Constitutive and induced subterranean plant volatiles attract both entomopathogenic and plant parasitic nematodes

The sky is not the limit: Successful foliar application of Steinernema spp. entomopathogenic nematodes to control Lepidopteran caterpillars

Entomopathogenic nematodes (EPNs) are ubiquitous soil-thriving organisms that use chemical cues to seek and infect soil-dwelling arthropods, yielding various levels of biological control. Going beyond soil application, scientists and practitioners started exploring the option of applying EPNs onto the foliage of crops in attempts to manage leaf-dwelling insect pests as well. Despite some success, particularly with protective formulations, it remains uncertain whether EPNs could indeed survive the phyllospheric environment, and successfully control foliar insect pests. In this context, we tested the potential of commercially produced Steinernema feltiae and S. carpocapsae, two of the most commonly used EPNs in the field of biological control, in controlling Lepidopteran foliar pests of economic importance, i.e. Tuta absoluta and Spodoptera spp. caterpillars as models. We first tested the survival and efficacy of both EPN species against the Lepidopteran caterpillars when applied onto tomato, sweet pepper and lettuce leaves, under controlled conditions and in commercial greenhouse conditions, respectively. Subsequently, we explored the behavioural responses of the EPNs to environmental cues typically encountered in the phyllosphere, and analysed plant volatile organic compounds (VOCs). Our results show that both S. feltiae and S. carpocapsae successfully survived and infected the foliar caterpillars, reaching similar level of control to a standard chemical pesticide in commercial practices. Remarkably, both EPN species survived and remained effective up to four days in the phyllosphere, and needed only a few hours to successfully penetrate the caterpillars. Interestingly, S. feltiae was attracted to VOCs from tomato plants, and tended to prefer those from caterpillar-induced plants, suggesting that the nematodes may actively forage toward its host, although it has never been exposed to leaf-borne volatiles during its evolution. The present study shows the high potential of steinernematids in managing major foliar pests in greenhouses and in becoming a key player in foliar biological control. In particular, the discovery that EPNs use foliar VOCs to locate caterpillar hosts opens up new opportunities in terms of application techniques and affordable effective doses.

Plants recruit insecticidal bacteria to defend against herbivore attacks

Pest feeding affects the rhizobacteria community. The rhizomicrobiota activates salicylic acid and jasmonic acid signaling pathways to help plants deal with pest infestation. However, whether plants can recruit special pesticidal microorganisms to deal with attack from herbivores is unclear. A system composed of peanuts and first-instar larvae of Holotrichia parallela were used to analyze whether peanuts truly enrich the insecticidal bacteria after feeding by larvae, and whether inoculation of the enriched bacteria promotes the resistance of plants to herbivore. In this study, high-throughput sequencing of 16 S rRNA gene amplicons was used to demonstrate that infestation of the subterranean pest H. parallela quickly changed the rhizosphere bacterial community structure within 24 h, and the abundance of Enterobacteriaceae, especially Enterobacter, was manifestly enriched. Root feeding induced rhizobacteria to form a more complex co-occurrence network than the control. Rhizosphere bacteria were isolated, and 4 isolates with high toxicity against H. parallela larvae were obtained by random forest analysis. In a back-inoculation experiment using a split-root system, green fluorescent protein (GFP)-labeled Enterobacter sp. IPPBiotE33 was observed to be enriched in uneaten peanut roots. Additionally, supplementation with IPPBiotE33 alleviated the adverse effects of H. parallela on peanuts. Our findings indicated that herbivore infestation could induce plants to assemble bacteria with specific larvicidal activity to address threats.

Behavioral and molecular response of the insect parasitic nematode Steinernema carpocapsae to plant volatiles

Entomopathogenic nematodes (EPNs) use the chemical cues emitted by insects and insect-damaged plants to locate their hosts. Steinernema carpocapsae, a species of EPN, is an established biocontrol agent used against insect pests. Despite its promising potential, the molecular mechanisms underlying its ability to detect plant volatiles remain poorly understood. In this study, we investigated the response of S. carpocapsae infective juveniles (IJs) to 8 different plant volatiles. Among these, carvone was found to be the most attractive volatile compound. To understand the molecular basis of the response of IJs to carvone, we used RNA-Seq technology to identify gene expression changes in response to carvone treatment. Transcriptome analysis revealed 721 differentially expressed genes (DEGs) between carvone-treated and control groups, with 403 genes being significantly upregulated and 318 genes downregulated. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the responsive DEGs to carvone attraction were mainly involved in locomotion, localization, behavior, response to stimulus, and olfactory transduction. We also identified four upregulated genes of chemoreceptor and response to stimulus that were involved in the response of IJs to carvone attraction. Our results provide insights into the potential transcriptional mechanisms underlying the response of S. carpocapsae to carvone, which can be utilized to develop environmentally friendly strategies for attracting EPNs.

The Chemical Ecology of Plant Natural Products

Plants are excellent chemists with an impressive capability of biosynthesizing a large variety of natural products (also known as secondary or specialized metabolites) to resist various biotic and abiotic stresses. In this chapter, 989 plant natural products and their ecological functions in plant-herbivore, plant-microorganism, and plant-plant interactions are reviewed. These compounds include terpenoids, phenols, alkaloids, and other structural types. Terpenoids usually provide direct or indirect defense functions for plants, while phenolic compounds play important roles in regulating the interactions between plants and other organisms. Alkaloids are frequently toxic to herbivores and microorganisms, and can therefore also provide defense functions. The information presented should provide the basis for in-depth research of these plant natural products and their natural functions, and also for their further development and utilization.

Collection of Hairy Roots as a Basis for Fundamental and Applied Research

Due to population growth, instability of climatic conditions, and reduction of the areas of natural ecosystems, it becomes necessary to involve modern biotechnological approaches to obtain highly productive plant material. This statement applies both to the creation of plant varieties and the production of new pharmaceutical raw materials. Genetic transformation of valuable medicinal plants using Agrobacterium rhizogenes ensures the production of stable and rapidly growing hairy roots cultures that have a number of advantages compared with cell culture and, above all, can synthesize root-specific substances at the level of the roots of the intact plant. In this regard, special attention should be paid to the collection of hairy roots of the Institute of Plant Physiology RAS, Russian Academy of Sciences, the founder of which was Dr. Kuzovkina I.N. Currently, the collection contains 38 hairy roots lines of valuable medicinal and forage plants. The review discusses the prospects of creating a hairy roots collection as a basis for fundamental research and commercial purposes.

