Herbivore-induced terpenoid emission in Medicago truncatula: concerted action of jasmonate, ethylene and calcium signaling

Impact of Methyl Jasmonate on Terpenoid Biosynthesis and Functional Analysis of Sesquiterpene Synthesis Genes in Schizonepeta tenuifolia

This study investigates the impact of methyl jasmonate (MeJA) on the volatile oil composition of Schizonepeta tenuifolia and elucidates the function of the StTPS45 gene, a key player in terpenoid biosynthesis. The effect of different concentrations of MeJA (0, 50, 100, 200, and 300 μmol/L) on the growth of S. tenuifolia adventitious bud clusters was analyzed over a 20 d period. Using gas chromatography-mass spectrometry (GC-MS), 17 compounds were identified from the adventitious bud clusters of S. tenuifolia. Significant changes in the levels of major monoterpenes, including increased contents of (+)-limonene and (+)-menthone, were observed, particularly at higher concentrations of MeJA. Analysis of transcriptome data from three groups treated with 0, 100, and 300 μmol/L MeJA revealed significant changes in the gene expression profiles following MeJA treatment. At 100 μmol/L MeJA, most terpene synthase (TPS) genes were overexpressed. Additionally, gene expression and functional predictions suggested that StTPS45 acts as germacrene D synthase. Therefore, StTPS45 was cloned and expressed in Escherichia coli, and enzyme activity assays confirmed its function as a germacrene D synthase. Molecular docking and structural prediction of StTPS45 further suggested specific interactions with farnesyl diphosphate (FPP), aligning with its role in the terpenoid synthesis pathway. These findings provide valuable insights into the modulation of secondary metabolite pathways by jasmonate signaling and underscore the potential of genetic engineering approaches to enhance the production of specific terpenoids in medicinal plants.

Feedback regulation of the isoprenoid pathway by SsdTPS overexpression has the potential to enhance plant tolerance to drought stress

In order to maintain the dynamic physiological balance, plants are compelled to adjust their energy metabolism and signal transduction to cope with the abiotic stresses caused by complex and changeable environments. The diterpenoid natural compound and secondary metabolites, sclareol, derived from Salvia sclarea, has gained significant attention owing to its economic value as a spice material and diverse physiological activities. Here, we focused on the roles and regulatory mechanisms of the sclareol diterpene synthase gene SsdTPS in the resistance of S. sclarea to abiotic stresses. Our results suggested that abiotic stresses could induce the response and upregulation of SsdTPS expression and isoprenoid pathway in S. sclarea. Ectopic expression of SsdTPS conferred drought tolerance in transgenic Arabidopsis, compared with wild-type. Overexpression of SsdTPS enhanced the transcription of ABA signal transduction synthetic regulators and induced the positive feedback upregulating key regulatory genes in the MEP pathway, thereby promoting the increase of ABA content and improving drought tolerance in transgenic plants. In addition, SsdTPS-overexpressed transgenic Arabidopsis improved the responses of stomatal regulatory genes and ROS scavenging enzyme activities and gene expression to drought stress. This promoted the stomatal closure and ROS reduction, thus enhancing water retention capacity and reducing oxidative stress damage. These findings unveil the potentially positive role of SsdTPS in orchestrating multiple regulatory mechanisms and maintaining homeostasis for improved abiotic stress resistance in S. sclarea, providing a novel insight into strategies for promoting drought resistance and cultivating highly tolerant plants.

Biochemical analysis of the TPS-a subfamily in Medicago truncatula

Terpenes are important mediators of plant chemical response to environmental cues. Here, we describe the genome-wide identification and biochemical characterization of TPS-a members in Medicago truncatula, a model legume crop. Genome mining identified thirty-nine full-length terpene synthases with a significant number predicted to produce monoterpenes and sesquiterpenes. Biochemical characterization of the TPS-a subfamily associated with sesquiterpene biosynthesis revealed such compounds, that exhibit substantial biological activity in other plants. Gene expression analysis using qPCR and the Medicago gene atlas illustrated distinct tissue and time-based variation in expression in leaves and roots. Together our work establishes the gene-to-metabolite relationships for sesquiterpene synthases in M. truncatula. Understanding the biosynthetic capacity is a foundational step to defining the ecological roles of this important family of compounds.

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 OsBDR1-MPK3 module negatively regulates blast resistance by suppressing the jasmonate signaling and terpenoid biosynthesis pathway

Receptor-like kinases (RLKs) may initiate signaling pathways by perceiving and transmitting environmental signals to cellular machinery and play diverse roles in plant development and stress responses. The rice genome encodes more than one thousand RLKs, but only a small number have been characterized as receptors for phytohormones, polypeptides, elicitors, and effectors. Here, we screened the function of 11 RLKs in rice resistance to the blast fungus Magnaporthe oryzae (M. oryzae) and identified a negative regulator named BDR1 (Blast Disease Resistance 1). The expression of BDR1 was rapidly increased under M. oryzae infection, while silencing or knockout of BDR1 significantly enhanced M. oryzae resistance in two rice varieties. Protein interaction and kinase activity assays indicated that BDR1 directly interacted with and phosphorylated mitogen-activated kinase 3 (MPK3). Knockout of BDR1 compromised M. oryzae-induced MPK3 phosphorylation levels. Moreover, transcriptome analysis revealed that M. oryzae-elicited jasmonate (JA) signaling and terpenoid biosynthesis pathway were negatively regulated by BDR1 and MPK3. Mutation of JA biosynthetic (allene oxide cyclase (AOC)/signaling (MYC2) genes decreased rice resistance to M. oryzae. Besides diterpenoid, the monoterpene linalool and the sesquiterpene caryophyllene were identified as unique defensive compounds against M. oryzae, and their biosynthesis genes (TPS3 and TPS29) were transcriptionally regulated by JA signaling and suppressed by BDR1 and MPK3. These findings demonstrate the existence of a BDR1-MPK3 cascade that negatively mediates rice blast resistance by affecting JA-related defense responses.

Calcium Signaling in Plant-Insect Interactions

In plant-insect interactions, calcium (Ca²⁺) variations are among the earliest events associated with the plant perception of biotic stress. Upon herbivory, Ca²⁺ waves travel long distances to transmit and convert the local signal to a systemic defense program. Reactive oxygen species (ROS), Ca²⁺ and electrical signaling are interlinked to form a network supporting rapid signal transmission, whereas the Ca²⁺ message is decoded and relayed by Ca²⁺-binding proteins (including calmodulin, Ca²⁺-dependent protein kinases, annexins and calcineurin B-like proteins). Monitoring the generation of Ca²⁺ signals at the whole plant or cell level and their long-distance propagation during biotic interactions requires innovative imaging techniques based on sensitive sensors and using genetically encoded indicators. This review summarizes the recent advances in Ca²⁺ signaling upon herbivory and reviews the most recent Ca²⁺ imaging techniques and methods.

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.

Tissue-Specific Expression of the Terpene Synthase Family Genes in Rosa chinensis and Effect of Abiotic Stress Conditions

Rose (Rosa chinensis) is one of the most famous ornamental plants worldwide, with a variety of colors and fragrances. Terpene synthases (TPSs) play critical roles in the biosynthesis of terpenes. In this work, we report a comprehensive study on the genome-wide identification and characterization of the TPS family in R. chinensis. We identified 49 TPS genes in the R. chinensis genome, and they were grouped into five subfamilies (TPS-a, TPS-b, TPS-c, TPS-g and TPS-e/f). Phylogenetics, gene structure and conserved motif analyses indicated that the RcTPS genes possessed relatively conserved gene structures and the RcTPS proteins contained relatively conserved motifs. Multiple putative cis-acting elements involved in the stress response were identified in the promoter region of RcTPS genes, suggesting that some could be regulated by stress. The expression profile of RcTPS genes showed that they were predominantly expressed in the petals of open flowers, pistils, leaves and roots. Under osmotic and heat stresses, the expression of most RcTPS genes was upregulated. These data provide a useful foundation for deciphering the functional roles of RcTPS genes during plant growth as well as addressing the link between terpene biosynthesis and abiotic stress responses in roses.

