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

Potato virus Y infection hinders potato defence response and renders plants more vulnerable to Colorado potato beetle attack

In the field, plants are challenged by more than one biotic stressor at the same time. In this study, the molecular interactions between potato (Solanum tuberosum L.), Colorado potato beetle (Leptinotarsa decemlineata Say; CPB) and Potato virus Y(NTN) (PVY(NTN) ) were investigated through analyses of gene expression in the potato leaves and the gut of the CPB larvae, and of the release of potato volatile compounds. CPB larval growth was enhanced when reared on secondary PVY(NTN) -infected plants, which was linked to decreased accumulation of transcripts associated with the antinutritional properties of potato. In PVY(NTN) -infected plants, ethylene signalling pathway induction and induction of auxin response transcription factors were attenuated, while no differences were observed in jasmonic acid (JA) signalling pathway. Similarly to rearing on virus-infected plants, CPB larvae gained more weight when reared on plants silenced in JA receptor gene (coi1). Although herbivore-induced defence mechanism is regulated predominantly by JA, response in coi1-silenced plants only partially corresponded to the one observed in PVY(NTN) -infected plants, confirming the role of other plant hormones in modulating this response. The release of β-barbatene and benzyl alcohol was different in healthy and PVY(NTN) -infected plants before CPB larvae infestation, implicating the importance of PVY(NTN) infection in plant communication with its environment. This was reflected in gene expression profiles of neighbouring plants showing different degree of defence response. This study thus contributes to our understanding of plant responses in agro-ecosystems.

Geranyllinalool synthases in solanaceae and other angiosperms constitute an ancient branch of diterpene synthases involved in the synthesis of defensive compounds

Many angiosperm plants, including basal dicots, eudicots, and monocots, emit (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene, which is derived from geranyllinalool, in response to biotic challenge. An Arabidopsis (Arabidopsis thaliana) geranyllinalool synthase (GLS) belonging to the e/f clade of the terpene synthase (TPS) family and two Fabaceae GLSs that belong to the TPS-g clade have been reported, making it unclear which is the main route to geranyllinalool in plants. We characterized a tomato (Solanum lycopersicum) TPS-e/f gene, TPS46, encoding GLS (SlGLS) and its homolog (NaGLS) from Nicotiana attenuata. The Km value of SlGLS for geranylgeranyl diphosphate was 18.7 µm, with a turnover rate value of 6.85 s(-1). In leaves and flowers of N. attenuata, which constitutively synthesize 17-hydroxygeranyllinalool glycosides, NaGLS is expressed constitutively, but the gene can be induced in leaves with methyl jasmonate. In tomato, SlGLS is not expressed in any tissue under normal growth but is induced in leaves by alamethicin and methyl jasmonate treatments. SlGLS, NaGLS, AtGLSs, and several other GLSs characterized only in vitro come from four different eudicot families and constitute a separate branch of the TPS-e/f clade that diverged from kaurene synthases, also in the TPS-e/f clade, before the gymnosperm-angiosperm split. The early divergence of this branch and the GLS activity of genes in this branch in diverse eudicot families suggest that GLS activity encoded by these genes predates the angiosperm-gymnosperm split. However, although a TPS sequence belonging to this GLS lineage was recently reported from a basal dicot, no representative sequences have yet been found in monocot or nonangiospermous plants.

Genome-wide analysis of terpene synthases in soybean: functional characterization of GmTPS3

Terpenes (terpenoids or isoprenoids) constitute a large class of plant natural products and play numerous functional roles in primary and secondary metabolism as well as inecological interactions. This study presents a genomic analysis of 23 putative soybean (Glycine max) terpene synthase genes (GmTPSs) distributed over 10 of 20 chromosomes. The GmTPSs are grouped into six types based on gene architecture and sequence identity. Sequence alignment indicates that most GmTPSs contain the conserved aspartate-rich DDX2D motif, and two clades encoded by TPS-a and TPS-b contain variations of an arginine-rich RRX8W motif. Quantitative real-time PCR analysis demonstrated that GmTPSs were predominantly expressed in reproductive organs. Heterologous expression followed by enzymatic assay suggested that GmTPS3 functions as a geraniol synthase. We also generated transgenic tobacco plants ectopically expressing GmTPS3. In dual-choice feeding-preference and force-feeding assays, the transgenic tobacco lines expressing GmTPS3 exhibited enhanced resistance to cotton leafworms and an increased level of geraniol. Taken together, these data provide a comprehensive understanding of the TPS family in soybeans and suggest a promising approach to engineering transgenic plants with enhanced insect resistance.

RNA sequencing on Solanum lycopersicum trichomes identifies transcription factors that activate terpene synthase promoters

Background

Glandular trichomes are production and storage organs of specialized metabolites such as terpenes, which play a role in the plant’s defense system. The present study aimed to shed light on the regulation of terpene biosynthesis in Solanum lycopersicum trichomes by identification of transcription factors (TFs) that control the expression of terpene synthases.

Results

A trichome transcriptome database was created with a total of 27,195 contigs that contained 743 annotated TFs. Furthermore a quantitative expression database was obtained of jasmonic acid-treated trichomes. Sixteen candidate TFs were selected for further analysis. One TF of the MYC bHLH class and one of the WRKY class were able to transiently transactivate S. lycopersicum terpene synthase promoters in Nicotiana benthamiana leaves. Strikingly, SlMYC1 was shown to act synergistically with a previously identified zinc finger-like TF, Expression of Terpenoids 1 (SlEOT1) in transactivating the SlTPS5 promoter.

Conclusions

High-throughput sequencing of tomato stem trichomes led to the discovery of two transcription factors that activated several terpene synthase promoters. Our results identified new elements of the transcriptional regulation of tomato terpene biosynthesis in trichomes, a largely unexplored field.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-402) contains supplementary material, which is available to authorized users.

Quantitative analysis of herbivore-induced cytosolic calcium by using a Cameleon (YC 3.6) calcium sensor in Arabidopsis thaliana

Ca(2+) is a key player in plant cell responses to biotic and abiotic stress. Owing to the central role of cytosolic Ca(2+) ([Ca(2+)]cyt) during early signaling and the need for precise determination of [Ca(2+)]cyt variations, we used a Cameleon YC 3.6 reporter protein expressed in Arabidopsis thaliana to quantify [Ca(2+)]cyt variations upon leaf mechanical damage (MD), herbivory by 3rd and 5th instar larvae of Spodoptera littoralis and S. littoralis oral secretions (OS) applied to MD. YC 3.6 allowed a clear distinction between MD and herbivory and discriminated between the two larvae instars. To our knowledge this is the first report of quantitative [Ca(2+)]cyt determination upon herbivory using a Cameleon calcium sensor.

