As illustrated by the previous paragraph, one of the crucial points of using plants as sources for essential oils is their heterogeneity. A first prerequisite for reproducible compositions is therefore an unambiguous botanical identification and characterization of the starting material. The first approach is the classical taxonomical identification of plant materials based on macro- and micromorphologi-cal features of the plant. The identification is followed by phytochemical analysis that may contribute to species identification as well as to the determination of the quality of the essential oil. This approach is now complemented by DNA-based identification.
DNA is a long polymer of nucleotides, the building units. One of four possible nitrogenous bases is part of each nucleotide and the sequence of the bases on the polymer strand is characteristic for each living individual. Some regions of the DNA, however, are conserved on the species or family level and can be used to study the relationship of taxa (Taberlet et al., 1991; Wolfe and Liston, 1998). DNA sequences conserved within a taxon but different between taxa can therefore be used to identify a taxon ("DNA-barcoding") (Hebert et al., 2003a; Kress et al., 2005). A DNA-barcoding consortium was founded in 2004 with the ambitious goal to build a barcode library for all eukaryotic life in the next 20 years (Ratnasingham and Hebert, 2007). New sequencing technologies (454, Solexa, SOLiD) enable a fast and representative analysis, but will be applied due to their high costs in the moment only in the next phase of DNA-barcoding (Frezal and Leblois, 2008). DNA-barcoding of animals has already become a routine task. DNA-barcoding of plants, however, are still not trivial and a scientific challenge (Pennisi, 2007).
Besides sequence information-based approaches, multilocus DNA methods [RAPD, amplified fragment length polymorphism (AFLP), etc.] are complementing in resolving complicated taxa and can become a barcode for the identification of populations and cultivars (Weising et al., 2005). With multilocus DNA methods, it is furthermore possible to tag a specific feature of a plant of which the genetic basis is still unknown. This approach is called molecular markers (in sensu strictu) because they mark the occurrence of a specific trait like a chemotype or flower color. The gene regions visualized, for example, on an agarose gel is not the specific gene responsible for a trait but is located on the genome in the vicinity of this gene and therefore co-occurs with the trait and is absent when the trait is absent. An example for such an inexpensive and fast polymerase chain reaction (PCR)-system was developed by Bradbury et al. (2005) to distinguish fragrant from nonfragrant rice cultivars. If markers would be developed for chemotypes in essential oil plants, species identification by DNA and the determination of a chemotype could be performed in one step.
Molecular biological methods to identify species are nowadays routinely used in feed- and foodstuffs to identify microbes, animals, and plants. Especially the discussion about traceability of genetically modified organisms (GMO's) throughout the complete chain ("from the living organism to the super-market") has sped up research in this area (Auer, 2003; Miraglia et al., 2004). One advantage of molecular biological methods is the possibility to be used in a number of processed materials like fatty oil (Pafundo et al., 2005) or even solvent extracts (Novak et al., 2007). The presence of minor amounts of DNA in an essential oil cannot be excluded a priori although distillation as separation technique would suggest the absence of DNA. However, small plant or DNA fragments could distill over or the essential oil could come in contact with plant material after distillation.
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