Methyl jasmonate-induced resistance to Delia platura (Diptera: Anthomyiidae) in Lupinus mutabilis

BACKGROUND

Andean lupin (Lupinus mutabilis Sweet) is an important leguminous crop from South America with a high protein content. In Ecuador, lupin yields are severely affected by the infestation of Delia platura larvae on germinating seeds. The application of elicitor molecules with activity against herbivorous insects to control D. platura infestation constitutes an unexplored and promising alternative for chemical insecticides. In this study, methyl jasmonate (MeJA), hexanoic acid, menadione sodium bisulfite, and DL-β-aminobutyric acid were evaluated for their ability to induce resistance against D. platura in three commercial lupin cultivars.

RESULTS

Only seeds pretreated with MeJA significantly impaired insect performance during choice and no-choice assays. Additionally, fitness indicators such as seed germination and growth were not affected by MeJA treatment. To investigate the molecular mechanisms behind the MeJA-mediated resistance, RT-qPCR assays were performed. First, RT-qPCR reference genes were validated, showing that LmUBC was the most stable reference gene. Next, expression analysis over time revealed that MeJA application up-regulated the activity of the jasmonic acid biosynthetic genes LmLOX2 and LmAOS, together with other jasmonate-related defense genes, such as LmTPS1, LmTPS4, LmPI2, LmMBL, LmL/ODC, LmCSD1, and LmPOD.

CONCLUSION

This study indicates that MeJA can be used as an environmentally friendly elicitor molecule to protect Andean lupin from D. platura attack without fitness cost. MeJA application induces plant defense responses to insects in Andean lupin that may be modulated by the onset of terpenoid biosynthesis, proteinase inhibitors, lectins, polyamines, and antioxidative enzymes. © 2021 Society of Chemical Industry.

Breaking Bad News: Dynamic Molecular Mechanisms of Wound Response in Plants

Recognition and repair of damaged tissue are an integral part of life. The failure of cells and tissues to appropriately respond to damage can lead to severe dysfunction and disease. Therefore, it is essential that we understand the molecular pathways of wound recognition and response. In this review, we aim to provide a broad overview of the molecular mechanisms underlying the fate of damaged cells and damage recognition in plants. Damaged cells release the so-called damage associated molecular patterns to warn the surrounding tissue. Local signaling through calcium (Ca2+), reactive oxygen species (ROS), and hormones, such as jasmonic acid, activates defense gene expression and local reinforcement of cell walls to seal off the wound and prevent evaporation and pathogen colonization. Depending on the severity of damage, Ca2+, ROS, and electrical signals can also spread throughout the plant to elicit a systemic defense response. Special emphasis is placed on the spatiotemporal dimension in order to obtain a mechanistic understanding of wound signaling in plants.

Protective Effects of (E)-β-Caryophyllene (BCP) in Chronic Inflammation

(E)-β-caryophyllene (BCP) is a bicyclic sesquiterpene widely distributed in the plant kingdom, where it contributes a unique aroma to essential oils and has a pivotal role in the survival and evolution of higher plants. Recent studies provided evidence for protective roles of BCP in animal cells, highlighting its possible use as a novel therapeutic tool. Experimental results show the ability of BCP to reduce pro-inflammatory mediators such as tumor necrosis factor-alfa (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), thus ameliorating chronic pathologies characterized by inflammation and oxidative stress, in particular metabolic and neurological diseases. Through the binding to CB2 cannabinoid receptors and the interaction with members of the family of peroxisome proliferator-activated receptors (PPARs), BCP shows beneficial effects on obesity, non-alcoholic fatty liver disease/nonalcoholic steatohepatitis (NAFLD/NASH) liver diseases, diabetes, cardiovascular diseases, pain and other nervous system disorders. This review describes the current knowledge on the biosynthesis and natural sources of BCP, and reviews its role and mechanisms of action in different inflammation-related metabolic and neurologic disorders.

Ethylene functions as a suppressor of volatile production in rice

We examined the role of ethylene in the production of rice (Oryza sativa) volatile organic compounds (VOCs), which act as indirect defense signals against herbivores in tritrophic interactions. Rice plants were exposed to exogenous ethylene (1 ppm) after simulated herbivory, which consisted of mechanical wounding supplemented with oral secretions (WOS) from the generalist herbivore larva Mythimna loreyi. Ethylene treatment highly suppressed VOCs in WOS-treated rice leaves, which was further corroborated by the reduced transcript levels of major VOC biosynthesis genes in ethylene-treated rice. In contrast, the accumulation of jasmonates (JA), known to control VOCs in higher plants, and transcript levels of primary JA response genes, including OsMYC2, were not largely affected by ethylene application. At the functional level, flooding is known to promote internode elongation in young rice via ethylene signaling. Consistent with the negative role of ethylene on VOC genes, the accumulation of VOCs in water-submerged rice leaves was suppressed. Furthermore, in mature rice plants, which naturally produce less volatiles, VOCs could be rescued by the application of the ethylene perception inhibitor 1-methylcyclopropene. Our data suggest that ethylene acts as an endogenous suppressor of VOCs in rice plants during development and under stress.

Functional Characterization of Terpene Synthases Accounting for the Volatilized-Terpene Heterogeneity in Lathyrus odoratus Cultivar Flowers

Lathyrus odoratus (sweet pea) is an ornamental plant with exceptional floral scent, previously used as an experimental organism in the early development of Mendelian genetics. However, its terpene synthases (TPSs), which act as metabolic gatekeepers in the biosynthesis of volatile terpenoids, remain to be characterized. Auto-Headspace Solid-phase Microextraction/Gas chromatography-mass spectrometry analysis of floral volatile terpene constituents from seven sweet pea cultivars identified α-bergamotene, linalool, (-)-α-cubebene, geraniol, β-caryophyllene and β-sesquiphellandrene as the dominant compounds. RNA sequencing was performed to profile the transcriptome of L. odoratus flowers. Bioinformatic analysis identified eight TPS genes (acronymed as LoTPS) that were successfully cloned, heterologously expressed and functionally analyzed. LoTPS4 and LoTPS7, belonging to the TPS-b clade, biochemically catalyzed the formation of monoterpenes and sesquiterpenes. LoTPS3 and LoTPS8, placed in the TPS-a clade, also generated monoterpenes and sesquiterpenes, while LoTPS12 belonging to the TPS-g clade showed linalool/nerolidol synthase activity. Notably, biochemical assays of the recombinant LoTPS proteins revealed their catalytic promiscuity, and the enzymatic products were basically consistent with major volatile compounds released from sweet pea flowers. The data from our study lay the foundation for the chemical ecology, molecular genetics and biotechnological improvement of sweet pea and other legumes (Fabaceae).

The phytotoxic air-pollutant O3 enhances the emission of herbivore-induced volatile organic compounds (VOCs) and affects the susceptibility of black mustard plants to pest attack

Stress-induced changes to plant biochemistry and physiology can influence plant nutritional quality and subsequent interactions with herbivorous pests. However, the effects of stress combinations are unpredictable and differ to the effects of individual stressors. Here we studied the effects of exposure to the phytotoxic air-pollutant ozone (O₃), feeding by larvae of the large cabbage white butterfly (Pieris brassicae), and a combination of the two stresses, on the emission of volatile organic compounds (VOCs) by black mustard plants (Brassica nigra) under field and laboratory conditions. Field-grown B. nigra plants were also measured for carbon-nitrogen (C-N) content, net photosynthetic activity (Pn), stomatal conductance (gs) and biomass. The effects of O₃ on interactions between plants and a herbivorous pest were addressed by monitoring the abundance of wild diamondback moth larvae (Plutella xylostella) and feeding-damage to B. nigra plants in an O₃-free air concentration enrichment (O₃-FACE) field site. Herbivore-feeding induced the emission of VOCs that were not emitted by undamaged plants, both under field and laboratory conditions. The combination of O₃ and herbivore-feeding stresses resulted in enhanced emission rates of several VOCs from field-grown plants. Short-term O₃ exposure (of 10 days) and P. brassicae-feeding did not affect C-N content, but chronic O₃ exposure (of 34 and 47 days) and P. brassicae-feeding exacerbated suppression of Pn. Ozone exposure also caused visible injury and decreased the plant biomass. Field-grown B. nigra under elevated O₃ were infested with fewer P. xylostella larvae and received significantly less feeding damage. Our results suggest that plants growing in a moderately polluted environment may be of reduced quality and less attractive to foraging herbivores.

