Plant volatiles as insect attractants

One of the most interesting aspects of coevolution deals with the interrelationship between the 250,000 odd species of flowering plants and the perhaps 500,000 species of insects that are associated with them in various ecosystems. These coevolutionary relationships began in the early Carboniferous period when land plants first began to diversify and insects began to diversify into modern orders. Chemical ecology lies at the interface between these two enormous groups of very different life forms. There are perhaps 100,000 different secondary plant compounds: terpenoids, alkaloids, phenyl propanoids, esters, acids, alcohols, ketones, and aldehydes produced in the chemical factories of the plant kingdom, and the great preponderance of these act as allomones, kairomones, and synomones in regulating and controlling ecology at the plant‐insect interface. This review will explore current knowledge in this area, with emphasis on the chemical communications involved between these two great groups of life forms. Discussion will be directed at the basic principles of chemical communication from emission of the chemical messenger, to receptor organs, and to the chemotactic responses that result.     

The role of volatiles in plant communication

Volatiles mediate the interaction of plants with pollinators, herbivores and their natural enemies, other plants and micro‐organisms. With increasing knowledge about these interactions the underlying mechanisms turn out to be increasingly complex. The mechanisms of biosynthesis and perception of volatiles are slowly being uncovered. The increasing scientific knowledge can be used to design and apply volatile‐based agricultural strategies.     

Plant cell cultivation as a biotechnological method

An overview is given about some aspects of history, principles and application of plant cell and tissue cultivation. The main topics of application discussed in the paper are aspects of plant propagation, plant breeding, techniques for in vitro cultivation, in vitro production of secondary plant products, biotransformations, application of immobilized cells as well as economic aspects of plant cell and tissue culture.     

Plant volatiles: new perspectives for research in Brazil

Agroecosystems consist on complex trophic relationships among host plants, herbivores and their natural enemies. This article reviews the research of plant volatiles in Brazil, in order to determine multiple resistance mechanisms of economically important crops and to contribute to the understanding of insect-plant interactions. Most pest management programs, including chemical and biological control, do not consider the impact of these chemicals on herbivores and their natural enemies. Alternative control methods are being developed in order to improve our understanding on the endogenous mechanisms of plant induced defenses against phytophagous arthropods. The use of plant volatiles technology as an additional tool in integrated pest management programs would offer a new and environmentally sound approach to crop protection. This technique involves the development of baits that attract beneficial organisms and the manipulation of biochemical processes that induce and regulate plant defenses, key factors in the improvement of control programs against economically important pests. The elucidation of the mechanisms involved in the indirect defenses of plants will result in useful tools for biological control of crop pests.     

Plant behaviour and communication

Plant behaviours are defined as rapid morphological or physiological responses to events, relative to the lifetime of an individual. Since Darwin, biologists have been aware that plants behave but it has been an underappreciated phenomenon. The best studied plant behaviours involve foraging for light, nutrients, and water by placing organs where they can most efficiently harvest these resources. Plants also adjust many reproductive and defensive traits in response to environmental heterogeneity in space and time. Many plant behaviours rely on iterative active meristems that allow plants to rapidly transform into many different forms. Because of this modular construction, many plant responses are localized although the degree of integration within whole plants is not well understood. Plant behaviours have been characterized as simpler than those of animals. Recent findings challenge this notion by revealing high levels of sophistication previously thought to be within the sole domain of animal behaviour. Plants anticipate future conditions by accurately perceiving and responding to reliable environmental cues. Plants exhibit memory, altering their behaviours depending upon their previous experiences or the experiences of their parents. Plants communicate with other plants, herbivores and mutualists. They emit cues that cause predictable reactions in other organisms and respond to such cues themselves. Plants exhibit many of the same behaviours as animals even though they lack central nervous systems. Both plants and animals have faced spatially and temporally heterogeneous environments and both have evolved plastic response systems.     

