Monoterpene concentrations in fresh, senescent, and decaying foliage of singleleaf pinyon (Pinus monophylla Torr. & Frem.: Pinaceae) from the western Great Basin

Bioprospection of the bacterial β-myrcene-biotransforming trait in the rhizosphere

The biocatalysis of β-myrcene into value-added compounds, with enhanced organoleptic/therapeutic properties, may be performed by resorting to specialized enzymatic machinery of β-myrcene-biotransforming bacteria. Few β-myrcene-biotransforming bacteria have been studied, limiting the diversity of genetic modules/catabolic pathways available for biotechnological research. In our model Pseudomonas sp. strain M1, the β-myrcene catabolic core-code was identified in a 28-kb genomic island (GI). The lack of close homologs of this β-myrcene-associated genetic code prompted a bioprospection of cork oak and eucalyptus rhizospheres, from 4 geographic locations in Portugal, to evaluate the environmental diversity and dissemination of the β-myrcene-biotransforming genetic trait (Myr⁺). Soil microbiomes were enriched in β-myrcene-supplemented cultures, from which β-myrcene-biotransforming bacteria were isolated, belonging to Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Sphingobacteriia classes. From a panel of representative Myr⁺ isolates that included 7 bacterial genera, the production of β-myrcene derivatives previously reported in strain M1 was detected in Pseudomonas spp., Cupriavidus sp., Sphingobacterium sp., and Variovorax sp. A comparative genomics analysis against the genome of strain M1 found the M1-GI code in 11 new Pseudomonas genomes. Full nucleotide conservation of the β-myrcene core-code was observed throughout a 76-kb locus in strain M1 and all 11 Pseudomonas spp., resembling the structure of an integrative and conjugative element (ICE), despite being isolated from different niches. Furthermore, the characterization of isolates not harboring the Myr⁺-related 76-kb locus suggested that they may biotransform β-myrcene via alternative catabolic loci, being thereby a novel source of enzymes and biomolecule catalogue for biotechnological exploitation. KEY POINTS: • The isolation of 150 Myr⁺ bacteria hints the ubiquity of such trait in the rhizosphere. • The Myr⁺ trait is spread across different bacterial taxonomic classes. • The core-code for the Myr⁺ trait was detected in a novel ICE, only found in Pseudomonas spp.

Amelioration potential of β-pinene on Cr(VI)-induced toxicity on morphology, physiology and ultrastructure of maize

Heavy metals' amassment in the soil environment is a threat to crop and agricultural sustainability and consequentially the global food security. For achieving enhancement of crop productivity in parallel to reducing chromium (Cr) load onto food chain demands continuous investigation and efforts to develop cost-effective strategies for maximizing crop yield and quality. In this context, we investigated the amelioration of Cr(VI) toxicity through β-pinene in experimental dome simulating natural field conditions. The protective role of β-pinene was determined on physiology, morphology and ultrastructure in Zea mays under Cr(VI) stress (250 and 500 μM). Results exhibited a marked reduction in the overall growth (shoot and root length and dry matter) of Z. mays plants subjected to Cr(VI) stress. Photosynthetic pigments (chlorophyll and carotenoids) were evidently reduced, and there was a loss of membrane integrity. Supplementation of β-pinene (100 μM), however, declined the toxicity induced by Cr(VI). Interestingly, Cr-tolerant abilities were improved in relation to plant growth, photosynthetic pigments and membrane integrity with the combined treatment of Cr(VI) and β-pinene. β-Pinene also reduced the root-mediated uptake of Cr(VI) and translocation to shoots. Moreover, significant ultrastructural damages recorded in roots and shoots under Cr(VI) stress were partially reverted upon addition of β-pinene. Our analyses revealed that β-pinene mitigates Cr(VI) toxicity in Z. mays, either by membrane stabilization or serving as a barrier to the uptake of Cr from soil. Thus, exogenous supply of β-pinene can be an effective alternative to mitigate Cr toxicity in soil. However, it is deemed essential to investigate further the responses throughout the life cycle of the plant on β-pinene supplementation under natural conditions.