Production of multiple terpenes of different chain lengths by subcellular targeting of multi-substrate terpene synthase in plants

Multi-substrate terpene synthases (TPSs) are distinct from typical TPSs that react with a single substrate. Although in vitro activity of few multi-substrate TPSs have been reported, in vivo characterization has not been well investigated for most of them. Here, a new TPS from Cananga odorata, CoTPS5, belonging to TPS-f subfamily was functionally characterized in vitro as well as in vivo. CoTPS5 reacted with multiple prenyl-pyrophosphate substrates of various chain lengths as a multi-substrate TPS. It catalyzed the formation of (E)-β-ocimene, (E,E)-α-farnesene and α-springene from geranyl pyrophosphate, (E,E)-farnesyl pyrophosphate and geranylgeranyl pyrophosphate, respectively. Upon transient expression in Nicotiana benthamiana, CoTPS5 localized to cytosol and produced only (E,E)-α-farnesene. However, expression of plastid-targeted CoTPS5 in N. benthamiana resulted in biosynthesis of all three compounds, (E)-β-ocimene, (E,E)-α-farnesene and α-springene. Similarly, transgenic Arabidopsis plants overexpressing plastid-targeted CoTPS5 showed stable and sustainable production of (E)-β-ocimene, (E,E)-α-farnesene and α-springene. Moreover, their production did not affect the growth and development of transgenic Arabidopsis plants. Our results demonstrate that redirecting multi-substrate TPS to a different intracellular compartment could be an effective way to prove in vivo activity of multi-substrate TPSs and thereby allowing for the production of multiple terpenoids simultaneously in plants.

Rahim MH, Sabri S, Lai KS, Song AAL, Abdul Rahim R, Wan Abdullah WMAN, Ong Abdullah J. Functional characterization of a new terpene synthase from Plectranthus amboinicus

Plectranthus amboinicus (Lour.) Spreng is an aromatic medicinal herb known for its therapeutic and nutritional properties attributed by the presence of monoterpene and sesquiterpene compounds. Up until now, research on terpenoid biosynthesis has focused on a few mint species with economic importance such as thyme and oregano, yet the terpene synthases responsible for monoterpene production in P. amboinicus have not been described. Here we report the isolation, heterologous expression and functional characterization of a terpene synthase involved in P. amboinicus terpenoid biosynthesis. A putative monoterpene synthase gene (PamTps1) from P. amboinicus was isolated with an open reading frame of 1797 bp encoding a predicted protein of 598 amino acids with molecular weight of 69.6 kDa. PamTps1 shares 60–70% amino acid sequence similarity with other known terpene synthases of Lamiaceae. The in vitro enzymatic activity of PamTps1 demonstrated the conversion of geranyl pyrophosphate and farnesyl pyrophosphate exclusively into linalool and nerolidol, respectively, and thus PamTps1 was classified as a linalool/nerolidol synthase. In vivo activity of PamTps1 in a recombinant Escherichia coli strain revealed production of linalool and nerolidol which correlated with its in vitro activity. This outcome validated the multi-substrate usage of this enzyme in producing linalool and nerolidol both in in vivo and in vitro systems. The transcript level of PamTps1 was prominent in the leaf during daytime as compared to the stem. Gas chromatography-mass spectrometry (GC-MS) and quantitative real-time PCR analyses showed that maximal linalool level was released during the daytime and lower at night following a diurnal circadian pattern which correlated with the PamTps1 expression pattern. The PamTps1 cloned herein provides a molecular basis for the terpenoid biosynthesis in this local herb that could be exploited for valuable production using metabolic engineering in both microbial and plant systems.

Transcriptome and Metabolite Conjoint Analysis Reveals that Exogenous Methyl Jasmonate Regulates Monoterpene Synthesis in Grape Berry Skin

Monoterpene is one of the important sources of varietal aroma, which provides a strong floral and fruity aroma in wines. Methyl jasmonate (MeJA) affects plant secondary metabolism. However, the regulatory mechanisms of monoterpene biosynthesis after MeJA application on grapes are not illuminated. In the present study, 10 mM MeJA was used as treatments in Italian Riesling grape at the preveraison stage in different ways, including grape cluster soaking, foliar spraying, and whole vine spraying, designated as T1, T2, and T3, respectively, while a blank group was used as the control (CK). HS-SPME/GC-MS and transcriptome sequencing analysis were performed to investigate the effect of exogenous MeJA on monoterpene synthesis in grape berry skin. The results of GC-MS showed that the application of MeJA induced the accumulation of volatile monoterpenes in grape berry skin, especially linalool, α-terpineol, and oxides. In addition, transcriptome analysis showed that differentially expressed genes were increased from T2 to T3 to T1 compared with CK, and significantly enriched in JA and monoterpene synthesis pathways. T1 application significantly upregulated the mRNA expression levels of LOX2S, AOS, OPR, and JMT involved in the JA biosynthesis pathway, as well as DXS, HMGCR, TPS14, and α-terpineol synthesis genes involved in the monoterpene synthesis pathway compared with T2, T3, and CK. Thus, grape cluster soaking treatment with MeJA could greatly activate volatile monoterpene synthesis. The results will deeply increase our understanding of the monoterpene biosynthesis of grape berry skin in response to MeJA.

Comparative (Within Species) Genomics of the Vitis vinifera L. Terpene Synthase Family to Explore the Impact of Genotypic Variation Using Phased Diploid Genomes

The Vitis vinifera L. terpene synthase (VviTPS) family was comprehensively annotated on the phased diploid genomes of three closely related cultivars: Cabernet Sauvignon, Carménère and Chardonnay. VviTPS gene regions were grouped to chromosomes, with the haplotig assemblies used to identify allelic variants. Functional predictions of the VviTPS subfamilies were performed using enzyme active site phylogenies resulting in the putative identification of the initial substrate and cyclization mechanism of VviTPS enzymes. Subsequent groupings into conserved catalytic mechanisms was coupled with an analysis of cultivar-specific gene duplications, resulting in the identification of conserved and unique VviTPS clusters. These findings are presented as a collection of interactive networks where any VviTPS of interest can be queried through BLAST, allowing for a rapid identification of VviTPS-subfamily, enzyme mechanism and degree of connectivity (i.e., extent of duplication). The comparative genomic analyses presented expands our understanding of the VviTPS family and provides numerous new gene models from three diploid genomes.

(3R,6E)-nerolidol, a fertility-related volatile secreted by the queens of higher termites (Termitidae: Syntermitinae)

The queens of advanced social insects maintain their reproductive monopoly by using exocrine chemicals. The chemistry of these "queen pheromones" in termites is poorly known. We show that primary queens of four higher termites from the subfamily Syntermitinae (Embiratermes neotenicus, Silvestritermes heyeri, Labiotermes labralis, and Cyrilliotermes angulariceps) emit significant amounts of the sesquiterpene alcohol (E)-nerolidol. It is the dominant analyte in queen body washes; it is present on the surface of eggs, but absent in kings, workers, and soldiers. In E. neotenicus, it is also produced by replacement neotenic queens, in quantities correlated with their fertility. Using newly synthesised (3R,6E)-nerolidol, we demonstrate that the queens of this species produce only the (R) enantiomer. It is distributed over the surface of their abdomen, in internal tissues, and in the haemolymph, as well as in the headspace of the queens. Both (R) and (S) enantiomers are perceived by the antennae of E. neotenicus workers. The naturally occurring (R) enantiomer elicited a significantly larger antennal response, but it did not show any behavioural effect. In spite of technical difficulties encountered in long-term experiments with the studied species, (3R,6E)-nerolidol remains among eventual candidates for the role in queen fertility signalling.

A herbivore-induced homoterpene volatile is emitted from Basella alba leaves

An irregular C11 homoterpene, (E)-4,8-dimethylnona-1,3,7-triene (DMNT) was identified as a major component of the volatile compounds emitted from Basella alba leaves induced by herbivore. The terpenes including DMNT were not detected from the leaves infected by Botrytis cinerea. These results suggested that volatile emission from B. alba leaves was induced by herbivory but not by a fungal infection.

