An overview of plant volatile metabolomics, sample treatment and reporting considerations with emphasis on mechanical damage and biological control of weeds

Better Together: Volatile-Mediated Intraguild Effects on the Preference of Tuta absoluta and Trialeurodes vaporariorum for Tomato Plants

Plant-herbivore interactions have been extensively studied in tomato plants and their most common pests. Tomato plant chemical defenses, both constitutive and inducible, play a role in mediating these interactions. Damaged tomato plants alter their volatile profiles, affecting herbivore preferences between undamaged and damaged plants. However, previous studies on tomato volatiles and herbivore preferences have yielded conflicting results, both in the volatile chemistry itself as well as in the attraction/repellent herbivore response. This study revisits the volatile-mediated interactions between tomato plants and two of their main herbivores: the leafminer Tuta absoluta and the whitefly Trialeurodes vaporariorum. Tomato plant volatiles were analyzed before and after damage by each of these herbivores, and the preference for oviposition (T. absoluta) and settling (T. vaporariorum) on undamaged and damaged plants was assessed both after conspecific and heterospecific damage. We found that both insects consistently preferred damaged plants over undamaged plants. The emission of herbivore-induced plant volatiles (HIPVs) increased after T. absoluta damage but decreased after T. vaporariorum damage. While some of our findings are in line with previous reports, T. absoluta preferred to oviposit on plants damaged by conspecifics, which differs from earlier studies. A comparison of HIPVs emitted after damage by T. absoluta and T. vaporariorum revealed differences in up- or down-regulation, as well as significant variations in specific compounds (12 for T. absoluta and 26 for T. vaporariorum damaged-plants). Only two compounds, β-caryophyllene and tetradecane, significantly varied because of damage by either herbivore, in line with the overall variation of the HIPV blend. Differences in HIPVs and herbivore preferences may be attributed to the distinct feeding habits of both herbivores, which activate different defensive pathways in plants. The plant's challenge in simultaneously activating both defensive pathways may explain the preference for heterospecific damaged plants found in this study, which are also in line with our own observations in greenhouses.

Spectroscopic Analyses Highlight Plant Biostimulant Effects of Baker's Yeast Vinasse and Selenium on Cabbage through Foliar Fertilization

The main aim of this study is to find relevant analytic fingerprints for plants' structural characterization using spectroscopic techniques and thermogravimetric analyses (TGAs) as alternative methods, particularized on cabbage treated with selenium-baker's yeast vinasse formulation (Se-VF) included in a foliar fertilizer formula. The hypothesis investigated is that Se-VF will induce significant structural changes compared with the control, analytically confirming the biofortification of selenium-enriched cabbage as a nutritive vegetable, and particularly the plant biostimulant effects of the applied Se-VF formulation on cabbage grown in the field. The TGA evidenced a structural transformation of the molecular building blocks in the treated cabbage leaves. The ash residues increased after treatment, suggesting increased mineral accumulation in leaves. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) evidenced a pectin-Iα-cellulose structure of cabbage that correlated with each other in terms of leaf crystallinity. FTIR analysis suggested the accumulation of unesterified pectin and possibly (seleno) glucosinolates and an increased network of hydrogen bonds. The treatment with Se-VF formulation induced a significant increase in the soluble fibers of the inner leaves, accompanied by a decrease in the insoluble fibers. The ratio of soluble/insoluble fibers correlated with the crystallinity determined by XRD and with the FTIR data. The employed analytic techniques can find practical applications as fast methods in studies of the effects of new agrotechnical practices, while in our particular case study, they revealed effects specific to plant biostimulants of the Se-VF formulation treatment: enhanced mineral utilization and improved quality traits.

Stress-Induced Volatile Emissions and Signalling in Inter-Plant Communication

The sessile plant has developed mechanisms to survive the “rough and tumble” of its natural surroundings, aided by its evolved innate immune system. Precise perception and rapid response to stress stimuli confer a fitness edge to the plant against its competitors, guaranteeing greater chances of survival and productivity. Plants can “eavesdrop” on volatile chemical cues from their stressed neighbours and have adapted to use these airborne signals to prepare for impending danger without having to experience the actual stress themselves. The role of volatile organic compounds (VOCs) in plant–plant communication has gained significant attention over the past decade, particularly with regard to the potential of VOCs to prime non-stressed plants for more robust defence responses to future stress challenges. The ecological relevance of such interactions under various environmental stresses has been much debated, and there is a nascent understanding of the mechanisms involved. This review discusses the significance of VOC-mediated inter-plant interactions under both biotic and abiotic stresses and highlights the potential to manipulate outcomes in agricultural systems for sustainable crop protection via enhanced defence. The need to integrate physiological, biochemical, and molecular approaches in understanding the underlying mechanisms and signalling pathways involved in volatile signalling is emphasised.