Leafminer attack accelerates the development of soil-dwelling conspecific pupae via plant-mediated changes in belowground volatiles

Herbivore population dynamics are strongly influenced by the interactions established through their shared host. Such plant-mediated interactions can occur between different herbivore species and different life developmental stages of the same herbivore. However, whether these interactions occur between leaf-feeding herbivores and their soil-dwelling pupae is unknown. We studied whether tomato (Solanum lycopersicum) leaf herbivory by the American serpentine leafminer Liriomyza trifolii affects the performance of conspecific pupae exposed to the soil headspace of the plant. To gain mechanistic insights, we performed insect bioassays with the jasmonate-deficient tomato mutant def-1 and its wild-type, along with phytohormones, gene expression and root volatiles analyses. Belowground volatiles accelerated leafminer metamorphosis when wild-type plants were attacked aboveground by conspecifics. The opposite pattern was observed for def-1 plants, in which aboveground herbivory slowed metamorphosis. Leafminer attack induced jasmonate and abscisic acid accumulation and modulated volatile production in tomato roots in a def-1-dependent manner. Our results demonstrate that aboveground herbivory triggers changes in root defence signalling and expression, which can directly or indirectly via changes in soil or microbial volatiles, alter pupal development time. This finding expands the repertoire of plant-herbivore interactions to herbivory-induced modulation of metamorphosis, with potential consequences for plant and herbivore community dynamics.

Identification of Repellents from Four Non-Host Asteraceae Plants for the Root Knot Nematode, Meloidogyne incognita

Olfactory cues guide plant parasitic nematodes (PPNs) to their host plants. We tested the hypothesis that non-host plant root volatiles repel PPNs. To achieve this, we compared the olfactory responses of infective juveniles (J2s) of the PPN Meloidogyne incognita to four non-host Asteraceae plants, namely, black-jack (Bidens pilosa), pyrethrum (Chrysanthemum cinerariifolium), marigold (Tagetes minuta), and sweet wormwood (Artemisia annua), traditionally used in sub-Saharan Africa for the management of PPNs. Chemical analysis by coupled gas chromatography-mass spectrometry (GC/MS) combined with random forest analysis, followed by behavioral assays, identified the repellents in the root volatiles of B. pilosa, T. minuta, and A. annua as (E)-β-farnesene and 1,8-cineole, whereas camphor was attractive. In contrast, random forest analysis predicted repellents for C. cinerariifolium and A. annua as β-patchoulene and isopropyl hexadecanoate. Our results suggested that terpenoids generally account for the repellency of non-host Asteraceae plants used in PPN management.

Attraction and toxicity: Ways volatile organic compounds released by Pochonia chlamydosporia affect Meloidogyne incognita

The production of volatile organic compounds (VOCs) acting against plant-parasitic nematodes has been characterized in different fungi; however, the role of VOCs emitted by Pochonia chlamydosporia in its trophic interaction with Meloidogyne incognita is still unknown. The aim of this study was to determine the effects of VOCs emitted by P. chlamydosporia strain Pc-10 on different stages (eggs, juveniles and female) of the M. incognita life cycle. Exposure of M. incognita eggs to VOCs released by Pc-10 resulted in a reduction up to 88 % in the nematode egg hatching, when compared to the control treatments. The VOCs emitted by Pc-10 also attracted M. incognita second-stage juveniles (J2). Through gas chromatography-mass spectrometry (GC-MS), three molecules were identified from the volatiles of the strain Pc-10, with 1,4-dimethoxybenzene being the major compound. In tests performed in vitro, 1,4-dimethoxybenzene at a concentration of 1050 μg mL-1 inhibited M. incognita egg hatching by up to 78.7 % compared to the control (0 μg mL-1) and attracted M. incognita J2 in all concentrations evaluated (1, 10, 100, 1000, and 10000 μg mL-1). The 1,4-dimethoxybenzene also showed fumigant and non-fumigant nematicidal activity against M. incognita. This compound presented lethal concentration for 50 % (LC50) of M. incognita J2 ranged from 132 to 136 μg mL-1. Fumigation with 1,4-dimethoxybenzene (100 mg) reduced egg hatching by up to 89 % and killed up to 86 % of M. incognita J2 compared to the control (0 μg mL-1). In vivo, the VOCs produced by Pc-10, 1,4-dimethoxybenzene, and the combination of both (Pc-10 + 1,4-dimethoxybenzene) attracted the M. incognita J2, compared to the respective controls. To the best of our knowledge, this is the first report on the attraction of M. incognita J2 and the toxicity to eggs and J2 by VOCs from P. chlamydosporia in which 1,4-dimethoxybenzene is the main toxin and attractant.

Beneficial worm allies warn plants of parasite attack below-ground and reduce above-ground herbivore preference and performance

Antagonistic interactions among different functional guilds of nematodes have been recognized for quite some time, but the underlying explanatory mechanisms are unclear. We investigated responses of tomato (Solanum lycopersicum) to two functional guilds of nematodes-plant parasite (Meloidogyne javanica) and entomopathogens (Heterorhabditis bacteriophora, Steinernema feltiae below-ground, and S. carpocapsae)-as well as a leaf mining insect (Tuta absoluta) above-ground. Our results indicate that entomopathogenic nematodes (EPNs): (1) reduced root knot nematode (RKN) infestation below-ground, (2) reduced herbivore (T. absoluta) host preference and performance above-ground, and (3) induced overlapping plant defence responses by rapidly activating polyphenol oxidase and guaiacol peroxidase activity in roots, but simultaneously suppressing this activity in above-ground tissues. Concurrently, we investigated potential plant signalling mechanisms underlying these interactions using transcriptome analyses. We found that both entomopathogens and plant parasites triggered immune responses in plant roots with shared gene expression. Secondary metabolite transcripts induced in response to the two nematode functional guilds were generally overlapping and showed an analogous profile of regulation. Likewise, we show that EPNs modulate plant defence against RKN invasion, in part, by suppressing active expression of antioxidant enzymes. Inoculations of roots with EPN triggered an immune response in tomato via upregulated phenylpropanoid metabolism and synthesis of protease inhibitors in plant tissues, which may explain decreased egg laying and developmental performance exhibited by herbivores on EPN-inoculated plants. Furthermore, changes induced in the volatile organic compound-related transcriptome indicated that M. javanica and/or S. carpocapsae inoculation of plants triggered both direct and indirect defences. Our results support the hypothesis that plants "mistake" subterranean EPNs for parasites, and these otherwise beneficial worms activate a battery of plant defences associated with systemic acquired resistance and/or induced systemic resistance with concomitant antagonistic effects on temporally co-occurring subterranean plant pathogenic nematodes and terrestrial herbivores.