Plant volatiles in the sexual communication of Melolontha hippocastani: response towards time-dependent bouquets and novel function of (Z)-3-hexen-1-ol as a sexual kairomone

1. Swarming males of Melolontha hippocastani are known to locate females that stay feeding within the host trees by orienting towards damage‐induced plant volatiles (green leaf volatiles) and a sex pheromone. Thus, volatiles emitted by freshly damaged leaves might indicate to a male the presence of currently feeding females. 2. The hypothesis was studied that volatiles from freshly damaged leaves are more attractive to males than volatiles from old damaged leaves. The odour bouquets of damaged leaves from three plant species that have been shown to attract male M. hippocastani in the field were analysed 10 min (fresh damage) and 1.5 h (old damage) after damaging, using coupled gas chromatography–mass spectrometry. The results showed clear differences between the bouquets: the bouquet of freshly damaged leaves of all species consisted of typical leaf aldehydes, i.e. hexanal, (Z)‐3‐hexenal, (Z)‐2‐hexenal, (E)‐2‐hexenal, the leaf alcohol (Z)‐3‐hexen‐1‐ol, and the corresponding acetate. One and a half hours after damage, aldehydes had almost vanished and (Z)‐3‐hexen‐1‐ol and (Z)‐3‐hexenyl acetate predominated; however males of M. hippocastani were equally attracted to traps baited with volatiles from old and freshly damaged leaves in field experiments. When traps were baited with synthetic volatile mixtures mimicking the bouquets of old and freshly damaged leaves, M. hippocastani males even preferred the old damage mixture. 3. Experiments addressing the role of individual green leaf volatiles revealed that only (Z)‐3‐hexen‐1‐ol was highly attractive while the other compounds tested individually were behaviourally inactive, however all tested compounds elicited comparable electrophysiological responses on male M. hippocastani antennae. 4. In analogy to the term aggregation kairomone used for feeding‐induced plant volatiles that attract both sexes of an insect, the term sexual kairomone is suggested to describe the novel function of (Z)‐3‐hexen‐1‐ol in the sexual communication of M. hippocastani.     

Plant volatiles inhibit restoration of plant species communities in dry grassland

Benefits from livestock grazing have declined in regions where vegetation has been degraded by overgrazing. The vegetation can be restored by excluding livestock for a period, but it takes longer in drier regions. Here we propose a possible mechanism for delays in the recovery of poor vegetation for livestock grazing in dry grassland, introducing a case in Mongolia where steppe vegetation dominated by Stipa krylovii, a palatable grass, can become dominated by Artemisia adamsii, an unpalatable forb, when the grassland is overgrazed. Our long-term field experiment shows that the exclusion of livestock has not enhanced the recovery of palatable species in 6 years, indicating that A. adamsii is a strong competitor in the plant community. To understand why livestock exclusion is ineffective, we examined the ecological significance of volatile organic compounds (VOCs) released by A. adamsii. In ex situ experiments, the VOCs promoted photosynthesis of S. krylovii with enhanced stomatal conductance, and S. krylovii grew faster and consumed more water when exposed to the VOCs even with water deficiency. These findings imply that S. krylovii would be more likely to face severe drought before the next rain falls. We therefore conclude that plant volatiles may reduce the resilience of overgrazed vegetation in arid environments.     

Plant volatiles moderate response to aggregation pheromone in Colorado potato beetle

Abstract:  The orientation of the Colorado potato beetle, Leptinotarsa decemlineata , to a male-produced aggregation pheromone, ( S )-3,7-dimethyl-2-oxo-oct-6-ene-1,3-diol, a three-component plant attractant blend [comprised of ( Z )-3-hexenyl acetate + (±)-linalool + methyl salicylate], and other potato volatiles (nonanal and 2-phenylethanol) were tested. All compounds were previously shown to be active in coupled gas chromatography/electroantennogram experiments. Both the three-component plant attractant blend and 2-phenylethanol were attractive to adult beetles. While male beetles oriented preferentially to both plant attractants vs. a control, females showed little preference. Combining the plant attractants with the pheromone resulted in sexually dimorphic responses similar to those seen with either plant attractant alone. Addition of nonanal abolished the sexually dimorphic response to the pheromone + 2-phenylethanol blend; the new three-component blend was attractive to both sexes. In both laboratory bioassays and field experiments, a combination of the pheromone + the three-component plant attractant was preferred over the plant attractant alone. Thus, it seems likely that combinations of pheromone + plant volatiles may be the most efficacious for field use.     

A method for the solvent extraction of low-boiling-point plant volatiles

A new method has been developed for the extraction of volatiles from plant materials and tested on seedling tissue and mature leaves of Arabidopsis thaliana, pine needles and commercial mixtures of plant volatiles. Volatiles were extracted with n-pentane and then subjected to quick distillation at a moderate temperature. Under these conditions, compounds such as pigments, waxes and non-volatile compounds remained undistilled, while short-chain volatile compounds were distilled into a receiving flask using a high-efficiency condenser. Removal of the n-pentane and concentration of the volatiles in the receiving flask was carried out using a Vigreux column condenser prior to GC-MS. The method is ideal for the rapid extraction of low-boiling-point volatiles from small amounts of plant material, such as is required when conducting metabolic profiling or defining biological properties of volatile components from large numbers of mutant lines.     