Behavioural regulator and molecular reception of a double-edge-sword hunter beetle

BACKGROUND

The black carabid beetle Calosoma maximoviczi is a successful predator that serves as both a beneficial insect and a severe threat to economic herbivores. Its hunting technique relies heavily on olfaction, but the underlying mechanism has not been studied. Here, we report the electrophysiological, ecological and molecular traits of bioactive components identified from a comprehensive panel of natural odorants in the beetle-prey-plant system. The aim of this work was to investigate olfactory perceptions and their influence on the behaviours of C. maximoviczi.

RESULTS

Among the 200 identified volatiles, 18 were concentrated in beetle and prey samples, and 14 were concentrated in plants. Insect feeding damage to plants led to a shift in the emission fingerprint. Twelve volatiles were selected using successive electrophysiological tests. Field trials showed that significant sex differences existed when trapping with a single chemical or chemical mixture. Expression profiles indicated that sex-biased catches were related to the expression of 15 annotated CmaxOBPs and 40 CmaxORs across 12 chemosensory organs. In silico evaluations were conducted with 16 CmaxORs using modelling and docking. Better recognition was predicted for the pairs CmaxOR5-(Z)-3-hexenyl acetate, CmaxOR6-β-caryophyllene, CmaxOR18-(E)-β-ocimene and CmaxOR18-tetradecane, with higher binding affinity and a suitable binding pocket. Lastly, 168Y in CmaxOR6 and 142Y in CmaxOR18 were predicted as key amino acid residues for binding β-caryophyllene and tetradecane, respectively.

CONCLUSION

This work provides an example pipeline for de novo investigation in C. maximoviczi baits and the underlying olfactory perceptions. The results will benefit the future development of trapping-based integrated pest management strategies and the deorphanization of odorant receptors in ground beetles. © 2022 Society of Chemical Industry.

Emerging Methods of Monitoring Volatile Organic Compounds for Detection of Plant Pests and Disease

Each year, unwanted plant pests and diseases, such as Hendel or potato soft rot, cause damage to crops and ecosystems all over the world. To continue to feed the growing population and protect the global ecosystems, the surveillance and management of the spread of these pests and diseases are crucial. Traditional methods of detection are often expensive, bulky and require expertise and training. Therefore, inexpensive, portable, and user-friendly methods are required. These include the use of different gas-sensing technologies to exploit volatile organic compounds released by plants under stress. These methods often meet these requirements, although they come with their own set of advantages and disadvantages, including the sheer number of variables that affect the profile of volatile organic compounds released, such as sensitivity to environmental factors and availability of soil nutrients or water, and sensor drift. Furthermore, most of these methods lack research on their use under field conditions. More research is needed to overcome these disadvantages and further understand the feasibility of the use of these methods under field conditions. This paper focuses on applications of different gas-sensing technologies from over the past decade to detect plant pests and diseases more efficiently.

Secondary or Specialized Metabolites, or Natural Products: A Case Study of Untargeted LC–QTOF Auto-MS/MS Analysis

The large structural diversity of specialized metabolites represents a substantial challenge in untargeted metabolomics. Modern LC–QTOF instruments can provide three- to four-digit numbers of auto-MS/MS spectra from sample sets. This case study utilizes twelve structurally closely related flavonol glycosides, characteristic specialized metabolites of plant tissues, some of them isomeric and isobaric, to illustrate the possibilities and limitations of their identification. This process requires specific software tools that perform peak picking and feature alignment after spectral deconvolution and facilitate molecular structure base searching with subsequent in silico fragmentation to obtain initial ideas about possible structures. The final assignment of a putative identification, so long as spectral databases are not complete enough, requires structure searches in a chemical reference database, such as SciFindern, in attempts to obtain additional information about specific product ions of a metabolite candidate or check its feasibility. The highlighted problems in this process not only apply to specialized metabolites in plants but to those occurring in other organisms as well. This case study is aimed at providing guidelines for all researchers who obtain data from such analyses but are interested in deeper information than just Venn diagrams of the feature distribution in their sample groups.