Biochemistry of Terpenes and Recent Advances in Plant Protection

Biodiversity is adversely affected by the growing levels of synthetic chemicals released into the environment due to agricultural activities. This has been the driving force for embracing sustainable agriculture. Plant secondary metabolites offer promising alternatives for protecting plants against microbes, feeding herbivores, and weeds. Terpenes are the largest among PSMs and have been extensively studied for their potential as antimicrobial, insecticidal, and weed control agents. They also attract natural enemies of pests and beneficial insects, such as pollinators and dispersers. However, most of these research findings are shelved and fail to pass beyond the laboratory and greenhouse stages. This review provides an overview of terpenes, types, biosynthesis, and their roles in protecting plants against microbial pathogens, insect pests, and weeds to rekindle the debate on using terpenes for the development of environmentally friendly biopesticides and herbicides.

Social networking in crop plants: Wired and wireless cross-plant communications

The plant-associated microbial community (microbiome) has an important role in plant-plant communications. Plants decipher their complex habitat situations by sensing the environmental stimuli and molecular patterns and associated with microbes, herbivores and dangers. Perception of these cues generates inter/intracellular signals that induce modifications of plant metabolism and physiology. Signals can also be transferred between plants via different mechanisms, which we classify as wired- and wireless communications. Wired communications involve direct signal transfers between plants mediated by mycorrhizal hyphae and parasitic plant stems. Wireless communications involve plant volatile emissions and root exudates elicited by microbes/insects, which enable inter-plant signalling without physical contact. These producer-plant signals induce microbiome adaptation in receiver plants via facilitative or competitive mechanisms. Receiver plants eavesdrop to anticipate responses to improve fitness against stresses. An emerging body of information in plant-plant communication can be leveraged to improve integrated crop management under field conditions.

Chemical host-seeking cues of entomopathogenic nematodes

Entomopathogenic nematodes (EPNs) are obligate parasites that infect a broad range of insect species. Host-seeking is a crucial step for EPN infection success and survival. Yet, the identity and ecological functions of chemicals involved in host-seeking by EPNs remain overlooked. In this review, we report known CO₂, plant-derived and insect-derived cues shaping EPN host-seeking and recognition. Despite species-specific response to environmental cues, we highlight a hierarchical integration of chemicals by EPNs. We further emphasize the impact of EPN selection pressure, age, and experience on their responsiveness to infochemicals. Finally, we feature that EPN chemical ecology can translate into powerful sustainable strategies to control insect herbivores in agriculture.

Calcium sulfate and calcium carbonate as root-knot-nematode attractants and possible trap materials to protect crop plants

Root-knot nematodes (RKNs, genus Meloidogyne) are a class of plant parasites that seek out and infect the roots of many plant species. The identification of RKN attractants can be used in agriculture in conjunction with nematode-trapping technology to redirect RKN movements and eventually reduce their prevalence in the field. Here, we discovered that some commercial silica gels can attract nematodes. Silica gels that attract nematodes contain calcium sulfate. Calcium sulfate and calcium carbonate showed strong nematode attraction properties. When plant seeds were surrounded by calcium sulfate or calcium carbonate, nematodes were not attracted to the plant seeds. We propose that calcium sulfate and calcium carbonate can be used in agriculture as a novel material to trap RKN.

Desorption Temperature, Solid-Phase Microextraction (SPME), and Natural Product Analyses, how Low Can we Go?

Solid phase microextraction (SPME) has become a common technique for volatile sampling due to its ease of use and limited technical requirements. The solvent-free nature of SPME is also exceptionally attractive for gas chromatography mass spectrometry (GC/MS) analysis. To ensure efficient transfer of the sample to the GC, the manufacturer recommend injector desorption temperatures in the range of 200 to 320 °C. A high desorption temperature can, however, have unwanted effects on analyses of plant and insect produced semiochemicals. By investigating the quantitative and qualitative chromatographic responses at varying inlet temperatures for a component blend of seven plant produced volatile compounds, we found the thermally labile plant-nematode signaling compound, pregeijerene to degrade to geijerene at all tested temperatures within the recommended range (200, 240, and 280 °C), but that it did not break down with an inlet temperature below 200 °C (100 °C and 150 °C). Degradation was also detected for the sesquiterpene germacrene D, but only at the highest inlet temperature tested (280 °C). Surprisingly, an inlet temperature of 200 °C gave the highest sample recovery, measured as total peak area while an inlet temperature of 100 °C as well as 280 °C gave the lowest total area values. An increase in desorption time from 3 to 5 min. Resulted in a recovery at 100 °C close to that obtained at 200 °C. Peak broadening was minimal, and only observed at the 100 °C inlet temperature. Based on these results, we highly recommend that SPME users include desorption temperature as one variable when developing sampling procedures for novel biological systems to ensure that potentially present thermally labile compounds are not degraded.

Hopping on: Conspecific traveller density within a vehicle regulates parasitic hitchhiking between ephemeral microcosms

Hitchhikers (phoretic organisms) identify their vehicles using species-specific visual, chemical and vibrational cues. However, what factors influence their choice between vehicles of the same species has rarely been investigated. Hitchhikers must not only avoid overcrowded vehicles but may also need to travel with conspecifics to ensure mates at their destination. Hence, a trade-off between overcrowding and presence of conspecifics likely determines the choice of a vehicle especially when destination sites are distant, ephemeral and unique. Here, we investigate whether a trade-off between the presence of conspecifics versus overcrowding by conspecifics or heterospecifics on a vehicle affects hitchhiker choice. We also investigate the sensory modality responsible for this choice. We experimentally examine these questions using a phoretic nematode community (containing plant- and animal-parasitic taxa) obligately associated with a brood-site pollination mutualism. In this model system, nematodes co-travel with conspecifics and heterospecifics on pollinators as vehicles, between ephemeral plant brood sites to complete their developmental life cycle. In this system, hitchhiker overcrowding has proven negative impacts on vehicle and plant fitness. We expected nematodes to respond to conspecifics and heterospecific density on offered vehicles when making their choice. We found that animal-parasitic nematodes preferred vehicles containing some conspecifics within a certain density range. However, plant-parasitic nematodes preferentially boarded vehicles that were devoid of conspecifics or had few conspecifics. Plant parasites that preferred empty vehicles likely hitchhiked in pairs. Both nematode types employed volatile cues to discriminate between vehicles with different conspecific nematode densities. Our results suggest that vehicle overcrowding by conspecifics, most likely, guaranteed access to mates at the destination determined hitchhiker choice. Surprisingly, and contrary to our expectations, plant- and animal-parasitic nematodes did not respond to heterospecific crowding on vehicles and did not discriminate between vehicles with different heterospecific nematode densities. The reason for this lack of response to heterospecific presence is unknown. This study not only shows that phoretic organisms use different strategies while choosing a vehicle but also confirms that density-dependent effects can ensure the stability and persistence of phoretic interactions in a mutualism by balancing overcrowding against reproductive assurance.

