Herbivore induced plant volatiles: their role in plant defense for pest management

Metabolites-induced co-evolutionary warfare between plants, viruses, and their associated vectors: So close yet so far away

Agriculture and global food security encounter significant challenges due to viral threats. In the following decades, several molecular studies have focused on discovering biosynthetic pathways of numerous defensive and signaling compounds, as key regulators of plant interactions, either with viruses or their associated vectors. Nevertheless, the complexities of specialized metabolites mediated plant-virus-vector tripartite viewpoint and the identification of their co-evolutionary crossroads toward antiviral defense system, remain elusive. The current study reviews the various roles of plant-specialized metabolites (PSMs) and how plants use these metabolites to defend against viruses. It discusses recent examples of specialized metabolites that have broad-spectrum antiviral properties. Additionally, the study presents the co-evolutionary basis of metabolite-mediated plant-virus-insect interactions as a potential bioinspired approach to combat viral threats. The prospects also show promising metabolic engineering strategies aimed at discovering a wide range of PSMs that are effective in fending off viruses and their related vectors. These advances in understanding the potential role of PSMs in plant-virus interactions not only serve as a cornerstone for developing plant antiviral systems, but also highlight essential principles of biological control.

Green Lacewing Chrysoperla externa Is Attracted to Volatile Organic Compounds and Essential Oils Extracted from Eucalyptus urograndis Leaves

Plant herbivore interactions have long been recognized as a complex interplay influenced by various factors, including plant volatile emissions. Understanding the role of these volatiles in mediating plant predator interactions is crucial for developing sustainable pest management strategies. This study investigated the olfactory preferences of Chrysoperla externa larvae for volatiles emitted by Eucalyptus urograndis leaves, focusing on both seedlings and essential oils (EOs). We used Y-tube olfactometry to compare larval preferences between the clean air and various plant treatments, including undamaged and herbivore-damaged leaves. Chemical analysis of EOs revealed higher concentrations of oxygenated monoterpenes and sesquiterpenes in young and damaged leaves, particularly linalool, which has been implicated in insect attraction. Our results showed a significant preference for volatiles emitted by young damaged leaves over clean air for both seedlings (χ2 = 11.03, p = 0.001) and EOs (χ2 = 9.76, p = 0.002). Chrysoperla externa larvae are significantly attracted to specific volatiles from damaged E. urograndis leaves, suggesting these compounds could serve as cues for natural enemy foraging. Our findings enhance the understanding of plant-predator dynamics and suggest potential applications of eucalyptus plantations to sustain C. externa populations for biocontrol purposes.

Multitrophic and Multilevel Interactions Mediated by Volatile Organic Compounds

Plants communicate with insects and other organisms through the release of volatile organic compounds (VOCs). Using Boolean operators, we retrieved 1093 articles from the Web of Science and Scopus databases, selecting 406 for detailed analysis, with approximately 50% focusing on herbivore-induced plant volatiles (HIPVs). This review examines the roles of VOCs in direct and indirect plant defense mechanisms and their influence on complex communication networks within ecosystems. Our research reveals significant functions of VOCs in four principal areas: activating insect antennae, attracting adult insects, attracting female insects, and attracting natural enemies. Terpenoids like α-pinene and β-myrcene significantly alter pest behavior by attracting natural enemies. β-ocimene and β-caryophyllene are crucial in regulating aboveground and belowground interactions. We emphasize the potential applications of VOCs in agriculture for developing novel pest control strategies and enhancing crop resilience. Additionally, we identify research gaps and propose new directions, stressing the importance of comparative studies across ecosystems and long-term observational research to better understand VOCs dynamics. In conclusion, we provide insights into the multifunctionality of VOCs in natural ecosystems, their potential for future research and applications, and their role in advancing sustainable agricultural and ecological practices, contributing to a deeper understanding of their mechanisms and ecological functions.

Volatile-mediated oviposition preference for healthy over root-infested plants by the European corn borer

The selection of oviposition sites by female moths is crucial in shaping their progeny performance and survival, and consequently in determining insect fitness. Selecting suitable plants that promote the performance of the progeny is referred to as the Preference-Performance hypothesis (or 'mother-knows-best'). While root infestation generally reduces the performance of leaf herbivores, little is known about its impact on female oviposition. We investigated whether maize root infestation by the Western corn rootworm (WCR) affects the oviposition preference and larval performance of the European corn borer (ECB). ECB females used leaf volatiles to select healthy plants over WCR-infested plants. Undecane, a compound absent from the volatile bouquet of healthy plants, was the sole compound to be upregulated upon root infestation and acted as a repellent for first oviposition. ECB larvae yet performed better on plants infested below-ground than on healthy plants, suggesting an example of 'bad motherhood'. The increased ECB performance on WCR-infested plants was mirrored by an increased leaf consumption, and no changes in the plant primary or secondary metabolism were detected. Understanding plant-mediated interactions between above- and below-ground herbivores may help to predict oviposition decisions, and ultimately, to manage pest outbreaks in the field.

Plant-derived monoterpene S-linalool and β-ocimene generated by CsLIS and CsOCS-SCZ are key chemical cues for attracting parasitoid wasps for suppressing Ectropis obliqua infestation in Camellia sinensis L

Insect-induced plant volatile organic compounds (VOCs) may function as either direct defence molecules to deter insects or indirect defence signals to attract the natural enemies of the invading insects. Tea (Camellia sinensis L.), an important leaf-based beverage crop, is mainly infested by Ectropis obliqua which causes the most serious damage. Here, we report a mechanistic investigation of tea plant-derived VOCs in an indirect defence mechanism against E. obliqua. Parasitoid wasp Parapanteles hyposidrae, a natural enemy of E. obliqua, showed strong electrophysiological response and selection behaviour towards S-linalool and β-ocimene, two monoterpenes with elevated emission from E. obliqua-damaged tea plants. Larvae frass of E. obliqua, which also released S-linalool and β-ocimene, was found to attract both mated female or male Pa. hyposidrae according to gas chromatography-electroantennogram detection and Y-tube olfactometer assays. In a field setting, both S-linalool and β-ocimene were effective in recruiting both female and male Pa. hyposidrae wasps. To understand the molecular mechanism of monoterpenes-mediated indirect defence in tea plants, two novel monoterpene synthase genes, CsLIS and CsOCS-SCZ, involved in the biosynthesis of S-linalool or β-ocimene, respectively, were identified and biochemically characterised. When the expression of these two genes in tea plants was inhibited by antisense oligodeoxynucleotide, both volatile emission and attraction of wasps were reduced. Furthermore, gene expression analysis suggested that the expression of CsLIS and CsOCS-SCZ is regulated by the jasmonic acid signalling pathway in the tea plant.

Belowground insect herbivory induces systemic volatile emissions that strengthen neighbouring plant resistance aboveground

Plants transmit ecologically relevant messages to neighbouring plants through chemical cues. For instance, insect herbivory triggers the production of herbivore-induced plant volatiles (HIPVs), which can enhance neighbouring plant defences. HIPVs are emitted from directly damaged plant tissues and from systemic, nondamaged tissues. Although volatile-mediated interplant interactions have been observed both above- and belowground, it remains unknown whether belowground herbivory induces systemic HIPVs aboveground that influence neighbouring plants. To explore how belowground herbivory affects interplant interactions aboveground, we characterised systemic HIPVs from squash induced by belowground striped cucumber beetle (Acalymma vittatum) larval herbivory. We exposed squash 'receiver plants' to systemic HIPVs or volatiles from nondamaged plants. We then measured herbivore resistance by challenging 'receiver plants' with aboveground-feeding herbivores: adult beetles (A. vittatum) or squash bugs (Anasa tristis). We discovered belowground-damaged plants emitted more (E)-β-ocimene, a key volatile from the systemic HIPV blend, than nondamaged controls, and that exposure to systemic HIPVs enhanced neighbouring plant resistance to aboveground squash bugs, but not adult beetles. Further investigations into the mechanism of interplant interaction revealed β-ocimene alone can elicit plant resistance against squash bugs. Overall, our findings reveal a novel form of volatile-mediated interactions between plants spanning across aboveground-belowground plant systems.

