Indirect Plant Defense Research Articles

Chemical and biological studies of natural and synthetic products for the highly selective control of pest insect species.

Tanacetum cinerariifolium was known to produce pyrethrins, but the mechanism of pyrethrin biosynthesis was largely unclear. The author showed that the nonmevalonate and oxylipin pathways underlie biosynthesis of the acid and alcohol moieties, respectively, and a GDSL lipase joins the products of these pathways. A blend of the green leaf volatiles and (E)-β-farnesene mediates the induction of wounding responses to neighboring intact conspecies by enhancing pyrethrin biosynthesis. Plants fight against herbivores underground as well as aboveground, and, in soy pulps, some fungi produce compounds selectively modulating ion channels in insect nervous system. The author proposed that indirect defense of plants occurs where microorganisms produce defense substances in the rhizosphere. Broad-spectrum pesticides, including neonicotinoids, may affect nontarget organisms. The author discovered cofactors enabling functional expression of insect nicotinic acetylcholine receptors (nAChRs). This led to understanding the mechanism of insect nAChR-neonicotinoid interactions, thus paving new avenues for controlling crop pests and disease vectors.

Feeding Volatiles of Larval Sparganothis pilleriana (Lepidoptera: Tortricidae) Attract Heterospecific Adults of the European Grapevine Moth.

Plants release volatiles in response to caterpillar feeding. These herbivore-induced plant volatiles (HIPVs) attract natural enemies of the herbivores and repel or attract conspecific adult herbivores in a tri-trophic interaction which has been considered to be an indirect plant defense against herbivores. Recently, we demonstrated the attraction of male and female European grapevine moth, Lobesia botrana (Denis & Schiffermüller) (Lepidoptera: Tortricidae) to a blend of phenylacetonitrile and acetic acid, two compounds identified as HIPVs in heterospecific apple-leafroller interactions. The ecological basis of our findings is not clearly understood. Thus, this work was undertaken to investigate HIPVs in the grapevine-leafroller interaction and study the response of heterospecific adults L. botrana, to these volatiles. We collected headspace volatiles emitted from uninfested grapevines and grapevines infested with larvae of a generalist herbivore, the grapevine leafroller moth, Sparganothis pilleriana (Denis & Schiffermüller), and analyzed them using gas chromatography/mass spectrometry. Infested grape leaves released three compounds (phenylacetonitrile, indole, and 2-phenylethanol) not found from uninfested leaves. Nine different blends, comprising a full factorial set of the three compounds with each blend containing acetic acid, were tested in a field-cage trial. Only lures containing phenylacetonitrile caused a significant increase in trap catches compared to the other lures and blank traps. Electroantennographic tests show that L. botrana can detect the compounds. The results confirm our hypothesis that phenylacetonitrile is released during grapevines infestation with herbivores, and attracts adult L. botrana.

Distance Dependent Contribution of Ants to Pollination but Not Defense in a Dioecious, Ambophilous Gymnosperm.

Dioecious plants are obligate outcrossers with separate male and female individuals, which can result in decreased seed set with increasing distance between the sexes. Wind pollination is a common correlate of dioecy, yet combined wind and insect pollination (ambophily) could be advantageous in compensating for decreased pollen flow to isolated females. Dioecious, ambophilous gymnosperms Ephedra (Gnetales) secrete pollination drops (PDs) in female cones that capture airborne pollen and attract ants that feed on them. Plant sugary secretions commonly reward ants in exchange for indirect plant defense against herbivores, and more rarely for pollination. We conducted field experiments to investigate whether ants are pollinators and/or plant defenders of South American Ephedra triandra, and whether their contribution to seed set and seed cone protection varies with distance between female and male plants. We quantified pollen flow in the wind and assessed the effectiveness of ants as pollinators by investigating their relative contribution to seed set, and their visitation rate in female plants at increasing distance from the nearest male. Ants accounted for most insect visits to female cones of E. triandra, where they consumed PDs, and pollen load was larger on bigger ants without reduction in pollen viability. While wind pollination was the main contributor to seed set overall, the relative contribution of ants was distance dependent. Ant contribution to seed set was not significant at shorter distances, yet at the farthest distance from the nearest male (23 m), where 20 times less pollen reached females, ants enhanced seed set by 30% compared to plants depending solely on wind pollination. We found no evidence that ants contribute to plant defense by preventing seed cone damage. Our results suggest that, despite their short-range movements, ants can offset pollen limitation in isolated females of wind-pollinated plants with separate sexes. We propose that ants enhance plant reproductive success via targeted delivery of airborne pollen, through frequent contact with ovule tips while consuming PDs. Our study constitutes the first experimental quantification of distance-dependent contribution of ants to pollination and provides a working hypothesis for ambophily in other dioecious plants lacking pollinator reward in male plants.

