Volatile organic compounds (VOCs) emitted by plant leaves can influence the physiology of neighbouring plants. In contrast to leaf VOCs, little is known about the role of root VOCs in plant–plant interactions. Here, we characterize constitutive root VOC emissions of the spotted knapweed (Centaurea stoebe) and explore the impact of these VOCs on the germination and growth of different sympatric plant species. We show that C. stoebe roots emit high amounts of sesquiterpenes, with estimated release rates of (E)‐β‐caryophyllene above 3 μg g−1 dw hr−1. Sesquiterpene emissions show little variation between different C. stoebe populations but vary substantially between different Centaurea species. Through root transcriptome sequencing, we identify six root‐expressed sesquiterpene synthases (TPSs). Two root‐specific TPSs, CsTPS4 and CsTPS5, are sufficient to produce the full blend of emitted root sesquiterpenes. VOC‐exposure experiments demonstrate that C. stoebe root VOCs have neutral to positive effects on the germination and growth of different sympatric neighbours. Thus, constitutive root sesquiterpenes produced by two C. stoebe TPSs are associated with facilitation of sympatric neighbouring plants. The release of root VOCs may thus influence plant community structure in nature.

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighbouring plants. However, little is known about the effects of root VOCs on plant–herbivore interactions of neighbouring plants. The spotted knapweed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on the primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant‐mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C.stoebe root VOCs does not affect the accumulation of defensive secondary metabolites but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)‐β‐caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant–herbivore interactions of neighbouring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.

Plant-soil feedbacks refer to effects on plants that are mediated by soil modifications caused by the previous plant generation. Maize conditions the surrounding soil by secretion of root exudates including benzoxazinoids (BXs), a class of bioactive secondary metabolites. Previous work found that a BX-conditioned soil microbiota enhances insect resistance while reducing biomass in the next generation of maize plants. Whether these BX-mediated and microbially driven feedbacks are conserved across different soils and response species is unknown. We found the BX-feedbacks on maize growth and insect resistance conserved between two arable soils, but absent in a more fertile grassland soil, suggesting a soil-type dependence of BX feedbacks. We demonstrated that wheat also responded to BX-feedbacks. While the negative growth response to BX-conditioning was conserved in both cereals, insect resistance showed opposite patterns, with an increase in maize and a decrease in wheat. Wheat pathogen resistance was not affected. Finally and consistent with maize, we found the BX-feedbacks to be cultivar-specific. Taken together, BXfeedbacks affected cereal growth and resistance in a soil and genotype-dependent manner. Cultivar-specificity of BX-feedbacks is a key finding, as it hides the potential to optimize crops that avoid negative plant-soil feedbacks in rotations.

Plant secondary metabolites that are released into the rhizosphere alter biotic and abiotic soil properties, which in turn affect the performance of other plants. How such plant-soil feedbacks affect agricultural productivity and food quality in crop rotations is unknown. Here, we assessed the impact of maize benzoxazinoids on the performance, yield and food quality of three winter wheat varieties in a two-year field experiment. Following maize cultivation, we detected benzoxazinoid-dependent chemical and microbial fingerprints in the soil. The chemical fingerprint was still visible during wheat growth, while the microbial fingerprint was no longer detected. Benzoxazinoid soil conditioning by wild-type maize led to increased wheat emergence, tillering, growth and biomass compared to soil conditioning by bx1 mutant plants. Weed cover remained unaffected, while insect damage decreased in a subset of varieties. Wheat yield was increased by over 4% without reduction in grain quality across variety. This improvement was directly associated with increased germination and tillering. Taken together, our experiments demonstrate that plant secondary metabolites can increase yield via plant-soil feedbacks under agronomically realistic conditions. If this phenomenon holds across different soils and environmental conditions, optimizing plant root exudation could be a powerful, genetically tractable strategy to enhance crop yields without additional inputs.

Plant secondary metabolites that are released into the rhizosphere alter biotic and abiotic soil properties, which in turn affect the performance of other plants. How this type of plant-soil feedback affects agricultural productivity and food quality in the field in the context of crop rotations is unknown. Here, we assessed the performance, yield and food quality of three winter wheat varieties growing in field plots whose soils had been conditioned by either wild type or benzoxazinoid-deficient bx1 maize mutant plants. Following maize cultivation, we detected benzoxazinoid-dependent chemical and microbial fingerprints in the soil. The benzoxazinoid fingerprint was still visible during wheat growth, but the microbial fingerprint was no longer detected. Wheat emergence, tillering, growth, and biomass increased in wild type conditioned soils compared to bx1 mutant conditioned soils. Weed cover was similar between soil conditioning treatments, but insect herbivore abundance decreased in benzoxazinoid-conditioned soils. Wheat yield was increased by over 4% without a reduction in grain quality in benzoxazinoid-conditioned soils. This improvement was directly associated with increased germination and tillering. Taken together, our experiments provide evidence that soil conditioning by plant secondary metabolite producing plants can increase yield via plant-soil feedbacks under agronomically realistic conditions. If this phenomenon holds true across different soils and environments, optimizing root exudation chemistry could be a powerful, genetically tractable strategy to enhance crop yields without additional inputs.