Terpene Synthase Gene Family in Chinese Chestnut (Castanea mollissima BL.) Harbors Two Sesquiterpene Synthase Genes Implicated in Defense against Gall Wasp Dryocosmus kuriphilus

Chinese chestnut (Castanea mollissima BL.) is a well-known fruit tree that has been cultivated in East Asia for millennia. Leaves and buds of the plant can become seriously infested by the gall wasp Dryocosmus kuriphilus (GWDK), which results in gall formation and associated significant losses in fruit production. Herbivore-induced terpenes have been reported to play an important role in plant-herbivory interactions, and in this study, we show that upon herbivory by GWDK, four terpene-related compounds were significantly induced, while the concentrations of these four compounds in intact buds were relatively low. Among these compounds, (E)-nerolidol and (E, E)-α-farnesene have frequently been reported to be involved in plant herbivory defenses, which suggests direct and/or indirect functions in chestnut GWDK defenses. Candidate terpene synthase (TPS) genes that may account for (E)-nerolidol and (E, E)-α-farnesene terpene biosynthesis were characterized by transcriptomics and phylogenetic approaches, which revealed altered transcript levels for two TPSs: CmAFS, a TPS-g subfamily member, and CmNES/AFS, a TPS-b clade member. Both genes were dramatically upregulated in gene expression upon GWDK infestation. Furthermore, Agrobacterium tumefaciens-mediated transient overexpression in Nicotiana benthamiana showed that CmAFS catalyzed the formation of (E, E)-α-farnesene, while CmNES/AFS showed dual (E)-nerolidol and (E, E)-α-farnesene synthase activity. Biochemical assays of the recombinant CmAFS and CmNES/AFS proteins confirmed their catalytic activity in vitro, and the enzymatic products were consistent with two of the major volatile compounds released upon GWDK-infested chestnut buds. Subcellular localization demonstrated that CmAFS and CmNES/AFS were both localized in the cytoplasm, the primary compartment for sesquiterpene synthesis. In summary, we show that two novel sesquiterpene synthase genes CmAFS and CmNES/AFS are inducible by herbivory and can account for the elevated accumulation of (E, E)-α-farnesene and (E)-nerolidol upon GWDK infestation and may be implicated in chestnut defense against GWDK herbivores.

Metabolic Response of Peach Fruit to Invasive Brown Marmorated Stink Bug (Halyomorpha halys Stål.)'s Infestation

The brown marmorated stink bug (BMSB; Halyomorpha halys Stål.) is a highly destructive and polyphagous invasive pest that poses a serious threat to more than a hundred reported host plants. In the current study, the metabolic response of peach fruit of two cultivars-'Maria Marta' and 'Redhaven'-to BMSB infestation was studied using high-performance liquid chromatography (HPLC) and mass spectrometry (MS). In general, a strong phenolic response to BMSB infestation in peach flesh in the injury zone was observed, with flavanol content increasing by 2.4-fold, hydroxycinnamic acid content by 5.0-fold, flavonol content by 3.2-fold, flavanone content by 11.3-fold, and dihydrochalcones content by 3.2-fold compared with the undamaged tissue in the cultivar 'Maria Marta'. The phenolic response in the 'Redhaven' cultivar was even stronger. Consequently, the total phenolic content in the injured flesh also increased, 3.3-fold in 'Maria Marta' and 6.9-fold in 'Redhaven', compared with the uninjured flesh. Infestation with BMSB induced the synthesis of cyanidin-3-glucoside, which is not normally present in peach flesh. In comparison, the phenolic response was lower in peach peel, especially in the cultivar 'Maria Marta'. The study showed that both peach cultivars reacted to BMSB infestation with an increase in phenolic content in the peach flesh, but in a limited area of injury.

Comparative analysis of defensive secondary metabolites in wild teosinte and cultivated maize under flooding and herbivory stress

Climate change is driving an alarming increase in the frequency and intensity of abiotic and biotic stress factors, negatively impacting plant development and agricultural productivity. To survive, plants respond by inducing changes in below and aboveground metabolism with concomitant alterations in defensive secondary metabolites. While plant responses to the isolated stresses of flooding and insect herbivory have been extensively studied, much less is known about their response in combination. Wild relatives of cultivated plants with robust stress tolerance traits provide an excellent system for comparing how diverse plant species respond to combinatorial stress, and provide insight into potential germplasms for stress-tolerant hybrids. In this study, we compared the below and aboveground changes in the secondary metabolites of maize (Zea mays) and a flood-tolerant wild relative Nicaraguan teosinte (Zea nicaraguensis) in response to flooding, insect herbivory, and their combination. Root tissue was analyzed for changes in belowground metabolism. Leaf total phenolic content and headspace volatile organic compound emission were analyzed for changes in aboveground secondary metabolism. Results revealed significant differences in the root metabolome profiles of teosinte and maize. Notably, the accumulation of the flavonoids apigenin, naringenin, and luteolin during flooding and herbivory differentiated teosinte from maize. Aboveground, terpenes, including trans-α-bergamotene and (E)-4,8-dimethylnona-1,3,7-triene, shaped compositional differences in their volatile profiles between flooding, herbivory, and their combination. Taken together, these results suggest teosinte may be more tolerant than maize due to dynamic metabolic changes during flooding and herbivory that help relieve stress and influence plant-insect interactions.

Monoterpene alcohols induced by sweet potato weevil larvae deter conspecific adults from feeding and oviposition

BACKGROUND

Intraspecific competition is shared in the insect world, especially under the condition of limited food and space resources. To avoid intraspecific competition and increase offspring survival, insects have evolved various effective strategies. A widely-accepted tactic is employing chemical cues, which are frequently utilized as indicators of conspecific colonization. The sweet potato weevil (SPW), Cylas formicarius, is a destructive pest of sweet potatoes. Its larvae bore into sweet potatoes and alter the emission of odors. The present study aimed to investigate whether volatiles associated with SPW larvae feeding influence the behavioral preference of conspecific adults.

RESULTS

Volatiles from SPW larvae-infested sweet potatoes were collected by a head-space method and analyzed using gas chromatography-electroantennogram detector (GC-EAD) and gas chromatography-mass spectrometry (GC-MS). Five compounds eliciting EAD responses from the antennae of both male and female adult SPW were identified from sweet potatoes with the third-instar larvae, including linalool, citronellol, nerol, geraniol, and ipomeamarone. Four monoterpene alcohols significantly repelled SPW adults from feeding and oviposition at higher doses in the behavioral preference bioassays. Among them, geraniol displayed the strongest repellent activities for SPW feeding and oviposition. These results suggested that SPW larvae could reduce colonization of adult SPWs by inducing monoterpene alcohols, thereby avoiding intraspecific competition.

CONCLUSION

The present study demonstrated that volatile monoterpene alcohols induced by SPW larvae are chemical cues of larvae occupation for SPW adults to change their behavioral preference. Unveiling factors that mediate avoidance of intraspecific competition could help develop repellents or oviposition deterrents for SPW control. © 2023 Society of Chemical Industry.

Transcriptomic Analysis of Tobacco Plants in Response to Whitefly Infection

The whitefly Bemisia tabaci is one of the most destructive pests worldwide, and causes tremendous economic losses. Tobacco Nicotiana tabacum serves as a model organism for studying fundamental biological processes and is severely damaged by whiteflies. Hitherto, our knowledge of how tobacco perceives and defends itself against whiteflies has been scare. In this study, we analyze the gene expression patterns of tobacco in response to whitefly infestation. A total of 244 and 2417 differentially expressed genes (DEGs) were identified at 12 h and 24 h post whitefly infestation, respectively. Enrichment analysis demonstrates that whitefly infestation activates plant defense at both time points, with genes involved in plant pattern recognition, transcription factors, and hormonal regulation significantly upregulated. Notably, defense genes are more intensely upregulated at 24 h post infestation than at 12 h, indicating an increased immunity induced by whitefly infestation. In contrast, genes associated with energy metabolism, carbohydrate metabolism, ribosomes, and photosynthesis are suppressed, suggesting impaired plant development. Taken together, our study provides comprehensive insights into how plants respond to phloem-feeding insects, and offers a theoretical basis for better research on plant-insect interactions.

Temporal transcriptomic profiling elucidates sorghum defense mechanisms against sugarcane aphids

BACKGROUND

The sugarcane aphid (SCA; Melanaphis sacchari) has emerged as a key pest on sorghum in the United States that feeds from the phloem tissue, drains nutrients, and inflicts physical damage to plants. Previously, it has been shown that SCA reproduction was low and high on sorghum SC265 and SC1345 plants, respectively, compared to RTx430, an elite sorghum male parental line (reference line). In this study, we focused on identifying the defense-related genes that confer resistance to SCA at early and late time points in sorghum plants with varied levels of SCA resistance.

RESULTS

We used RNA-sequencing approach to identify the global transcriptomic responses to aphid infestation on RTx430, SC265, and SC1345 plants at early time points 6, 24, and 48 h post infestation (hpi) and after extended period of SCA feeding for 7 days. Aphid feeding on the SCA-resistant line upregulated the expression of 3827 and 2076 genes at early and late time points, respectively, which was relatively higher compared to RTx430 and SC1345 plants. Co-expression network analysis revealed that aphid infestation modulates sorghum defenses by regulating genes corresponding to phenylpropanoid metabolic pathways, secondary metabolic process, oxidoreductase activity, phytohormones, sugar metabolism and cell wall-related genes. There were 187 genes that were highly expressed during the early time of aphid infestation in the SCA-resistant line, including genes encoding leucine-rich repeat (LRR) proteins, ethylene response factors, cell wall-related, pathogenesis-related proteins, and disease resistance-responsive dirigent-like proteins. At 7 days post infestation (dpi), 173 genes had elevated expression levels in the SCA-resistant line and were involved in sucrose metabolism, callose formation, phospholipid metabolism, and proteinase inhibitors.