Metabolic engineering of the C16 homoterpene TMTT in Lotus japonicus through overexpression of (E,E)-geranyllinalool synthase attracts generalist and specialist predators in different manners

Plant defenses against herbivores include the emission of specific blends of volatiles, which enable plants to attract natural enemies of herbivores. We characterized a plastidial terpene synthase gene, PlTPS2, from lima bean (Phaseolus lunatus). The recombinant PlTPS2 protein was multifunctional, producing linalool, (E)-nerolidol and (E,E)-geranyllinalool, precursors of (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene [TMTT]. Transgenic Lotus japonicus and Nicotiana tabacum plants, expressing PlTPS2 or its homolog Medicago truncatula TPS3 (MtTPS3), were produced and used for bioassays with herbivorous and predatory mites. Transgenic L. japonicus plants expressing PlTPS2 produced (E,E)-geranyllinalool and TMTT, whereas wild-type plants and transgenic plants expressing MtTPS3 did not. Transgenic N. tabacum expressing PlTPS2 produced (E,E)-geranyllinalool but not TMTT. Moreover, in olfactory assays, the generalist predatory mite Neoseiulus californicus but not the specialist Phytoseiulus persimilis was attracted to uninfested, transgenic L. japonicus plants expressing PlTPS2 over wild-type plants. The specialist P. persimilis was more strongly attracted by the transgenic plants infested with spider mites than by infested wild-type plants. Predator responses to transgenic plant volatile TMTT depend on various background volatiles endogenously produced by the transgenic plants. Therefore, the manipulation of TMTT is an ideal platform for pest control via the attraction of generalist and specialist predators in different manners.

Identification and characterization of (E)-β-caryophyllene synthase and α/β-pinene synthase potentially involved in constitutive and herbivore-induced terpene formation in cotton

Cotton (Gossypium hirsutum L.) plants damaged by insects emit a blend of volatiles, including monoterpenes and sesquiterpenes, which can directly repel herbivores and/or indirectly protect the plant by attracting natural enemies of the herbivores. To understand the molecular basis of terpene biosynthesis and regulation in cotton, two terpene synthase genes, GhTPS1 and GhTPS2, were heterologously expressed and characterized. Recombinant GhTPS1 accepted farnesyl pyrophosphate as substrate and produced (E)-β-caryophyllene and α-humulene. GhTPS2 was characterized as a monoterpene synthase which formed α-pinene and β-pinene using geranyl pyrophosphate as substrate. Quantitative real-time PCR analysis revealed that GhTPS1 and GhTPS2 gene expression was elevated after methyl jasmonate (MeJA) treatment in cotton leaves. Moreover, feeding of the green plant bug Apolygus lucorum, a major cotton pest in northern China, resulted in increased GhTPS2 expression in young leaves, suggesting that GhTPS2 might be involved in plant defense in cotton.

Plant defense against insect herbivores

Plants have been interacting with insects for several hundred million years, leading to complex defense approaches against various insect feeding strategies. Some defenses are constitutive while others are induced, although the insecticidal defense compound or protein classes are often similar. Insect herbivory induce several internal signals from the wounded tissues, including calcium ion fluxes, phosphorylation cascades and systemic- and jasmonate signaling. These are perceived in undamaged tissues, which thereafter reinforce their defense by producing different, mostly low molecular weight, defense compounds. These bioactive specialized plant defense compounds may repel or intoxicate insects, while defense proteins often interfere with their digestion. Volatiles are released upon herbivory to repel herbivores, attract predators or for communication between leaves or plants, and to induce defense responses. Plants also apply morphological features like waxes, trichomes and latices to make the feeding more difficult for the insects. Extrafloral nectar, food bodies and nesting or refuge sites are produced to accommodate and feed the predators of the herbivores. Meanwhile, herbivorous insects have adapted to resist plant defenses, and in some cases even sequester the compounds and reuse them in their own defense. Both plant defense and insect adaptation involve metabolic costs, so most plant-insect interactions reach a stand-off, where both host and herbivore survive although their development is suboptimal.

Functional genomics reveals that a compact terpene synthase gene family can account for terpene volatile production in apple

Terpenes are specialized plant metabolites that act as attractants to pollinators and as defensive compounds against pathogens and herbivores, but they also play an important role in determining the quality of horticultural food products. We show that the genome of cultivated apple (Malus domestica) contains 55 putative terpene synthase (TPS) genes, of which only 10 are predicted to be functional. This low number of predicted functional TPS genes compared with other plant species was supported by the identification of only eight potentially functional TPS enzymes in apple 'Royal Gala' expressed sequence tag databases, including the previously characterized apple (E,E)-α-farnesene synthase. In planta functional characterization of these TPS enzymes showed that they could account for the majority of terpene volatiles produced in cv Royal Gala, including the sesquiterpenes germacrene-D and (E)-β-caryophyllene, the monoterpenes linalool and α-pinene, and the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene. Relative expression analysis of the TPS genes indicated that floral and vegetative tissues were the primary sites of terpene production in cv Royal Gala. However, production of cv Royal Gala floral-specific terpenes and TPS genes was observed in the fruit of some heritage apple cultivars. Our results suggest that the apple TPS gene family has been shaped by a combination of ancestral and more recent genome-wide duplication events. The relatively small number of functional enzymes suggests that the remaining terpenes produced in floral and vegetative and fruit tissues are maintained under a positive selective pressure, while the small number of terpenes found in the fruit of modern cultivars may be related to commercial breeding strategies.