Compounds Associated with Infection by the Root-Knot Nematode, Meloidogyne javanica, Influence the Ability of Infective Juveniles to Recognize Host Plants

Plant root chemistry is altered by the parasitism of plant-parasitic nematodes (PPN). Here, we investigated the influence of the infective stage juveniles (J2) of Meloidogyne javanica in inducing tomato (Solanum lycopersicum) root volatiles and chemotactic effect on conspecifics. In olfactometer assays, J2 avoided the roots of 2-day infected plants but preferred 7-day-infected tomato compared to healthy plants. Chemical analysis showed a 2-7-fold increase in the amounts of monoterpenes emitted from tomato roots infected with M. javanica relative to healthy roots. In further bioassays, the monoterpenes β-pinene, (+)-(2)-carene, α-phellandrene, and β-phellandrene differentially attracted (51-87%) J2 relative to control. Concurrent reduction and increase in the levels of methyl salicylate and (Z)-methyl dihydrojasmonate, respectively, in the root volatiles reduced J2 responses. These results demonstrate that the host plant can alter its root volatile composition to inhibit PPN attack. The observed plant-produced inhibition of J2 warrants further investigation as a potential management tool for growers.

Constituents Leached by Tomato Seeds Regulate the Behavior of Root-Knot Nematodes and Their Antifungal Effects against Seed-Borne Fungi

Germinating seeds can release diverse phytochemicals that repel, inhibit, or kill pathogens such as root-knot nematodes and seed-borne fungi. However, little is known about the composition of these phytochemicals and their effects on pathogens. In this study, we demonstrated that tomato seed exudates can attract the nematode Meloidogyne incognita using a dual-choice assay. Eighteen compounds were then isolated and identified from the exudates. Of these, esters (1-3), fatty acids (4-6), and phenolic acids (10-12) were proven to be the signaling molecules that facilitated the host-seeking process of second-stage juveniles (J2s) of nematodes, while alkaloids (17 and 18) disrupted J2s in locating their host. Furthermore, some phenolic acids and alkaloids showed antifungal effects against seed-borne fungi. In particular, ferulic acid (12) showed obvious activity against Aspergillus flavus (minimum inhibitory concentration (MIC), 32 μg/mL), while dihydrocapsaicin (17) showed noticeable activity against Fusarium oxysporum (MIC, 16 μg/mL). Overall, this study presents the first evidence that M. incognita can be attracted to or deterred by various compounds in seed exudates through identification of the structures of the compounds in the exudates and analysis of their effects on nematodes. Furthermore, some antifungal compounds were also found. The findings of this work suggest that seed exudates are new source for finding insights into the development of plant protective substances with nematocidal and antifungal effects.

Chemotactic Host-Finding Strategies of Plant Endoparasites and Endophytes

Plants interact with microorganisms in the environment during all stages of their development and in most of their organs. These interactions can be either beneficial or detrimental for the plant and may be transient or long-term. In extreme cases, microorganisms become endoparastic or endophytic and permanently reside within a plant, while the host plant undergoes developmental reprogramming and produces new tissues or organs as a response to the invasion. Events at the cellular and molecular level following infection have been extensively described, however the mechanisms of how these microorganisms locate their plant hosts via chemotaxis remain largely unknown. In this review, we summarize recent findings concerning the signalling molecules that regulate chemotaxis of endoparasitic/endophytic bacteria, fungi, and nematodes. In particular, we will focus on the molecules secreted by plants that are most likely to act as guidance cues for microorganisms. These compounds are found in a wide range of plant species and show a variety of secondary effects. Interestingly, these compounds show different attraction potencies depending on the species of the invading organism, suggesting that cues perceived in the soil may be more complex than anticipated. However, what the cognate receptors are for these attractants, as well as the mechanism of how these attractants influence these organisms, remain important outstanding questions. Host-targeting marks the first step of plant-microorganism interactions, therefore understanding the signalling molecules involved in this step plays a key role in understanding these interactions as a whole.

Dihydroxyacetone of wheat root exudates serves as an attractant for Heterodera avenae

Heterodera avenae, as an obligate endoparasite, causes severe yield loss in wheat (Triticum aestivum). Investigation on the mechanisms how H. avenae perceives wheat roots is limited. Here, the attractiveness of root exudates from eight plant genotypes to H. avenae were evaluated on agar plates. Results showed that the attraction of H. avenae to the root exudates from the non-host Brachypodium distachyon variety Bd21-3 was the highest, approximately 50 infective second-stage juveniles (J2s) per plate, followed by that from three H. avenae-susceptible wheat varieties, Zhengmai9023, Yanmai84 and Xiangmai25, as well as the resistant one of Xinyuan958, whereas the lowest attractive activity was observed in the two H. avenae-resistant wheat varieties, Xianmai20 (approximately 12 J2s/plate) and Liangxing66 (approximately 11 J2s/plate). Then Bd21-3, Zhengmai9023 and Heng4399 were selected for further assays as their different attractiveness and resistance to H. avenae, and attractants for H. avenae in their root exudates were characterized to be heat-labile and low-molecular compounds (LM) by behavioral bioassay. Based on these properties of the attractants, a principle of identifying attractants for H. avenae was set up. Then LM of six root exudates from the three plants with and without heating were separated and analyzed by HPLC-MS. Finally, dihydroxyacetone (DHA), methylprednisolone succinate, embelin and diethylpropionin in the root exudates were identified to be putative attractants for H. avenae according to the principle, and the attraction of DHA to H. avenae was validated by behavioral bioassay on agar. Our study enhances the recognition to the orientation mechanism of H. avenae towards wheat roots.

