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ScienceWeek
PLANT BIOLOGY: ON PLANT VOLATILES
The following points are made by Eran Pichersky (American Scientist 2004 92:514):
1) Plant scents are a subset of the tens of thousands of plant metabolites. Plant fragrances consist of small organic molecules with high vapor pressures. This physical property means that although a scent compound would be in the liquid or even in the solid phase (most often dissolved in the fluid inside cells) at a plant's normal growing temperatures, it evaporates easily if exposed to the air. Chemicals with this property are described as volatile. A flower's perfume is a familiar example of a volatile compound, but characteristic scents can also be released by breaking the vegetative parts of many plants, e.g., freshly cut grass, herbs or pine needles.
2) Volatile plant compounds probably evolved to repel herbivores, but they now perform a remarkable range of functions. Most of the animals that interact with plants are insects, which detect volatiles through the antennae on their heads and, in some species, certain mouthparts called maxillary palps. On the surface of the antennae, specialized cells each contain a single type of protein receptor that recognizes and binds specific volatile compounds. The array of receptor-decorated cells reports to the brain by way of the nervous system. Although each cell contains only one receptor type, a single compound can be recognized (to a greater or lesser degree) by more than one receptor. As a result, the pattern of neuronal firing that is elicited by a specific compound or mixture will be unique. Furthermore, this system is extremely sensitive -- some receptors can detect an airborne volatile at concentrations of a few parts per billion.
3) In mammals (including us), the sense of smell requires volatile compounds to travel through the nasal passages to an organ deep inside the nose called the olfactory epithelium. Although the gross anatomy of the olfactory epithelium is quite different from that of insect antennae, the two organs are similar at a cellular level. Like the antennae, the olfactory epithelium contains neuronal cells studded with receptors that bind specific molecules. And because a single compound can be recognized to a varying degree by different receptors, the complex pattern of neuronal activity conveys both qualitative and quantitative information to the brain.
4) Volatiles can also reach the olfactory epithelium indirectly through the mouth. As food is chewed, mixed with saliva and warmed, volatiles from the food evaporate and ascend through the retro-nasal route. The detection of these aromas is basically instantaneous, giving each food its distinctive flavor. In fact, the tongue only perceives five basic tastes (sweet, sour, salty, bitter and the recently discovered umami), but the number of aroma mixtures is practically unlimited.
5) Although we generally think of plant aromas as pleasant, many plant volatiles are toxic when eaten. These compounds may be used by plants to protect vulnerable organs (such as sugar-laden fruits) from microbial assault. Humans have recognized and taken advantage of these antimicrobials since antiquity, when they were used to retard spoilage. For example, the spice clove, whose major active ingredient is the compound eugenol, was used in baked goods and prepared meats to prevent mold growth. Indeed, before the development of modern food-preservation techniques, European civilization was heavily dependent on clove and other tropical spices to ensure a lasting supply of food, particularly during the winter months. Many of these spices could only be obtained by long-distance trade with Asia, making them extremely expensive. The potential for profit was a major reason why Columbus and other explorers began looking for a shorter route to the East Indies. In fact, it can be said that spices -- plant volatiles -- were one of the ultimate motivations for the discovery of the "new" world.[1-5]
References (abridged):
1. Dicke, M., and J. J. A. van Loon. 2000. Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context. Entomologia Experimentalis et Applicata 97:237-249
2. Dudareva, N., L. Cseke, V. M. Blanc and E. Pichersky. 1996. Evolution of floral scent in Clarkia: Novel patterns of S-linalool synthase gene expression in the C. breweri flower. Plant Cell 8:1137-1148
3. Dudareva, N., E. Pichersky and J. Gershenzon. 2004. Biochemistry of plant volatiles. Plant Physiology 135:1893-1902
4. Kessler, A., and I. T. Baldwin. 2001. Defensive function of herbivore-induced plant volatile emissions in nature. Science 291:2141-2144
5. Pare, P. W., and J. H. Tumlinson. 1999. Plant volatiles as a defense against insect herbivores. Plant Physiology 121:325-332
American Scientist http://www.americanscientist.org
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Related Material:
ON PLANT-INSECT BIOCHEMICAL WARFARE
The following points are made by Jack C. Schultz (Nature 2002 416:267):
1) Each of the world's 300,000 plant species is a target for attack from a range of nearly 400,000 species of plant-eating insects. Plant-eating (herbivory) is among the Earth's most important interactions in terms of number of taxa, biomass and mass transfer, as well as evolutionary impact on plant traits, community structure and ecosystem function. The population biologists Paul Ehrlich and Peter Raven have even claimed that insect herbivory has generated much of terrestrial biodiversity. But we are now realizing that the interactions between plants and insects are far more dynamic than was previously thought, and involve much shared chemistry.
2) Insects are responsible for 15 percent of the world's crop losses. Even in natural systems, they consume 10 percent of plant production each year. Herbivory is limited directly or indirectly by plant chemicals, which at first glance seem to have no role in normal plant metabolism but are highly active in animal tissues. Plant-herbivore interactions are often described as an ongoing biochemical warfare that occurs on an evolutionary timescale.
3) Our perspective of plant defenses and plant-insect interactions is now changing. Plants are not static, chemically defended fortresses ù they respond to attack with rapid, long-lasting, variable, and often specific biochemical, physiological and developmental changes. Plants respond differentially to many stimuli, including various insect species. Every class of constitutive chemical defense responds to a physical insect attack, or to insect regurgitant or saliva, over periods of minutes to days. The few 'stealthy' insect species that fail to elicit any response at all are now considered exceptional.