Reprogramming of plants during systemic acquired resistance

Genome-wide microarray analyses revealed that during biological activation of systemic acquired resistance (SAR) in Arabidopsis, the transcript levels of several hundred plant genes were consistently up- (SAR(+) genes) or down-regulated (SAR(-) genes) in systemic, non-inoculated leaf tissue. This transcriptional reprogramming fully depended on the SAR regulator FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1). Functional gene categorization showed that genes associated with salicylic acid (SA)-associated defenses, signal transduction, transport, and the secretory machinery are overrepresented in the group of SAR(+) genes, and that the group of SAR(-) genes is enriched in genes activated via the jasmonate (JA)/ethylene (ET)-defense pathway, as well as in genes associated with cell wall remodeling and biosynthesis of constitutively produced secondary metabolites. This suggests that SAR-induced plants reallocate part of their physiological activity from vegetative growth towards SA-related defense activation. Alignment of the SAR expression data with other microarray information allowed us to define three clusters of SAR(+) genes. Cluster I consists of genes tightly regulated by SA. Cluster II genes can be expressed independently of SA, and this group is moderately enriched in H2O2- and abscisic acid (ABA)-responsive genes. The expression of the cluster III SAR(+) genes is partly SA-dependent. We propose that SA-independent signaling events in early stages of SAR activation enable the biosynthesis of SA and thus initiate SA-dependent SAR signaling. Both SA-independent and SA-dependent events tightly co-operate to realize SAR. SAR(+) genes function in the establishment of diverse resistance layers, in the direct execution of resistance against different (hemi-)biotrophic pathogen types, in suppression of the JA- and ABA-signaling pathways, in redox homeostasis, and in the containment of defense response activation. Our data further indicated that SAR-associated defense priming can be realized by partial pre-activation of particular defense pathways.

Network analysis of the MVA and MEP pathways for isoprenoid synthesis

Isoprenoid biosynthesis is essential for all living organisms, and isoprenoids are also of industrial and agricultural interest. All isoprenoids are derived from prenyl diphosphate (prenyl-PP) precursors. Unlike isoprenoid biosynthesis in other living organisms, prenyl-PP, as the precursor of all isoprenoids in plants, is synthesized by two independent pathways: the mevalonate (MVA) pathway in the cytoplasm and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway in plastids. This review focuses on progress in our understanding of how the precursors for isoprenoid biosynthesis are synthesized in the two subcellular compartments, how the underlying pathway gene networks are organized and regulated, and how network perturbations impact each pathway and plant development. Because of the wealth of data on isoprenoid biosynthesis, we emphasize research in Arabidopsis thaliana and compare the synthesis of isoprenoid precursor molecules in this model plant with their synthesis in other prokaryotic and eukaryotic organisms.

Perception of insect feeding by plants

The recognition of phytophagous insects by plants induces a set of very specific responses aimed at deterring tissue consumption and reprogramming metabolism and development of the plant to tolerate the herbivore. The recognition of insects by plants requires the plant's ability to perceive chemical cues generated by the insects and to distinguish a particular pattern of tissue disruption. Relatively little is known about the molecular basis of insect perception by plants and the signalling mechanisms directly associated with this perception. Importantly, the insect feeding behaviour (piercing-sucking versus chewing) is a decisive determinant of the plant's defence response, and the mechanisms used to perceive insects from different feeding guilds may be distinct. During insect feeding, components of the saliva of chewing or piercing-sucking insects come into contact with plant cells, and elicitors or effectors present in this insect-derived fluid are perceived by plant cells to initiate the activation of specific signalling cascades. Although receptor-ligand interactions controlling insect perception have yet not been molecularly described, a significant number of regulatory components acting downstream of receptors and involved in the activation of defence responses against insects has been reported. Some of these regulators mediate changes in the phytohormone network, while others directly control gene expression or the redox state of the cell. These processes are central in the orchestration of plant defence responses against insects.

Virus infection decreases the attractiveness of white clover plants for a non-vectoring herbivore

Plant pathogens and insect herbivores are prone to share hosts under natural conditions. Consequently, pathogen-induced changes in the host plant can affect herbivory, and vice versa. Even though plant viruses are ubiquitous in the field, little is known about plant-mediated interactions between viruses and non-vectoring herbivores. We investigated the effects of virus infection on subsequent infestation by a non-vectoring herbivore in a natural genotype of Trifolium repens (white clover). We tested whether infection with White clover mosaic virus (WClMV) alters (1) the effects of fungus gnat feeding on plant growth, (2) the attractiveness of white clover for adult fungus gnat females, and (3) the volatile emission of white clover plants. We observed only marginal effects of WClMV infection on the interaction between fungus gnat larvae and white clover. However, adult fungus gnat females clearly preferred non-infected over WClMV-infected plants. Non-infected and virus-infected plants could easily be discriminated based on their volatile blends, suggesting that the preference of fungus gnats for non-infected plants may be mediated by virus-induced changes in volatile emissions. The compound β-caryophyllene was exclusively detected in the headspace of virus-infected plants and may hence be particularly important for the preference of fungus gnat females. Our results demonstrate that WClMV infection can decrease the attractiveness of white clover plants for fungus gnat females. This suggests that virus infections may contribute to protecting their hosts by decreasing herbivore infestation rates. Consequently, it is conceivable that viruses play a more beneficial role in plant-herbivore interactions than generally thought.

The organ-specific expression of terpene synthase genes contributes to the terpene hydrocarbon composition of chamomile essential oils

BACKGROUND

The essential oil of chamomile, one of the oldest and agronomically most important medicinal plant species in Europe, has significant antiphlogistic, spasmolytic and antimicrobial activities. It is rich in chamazulene, a pharmaceutically active compound spontaneously formed during steam distillation from the sesquiterpene lactone matricine. Chamomile oil also contains sesquiterpene alcohols and hydrocarbons which are produced by the action of terpene synthases (TPS), the key enzymes in constructing terpene carbon skeletons.

RESULTS

Here, we present the identification and characterization of five TPS enzymes contributing to terpene biosynthesis in chamomile (Matricaria recutita). Four of these enzymes were exclusively expressed in above-ground organs and produced the common terpene hydrocarbons (-)-(E)-β-caryophyllene (MrTPS1), (+)-germacrene A (MrTPS3), (E)-β-ocimene (MrTPS4) and (-)-germacrene D (MrTPS5). A fifth TPS, the multiproduct enzyme MrTPS2, was mainly expressed in roots and formed several Asteraceae-specific tricyclic sesquiterpenes with (-)-α-isocomene being the major product. The TPS transcript accumulation patterns in different organs of chamomile were consistent with the abundance of the corresponding TPS products isolated from these organs suggesting that the spatial regulation of TPS gene expression qualitatively contribute to terpene composition.

