Synergism in the effect of prior jasmonic acid application on herbivore-induced volatile emission by Lima bean plants: transcription of a monoterpene synthase gene and volatile emission

Metabolic engineering of a 1,8-cineole synthase enhances aphid repellence and increases trichome density in transgenic tobacco (Nicotiana tabacum L.)

BACKGROUND

The green peach aphid (Myzus persicae Sulzer) is a harmful agricultural pest that causes severe crop damage by directly feeding or indirectly vectoring viruses. 1,8-cineole synthase (CINS) is a multiproduct enzyme that synthesizes monoterpenes, with 1,8-cineole dominating the volatile organic compound profile. However, the relationship between aphid preference and CINS remains elusive.

RESULTS

Here, we present evidence that SoCINS, a protein from garden sage (Salvia officinalis), enhanced aphid repellence and increased trichome density in transgenic tobacco. Our results demonstrated that overexpression of SoCINS (SoCINS-OE) led to the emission of 1,8-cineole at a level of up to 181.5 ng per g fresh leaf. Subcellular localization assay showed that SoCINS localized to chloroplasts. A Y-tube olfactometer assay and free-choice assays revealed that SoCINS-OE plants had a repellent effect on aphids, without incurring developmental or fecundity-related penalties. Intriguingly, the SoCINS-OE plants displayed an altered trichome morphology, showing increases in trichome density and in the relative proportion of glandular trichomes, as well as enlarged glandular cells. We also found that SoCINS-OE plants had significantly higher jasmonic acid (JA) levels than wild-type plants. Furthermore, application of 1,8-cineole elicited increased JA content and trichome density.

CONCLUSION

Our results demonstrate that SoCINS-OE plants have a repellent effect on aphids, and suggest an apparent link between 1,8-cineole, JA and trichome density. This study presents a viable and sustainable approach for aphid management by engineering the expression of 1,8-cineole synthase gene in plants, and underscores the potential usefulness of monoterpene synthase for pest control. © 2023 Society of Chemical Industry.

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.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Attraction of the Biocontrol Agent, Galerucella placida Towards Volatile Blends of Two Polygonaceae Weeds, Rumex dentatus and Polygonum glabrum

The Polygonaceae weed, Rumex dentatus L. grows in association with wheat, mustard and potato, while Polygonum glabrum Willd. grows in association with rice in India. Both larvae and adults of Galerucella placida Baly (Coleoptera: Chrysomelidae) voraciously consume these weeds. Applications of synthetic herbicides to control weeds are harmful to the environment including beneficial organisms. We propose to find volatile organic compounds (VOCs) from both weeds causing attraction of the biocontrol agent, G. placida, in order to attempt to use the insect as a biological weed control. Behavioral responses of G. placida towards volatile blends characteristic of undamaged (UD), insect-damaged (ID), jasmonic acid-treated (JA) or mechanically-damaged (MD) plants were conducted by Y-tube olfactometer bioassays. Cuminaldehyde was predominant in VOCs of UD R. dentatus, ID P. glabrum, and both JA and MD R. dentatus and P. glabrum. Geraniol was predominant in VOCs of UD P. glabrum, while 1,3-diethylbenzene predominated in VOCs of ID R. dentatus. Females were more attracted towards volatile blends of ID plants compared to UD or JA plants. Females did not show attraction towards volatile blends of JA plants. We identified two bioactive synthetics blends, one comprised of seven compounds - 16.65 µg 1,3-diethylbenzene, 10.72 µg acetophenone, 6.52 µg 2,6-(E,Z)-nonadienal, 2.46 µg 1-nonanol, 4.19 µg decanal, 9.86 µg 4-ethylacetophenone and 3.34 µg 1-hexadecene dissolved in 25 µl CH₂Cl₂ and the other containing five compounds - 2.50 µg 2-octanol, 6.84 µg limonene, 0.64 µg dodecane, 6.63 µg 4-ethylacetophenone and 0.24 µg geranyl acetone dissolved in 25 µl CH₂Cl₂. These two blends of volatile compounds could be used to attract the biocontrol agent during early vegetative period of these two weeds, which could lead to eradication of weeds from crop fields.