The enemy of my enemy is not always my friend: Negative effects of carnivorous arthropods on plants

Abstract Plants are members of complex communities of which arthropods are the most speciose members. The role of carnivores in shaping the outcome of multi‐trophic interactions by top‐down control of herbivores has been well studied. Particularly, the positive impacts of natural enemies of herbivores on plants through direct (consumptive) and indirect (non‐consumptive) effects on their prey and hosts have received considerable interest, while multi‐trophic interactions that result in negative effects on plants have received little attention. Negative impacts of carnivorous arthropods have been documented and arise when carnivores directly affect plants and/or their interactions with beneficial arthropods. In general, negative effects may be compensated by positive effects of other carnivorous arthropods, but their presence and significance is likely to be underestimated in tri‐trophic interactions. Recent studies have revealed that the composition and dynamics of the plant and arthropod community have a significant effect on plant fitness. Therefore, we encourage an approach that accounts for a larger community of species and interactions associated with plants, including interaction types in which carnivores may negatively affect plants. This review highlights specific interaction types that ultimately lead to negative effects of carnivores on plants. This synthesis presents alternative hypotheses to those that predict that carnivores invariably benefit plants. Testing potential costs and benefits of carnivores to plants will advance our understanding of indirect plant defence. A free Plain Language Summary can be found within the Supporting Information of this article.

Volatile emission and biosynthesis in endophytic fungi colonizing black poplar leaves.

Plant volatiles play a major role in plant–insect interactions as defense compounds or attractants for insect herbivores. Recent studies have shown that endophytic fungi are also able to produce volatiles and this raises the question of whether these fungal volatiles influence plant–insect interactions. Here, we qualitatively investigated the volatiles released from 13 endophytic fungal species isolated from leaves of mature black poplar (Populus nigra) trees. The volatile blends of these endophytes grown on agar medium consist of typical fungal compounds, including aliphatic alcohols, ketones and esters, the aromatic alcohol 2-phenylethanol and various sesquiterpenes. Some of the compounds were previously reported as constituents of the poplar volatile blend. For one endophyte, a species of Cladosporium, we isolated and characterized two sesquiterpene synthases that can produce a number of mono- and sesquiterpenes like (E)-β-ocimene and (E)-β-caryophyllene, compounds that are dominant components of the herbivore-induced volatile bouquet of black poplar trees. As several of the fungus-derived volatiles like 2-phenylethanol, 3-methyl-1-butanol and the sesquiterpene (E)-β-caryophyllene, are known to play a role in direct and indirect plant defense, the emission of volatiles from endophytic microbial species should be considered in future studies investigating tree-insect interactions.

Indirect plant defenses: volatile organic compounds and extrafloral nectar

Indirect plant defense is an important component in regulating population dynamics and the structure of numerous communities in different ecosystems. These defenses mainly involve the emission of volatile organic compounds (VOCs) induced after damage by an herbivore. VOCs can play several ecological roles, such as help natural enemies locate prey or hosts, attract or repel other herbivores, mediate communication between neighboring plants and among different parts of the same plant, and affect the behavior of pollinators and seed dispersers. In addition to VOCs, some plants produce extrafloral nectar, which can enhance plant defense, by attracting, retaining, and increasing the efficiency of some natural enemies; however, among the associated costs, these compounds can attract other herbivorous species and exclude some natural enemies due to competition. Many factors can influence the production of indirect defenses by plants, such as the individual species, life stage, density of herbivores, age, abiotic factors, as well as the association of plants with symbiotic microorganisms. The potential of indirect plant defenses to reduce herbivory and increase the plant fitness has been well demonstrated. Indirect plant defenses may have ecological costs but can express phenotypic plasticity, as plants can reduce or increase the production of defenses according to the associated herbivory rate. Such variable expression of characteristics provides a barrier against the evolution of resistance by the associated herbivores. In this article, we intend to provide a review on volatile organic compounds and extrafloral nectar as indirect plant defenses, including some costs and benefits of these defense mechanisms.