CONCLUSIONS

The terpene synthases characterized in this study are involved in the organ-specific formation of essential oils in chamomile. While the products of MrTPS1, MrTPS2, MrTPS4 and MrTPS5 accumulate in the oils without further chemical alterations, (+)-germacrene A produced by MrTPS3 accumulates only in trace amounts, indicating that it is converted into another compound like matricine. Thus, MrTPS3, but also the other TPS genes, are good markers for further breeding of chamomile cultivars rich in pharmaceutically active essential oils.

Plant communication: mediated by individual or blended VOCs?

Plants emit volatile organic compounds (VOCs) as a means to warn other plants of impending danger. Nearby plants exposed to the induced VOCs prepare their own defense weapons in response. Accumulated data supports this assertion, yet much of the evidence has been obtained in laboratories under artificial conditions where, for example, a single VOC might be applied at a concentration that plants do not actually experience in nature. Experiments conducted outdoors suggest that communication occurs only within a limited distance from the damaged plants. Thus, the question remains as to whether VOCs work as a single component or a specific blend, and at which concentrations VOCs elicit insect and pathogen defenses in undamaged plants. We discuss these issues based on available literature and our recent work, and propose future directions in this field.

Plant communication: mediated by individual or blended VOCs?

Plants emit volatile organic compounds (VOCs) as a means to warn other plants of impending danger. Nearby plants exposed to the induced VOCs prepare their own defense weapons in response. Accumulated data supports this assertion, yet much of the evidence has been obtained in laboratories under artificial conditions where, for example, a single VOC might be applied at a concentration that plants do not actually experience in nature. Experiments conducted outdoors suggest that communication occurs only within a limited distance from the damaged plants. Thus, the question remains as to whether VOCs work as a single component or a specific blend, and at which concentrations VOCs elicit insect and pathogen defenses in undamaged plants. We discuss these issues based on available literature and our recent work, and propose future directions in this field.

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

RNA-seq discovery, functional characterization, and comparison of sesquiterpene synthases from Solanum lycopersicum and Solanum habrochaites trichomes

Solanum lycopersicum and Solanum habrochaites (f. typicum) accession PI127826 emit a variety of sesquiterpenes. To identify terpene synthases involved in the production of these volatile sesquiterpenes, we used massive parallel pyrosequencing (RNA-seq) to obtain the transcriptome of the stem trichomes from these plants. This approach resulted initially in the discovery of six sesquiterpene synthase cDNAs from S. lycopersicum and five from S. habrochaites. Searches of other databases and the S. lycopersicum genome resulted in the discovery of two additional sesquiterpene synthases expressed in trichomes. The sesquiterpene synthases from S. lycopersicum and S. habrochaites have high levels of protein identity. Several of them appeared to encode for non-functional proteins. Functional recombinant proteins produced germacrenes, β-caryophyllene/α-humulene, viridiflorene and valencene from (E,E)-farnesyl diphosphate. However, the activities of these enzymes do not completely explain the differences in sesquiterpene production between the two tomato plants. RT-qPCR confirmed high levels of expression of most of the S. lycopersicum sesquiterpene synthases in stem trichomes. In addition, one sesquiterpene synthase was induced by jasmonic acid, while another appeared to be slightly repressed by the treatment. Our data provide a foundation to study the evolution of terpene synthases in cultivated and wild tomato.

The biochemistry of homoterpenes--common constituents of floral and herbivore-induced plant volatile bouquets

Volatile organic compounds emitted by plants mediate a variety of interactions between plants and other organisms. The irregular acyclic homoterpenes, 4,8-dimethylnona-1,3,7-triene (DMNT) and 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT), are among the most widespread volatiles produced by angiosperms with emissions from flowers and from vegetative tissues upon herbivore feeding. Special attention has been placed on the role of homoterpenes in attracting parasitoids and predators of herbivores and has sparked interest in engineering homoterpene formation to improve biological pest control. The biosynthesis of DMNT and TMTT proceeds in two enzymatic steps: the formation of the tertiary C₁₅₋, and C₂₀₋ alcohols, (E)-nerolidol and (E,E)-geranyl linalool, respectively, catalyzed by terpene synthases, and the subsequent oxidative degradation of both alcohols by a single cytochrome P450 monooxygenase (P450). In Arabidopsis thaliana, the herbivore-induced biosynthesis of TMTT is catalyzed by the concerted activities of the (E,E)-geranyllinalool synthase, AtGES, and CYP82G1, a P450 of the so far uncharacterized plant CYP82 family. TMTT formation is in part controlled at the level of AtGES expression. Co-expression of AtGES with CYP82G1 at wound sites allows for an efficient conversion of the alcohol intermediate. The identified homoterpene biosynthesis genes in Arabidopsis and related genes from other plant species provide tools to engineer homoterpene formation and to address questions of the regulation and specific activities of homoterpenes in plant-herbivore interactions.

Recent advances in plant early signaling in response to herbivory

Plants are frequently attacked by herbivores and pathogens and therefore have acquired constitutive and induced defenses during the course of their evolution. Here we review recent progress in the study of the early signal transduction pathways in host plants in response to herbivory. The sophisticated signaling network for plant defense responses is elicited and driven by both herbivore-induced factors (e.g., elicitors, effectors, and wounding) and plant signaling (e.g., phytohormone and plant volatiles) in response to arthropod factors. We describe significant findings, illuminating the scenario by providing broad insights into plant signaling involved in several arthropod-host interactions.

A multiproduct terpene synthase from Medicago truncatula generates cadalane sesquiterpenes via two different mechanisms

Terpene synthases are responsible for a large diversity of terpene carbon skeletons found in nature. The multiproduct sesquiterpene synthase MtTPS5 isolated from Medicago truncatula produces 27 products from farnesyl diphosphate (1, FDP). In this paper, we report the reaction steps involved in the formation of these products using incubation experiments with deuterium-containing substrates; we determined the absolute configuration of individual products to establish the stereochemical course of the reaction cascade and the initial conformation of the cycling substrate. Additional labeling experiments conducted with deuterium oxide showed that cadalane sesquiterpenes are mainly produced via the protonation of the neutral intermediate germacrene D (5). These findings provide an alternative route to the general accepted pathway via nerolidyl diphosphate (2, NDP) en route to sesquiterpenes with a cadalane skeleton. Mutational analysis of the enzyme demonstrated that a tyrosine residue is important for the protonation process.