CONCLUSIONS

In summary, our results indicate that the SCA-resistant line is better adapted to activate early defense signaling mechanisms in response to SCA infestation because of the rapid activation of the defense mechanisms by regulating genes involved in monolignol biosynthesis pathway, oxidoreductase activity, biosynthesis of phytohormones, and cell wall composition. This study offers further insights to better understand sorghum defenses against aphid herbivory.

Dynamic distress calls: volatile info chemicals induce and regulate defense responses during herbivory

Plants are continuously threatened by a plethora of biotic stresses caused by microbes, pathogens, and pests, which often act as the major constraint in crop productivity. To overcome such attacks, plants have evolved with an array of constitutive and induced defense mechanisms- morphological, biochemical, and molecular. Volatile organic compounds (VOCs) are a class of specialized metabolites that are naturally emitted by plants and play an important role in plant communication and signaling. During herbivory and mechanical damage, plants also emit an exclusive blend of volatiles often referred to as herbivore-induced plant volatiles (HIPVs). The composition of this unique aroma bouquet is dependent upon the plant species, developmental stage, environment, and herbivore species. HIPVs emitted from infested and non-infested plant parts can prime plant defense responses by various mechanisms such as redox, systemic and jasmonate signaling, activation of mitogen-activated protein (MAP) kinases, and transcription factors; mediate histone modifications; and can also modulate the interactions with natural enemies via direct and indirect mechanisms. These specific volatile cues mediate allelopathic interactions leading to altered transcription of defense-related genes, viz., proteinase inhibitors, amylase inhibitors in neighboring plants, and enhanced levels of defense-related secondary metabolites like terpenoids and phenolic compounds. These factors act as deterrents to feeding insects, attract parasitoids, and provoke behavioral changes in plants and their neighboring species. This review presents an overview of the plasticity identified in HIPVs and their role as regulators of plant defense in Solanaceous plants. The selective emission of green leaf volatiles (GLVs) including hexanal and its derivatives, terpenes, methyl salicylate, and methyl jasmonate (MeJa) inducing direct and indirect defense responses during an attack from phloem-sucking and leaf-chewing pests is discussed. Furthermore, we also focus on the recent developments in the field of metabolic engineering focused on modulation of the volatile bouquet to improve plant defenses.

The Role of Ascorbate-Glutathione System and Volatiles Emitted by Insect-Damaged Lettuce Roots as Navigation Signals for Insect and Slug Parasitic Nematodes

The effect of wireworm-damaged lettuce roots on the antioxidative defense system (ascorbate-glutathione cycle, photosynthetic pigments) and movement of insect/slug parasitic nematodes towards determined root exudates was studied in a glasshouse experiment. Lettuce seedlings were grown in a substrate soil in the absence/presence of wireworms (Elateridae). The ascorbate-glutathione system and photosynthetic pigments were analyzed by HPLC, while volatile organic compounds (VOC) emitted by lettuce roots were investigated by GC-MS. Herbivore-induced root compounds, namely 2,4-nonadienal, glutathione, and ascorbic acid, were selected for a chemotaxis assay with nematodes Steinernema feltiae, S. carpocapsae, Heterorhabditis bacteriophora, Phasmarhabditis papillosa, and Oscheius myriophilus. Root pests had a negative effect on the content of photosynthetic pigments in the leaves of infested plants, indicating that they reacted to the presence of reactive oxygen species (ROS). Using lettuce as a model plant, we recognized the ascorbate-glutathione system as a redox hub in defense response against wireworms and analyzed its role in root-exudate-mediated chemotaxis of nematodes. Infected plants also demonstrated increased levels of volatile 2,4-nonadienal. Entomopathogenic nematodes (EPNs, S. feltiae, S. carpocapsae, and H. bacteriophora) proved to be more mobile than parasitic nematodes O. myriophilus and P. papillosa towards chemotaxis compounds. Among them, 2,4-nonadienal repelled all tested nematodes. Most exudates that are involved in belowground tritrophic interactions remain unknown, but an increasing effort is being made in this field of research. Understanding more of these complex interactions would not only allow a better understanding of the rhizosphere but could also offer ecologically sound alternatives in the pest management of agricultural systems.

Herbivore-Dependent Induced Volatiles in Pear Plants Cause Differential Attractive Response by Lacewing Larvae

Biological control may benefit from the behavioral manipulation of natural enemies using volatile organic compounds (VOCs). Among these, herbivore-induced plant volatiles (HIPVs) provide potential tools for attracting or retaining predators and parasitoids of insect pests. This work aimed to characterize the VOCs emitted by pear plants in response to attack by Cacopsylla bidens (Hemiptera: Psyllidae), a major pest in pear orchards, to compare these with VOCs induced by a leaf chewing insect, Argyrotaenia sphaleropa (Lepidoptera: Tortricidae), and to evaluate the behavioral response of Chrysoperla externa (Neuroptera: Chrysopidae) to HIPVs from pear plants damaged by either herbivore. The results demonstrated that plants damaged by the pear psylla emitted VOC blends with increased amounts of aliphatic aldehydes. Leafroller damage resulted in increased amounts of benzeneacetonitrile, (E)-4,8-dimethylnona-1,3,7-triene, β-ocimene and caryophyllene. In olfactometer bioassays, larvae of C. externa were attracted to herbivore-damaged plants when contrasted with undamaged plants. When plant odors from psylla-damaged were contrasted with those of leafroller-damaged plants, C.externa preferred the former, also showing shorter response lag-times and higher response rates when psylla-damaged plants were present. Our results suggest that pear plants respond to herbivory by modifying their volatile profile, and that psylla-induced volatiles may be used as prey-specific chemical cues by chrysopid larvae. Our study is the first to report HIPVs in pear plants attacked by C. bidens, as well as the attraction of C. externa to psyllid-induced volatiles.

Plant Secondary Metabolites: The Weapons for Biotic Stress Management

The rise in global temperature also favors the multiplication of pests and pathogens, which calls into question global food security. Plants have developed special coping mechanisms since they are sessile and lack an immune system. These mechanisms use a variety of secondary metabolites as weapons to avoid obstacles, adapt to their changing environment, and survive in less-than-ideal circumstances. Plant secondary metabolites include phenolic compounds, alkaloids, glycosides, and terpenoids, which are stored in specialized structures such as latex, trichomes, resin ducts, etc. Secondary metabolites help the plants to be safe from biotic stressors, either by repelling them or attracting their enemies, or exerting toxic effects on them. Modern omics technologies enable the elucidation of the structural and functional properties of these metabolites along with their biosynthesis. A better understanding of the enzymatic regulations and molecular mechanisms aids in the exploitation of secondary metabolites in modern pest management approaches such as biopesticides and integrated pest management. The current review provides an overview of the major plant secondary metabolites that play significant roles in enhancing biotic stress tolerance. It examines their involvement in both indirect and direct defense mechanisms, as well as their storage within plant tissues. Additionally, this review explores the importance of metabolomics approaches in elucidating the significance of secondary metabolites in biotic stress tolerance. The application of metabolic engineering in breeding for biotic stress resistance is discussed, along with the exploitation of secondary metabolites for sustainable pest management.

Plant viruses induce plant volatiles that are detected by aphid parasitoids

Aphis gossypii (Sternorrhyncha: Aphididae) aphids are vectors of important plant viruses among which cucumber mosaic virus (CMV) and potato virus Y (PVY). Virus-infected plants attract aphid vectors and affect their behavior and growth performance either positively or negatively depending on mode of transmission. Viruses cause changes in the composition and the amount of volatile organic compounds (VOCs) released by the plant that attract aphids. The aphid parasitoid Aphidius colemani (Hymenoptera: Aphelinidae) has been shown to have higher parasitism and survival rates on aphids fed on virus-infected than aphids fed on non-infected plants. We hypothesized that parasitoids distinguish virus-infected plants and are attracted to them regardless of the presence of their aphid hosts. Herein, we examined the attraction of the A. colemani parasitoid to infected pepper plants with each of CMV or PVY without the presence of aphids. The dynamic headspace technique was used to collect VOCs from non-infected and CMV or PVY-infected pepper plants. Identification was performed with gas chromatography-mass spectrometry (GC-MS). The response of the parasitoids on virus-infected vs non-infected pepper plants was tested by Y-tube olfactometer assays. The results revealed that parasitoids displayed a preference to CMV and PVY infected plants compared to those that were not infected.