Transcriptomic and metabolomic reprogramming in cotton after Apolygus lucorum feeding implicated in enhancing recruitment of the parasitoid Peristenus spretus

Different types of insect feeding can activate distinct plant resistance mechanisms and trigger the generation of specific volatile compounds. Transcriptomic changes and the genomic basis underlying plant defense in response to chewing herbivores or phloem-feeding insects have been relatively well investigated, while better insight into molecular mechanisms underlying the plant defense response, in particular, volatile emissions triggered by sap-feeding insects such as the green plant bug Apolygus lucorum is needed. Here, we monitored transcriptomic and volatile metabolomic changes in cotton over time during A. lucorum infestation. RNA-seq analysis showed that 1614 transcripts were differentially expressed (log2|Ratio| ≥ 1; q ≤ 0.05) in cotton leaves infested by A. lucorum. All differentially expressed genes (DEGs) in jasmonic acid (JA; 48 genes) and salicylic acid (SA; 5 genes) pathways were upregulated, highlighting a central role of JA in A. lucorum-induced signaling without attenuating the SA pathway. Moreover, all DEGs (30 genes) involved in herbivore-induced volatile biosynthesis were upregulated. Consistently, A. lucorum-induced cotton volatile blends and synthetic methyl salicylate significantly attracted the parasitoid Peristenus spretus. The present data indicated that cotton plants after A. lucorum infestation undergo rapid, extensive transcriptome reprogramming mediated by complex signaling networks in which the JA and SA pathways act synergistically, leading to a specific volatile profile involved in an indirect plant defense.

Engineering Nicotiana tabacum for the de novo biosynthesis of DMNT to regulate orientation behavior of the parasitoid wasps Microplitis mediator.

(E)-4,8-dimethylnona-1,3,7-triene (DMNT), one of the homoterpenes, is thought to contribute to plant indirect defense against insect herbivores. DMNT-enriched plants have great application potential to regulate insect behavior in the 'push & pull' strategy of pest management. However, de novo biosynthesis of DMNT in plants without a homoterpene metabolic pathway in their wild type is still not achieved, and the role of DMNT played in these plants and their interacted insects remains unclear. Cytochrome P450s and terpene synthases involved in homoterpenes biosynthesis in cotton plants were employed to generate DMNT-releasing tobacco plants. Single GhTPS14 transgenic Nicotiana tabacum only emitted (E)-nerolidol, the precursor of DMNT. Transgenic tobaccos expressing single GhCYP82Ls were unable to produce DMNT or TMTT, while DMNT was detected when exogenous (E)-nerolidol was added. Compared to wild-type plants, only co-expression of GhCYP82Ls and GhTPS14 in transgenic tobaccos triggered the constitutive release of single-component DMNT. Furthermore, DMNT-emitting transgenic tobacco plants, whether infested with Helicoverpa armigera larvae or not, significantly incited orientation behavior of parasitoid wasps Microplitis mediator. Wild type N. tabacum plants have no DMNT metabolic pathway. DMNT could be de novo biosynthesized via co-expression of GhCYP82Ls and GhTPS14. What is more, the parasitoid wasp M. mediator could be recruited by DMNT-releasing transgenic tobaccos, especially by H. armigera-infested transgenic tobaccos, suggesting the potential roles of engineered N. tabacum in regulating the behavioral preference of M. mediator.