Is biocontrol efficacy rather driven by the plant or the antagonist genotypes? A conceptual bioassay approach

In the new range, invasive species lack their specialist co-evolved natural enemies, which then might be used as biocontrol agents. Populations of both a plant invader in the introduced range and its potential biocontrol agents in the native range may be genetically differentiated among geographically distinct regions. This, in turn, is expected to affect the outcome of their interaction when brought together, and by this the efficacy of the control. It further raises the question, is the outcome of such interactions mainly driven by the genotype of the plant invader (some plant genotypes being more resistant/tolerant to most of the antagonist genotypes), or by the antagonist genotype (some antagonist genotypes being more effective against most of the plant genotypes)? This is important for biocontrol management, as only the latter is expected to result in more effective control, when introducing the right biocontrol agent genotypes. In a third scenario, where the outcome of the interaction is driven by a specific plant by antagonist genotype interactions, an effective control will need the introduction of carefully selected multiple antagonist genotypes. Here, we challenged in a complete factorial design 11 plant genotypes (mainly half-siblings) of the invasive Ambrosia artemisiifolia with larvae of eight genotypes (full-siblings) of the leaf beetle Ophraella communa, a potential biocontrol insect, and assessed larval and adult performance and leaf consumption as proxies of their expected impact on the efficacy of biological control. Both species were collected from several locations from their native (USA) and introduced ranges (Europe and China). In summary, we found O. communa genotype to be the main driver of this interaction, indicating the potential for at least short-term control efficacy when introducing the best beetle genotypes. Besides the importance of investigating the genetic structure both among and within populations of the plant invader and the biocontrol agent during the pre-release phase of a biocontrol program, we advocate integrating such bioassays, as this will give a first indication of the probability for an – at least – short- to mid-term efficacy when introducing a potential biocontrol agent, and on where to find the most efficient agent genotypes.

Real-Time Volatilomics: A Novel Approach for Analyzing Biological Samples

The use of the 'omics techniques in environmental research has become common-place. The most widely implemented of these include metabolomics, proteomics, genomics, and transcriptomics. In recent years, a similar approach has also been taken with the analysis of volatiles from biological samples, giving rise to the so-called 'volatilomics' in plant analysis. Developments in direct infusion mass spectrometry (DI-MS) techniques have made it possible to monitor the changes in the composition of volatile flux from parts of plants, single specimens, and entire ecosystems in real-time. The application of these techniques enables a unique insight into the dynamic metabolic processes that occur in plants. Here, we provide an overview of the use of DI-MS in real-time volatilomics research involving plants.

Determination of the metabolic profile of Solidago canadensis using UFLC-PDA-ESI-TOF

Abstract Species in the Solidago genus are used worldwide due to the therapeutic activities of their flavonoids and terpenoids. Its anti-inflammatory, antirheumatic, analgesic, diuretic, aquaretic, wound-healing properties as well as its ability to relieve burn and insect bites allow use in a variety of situations. This study was carried out to determine the metabolic profile of Solidago canadensis inflorescences (SCAI) to identify the species commercialized in Rio de Janeiro, RJ, Brazil, and evaluate the medicinal potential compared to S. chilensis and SCAI, which is used in North America and Europe. The UFLC-PDA-ESI-TOF revealed the metabolic profile of SCAI, finding the pseudo-molecular ions and characteristic fragments, mainly flavonols (10) such as quercetin, kaempferol, isorhamnetin, and its glycosides, as well as chlorogenic acid (CA). Eleven substances were identified, confirming the compatibility of the metabolic profile presented in varieties canadensis and scabra of european S. canadensis. The both contain the same flavonoid aglycones, and their glycosides are important chemical and biological markers for extracts and products based on species of the genus Solidago. UFLC-PDA-ESI-TOF characterized the extracts, which can help with quality control of raw plant materials and commercialized phytotherapeutics as well as for development of new products and phytomedicines.