Exogenous Application of Methyl Salicylate Induces Defence in Brassica against Peach Potato Aphid Myzus persicae

Plants use a variety of secondary metabolites to defend themselves against herbivore insects. Methyl salicylate (MeSA) is a natural plant-derived compound that has been used as a plant defence elicitor and a herbivore repellent on several crop plants. The aim of this study was to investigate the effect of MeSA treatment of Brassica rapa subsp. chinensis ('Hanakan' pak choi) on its interactions with peach potato aphids, Myzus persicae, and their natural enemy, Diaeretiella rapae. For this, we selected two concentrations of MeSA (75 mg/L and 100 mg/L). Our results showed that aphid performance was significantly reduced on plants treated with MeSA (100 mg/L). In a cage bioassay, the MeSA (100 mg/L)-treated plants showed lower adult survival and larviposition. Similarly, the MeSA (100 mg/L)-treated plants had a significantly lower aphid settlement in a settlement bioassay. In contrast, the M. persicae aphids did not show any significant difference between the MeSA (75 mg/L)-treated and control plants. In a parasitoid foraging bioassay, the parasitoid D. rapae also did not show any significant difference in the time spent on MeSA-treated and control plants. A volatile analysis showed that the MeSA treatment induced a significant change in volatile emissions, as high numbers of volatile compounds were detected from the MeSA-treated plants. Our results showed that MeSA has potential to induce defence in Brassica against M. persicae and can be utilised in developing sustainable approaches for the management of peach potato aphids.

Natural Compounds in Plant-Based Food

Plant-based foods include a wide range of products, such as fruits, vegetables, herbs and spices, as well as food products based on them, such as sauces, soups, or beverages [...].

Different responses of two maize cultivars to Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae infestation provide insights into their differences in resistance

Spodoptera frugiperda (Lepidoptera: Noctuidae), a pest with an amazing appetite, damages many crops and causes great losses, especially maize. Understanding the differences in different maize cultivars' responses to S. frugiperda infestation is very important for revealing the mechanisms involved in the resistance of maize plants to S. frugiperda. In this study, a comparative analysis of two maize cultivars, the common cultivar 'ZD958' and the sweet cultivar 'JG218', was used to investigate their physico-biochemical responses to S. frugiperda infestation by a pot experiment. The results showed that the enzymatic and non-enzymatic defense responses of maize seedlings were rapidly induced by S. frugiperda. Frist, the hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents of infested maize leaves were significantly increased and then decreased to the level of the control. Furthermore, compared with the control leaves, the puncture force values and the total phenolics, total flavonoids, and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one contents of infested leaves were significantly increased within a certain time. The superoxide dismutase and peroxidase activities of infested leaves were significantly increased in a certain period of time, while the catalase activities decreased significantly and then increased to the control level. The jasmonic acid (JA) levels of infested leaves were significantly improved, whereas the salicylic acid and abscisic acid levels changed less. Signaling genes associated with phytohormones and defensive substances including PAL4, CHS6, BX12, LOX1, and NCED9 were significantly induced at certain time points, especially LOX1. Most of these parameters changed greater in JG218 than in ZD958. Moreover, the larvae bioassay showed that S. frugiperda larvae weighed more on JG218 leaves than those on ZD958 leaves. These results suggested that JG218 was more susceptible to S. frugiperda than ZD958. Our findings will make it easier to develop strategies for controlling S. frugiperda for sustainable maize production and breeding of new maize cultivars with increased resistance to herbivores.

Differential responses of Bactrocera dorsalis and its parasitoids to headspaces of different varieties of tree-attached mango fruits and the associated chemical profiles

Bactrocera dorsalis (Hendel) is a major pest of fruits and vegetables worldwide with documented losses of up to 100%. Various management techniques including the use of parasitoids, such as Fopius arisanus (Sonan) and Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae) within the context of the Integrated Pest Management (IPM) approach have been deployed for its control. The effectiveness of parasitoids is well understood, but knowledge of the semiochemicals that mediate their behavior, as well as that of the host fruit fly to tree-attached mangoes, is lacking. Here, we first compared the attractiveness of the above-mentioned fruit fly and its parasitoids to volatiles of different treatments (non-infested physiologically mature unripe and ripe mangoes, mangoes newly exposed to ovipositing B. dorsalis, and mangoes on day 7 and day 9 post-oviposition) of tree-attached Kent, Apple, and Haden mango varieties relative to control (clean air). The fruit fly was significantly more attracted to the mango volatiles (up to 93% of responsive insects) compared to the control (clean air). Fopius arisanus was significantly more attracted to mangoes with ovipositing fruit flies (68–76%) while D. longicaudata was significantly more attracted to day 9 post-oviposited mangoes (64–72%) compared to the control. Secondly, we elucidated the headspace volatile profiles of the non-infested and infested tree-attached mangoes using gas chromatography linked to mass spectrometry (GC-MS). The volatiles revealed various types of organic compounds with qualitative and quantitative differences. The majority of the compounds were esters making 33.8% of the total number, followed by sesquiterpenes-16.4%, and monoterpenes-15.4% among others. Most compounds had higher release rates in headspace volatiles of fruit fly-infested mangoes. Lastly, we harvested the infested mangoes and incubated them for puparia recovery. The number of puparia recovered varied according to the mango variety with Apple mango registering 81.7% of the total, while none was recovered from Kent. These results represent the first report of the changes in the headspace components of non-infested and infested tree-attached mangoes and the associated differential responses of the mentioned insects. A follow-up study can reveal whether there is a convergence in olfactomes which is significant when developing baits that selectively attract the fruit fly and not its natural enemies and fill the knowledge gap from an evolutionary ecological perspective.

The role of methyl salicylate in plant growth under stress conditions

Methyl salicylate is a volatile compound, the synthesis of which takes place via the salicylic acid pathway in plants. Both compounds can be involved in the development of systemic acquired resistance and they play their role partly independently. Salicylic acid transport has an important role in long-distance signalling, but methyl salicylate has also been suggested as a phloem-based mobile signal, which can be demethylated to form salicylic acid, inducing the de-novo synthesis of salicylic acid in distal tissue. Despite the fact that salicylic acid has a protective role in abiotic stress responses and tolerance, very few investigations have been reported on the similar effects of methyl salicylate. In addition, as salicylic acid and methyl salicylate are often treated simply as the volatile and non-volatile forms of the same compound, and in several cases they also act in the same way, it is hard to highlight the differences in their mode of action. The main aim of the present review is to reveal the individual role and action mechanism of methyl salicylate in systemic acquired resistance, plant-plant communication and various stress conditions in fruits and plants.

Infestation of the gall midge Dasineura oleae provides first evidence of induced plant volatiles in olive leaves

In this study, we present the first characterization of herbivore-induced plant volatiles (HIPVs) released from infested olive leaves. The gall midge Dasineura oleae is a specific pest of Olea europaea and endemic of the Mediterranean Basin, an area in which severe outbreaks currently occurred. Little is known about the damage caused by the pest and the relationship with its host. Since gall formation and larval feeding activity may lead to the release of specific plant volatile compounds, we investigated the volatile profiles emitted from infested plants compared with healthy plants under both laboratory and field conditions. Additionally, the volatiles emitted from mechanically damaged plants were considered. A blend of 12 volatiles was emitted from olive trees infested by D. oleae. Of these, β-copaene, β-ocimene, cosmene, unknown 1 and unknown 3 were found to be exclusively emitted in infested plants. The emission of germacrene-D, (E,E)-α-farnesene, and (Z,E)-α-farnesene, α-copaene, (E)-4,8-dimethylnona-1,3,7-triene, (E)-β-guaiene and heptadecane significantly increased in infested trees. Linalool, β-copaen-4-α-ol, β-bourbonene, β-cubebene, β-elemene, β-copaene and δ-amorphene were found only in the field trial and showed differences depending on the level of infestation and the plant stage. (Z)-3-Hexenol, (E)-4-oxohen-2-enal, and 2-(2-butoxyethoxy)-ethanol, were exclusively emitted from the leaves after mechanical damage. In a field trial in Italy, we also demonstrated spring synchronization between adults of D. oleae and O. europaea trees. Analyses of morphoanatomical malformations of gall leaves showed that tissue alterations occur at the spongy parenchyma causing an increase of the leaf blade thickness. We speculate that tissue alterations may lead to HIPV release, in turn potentially attracting D. oleae natural enemies.

Caterpillar-Induced Rice Volatile (E)-β-Farnesene Impairs the Development and Survival of Chilo suppressalis Larvae by Disrupting Insect Hormone Balance

Significant research progress has recently been made on establishing the roles of tps46 in rice defense. (E)-β-farnesene (Eβf) is a major product of tps46 activity but its physiological functions and potential mechanisms against Chilo suppressalis have not yet been clarified. In the present study, C. suppressalis larvae were artificially fed a diet containing 0.8 g/kg Eβf and the physiological performance of the larvae was evaluated. In response to Eβf treatment, the average 2nd instar duration significantly increased from 4.78 d to 6.31 d while that of the 3rd instar significantly increased from 5.70 d to 8.00 d compared with the control. There were no significant differences between the control and Eβf-fed 4th and 5th instars in terms of their durations. The mortalities of the 2nd and 3rd Eβf-fed instars were 21.00-fold and 6.39-fold higher, respectively, than that of the control. A comparative transcriptome analysis revealed that multiple differentially expressed genes are involved in insect hormone biosynthesis. An insect hormone assay on the 3rd instars disclosed that Eβf disrupted the balance between the juvenile hormone and ecdysteroid levels. Eβf treatment increased the juvenile hormones titers but not those of the ecdysteroids. The qPCR results were consistent with those of the RNA-Seq. The foregoing findings suggested that Eβf impairs development and survival in C. suppressalis larvae by disrupting their hormone balance. Moreover, Eβf altered the pathways associated with carbohydrate and xenobiotic metabolism as well as those related to cofactors and vitamins in C. suppressalis larvae. The discoveries of this study may contribute to the development and implementation of an integrated control system for C. suppressalis infestations in rice.