Identification and characterization of two sesquiterpene synthase genes involved in volatile-mediated defense in tea plant (Camellia sinensis)

Terpenes and their derivatives are vital components of tea aroma. Their constitution and quantity are highly important criteria for the sensory evaluation of teas. Biologically, terpenes are involved in chemical resistance of tea plant against biotic and/or abiotic stresses. The goal of this study is to identify volatile terpenes of tea plants implicated in defense against herbivores and to identify terpene synthase (TPS) genes for their biosynthesis. Upon herbivory by tea geometrid (Ectropis obliqua Prout), tea plants were found to emit two sesquiterpenes, (E, E)-α-farnesene and (E)-nerolidol, which were undetectable in intact tea plants. The induced emission of (E, E)-α-farnesene and (E)-nerolidol suggests that they function in either direct or indirect defense of tea plants against the tea geometrid. Candidate TPS genes were identified from the transcriptomes of tea plants infested by tea geometrids. Two dedicated sesquiterpene synthases, CsAFR and CsNES2, were identified. CsAFR belongs to the TPS-b clade and can catalyze the formation of (E, E)-α-farnesene from (E, E)-FPP. CsNES2 belongs to the TPS-g clade and can synthesize (E)-nerolidol using (E, E)-FPP. The two genes were also both dramatically upregulated by herbivore damage. In summary, we showed that two novel sesquiterpene synthase genes CsAFR and CsNSE2 are inducible by herbivory and responsible for the elevated emission of herbivore-induced (E, E)-α-farnesene and (E)-nerolidol, which are implicated in tea plant defense against herbivores.

Two farnesyl pyrophosphate synthases, GhFPS1–2, in Gossypium hirsutum are involved in the biosynthesis of farnesol to attract parasitoid wasps

Sesquiterpenoids play an import role in the direct or indirect defense of plants. Farnesyl pyrophosphate synthases (FPSs) catalyze the biosynthesis of farnesyl pyrophosphate, which is a key precursor of farnesol and (E)-β-farnesene. In the current study, two FPS genes in Gossypium hirsutum, GhFPS1 and GhFPS2, were heterologously cloned and functionally characterized in a greenhouse setting. The open reading frames for full-length GhFPS1 and GhFPS2 were each 1 029 nucleotides, and encoded two proteins of 342 amino acids with molecular weights of 39.4 kDa. The deduced amino acid sequences of GhFPS1–2 showed high identity to FPSs of other plants. Quantitative real-time PCR analysis revealed that GhFPS1 and GhFPS2 were highly expressed in G. hirsutum leaves, and were upregulated in methyl jasmonate (MeJA)-, methyl salicylate (MeSA)- and aphid infestation-treated cotton plants. The recombinant proteins of either GhFPS1 or GhFPS2 plus calf intestinal alkaline phosphatase could convert geranyl diphosphate (GPP) or isopentenyl diphosphate (IPP) to one major product, farnesol. Moreover, in electrophysiological response and Y-tube olfactometer assays, farnesol showed obvious attractiveness to female Aphidius gifuensis, which is an important parasitic wasp of aphids. Our findings suggest that two GhFPSs are involved in farnesol biosynthesis and they play a crucial role in indirect defense of cotton against aphid infestation.

Risky roots and careful herbivores: Sustained herbivory by a root‐feeding herbivore attenuates indirect plant defences

Abstract Above‐ground plant tissues produce characteristic blends of volatile compounds in response to insect herbivory. These herbivore‐induced plant volatiles (HIPVs) function in plant defence and mediate foraging decisions by herbivores and their natural enemies. The ecological roles of HIPVs as foraging cues for different trophic levels highlight an important conflict for herbivores that need to locate suitable host plants while avoiding competition and predation. Plant roots also emit HIPVs following herbivory, but our understanding of root‐produced volatiles and their ecological functions in soil environments remains limited. Moreover, recent studies have documented the effects of temporal dynamics of plant volatile production on ecological interactions, but little is known about how root HIPVs change throughout herbivory or the resulting ecological implications from such changes. In this study, we examined the roles of HIPVs from roots of cucumber plants Cucumis sativus as foraging cues for a specialist herbivore, striped cucumber beetle Acalymma vittatum and its natural enemies, entomopathogenic nematodes (EPNs). We predicted HIPVs from A. vittatum‐damaged roots would attract EPNs, while repelling conspecific larvae that avoid competition, induced plant defences and increased risk of predation by EPNs. To capture the temporal dynamics of root HIPVs, we determined how HIPV‐mediated interactions change over time with sustained herbivory. Initially (after 24 hr), A. vittatum herbivory on C. sativus, or mechanical wounding, induced greater production of root volatiles. These root HIPVs recruited EPNs and repelled foraging A. vittatum larvae, although larval performance was not affected by prior damage. Sustained (7 days) herbivory by larvae reduced HIPVs to levels indistinguishable from undamaged control roots while mechanically damaged roots continued to produce higher levels of volatiles. Attenuation of HIPVs impaired indirect defence responses of C. sativus by reducing recruitment of EPNs and deterrence of A. vittatum larvae. These results suggest that root HIPVs function as honest signals that indicate the presence of herbivores, induction of indirect plant defences and increased risk of predation by natural enemies. However, some herbivores may overcome this line of plant defence by attenuating production of HIPVs and thus altering the outcomes of subsequent interactions among plants, herbivores and natural enemies. A free Plain Language Summary can be found within the Supporting Information of this article.