Genomic characterization of the complete terpene synthase gene family from Cannabis sativa

Terpenes are responsible for most or all of the odor and flavor properties of Cannabis sativa, and may also impact effects users experience either directly or indirectly. We report the diversity of terpene profiles across samples bound for the Washington dispensary market. The remarkable degree of variation in terpene profiles ultimately results from action of a family of terpene synthase genes, only some of which have been described. Using a recently available genome assembly we describe 55 terpene synthases with genomic context, and tissue specific expression. The family is quite diverse from a protein similarity perspective, and subsets of the family are expressed in all tissues in the plant, including a set of root specific monoterpene synthases that could well have agronomic importance. Ultimately understanding and breeding for specific terpene profiles will require a good understanding of the gene family that underlies it. We intend for this work to serve as a foundation for that.

Plant terpenes that mediate below-ground interactions: prospects for bioengineering terpenoids for plant protection

Plants are sessile organisms that have evolved various mechanisms to adapt to complex and changing environments. One important feature of plant adaption is the production of specialised metabolites. Terpenes are the largest class of specialised metabolites, with over 80 000 structures reported so far, and they have important ecological functions in plant adaptation. Here, we review the current knowledge on plant terpenes that mediate below-ground interactions between plants and other organisms, including microbes, herbivores and other plants. The discovery, functions and biosynthesis of these terpenes are discussed, and prospects for bioengineering terpenoids for plant protection are considered. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Role and exploitation of underground chemical signaling in plants

The soil ecosystem is composed of a mixture of living organisms and non-living matter as well as the complex interactions between them. In the past 100 years or so, agricultural soil ecosystems have been strongly affected by agricultural practices such as tillage and the use of pesticides and fertilizers, which strongly affect soil nutrient composition, pH and biodiversity. In modern pest management, however, the focus is gradually shifting from crop production through agricultural practices to soil ecosystem protection. In this review we discuss how the underground chemical signals secreted by plant roots play a role in keeping the soil ecosystem in balance and how they affect plant fitness by shaping the root biome, increasing nutrient availability, promoting symbiosis, and attracting beneficial organisms and repelling harmful ones, including other plants. We review a number of fascinating cases, such as signaling molecules with dual, positive and negative, functions and bacterial quorum sensing mimicking molecules. Finally, examples of how these compounds can be exploited in modern pest management are reviewed, and the prospects for future developments discussed. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Getting confused: learning reduces parasitoid foraging efficiency in some environments with non-host-infested plants

Foraging animals face the difficult task to find resources in complex environments that contain conflicting information. The presence of a non-suitable resource that provides attractive cues can be expected to confuse foraging animals and to reduce their foraging efficiency. We used the parasitoid Cotesia glomerata to study the effect of non-host-infested plants and associative learning on parasitoid foraging efficiency. Inexperienced C. glomerata did not prefer volatiles emitted from host (Pieris brassicae)-infested plants over volatiles from non-host (Mamestra brassicae)-infested plants and parasitoids that had to pass non-host-infested plants needed eight times longer to reach the host-infested plant compared to parasitoids that had to pass undamaged plants. Contrary to our expectations, oviposition experience on a host-infested leaf decreased foraging efficiency due to more frequent visits of non-host-infested plants. Oviposition experience did not only increase the responsiveness of C. glomerata to the host-infested plants, but also the attraction towards herbivore-induced plant volatiles in general. Experience with non-host-infested leaves on the contrary resulted in a reduced attraction towards non-host-infested plants, but did not increase foraging efficiency. Our study shows that HIPVs emitted by non-host-infested plants can confuse foraging parasitoids and reduce their foraging efficiency when non-host-infested plants are abundant. Our results further suggest that the effect of experience on foraging efficiency in the presence of non-host-infested plants depends on the similarity between the rewarding and the non-rewarding cue as well as on the completeness of information that parasitoids have acquired about the rewarding and non-rewarding cues.

Study on host-seeking behavior and chemotaxis of entomopathogenic nematodes using Pluronic F-127 gel

Pluronic F-127 gel (PF127) has proven to be a powerful medium in which to study host-finding behavior and chemotaxis for plant-parasitic nematodes. Pluronic gel can also be used to study host-habitat seeking behavior of entomopathogenic nematodes (EPN), which are natural enemies of root-feeding insect pests. In this study, PF127 was used to study tritrophic interactions among EPNs, host-habitat roots and insects. We also tested whether EPN aggregated to acetic acid (pH gradient) which mimicked the conditions near the roots. The chive root gnat Bradysia odoriphaga alone significantly attracted more nematodes than chive roots alone or the combination of roots plus insects. The attractiveness of B. odoriphaga differed (3.7-15.4%) among all tested species/strains of EPNs. In addition, we found that Heterorhabditis spp. and Steinernema spp. infective juveniles responded to pH gradients formed by acetic acid in Pluronic gel. The preferred pH ranges for strains of H. bacteriophora and H. megidis were from 4.32-5.04, and from 5.37-6.92 for Steinernema species, indicating that Heterorhabditis spp. prefer low pH conditions than Steinernema species. A narrow pH gradient between 6.84 and 7.05 was detected around chive root tips in which EPN was attracted. These results suggest that Pluronic gel can be broadly used for the study of host or host-habitat seeking behaviors and chemotaxis of nematodes.

Temperature-dependent behaviors of parasitic helminths

Parasitic helminth infections are the most common source of neglected tropical disease among impoverished global communities. Many helminths infect their hosts via an active, sensory-driven process in which environmentally motile infective larvae position themselves near potential hosts. For these helminths, host seeking and host invasion can be divided into several discrete behaviors that are regulated by both host-emitted and environmental sensory cues, including heat. Thermosensation is a critical sensory modality for helminths that infect warm-blooded hosts, driving multiple behaviors necessary for host seeking and host invasion. Furthermore, thermosensory cues influence the host-seeking behaviors of both helminths that parasitize endothermic hosts and helminths that parasitize insect hosts. Here, we discuss the role of thermosensation in guiding the host-seeking and host-infection behaviors of a diverse group of helminths, including mammalian-parasitic nematodes, entomopathogenic nematodes, and schistosomes. We also discuss the neural circuitry and molecular pathways that underlie thermosensory responses in these species.

