Manipulation of VOC emissions with methyl jasmonate and carrageenan in the evergreen conifer Pinus sylvestris and evergreen broadleaf Quercus ilex

Simulated Herbivory Affects the Volatile Emissions of Oak Saplings, while Neighbourhood Affects Flavan-3-ols Content of Their Leaves

To what extent particular plant defences against herbivorous insects are constitutive or inducible will depend on the costs and benefits in their neighbourhood. Some defensive chemicals in leaves are thought to be costly and hard to produce rapidly, while others, including volatile organic compounds that attract natural enemies, might be cheaper and can be released rapidly. When surrounding tree species are more closely related, trees can face an increased abundance of both specialist herbivores and their parasitoids, potentially increasing the benefits of constitutive and inducible defences. To test if oaks (Quercus robur) respond more to herbivore attacks with volatile emission than with changes in leaf phenolic chemistry and carbon to nitrogen ratio (C: N), and whether oaks respond to the neighbouring tree species, we performed an experiment in a forest in Poland. Oak saplings were placed in neighbourhoods dominated by oak, beech, or pine trees, and half of them were treated with the phytohormone methyl jasmonate (elicitor of anti-herbivore responses). Oaks responded to the treatment by emitting a different volatile blend within 24 h, while leaf phenolic chemistry and C: N remained largely unaffected after 16 days and multiple treatments. Leaf phenolics were subtly affected by the neighbouring trees with elevated flavan-3-ols concentrations in pine-dominated plots. Our results suggest that these oaks rely on phenols as a constitutive defence and when attacked emit volatiles to attract natural enemies. Further studies might determine if the small effect of the neighbourhood on leaf phenolics is a response to different levels of shading, or if oaks use volatile cues to assess the composition of their neighbourhood.

Engineering DMNT emission in cotton enhances direct and indirect defense against mirid bugs

INTRODUCTION

As an important herbivore-induced plant volatile, (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) is known for its defensive role against multiple insect pests, including attracting natural enemies. A terpene synthase (GhTPS14) and two cytochrome P450 (GhCYP82L1, GhCYP82L2) enzymes are involved in the de novo synthesis of DMNT in cotton. We conducted a study to test the potential of manipulating DMNT-synthesizing enzymes to enhance plant resistance to insects.

OBJECTIVES

To manipulate DMNT emissions in cotton and generate cotton lines with increased resistance to mirid bug Apolygus lucorum.

METHODS

Biosynthesis and emission of DMNT by cotton plants were altered using CRISPR/Cas9 and overexpression approaches. Dynamic headspace sampling and GC-MS analysis were used to collect, identify and quantify volatiles. The attractiveness and suitability of cotton lines against mirid bug and its parasitoid Peristenus spretus were evaluated through various assays.

RESULTS

No DMNT emission was detected in knockout CAS-L1L2 line, where both GhCYP82L1 and GhCYP82L2 were knocked out. In contrast, gene-overexpressed lines released higher amounts of DMNT when infested by A. lucorum. At the flowering stage, L114 (co-overexpressing GhCYP82L1 and GhTPS14) emitted 10-15-fold higher amounts than controls. DMNT emission in overexpressed transgenic lines could be triggered by methyl jasmonate (MeJA) treatment. Apolygus lucorum and its parasitoid were far less attracted to the double edited CAS-L1L2 plants, however, co-overexpressed line L114 significantly attracted bugs and female wasps. A high dose of DMNT, comparable to the emission of L114, significantly inhibited the growth of A. lucorum, and further resulted in higher mortalities.

CONCLUSION

Turning down DMNT emission attenuated the behavioral preferences of A. lucorum to cotton. Genetically modified cotton plants with elevated DMNT emission not only recruited parasitoids to enhance indirect defense, but also formed an ecological trap to kill the bugs. Therefore, manipulation of DMNT biosynthesis and emission in plants presents a promising strategy for controlling mirid bugs.

MeJA-mediated enhancement of salt-tolerance of Populus wutunensis by 5-aminolevulinic acid

BACKGROUND

5-Aminolevulinic acid (ALA) is a natural and environmentally benign multifunctional plant growth regulator involved in the regulation of plant tolerance to various environmental stresses. This research aimed to explore the molecular mechanisms of salt tolerance in Populus wutunensis induced by exogenous ALA using physiological and transcriptomic analyses.