Elevated CO2 Altered Rice VOCs Aggravate Population Occurrence of Brown Planthoppers by Improving Host Selection Ability

Simple Summary

In recent years, the atmospheric CO₂ concentration was increasing continuously, which has led to the change in the photosynthesis and chemical composition of rice plants. The growth and development of brown planthopper (BPH) Nilaparvata lugens are further affected. Plants release volatile organic compounds (VOCs) to mediate intra- and inter-specific interactions with other organisms in the surrounding environment. Therefore, here we aim to explore the effect of rice VOCs on the host selection ability of BPH under elevated CO₂. Among the identified thirty-six rice VOCs, the contents of heptadecane, linalool and limonene from rice plants were significantly decreased under elevated CO₂. Moreover, we found that the VOCs of rice damaged by BPH were also changed. Undecane, hexadecane, nonanal and 2,6-diphenylphenol from BPH-damaged rice plants under elevated CO₂ were all significantly higher than those from healthy rice plants, which might lead to enhancement of the host selection ability of BPH, eventually aggravating the damage caused by BPH. However, the role of these VOCs in host selection ability of BPH is not clear, and more experiments are needed to verify their function.

Abstract

It is predicted that plant volatile organic compounds (VOCs) are affected by the atmospheric CO₂ levels rising globally, which further affects the interaction between plants and herbivorous insects, especially the host selection behavior of herbivorous insects. In this study, the effects of elevated CO₂ on the host-selection behavior of the brown planthopper (BPH) Nilaparvata lugens, and the emission of VOCs from the healthy and BPH-damaged rice plants were studied simultaneously to make clear the population occurrence of BPH under global climate change. Compared with ambient CO₂, elevated CO₂ significantly increased the host selection percent of BPH for the healthy (CK) and BPH-damaged rice plants, and the host selection percent of BPH for the BPH-damaged rice plants was significantly higher than that for the healthy rice plants under elevated CO₂, which might be regulated by the transcription levels of OBP1, OBP2 and CSP8 in BPH due to the upregulated transcriptional levels of these three genes of BPH under elevated CO₂. In addition, we analyzed and quantified the emission of VOCs in rice plants grown under ambient CO₂ and elevated CO₂ by GS-MS. A total of 36 VOCs from rice plants were identified into eight categories, including alkanes, alkenes, alcohols, aldehydes, ketones, esters, phenols and aromatic hydrocarbons. Elevated CO₂ significantly decreased the contents of heptadecane, linalool and limonene from rice plants compared with ambient CO₂. Besides, the contents of linalool, phytol, decanal, 1-methyldecalin and 2,6-diphenylphenol from BPH-damaged rice plants under ambient CO₂, and undecane, hexadecane, nonanal and 2,6-diphenylphenol from BPH-damaged rice plants under elevated CO₂ were all significantly higher than those from healthy rice plants. The percentage composition of phenols was positively correlated with the host selection rate of BPH. Our study indicates that elevated CO₂ is beneficial to promote the host selection ability of BPH for rice plants damaged by BPHs due to the changed plant VOCs.

Sesquiterpene Induction by the Balsam Woolly Adelgid (Adelges piceae) in Putatively Resistant Fraser Fir (Abies fraseri)

Fraser fir, Abies fraseri (Pursh) Poir., is a tree endemic to the Southern Appalachians and is found only in a few isolated populations at high elevations. Fraser firs are also cultivated on a commercial scale as Christmas trees. The species is imperiled by an introduced insect, the balsam woolly adelgid, Adelges piceae Ratzeburg (BWA). The insect severely damages Christmas tree crops and has caused substantial Fraser fir mortality in natural stands. Foliar terpenoids are one mechanism of host plant defense against invading insects and may be one focus of future Christmas tree breeding efforts. This study examines the correlation of foliar terpenoids with Fraser fir performance when infested with BWA. GC-MS and GC-FID analysis of artificially infested Fraser fir foliage reveals that increased concentrations of four terpenoid compounds are associated with BWA infestations. Foliar concentrations of two sesquiterpenes, camphene and humulene, are significantly higher in putatively resistant Fraser fir clones than in more susceptible clones after sustained adelgid feeding for a period of 20 weeks. Although it is unclear if the induction of these sesquiterpenes in the host fir is directly contributing to adelgid resistance, these compounds could serve as effective indicators while screening for BWA resistance in future Christmas tree breeding programs.

Herbivory Protection via Volatile Organic Compounds Is Influenced by Maize Genotype, Not Bacillus altitudinis-Enriched Bacterial Communities

Belowground, plants interact with beneficial soil microbes such as plant growth-promoting rhizobacteria (PGPR). PGPR are rhizosphere bacteria that colonize roots and elicit beneficial effects in plants such as improved plant growth, pathogen resistance, abiotic stress tolerance, and herbivore protection. Treatment of plants with PGPR has been shown to trigger the emission of volatile organic compounds (VOCs). Volatile emissions can also be triggered by herbivory, termed herbivore-induced plant volatiles (HIPV), with important ramifications for chemical-mediated plant and insect interactions. Much of our current understanding of PGPR and herbivore-induced volatiles is based on studies using one plant genotype, yet domestication and modern breeding has led to the development of diverse germplasm with altered phenotypes and chemistry. In this study, we investigated if volatile emissions triggered by PGPR colonization and herbivory varies by maize genotype and microbial community assemblages. Six maize genotypes representing three decades of crop breeding and two heterotic groups were used, with four microbiome treatments: live or sterilized soil, with or without a Bacillus inoculant. Soil sterilization was used to delay microbiome establishment, resulting in low-diversity treatments. At planting, maize seeds were inoculated with PGPR Bacillus altitudinis AP-283 and grown under greenhouse conditions. Four weeks post planting, plants were subjected to feeding by third instar Helicoverpa zea (Lepidoptera: Noctuidae) larvae. Volatiles were collected using solid phase microextraction and analyzed with gas chromatography-mass spectrometry. Illumina NovaSeq 16S rRNA amplicon sequencing was carried out to characterize the rhizosphere microbiome. Maize genotype significantly influenced total volatile emissions, and relative abundance of volatile classes. We did not document a strong influence of microbe treatment on plant VOC emissions. However, inoculating plants with PGPR improved plant growth under sterile conditions. Taken together, our results suggest that genotypic variation is the dominant driver in HIPV composition and individual HIPV abundances, and any bacterial-mediated benefit is genotype and HIPV-specific. Therefore, understanding the interplay of these factors is necessary to fully harness microbially-mediated benefits and improve agricultural sustainability.

Volatiles from Cotton Plants Infested by Agrotis segetum (Lep.: Noctuidae) Attract the Larval Parasitoid Microplitis mediator (Hym.: Braconidae)

Herbivore-induced plant volatiles (HIPVs), chemicals produced by plants infested by herbivorous insects, can act as kairomones that recruit natural enemies of the pest herbivore. Agrotis segetum (Denis and Schiffermüller) is a common, important pest of seedling cotton in Xinjiang Province, China, and the braconid Microplitis mediator (Haliday) is an important mortality factor of this pest's larvae. In olfactometer tests, which included healthy foliage, infested foliage, or infested roots, M. mediator preferred A. segetum-infested cotton plants to healthy cotton plants. In GC-MS analyses of plant-emitted volatiles, we found that compounds emitted increased 14.9- and 13.3- fold after leaf infestation and root infestation, respectively, compared to healthy control plants. The volatiles were mainly p-xylene, nonanal, tetradecane, decanal, benzaldehyde, β-caryophyllene, and humulene, while linalool was only present in the leaf-infestation treatment. In addition, principal component analysis indicated that all 18 compounds were associated with the infested plants, especially β-caryophyllene, p-xylene, and decanal. Based on the above studies and previous functional evaluations of the volatile compounds, it can be demonstrated that these compounds play a crucial role in modulating the interactions between A. segetum and M. mediator and regulating parasitoid behavior. It may be possible to enhance the biological control of A. segetum by M. mediator through the application of HIPVs.