Microbe-induced plant volatiles

Plants emit a plethora of volatile organic compounds in response to biotic and abiotic stresses. These compounds act as infochemicals for ecological communication in the phytobiome. This study reviews the role of microbe-induced plant volatiles (MIPVs) in plant-microbe interactions. MIPVs are affected by the taxonomic position of the microbe, the identity of the plant and the type of interaction. Plants also emit exclusive blends of volatiles in response to nonhost and host interactions, as well as to beneficial microbes and necrotrophic/biotrophic pathogens. These MIPVs directly inhibit pathogen growth and indirectly promote resistance/susceptibility to subsequent plant pathogen attack. Viruses and phloem-limiting bacteria modify plant volatiles to attract insect vectors. Susceptible plants can respond to MIPVs from resistant plants and become resistant. Recent advances in our understanding of the molecular mechanisms of MIPV synthesis in plants and how plant pathogen effectors manipulate their biosynthesis are discussed. This knowledge will help broaden our understanding of plant-microbe interactions and should facilitate the development of new emerging techniques for sustainable plant disease management.

Elicitation of Differential Responses in the Root-Knot Nematode Meloidogyne incognita to Tomato Root Exudate Cytokinin, Flavonoids, and Alkaloids

Root exudates of plants mediate interactions with a variety of organisms in the rhizosphere, including root-knot nematodes (RKNs, Meloidogyne spp.) We investigated the responses of the motile stage second-stage juveniles (J2s) of Meloidogyne incognita to non-volatile components identified in the root exudate of tomato. Using stylet thrusting, chemotaxis assays, and chemical analysis, we identified specific metabolites in the root exudate that attract and repel J2s. Liquid chromatography quadrupole time-of-flight mass spectrometry analysis of bioactive fractions obtained from the root exudate revealed a high diversity of compounds, of which five were identified as the phytohormone zeatin (cytokinin), the flavonoids quercetin and luteolin, and alkaloids solasodine and tomatidine. In stylet thrusting and chemotaxis assays, the five compounds elicited concentration-dependent responses in J2s relative to 2% dimethyl sulfoxide (negative control) and methyl salicylate (positive control). These results indicate that J2 herbivory is influenced by root exudate chemistry and concentrations of specific compounds, which may have potential applications in RKN management.

Risk taking of educated nematodes

Nematode parasites rely on successful host infection to perpetuate their species. Infection by individual nematode parasites can be risky, however; any one individual could be killed by the host's immune response. Here we use a model system to show that environmental cues and parasite past experience can be used by entomopathogenic nematodes to reduce individual risk of infection. Past parasite experience can more than double the infective virulence (number of host invaders) of a given cohort of entomopathogenic nematode parasites. This plasticity in individual parasite risk-taking and associated infection can be used to manage infection of parasitic nematodes: enhancing biological control with entomopathogenic nematodes and developing behavioral and chemical strategies to reduce infection by vertebrate and plant parasitic nematodes.

Indirect Root Defenses Cause Induced Fitness Costs in Bt-Resistant Western Corn Rootworm

Plants genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis Berliner (Bt) have been extensively used to manage the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) in the United States. Evolution of WCR resistance to Bt toxins has forced the consideration of alternative pest management and improved insect resistance management. Entomopathogenic nematodes (EPNs), obligate insect parasites, are attracted toward volatile organic compounds (VOCs) emitted by maize roots after WCR herbivory. The production of VOCs of two types of Bt maize (MON88017 and MIR604) and their near-isolines was evaluated after induction with Bt-susceptible and resistant WCR. The attraction of EPNs toward the Bt hybrids was tested in the laboratory and the field. Bt hybrids emitted VOCs when induced by Bt-resistant insects whereas induction by Bt-susceptible WCR did not elicit a plant response. Survival of Bt-resistant WCR was lower on the hybrid attracting EPNs and similar to the survival of Bt-susceptible WCR without EPNs. This trade-off of Bt-resistance is defined here as an induced fitness cost, and offers a viable tool to management of Bt-resistant WCR.

Management of Cyst and Root Knot Nematodes: A Chemical Ecology Perspective

Plant parasitic nematode infection of crops can be highly detrimental to agricultural production. Since the discovery that plant roots release chemicals that attract the infective stage of plant parasitic nematodes some 80 years ago, significant progress in identifying the signaling molecules has occurred only relatively recently. Here, we review the literature on chemical ecological studies of two major plant parasitic nematode groups: root knot nematodes in the genus Meloidogyne and cyst nematodes in the genus Globodera because of the negative impact their parasitism has on farming systems in Africa. We then highlight perspectives for future directions for their management.

Belowground Microbiota and the Health of Tree Crops

Trees are crucial for sustaining life on our planet. Forests and land devoted to tree crops do not only supply essential edible products to humans and animals, but also additional goods such as paper or wood. They also prevent soil erosion, support microbial, animal, and plant biodiversity, play key roles in nutrient and water cycling processes, and mitigate the effects of climate change acting as carbon dioxide sinks. Hence, the health of forests and tree cropping systems is of particular significance. In particular, soil/rhizosphere/root-associated microbial communities (known as microbiota) are decisive to sustain the fitness, development, and productivity of trees. These benefits rely on processes aiming to enhance nutrient assimilation efficiency (plant growth promotion) and/or to protect against a number of (a)biotic constraints. Moreover, specific members of the microbial communities associated with perennial tree crops interact with soil invertebrate food webs, underpinning many density regulation mechanisms. This review discusses belowground microbiota interactions influencing the growth of tree crops. The study of tree-(micro)organism interactions taking place at the belowground level is crucial to understand how they contribute to processes like carbon sequestration, regulation of ecosystem functioning, and nutrient cycling. A comprehensive understanding of the relationship between roots and their associate microbiota can also facilitate the design of novel sustainable approaches for the benefit of these relevant agro-ecosystems. Here, we summarize the methodological approaches to unravel the composition and function of belowground microbiota, the factors influencing their interaction with tree crops, their benefits and harms, with a focus on representative examples of Biological Control Agents (BCA) used against relevant biotic constraints of tree crops. Finally, we add some concluding remarks and suggest future perspectives concerning the microbiota-assisted management strategies to sustain tree crops.