X-ray structure of linalool dehydratase/isomerase from Castellaniella defragrans reveals enzymatic alkene synthesis

Linalool dehydratase/isomerase (Ldi), an enzyme of terpene degradation in Castellaniella defragrans, isomerizes the primary monoterpene alcohol geraniol into the tertiary alcohol (S)-linalool and dehydrates (S)-linalool to the alkene β-myrcene. Here we report on the crystal structures of Ldi with and without terpene substrates, revealing a cofactor-free homopentameric enzyme. The substrates were embedded inside a hydrophobic channel between two monomers of the (α,α)6 barrel fold class and flanked by three clusters of polar residues involved in acid-base catalysis. The detailed view into the active site will guide future biotechnological applications of Ldi, in particular, for industrial butadiene and isoprene production from renewable sources.

Linalool isomerase, a membrane-anchored enzyme in the anaerobic monoterpene degradation in Thauera linaloolentis 47Lol

Background

Thauera linaloolentis 47Lol uses the tertiary monoterpene alcohol (R,S)-linalool as sole carbon and energy source under denitrifying conditions. The conversion of linalool to geraniol had been observed in carbon-excess cultures, suggesting the presence of a 3,1-hydroxyl-Δ¹-Δ²-mutase (linalool isomerase) as responsible enzyme. To date, only a single enzyme catalyzing such a reaction is described: the linalool dehydratase/isomerase (Ldi) from Castellaniella defragrans 65Phen acting only on (S)-linalool.

Results

The linalool isomerase activity was located in the inner membrane. It was enriched by subcellular fractionation and sucrose gradient centrifugation. MALDI-ToF MS analysis of the enriched protein identified the corresponding gene named lis that codes for the protein in the strain with the highest similarity to the Ldi. Linalool isomerase is predicted to have four transmembrane helices at the N-terminal domain and a cytosolic domain. Enzyme activity required a reductant for activation. A specific activity of 3.42 ± 0.28 nkat mg * protein⁻¹ and a k_M value of 455 ± 124 μM were determined for the thermodynamically favored isomerization of geraniol to both linalool isomers at optimal conditions of pH 8 and 35 °C.

Conclusion

The linalool isomerase from T. linaloolentis 47Lol represents a second member of the enzyme class 5.4.4.4, next to the linalool dehydratase/isomerase from C. defragrans 65Phen. Besides considerable amino acid sequence similarity both enzymes share common characteristics with respect to substrate affinity, pH and temperature optima, but differ in the dehydratase activity and the turnover of linalool isomers.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-016-0062-0) contains supplementary material, which is available to authorized users.

Microbial monoterpene transformations-a review

Isoprene and monoterpenes constitute a significant fraction of new plant biomass. Emission rates into the atmosphere alone are estimated to be over 500 Tg per year. These natural hydrocarbons are mineralized annually in similar quantities. In the atmosphere, abiotic photochemical processes cause lifetimes of minutes to hours. Microorganisms encounter isoprene, monoterpenes, and other volatiles of plant origin while living in and on plants, in the soil and in aquatic habitats. Below toxic concentrations, the compounds can serve as carbon and energy source for aerobic and anaerobic microorganisms. Besides these catabolic reactions, transformations may occur as part of detoxification processes. Initial transformations of monoterpenes involve the introduction of functional groups, oxidation reactions, and molecular rearrangements catalyzed by various enzymes. Pseudomonas and Rhodococcus strains and members of the genera Castellaniella and Thauera have become model organisms for the elucidation of biochemical pathways. We review here the enzymes and their genes together with microorganisms known for a monoterpene metabolism, with a strong focus on microorganisms that are taxonomically validly described and currently available from culture collections. Metagenomes of microbiomes with a monoterpene-rich diet confirmed the ecological relevance of monoterpene metabolism and raised concerns on the quality of our insights based on the limited biochemical knowledge.