Identification and Functional Analysis of Two Alcohol Dehydrogenase Genes Involved in Catalyzing the Reduction of (Z)-3-Hexenal into (Z)-3-Hexenol in Tea Plants (Camellia sinensis)

Alcohol dehydrogenase (ADH) is a vital enzyme in the biosynthesis pathway of six-carbon volatiles in plants. However, little is known about its functions in tea plants. Here, we identified two ADH genes (CsADH1 and CsADH2). An in vitro protein expression assay showed that both CsADH1 and CsADH2 proteins can catalyze the reduction of (Z)-3-hexenal into (Z)-3-hexenol. Subcellular localization revealed that both CsADH1 and CsADH2 proteins were predominantly localized in the nucleus and cytosol. CsADH1 had high transcripts in young stems in autumn, while CsADH2 showed extremely high expression levels in stems and roots. The expression of CsADH2 was mainly downregulated under ABA treatment, while CsADH1 and CsADH2 transcripts were significantly lower under MeJA treatment at 12 and 24 h. Under cold treatment, CsADH1 transcripts first decreased and then increased, while CsADH2 demonstrated an almost opposite expression pattern. Notably, CsADH2 was significantly upregulated under simulated Ectropis obliqua invasion. Gene suppression by antisense oligonucleotides (AsODNs) demonstrated that AsODN_ADH2 treatment significantly reduced CsADH2 transcripts and the abundance of (Z)-3-hexenol products. The results indicate that the two CsADH genes may play an important role in response to (a)biotic stresses and in the process of (Z)-3-hexenol biosynthesis.

The Cellular and Subcellular Organization of the Glucosinolate–Myrosinase System against Herbivores and Pathogens

Glucosinolates are an important class of secondary metabolites in Brassicales plants with a critical role in chemical defense. Glucosinolates are chemically inactive but can be hydrolyzed by myrosinases to produce a range of chemically active compounds toxic to herbivores and pathogens, thereby constituting the glucosinolate–myrosinase defense system or the mustard oil bomb. During the evolution, Brassicales plants have developed not only complex biosynthetic pathways for production of a large number of glucosinolate structures but also different classes of myrosinases that differ in catalytic mechanisms and substrate specificity. Studies over the past several decades have made important progress in the understanding of the cellular and subcellular organization of the glucosinolate–myrosinase system for rapid and timely detonation of the mustard oil bomb upon tissue damage after herbivore feeding and pathogen infection. Progress has also been made in understanding the mechanisms that herbivores and pathogens have evolved to counter the mustard oil bomb. In this review, we summarize our current understanding of the function and organization of the glucosinolate–myrosinase system in Brassicales plants and discuss both the progresses and future challenges in addressing this complex defense system as an excellent model for analyzing plant chemical defense.

Herbivore-Induced Rice Volatiles Attract and Affect the Predation Ability of the Wolf Spiders, Pirata subpiraticus and Pardosa pseudoannulata

Spiders are important natural enemies of rice pests. Studying the effects of herbivore-induced rice volatiles on spider attraction and predation ability may lead to safer methods for pest prevention and control. In this study, four-arm olfactometer, predation ability experiment, and field trapping experiment were used to evaluate the effects of herbivore-induced rice volatiles on Pirata subpiraticus Bösenberg et Strand (Araneae: Lycosidae) and Pardosa pseudoannulata Bösenberg et Strand (Araneae: Lycosidae). The 0.5 μg/μL linalool concentration was attractive, and also shortened the predation latency in male P. subpiraticus and female P. pseudoannulata. The 0.5 μg/μL linalool concentration increased the daily predation capacity of female P. pseudoannulata. Male P. pseudoannulata were attracted to 1.0 g/L methyl salicylate, which also shortened their predation latency. In field experiments, methyl salicylate and linalool were effective for trapping spiders. Herbivore-induced rice volatiles attract rice field spiders and affect their predatory ability. These results suggest that herbivore-induced rice volatiles can be used to attract spiders and provide improved control of rice pests.

Chinese Cabbage Changes Its Release of Volatiles to Defend against Spodoptera litura

Simple Summary

Biological control is an important direction for pest control in the future, and chemical ecology is an indispensable part of biological control. Therefore, we tested the selection of Spodoptera litura and parasitic wasps on the volatiles of different treatments of cabbage and collected and analyzed the volatiles of different treatments of cabbage. This study found that cabbage was fed by Spodoptera litura to produce volatiles to avoid Spodoptera litura while also attracting Microplitis similis. As a result, some compounds were found to be related to the behavior of Spodoptera litura and Microplitis similis. These results provide a theoretical basis for searching for biological control resources and chemical control.

Abstract

Plants respond to herbivorous insect attacks by releasing volatiles that directly harm the herbivore or that indirectly harm the herbivore by attracting its natural enemies. Although the larvae of Spodoptera litura (the tobacco cutworm) are known to induce the release of host plant volatiles, the effects of such volatiles on host location by S. litura and by the parasitoid Microplitis similis, a natural enemy of S. litura larvae, are poorly understood. Here, we found that both the regurgitate of S. litura larvae and S. litura-infested cabbage leaves attracted M. similis. S. litura had a reduced preference for cabbage plants that had been infested with S. litura for 24 or 48 h. M. similis selection of plants was positively correlated with the release of limonene; linalool and hexadecane, and was negatively correlated with the release of (E)-2-hexenal and 1-Butene, 4-isothiocyanato. S. litura selection of plants was positively correlated with the release of (E)-2-hexenal, 1-Butene, 4-isothiocyanato, and decanal, and was negatively correlated with the release of limonene, nonanal, hexadecane, heptadecane, and octadecane. Our results indicate that host plant volatiles can regulate the behavior of S. litura and M. similis.

Eliciting Plant Defenses Through Herbivore-Induced Plant Volatiles’ Exposure in Sweet Peppers

Insect herbivory activates plant defense mechanisms and releases a blend of herbivore-induced plant volatiles (HIPVs). These volatile compounds may be involved in plant-plant communication and induce defense response in undamaged plants. In this work, we investigated whether the exposure of sweet pepper plants to HIPVs [(Z)-3-hexenol, (Z)-3-hexenyl acetate, (Z)-3-hexenyl propanoate, (Z)-3-hexenyl butanoate, hexyl butanoate, methyl salicylate and methyl jasmonate] activates the sweet pepper immune defense system. For this, healthy sweet pepper plants were individually exposed to the each of the above mentioned HIPVs over 48 h. The expression of jasmonic acid and salicylic acid related genes was quantified. Here, we show that all the tested volatiles induced plant defenses by upregulating the jasmonic acid and salicylic acid signaling pathway. Additionally, the response of Frankliniella occidentalis, a key sweet pepper pest, and Orius laevigatus, the main natural enemy of F. occidentalis, to HIPV-exposed sweet pepper plants were studied in a Y-tube olfactometer. Only plants exposed to (Z)-3-hexenyl propanoate and methyl salicylate repelled F. occidentalis whereas O. laevigatus showed a strong preference to plants exposed to (Z)-3-hexenol, (Z)-3-hexenyl propanoate, (Z)-3-hexenyl butanoate, methyl salicylate and methyl jasmonate. Our results show that HIPVs act as elicitors to sweet pepper plant defenses by enhancing defensive signaling pathways. We anticipate our results to be a starting point for integrating HIPVs-based approaches in sweet pepper pest management systems which may provide a sustainable strategy to manage insect pests in horticultural plants.

Priming seeds for the future: Plant immune memory and application in crop protection

Plants have evolved adaptive strategies to cope with pathogen infections that seriously threaten plant viability and crop productivity. Upon the perception of invading pathogens, the plant immune system is primed, establishing an immune memory that allows primed plants to respond more efficiently to the upcoming pathogen attacks. Physiological, transcriptional, metabolic, and epigenetic changes are induced during defense priming, which is essential to the establishment and maintenance of plant immune memory. As an environmental-friendly technique in crop protection, seed priming could effectively induce plant immune memory. In this review, we highlighted the recent advances in the establishment and maintenance mechanisms of plant defense priming and the immune memory associated, and discussed strategies and challenges in exploiting seed priming on crops to enhance disease resistance.

Nanoselenium Enhanced Wheat Resistance to Aphids by Regulating Biosynthesis of DIMBOA and Volatile Components

The mechanism of nanoselenium (nano-Se) improving the resistance induced by plant components to aphids is unclear. In this study, foliar sprayed nano-Se (5.0 mg/L) could significantly reduce the Sitobion avenae number (36%) compared with that in the control. Foliar application of nano-Se enhanced the antioxidant capacity by reducing malondialdehyde (MDA) and increasing GSH-Px, CAT, GSH, Pro, and VE concentrations in wheat seedlings. The phenylpropane pathway was activated by nano-Se biofortification, which increased apigenin and caffeic acid concentrations. The high-level expression of the related genes (TaBx1A, TaBx3A, TaBx4A, TaASMT2, and TaCOMT) induced the promotion of melatonin (88.6%) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) (64.3%). Different ratios of the secondary metabolites to nano-Se were taken to examine the effects on resistance of wheat to S. avenae. The results revealed that the combination of nano-Se and melatonin could achieve the best overall performance by reducing the S. avenae number by 52.2%. The study suggests that the coordinated applications of nano-Se and melatonin could more effectively improve the wheat resistance to aphids via the promotion of volatile organic compound synthesis and modulation in phenylpropane and indole metabolism pathways.