RESULTS

Physiological results showed that 50 mg·L^- 1 ALA-treatment significantly reduced the malondialdehyde (MDA) content and the relative electrical conductivity (REC) and enhanced antioxidant activities of enzymes such as SOD, POD and CAT in salt-stressed P. wutunensis seedlings. Transcriptome analysis identified ALA-induced differentially expressed genes (DEGs) associating with increased salt-tolerance in P. wutunensis. GO and KEGG enrichment analyses showed that ALA activated the jasmonic acid signaling and significantly enhanced the protein processing in endoplasmic reticulum and the flavonoid biosynthesis pathways. Results of the hormone-quantification by LC-MS/MS-based assays showed that ALA could increase the accumulation of methyl jasmonate (MeJA) in salt-stressed P. wutunensis. Induced contents of soluble proteins and flavonoids by exogenous ALA in salt-treated seedlings were also correlated with the MeJA content.

CONCLUSION

5-aminolevulinic acid improved the protein-folding efficiency in the endoplasmic reticulum and the flavonoid-accumulation through the MeJA-activated jasmonic acid signaling, thereby increased salt-tolerance in P. wutunensis.

Acute methyl jasmonate exposure results in major bursts of stress volatiles, but in surprisingly low impact on specialized volatile emissions in the fragrant grass Cymbopogon flexuosus

Methyl jasmonate (MeJA) is an airborne hormonal elicitor that induces a fast rise of emissions of characteristic stress marker compounds methanol and green leaf volatiles (GLV), and a longer-term release of volatile terpenoids, but there is limited information of how terpene emissions respond to MeJA in terpene-storing species. East-Indian lemongrass (Cymbopogon flexuosus), an aromatic herb with a large terpenoid storage pool in idioblasts, was used to investigate the short- (0-1 h) and long-term (1-16 h) responses of leaf net assimilation rate (A), stomatal conductance (G_s) and volatile emissions to MeJA concentrations ranging from moderate to lethal. Both A and G_s were increasingly inhibited with increasing MeJA concentration in both short and long term. MeJA exposure resulted in a rapid elicitation, within 1 h after exposure, of methanol and GLV emissions. Subsequently, a secondary rise of GLV emissions was observed, peaking at 2 h after MeJA exposure for the highest and at 8 h for the lowest application concentration. The total amount and maximum emission rate of methanol and the first and second GLV emission bursts were positively correlated with MeJA concentration. Unexpectedly, no de novo elicitation of terpene emissions was observed through the experiment. Although high MeJA application concentrations led to visible lesions and desiccation in extensive leaf regions, this did not result in breakage of terpene-storing idioblasts. The study highlights an overall insensitivity of lemongrass to MeJA and indicates that differently from mechanical wounding, MeJA-driven cellular death does not break terpene-storing cells. Further studies are needed to characterize the sensitivity of induced defense responses in species with strongly developed constitutive defenses.

Dose-dependent methyl jasmonate effects on photosynthetic traits and volatile emissions: biphasic kinetics and stomatal regulation

Exogenous application of methyl jasmonate (MeJA) has been extensively used to study jasmonate-dependent signaling events triggered by biotic stresses. MeJA application leads to complex jasmonate-dependent physiological responses, including changes in stomatal openness and induction of emissions of a multitude of volatile compounds. Whether the alterations in stomatal conductance and emissions of MeJA-induced volatiles are quantitatively associated with MeJA dose, and whether the induced volatile emissions are regulated by modifications in stomatal conductance had been poorly known until recently. Our latest studies highlighted a biphasic kinetics of jasmonate-dependent volatile emissions induced by MeJA treatment in the model species cucumber (Cucumis sativus), indicating induction of an immediate stress response and subsequent gene-expression level response. Both the immediate and delayed responses were MeJA dose-dependent. The studies further demonstrated that stomata modulated the kinetics of emissions of water-soluble volatiles in a MeJA dose-dependent manner. These studies contribute to understanding of plant short- and long-term responses to different biotic stress severities as simulated by treatments with a range of MeJA doses corresponding to mild to acute stress.

Variation in Methyl Jasmonate-Induced Defense Among Norway Spruce Clones and Trade-Offs in Resistance Against a Fungal and an Insect Pest