Transcriptome-guided identification and functional characterization of key terpene synthases involved in constitutive and methyl jasmonate-inducible volatile terpene formation in Eremochloa ophiuroides (Munro) Hack

Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) is a warm-season turfgrass, widely planted in residential lawns and recreational fields. Here, we uncovered three major terpenes released from the shoots of Eo: (E)-β-ocimene (6%), α-muurolene (87.8%), and eremophilene (6.2%). Methyl jasmonate (MeJA) treatment increased the emission of monoterpenes, including (E)- and (Z)-β-ocimene, limonene, and myrcene, as well as sesquiterpene blends of (E)-caryophyllene, α-copaene, (+)-cyclosativene, and α-farnesene. RNA sequencing analysis predicted 14 putative Eo terpene synthase (EoTPS) genes, and two full-length EoTPS were successfully amplified: Eo7816 (1722 bp) and Eo6039 (1701 bp). Phylogenetic analysis revealed that Eo7816 and Eo6039 belonged to the clades of TPS-b and TPS-a, respectively. The Arabidopsis transgenic plants overexpressing Eo7816 exclusively released (E)-β-ocimene (96%) with (Z)-β-ocimene and myrcene. In contrast, Eo6039-overexpressing Arabidopsis plants emitted significant amounts of α-muurolene (69.4%) and eremophilene (21.8%). Together, we demonstrated that the two TPSs play roles in producing major volatile terpenes in Eo.

Medicago TERPENE SYNTHASE 10 Is Involved in Defense Against an Oomycete Root Pathogen

In nature, plants interact with numerous beneficial or pathogenic soil-borne microorganisms. Plants have developed various defense strategies to expel pathogenic microbes, some of which function soon after pathogen infection. We used Medicago truncatula and its oomycete pathogen Aphanomyces euteiches to elucidate early responses of the infected root. A. euteiches causes root rot disease in legumes and is a limiting factor in legume production. Transcript profiling of seedlings and adult plant roots inoculated with A. euteiches zoospores for 2 h revealed specific upregulation of a gene encoding a putative sesquiterpene synthase (M. truncatula TERPENE SYNTHASE 10 [MtTPS10]) in both developmental stages. MtTPS10 was specifically expressed in roots upon oomycete infection. Heterologous expression of MtTPS10 in yeast led to production of a blend of sesquiterpenes and sesquiterpene alcohols, with NMR identifying a major peak corresponding to himalachol. Moreover, plants carrying a tobacco (Nicotiana tabacum) retrotransposon Tnt1 insertion in MtTPS10 lacked the emission of sesquiterpenes upon A. euteiches infection, supporting the assumption that the identified gene encodes a multiproduct sesquiterpene synthase. Mttps10 plants and plants with reduced MtTPS10 transcript levels created by expression of an MtTPS10-artificial microRNA in roots were more susceptible to A. euteiches infection than were the corresponding wild-type plants and roots transformed with the empty vector, respectively. Sesquiterpenes produced by expression of MtTPS10 in yeast also inhibited mycelial growth and A. euteiches zoospore germination. These data suggest that sesquiterpene production in roots by MtTPS10 plays a previously unrecognized role in the defense response of M. truncatula against A. euteiches.

Methylobacterium oryzae CBMB20 influences photosynthetic traits, volatile emission and ethylene metabolism in Oryza sativa genotypes grown in salt stress conditions

MAIN CONCLUSION

Inoculation of endophytic Methylobacterium oryzae CBMB20 in salt-stressed rice plants improves photosynthesis and reduces stress volatile emissions due to mellowing of ethylene-dependent responses and activating vacuolar H+-ATPase. The objective of this study was to analyze the impact of ACC (1-aminocyclopropane-1-carboxylate) deaminase-producing Methylobacterium oryzae CBMB20 in acclimation of plant to salt stress by controlling photosynthetic characteristics and volatile emission in salt-sensitive (IR29) and moderately salt-resistant (FL478) rice (Oryza sativa L.) cultivars. Saline levels of 50 mM and 100 mM NaCl with and without bacteria inoculation were applied, and the temporal changes in stress response and salinity resistance were assessed by monitoring photosynthetic characteristics, ACC accumulation, ACC oxidase activity (ACO), vacuolar H+ ATPase activity, and volatile organic compound (VOC) emissions. Salt stress considerably reduced photosynthetic rate, stomatal conductance, PSII efficiency and vacuolar H+ ATPase activity, but it increased ACC accumulation, ACO activity, green leaf volatiles, mono- and sesquiterpenes, and other stress volatiles. These responses were enhanced with increasing salt stress and time. However, rice cultivars treated with CBMB20 showed improved plant vacuolar H+ ATPase activity, photosynthetic characteristics and decreased ACC accumulation, ACO activity and VOC emission. The bacteria-dependent changes were greater in the IR29 cultivar. These results indicate that decreasing photosynthesis and vacuolar H+ ATPase activity rates and increasing VOC emission rates in response to high-salinity stress were effectively mitigated by M. oryzae CBMB20 inoculation.

Enhanced structural diversity in terpenoid biosynthesis: enzymes, substrates and cofactors

The enormous diversity of terpenes found in nature is generated by enzymes known as terpene synthases, or cyclases. Some are also known for their ability to convert a single substrate into multiple products. This review comprises monoterpene and sesquiterpene synthases that are multiproduct in nature along with the regulation factors that can alter the product specificity of multiproduct terpene synthases without genetic mutations. Variations in specific assay conditions with focus on shifts in product specificity based on change in metal cofactors, assay pH and substrate geometry are described. Alterations in these simple cellular conditions provide the organism with enhanced chemodiversity without investing into new enzymatic architecture. This versatility to modulate product diversity grants organisms, especially immobile ones like plants with access to an enhanced defensive repertoire by simply altering cofactors, pH level and substrate geometry.

Functional characterization and expression analysis of two terpene synthases involved in floral scent formation in Lilium 'Siberia'

Floral scent formation in Lilium 'Siberia' is mainly due to monoterpene presence in the floral profile. LoTPS1 and LoTPS3 are responsible for the formation of (±)-linalool and β-ocimene in Lilium 'Siberia'. Lilium 'Siberia' is a perennial herbaceous plant belonging to Liliaceae family, cultivated both as a cut flower and garden plant. The snowy white flower emits a pleasant aroma which is mainly caused by monoterpenes present in the floral volatile profile. Previously terpene synthase (TPS) genes have been isolated and characterized from various plant species but less have been identified from Liliaceae family. Here, two terpene synthase genes (LoTPS1 and LoTPS3), which are highly expressed in sepals and petals of Lilium 'Siberia' flower were functionally characterized recombinant LoTPS1 specifically catalyzes the formation of (Z)-β-ocimene and (±)-linalool as its main volatile compounds from geranyl pyrophosphate (GPP), whereas LoTPS3 is a promiscuous monoterpene synthase which utilizes both GPP and farnesyl pyrophosphate (FPP) as a substrate to generate (±)-linalool and cis-nerolidol, respectively. Transcript levels of both genes were prominent in flowering parts, especially in sepals and petals which are the main source of floral scent production. The gas chromatography-mass spectrometry (GC-MS) and quantitative real-time PCR analysis revealed that the compounds were emitted throughout the day, prominently during the daytime and lower levels at night following a strong circadian rhythm in their emission pattern. Regarding mechanical wounding, both genes showed considerable involvement in floral defense by inducing the emission of (Z)-β-ocimene and (±)-linalool, elevating the transcript accumulation of LoTPS1 and LoTPS3. Furthermore, the subcellular localization experiment revealed that LoTPS1 was localized in plastids, whilst LoTPS3 in mitochondria. Our findings on these two TPSs characterized from Lilium 'Siberia' provide new insights into molecular mechanisms of terpene biosynthesis in this species and also provide an opportunity for biotechnological modification of floral scent profile of Lilium.

Evolutionary analysis and structural characterization of Aquilaria sinensis sesquiterpene synthase in agarwood formation: A computational study

Agarwood originating from Aquilaria sinensis contains sesquiterpenoids that have tremendous commercial value in the pharmaceutical and fragrance industries. Aquilaria sinensis sesquiterpene synthase (AsSTS) is the key enzyme in the agarwood biosynthesis pathway, and its activity directly affects the chemical composition of agarwood; however, its role in species evolution remains unclear. In this study, we performed an evolutionary analysis based on 68 plant sesquiterpene synthase (STS) genes and further structural characterization of the gene encoding AsSTS to explore its molecular evolution. The phylogenetic tree indicated that these STS genes included three subfamilies. Additionally, 23 positively selected sites were detected, and no influence of recombination was found. Furthermore, the protein structure of AsSTS was characterized using primary sequence and structural analyses as having a functional active site lid domain, a substrate binding site, two post-translational modification sites and four conserved motifs. Finally, most virtual mutations of positively selected sites could be stabilized against thermal denaturation by a decrease in free energy, and three virtual mutations (D403R, G470Q and S538K) were shown to play important roles in the function and stability of AsSTS. The molecular evolutionary analysis of plant STSs provides essential clues for further experimental site-directed mutagenesis and molecular modification of AsSTS.