Phaeophyceaean (Brown Algal) Extracts Activate Plant Defense Systems in Arabidopsis thaliana Challenged With Phytophthora cinnamomi.

Seaweed extracts are important sources of plant biostimulants that boost agricultural productivity to meet current world demand. The ability of seaweed extracts based on either of the Phaeophyceaean species Ascophyllum nodosum or Durvillaea potatorum to enhance plant growth or suppress plant disease have recently been shown. However, very limited information is available on the mechanisms of suppression of plant disease by such extracts. In addition, there is no information on the ability of a combination of extracts from A. nodosum and D. potatorum to suppress a plant pathogen or to induce plant defense. The present study has explored the transcriptome, using RNA-seq, of Arabidopsis thaliana following treatment with extracts from the two species, or a mixture of both, prior to inoculation with the root pathogen Phytophthora cinnamomi. Following inoculation, five time points (0−24 h post-inoculation) that represented early stages in the interaction of the pathogen with its host were assessed for each treatment and compared with their respective water controls. Wide scale transcriptome reprogramming occurred predominantly related to phytohormone biosynthesis and signaling, changes in metabolic processes and cell wall biosynthesis, there was a broad induction of proteolysis pathways, a respiratory burst and numerous defense-related responses were induced. The induction by each seaweed extract of defense-related genes coincident with the time of inoculation showed that the plants were primed for defense prior to infection. Each seaweed extract acted differently in inducing plant defense-related genes. However, major systemic acquired resistance (SAR)-related genes as well as salicylic acid-regulated marker genes (PR1, PR5, and NPR1) and auxin associated genes were found to be commonly up-regulated compared with the controls following treatment with each seaweed extract. Moreover, each seaweed extract suppressed P. cinnamomi growth within the roots of inoculated A. thaliana by the early induction of defense pathways and likely through ROS-based signaling pathways that were linked to production of ROS. Collectively, the RNA-seq transcriptome analysis revealed the induction by seaweed extracts of suites of genes that are associated with direct or indirect plant defense in addition to responses that require cellular energy to maintain plant growth during biotic stress.

Generalising indirect defence and resistance of plants

Indirect defence, the adaptive top-down control of herbivores by plant traits that enhance predation, is a central component of plant-herbivore interactions. However, the scope of interactions that comprise indirect defence and associated ecological and evolutionary processes has not been clearly defined. We argue that the range of plant traits that mediate indirect defence is much greater than previously thought, and wefurther organise major concepts surrounding their ecological functioning. Despite the wide range of plant traits and interacting organisms involved, indirect defences show commonalities when grouped. These categories are based on whether indirect defences boost natural enemy abundance via food or shelter resources, or, alternatively, increase natural enemy foraging efficiency via information or alteration of habitat complexity. The benefits of indirect defences to natural enemies should be further explored to establish the conditions in which indirect defence generates a plant-natural enemy mutualism. By considering the broader scope of plant-herbivore-natural enemy interactions that comprise indirect defence, we can better understand plant-based food webs, as well as the evolutionary processes that have shaped them.

Nonglandular silicified trichomes are essential for rice defense against chewing herbivores.