Sensing of Airborne Infochemicals for Green Pest Management: What Is the Challenge?

One of the biggest global challenges for our societies is to provide natural resources to the rapidly expanding population while maintaining sustainable and ecologically friendly products. The increasing public concern about toxic insecticides has resulted in the rapid development of alternative techniques based on natural infochemicals (ICs). ICs (e.g., pheromones, allelochemicals, volatile organic compounds) are secondary metabolites produced by plants and animals and used as information vectors governing their interactions. Such chemical language is the primary focus of chemical ecology, where behavior-modifying chemicals are used as tools for green pest management. The success of ecological programs highly depends on several factors, including the amount of ICs that enclose the crop, the range of their diffusion, and the uniformity of their application, which makes precise detection and quantification of ICs essential for efficient and profitable pest control. However, the sensing of such molecules remains challenging, and the number of devices able to detect ICs in air is so far limited. In this review, we will present the advances in sensing of ICs including biochemical sensors mimicking the olfactory system, chemical sensors, and sensor arrays (e-noses). We will also present several mathematical models used in integrated pest management to describe how ICs diffuse in the ambient air and how the structure of the odor plume affects the pest dynamics.

Managing the invasive plant Carpobrotus edulis: is mechanical control or specialized natural enemy more effective?

Carpobrotus edulis is an invasive clonal plant with drastic effects on biodiversity and functioning of coastal ecosystems. In recent years, authorities and land managers have implemented diverse management strategies that usually focus on mechanical removal and chemical control. However, applying mechanical control to remove C. edulis may cause indirect adverse effects since it could increase the probability of spreading new propagules, which do not lose their physiological activity. Therefore, reducing the physiological activity of these plant fragments should be a priority to avoid their spread and re-rooting. Our goal was to assess the plant regeneration capacity after applying mechanical control (i) when placing the plant material on different types of ground surface (on sand, on stones and using rooted plants as control) and (ii) combined with the attack of specialized herbivores (the soft scale Pulvinariella mesembryanthemi). To achieve this, we evaluated how these two factors (ground surface and herbivory) affected the plant physiological activity, its survival and re-rooting, biometric measurements, shoot and root nutrient composition and biochemical parameters (total phenols and tannins). Regardless of the ground surface type, our results indicated that the specialist herbivore greatly affected the C. edulis parameters studied. The attack of P. mesembryanthemi stimulated the plant defence mechanisms, even in those individuals with less photosynthetic activity. Furthermore, P. mesembryanthemi severely reduced the biomass and volume of plant material. Decomposition of C. edulis was accelerated by the combination between the inoculation of P. mesembryanthemi and placing the plants on the stones ground surface. Overall, preventing plant re-rooting by avoiding connection to the soil is an effective method of reducing its viability after the eighth-tenth month. After applying mechanical control, we recommend placing C. edulis fragments over an inert ground surface to avoid re-rooting, which would favour its death. We conclude that the combination of mechanical control and P. mesembryanthemi or even direct inoculation with this specialist herbivore could help authorities and land managers to improve management strategies for C. edulis.

Genome-wide transcriptome signatures of ant-farmed Squamellaria epiphytes reveal key functions in a unique symbiosis

Farming of fungi by ants, termites, or beetles has led to ecologically successful societies fueled by industrial-scale food production. Another type of obligate insect agriculture in Fiji involves the symbiosis between the ant Philidris nagasau and epiphytes in the genus Squamellaria (Rubiaceae) that the ants fertilize, defend, harvest, and depend on for nesting. All farmed Squamellaria form tubers (domatia) with preformed entrance holes and complex cavity networks occupied by P. nagasau. The inner surface of the domatia consists of smooth-surfaced walls where the ants nest and rear their brood, and warty-surfaced walls where they fertilize their crop by defecation. Here, we use RNA sequencing to identify gene expression patterns associated with the smooth versus warty wall types. Since wall differentiation occurred in the most recent common ancestor of all farmed species of Squamellaria, our study also identifies genetic pathways co-opted following the emergence of agriculture. Warty-surfaced walls show many upregulated genes linked to auxin transport, root development, and nitrogen transport consistent with their root-like function; their defense-related genes are also upregulated, probably to protect these permeable areas from pathogen entry. In smooth-surfaced walls, genes functioning in suberin and wax biosynthesis are upregulated, contributing to the formation of an impermeable ant-nesting area in the domatium. This study throws light on a number of functional characteristics of plant farming by ants and illustrates the power of genomic studies of symbiosis.

Soybean Cyst Nematodes Influence Aboveground Plant Volatile Signals Prior to Symptom Development

Soybean cyst nematode (SCN), Heterodera glycines, is one of the most destructive soybean pests worldwide. Unlike many diseases, SCN doesn't show above ground evidence of disease until several weeks after infestation. Knowledge of Volatile Organic Compounds (VOCs) related to pests and pathogens of foliar tissue is extensive, however, information related to above ground VOCs in response to root damage is lacking. In temporal studies, gas chromatography-mass spectrometry analysis of VOCs from the foliar tissues of SCN infested plants yielded 107 VOCs, referred to as Common Plant Volatiles (CPVs), 33 with confirmed identities. Plants showed no significant stunting until 10 days after infestation. Total CPVs increased over time and were significantly higher from SCN infested plants compared to mock infested plants post 7 days after infestation (DAI). Hierarchical clustering analysis of expression ratios (SCN: Mock) across all time points revealed 5 groups, with the largest group containing VOCs elevated in response to SCN infestation. Linear projection of Principal Component Analysis clearly separated SCN infested from mock infested plants at time points 5, 7, 10 and 14 DAI. Elevated Styrene (CPV11), D-Limonene (CPV32), Tetradecane (CPV65), 2,6-Di-T-butyl-4-methylene-2,5-cyclohexadiene-1-one (CPV74), Butylated Hydroxytoluene (CPV76) and suppressed Ethylhexyl benzoate (CPV87) levels, were associated with SCN infestation prior to stunting. Our findings demonstrate that SCN infestation elevates the release of certain VOCs from foliage and that some are evident prior to symptom development. VOCs associated with SCN infestations prior to symptom development may be valuable for innovative diagnostic approaches.

Herbivory-Induced Emission of Volatile Terpenes in Chrysanthemum morifolium Functions as an Indirect Defense against Spodoptera litura Larvae by Attracting Natural Enemies

Indirect defense is an important strategy employed by sessile plants to defend against herbivorous insects by recruiting the natural enemies of herbivores mediated by herbivore-induced plant volatiles (HIPVs). We aimed to determine whether indirect defense occurs in Compositae with Chrysanthemum morifolium as the model and elucidate the mechanisms underlying the biosynthesis of HIPVs. Using two-choice olfactometer bioassays, we showed that C. morifolium plants following infestation by larvae of the tobacco cutworm (Spodoptera litura, TCW) were significantly more attractive to two natural enemies of TCW larvae than control plants, indicating that indirect defense is an active defense strategy of C. morifolium. The chemical cue responsible for indirect defense in C. morifolium was identified as a complex blend of volatiles predominated by sesquiterpenes and monoterpenes. A total of 11 candidate terpene synthase (TPS) genes were identified by comparing the transcriptomes of healthy and TCW-infested plants. Gene expression analysis confirmed that up-regulated CmjTPS genes are consistent with the elevated emission of terpenes after herbivory treatment. Our study showed that the herbivore-induced emission of JA-dependent volatile terpenes attracted both predatory and parasitic enemies of herbivores. Generally, our findings indicate that indirect defense might be an important defense mechanism against insects in C. morifolium.

Changes in plant responses induced by an arthropod influence the colonization behavior of a subsequent herbivore

BACKGROUND

Plants in nature can be sequentially attacked by different arthropod herbivores. Feeding by one arthropod species may induce plant-defense responses that might affect the performance of a later-arriving herbivorous species. Understanding these interactions can help in developing pest-management strategies. In tomato, the sweet-potato whitefly Bemisia tabaci and the two-spotted spider mite Tetranychus urticae are key pests that frequently cohabit on the same plant. We studied whether colonization by one species can either facilitate or impede later colonization of tomato plants by conspecific or heterospecific individuals.

RESULTS

B. tabaci females showed a strong preference for and increased oviposition on plants previously colonized by conspecifics. In contrast, plants infested with T. urticae repelled B. tabaci females and reduced their oviposition rate by 86%. Although females of T. urticae showed no preference between conspecific-infested or uninfested plants, we observed a 50% reduction in the number of eggs laid on conspecific-infested plants. Both herbivorous arthropods up-regulated the expression of genes involving the jasmonic acid and abscisic acid pathways, increasing emissions of fatty-acid derivatives, but only B. tabaci increased the expression of genes related to the salicylic acid pathway and the total amount of phenylpropanoids released. Terpenoids were the most abundant compounds in the volatile blends; many terpenoids were emitted at different rates, which might have influenced the arthropods' host selection.