An essential component of plant defense is the change that occurs from a constitutive to an induced state following damage or infection. Exogenous application of the plant hormone methyl jasmonate (MeJA) has shown great potential to be used as a defense inducer prior to pest exposure, and could be used as a plant protection measure. Here, we examined (1) the importance of MeJA-mediated induction for Norway spruce (Picea abies) resistance against damage by the pine weevil Hylobius abietis, which poses a threat to seedling survival, and infection by the spruce bark beetle-associated blue-stain fungus Endoconidiophora polonica, (2) genotypic variation in MeJA-induced defense (terpene chemistry), and (3) correlations among resistance to each pest. In a semi-field experiment, we exposed rooted-cuttings from nine different Norway spruce clones to insect damage and fungal infection separately. Plants were treated with 0, 25, or 50 mM MeJA, and planted in blocks where only pine weevils were released, or in a separate block in which plants were fungus-inoculated or not (control group). As measures of resistance, stem area debarked and fungal lesion lengths were assessed, and as a measure of defensive capacity, terpene chemistry was examined. We found that MeJA treatment increased resistance to H. abietis and E. polonica, but effects varied with clone. Norway spruce clones that exhibited high constitutive resistance did not show large changes in area debarked or lesion length when MeJA-treated, and vice versa. Moreover, insect damage negatively correlated with fungal infection. Clones receiving little pine weevil damage experienced larger lesion lengths, and vice versa, both in the constitutive and induced states. Changes in absolute terpene concentrations occurred with MeJA treatment (but not on proportional terpene concentrations), however, variation in chemistry was mostly explained by differences between clones. We conclude that MeJA can enhance protection against H. abietis and E. polonica, but the extent of protection will depend on the importance of constitutive and induced resistance for the Norway spruce clone in question. Trade-offs among resistances do not necessarily hinder the use of MeJA, as clones that are constitutively more resistant to either pest, should show greater MeJA-induced resistance against the other.

Foliage inoculation by Burkholderia vietnamiensis CBMB40 antagonizes methyl jasmonate-mediated stress in Eucalyptus grandis

Methyl jasmonate (MeJA) is widely used as a model chemical to study hypersensitive responses to biotic stress impacts in plants. Elevated levels of methyl jasmonate induce jasmonate-dependent defense responses, associated with a decline in primary metabolism and enhancement of secondary metabolism of plants. However, there is no information of how stress resistance of plants, and accordingly the sensitivity to exogenous MeJA can be decreased by endophytic plant growth promoting rhizobacteria (PGPR) harboring ACC (1-aminocyclopropane-1-carboxylate) deaminase. In this study, we estimated stress alleviating potential of endophytic PGPR against MeJA-induced plant perturbations through assessing photosynthetic traits and stress volatile emissions. We used mild (5 mM) to severe (20 mM) MeJA and endophytic plant growth promoting rhizobacteria Burkholderia vietnamiensis CBMB40 and studied how MeJA and B. vietnamiensis treatments influenced temporal changes in photosynthetic characteristics and stress volatile emissions. Separate application of MeJA markedly decreased photosynthetic characteristics and increased lipoxygenase pathway (LOX) volatiles, volatile isoprenoids, saturated aldehydes, lightweight oxygenated compounds (LOC), geranyl-geranyl diphosphate pathway (GGDP) volatiles, and benzenoids. However, MeJA-treated leaves inoculated by endophytic bacteria B. vietnamiensis had substantially increased photosynthetic characteristics and decreased emissions of LOX, volatile isoprenoids and other stress volatiles compared with non-inoculated MeJA treatments, especially at later stages of recovery. In addition, analysis of leaf terpenoid contents demonstrated that several mono- and sesquiterpenes were de novo synthesized upon MeJA and B. vietnamiensis applications. This study demonstrates that foliar application of endophytic bacteria B. vietnamiensis can potentially enhance resistance to biotic stresses and contribute to the maintenance of the integrity of plant metabolic activity.

Methyl jasmonate-induced compositional changes of volatile organic compounds in Polygonum minus leaves

Polygonum minus Huds. is a medicinal aromatic plant rich in terpenes, aldehydes, and phenolic compounds. Methyl jasmonate (MeJA) is a plant signaling molecule commonly applied to elicit stress responses to produce plant secondary metabolites. In this study, the effects of exogenous MeJA treatment on the composition of volatile organic compounds (VOCs) in P. minus leaves were investigated by using a metabolomic approach. Time-course changes in the leaf composition of VOCs on days 1, 3, and 5 after MeJA treatment were analyzed through solid-phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). The VOCs found in MeJA-elicited leaves were similar to those found in mock-treated leaves but varied in quantity at different time points. We focused our analysis on the content and composition of monoterpenes, sesquiterpenes, and green leaf volatiles (GLVs) within the leaf samples. Our results suggest that MeJA enhances the activity of biosynthetic pathways for aldehydes and terpenes in P. minus. Hence, the production of aromatic compounds in this medicinal herb can be increased by MeJA elicitation. Furthermore, the relationship between MeJA elicitation and terpene biosynthesis in P. minus was shown through SPME-GC-MS analysis of VOCs combined with transcriptomic analysis of MeJA-elicited P. minus leaves from our previous study.