Interspecific New Rice for Africa (NERICA) varieties have been recently developed and used in Sub-Saharan Africa but herbivore resistance properties of these plants remain poorly understood. Here we report that, compared to a local Japanese cultivar Nipponbare, NERICA 1, 4 and 10 are significantly more damaged by insect herbivores in the paddy fields. In contrast to high levels of leaf damage from rice skippers and grasshoppers, constitutive and induced volatile organic compounds for indirect plant defense were higher or similar in NERICAs and Nipponbare. Accumulation of direct defense secondary metabolites, momilactones A and B, and p-coumaroylputrescine (CoP) was reduced in NERICAs, while feruloylputrescine accumulated at similar levels in all varieties. Finally, we found that Nipponbare leaves were covered with sharp nonglandular trichomes impregnated with silicon but comparable defense structures were virtually absent in herbivory-prone NERICA plants. As damage to the larval gut membranes by Nipponbare silicified trichomes that pass intact through the insect digestive system, occurs, and larval performance is enhanced by trichome removal from otherwise chemically defended Nipponbare plants, we propose that silicified trichomes work as an important defense mechanism of rice against chewing insect herbivores.

Metabolome Analysis Identified Okaramines in the Soybean Rhizosphere as a Legacy of Hairy Vetch.

Inter-organismal communications below ground, such as plant–microbe interactions in the rhizosphere, affect plant growth. Metabolites are shown to play important roles in biological communication, but there still remain a large number of metabolites in soil to be uncovered. Metabolomics, a technique for the comprehensive analysis of metabolites in samples, may uncover the molecules that intermediate these interactions. We conducted a multivariate analysis using liquid chromatography (LC)—mass spectrometry (MS)-based untargeted metabolomics in several soil samples and also targeted metabolome analysis for the identification of the candidate compounds in soil. We identified okaramine A, B, and C in the rhizosphere soil of hairy vetch. Okaramines are indole alkaloids first identified in soybean pulp (okara) inoculated with Penicillium simplicissimum AK-40 and are insecticidal. Okaramine B was detected in the rhizosphere from an open field growing hairy vetch. Okaramine B was also detected in both bulk and rhizosphere soils of soybean grown following hairy vetch, but not detected in soils of soybean without hairy vetch growth. These results suggested that okaramines might be involved in indirect defense of plants against insects. To our knowledge, this is the first report of okaramines in the natural environment. Untargeted and targeted metabolomics would be useful to uncover the chemistry of the rhizosphere.

Tritrophic interactions follow phylogenetic escalation and climatic adaptation

One major goal in plant evolutionary ecology is to address how and why tritrophic interactions mediated by phytochemical plant defences vary across species, space, and time. In this study, we tested three classical hypotheses about plant defences: (i) the resource-availability hypothesis, (ii) the altitudinal/elevational gradient hypothesis and (iii) the defence escalation hypothesis. For this purpose, predatory soil nematodes were challenged to hunt for root herbivores based on volatile cues from damaged or intact roots of 18 Alpine Festuca grass species adapted to distinct climatic niches spanning 2000 meters of elevation. We found that adaptation into harsh, nutrient-limited alpine environments coincided with the production of specific blends of volatiles, highly attractive for nematodes. We also found that recently-diverged taxa exposed to herbivores released higher amounts of volatiles than ancestrally-diverged species. Therefore, our model provides evidence that belowground indirect plant defences associated with tritrophic interactions have evolved under two classical hypotheses in plant ecology. While phylogenetic drivers of volatile emissions point to the defence-escalation hypothesis, plant local adaptation of indirect defences is in line with the resource availability hypothesis.

Sensing the Danger Signals: cis-Jasmone Reduces Aphid Performance on Potato and Modulates the Magnitude of Released Volatiles