Herbivore-induced and floral homoterpene volatiles are biosynthesized by a single P450 enzyme (CYP82G1) in Arabidopsis

Terpene volatiles play important roles in plant-organism interactions as attractants of pollinators or as defense compounds against herbivores. Among the most common plant volatiles are homoterpenes, which are often emitted from night-scented flowers and from aerial tissues upon herbivore attack. Homoterpene volatiles released from herbivore-damaged tissue are thought to contribute to indirect plant defense by attracting natural enemies of pests. Moreover, homoterpenes have been demonstrated to induce defensive responses in plant-plant interaction. Although early steps in the biosynthesis of homoterpenes have been elucidated, the identity of the enzyme responsible for the direct formation of these volatiles has remained unknown. Here, we demonstrate that CYP82G1 (At3g25180), a cytochrome P450 monooxygenase of the Arabidopsis CYP82 family, is responsible for the breakdown of the C(20)-precursor (E,E)-geranyllinalool to the insect-induced C(16)-homoterpene (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). Recombinant CYP82G1 shows narrow substrate specificity for (E,E)-geranyllinalool and its C(15)-analog (E)-nerolidol, which is converted to the respective C(11)-homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). Homology-based modeling and substrate docking support an oxidative bond cleavage of the alcohol substrate via syn-elimination of the polar head, together with an allylic C-5 hydrogen atom. CYP82G1 is constitutively expressed in Arabidopsis stems and inflorescences and shows highly coordinated herbivore-induced expression with geranyllinalool synthase in leaves depending on the F-box protein COI-1. CYP82G1 represents a unique characterized enzyme in the plant CYP82 family with a function as a DMNT/TMTT homoterpene synthase.

The isogene 1-deoxy-D-xylulose 5-phosphate synthase 2 controls isoprenoid profiles, precursor pathway allocation, and density of tomato trichomes

Plant isoprenoids are formed from precursors synthesized by the mevalonate (MVA) pathway in the cytosol or by the methyl-D-erythritol 4-phosphate (MEP) pathway in plastids. Although some exchange of precursors occurs, cytosolic sesquiterpenes are assumed to derive mainly from MVA, while plastidial monoterpenes are produced preferentially from MEP precursors. Additional complexity arises in the first step of the MEP pathway, which is typically catalyzed by two divergent 1-deoxy-D-xylulose 5-phosphate synthase isoforms (DXS1, DXS2). In tomato (Solanum lycopersicum), the SlDXS1 gene is ubiquitously expressed with highest levels during fruit ripening, whereas SlDXS2 transcripts are abundant in only few tissues, including young leaves, petals, and isolated trichomes. Specific down-regulation of SlDXS2 expression was performed by RNA interference in transgenic plants to investigate feedback mechanisms. SlDXS2 down-regulation led to a decrease in the monoterpene β-phellandrene and an increase in two sesquiterpenes in trichomes. Moreover, incorporation of MVA-derived precursors into residual monoterpenes and into sesquiterpenes was elevated as determined by comparison of ¹³C to ¹²C natural isotope ratios. A compensatory up-regulation of SlDXS1 was not observed. Down-regulated lines also exhibited increased trichome density and showed less damage by leaf-feeding Spodoptera littoralis caterpillars. The results reveal novel, non-redundant roles of DXS2 in modulating isoprenoid metabolism and a pronounced plasticity in isoprenoid precursor allocation.

Regulation of Arabidopsis defense responses against Spodoptera littoralis by CPK-mediated calcium signaling

BACKGROUND

Plant Ca2+ signals are involved in a wide array of intracellular signaling pathways after pest invasion. Ca2+-binding sensory proteins such as Ca2+-dependent protein kinases (CPKs) have been predicted to mediate the signaling following Ca2+ influx after insect herbivory. However, until now this prediction was not testable.

RESULTS

To investigate the roles CPKs play in a herbivore response-signaling pathway, we screened the characteristics of Arabidopsis CPK mutants damaged by a feeding generalist herbivore, Spodoptera littoralis. Following insect attack, the cpk3 and cpk13 mutants showed lower transcript levels of plant defensin gene PDF1.2 compared to wild-type plants. The CPK cascade was not directly linked to the herbivory-induced signaling pathways that were mediated by defense-related phytohormones such as jasmonic acid and ethylene. CPK3 was also suggested to be involved in a negative feedback regulation of the cytosolic Ca2+ levels after herbivory and wounding damage. In vitro kinase assays of CPK3 protein with a suite of substrates demonstrated that the protein phosphorylates transcription factors (including ERF1, HsfB2a and CZF1/ZFAR1) in the presence of Ca2+. CPK13 strongly phosphorylated only HsfB2a, irrespective of the presence of Ca2+. Furthermore, in vivo agroinfiltration assays showed that CPK3-or CPK13-derived phosphorylation of a heat shock factor (HsfB2a) promotes PDF1.2 transcriptional activation in the defense response.

CONCLUSIONS

These results reveal the involvement of two Arabidopsis CPKs (CPK3 and CPK13) in the herbivory-induced signaling network via HsfB2a-mediated regulation of the defense-related transcriptional machinery. This cascade is not involved in the phytohormone-related signaling pathways, but rather directly impacts transcription factors for defense responses.

Genetic variation in jasmonic acid- and spider mite-induced plant volatile emission of cucumber accessions and attraction of the predator Phytoseiulus persimilis

Cucumber plants (Cucumis sativus L.) respond to spider-mite (Tetranychus urticae) damage with the release of specific volatiles that are exploited by predatory mites, the natural enemies of the spider mites, to locate their prey. The production of volatiles also can be induced by exposing plants to the plant hormone jasmonic acid. We analyzed volatile emissions from 15 cucumber accessions upon herbivory by spider mites and upon exposure to jasmonic acid using gas chromatography-mass spectrometry. Upon induction, cucumber plants emitted over 24 different compounds, and the blend of induced volatiles consisted predominantly of terpenoids. The total amount of volatiles was higher in plants treated with jasmonic acid than in those infested with spider mites, with (E)-4,8-dimethyl-1,3,7-nonatriene, (E,E)-alpha-farnesene, and (E)-beta-ocimene as the most abundant compounds in all accessions in both treatments. Significant variation among the accessions was found for the 24 major volatile compounds. The accessions differed strongly in total amount of volatiles emitted, and displayed very different odor profiles. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which is suggestive of common metabolic pathways. A number of cucumber accessions also were tested for their attractiveness to Phytoseiulus persimilis, a specialist predator of spider mites. Differences in the attraction of predatory mites by the various accessions correlated to differences in the individual chemical profiles of these accessions. The presence of genetic variation in induced plant volatile emission in cucumber shows that it is possible to breed for cucumber varieties that are more attractive to predatory mites and other biological control agents.