Monoterpenes: Their effects on ecosystem nutrient cycling

This article explores the evidence for monoterpenes to alter rates of nutrient cycling, with particular emphasis on the nitrogen (N) cycle, from an ecosystem perspective. The general N cycle is reviewed and particular processes are noted where monoterpenes could exert control. The theoretical and conceptual basis for a proposed mode of action by which monoterpenes effect the processes of N mineralization and nitrification is presented, along with recent developments. It is hypothesized that monoterpenes retained in litter enhance the frequency of fire, which in turn changes many N-cycling processes. Experimental support for these roles is presented that includes effects at the cellular level and progresses through populations and communities (microbial and invertebrate) involved in N mineralization and immobilization processes. Since many inhibitors of ammonium oxidation also inhibit methane oxidation, monoterpenes also may alter processes within the carbon cycle. Finally, areas for future research that appear most promising are suggested.

Quantitative structure-activity relationships of monoterpenoid binding activities to the housefly GABA receptor

BACKGROUND

Monoterpenoids are a large group of plant secondary metabolites. Many of these naturally occurring compounds have shown good insecticidal potency on pest insects. Previous studies in this laboratory have indicated that some monoterpenoids have positive modulatory effects on insect GABA receptors. In this study, the key properties of monoterpenoids involved in monoterpenoid binding activity at the housefly GABA receptor were determined by developing quantitative structure-activity relationship (QSAR) models, and the relationship between the toxicities of these monoterpenoids and their GABA receptor binding activities was evaluated.

RESULTS

Two QSAR models were determined for nine monoterpenoids showing significant effects on [³H]-TBOB binding and for nine p-menthane analogs with at least one oxygen atom attached to the ring. The Mulliken charges on certain carbon atoms, the log P value and the total energy showed significant relationships with binding activities to the housefly GABA receptor in these two QSAR models.

CONCLUSIONS

From the QSAR models, some chemical and structural parameters, including the electronic properties, hydrophobicity and stability of monoterpenoid molecules, were suggested to be strongly involved in binding activities to the housefly GABA receptor. These findings will help to understand the mode of action of these natural insecticides, and provide guidance to predict more monoterpenoid insecticides.

Soil microorganisms alleviate the allelochemical effects of a thyme monoterpene on the performance of an associated grass species

BACKGROUND

Plant allelochemicals released into the soil can significantly impact the performance of associated plant species thereby affecting their competitive ability. Soil microbes can potentially affect the interaction between plant and plant chemicals by degrading the allelochemicals. However, most often plant-plant chemical interactions are studied using filter paper bioassays examining the pair-wise interaction between a plant and a plant chemical, not taking into account the potential role of soil microorganisms.

METHODOLOGY/PRINCIPAL FINDINGS

To explore if the allelopathic effects on a grass by the common thyme monoterpene "carvacrol" are affected by soil microorganisms. Seedlings of the grass Agrostis capillaris originating from 3 different thyme sites were raised in the greenhouse. Seedlings were grown under four different soil treatments in a 2*2 fully factorial experiment. The monoterpene carvacrol was either added to standard greenhouse soil or left out, and soil was either sterilized (no soil microorganisms) or not (soil microorganisms present in soil). The presence of carvacrol in the soil strongly increased mortality of Agrostis plants, and this increase was highest on sterile soil. Plant biomass was reduced on soil amended with carvacrol, but only when the soil was also sterilized. Plants originating from sites where thyme produces essential oils containing mostly carvacrol had higher survival on soil treated with that monoterpene than plants originating from a site where thyme produced different types of terpenes, suggesting an adaptive response to the locally occurring terpene.

CONCLUSIONS/SIGNIFICANCE

The study shows that presence of soil microorganisms can alleviate the negative effect of a common thyme monoterpene on the performance of an associated plant species, emphasizing the role of soil microbes in modulating plant-plant chemical interactions.