Plant volatiles induced by herbivore egg deposition affect insects of different trophic levels

Plants release volatiles induced by herbivore feeding that may affect the diversity and composition of plant-associated arthropod communities. However, the specificity and role of plant volatiles induced during the early phase of attack, i.e. egg deposition by herbivorous insects, and their consequences on insects of different trophic levels remain poorly explored. In olfactometer and wind tunnel set-ups, we investigated behavioural responses of a specialist cabbage butterfly (Pieris brassicae) and two of its parasitic wasps (Trichogramma brassicae and Cotesia glomerata) to volatiles of a wild crucifer (Brassica nigra) induced by oviposition of the specialist butterfly and an additional generalist moth (Mamestra brassicae). Gravid butterflies were repelled by volatiles from plants induced by cabbage white butterfly eggs, probably as a means of avoiding competition, whereas both parasitic wasp species were attracted. In contrast, volatiles from plants induced by eggs of the generalist moth did neither repel nor attract any of the tested community members. Analysis of the plant's volatile metabolomic profile by gas chromatography-mass spectrometry and the structure of the plant-egg interface by scanning electron microscopy confirmed that the plant responds differently to egg deposition by the two lepidopteran species. Our findings imply that prior to actual feeding damage, egg deposition can induce specific plant responses that significantly influence various members of higher trophic levels.     

Open communication as an effective stress management method for breast cancer patients

The social and emotional adaptation of 51 breast cancer patients was assessed four times during the first year after mastectomy according to Weissman and Paykel's Social Adaptation Scale (SAS) and judges' ratings. Openness of communication was measured by eight indices during the first two interviews. It was predicted that they would correlate positively with successful adaptation as measured at the third and fourth interviews. Most patients were aware of their diagnosis, and their communication about their plight was found to be multifaceted. Successful copers sought information; less successful copers avoided it. However, where emotions and not facts were the issue, palliative measures such as avoidance of speaking about the threat and refusal to accept its further implications were connected with better adjustment. The findings indicate that palliation is a prerequisite to good instrumental adjustment when the emotional reactions are intense and countermeasures are limited. More research is needed for assessing communication with specific others and change over time.     