Monoterpene and sesquiterpene synthases and the origin of terpene skeletal diversity in plants

The multitude of terpene carbon skeletons in plants is formed by enzymes known as terpene synthases. This review covers the monoterpene and sesquiterpene synthases presenting an up-to-date list of enzymes reported and evidence for their ability to form multiple products. The reaction mechanisms of these enzyme classes are described, and information on how terpene synthase proteins mediate catalysis is summarized. Correlations between specific amino acid motifs and terpene synthase function are described, including an analysis of the relationships between active site sequence and cyclization type and a discussion of whether specific protein features might facilitate multiple product formation.

Insect-induced gene expression at the core of volatile terpene release in Medicago truncatula

The blends of induced volatiles released by higher plants in response to herbivory regularly contain terpenoids. The precursors of volatile terpenoids can be synthesized via two pathways, the mevalonate (MVA) and the methyl erythritol 4-phosphate (MEP) pathways localized in the cytosol and in plastids, respectively. Terpenes are important players in interactions between plants and herbivorous insects, by acting in both direct and indirect defenses. We recently characterized a gene encoding an (E)-beta-ocimene synthase (MtEBOS) in the legume Medicago truncatula Gaertn. Compared to undamaged plants, caterpillar-damaged M. truncatula emitted (E)-beta-ocimene at an elevated level and this increase is associated with high levels of expression of MtEBOS mRNA. Exogenous treatment with jasmonic acid also increases transcript accumulation of MtEBOS. These results indicate that transcript accumulation is used as a tightly regulated mechanism to control (E)-beta-ocimene emission. The data, along with additional findings in other species, illustrate that like most plant families legumes regulate the final steps of volatile terpene biosynthesis at the level of transcript induction.

Belowground ABA boosts aboveground production of DIMBOA and primes induction of chlorogenic acid in maize

Plants are important mediators between above- and belowground herbivores. Consequently, interactions between root and shoot defenses can have far-reaching impacts on entire food webs. We recently reported that infestation of maize roots by larvae of the beetle Diabrotica virgifera virgifera induced shoot resistance against herbivores and pathogens. Root herbivory also enhanced aboveground DIMBOA and primed for enhanced induction of chlorogenic acid, two secondary metabolites that have been associated with plant stress resistance. Interestingly, the plant hormone abscisic acid (ABA) emerged as a putative long-distance signal in the regulation of these systemic defenses. In this addendum, we have investigated the role of root-derived ABA in aboveground regulation of DIMBOA and the phenolic compounds chlorogenic acid, caffeic and ferulic acid. Furthermore, we discuss the relevance of ABA in relation to defense against the leaf herbivore Spodoptera littoralis. Soil-drench treatment with ABA mimicked root herbivore-induced accumulation of DIMBOA in the leaves. Similarly, ABA mimicked aboveground priming of chlorogenic acid production, causing augmented induction of this compound after subsequent shoot attack by S. littoralis caterpillars. These findings confirm our notion that ABA acts as an important signal in the regulation of aboveground defenses during belowground herbivory. However, based on our previous finding that ABA alone is not sufficient to trigger aboveground resistance against S. littoralis caterpillars, our results also suggest that the ABA-inducible effects on DIMBOA and chlorogenic acid are not solely responsible for root herbivore-induced resistance against S. littoralis.

Chemical complexity of volatiles from plants induced by multiple attack

The attack of a plant by herbivorous arthropods can result in considerable changes in the plant's chemical phenotype. The emission of so-called herbivore-induced plant volatiles (HIPV) results in the attraction of carnivorous enemies of the herbivores that induced these changes. HIPV induction has predominantly been investigated for interactions between one plant and one attacker. However, in nature plants are exposed to a variety of attackers, either simultaneously or sequentially, in shoots and roots, causing much more complex interactions than have usually been investigated in the context of HIPV. To develop an integrated view of how plants respond to their environment, we need to know more about the ways in which multiple attackers can enhance, attenuate, or otherwise alter HIPV responses. A multidisciplinary approach will allow us to investigate the underlying mechanisms of HIPV emission in terms of phytohormones, transcriptional responses and biosynthesis of metabolites in an effort to understand these complex plant-arthropod interactions.

Chemical and molecular ecology of herbivore-induced plant volatiles: proximate factors and their ultimate functions

In response to herbivory, plants emit specific blends of herbivore-induced plant volatiles (HIPVs). HIPVs mediate sizable arrays of interactions between plants and arthropods, microorganisms, undamaged neighboring plants or undamaged sites within the plant in various ecosystems. HIPV profiles vary according to the plant and herbivore species, and the developmental stages and conditions of the live plants and herbivores. To understand the regulatory mechanisms underling HIPV biosynthesis, the following issues are reviewed here: (i) herbivore-induced formation of plant volatile terpenoids and green leaf volatiles; (ii) initial activation of plant responses by feeding herbivores; and (iii) the downstream network of the signal transduction. To understand the ecological significance of HIPVs, we also review case studies of insect-plant and inter-/intraplant interactions mediated by HIPVs that have been documented in the field and laboratory in recent years.

Chemical complexity of volatiles from plants induced by multiple attack

The attack of a plant by herbivorous arthropods can result in considerable changes in the plant's chemical phenotype. The emission of so-called herbivore-induced plant volatiles (HIPV) results in the attraction of carnivorous enemies of the herbivores that induced these changes. HIPV induction has predominantly been investigated for interactions between one plant and one attacker. However, in nature plants are exposed to a variety of attackers, either simultaneously or sequentially, in shoots and roots, causing much more complex interactions than have usually been investigated in the context of HIPV. To develop an integrated view of how plants respond to their environment, we need to know more about the ways in which multiple attackers can enhance, attenuate, or otherwise alter HIPV responses. A multidisciplinary approach will allow us to investigate the underlying mechanisms of HIPV emission in terms of phytohormones, transcriptional responses and biosynthesis of metabolites in an effort to understand these complex plant-arthropod interactions.

