Biochemistry of Plant Volatiles

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Abstract
Plants have a penchant for perfuming the atmo- sphere around them. Since antiquity it has been known that both floral and vegetative parts of many species emit substances with distinctive smells. The discovery of the gaseous hormone ethylene 70 years ago brought the realization that at least some of the compounds emitted may have physiological signifi- cance without any distinctive smell to humans. At present, more than 1,000 low Mr organic compounds have been reported to be emitted from plants, al- though a comprehensive list is available only for floral volatiles (Knudsen et al., 1993). Our knowledge of the occurrence and distribution of plant volatiles has been significantly extended in the last 15 years thanks to the adoption of simple, sensitive methods for headspace sampling and the availability of relatively inexpensive bench-top instruments for gas chromatography-mass spectrometry. The substances reported are largely lipophilic products with molecular masses under 300. Most can be assigned to the follow- ing classes (in order of decreasing size): terpenoids, fatty acid derivatives including lipoxygenase path- way products, benzenoids and phenylpropanoids, C5-branched compounds, and various nitrogen and sulfur containing compounds. Nearly all of these classes are emitted from vegetative parts as well as flowers (Knudsen et al., 1993), and some are even emitted from roots (Steeghs et al., 2004). A major discovery of the last decade is that plants commonly emit much greater amounts and varieties of volatiles after herbivore damage, and not just from the site of injury (Pare and Tumlinson, 1999). Major progress in plant volatile research, as in other areas of plant biology, has come from the use of molecular and biochemical techniques. A large number of genes encoding enzymes of volatile biosynthesis have recently been reported. In vitro characterization of the heterologously expressed enzymes, especially de- termination of their substrate and product specificity, has helped clarify the pathways of volatile formation. In addition, investigation of the spatial and temporal patterns of gene expression has provided new infor- mation on the factors regulating the emission of plant volatile compounds. In this update, we survey the latest advances on the biosynthesis and regulation of plant volatiles, beginning with a brief review of the function of these substances.