It is now becoming possible to measure the fluctuations of key metabolites in some tissues. Characterization of the metabolic profiles of plants will help us to identify the biochemical perturbations underlying altered gene expression, which may not be straightforward in the absence of a clear phenotype. For a wide variety of solutes, the further development of capillary electrophoresis (CE) has enormous potential in this direction because of its high peak capacities, resolution and sensitivity of detection. In combination with mass spectrometry (CE-MS), this raises the possibility of the rapid and unequivocal identification of the analyzed compounds. Already, analysis of the contents of a single cell of a citrus fruit has been achieved, and studies on the release of biogenic amines from individual nerve cells have been published. There is no doubt that in the future CE-MS will become an essential tool in helping to identify the biosynthetic pathways of plant secondary metabolites. This identification is essential if one wants to explore and secure the value of the compounds extracted from tropical forest species. Indeed, a combination of characterization (by CE-MS) of the intermediates in a biosynthesis and the identification of the enzymes present in the biosynthetic tissues must allow us to unravel the pathway and to clone the genes encoding it. Further recombinant technology work can then allow the engineering of the pathway in more economically favorable plants.
Another aspect of metabolite profiling is the analysis of the volatile molecules of a plant. Often such volatiles play an important role in plant signaling. An extensive literature is already available on the ecological significance of plant volatiles, for instance during pathogen interactions, as attractant of pollinators or in plant-plant communication. There are even indications that the capacity of some plants to conquer a given territory and to form an ecological niche depends on their ability to release volatile molecules. To really substantiate these data, we urgently need the molecular proof through the identification of the genes encoding the information for the regulated synthesis of these compounds, as well as to identify the receptor molecules and the signal transduction pathways involved. This ambitious project is feasible thanks to the high separation capacities and sensitivity of gas chromatography, when coupled to mass spec-trometry (GC-MS), and particularly thanks to new approaches in head-space analysis to detect minimum amounts of chemicals in the gas phase surrounding plant organs. Traditionally, these compounds were adsorbed and concentrated on the surface of, for instance, active carbon. A temperature chase transported them to the GC-MS analyzer. Such a procedure functions well for hydrophobic compounds that can be released from the adsorbent at nondestructive temperatures. More recently, gum phase columns are being used for the absorption of gaseous compounds, and this requires only a mild temperature increase to release polar and hydrophobic compounds with a minimum of degradation.
One may safely predict that many new compounds, other than the already well studied terpenoids, aromatic compounds or esters, will be identified. Coupling this compound identification with judicious field work and with the introduction of the molecular biologist's tools will open a completely new area of molecular ecology.
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