Medicago truncatula (E)-beta-ocimene synthase is induced by insect herbivory with corresponding increases in emission of volatile ocimene

Virtually all plants are able to recognize attack by herbivorous insects and release volatile organic compounds (VOC) in response. Terpenes are the most abundant and varied class of insect-induced VOC from plants. Four genes encoding putative terpene synthases (MtTps1, MtTps2, MtTps3 and MtTps4) were shown to accumulate in Medicago truncatula Gaertn. in response to Spodoptera exigua (Hübner) feeding and methyl jasmonate treatment in a previous study [S.K. Gomez, M.M. Cox, J.C. Bede, K.K. Inoue, H.T. Alborn, J.H. Tumlinson, K.L. Korth, Lepidopteran herbivory and oral factors induce transcripts encoding novel terpene synthases in Medicago truncatula, Arch. Insect Biochem. Physiol. 58 (2005) 114-127.] The focus of the current study is the functional characterization of one (MtTps4) of these four genes. Using an M. truncatula cDNA clone, the insect-inducible putative terpene synthase was expressed in Escherichiacoli and shown to convert geranyl diphosphate (GPP) into the monoterpene (E)-beta-ocimene as the major product. The clone was therefore designated M. truncatula (E)-beta-ocimene synthase (MtEBOS). Transcripts encoding this enzyme accumulate in M. truncatula leaves in response to exogenous jasmonic acid treatments, lepidopteran herbivory, and lepidopteran oral secretions. Treatment with the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) did not cause an increase in MtEBOS transcripts. The volatile (E)-beta-ocimene was released from leaves of both undamaged and insect-damaged plants, but at levels two-fold higher in insect-damaged M. truncatula. Although leaves have low constitutive levels of MtEBOS transcripts, RNA blot analysis indicates no constitutive expression in flowers, stems or roots. The strong insect-induced expression of this gene, and its correspondence with release of volatile ocimene, suggest that it plays an active role in indirect insect defenses in M. truncatula.

Methyl salicylate production and jasmonate signaling are not essential for systemic acquired resistance in Arabidopsis

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.

Phytohormone-based activity mapping of insect herbivore-produced elicitors

In response to insect attack, many plants exhibit dynamic biochemical changes, resulting in the induced production of direct and indirect defenses. Elicitors present in herbivore oral secretions are believed to positively regulate many inducible plant defenses; however, little is known about the specificity of elicitor recognition in plants. To investigate the phylogenic distribution of elicitor activity, we tested representatives from three different elicitor classes on the time course of defense-related phytohormone production, including ethylene (E), jasmonic acid (JA), and salicylic acid, in a range of plant species spanning angiosperm diversity. All families examined responded to at least one elicitor class with significant increases in E and JA production within 1 to 2 h after treatment, yet elicitation activity among species was highly idiosyncratic. The fatty-acid amino acid conjugate volicitin exhibited the widest range of phytohormone and volatile inducing activity, which spanned maize (Zea mays), soybean (Glycine max), and eggplant (Solanum melongena). In contrast, the activity of inceptin-related peptides, originally described in cowpea (Vigna unguiculata), was limited even within the Fabaceae. Similarly, caeliferin A16:0, a disulfooxy fatty acid from grasshoppers, was the only elicitor with demonstrable activity in Arabidopsis thaliana. Although precise mechanisms remain unknown, the unpredictable nature of elicitor activity between plant species supports the existence of specific receptor-ligand interactions mediating recognition. Despite the lack of an ideal plant model for studying the action of numerous elicitors, E and JA exist as highly conserved and readily quantifiable markers for future discoveries in this field.

Biogenic volatile organic compounds in the Earth system

Biogenic volatile organic compounds produced by plants are involved in plant growth, development, reproduction and defence. They also function as communication media within plant communities, between plants and between plants and insects. Because of the high chemical reactivity of many of these compounds, coupled with their large mass emission rates from vegetation into the atmosphere, they have significant effects on the chemical composition and physical characteristics of the atmosphere. Hence, biogenic volatile organic compounds mediate the relationship between the biosphere and the atmosphere. Alteration of this relationship by anthropogenically driven changes to the environment, including global climate change, may perturb these interactions and may lead to adverse and hard-to-predict consequences for the Earth system.

Pseudomonas syringae elicits emission of the terpenoid (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene in Arabidopsis leaves via jasmonate signaling and expression of the terpene synthase TPS4

Volatile, low-molecular weight terpenoids have been implicated in plant defenses, but their direct role in resistance against microbial pathogens is not clearly defined. We have examined a possible role of terpenoid metabolism in the induced defense of Arabidopsis thaliana plants against leaf infection with the bacterial pathogen Pseudomonas syringae. Inoculation of plants with virulent or avirulent P. syringae strains induces the emission of the terpenoids (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), beta-ionone and alpha-farnesene. While the most abundant volatile, the C16-homoterpene TMTT, is produced relatively early in compatible and incompatible interactions, emission of both beta-ionone and alpha-farnesene only increases in later stages of the compatible interaction. Pathogen-induced synthesis of TMTT is controlled through jasmonic acid (JA)-dependent signaling but is independent of a functional salicylic acid (SA) pathway. We have identified Arabidopsis T-DNA insertion lines with defects in the terpene synthase gene TPS4, which is expressed in response to P. syringae inoculation. The tps4 knockout mutant completely lacks induced emission of TMTT but is capable of beta-ionone and alpha-farnesene production, demonstrating that TPS4 is specifically involved in TMTT formation. The tps4 plants display at least wild type-like resistance against P. syringae, indicating that TMTT per se does not protect against the bacterial pathogen in Arabidopsis leaves. Similarly, the ability to mount SA-dependent defenses and systemic acquired resistance (SAR) is barely affected in tps4, which excludes a signaling function of TMTT during SAR. Besides P. syringae challenge, intoxication of Arabidopsis leaves with copper sulfate, a treatment that strongly activates JA biosynthesis, triggers production of TMTT, beta-ionone, and alpha-farnesene. Taken together, our data suggest that induced TMTT production in Arabidopsis is a by-product of activated JA signaling, rather than an effective defense response that contributes to resistance against P. syringae.

