ABSTRACT
Plants produce a high diversity of secondary metabolites with a prominent function of protecting plants against predators and microbial pathogens due to their biocidal properties against microbes or repellence to herbivores. Some metabolites are also involved in defense mechanisms against abiotic stress (e.g., UV-B exposure) and are important in the interaction of plants with other organisms (e.g., attraction of pollinators) (Schafer and Wink, 2009). It is believed that most of the 100,000 known secondary metabolites are involved in plant chemical defense systems; they seem to have appeared as a response of plants to the interactions with predators throughout the millions of years of coevolution. There are three major groups of secondary metabolites, including terpenes (Figure 10.1a-d), phenylpropenoids, and N-and S-containing compounds. Among these secondary metabolites, it is estimated that more than 3000 essential oils (EOs) are known, of which about 300 are commercially important and used by the avor and fragrance industries.