Metabolism and transcriptome profiling provides insight into the genes and transcription factors involved in monoterpene biosynthesis of borneol chemotype of Cinnamomum camphora induced by mechanical damage

Background

The borneol chemotype of Cinnamomum camphora (BCC), a monoterpene-rich woody plant species, is the sole source prescribed by the Chinese Pharmacopoeia for the production of natural D-borneol, a major monoterpene in BCC used for millennia as a topical analgesic in China. Nevertheless, the possible gene-regulatory roles of transcription factors (TFs) in BCC’s monoterpenoid biosynthesis remained unknown. Here, a joint analysis of the transcriptome and terpenoid metabolome of BCC induced by mechanical damage (MD) was used to comprehensively explore the interaction between TFs and terpene synthase (TPS) unigenes that might participate in monoterpene biosynthesis in BCC.

Results

Gas chromatography–mass spectrometry analysis detected 14 monoterpenes and seven sesquiterpenes. All but two monoterpenes underwent a significantly increased accumulation after the MD treatment. RNA sequencing data revealed that 10 TPS, 82 MYB, 70 AP2/ERF, 38 BHLH, 31 WRKY, and 29 bZIP unigenes responded to the MD treatment. A correlation analysis revealed that three monoterpene synthase genes (CcTPS1, CcTPS3, CcTPS4) highly correlated with multiple monoterpenes, namely D-borneol, camphor, and bornyl acetate, which could be responsible for monoterpenoid biosynthesis in BCC. Furthermore, five WRKY, 15 MYB, 10 ERF/AP2, five bZIP, and two BHLH genes had strong, positive correlations with CcTPS1 or CcTPS4, judging by their high coefficient values (R² > 0.8). The bioinformatics results were verified by quantitative real-time PCR.

Conclusion

This study provides insight into the genes involved in the biosynthesis and regulation of monoterpene in BCC and thus provides a pool of candidate genes for future mechanistic analyses of how monoterpenes accumulate in BCC.

Specialized 16SrX phytoplasmas induce diverse morphological and physiological changes in their respective fruit crops

The host-pathogen combinations-Malus domestica (apple)/`Candidatus Phytoplasma mali´, Prunus persica (peach)/`Ca. P. prunorum´ and Pyrus communis (pear)/`Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within weeks to few years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

Functional characterization of an Indian sandalwood (Santalum album L.) dual-localized bifunctional nerolidol/linalool synthase gene involved in stress response

Essential oils extracted from the heartwood of Indian sandalwood (Santalum album L.) contain linalool and nerolidol as minor components. However, nerolidol/linalool synthase (NES/LIS), which produce linalool and nerolidol, have yet to be characterized in sandalwood. Using a transcriptomic-based approach, a terpene synthase gene was screened from unigenes of transcriptome data derived from S. album seedlings exposed to low temperature (4 °C). The enzyme encoded by these complementary DNAs belongs to the TPS-b clade. Recombinant SaNES/LIS is a bifunctional enzyme that can catalyze the formation of (E)-nerolidol from farnesyl diphosphate and linalool from geranyl diphosphate, respectively. Whereas SaNES/LIS was primarily localized in chloroplastids, both as granular fluorescence and as diffuse fluorescence, it was also detected in the cytosol of a limited number of cells. Agrobacterium tumefaciens-mediated transient gene expression in planta produced the same terpene products as those obtained in vitro. Real-time PCR analysis showed the highest expression of SaNES/LIS in fruits, with about a three-fold higher level than in leaves, followed by flowers, heartwood and roots. SaNES/LIS transcripts were differentially activated in different tissues in response to methyl jasmonate, cold, high temperature, strong illumination, and drought stress. Our results provide novel insight into the role of sandalwood terpenoids in response to various environmental stresses.