Metabolomics Analysis Reveals that Ethylene and Methyl Jasmonate Regulate Different Branch Pathways to Promote the Accumulation of Terpenoid Indole Alkaloids in Catharanthus roseus

The medicinal plant Catharanthus roseus accumulates large numbers of terpenoid indole alkaloids (TIAs), including the pharmaceutically important vinblastine, vincristine, ajmalicine, and serpentine. The phytohormone ethylene or methyl jasmonate (MeJA) can markedly enhance alkaloid accumulation. The interaction between ethylene or MeJA in the regulation of TIA biosynthesis in C. roseus is unknown. Here, a metabolomics platform is reported that is based on liquid chromatography (LC) coupled with time-of-flight mass spectrometry to study candidate components for TIA biosynthesis, which is controlled by ethylene or MeJA in C. roseus. Multivariate analysis identified 16 potential metabolites mostly associated with TIA metabolic pathways and seven targeted metabolites, outlining the TIA biosynthesis metabolic networks controlled by ethylene or MeJA. Interestingly, ethylene and MeJA regulate the 2-C-methyl-d-erythritol 4-phosphate (MEP) and acetate-mevalonate (MVA) pathways through AACT and HMGS and through DXS, respectively, to induce TIA biosynthesis in C. roseus. Overall, both nontargeted and targeted metabolomics, as well as transcript analysis, were used to reveal that MeJA and ethylene control different metabolic networks to induce TIA biosynthesis.

Mechanistic studies of sesquiterpene cyclases based on their carbon isotope ratios at natural abundance

During the process of terpene biosynthesis, C-C bond breaking and forming steps are subjected to kinetic carbon isotope effects, leading to distinct carbon isotopic signatures of the products. Accordingly, carbon isotopic signatures could be used to reveal the 'biosynthetic history' of the produced terpenoids. Five known sesquiterpene cyclases, regulating three different pathways, representing simple to complex biosynthetic sequences, were heterologously expressed and used for in vitro assays with farnesyl diphosphate as substrate. Compound specific isotope ratio mass spectrometry measurements of the enzyme substrate farnesyl diphosphate (FDP) and the products of all the five cyclases were performed. The calculated δ¹³ C value for FDP, based on δ¹³ C values and relative amounts of the products, was identical with its measured δ¹³ C value, confirming the reliability of the approach and the precision of measurements. The different carbon isotope ratios of the products reflect the complexity of their structure and are correlated with the frequency of carbon-carbon bond forming and breaking steps on their individual biosynthetic pathways. Thus, the analysis of carbon isotopic signatures of terpenes at natural abundance can be used as a powerful tool in elucidation of associated biosynthetic mechanisms of terpene synthases and in future in vivo studies even without 'touching' the plant.

Expression of lima bean terpene synthases in rice enhances recruitment of a beneficial enemy of a major rice pest

Volatile terpenoids play a key role in plant defence against herbivory by attracting parasitic wasps. We identified seven terpene synthase genes from lima bean, Phaseolus lunatus L. following treatment with either the elicitor alamethicin or spider mites, Tetranychus cinnabarinus. Four of the genes (Pltps2, Pltps3, Pltps4 and Pltps5) were up-regulated with their derived proteins phylogenetically clustered in the TPS-g subfamily and PlTPS3 positioned at the base of this cluster. Recombinant PlTPS3 was able to convert geranyl diphosphate and farnesyl diphosphate to linalool and (E)-nerolidol, the latter being precursor of the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). Recombinant PlTPS4 showed a different substrate specificity and produced linalool and (E)-nerolidol, as well as (E,E)-geranyllinalool from geranylgeranyl diphosphate. Transgenic rice expressing Pltps3 emitted significantly more (S)-linalool and DMNT than wild-type plants, whereas transgenic rice expressing Pltps4 produced (S)-linalool, DMNT and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT). In laboratory bioassays, female Cotesia chilonis, the natural enemy of the striped rice stemborer, Chilo suppressalis, were significantly attracted to the transgenic plants and their volatiles. We further confirmed this with synthetic blends mimicking natural rice volatile composition. Our study demonstrates that the transformation of rice to produce volatile terpenoids has the potential to enhance plant indirect defence through natural enemy recruitment.

Methyl jasmonate-induced emission of biogenic volatiles is biphasic in cucumber: a high-resolution analysis of dose dependence

Methyl jasmonate (MeJA) is a key airborne elicitor activating jasmonate-dependent signaling pathways, including induction of stress-related volatile emissions, but how the magnitude and timing of these emissions scale with MeJA dose is not known. Treatments with exogenous MeJA concentrations ranging from mild (0.2 mM) to lethal (50 mM) were used to investigate quantitative relationships among MeJA dose and the kinetics and magnitude of volatile release in Cucumis sativus by combining high-resolution measurements with a proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and GC-MS. The results highlighted biphasic kinetics of elicitation of volatiles. The early phase, peaking in 0.1-1 h after the MeJA treatment, was characterized by emissions of lipoxygenase (LOX) pathway volatiles and methanol. In the subsequent phase, starting in 6-12 h and reaching a maximum in 15-25 h after the treatment, secondary emissions of LOX compounds as well as emissions of monoterpenes and sesquiterpenes were elicited. For both phases, the maximum emission rates and total integrated emissions increased with applied MeJA concentration. Furthermore, the rates of induction and decay, and the duration of emission bursts were positively, and the timing of emission maxima were negatively associated with MeJA dose for LOX compounds and terpenoids, except for the duration of the first LOX burst. These results demonstrate major effects of MeJA dose on the kinetics and magnitude of volatile response, underscoring the importance of biotic stress severity in deciphering the downstream events of biological impacts.

Terpenoids in plant and arbuscular mycorrhiza-reinforced defence against herbivorous insects

BACKGROUND

Plants, though sessile, employ various strategies to defend themselves against herbivorous insects and convey signals of an impending herbivore attack to other plant(s). Strategies include the production of volatiles that include terpenoids and the formation of symbiotic associations with fungi, such as arbuscular mycorrhiza (AM). This constitutes a two-pronged above-ground/below-ground attack-defence strategy against insect herbivores.

SCOPE

Terpenoids represent an important constituent of herbivore-induced plant volatiles that deter herbivores and/or attract their predators. Terpenoids serve as airborne signals that can induce defence responses in systemic undamaged parts of the plant and also prime defence responses in neighbouring plants. Colonization of roots by AM fungi is known to influence secondary metabolism in plants; this includes alteration of the concentration and composition of terpenoids, which can boost both direct and indirect plant defence against herbivorous insects. Enhanced nutrient uptake facilitated by AM, changes in plant morphology and physiology and increased transcription levels of certain genes involved in the terpenoid biosynthesis pathway result in alterations in plant terpenoid profiles. The common mycorrhizal networks of external hyphae have added a dimension to the two-pronged plant defence strategy. These act as conduits to transfer defence signals and terpenoids.

CONCLUSION

Improved understanding of the roles of terpenoids in plant and AM defences against herbivory and of interplant signalling in natural communities has significant implications for sustainable management of pests in agricultural ecosystems.

Characterization of Biosynthetic Pathways for the Production of the Volatile Homoterpenes DMNT and TMTT in Zea mays

Plant volatiles not only have multiple defense functions against herbivores, fungi, and bacteria, but also have been implicated in signaling within the plant and toward other organisms. Elucidating the function of individual plant volatiles will require more knowledge of their biosynthesis and regulation in response to external stimuli. By exploiting the variation of herbivore-induced volatiles among 26 maize (Zea mays) inbred lines, we conducted a nested association mapping and genome-wide association study (GWAS) to identify a set of quantitative trait loci (QTLs) for investigating the pathways of volatile terpene production. The most significant identified QTL affects the emission of (E)-nerolidol, linalool, and the two homoterpenes (E)-3,8-dimethyl-1,4,7-nonatriene (DMNT) and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). GWAS associated a single nucleotide polymorphism in the promoter of the gene encoding the terpene synthase TPS2 with this QTL Biochemical characterization of TPS2 verified that this plastid-localized enzyme forms linalool, (E)-nerolidol, and (E,E)-geranyllinalool. The subsequent conversion of (E)-nerolidol into DMNT maps to a P450 monooxygenase, CYP92C5, which is capable of converting nerolidol into DMNT by oxidative degradation. A QTL influencing TMTT accumulation corresponds to a similar monooxygenase, CYP92C6, which is specific for the conversion of (E,E)-geranyllinalool to TMTT The DMNT biosynthetic pathway and both monooxygenases are distinct from those previously characterized for DMNT and TMTT synthesis in Arabidopsis thaliana, suggesting independent evolution of these enzymatic activities.