α-Farnesene and ocimene induce metabolite changes by volatile signaling in neighboring tea (Camellia sinensis) plants

Herbivore-induced plant volatiles (HIPVs) act as direct defenses against herbivores and as indirect defenses by attracting herbivore enemies. However, the involvement of HIPVs in within-plant or plant-to-plant signaling is not fully clarified. Furthermore, in contrast to model plants, HIPV signaling roles in crops have hardly been reported. Here, we investigated HIPVs emitted from tea (Camellia sinensis) plants, an important crop used for beverages, and their involvement in tea plant-to-plant signaling. To ensure uniform and sufficient exposure to HIPVs, jasmonic acid combined with mechanical damage (JAMD) was used to simulate herbivore attacks. Metabonomics techniques based on ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry and gas chromatography-mass spectrometry were employed to determine metabolite changes in undamaged tea plants exposed to JAMD-stimulated volatiles. JAMD-stimulated volatiles mainly enhanced the amounts of 1-O-galloyl-6-O-luteoyl-α-d-glucose, assamicain C, 2,3,4,5-tetrahydroxy-6-oxohexyl gallate, quercetagitrin, 2-(2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4-oxo-4H-chromen-8-yl)-4,5-dihydroxy-6-(hydroxymethyl)-tetrahydro-2H-pyran-3-yl, 3,4-dimethoxybenzoate, 1,3,4,5,6,7-hexahydroxyheptan-2-one, and methyl gallate in neighboring undamaged tea leaves. Furthermore, α-farnesene and β-ocimene, which were produced after JAMD treatments, were identified as two main JAMD-stimulated volatiles altering metabolite profiles of the neighboring undamaged tea leaves. This research advances our understanding of the ecological functions of HIPVs and can be used to develop crop biological control agents against pest insects in the future.

In-field Volatile Analysis Employing a Hand-held Portable GC-MS: Emission Profiles Differentiate Damaged and Undamaged Yellow Starthistle Flower Heads

INTRODUCTION

Understanding the complex chemical signalling of plants and insects is an important component of chemical ecology. Accordingly, the collection and analysis of chemical cues from plants in their natural environment is integral to elucidation of plant-insect communications. Remote plant locations and the need for a large number of replicates make in situ headspace analyses a daunting logistical challenge. A hand-held, portable GC-MS system was used to discriminate between damaged and undamaged Centaurea solstitialis (yellow starthistle) flower heads in both a potted-plant and natural setting.

OBJECTIVE

To determine if a portable GC-MS system was capable of distinguishing between undamaged and mechanically damaged plant treatments, and plant environments.

METHODOLOGY

A portable GC-MS utilising needle trap adsorbent technology was used to collect and analyse in situ headspace volatiles of varying yellow starthistle treatments. Principal component analysis (PCA) was used to distinguish treatments and identify biomarker volatiles. Analysis of variance (ANOVA) was used to determine differences between treatment volatile amounts.

RESULTS

The portable GC-MS system detected 31 volatiles from the four treatments. Each GC-MS run was completed in less than 3 min. PCA showed four distinct clusters representing the four treatments - damaged and undamaged potted plant, and damaged and undamaged natural plant. Damage-specific volatiles were identified.

CONCLUSION

The portable GC-MS system distinguished the treatments based on their detected volatile profiles. Additional statistical analysis identified five possible biomarker volatiles for the treatments, among them cyclosativene and copaene, which indicated damaged flower heads.

Improving our chemistry: challenges and opportunities in the interdisciplinary study of floral volatiles

The field of chemical ecology was established, in large part, through collaborative studies between biologists and chemists with common interests in the mechanisms that mediate chemical communication in ecological and evolutionary contexts. Pollination is one highly diverse and important category of such interactions, and there is growing evidence that floral volatiles play important roles in mediating pollinator behaviour and its consequences for plant reproductive ecology and evolution. Here we outline next-generation questions emerging in the study of plants and pollinators, and discuss the potential for strengthening collaboration between biologists and chemists in answering such questions.