Short-chain isoprenyl diphosphate synthases of lavender (Lavandula)

KEY MESSAGE

We reported the functional characterization of cDNAs encoding short-chain isoprenyl diphosphate synthases that control the partitioning of precursors for lavender terpenoids. Lavender essential oil is composed of regular and irregular monoterpenes, which are derived from linear precursors geranyl diphosphate (GPP) and lavandulyl diphosphate (LPP), respectively. Although this plant strongly expresses genes responsible for the biosynthesis of both monoterpene classes, it is unclear why regular monoterpenes dominate the oil. Here, we cloned and characterized Lavandula x intermedia cDNAs encoding geranyl diphosphate synthase (LiGPPS), geranylgeranyl diphosphate synthase (LiGGPPS) and farnesyl diphosphate synthase (LiFPPS). LiGPPS was heteromeric protein, consisting of a large subunit (LiGPPS.LSU) and a small subunit for which two different cDNAs (LiGPPS.SSU1 and LiGPPS.SSU2) were detected. Neither recombinant LiGPPS subunits was active by itself. However, when co-expressed in E. coli LiGPPS.LSU and LiGPPS.SSU1 formed an active heteromeric GPPS, while LiGPPS.LSU and LiGPPS.SSU2 did not form an active protein. Recombinant LiGGPPS, LiFPPS and LPP synthase (LPPS) proteins were active individually. Further, LiGPPS.SSU1 modified the activity of LiGGPPS (to produce GPP) in bacterial cells co-expressing both proteins. Given this, and previous evidence indicating that GPPS.SSU can modify the activity of GGPPS to GPPS in vitro and in plants, we hypothesized that LiGPPS.SSU1 modifies the activity of L. x intermedia LPP synthase (LiLPPS), thus accounting for the relatively low abundance of LPP-derived irregular monoterpenes in this plant. However, LiGPPS.SSU1 did not affect the activity of LiLPPS. These results, coupled to the observation that LiLPPS transcripts are more abundant than those of GPPS subunits in L. x intermedia flowers, suggest that regulatory mechanisms other than transcriptional control of LPPS regulate precursor partitioning in lavender flowers.

Identification and functional characterization of three new terpene synthase genes involved in chemical defense and abiotic stresses in Santalum album

BACKGROUND

It is well known that aromatic essential oils extracted from the heartwood of Santalum album L. have wide economic value. However, little is known about the role of terpenoids in response to various adverse environmental stresses as other plants do in the form of signals during plant-environment interactions.

RESULTS

In this study, trace amounts of volatiles consisting of α-santalene, epi-β-santalene, β-santalene, α-santalol, β-santalol, (E)-α-bergamotene, (E)-β-farnesene and β-bisabolene were found in the leaves of mature S. album trees. We identified more than 40 candidate terpene synthase (TPS) unigenes by mining publicly-available RNA-seq data and characterized the enzymes encoded by three cDNAs: one mono-TPS catalyzes the formation of mostly α-terpineol, and two multifunctional sesqui-TPSs, one of which produces (E)-α-bergamotene and sesquisabinene as major products and another which catalyzes the formation of (E)-β-farnesene, (E)-nerolidol and (E,E)-farnesol as main products. Metabolite signatures and gene expression studies confirmed that santalol content is closely related with santalene synthase (SaSSY) transcripts in heartwood, which is key enzyme responsible for santalol biosynthesis. However, the expression of three new SaTPS genes differed significantly from SaSSY in the essential oil-producing heartwood. Increased activities of antioxidant enzymes, superoxide dismutase, catalase, peroxidase and ascorbate peroxidase, were detected in different tissues of S. album plants after applying 1 mM methyl jasmonate (MeJA) and 1 mM salicylic acid (SA), or exposure to 4°C, 38°C and high light intensity. MeJA and SA dramatically induced the expression of SaTPS1 and SaTPS2 in leaves. SaTPS1 to 3 transcripts were differentially activated among different tissues under adverse temperature and light stresses. In contrast, almost all SaSSY transcripts decreased in response to these environmental stresses, unlike SaTPS1 to 3.

CONCLUSIONS

Multifunctional enzymes were biochemically characterized, including one chloroplastic mono-TPS and two cytosolic sesqui-TPSs in sandalwood. Our results suggest the ecological importance of these three new SaTPS genes in defensive response to biotic attack and abiotic stresses in S. album.