Overexpression of Panax ginseng sesquiterpene synthase gene confers tolerance against Pseudomonas syringae pv. tomato in Arabidopsis thaliana

Sesquiterpenes are an abundant group belonging to the terpenoid family, with a C15 structure comprise of three isoprene units. Many sesquiterpenes are volatile compounds and it act as chemical messenger in plant signalling, particularly in the defense mechanism against biotic and abiotic stresses. Panax ginseng Meyer is important medicinal herbs with various reported pharmacological efficacies in which its triterpenoid saponins, called ginsenosides, were mostly studied. However, there have been few studies on volatile sesquiterpenes compounds regulation on P. ginseng. As slow-growing perennial plant, P. ginseng received many kind of stresses during its cultivation. The pathogen attack is one of the most devastated perturbation for ginseng yield. Thus, we aimed to analyze P. ginseng STS gene (PgSTS) expressions in ginseng organs as well as mono-, sesquiterpenes contents from ginseng seedlings treated with elicitors. qRT-PCR and GC-MS analysis showed that two elicitors- salicylic acid (SA) and methyl jasmonate (MeJA) triggered PgSTS expression at different time points and significantly induced mono-, sesquiterpene yield. Overexpression of PgSTS in Arabidopsis also induced high terpene content and conferred tolerance against Pseudomonas syringae pv. tomato infection. These results suggested that PgSTS transcripts are involved in terpenoid biosynthesis in response to environmental stress mediated by MeJA and SA elicitors; thus, generate tolerance against pathogen attack.

Sources of specificity in plant damaged-self recognition

Plants perceive injury and herbivore attack via the recognition of damage-associated molecular patterns (DAMPs) and herbivore-associated molecular patterns (HAMPs). Although HAMPs in particular are cues that can indicate the presence of a specific enemy, the application of pure DAMPs or HAMPs frequently activates general downstream responses: membrane depolarization, Ca(2+) influxes, oxidative stress, MAPKinase activation and octadecanoid signaling at the molecular level, and the expression of digestion inhibitors, cell wall modifications and other general defenses at the phenotypic level. We discuss the relative benefits of perceiving the non-self versus the damaged-self and of specific versus non-specific responses and suggest that the perception of a complex mixture of DAMPs and HAMPs triggers fine-tuned plant responses. DAMPs such as extracellular ATP (eATP), cell wall fragments, signaling peptides, herbivore-induced volatile organic compounds (HI-VOCs) and eDNA hold the key for a more complete understanding of how plants perceive that and by whom they are attacked.

Multi-Substrate Terpene Synthases: Their Occurrence and Physiological Significance

Terpene synthases are responsible for synthesis of a large number of terpenes in plants using substrates provided by two distinct metabolic pathways, the mevalonate-dependent pathway that is located in cytosol and has been suggested to be responsible for synthesis of sesquiterpenes (C15), and 2-C-methyl-D-erythritol-4-phosphate pathway located in plastids and suggested to be responsible for the synthesis of hemi- (C5), mono- (C10), and diterpenes (C20). Recent advances in characterization of genes and enzymes responsible for substrate and end product biosynthesis as well as efforts in metabolic engineering have demonstrated existence of a number of multi-substrate terpene synthases. This review summarizes the progress in the characterization of such multi-substrate terpene synthases and suggests that the presence of multi-substrate use might have been significantly underestimated. Multi-substrate use could lead to important changes in terpene product profiles upon substrate profile changes under perturbation of metabolism in stressed plants as well as under certain developmental stages. We therefore argue that multi-substrate use can be significant under physiological conditions and can result in complicate modifications in terpene profiles.

Scaling of photosynthesis and constitutive and induced volatile emissions with severity of leaf infection by rust fungus (Melampsora larici-populina) in Populus balsamifera var. suaveolens

Fungal infections result in decreases in photosynthesis, induction of stress and signaling volatile emissions and reductions in constitutive volatile emissions, but the way different physiological processes scale with the severity of infection is poorly known. We studied the effects of infection by the obligate biotrophic fungal pathogen Melampsora larici-populina Kleb., the causal agent of poplar leaf rust disease, on photosynthetic characteristics, and constitutive isoprene and induced volatile emissions in leaves of Populus balsamifera var. suaveolens (Fisch.) Loudon. exhibiting different degrees of damage. The degree of fungal damage, quantified by the total area of chlorotic and necrotic leaf areas, varied between 0 (noninfected control) and ∼60%. The rates of all physiological processes scaled quantitatively with the degree of visual damage, but the scaling with damage severity was weaker for photosynthetic characteristics than for constitutive and induced volatile release. Over the whole range of damage severity, the net assimilation rate per area (AA) decreased 1.5-fold, dry mass per unit area 2.4-fold and constitutive isoprene emissions 5-fold, while stomatal conductance increased 1.9-fold and dark respiration rate 1.6-fold. The emissions of key stress and signaling volatiles (methanol, green leaf volatiles, monoterpenes, sesquiterpenes and methyl salicylate) were in most cases nondetectable in noninfested leaves, and increased strongly with increasing the spread of infection. The moderate reduction in AA resulted from the loss of photosynthetically active biomass, but the reduction in constitutive isoprene emissions and the increase in induced volatile emissions primarily reflected changes in the activities of corresponding biochemical pathways. Although all physiological alterations in fungal-infected leaves occurred in a stress severity-dependent manner, modifications in primary and secondary metabolic pathways scaled differently due to contrasting operational mechanisms.

Direct suppression of a rice bacterial blight (Xanthomonas oryzae pv. oryzae) by monoterpene (S)-limonene

Rice bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is a severe disease of rice plants. Upon pathogen infection, rice biosynthesizes phytoalexins, including diterpenoids such as momilactones, phytocassanes, and oryzalexins. However, information on headspace volatiles in response to Xoo infection is limited. We have examined headspace volatile terpenes, induced by the infection of Xoo, and investigated their biological roles in the rice plant. Monoterpenes α-thujene, α-pinene, sabinene, myrcene, α-terpene, and (S)-limonene and sesquiterpenes cyclosativene, α-copaene, and β-elemene were detected from 1-week-old Xoo-infected rice seedlings, by solid-phase microextraction-gas chromatography-mass spectrometry. All monoterpenes were constitutively released from rice seedlings before Xoo infection. However, (S)-limonene emission was further elicited after exposure of the seedlings to Xoo in coincidence with upregulation of limonene synthase gene (OsTPS20) transcripts. Only the stereospecific (S)-limonene [and not (R)-limonene or other monoterpenes] severely inhibited Xoo growth, as confirmed by disc diffusion and liquid culture assays. Rice seedlings showed suppressed pathogenic symptoms suggestive of resistance to Xoo infection after foliar treatment with (S)-limonene. Collectively, our findings suggest that (S)-limonene is a volatile phytoanticipin, which plays a significant role in suppressing Xoo growth in rice seedlings.

Tomato Fruits-A Platform for Metabolic Engineering of Terpenes

Terpenoids are a large and diverse class of plant metabolites including mono-, sesqui-, and diterpenes. They have numerous functions in basic physiological processes as well as the interaction of plants with their biotic and abiotic environment. Due to the tight regulation of biosynthetic pathways and the resulting limited natural availability of terpenes, there is a strong interest in increasing their production in plants by metabolic engineering for agricultural, pharmaceutical, and industrial applications. The tomato fruit system was developed as a platform for metabolic engineering of terpenes to overcome detrimental effects on overall plant growth and photosynthesis traits, which are affected when terpenoid engineering is performed in vegetative tissues. Here we describe how the use of fruit-specific promoters for transgene expression can avoid these unwanted effects. In addition, targeting the expression of the introduced terpene biosynthetic gene to fruit tissue can take advantage of the large precursor pool provided by the methylerythritol-phosphate (MEP) pathway, which is highly active during tomato fruit ripening to facilitate the accumulation of carotenoids. We also discuss how the production of high levels of target terpene compounds can be achieved in fruits by the expression of individual or a combination of (i) the MEP or mevalonic acid pathway enzymes, (ii) prenyltransferases, and/or (iii) terpene synthases. Finally, we provide a brief outline of how the emitted as well as internal pools of terpenes can be analyzed in transgenic tomato fruits.