Knock-down of the MEP pathway isogene 1-deoxy-D-xylulose 5-phosphate synthase 2 inhibits formation of arbuscular mycorrhiza-induced apocarotenoids, and abolishes normal expression of mycorrhiza-specific plant marker genes

The first step of the plastidial methylerythritol phosphate (MEP) pathway is catalyzed by two isoforms of 1-deoxy-D-xylulose 5-phosphate synthase (DXS1 and DXS2). In Medicago truncatula, MtDXS1 and MtDXS2 genes exhibit completely different expression patterns. Most prominently, colonization by arbuscular mycorrhizal (AM) fungi induces the accumulation of certain apocarotenoids (cyclohexenone and mycorradicin derivatives) correlated with the expression of MtDXS2 but not of MtDXS1. To prove a distinct function of DXS2, a selective RNAi approach on MtDXS2 expression was performed in transgenic hairy roots of M. truncatula. Repression of MtDXS2 consistently led to reduced transcript levels in mycorrhizal roots, and to a concomitant reduction of AM-induced apocarotenoid accumulation. The transcript levels of MtDXS1 remained unaltered in RNAi plants, and no phenotypical changes in non-AM plants were observed. Late stages of the AM symbiosis were adversely affected, but only upon strong repression with residual MtDXS2-1 transcript levels remaining below approximately 10%. This condition resulted in a strong decrease in the transcript levels of MtPT4, an AM-specific plant phosphate transporter gene, and in a multitude of other AM-induced plant marker genes, as shown by transcriptome analysis. This was accompanied by an increased proportion of degenerating and dead arbuscules at the expense of mature ones. The data reveal a requirement for DXS2-dependent MEP pathway-based isoprenoid products to sustain mycorrhizal functionality at later stages of the symbiosis. They further validate the concept of a distinct role for DXS2 in secondary metabolism, and offer a novel tool to selectively manipulate the levels of secondary isoprenoids by targeting their precursor supply.

Early herbivore-elicited events in terpenoid biosynthesis

Volatile terpenoids, the major products among the herbivore-induced plant volatiles in the legume, mediate interactions that attract herbivores' natural enemies and serve as signals to neighboring plants. We recently demonstrated cross-talk among the signaling components involving Ca(2+), jasmonic acid and ethylene, which are altogether responsible for volatile terpenoid formation in Medicago truncatula. Herbivore-stimulated Ca(2+) transients are an additional element that has an impact on the composition of the blend of terpenoids, whose biosynthesis depends on the jasmonic acid/ethylene pathway. The molecular diversity of the blend is expanded and modulated by the transcriptional regulation of terpene synthases, some of which are multi-functional enzymes producing a large set of sesqui- and monotepenes or precursors of C(11) and C16 homoterpenes from different prenyl diphosphates. In this addendum, we discuss a new perspective on early events leading to terpenoid biosynthesis.

Effects of feeding Spodoptera littoralis on lima bean leaves: IV. Diurnal and nocturnal damage differentially initiate plant volatile emission

Continuous mechanical damage initiates the rhythmic emission of volatiles in lima bean (Phaseolus lunatus) leaves; the emission resembles that induced by herbivore damage. The effect of diurnal versus nocturnal damage on the initiation of plant defense responses was investigated using MecWorm, a robotic device designed to reproduce tissue damage caused by herbivore attack. Lima bean leaves that were damaged by MecWorm during the photophase emitted maximal levels of beta-ocimene and (Z)-3-hexenyl acetate in the late photophase. Leaves damaged during the dark phase responded with the nocturnal emission of (Z)-3-hexenyl acetate, but with only low amounts of beta-ocimene; this emission was followed by an emission burst directly after the onset of light. In the presence of (13)CO(2), this light-dependent synthesis of beta-ocimene resulted in incorporation of 75% to 85% of (13)C, demonstrating that biosynthesis of beta-ocimene is almost exclusively fueled by the photosynthetic fixation of CO(2) along the plastidial 2-C-methyl-D-erythritol 4-P pathway. Jasmonic acid (JA) accumulated locally in direct response to the damage and led to immediate up-regulation of the P. lunatus beta-ocimene synthase gene (PlOS) independent of the phase, that is, light or dark. Nocturnal damage caused significantly higher concentrations of JA (approximately 2-3 times) along with enhanced expression levels of PlOS. Transgenic Arabidopsis thaliana transformed with PlOS promoter :: beta-glucuronidase fusion constructs confirmed expression of the enzyme at the wounded sites. In summary, damage-dependent JA levels directly control the expression level of PlOS, regardless of light or dark conditions, and photosynthesis is the major source for the early precursors of the 2-C-methyl-D-erythritol 4-P pathway.

Mechanostimulation of Medicago truncatula leads to enhanced levels of jasmonic acid

Wounding of plants leads to endogenous rise of jasmonic acid (JA) accompanied with the expression of a distinct set of genes. Among them are those coding for the allene oxide cyclase (AOC) that catalyses a regulatory step in JA biosynthesis, and for 1-deoxy-D-xylulose 5-phosphate synthase 2 (DXS2), an enzyme involved in isoprenoid biosynthesis. To address the question how roots and shoots of Medicago truncatula respond to mechanostimulation and wounding, M. truncatula plants were analysed in respect to JA levels as well as MtAOC1 and MtDXS2-1 transcript accumulation. Harvest-caused mechanostimulation resulted in a strong, but transient increase in JA level in roots and shoots followed by a transient increase in MtAOC1 transcript accumulation. Additional wounding of either shoots or roots led to further increased JA and MtAOC1 transcript levels in shoots, but not in roots. In situ hybridization revealed a cell-specific transcript accumulation of MtAOC1 after mechanostimulation in companion cells of the vascular tissue of the stem. AOC protein, however, was found to occur constitutively in vascular bundles. Further, transcript accumulation of MtDXS2-1 was similar to that of MtAOC1 in shoots, but its transcript levels were not enhanced in roots. Repeated touching of shoots increased MtAOC1 transcript levels and led to significantly shorter shoots and increased biomass. In conclusion, M. truncatula plants respond very sensitively to mechanostimulation with enhanced JA levels and altered transcript accumulation, which might contribute to the altered phenotype after repeated touching of plants.