity and quality.

Roots deploy their volatile chemistry defensively as well. Isothiocyanates released from glucosinolates in Brassicaceae species contribute substantially to belowground defense against soil herbivores, and volatiles from Asarum sieboldii roots show lethal activity against the root-knot nematode Meloidogyne incognita, reducing galls and egg masses on tomato roots. Antimicrobial effects have also been documented: volatiles from Fusarium-infected barley roots suppressed the growth of the pathogen Cochliobolus sativus by 13 to 17 percent in coculture assays, and compounds such as octanol, nonanal, 1,8-cineole, and benzothiazole inhibit multiple soil-borne pathogens in vitro. The review’s authors caution, however, that much of this evidence derives from essential oils or synthetic standards rather than the natural blends emitted by living roots, leaving a gap between laboratory bioactivity and ecological reality.

Perhaps the most celebrated example of root volatile signaling is indirect defense. When western corn rootworm larvae attack maize roots, the damaged tissue releases (E)-beta-caryophyllene into the soil, which does not repel the pest but instead recruits entomopathogenic nematodes that infect and kill rootworm larvae. Maize varieties that lack this sesquiterpene fail to recruit these allies and suffer greater damage. Similar recruitment strategies appear in sugarcane, whose spittlebug-damaged roots attract the nematodes Heterorhabditis indica and Steinernema carpocapsae, and in blueberry, whose weevil-attacked roots release methyl salicylate and other compounds that lure Steinernema australe. Notably, natural blends can outperform single compounds: predatory beetles were more strongly attracted to the full volatile mixture from oilseed rape roots than to dimethyl disulfide alone, hinting that complex mixtures carry richer ecological information.

Root volatiles also orchestrate relationships with beneficial microbes. Roots of the sedge Carex arenaria infected with Fusarium culmorum emit volatiles that recruit antifungal Burkholderia and Paenibacillus species, an attraction that intensifies under nutrient limitation because compounds like isoprene and alpha-pinene can serve as carbon substrates for soil bacteria. Tomato root volatiles promote the pre-symbiotic sporulation of arbuscular mycorrhizal fungi, and healthy tomato roots attract beneficial bacteria even without pathogen pressure. Ethylene emitted by roots can reshape entire soil microbial networks in ways that enhance seed production, while methyl jasmonate rapidly triggers biofilm formation in host-beneficial soil microbiomes, delivering distance-dependent growth benefits. These interactions weave plants, microbes, and pathogens into dynamic networks that simultaneously suppress disease and promote growth.

Plants also eavesdrop on one another through root-borne chemicals. Volatile terpenes from the invasive shrub Chrysanthemoides monilifera inhibit seedling growth of the native sedge Isolepis nodosa, a classic allelopathic suppression, and similar effects have been documented for sagebrush and Aleppo pine. Yet root volatiles can be generous as well as hostile: beta-caryophyllene released from spotted knapweed roots increased germination and biomass in neighboring plants, with stronger effects on heterospecific than conspecific neighbors, suggesting a role in species recognition. Dandelion plants exposed to heterospecific root volatiles allocated more biomass belowground and altered their root metabolite profiles, indicating that receivers can identify who is next door and adjust their competitive strategy accordingly. Stress information travels through this channel too: jasmonic acid-treated Norway spruce released enhanced root volatile signals that primed early herbivore defenses in neighboring beech saplings.

Signaling extends across the whole plant. Belowground herbivory can trigger volatile emission from undamaged roots and even shoots, while aboveground feeding can change what roots release. Leafminer attack on foliage accelerated the development of soil-dwelling conspecific pupae via changes in root volatiles, and dual attack by wireworms and foliar caterpillars produced the strongest attraction of wireworms to maize roots, revealing integrated whole-plant defense strategies. But these chemical investments carry costs. Chinese fir seedlings reduced root volatile release under phosphorus scarcity to preserve growth, and maize engineered to constitutively emit (E)-beta-caryophyllene and alpha-humulene suffered impaired growth and yield, illustrating the classical trade-off between defense and growth. In milkweed, constitutive root volatile levels were negatively correlated with inducibility after herbivore attack, evidence that plants cannot maximize both strategies at once.

Evolution has left its fingerprints on this chemistry. Wild and domesticated tomatoes emit different defense-related volatiles after insect attack, and cultivated peppers diverge from wild relatives in their root volatile profiles, showing that artificial selection has reshaped belowground signaling. Soil organisms, in turn, exploit these cues: tobacco rattle virus infection induces root volatiles in Nicotiana benthamiana that attract nematode vectors, facilitating viral spread, and the fungus Metarhizium robertsii metabolizes a root-derived compound into a product that lures host insects into the rhizosphere. These reciprocal adaptations suggest coevolutionary dynamics, though direct long-term evidence remains scarce.

The translational promise is considerable. Field trials have shown that restoring (E)-beta-caryophyllene signaling in non-emitting maize varieties reduces root damage, that dimethyl disulfide attracts multiple natural enemy species under field conditions, and that intercropping with Chinese chive suppresses Panama disease in banana through root volatiles. Brassica nigra plants shift their root volatile profiles within one to six hours of root fly infestation, opening the door to volatile-based crop damage monitoring. Controlled-release nanocarriers may stabilize volatile-based products in variable soils, and CRISPR-based editing of terpene synthase genes offers targeted crop improvement. Yet the review emphasizes that success depends on resolving fundamental unknowns: how volatiles move through soil pore networks, where root-derived signals end and microbial ones begin, how soil biota perceive these molecules, and how engineered emissions ripple through multitrophic networks. As the authors conclude, root volatiles should be viewed as dynamic traits integrating aboveground and belowground cues, and only systematic study of their biosynthesis, diffusion, perception, and evolution will unlock their full potential for sustainable agriculture.

Subject of Research: Ecological functions and agricultural applications of root-emitted volatile organic compounds in belowground chemical communication

Article Title: Underground Signals: The Ecological Power of Root Volatiles

Article References: Underground Signals: The Ecological Power of Root Volatiles. (n.d.). https://doi.org/10.1002/advs.202522148

Image Credits: AI Generated

DOI: 10.1002/advs.202522148

Keywords: root volatiles, plant chemical ecology, rhizosphere, entomopathogenic nematodes, terpenoids, plant-plant communication, soil microbes, biological control, allelopathy, plant defense, volatile organic compounds, sustainable agriculture