Specificity of Herbivore Defense Responses in a Woody Plant, Black Poplar (Populus nigra)

The specificity of woody plant defense responses to different attacking herbivores is poorly known. We investigated the responses of black poplar (Populus nigra) to leaf feeding by three lepidopteran species (Lymantria dispar, Laothoe populi and Amata mogadorensis) and two leaf beetle species (Phratora vulgatissima and Chrysomela populi). Of the direct defenses monitored, increases in trypsin protease inhibitor activity and the salicinoid salicin were triggered by herbivore damage, but this was not herbivore-specific. Moreover, the majority of leaf salicinoid content was present constitutively and not induced by herbivory. On the other hand, volatile emission profiles did vary among herbivore species, especially between coleopterans and lepidopterans. Monoterpenes and sesquiterpenes were induced in damaged and adjacent undamaged leaves, while the emission of green leaf volatiles, aromatic and nitrogen-containing compounds (known to attract herbivore enemies) was restricted to damaged leaves. In conclusion, indirect defenses appear to show more specific responses to attacking herbivores than direct defenses in this woody plant.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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.

Regulation of Floral Terpenoid Emission and Biosynthesis in Sweet Basil (Ocimum basilicum)

Past studies have focused on the composition of essential oil of Ocimum basilicum leaves, but data on composition and regulation of its aerial emissions, especially floral volatile emissions are scarce. We studied the chemical profile, within-flower spatial distribution (sepals, petals, pistils with stamina and pedicels), diurnal emission kinetics and effects of exogenous methyl jasmonate (MeJA) application on the emission of floral volatiles by dynamic headspace collection and identification using gas chromatography-mass spectrometry (GC-MS) and proton transfer reaction mass spectrometry (PTR-MS). We observed more abundant floral emissions from flowers compared with leaves. Sepals were the main emitters of floral volatiles among the flower parts studied. The emissions of lipoxygenase compounds (LOX) and monoterpenoids, but not sesquiterpene emissions, displayed a diurnal variation driven by light. Response to exogenous MeJA treatment of flowers consisted of a rapid stress response and a longer-term acclimation response. The initial response was associated with enhanced emissions of fatty acid derivatives, monoterpenoids, and sesquiterpenoids without variation of the composition of individual compounds. The longer-term response was associated with enhanced monoterpenoid and sesquiterpenoid emissions with profound changes in the emission spectrum. According to correlated patterns of terpenoid emission changes upon stress, highlighted by a hierarchical cluster analysis, candidate terpenoid synthases responsible for observed diversity and complexity of released terpenoid blends were postulated. We conclude that flower volatile emissions differ quantitatively and qualitatively from leaf emissions, and overall contribute importantly to O. basilicum flavor, especially under stress conditions.

Oak powdery mildew (Erysiphe alphitoides)-induced volatile emissions scale with the degree of infection in Quercus robur

Oak powdery mildew (Erysiphe alphitoides) is a major foliar pathogen of Quercus robur often infecting entire tree stands. In this study, foliage photosynthetic characteristics and constitutive and induced volatile emissions were studied in Q. robur leaves, in order to determine whether the changes in foliage physiological traits are quantitatively associated with the degree of leaf infection, and whether infection changes the light responses of physiological traits. Infection by E. alphitoides reduced net assimilation rate by 3.5-fold and isoprene emission rate by 2.4-fold, and increased stomatal conductance by 1.6-fold in leaves with the largest degree of infection of ∼60%. These alterations in physiological activity were quantitatively associated with the fraction of leaf area infected. In addition, light saturation of net assimilation and isoprene emission was reached at lower light intensity in infected leaves, and infection also reduced the initial quantum yield of isoprene emission. Infection-induced emissions of lipoxygenase pathway volatiles and monoterpenes were light-dependent and scaled positively with the degree of infection. Overall, this study indicates that the reduction of foliage photosynthetic activity and constitutive emissions and the onset of stress volatile emissions scale with the degree of infection, but also that the infection modifies the light responses of foliage physiological activities.

Plant volatiles in polluted atmospheres: stress responses and signal degradation

Plants emit a plethora of volatile organic compounds, which provide detailed information on the physiological condition of emitters. Volatiles induced by herbivore feeding are among the best studied plant responses to stress and may constitute an informative message to the surrounding community and further function in plant defence processes. However, under natural conditions, plants are potentially exposed to multiple concurrent stresses with complex effects on the volatile emissions. Atmospheric pollutants are an important facet of the abiotic environment and can impinge on a plant's volatile-mediated defences in multiple ways at multiple temporal scales. They can exert changes in volatile emissions through oxidative stress, as is the case with ozone pollution. The pollutants, in particular, ozone, nitrogen oxides and hydroxyl radicals, also react with volatiles in the atmosphere. These reactions result in volatile breakdown products, which may themselves be perceived by community members as informative signals. In this review, we demonstrate the complex interplay among stresses, emitted signals, and modification in signal strength and composition by the atmosphere, collectively determining the responses of the biotic community to elicited signals.