Methyl salicylate differently affects benzenoid and terpenoid volatile emissions in Betula pendula

Methyl salicylate (MeSA) is a long-distance signal transduction chemical that plays an important role in plant responses to abiotic stress and herbivore and pathogen attacks. However, it is unclear how photosynthesis and elicitation of plant volatile organic compounds (VOC) from different metabolic pathways respond to the dose of MeSA. We applied different MeSA concentrations (0-50 mM) to study how exogenous MeSA alters VOC profiles of silver birch (Betula pendula Roth) leaves from application through recovery (0.5-23 h). Methyl salicylate application significantly reduced net assimilation rate in 10 mM and 20 mM MeSA-treated plants. No significant effects of MeSA were observed on the stomatal conductance at any MeSA concentration. Methyl salicylate elicited emissions of benzenoids (BZ), monoterpenes (MT) and fatty acid derived compounds (LOX products). Emission rates of BZ were positively, but emission rates of MT were negatively correlated with MeSA concentration. Total emission of LOX products was not influenced by MeSA concentration. Emission rate of MT was negatively correlated with BZ and the share of MT in the total emission blend decreased and the share of BZ increased with increasing MeSA concentration. Although the share of LOX products was similar across MeSA treatments, some LOX products responded differently to MeSA concentration, ultimately resulting in unique VOC blends. Overall, this study demonstrates inverse responses of MT and BZ to different MeSA doses such that plant defense mechanisms induced by lower MeSA doses mainly lead to enhanced MT synthesis, whereas greater MeSA doses trigger BZ-related defense mechanisms. Our results will contribute to improving the understanding of birch defenses induced upon regular herbivore attacks and pathogen infections in boreal forests.

Priming of cowpea volatile emissions with defense inducers enhances the plant's attractiveness to parasitoids when attacked by caterpillars

BACKGROUND

The manipulation of herbivore-induced volatile organic compounds (HI-VOCs) via the application of the inducers benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) and laminarin (β-1,3-glucan) is known to enhance the attractiveness of caterpillar-damaged cotton and maize plants to parasitoids. To test if this is also the case for legumes, we treated cowpea (Vigna unguiculata var. unguiculata) with these inducers and studied the effects on HI-VOC emissions and the attraction of three generalist endoparasitoids.

RESULTS

After the inducers had been applied and the plants subjected to either real or mimicked herbivory by Spodoptera littoralis caterpillars, females of the parasitoids Campoletis sonorensis and Microplitis rufiventris showed a strong preference for BTH-treated plants, whereas Cotesia females were strongly attracted to both BTH- and laminarin-treated plants with real or mimicked herbivory. Treated plants emitted more of certain HI-VOCs, but considerably less indole and linalool and less of several sesquiterpenes. Multivariate data analysis revealed that enhanced wasp attraction after treatment was correlated with high relative concentrations of nonanal, α-pinene, (E)-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and with low relative concentrations of indole, (S)-linalool and (E)-β-farnesene. Inducer treatments had no significant effect on leaf consumption by the caterpillars.

CONCLUSION

Our findings confirm that treating cowpea plants with inducers can enhance their attractiveness to biological control agents. © 2017 Society of Chemical Industry.

Response of Brassica oleracea to temporal variation in attack by two herbivores affects preference and performance of a third herbivore

1. Plants are frequently under attack by multiple insect herbivores, which may interact indirectly through herbivore-induced changes in the plant's phenotype. The identity, order, and timing of herbivore arrivals may influence the outcome of interactions between two herbivores. How these aspects affect, in turn, subsequently arriving herbivores that feed on double herbivore-induced plants has not been widely investigated. 2. This study tested whether the order and timing of arrival of two inducing herbivores from different feeding guilds affected the preference and performance of a subsequently arriving third herbivore, caterpillars of Mamestra brassicae L. (Lepidoptera: Noctuidae). Aphids [Brevicoryne brassicae L. (Hemiptera: Aphididae)] and caterpillars [Plutella xylostella L. (Lepidoptera: Yponomeutidae)] were introduced onto wild Brassica oleracea L. (Brassicaceae) plants in different sequences and with different arrival times. The effects of these plant treatments on M. brassicae caterpillars were assessed in pairwise preference tests and no-choice performance tests. 3. The caterpillars of M. brassicae preferred to feed from undamaged plants rather than double herbivore-induced plants. Compared with undamaged plants, they preferred plant material on which aphids had arrived first followed by caterpillars, whereas they avoided plant material with the reverse order of herbivore arrival. Performance of the caterpillars increased with increasing arrival time between herbivore infestations in double herbivore-induced plants. Although M. brassicae grew faster on plants induced by aphids than on those induced by caterpillars alone, its performance was not affected by the order of previous herbivore arrival. 4. These results imply that the timing of colonisation by multiple herbivores determines the outcome of plant-mediated herbivore-herbivore interactions.