The Mechanism of Ethylene Signaling Induced by Endophytic Fungus Gilmaniella sp. AL12 Mediating Sesquiterpenoids Biosynthesis in Atractylodes lancea

Ethylene, the first known gaseous phytohormone, is involved in plant growth, development as well as responses to environmental signals. However, limited information is available on the role of ethylene in endophytic fungi induced secondary metabolites biosynthesis. Atractylodes lancea is a traditional Chinese herb, and its quality depends on the main active compounds sesquiterpenoids. This work showed that the endophytic fungus Gilmaniella sp. AL12 induced ethylene production in Atractylodes lancea. Pre-treatment of plantlets with ethylene inhibiter aminooxyacetic acid (AOA) suppressed endophytic fungi induced accumulation of ethylene and sesquiterpenoids. Plantlets were further treated with AOA, salicylic acid (SA) biosynthesis inhibitor paclobutrazol (PAC), jasmonic acid inhibitor ibuprofen (IBU), hydrogen peroxide (H2O2) scavenger catalase (CAT), nitric oxide (NO)-specific scavenger 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO). With endophytic fungi inoculation, IBU or PAC did not inhibit ethylene production, and JA and SA generation were suppressed by AOA, showing that ethylene may act as an upstream signal of JA and SA pathway. With endophytic fungi inoculation, CAT or cPTIO suppressed ethylene production, and H2O2 or NO generation was not affected by 1-aminocyclopropane-1-carboxylic acid (ACC), showing that ethylene may act as a downstream signal of H2O2 and NO pathway. Then, plantlets were treated with ethylene donor ACC, JA, SA, H2O2, NO donor sodium nitroprusside (SNP). Exogenous ACC could trigger JA and SA generation, whereas exogenous JA or SA did not affect ethylene production, and the induced sesquiterpenoids accumulation triggered by ACC was partly suppressed by IBU and PAC, showing that ethylene acted as an upstream signal of JA and SA pathway. Exogenous ACC did not affect H2O2 or NO generation, whereas exogenous H2O2 and SNP induced ethylene production, and the induced sesquiterpenoids accumulation triggered by SNP or H2O2 was partly suppressed by ACC, showing that ethylene acted as a downstream signal of NO and H2O2 pathway. Taken together, this study demonstrated that ethylene is an upstream signal of JA and SA, and a downstream signal of NO and H2O2 signaling pathways, and acts as an important signal mediating sesquiterpenoids biosynthesis of Atractylodes lancea induced by the endophytic fungus.

Rice terpene synthase 20 (OsTPS20) plays an important role in producing terpene volatiles in response to abiotic stresses

This study examined the volatile terpenes produced by rice seedlings in response to oxidative stress induced by various abiotic factors. Solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) analyses revealed that when exposed to UV-B radiation, rice seedlings emitted a bouquet of monoterpene mixtures in a time-dependent manner. The mixtures comprised limonene, sabinene, myrcene, α-terpinene, β-ocimene, γ-terpinene, and α-terpinolene. Among them, (S)-limonene was the most abundant volatile, discriminated by chiral SPME-GC-MS. The volatile profiles collected from rice plants treated with both γ-irradiation and H2O2 were identical to those observed in the UV-B irradiated plants, thus indicating that the volatile mixtures were specifically produced in response to oxidative stress, particularly in the presence of H2O2. Using a reverse genetics approach, we isolated full-length rice terpene synthase 20 (OsTPS20, 599 amino acids, 69.39 kDa), which was further characterized as an (S)-limonene synthase by removing the N-terminal signal peptide (63 amino acids) of the protein. The recombinant OsTPS20 protein catalyzed the conversion of geranyl diphosphate to (S)-limonene and other minor monoterpenes, essentially covering all of the volatile compounds detected from the plant. Moreover, qRT-PCR revealed that the transcript levels of OsTPS20 were significantly induced in response to oxidative stress, thereby suggesting that OsTPS20 plays a major role in producing terpene volatiles during abiotic stress. Detailed biochemical analyses and the unusual domain characteristics of OsTPS20 are also discussed in this report.

The floral transcriptome of ylang ylang (Cananga odorata var. fruticosa) uncovers biosynthetic pathways for volatile organic compounds and a multifunctional and novel sesquiterpene synthase

The pleasant fragrance of ylang ylang varieties (Cananga odorata) is mainly due to volatile organic compounds (VOCs) produced by the flowers. Floral scents are a key factor in plant-insect interactions and are vital for successful pollination. C. odorata var. fruticosa, or dwarf ylang ylang, is a variety of ylang ylang that is popularly grown in Southeast Asia as a small shrub with aromatic flowers. Here, we describe the combined use of bioinformatics and chemical analysis to discover genes for the VOC biosynthesis pathways and related genes. The scented flowers of C. odorata var. fruticosa were analysed by gas chromatography/mass spectrometry and a total of 49 VOCs were identified at four different stages of flower development. The bulk of these VOCs were terpenes, mainly sesquiterpenes. To identify the various terpene synthases (TPSs) involved in the production of these essential oils, we performed RNA sequencing on mature flowers. From the RNA sequencing data, four full-length TPSs were functionally characterized. In vitro assays showed that two of these TPSs were mono-TPSs. CoTPS1 synthesized four products corresponding to β-thujene, sabinene, β-pinene, and α-terpinene from geranyl pyrophosphate and CoTPS4 produced geraniol from geranyl pyrophosphate. The other two TPSs were identified as sesqui-TPSs. CoTPS3 catalysed the conversion of farnesyl pyrophosphate to α-bergamotene, whereas CoTPS2 was found to be a multifunctional and novel TPS that could catalyse the synthesis of three sesquiterpenes, β-ylangene, β-copaene, and β-cubebene. Additionally, the activities of the two sesqui-TPSs were confirmed in planta by transient expression of these TPS genes in Nicotiana benthamiana leaves by Agrobacterium-mediated infiltration.

Rice terpene synthase 20 (OsTPS20) plays an important role in producing terpene volatiles in response to abiotic stresses

This study examined the volatile terpenes produced by rice seedlings in response to oxidative stress induced by various abiotic factors. Solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) analyses revealed that when exposed to UV-B radiation, rice seedlings emitted a bouquet of monoterpene mixtures in a time-dependent manner. The mixtures comprised limonene, sabinene, myrcene, α-terpinene, β-ocimene, γ-terpinene, and α-terpinolene. Among them, (S)-limonene was the most abundant volatile, discriminated by chiral SPME-GC-MS. The volatile profiles collected from rice plants treated with both γ-irradiation and H2O2 were identical to those observed in the UV-B irradiated plants, thus indicating that the volatile mixtures were specifically produced in response to oxidative stress, particularly in the presence of H2O2. Using a reverse genetics approach, we isolated full-length rice terpene synthase 20 (OsTPS20, 599 amino acids, 69.39 kDa), which was further characterized as an (S)-limonene synthase by removing the N-terminal signal peptide (63 amino acids) of the protein. The recombinant OsTPS20 protein catalyzed the conversion of geranyl diphosphate to (S)-limonene and other minor monoterpenes, essentially covering all of the volatile compounds detected from the plant. Moreover, qRT-PCR revealed that the transcript levels of OsTPS20 were significantly induced in response to oxidative stress, thereby suggesting that OsTPS20 plays a major role in producing terpene volatiles during abiotic stress. Detailed biochemical analyses and the unusual domain characteristics of OsTPS20 are also discussed in this report.