4) Several signaling pathways coordinate these responses. Fatty-acid signals (for example, oxylipins, which are synthesized from linolenic acid released from membranes by lipases) regulate expression of defense-related genes and are central to most wound-mediated plant responses. Peptides, phenolics, terpenoids and classical plant hormones (such as cytokinins and ethylene) also can help to coordinate plant responses.
References (abridged):
1. Ehrlich, P. R. & Raven, P. H. Evolution 18, 586-608 (1964).
2. Karban, R. & Baldwin, I. T. Induced Responses to Herbivory (Univ. Chicago Press, Chicago, 1997).
3. Shiu, S-H. & Bleecker, A. B. Proc. Natl Acad. Sci. USA 98, 10763-10768 (2001).
4. Chiu, J., et al. Mol. Biol. Evol. 16, 826-838 (1999).
Nature http://www.nature.com/nature
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Related Material:
VOLATILES AS PLANT DEFENSE MECHANISMS
The following points are made by Sophie L. Wilkinson (Chem. Engin. News 2001 30 Jul):
1) Plants are not passive in the face of an attack by insects, but they can marshal elegant defenses and perhaps even inform neighboring plants of danger. Plant security measures are of two types: a) Direct defense involves the expression of defense genes that lead to production of chemicals, such as nicotine or protease inhibitors, that are unpalatable or harmful to insects. b) Alternatively, a plant under attack can rely on indirect defense, which involves the emission of volatile chemicals such as terpenes that attract predatory or parasitic insects.
2) Insect protectors recruited by volatiles, for example, might be predatory wasps that lay eggs in plant-eating caterpillars; when the eggs hatch, the wasp larvae eat the caterpillars. Researchers have been unraveling such complex interactions between plants and insects since the 1980s, when Marcel Dicke (Wageningen University, NL) demonstrated that plants communicate with the enemies of their enemies. Apparently, terpenes and methyl salicylate are involved. In many plant species, the hormone methylsalicylate is emitted only when the plant is attacked by insects but not when other types of damage occur. Plants apparently recognize chemicals in herbivore oral secretions and thus can discriminate between mechanical damage and an attacking herbivore insect.
Chem. & Eng. News http://www.cen-online.org
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Related Material:
DEFENSIVE FUNCTION OF HERBIVORE-INDUCED PLANT VOLATILE EMISSIONS
"Ecology" (environmental biology) is the study of the relationship between organisms and their environment, with the term "environment" including both other organisms and the physical surroundings. The word "ecology" was introduced by the zoologist Ernst Haeckel (1834-1919), who applied the term "oekologie" to "the relation of the animal both to its organic as well as its inorganic environment."
Of major importance in ecology is the categorization of organisms as "autotrophs" or "heterotrophs". All biological communities have a basic structure of interaction that forms a "trophic pyramid". The trophic pyramid consists of trophic levels, with food energy passed from one level to the next along the food chain. The base of the pyramid is composed of autotrophs, the primary producers of the ecosystem, those biological organisms (e.g., plants) that do not obtain energy and nutrients by eating other organisms. Instead, autotrophs harness solar energy by photosynthesis (photoautotrophs) or, more rarely, they harness chemical energy by oxidation (chemoautotrophs), both cases involving the production of organic substances from inorganic substances. All other organisms in the ecosystem are consumers called "heterotrophs", which either directly or indirectly depend on the autotrophs for food energy.
In many land-based ecosystems, the major autotrophs are plants, and the major heterotrophs feeding on plants are insects. One of the more intriguing ecological relationships in nature is that of the "trophic triangle", an often remarkable instance of co-evolution among different species. One example is the insect-plant-insect trophic triangle, in which a species of plant, prey to a predatory insect species that destructively feeds on it, evolves a defense mechanism that sends out a signal that attracts another species of insect that feeds on the first species of insect. Such co-evolutionary triangles, delicate minuets of prey and predation, have become an important focus of experimental ecology.
The following points are made by A. Kessler and I.T. Baldwin (Science 2001 291:2141):
1) Plants defend themselves against plant-eating animals (herbivores) with chemical and physical defenses that directly influence herbivore performance and indirectly influence such performance through traits that attract the natural enemies of herbivores. One such indirect defense, the release of volatile organic compounds specifically after herbivory, is known to attract parasitoids and predators to actively feeding larvae in the laboratory, and evidence from agricultural systems suggests a role for herbivore-induced volatile organic compounds in increasing predation evolutionary selection pressure. But exclusive evidence has been lacking, and it is not even known whether plants growing in natural populations increase volatile organic compound emissions after herbivore attack.
2) The authors report a study in which they quantified volatile emissions from Nicotania attenuata plants growing in natural populations during attack by 3 species of leaf-feeding herbivores (the caterpillars of Manduca quinquemaculata [Lepidoptera]; the leaf bug Dicyphus minimus [Heteroptera]; the flea beetle Epitrix hirtipennis [Coleoptera]). In the experiment, the authors mimicked the individual release of 5 commonly emitted volatiles. Three compounds (cis-3-hexen-1-ol; linalool; cis-alpha-bergamotene) increased egg predation rates by a generalist predator (Geocoris pallens [Heteroptera; a bug]). Linalool and the complete blend decreased lepidopteran egg-deposition rates. As a consequence, a plant could reduce the number of herbivores by more than 90 percent by releasing volatiles. The authors suggest these results confirm that indirect defenses can operate in nature.
Science http://www.sciencemag.org
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