Resistance of Seven Cabbage Cultivars to Green Peach Aphid (Hemiptera: Aphididae)

The green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), is an important pest of many crops in the world and a vector of more than 100 plant viruses. It is a major pest of Brassica vegetables such as Chinese cabbage in northern China. Chemical control is extensively used to manage this aphid around the world; however, development of insecticide resistance has been a major obstacle facing growers. Host plant resistance in Chinese cabbage against M. persicae has not been reported yet. In this study, we investigated the resistance categories in seven Chinese cabbage cultivars against M. persicae. The resistance categories of these cultivars included antixenosis, antibiosis, and tolerance related to leaf color and wax content. The cultivar 'Yuanbao' had antibiotic and tolerance effects on the aphid. The rate of intrinsic increase (rm) of M. persicae was lower on Yuanbao compared with the other six cultivars. Yuanbao also had the highest antibiosis against the aphid. The aphid preferred 'Qingan 80', which had the highest wavelength (green) in leaf color. The highest wax content was found in Yuanbao, which had a significantly negative correlation with the preference of M. persicae. The cabbage cultivar Yuanbao was resistant to M. persicae and could be used in the development of integrated pest management (IPM) programs against the aphid in the field.

Aboveground herbivory induced jasmonates disproportionately reduce plant reproductive potential by facilitating root nematode infestation

Different plant feeders, including insects and parasitic nematodes, can influence each other by triggering systemic changes in their shared host plants. In most cases, however, the underlying mechanisms are unclear, and the consequences for plant fitness are not well understood. We studied the interaction between leaf feeding Manduca sexta caterpillars and root parasitic nematodes in Nicotiana attenuata. Simulated M. sexta attack increased the abundance of root parasitic nematodes in the field and facilitated Meloidogyne incognita reproduction in the glasshouse. Intact jasmonate biosynthesis was found to be required for both effects. Flower counts revealed that the jasmonate-dependent facilitation of nematode infestation following simulated leaf attack reduces the plant's reproductive potential to a greater degree than would be expected from the additive effects of the individual stresses. This work reveals that jasmonates mediate the interaction between a leaf herbivore and root parasitic nematodes and illustrates how plant-mediated interactions can alter plant's reproductive potential. The selection pressure resulting from the demonstrated fitness effects is likely to influence the evolution of plant defense traits in nature.

Root-colonizing bacteria enhance the levels of (E)-β-caryophyllene produced by maize roots in response to rootworm feeding

When larvae of rootworms feed on maize roots they induce the emission of the sesquiterpene (E)-β-caryophyllene (EβC). EβC is attractive to entomopathogenic nematodes, which parasitize and rapidly kill the larvae, thereby protecting the roots from further damage. Certain root-colonizing bacteria of the genus Pseudomonas also benefit plants by promoting growth, suppressing pathogens or inducing systemic resistance (ISR), and some strains also have insecticidal activity. It remains unknown how these bacteria influence the emissions of root volatiles. In this study, we evaluated how colonization by the growth-promoting and insecticidal bacteria Pseudomonas protegens CHA0 and Pseudomonas chlororaphis PCL1391 affects the production of EβC upon feeding by larvae of the banded cucumber beetle, Diabrotica balteata Le Conte (Coleoptera: Chrysomelidae). Using chemical analysis and gene expression measurements, we found that EβC production and the expression of the EβC synthase gene (tps23) were enhanced in Pseudomonas protegens CHA0-colonized roots after 72 h of D. balteata feeding. Undamaged roots colonized by Pseudomonas spp. showed no measurable increase in EβC production, but a slight increase in tps23 expression. Pseudomonas colonization did not affect root biomass, but larvae that fed on roots colonized by P. protegens CHA0 tended to gain more weight than larvae that fed on roots colonized by P. chlororaphis PCL1391. Larvae mortality on Pseudomonas spp. colonized roots was slightly, but not significantly higher than on non-colonized control roots. The observed enhanced production of EβC upon Pseudomonas protegens CHA0 colonization may enhance the roots' attractiveness to entomopathogenic nematodes, but this remains to be tested.

Movement patterns in Entomopathogenic nematodes: Continuous vs. temporal

To exploit resources, animals implement various foraging behaviors to increase their fitness. Entomopathogenic nematodes are obligate parasites of insects in nature. In previous studies, entomopathogenic nematodes were reported to exhibit group movement behavior in the presence and absence of insect hosts. However, it was not determined if group movement is continuous or temporal. For example, nematode movement behavior upon emergence from the host might start out in an independent fashion prior to aggregation, or group movement may be exhibited continuously. In the present study, we explored the propensity for innate group movement behavior of two insect parasitic nematodes in two families and genera: Heterorhabditis indica and Steinernema carpocapsae. We hypothesized the nematode populations would initially move independently from their origin and then come together for group movement. Movement patterns were investigated in sand when nematodes were applied in aqueous suspension (via filter paper) to a specific locus or when the nematodes emerged naturally from infected insect hosts. To compare nematode movement behavior over time and space, nematode dispersal was monitored at three distances (2.5, 4.5 and 8.0 cm) from the center (origin) and at two different time periods, 2 days and 3 days after nematode addition. We discovered that nematode dispersal continuously exhibited an aggregative pattern (independent movement was not observed). Results from both nematode species as well as the host-cadaver and filter paper (aqueous nematode suspension) application methods indicated a continuous aggregative pattern. The discovery of continuous aggregative movement patterns in steinernematid and heterorhabditid nematodes elucidates further the complexity of their foraging behavior and may serve as basis for exploring foraging behavior in other host-parasite systems.

Transcriptional responses of wheat and the cereal cyst nematode Heterodera avenae during their early contact stage

Cereal cyst nematode (Heterodera avenae) is attracted to and aggregated around wheat roots to initiate infection, but this interaction between wheat and the nematode is not fully understood. The transcriptional responses of both wheat and H. avenae were examined during their early contact stage by mRNA sequencing analysis; certain numbers of the differentially expressed genes (DEGs) were validated using quantitative real-time PCR. The immobile host wheat root only had 93 DEGs (27 up-regulated and 66 down-regulated), while the mobile plant parasitic nematode reacted much more actively with 879 DEGs (867 up-regulated and 12 down-regulated). Among them, a number of wheat DEGs (mostly down-regulated) were involved in biotic stress pathways, while several putative effector genes were up-regulated in the nematode DEGs. One putative chitinase-like effector gene of H. avenae was able to suppress BAX-triggered programmed cell death in Nicotiana benthamiana. Results of these experiments demonstrated that nematode responded more actively than wheat during the contact stage of parasitism. The parasite's responses mainly involved up-regulation of genes including at least one anti-plant-defence effector gene, whereas the host responses mainly involved down-regulation of certain defence-related genes.