Response to Host Volatiles by Native and Introduced Populations of Dendroctonus valens (Coleoptera: Curculionidae, Scolytinae) in North America and China

Bark beetles (Coleoptera: Curculionidae, Scolytinae) have specialized feeding habits, and commonly colonize only one or a few closely related host genera in their geographical ranges. The red turpentine beetle, Dendroctonus valens LeConte, has a broad geographic distribution in North America and exploits volatile cues from a wide variety of pines in selecting hosts. Semiochemicals have been investigated for D. valens in North America and in its introduced range in China, yielding apparent regional differences in response to various host volatiles. Testing volatiles as attractants for D. valens in its native and introduced ranges provides an opportunity to determine whether geographic separation promotes local adaptation to host compounds and to explore potential behavioral divergence in native and introduced regions. Furthermore, understanding the chemical ecology of host selection facilitates development of semiochemicals for monitoring and controlling bark beetles, especially during the process of expansion into new geographic ranges. We investigated the responses of D. valens to various monoterpenes across a wide range of sites across North America and one site in China, and used the resulting information to develop an optimal lure for monitoring populations of D. valens throughout its Holarctic range. Semiochemicals were selected based on previous work with D. valens: (R)-(+)-alpha-pinene, (S)-(-)-alpha-pinene, (S)-(-)-beta-pinene, (S)-(+)-3-carene, a commercially available lure [1:1:1 ratio of (R)-(+)-alpha-pinene:(S)-(-)-beta-pinene:(S)-(+)-3-carene], and a blank control. At the release rates used, (+)-3-carene was the most attractive monoterpene tested throughout the native range in North America and introduced range in China, confirming results from Chinese studies. In addition to reporting a more effective lure for D. valens, we present a straightforward statistical procedure for analysis of insect trap count data yielding cells with zero counts, an outcome that is common but makes the estimation of the variance with a Generalized Linear Model unreliable because of the variability/mean count dependency.

Effect of selected monoterpenes on methane oxidation, denitrification, and aerobic metabolism by bacteria in pure culture

Selected monoterpenes inhibited methane oxidation by methanotrophs (Methylosinus trichosporium OB3b, Methylobacter luteus), denitrification by environmental isolates, and aerobic metabolism by several heterotrophic pure cultures. Inhibition occurred to various extents and was transient. Complete inhibition of methane oxidation by Methylosinus trichosporium OB3b with 1.1 mM (-)-alpha-pinene lasted for more than 2 days with a culture of optical density of 0.05 before activity resumed. Inhibition was greater under conditions under which particulate methane monooxygenase was expressed. No apparent consumption or conversion of monoterpenes by methanotrophs was detected by gas chromatography, and the reason that transient inhibition occurs is not clear. Aerobic metabolism by several heterotrophs was much less sensitive than methanotrophy was; Escherichia coli (optical density, 0.01), for example, was not affected by up to 7.3 mM (-)-alpha-pinene. The degree of inhibition was monoterpene and species dependent. Denitrification by isolates from a polluted sediment was not inhibited by 3.7 mM (-)-alpha-pinene, gamma-terpinene, or beta-myrcene, whereas 50 to 100% inhibition was observed for isolates from a temperate swamp soil. The inhibitory effect of monoterpenes on methane oxidation was greatest with unsaturated, cyclic hydrocarbon forms [e.g., (-)-alpha-pinene, (S)-(-)-limonene, (R)-(+)-limonene, and gamma-terpinene]. Lower levels of inhibition occurred with oxide and alcohol derivatives [(R)-(+)-limonene oxide, alpha-pinene oxide, linalool, alpha-terpineol] and a noncyclic hydrocarbon (beta-myrcene). Isomers of pinene inhibited activity to different extents. Given their natural sources, monoterpenes may be significant factors affecting bacterial activities in nature.