Plant polyphenols in cell-cell interaction and communication

Plant polyphenols including flavonoids and tannins are important constituent of our everyday diet and medical herbals. It is broadly accepted that polyphenols may protect us from toxins, carcinogens and pollutants though the mechanisms of the polyphenols action is still not clear. Here we discuss the ability of polyphenols and especially gallate rich compounds like tannins and catechin gallates to interact with proteins and lipids, establish binding between adjacent bilayer surfaces and initiate membrane aggregation. This phenomena discovered in model experiments could also influence lateral segregation and compartmentalization of cell surface compounds and assist the cell-cell interaction and signal transduction. The involvement of plant polyphenols in communication between cells could be an important factor responsible for anticarcinogenic, vascular and cardioprotective activity of these compounds and speculated to be implicated in the evolution of human brain and intelligence.Key words: human evolution, polyphenols, flavonoids, tannins, membranes, lipid rafts, bilayer structure, cell communicationPlant polyphenols including flavonoids and tannins are present on our daily diet. The opinion on their influence on the human health is contradictory and ranges widely from positive to skeptic and even to alarming.^1–3 Obviously the processes of their functioning in our body should be studied in more details.The polyphenols are know not only as patent antioxidants but also as cell metabolism regulators.⁴ The biological functioning of these compounds begins from their interaction with the cell surface and penetration through the plasma membrane into the cytoplasm. They can influence various physical properties of membrane lipids including diffusion, melting temperature, detergent solubility, osmotic stability, permeability to water soluble compounds^5–7 and general parameters of lipid packing and intrinsic bilayer curvature related to membrane interaction and fusion. Their penetration through the lipid bilayer inversive correlates with the number of hydroxyl groups and accordingly with the hydrophobicity of molecules.The background of polyphenols functioning is attributed to the ability of these compounds to interact with proteins, lipids and polynucleotides through electrostatic, hydrophobic, and even covalent binding.^6–9 The polyphenolic compounds are generally not very active chemically and only electrostatic and hydrophobic forces are responsible for their interaction within cells. However, after oxidation of catechol and gallate¹⁰ moieties to quinones (Fig. 1) in presence of peroxidases, polyphenol oxidases, alkaline pH, or transition metals they could be involved in reactions with free nucleophilic functional groups such as sulfhydryl, amine, amide, indole and imidazole substituents;¹¹ and result in covalent binding with different residues including lysine, methionine, cysteine and tryptophane.^8,12 We expect that terminal amino groups of phosphatidylethanolamines or sulfolipids of biological membranes can also participate in covalent binding with quinones. The consequence of chemical derivatization of proteins may lead to changes in a-helix and random coiled constituents and be accompanied by modulation of protein physical properties and functioning.⁸Open in a separate windowFigure 1A simplified scheme of catechol and gallate moieties oxidation to quinones and subsequent quinones reaction with terminal amino- and sulfa-groups of proteins and lipids (R₂ or R₃). The catechol and gallate moieties are attached to a larger molecule of polyphenol (R₁). For details see reviews.^8,12Among the polyphenolic compounds, tannins express the strongest influence on the lipid bilayer properties. The influence depends on the presence of numerous gallic moieties in the tannin molecule.¹³ Tannins are able to bind on the membrane surface, establish bridges between apposing bilayers and initiate their interaction and adhesion.¹⁴ The process is accompanied by reduction of the membrane dipole potential, lipid interdigitation and the decrease of interbilayer spacing from 15 Å to 5 Å. According to authors¹³ this happens because tannins (1) are amphipathic and partition into the bilayers interfacial region, (2) are long enough to span the interbilayer space, (3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and (4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. The electrostatic interaction between the π electrons in the phenol ring and -N⁺(CH₃)₃ groups of phosphatidylcholines is also should be considered.It is necessary to mention that besides tannins the gallate moiety is present also in some antimalarial agents as rufigallol and exifone.¹⁵ The tea catechin gallate and a number of related compounds including gallocatechin gallate, epicatechin gallate, epigallocatechin gallate contain the gallic moiety and was found to influence the membrane fluidity.¹⁶ Some of the tea gallates reveal anticancer activity correlated with membrane rigidifying effect.¹⁷ Not long ago novel black tea pigments rich with gallate moieties and produced by oxidation of tea epagallocatechin-3-o-gallate were discovered (Fig. 2).¹⁸ The appearance of this kind of pigments in food is very typical example of enzymatic and thermal browning process accompanying various traditional preparation of beverages (tea, coffee, cacao, beer and so on) and meal (roasted or baked vegetables and meet).¹¹Open in a separate windowFigure 2Tannin (A) and a fragment of polymeric chain of gallate rich black tea pigment (B). The triplets of neighboring hydroxyl groups of gallic acid are emphasized by a larger font.According to our hypotheses (Fig. 3) the polyphenols rich with gallate moieties may attach to the cell surface, change physical properties of lipids, initiate lateral segregation and formation of lipid and protein clusters, and finally, initiate binding of neighbouring cells. The lateral segregation of the bilayer compounds is known as a physical background of membranes compartmentalization and lipid rafts formation.^19,20 This process is relevant for cell-cell communication, signalling and metabolism regulation; and, thus, responsible for numerous medically recognizable processes.²¹ It is known that lipid rafts are enriched with cholesterol and sphingolipids that serve as a platform for gathering of signalling and trafficking proteins.²² Tannins and other gallate rich molecules may specifically interact with -N⁺(CH₃)₃ groups of sphingomyelin present in the outer leaflet of plasma membrane. High reactivity of gallates towards nucleophilic moieties of proteins especially in presence of peroxidases may facilitate formation of covalent bridges between adjacent cells.Open in a separate windowFigure 3Polyphenols rich of gallate moieties can interact with the cell surface (A). Then gallates may initiate the phospholipid rigidifying process (physically modified lipids are emphasized by shape and color) (B) and lead to aggregation of some proteins and lipids in clusters (D). Polyphenol molecules could serve as bridges between surfaces of two neighbor cells and initiate cells binding and formation of similar clusters in the membrane of the opposite cell.Plant polyphenols as catechins of tea^23,24 are involved in cell-cell interactions responsible for their anticarcinogenic activity by upregulating the receptor and signaling cascades of communication between cells. Well known cardiovascular protective effect of plant polyphenols can also be explained by influence of these compounds on interaction between endothelial cells.²⁵The cell-cell communication and signal transduction is very important also for functioning of neural networks. In human evolution the plant polyphenol consumption would be considerable improved after invention of the fire cooking. It was supposed that the thermal treatment of meat and vegetables, mostly wild tubers of antic savanna, was an important factor of the brain evolution.²⁶ The increase of protein, lipid and carbohydrates availability contributes the most noticeable gain in human intellectual development.²⁷ The role of polyphenols in neuronal communication and brain evolution is still has to be uncovered. Flavonoids are known to protect the brain from oxidation stress and prevent the aging and numerous pathological processes including neuronal damage during Alzheimer''s disease, Parkinson''s disease, amyotrophic lateral sclerosis.^28,29 It seems the mechanism of polyphenols action on the brain functioning could not be restricted by prevention against oxidation stress because these compounds could also participate in the intracellular signal transduction cascades and modulation of brain functioning.³⁰ This may be explained by interactions of polyphenols with specific proteins and plasma membrane compartments responsible for signal transduction as well as for memory formation and storage.     