Isolation and functional characterization of a methyl jasmonate-responsive 3-carene synthase from Lavandula x intermedia

A methyl jasmonate responsive 3-carene synthase (Li3CARS) gene was isolated from Lavandula x intermedia and functionally characterized in vitro. Lavenders produce essential oils consisting mainly of monoterpenes, including the potent antimicrobial and insecticidal monoterpene 3-carene. In this study we isolated and functionally characterized a leaf-specific, methyl jasmonate (MeJA)-responsive monoterpene synthase (Li3CARS) from Lavandula x intermedia. The ORF excluding transit peptides encoded a 64.9 kDa protein that was expressed in E. coli, and purified with Ni-NTA agarose affinity chromatography. The recombinant Li3CARS converted GPP into 3-carene as the major product, with K _m and k _cat of 3.69 ± 1.17 µM and 2.01 s^-1 respectively. Li3CARS also accepted NPP as a substrate to produce multiple products including a small amount of 3-carene. The catalytic efficiency of Li3CARS to produce 3-carene was over ten fold higher for GPP (k _cat /K _m = 0.56 µM^-1s^-1) than NPP (k _cat /K _m = 0.044 µM^-1s^-1). Production of distinct end product profiles from different substrates (GPP versus NPP) by Li3CARS indicates that monoterpene metabolism may be controlled in part through substrate availability. Li3CARS transcripts were found to be highly abundant in leaves (16-fold) as compared to flower tissues. The transcriptional activity of Li3CARS correlated with 3-carene production, and was up-regulated (1.18- to 3.8-fold) with MeJA 8-72 h post-treatment. The results suggest that Li3CARS may have a defensive role in Lavandula.

cis-Jasmone Elicits Aphid-Induced Stress Signalling in Potatoes

Elicitation of plant defense signaling that results in altered emission of volatile organic compounds (VOCs) offers opportunities for protecting plants against arthropod pests. In this study, we treated potato, Solanum tuberosum L., with the plant defense elicitor cis-jasmone (CJ), which induces the emission of defense VOCs and thus affects the behavior of herbivores. Using chemical analysis, electrophysiological and behavioral assays with the potato-feeding aphid Macrosiphum euphorbiae, we showed that CJ treatment substantially increased the emission of defense VOCs from potatoes compared to no treatment. Coupled GC-electroantennogram (GC-EAG) recordings from the antennae of M. euphorbiae showed robust responses to 14 compounds present in induced VOCs, suggesting their behavioral role in potato/aphid interactions. Plants treated with CJ and then challenged with M. euphorbiae were most repellent to alate M. euphorbiae. Principal component analysis (PCA) of VOC collections suggested that (E)-2-hexenal, (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), (E)-β-farnesene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), methyl salicylate (MeSA), CJ, and methyl benzoate (MeBA) were the main VOCs contributing to aphid behavioral responses, and that production of TMTT, (E)-β-farnesene, CJ, and DMNT correlated most strongly with aphid repellency. Our findings confirm that CJ can enhance potato defense against aphids by inducing production of VOCs involved in aphid-induced signalling.