Ethylene Signaling Modulates Herbivore-Induced Defense Responses in the Model Legume Medicago truncatula

One or more effectors in the labial saliva (LS) of generalist Noctuid caterpillars activate plant signaling pathways to modulate jasmonate (JA)-dependent defense responses; however, the exact mechanisms involved have yet to be elucidated. A potential candidate in this phytohormone interplay is the ethylene (ET) signaling pathway. We compared the biochemical and molecular responses of the model legume Medicago truncatula and the ET-insensitive skl mutant to herbivory by fourth instar Spodoptera exigua (Hübner) caterpillars with intact or impaired LS secretions. Cellular oxidative stress increases rapidly after herbivory, as evidenced by changes in oxidized-to-reduced ascorbate (ASC) and glutathione (GSH) ratios. The caterpillar-specific increase in GSH ratios and the LS-specific increase in ASC ratios are alleviated in the skl mutant, indicating that ET signaling is required. Ten hours postherbivory, markers of the JA and JA/ET pathways are differentially expressed; MtVSP is induced and MtHEL is repressed in a caterpillar LS- and ET-independent manner. In contrast, expression of the classic marker of the systemic acquired resistance pathway, MtPR1, is caterpillar LS-dependent and requires ET signaling. Caterpillar LS further suppresses the induction of JA-related trypsin inhibitor activity in an ET-dependent manner. Findings suggest that ET is involved in the caterpillar LS-dependent, salicylic acid/NPR1-mediated attenuation of JA-dependent induced responses.

Direct Contact - Sorptive Tape Extraction coupled with Gas Chromatography - Mass Spectrometry to reveal volatile topographical dynamics of lima bean (Phaseolus lunatus L.) upon herbivory by Spodoptera littoralis Boisd

BACKGROUND

The dynamics of plant volatile (PV) emission, and the relationship between damaged area and biosynthesis of bioactive molecules in plant-insect interactions, remain open questions. Direct Contact-Sorptive Tape Extraction (DC-STE) is a sorption sampling technique employing non adhesive polydimethylsiloxane tapes, which are placed in direct contact with a biologically-active surface. DC-STE coupled to Gas Chromatography - Mass Spectrometry (GC-MS) is a non-destructive, high concentration-capacity sampling technique able to detect and allow identification of PVs involved in plant responses to biotic and abiotic stresses. Here we investigated the leaf topographical dynamics of herbivory-induced PV (HIPV) produced by Phaseolus lunatus L. (lima bean) in response to herbivory by larvae of the Mediterranean climbing cutworm (Spodoptera littoralis Boisd.) and mechanical wounding by DC-STE-GC-MS.

RESULTS

Time-course experiments on herbivory wounding caused by larvae (HW), mechanical damage by a pattern wheel (MD), and MD combined with the larvae oral secretions (OS) showed that green leaf volatiles (GLVs) [(E)-2-hexenal, (Z)-3-hexen-1-ol, 1-octen-3-ol, (Z)-3-hexenyl acetate, (Z)-3-hexenyl butyrate] were associated with both MD and HW, whereas monoterpenoids [(E)-β-ocimene], sesquiterpenoids [(E)-nerolidol] and homoterpenes (DMNT and TMTT) were specifically associated with HW. Up-regulation of genes coding for HIPV-related enzymes (Farnesyl Pyrophosphate Synthase, Lipoxygenase, Ocimene Synthase and Terpene Synthase 2) was consistent with HIPV results. GLVs and sesquiterpenoids were produced locally and found to influence their own gene expression in distant tissues, whereas (E)-β-ocimene, TMTT, and DMNT gene expression was limited to wounded areas.

CONCLUSIONS

DC-STE-GC-MS was found to be a reliable method for the topographical evaluation of plant responses to biotic and abiotic stresses, by revealing the differential distribution of different classes of HIPVs. The main advantages of this technique include: a) in vivo sampling; b) reproducible sampling; c) ease of execution; d) simultaneous assays of different leaf portions, and e) preservation of plant material for further "omic" studies. DC-STE-GC-MS is also a low-impact innovative method for in situ PV detection that finds potential applications in sustainable crop management.

Inhibition of a multiproduct terpene synthase from Medicago truncatula by 3-bromoprenyl diphosphates

3-Bromo prenyl analogues bind to the active site and act as competitive inhibitors for terpene cyclases and -synthases.

A putative role for γ-aminobutyric acid (GABA) in vascular development in pine seedlings

A model for GABA synthesis in stems of pine seedlings is proposed. The localization of GABA in differentiating tracheids suggests a link between GABA production and vascular development. γ-aminobutyric acid (GABA) is a non-proteinogenic amino acid present in both prokaryotic and eukaryotic organisms. GABA plays a fundamental role as a signal molecule in the central nervous system in animals. In plants, GABA has been correlated with cellular elongation, plant development, gene expression regulation, synthesis of ethylene and other hormones, and signaling. Considering the physiological importance of GABA in plants, the lack of works about GABA localization in this kingdom seems surprising. In this work, the immunolocalization of GABA in root and hypocotyl during seedling development and in bent stem showing compression xylem has been studied. In the seedling root, the GABA signal was very high and restricted to the stele supporting previous evidences indicating a potential role for this amino acid in root growth and nutrient transport. In hypocotyl, GABA was localized in vascular tissues, including differentiating xylem, ray parenchyma and epithelial resin duct cells, drawing also a role for GABA in vascular development, communication and defense. During the production of compression wood, a special lignified wood produced when the stem loss its vertical position, a clear GABA signal was found in the new differentiating xylem cells showing a gradient-like pattern with higher signal in less differentiated elements. The results are in accordance with a previous work indicating that glutamate decarboxylase and GABA production are associated to vascular differentiation in pine Molina-Rueda et al. (Planta 232: 1471-1483, 2010). A model for GABA synthesis in vascular differentiation, communication, and defense is proposed in the stem of pine seedlings.

Comparative genomic and transcriptomic analysis of terpene synthases in Arabidopsis and Medicago

This study provides a timely comparative genomic and transcriptomic analysis of the terpene synthase (TPS) gene family in Medicago truncatula (bears glandular and non-glandular trichomes) and Arabidopsis thaliana (bears non-glandular trichomes). The authors' efforts aimed to gain insight into TPS function, phylogenetic relationships and the role of trichomes in terpene biosynthesis and function. In silico analysis identified 33 and 23 putative full-length TPS genes in Arabidopsis and Medicago, respectively. All AtTPS and MtTPS fall into the five established angiosperm TPS subfamilies, with lineage-specific expansion of Subfamily A in Arabidopsis and Subfamily G in Medicago. Large amounts of tandem duplication have occurred in both species, but only one syntenic duplication seems to have occurred in Arabidopsis, with no such duplication apparent in Medicago. Expression analysis indicates that there is much more trichome-localised TPS expression in Medicago than in Arabidopsis. However, TPS genes were expressed in non-glandular trichomes in both species. One trichome-specific gene has been identified in each Medicago and Arabidopsis along with flower-, seed-, stem- and root-specific genes. Of these, MtTPS11 is a promising candidate for trichome-specific genetic engineering, a technology that may be possible for both plants according to the findings of this manuscript. These results suggest that non-glandular trichomes may play a role in plant chemical defense and/or ecological communication instead of only in physical defence. Finally, the general lack of correlation between expression patterns and phylogenetic relationships in both species suggests that phylogenetic analysis alone is insufficient to predict gene function even for phylogenetically close paralogs.

A method for the simultaneous determination of chlorogenic acid and sesquiterpene lactone content in industrial chicory root foodstuffs

A method for the simultaneous determination of free chlorogenic acids (CGA) and sesquiterpene lactones (STL) in chicory root and its dried (flour) and roasted (grain) forms is described. The method uses one extraction and one analysis for all chicory root products. Various solvents with low to high polarity, such as methanol, chloroform, or n-hexane, were tested alone, in combination in different proportions or with acidified or neutral aqueous solvent. The water/chloroform/methanol (30/30/40, v/v/v) mixture generated the best extraction yield, 21% higher than alcohol mixtures. The profiling of CGA and STL content was performed through a conventional HPLC-DAD method using a PFP core shell column in a fast single run. Good retention time and area repeatability (RDD mean % 0.46 and 5.6, resp.) and linearity (R2≥0.96) were obtained. The STL and chlorogenic acids levels determined were 254.7 and 100.2 μg/g of dry matter in the root, 792.5 and 1,547 μg/g in flour, and 160.4 and 822.5 μg/g in the roasted grains, respectively. With an average recovery of 106% and precision of 90%, this method is rapid, reproducible, and straightforward way to quantify the chlorogenic acids and STL in chicory raw material and end products.