Duration of emission of volatile organic compounds from mechanically damaged plant leaves

Classical biological control of invasive alien weeds depends on the use of arthropod herbivores that are sufficiently host specific to avoid risk of injuring nontarget plants. Host plant specificity is usually evaluated by using a combination of behavioral and developmental experiments under choice, no-choice and field conditions. Secondary plant compounds are likely to have an important influence on host plant specificity. However, relatively little is known about the volatile organic compounds (VOCs) that are emitted by target and nontarget plants, and how environmental conditions may affect their emission. Previous studies have shown that mechanical damage of leaves increases the composition and content of VOCs emitted. In this study we measured the VOC emissions of five species of plants in the subtribe Centaureinae (Asteraceae)--Carthamus tinctorius, Centaurea cineraria, Centaurea melitensis, Centaurea rothrockii, and Centaurea solstitialis--that have previously been used in host specificity experiments for a prospective biological control agent of yellow starthistle (C. solstitialis). Leaves of each plant were punctured with a needle and the VOCs were collected by solid-phase microextraction (SPME) periodically over 48 h and analyzed by GC-MS. A total of 49 compounds were detected. Damage caused an immediate increase of 200-600% in the composition of VOCs emitted from each plant species, and the amounts generally remained high for at least 48 h. The results indicate that a very unspecific mechanical damage can cause a prolonged change in the VOC profile of plants.

Metabolic changes in Citrus leaf volatiles in response to environmental stress

Citrus plants are well known as a rich source of VOCs, and several have important roles in defense responses. However, how VOCs are regulated in response to environmental stress is not yet well understood. In this study, we investigated dynamic changes of VOCs present in leaves of seven Citrus species (Citrus sinensis, C. limon, C. paradisi, C. unshiu, C. kinokuni, C. grandis, and C. hassaku) in response to mechanical wounding, jasmonic acid (JA), and salicylic acid (SA) as determined by gas chromatography/mass spectrometric analysis followed by multivariate analysis (principal component analysis, PCA, and orthogonal partial least squares-discriminant analysis, OPLS-DA). PCA and OPLS-DA suggested that changes in VOC profiles against stress stimuli were much diverse among Citrus species. OPLS-DA showed that C6 volatiles, such as hexanal and trans-2-hexenal, were induced in response to JA and SA stimuli in C. sinensis and C. grandis, while the other VOCs were decreased under all tested stress conditions. α-Farnesene was induced in all species except C. hassaku after wounding or JA treatment. In addition, α-farnesene was also induced in response to SA stimuli in C. unshiu and C. kinokuni. Therefore these volatiles can be candidates of the common stress biomarkers in Citrus. Our results will give a new insight into defense mechanisms in Citrus species.

Kiwifruit Flower Odor Perception and Recognition by Honey Bees, Apis mellifera

Volatile organic compounds (VOCs) from male and female kiwifruit (Actinidia deliciosa 'Hayward') flowers were collected by dynamic headspace sampling. Honey bee (Apis mellifera) perception of the flower VOCs was tested using gas chromatography coupled to electroantennogram detection. Honey bees consistently responded to six compounds present in the headspace of female kiwifruit flowers and five compounds in the headspace of male flowers. Analysis of the floral volatiles by gas chromatography-mass spectrometry and microscale chemical derivatization showed the compounds to be nonanal, 2-phenylethanol, 4-oxoisophorone, (3E,6E)-α-farnesene, (6Z,9Z)-heptadecadiene, and (8Z)-heptadecene. Bees were then trained via olfactory conditioning of the proboscis extension response (PER) to synthetic mixtures of these compounds using the ratios present in each flower type. Honey bees trained to the synthetic mixtures showed a high response to the natural floral extracts, indicating that these may be the key compounds for honey bee perception of kiwifruit flower odor.

Recent Advances in the Application of Metabolomics to Studies of Biogenic Volatile Organic Compounds (BVOC) Produced by Plant

In many plants, biogenic volatile organic compounds (BVOCs) are produced as specialized metabolites that contribute to the characteristics of each plant. The varieties and composition of BVOCs are chemically diverse by plant species and the circumstances in which the plants grow, and also influenced by herbivory damage and pathogen infection. Plant-produced BVOCs are receptive to many organisms, from microorganisms to human, as both airborne attractants and repellants. In addition, it is known that some BVOCs act as signals to prime a plant for the defense response in plant-to-plant communications. The compositional profiles of BVOCs can, thus, have profound influences in the physiological and ecological aspects of living organisms. Apart from that, some of them are commercially valuable as aroma/flavor compounds for human. Metabolomic technologies have recently revealed new insights in biological systems through metabolic dynamics. Here, the recent advances in metabolomics technologies focusing on plant-produced BVOC analyses are overviewed. Their application markedly improves our knowledge of the role of BVOCs in chemosystematics, ecological influences, and aroma research, as well as being useful to prove the biosynthetic mechanisms of BVOCs.