Volatile Organic Compounds Induced by Herbivory of the Soybean Looper Chrysodeixis includens in Transgenic Glyphosate-Resistant Soybean and the Behavioral Effect on the Parasitoid, Meteorus rubens

Transgenic soybean plants (RR) engineered to express resistance to glyphosate harbor a variant of the enzyme EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) involved in the shikimic acid pathway, the biosynthetic route of three aromatic amino acids: phenylalanine, tyrosine, and tryptophan. The insertion of the variant enzyme CP4 EPSPS confers resistance to glyphosate. During the process of genetic engineering, unintended secondary effects are likely to occur. In the present study, we quantified volatile organic compounds (VOCs) emitted constitutively or induced in response to herbivory by the soybean looper Chrysodeixis includens in transgenic soybean and its isogenic (untransformed) line. Since herbivore-induced plant volatiles (HIPVs) are known to play a role in the recruitment of natural enemies, we assessed whether changes in VOC profiles alter the foraging behavior of the generalist endoparasitic larval parasitoid, Meteorus rubens in the transgenic line. Additionally, we assessed whether there was a difference in plant quality by measuring the weight gain of the soybean looper. In response to herbivory, several VOCs were induced in both the conventional and the transgenic line; however, larger quantities of a few compounds were emitted by transgenic plants. Meteorus rubens females were able to discriminate between the odors of undamaged and C. includens-damaged plants in both lines, but preferred the odors emitted by herbivore-damaged transgenic plants over those emitted by herbivore-damaged conventional soybean plants. No differences were observed in the weight gain of the soybean looper. Our results suggest that VOC-mediated tritrophic interactions in this model system are not negatively affected. However, as the preference of the wasps shifted towards damaged transgenic plants, the results also suggest that genetic modification affects that tritrophic interactions in multiple ways in this model system.

AtWRKY22 promotes susceptibility to aphids and modulates salicylic acid and jasmonic acid signalling

Aphids induce many transcriptional perturbations in their host plants, but the signalling cascades responsible and the effects on plant resistance are largely unknown. Through a genome-wide association (GWA) mapping study in Arabidopsis thaliana, we identified WRKY22 as a candidate gene associated with feeding behaviour of the green peach aphid, Myzus persicae The transcription factor WRKY22 is known to be involved in pathogen-triggered immunity, and WRKY22 gene expression has been shown to be induced by aphids. Assessment of aphid population development and feeding behaviour on knockout mutants and overexpression lines showed that WRKY22 increases susceptibility to M. persicae via a mesophyll-located mechanism. mRNA sequencing analysis of aphid-infested wrky22 knockout plants revealed the up-regulation of genes involved in salicylic acid (SA) signalling and down-regulation of genes involved in plant growth and cell-wall loosening. In addition, mechanostimulation of knockout plants by clip cages up-regulated jasmonic acid (JA)-responsive genes, resulting in substantial negative JA-SA crosstalk. Based on this and previous studies, WRKY22 is considered to modulate the interplay between the SA and JA pathways in response to a wide range of biotic and abiotic stimuli. Its induction by aphids and its role in suppressing SA and JA signalling make WRKY22 a potential target for aphids to manipulate host plant defences.

Differential Costs of Two Distinct Resistance Mechanisms Induced by Different Herbivore Species in Arabidopsis

Plants respond to herbivory with the induction of resistance, mediated by distinct phytohormonal signaling pathways and their interactions. Phloem feeders are known to induce plant resistance via the salicylic acid pathway, whereas biting-chewing herbivores induce plant resistance mainly via the jasmonate pathway. Here, we show that a specialist caterpillar (biting-chewing herbivore) and a specialist aphid (phloem feeder) differentially induce resistance against Pieris brassicae caterpillars in Arabidopsis (Arabidopsis thaliana) plants. Caterpillar feeding induces resistance through the jasmonate signaling pathway that is associated with the induction of kaempferol 3,7-dirhamnoside, whereas aphid feeding induces resistance via a novel mechanism involving sinapoyl malate. The role of sinapoyl malate is confirmed through the use of a mutant compromised in the biosynthesis of this compound. Caterpillar-induced resistance is associated with a lower cost in terms of plant growth reduction than aphid-induced resistance. A strong constitutive resistance against P. brassicae caterpillars in combination with a strong growth attenuation in plants of a transfer DNA (T-DNA) insertion mutant of WRKY70 (wrky70) suggest that the WRKY70 transcription factor, a regulator of downstream responses mediated by jasmonate-salicylic acid signaling cross talk, is involved in the negative regulation of caterpillar resistance and in the tradeoff between growth and defense. In conclusion, different mechanisms of herbivore-induced resistance come with different costs, and a functional WRKY70 transcription factor is required for the induction of low-cost resistance.