In response to herbivory, plants synthesize and release variable mixtures of herbivore-induced plant volatiles (HIPVs) as indirect defense traits. Such induction of indirect plant defense can also be “switched on” by certain chemicals known as priming agents. Preceding work showed that the plant HIPV cis-jasmone (CJ) induced the emission of aphid defense-related volatiles affecting their behavioral response. However, little is known about the extent to which CJ-induced volatiles impacts aphid performance. In the current study, we conducted growth assays of potato aphids, Macrosiphum euphorbiae, observing their reproduction, development, and survival on CJ-primed potato plants. Adult M. euphoribae produced fewer neonates on CJ-treated plants compared to untreated plants. The weight and survival of M. euphorbiae reproduced neonates were significantly lower on CJ-treated plants. Additionally, there was a significant reduction in mean relative growth rate (MRGR) of M. euphoribae nymphs that fed on CJ-treated plants. Furthermore, the intrinsic rate of population increase (rm) of M. euphoribae was significantly reduced on CJ-treated plants. Volatile analysis showed that CJ treatment significantly increased the emission of differently assigned volatile groups that have functional or biosynthetic characteristics, i.e., alcohols, benzenoids, homoterpenes, ketones, and sesquiterpenes at all sampling periods. Such enhanced volatile emissions were persistent over 7 days, suggesting a long-lasting effect of CJ defense priming. A negative correlation was found between volatile emission and MRGR of M. euphoribae. Principal component analysis (PCA) of data for the volatiles showed that (Z)-3-hexen-1-ol, α-pinene, (E)-ocimene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), cis-jasmone, indole, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) were the main volatiles contributing to the emitted blends, suggesting possible involvement in the reduced performance of M. euphorbiae. Overall, our findings demonstrate that priming potato with CJ significantly results in elevated emission of known biologically active volatiles, which may negatively impact aphid settling and other performance traits on primed plants.

Plant structural complexity mediates trade-off in direct and indirect plant defense by birds.

Direct and indirect defenses are predicted to trade-off due to costs associated with redundancy in plant defense, but the factors mediating a plant's position along this trade-off axis are unknown. We conducted a bird exclusion experiment of nine sympatric shrub species to assess convergent associations among direct defense, indirect defense from birds, and shrub structural complexity, a trait predicted to influence bird foraging. We found high variation in defense; direct resistance varied four-fold, with indirect defense ranging from a 59% reduction to a 32% increase in herbivore density. These resistance strategies traded off and were mediated by plant structure; high complexity was associated with weaker indirect defense from birds, strong direct defense, and more predatory arthropods. Our findings suggest that species with growth forms that inhibit bird foraging invest more in direct defense and may provide refuge for arthropod predators. Accordingly, we provide evidence for a potentially widespread mechanism underlying the evolution of plant defenses.

Plant Volatiles Modulate Immune Responses of Spodoptera litura

Plants emit a specific blend of volatiles in response to herbivory and these volatiles, which often attract predators and parasitoids function as an indirect plant defense. The impact of plant volatiles in shaping herbivore defenses is unclear. Here, we report that specific plant volatiles induce immune responses in the polyphagous herbivore, Spodoptera litura. We characterized the hemocyte profile and established their functional significance with respect to ontogeny and exposure to specific plant volatiles. Fifth instar larvae showed the highest number and hemocytes diversity. We characterized seven different types of hemocytes, of which granulocytes performed phagocytosis, oenocytoids showed melanization activity, and plasmatocytes along with granulocytes and oenocytoids were found to be involved in encapsulation. Among the six volatiles tested, exposure to (E)-β-ocimene caused the highest increase in total hemocytes number (THC) followed by linalool and (Z)-3-hexenyl acetate exposure. Although THC did not differ between these three volatile treatments, circulating hemocytes diversity varied significantly. (E)-β-ocimene exposure showed higher number of plasmatocytes and phenol oxidase activity. The interaction of the parasitic wasp Bracon brevicornis with (E)-β-ocimene exposed larvae was poor in terms of delayed paralysis and lower egg deposition. In choice assays, the wasp showed clear preference towards control larvae indicating (E)-β-ocimene treatment renders the host unattractive. Hemocyte profiles post-parasitoid exposure and (E)-β-ocimene treatment were similar indicating cue-based priming. When challenged with Bacillus thuringiensis, linalool exposure resulted in the highest survival as compared to other volatiles. Our results show that specific HIPVs can modulate cellular immunity of S. litura, revealing a new role for HIPVs in tri-trophic interactions.

Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton.

The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number ofstudies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap,here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum). We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant. Single events of 25% and 50% leaf damagedid not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar productionwas stronger than systemic induction, whereas the reverse pattern was observed for phenolics. Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.