Bacterial phytopathogen infection disrupts belowground plant indirect defense mediated by tritrophic cascade

Plants can defend themselves against herbivores through activation of defensive pathways and attraction of third-trophic-level predators and parasites. Trophic cascades that mediate interactions in the phytobiome are part of a larger dynamic including the pathogens of the plant itself, which are known to greatly influence plant defenses. As such, we investigated the impact of a phloem-limited bacterial pathogen, Candidatus Liberibacter asiaticus (CLas), in cultivated citrus rootstock on a well-studied belowground tritrophic interaction involving the attraction of an entomopathogenic nematode (EPN), Steinernema diaprepesi, to their root-feeding insect hosts, Diaprepes abbreviatus larvae. Using belowground olfactometers, we show how CLas infection interferes with this belowground interaction by similarly inducing the release of a C12 terpene, pregeijerene, and disconnecting the association of the terpene with insect presence. D. abbreviatus larvae that were not feeding but in the presence of a CLas-infected plant were more likely to be infected by EPN than those near uninfected plants. Furthermore, nonfeeding larvae associated with CLas-infected plants were just as likely to be infected by EPN as those near noninfected plants with D. abbreviatus larval damage. Larvae of two weevil species, D. abbreviatus and Pachnaeus litus, were also more attracted to plants with infection than to uninfected plants. D. abbreviatus larvae were most active when exposed to pregeijerene at a concentration of 0.1 μg/μl. We attribute this attraction to CLas-infected plants to the same signal previously thought to be a herbivore-induced plant volatile specifically induced by root-feeding insects, pregeijerene, by assessing volatiles collected from the roots of infected plants and uninfected plants with and without feeding D. abbreviatus. Synthesis. Phytopathogens can influence the structuring of soil communities extending to the third trophic level. Field populations of EPN may be less effective at host-finding using pregeijerene as a cue in citrus grove agroecosystems with high presence of CLas infection.

Multitrophic Effects of Belowground Parasitoid Learning

The ability to learn allows organisms to take advantage of dynamic and ephemeral opportunities in their environment. Here we show that learning in belowground entomopathogenic nematodes has cascading multitrophic effects on their hosts, other nematodes, and nematophagous fungal predators. In addition to quantifying these effects, we show that social behavioral plasticity in these belowground parasitoids can amplify signaling by plant defense pathways and results in an almost doubling of insect herbivore infection by entomopathogenic nematodes. Cumulatively, these effects point to the critical role of plant signaling in regulating community structure while suggesting an equally important role for behavioral plasticity in shaping community dynamics.

Small molecules below-ground: the role of specialized metabolites in the rhizosphere

Soil communities are diverse taxonomically and functionally. This ecosystem experiences highly complex networks of interactions, but may also present functionally independent entities. Plant roots, a metabolically active hotspot in the soil, take an essential part in below-ground interactions. While plants are known to release an extremely high portion of the fixated carbon to the soil, less information is known about the composition and role of C-containing compounds in the rhizosphere, in particular those involved in chemical communication. Specialized metabolites (or secondary metabolites) produced by plants and their associated microbes have a critical role in various biological activities that modulate the behavior of neighboring organisms. Thus, elucidating the chemical composition and function of specialized metabolites in the rhizosphere is a key element in understanding interactions in this below-ground environment. Here, we review key classes of specialized metabolites that occur as mostly non-volatile compounds in root exudates or are emitted as volatile organic compounds (VOCs). The role of these metabolites in below-ground interactions and response to nutrient deficiency, as well as their tissue and cell type-specific biosynthesis and release are discussed in detail.

Eliciting maize defense pathways aboveground attracts belowground biocontrol agents

Plant defense pathways mediate multitrophic interactions above and belowground. Understanding the effects of these pathways on pests and natural enemies above and belowground holds great potential for designing effective control strategies. Here we investigate the effects of aboveground stimulation of plant defense pathways on the interactions between corn, the aboveground herbivore adult Diabrotica speciosa, the belowground herbivore larval D. speciosa, and the subterranean ento-mopathogenic nematode natural enemy Heterorhabditis amazonensis. We show that adult D. speciosa recruit to aboveground herbivory and methyl salicylate treatment, that larval D. speciosa are relatively indiscriminate, and that H. amazonensis en-tomopathogenic nematodes recruit to corn fed upon by adult D. speciosa. These results suggest that entomopathogenicnematodes belowground can be highly attuned to changes in the aboveground parts of plants and that biological control can be enhanced with induced plant defense in this and similar systems.

Olfactory circuits and behaviors of nematodes

Over one billion people worldwide are infected with parasitic nematodes. Many parasitic nematodes actively search for hosts to infect using volatile chemical cues, so understanding the olfactory signals that drive host seeking may elucidate new pathways for preventing infections. The free-living nematode Caenorhabditis elegans is a powerful model for parasitic nematodes: because sensory neuroanatomy is conserved across nematode species, an understanding of the microcircuits that mediate olfaction in C. elegans may inform studies of olfaction in parasitic nematodes. Here we review circuit mechanisms that allow C. elegans to respond to odorants, gases, and pheromones. We also highlight work on the olfactory behaviors of parasitic nematodes that lays the groundwork for future studies of their olfactory microcircuits.

Root signals that mediate mutualistic interactions in the rhizosphere

A recent boom in research on belowground ecology is rapidly revealing a multitude of fascinating interactions, in particular in the rhizosphere. Many of these interactions are mediated by photo-assimilates that are excreted by plant roots. Root exudates are not mere waste products, but serve numerous functions to control abiotic and biotic processes. These functions range from changing the chemical and physical properties of the soil, inhibiting the growth of competing plants, combatting herbivores, and regulating the microbial community. Particularly intriguing are root-released compounds that have evolved to serve mutualistic interactions with soil-dwelling organisms. These mutually beneficial plant-mediated signals are not only of fundamental ecological interest, but also exceedingly important from an agronomical perspective. Here, we attempt to provide an overview of the plant-produced compounds that have so far been implicated in mutualistic interactions. We propose that these mutualistic signals may have evolved from chemical defenses and we point out that they can be (mis)used by specialized pathogens and herbivores. We speculate that many more signals and interactions remain to be uncovered and that a good understanding of the mechanisms and ecological implications can be the basis for exploitation and manipulation of the signals for crop improvement and protection.