Fungal volatiles as indicators of food and feeds spoilage.

Fungal growth leads to spoilage of food and animal feeds and to formation of mycotoxins and potentially allergenic spores. Fungi produce volatile compounds, during both primary and secondary metabolism, which can be used for detection and identification. Fungal volatiles from mainly Aspergillus, Fusarium, and Penicillium have been characterized with gas chromatography, mass spectrometry, and sensory analysis. Common volatiles are 2-methyl-1-propanol, 3-methyl-1-butanol, 1-octen-3-ol, 3-octanone, 3-methylfuran, ethyl acetate, and the malodorous 2-methyl-isoborneol and geosmin. Volatile sesquiterpenes can be used for taxonomic classification and species identification in Penicillium, as well as to indicate mycotoxin formation in Fusarium and Aspergillus. Developments in sensor technology have led to the construction of "electronic noses" (volatile compound mappers). Exposure of different nonspecific sensors to volatile compounds produces characteristic electrical signals. These are collected by a computer and processed by multivariate statistical methods or in an artificial neural network (ANN). Such systems can grade cereal grain with regard to presence of molds as efficiently as sensory panels evaluating grain odor. Volatile compound mapping can also be used to predict levels of ergosterol and fungal colony-forming units in grain. Further developments should make it possible to detect individual fungal species as well as the degree of mycotoxin contamination of food and animal feeds.     

Plant volatiles, rather than light, determine the nocturnal behavior of a caterpillar

Although many organisms show daily rhythms in their activity patterns, the mechanistic causes of these patterns are poorly understood. Here we show that host plant volatiles affect the nocturnal behavior of the caterpillar Mythimna separata. Irrespective of light status, the caterpillars behaved as if they were in the dark when exposed to volatiles emitted from host plants (either uninfested or infested by conspecific larvae) in the dark. Likewise, irrespective of light status, the caterpillars behaved as if they were in the light when exposed to volatiles emitted from plants in the light. Caterpillars apparently utilize plant volatile information to sense their environment and modulate their daily activity patterns, thereby potentially avoiding the threat of parasitism.     

Plant responsiveness to root-root communication of stress cues

Background and Aims

Phenotypic plasticity is based on the organism''s ability to perceive, integrate and respond to multiple signals and cues informative of environmental opportunities and perils. A growing body of evidence demonstrates that plants are able to adapt to imminent threats by perceiving cues emitted from their damaged neighbours. Here, the hypothesis was tested that unstressed plants are able to perceive and respond to stress cues emitted from their drought- and osmotically stressed neighbours and to induce stress responses in additional unstressed plants.

Methods

Split-root Pisum sativum, Cynodon dactylon, Digitaria sanguinalis and Stenotaphrum secundatum plants were subjected to osmotic stress or drought while sharing one of their rooting volumes with an unstressed neighbour, which in turn shared its other rooting volume with additional unstressed neighbours. Following the kinetics of stomatal aperture allowed testing for stress responses in both the stressed plants and their unstressed neighbours.

Key Results

In both P. sativum plants and the three wild clonal grasses, infliction of osmotic stress or drought caused stomatal closure in both the stressed plants and in their unstressed neighbours. While both continuous osmotic stress and drought induced prolonged stomatal closure and limited acclimation in stressed plants, their unstressed neighbours habituated to the stress cues and opened their stomata 3–24 h after the beginning of stress induction.

Conclusions

The results demonstrate a novel type of plant communication, by which plants might be able to increase their readiness to probable future osmotic and drought stresses. Further work is underway to decipher the identity and mode of operation of the involved communication vectors and to assess the potential ecological costs and benefits of emitting and perceiving drought and osmotic stress cues under various ecological scenarios.