CONCLUSION

Our results indicate that B. tabaci infestation facilitated subsequent infestations by conspecifics and mites, while T. urticae infestation promoted herbivore-induced resistance. Based on both the molecular and behavioral findings, a novel sustainable pest-management strategy is discussed.

Characterization of Eggplant Genotypes for Different Resistance Mechanisms Against Leucinodes orbonalis

Host plant resistance mechanisms play an important role in developing cultivars with resistance to the target pests; information regarding morphological and biochemical factors contributing to the resistance is essential for developing pest-resistant cultivars. As a result, we investigated the contribution of various morphological and biochemical characters in forty-two eggplant genotypes against Leucinodes orbonalis Guenée, in Himachal Pradesh, India. Out of all the phenotypic parameters evaluated, pericarp thickness (r = 0.89) has significantly positive correlation with fruit infestation, whereas trichome density had significantly negative correlation (r = - 0.89). Analysis of the biochemical compounds in the eggplant genotypes revealed that total phenols (r = - 0.71), polyphenol oxidase (r = - 0.63), peroxidases (r = - 0.35), phenylalanine ammonium lyase (r = - 0.71) and solasodine (r = - 0.81) had significantly negative correlation with the per cent fruit infestation by L. orbonalis while the reducing sugars (r = 0.66) and non-reducing sugars (r = 0.62) showed a significantly positive correlation. Molecular characterization by random amplified polymorphic DNA (RAPD) primers also revealed the presence of high genetic diversity among different eggplant genotypes, where 17 polymorphic RAPD primers produced a total of 167 amplicons, among which 144 amplicons were polymorphic and 23 monomorphic bands. PCR-amplified DNA fragment size ranged from 100 to 2500 bp, mean polymorphism was 86.42% and the average PIC value was 0.444. Jaccards coefficient-based dendrogram grouped 40 eggplant genotypes into two major clusters. Results also revealed that the resistant genotypes accumulated higher levels of defensive biochemical enzymes such as phenols, PO, PPO, PAL and solasodine to confer insect resistance. Molecular characterization also revealed that genotypes in the present study were genetically diverse and could be used in future breeding and improvement programmes in this crop. Genotypes, IC411485 and IC090951, in particular, can be used as varied parents in breeding programmes to generate improved lines in terms of resistance to L. orbonalis.

Induced plasticity alters responses to conspecific interactions in seedlings of a perennial grass

Plants can interact with different individuals in their lifetime which may lead to plastic response that affect performance. If conspecific interactions are altered through previous plastic responses that could affect stabilizing niche mechanisms, in which conspecifics compete more intensely to promote diversity and coexistence. Here, I show interactions between Pascopyrum smithii and conspecifics resulted in largely canalized traits, whereas P. smithii with an invasive grass, Bromus tectorum resulted in plastic responses for root mass (p = 0.02), shoot mass (p < 0.0001), root mass fraction (p = 0.003) and plant height (p < 0.0001). A subset of individuals transplanted from these two interaction treatments which were moved with new, same aged conspecifics showed that previous interactions led to differing trait relationships: increases in the number of leaves for the interspecific-induced plants were related to increases in non-focal leaf production, whereas increases in the number of leaves for the intraspecific-induced plants were related to decreases in the non-focal plants (R² = 0.52, p = 0.006). These results suggest that previous intraspecific interactions intensify conspecific competition and stabilize subsequent interactions with conspecifics by imposing greater competition, and that invasive-interspecific interactions can weaken stabilizing niche mechanisms, thus negatively influencing species coexistence.

Induced volatiles in the interaction between soybean (Glycine max) and the Mexican soybean weevil (Rhyssomatus nigerrimus)

The present study analyzed the volatile compounds emitted by Glycine max (cv. FT-Cristalina-RCH) soybean plants: healthy plants and plants damaged mechanically or by the Mexican soybean weevil Rhyssomatus nigerrimus. The SPME method was used to compare the volatile profile of soybean plants in four different conditions. The volatile profile of G. max plants infested by R. nigerrimus was qualitatively and quantitatively different from that of healthy and mechanically damaged plants. Emission of 59 compounds was detected in the four treatments. Of these compounds, 19 were identified by comparison of the Kovats index, mass spectrum and retention times with those of synthetic standards. An increase in concentration of the volatiles (Z)-3-hexenyl acetate and the compound 1-octen-3-ol was observed when the soybean plants were mechanically damaged. The compounds mostly produced by the soybean plant during infestation by male and female R. nigerrimus were 1-octen-3-ol, 6-methyl-5-hepten-2-one, (E)-β-ocimene, salicylaldehyde, unknown 10, linalool, methyl salicylate, (Z)-8-dodecenyl acetate (ester 5), ketone 2 and geranyl acetone. Behavioral effects of the identified compounds during the insect-plant interaction and their conspecifics are discussed.

In the tripartite combination Botrytis cinerea-Arabidopsis-Eurydema oleracea, the fungal pathogen alters the plant-insect interaction via jasmonic acid signalling activation and inducible plant-emitted volatiles

In ecosystems, plants are continuously challenged by combined stress conditions more than by a single biotic or abiotic factor. Consequently, in recent years studies on plant relationships with multiple stresses have aroused increasing interest. Here, the impact of inoculation with fungal pathogens with different lifestyles on Arabidopsis plants response to the following infestation with the invasive crop pest Eurydema oleracea was investigated. In particular, as fungal pathogens the necrotroph Botrytis cinerea and the biotroph Golovinomyces orontii were used. Plants exposed to B. cinerea, but not to G. orontii, showed reduced herbivore feeding damage. This difference was associated to different hormonal pathways triggered by the pathogens: G. orontii only induced the salicylate-mediated pathway, while B. cinerea stimulated also the jasmonate-dependent signalling, which persisted for a long time providing a long-term defence to further herbivore attack. In particular, the lower susceptibility of B. cinerea-infected Arabidopsis plants to E. oleracea was related to the stimulation of the JA-induced pathway on the production of plant volatile compounds, since treatment with VOCs emitted by B. cinerea inoculated plants inhibited both insect plant choice and feeding damage. These results indicate that necrotrophic plant pathogenic fungi modulate host volatile emission, thus affecting plant response to subsequent insect, thereby increasing the knowledge on tripartite plant-microbe-insect interactions in nature.

Plant volatiles as regulators of plant defense and herbivore immunity: molecular mechanisms and unanswered questions

Plants release distinct blends of herbivore-induced plant volatiles (HIPVs) upon herbivore attack. HIPVs have long been known to influence the behavior of herbivores and natural enemies. In addition, HIPVs can act as physiological regulators that induce or prime plant defenses. Recent work indicates that the regulatory capacity of HIPVs may extend to herbivore immunity: herbivores that are exposed to HIPVs can become more resistant or susceptible to parasitoids and pathogens. While the mechanisms of HIPV-mediated plant defense regulation are being unraveled, the mechanisms underlying the regulation of herbivore immunity are unclear. Evidence so far suggests a high degree of context dependency. Here, we review the mechanisms by which HIPVs regulate plant defense and herbivore immunity. We address major gaps of knowledge and discuss directions for future mechanistic research to facilitate efforts to use the regulatory capacity of HIPVs for the biological control of insect pests.

Exposure of Helicoverpa armigera Larvae to Plant Volatile Organic Compounds Induces Cytochrome P450 Monooxygenases and Enhances Larval Tolerance to the Insecticide Methomyl

Plants release an array of volatile chemicals into the air to communicate with other organisms in the environment. Insect attack triggers emission of herbivore-induced plant volatiles (HIPVs). How insect herbivores use these odors to plan their detoxification systems is vital for insect adaptation to environmental xenobiotics. Here we show that the larvae of Helicoverpa armigera (Hübner), a broadly polyphagous lepidopteran herbivore, have the capacity to use plant volatiles as cues to upregulate multiple detoxification systems, including cytochrome P450 monooxygenases (P450s), for detoxification of insecticides. Olfactory exposure of the fifth instars to two terpene volatiles limonene and nerolidol, and two green-leaf volatiles 2-heptanone and cis-3-hexenyl acetate significantly reduced larval susceptibility to the insecticide methomyl. However, larval pretreatment with piperonyl butoxide (PBO), a known P450 inhibitor, neutralized the effects of volatile exposure. Furthermore, larval exposure to the four plant volatiles enhanced activities of P450 enzymes in midguts and fatbodies, and upregulated expression of CYP6B2, CYP6B6 and CYP6B7, P450s involved in detoxification of the insecticide. Larval exposure to 2-heptanone and limonene volatiles also enhanced activities of glutathione-s-transferase and carboxylesterase. Our findings suggest that olfactory exposure to HIPVs enhances larval insecticide tolerance via induction of detoxification P450s.