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Defensive compounds in plants - Biosynthesis of phenolic monoterpenes in oregano and thyme

Plant secondary compounds are of great importance not only to the plant as defense but also for pharmaceutical and medicinal purposes. Understanding the mechanisms underlying the formation and regulation of plant secondary compounds is essential to further investigate their roles in plant defense and develop new strategies to make these compounds more available for pharmaceutical and nutritional usage.
Terpenes form the largest group of natural products with more than 30,000 different structures spread over the widest assortment of structural types with hundreds of different monoterpene, sesquiterpene, diterpene and triterpene carbon skeletons. The majority of terpenes have been isolated from plants where their enormous structural variability leads to a great functional diversity. Terpenes play important roles in almost all basic plant processes, including growth, development, reproduction and defense.
Thymol and carvacrol are two phenolic monoterpenes often found in the essential oil of two culinary herbs, oregano and thyme but also in a great diversity of other plant species. These compounds have a broad range of biological activities acting as antimicrobial compounds, insecticides, anti-oxidants and pharmaceutical agents. A pathway for the biosynthesis of thymol from the monoterpene γ-terpinene via an intermediate p-cymene was proposed in the late 1970s which has never been validated.
The plant system in use is the common oregano (Origanum vulgare L.) and closely related species such as thyme (Thymus vulgaris L.) and marjoram (Origanum majoranaL.). These plants produce aromatic essential oils that consist mainly of mono- and sesquiterpenes which are formed and stored in glandular trichomes on the aerial parts of the plants.
In two cultivars of oregano, γ-terpinene was recently demonstrated to be formed from geranyl diphosphate by a monoterpene synthase (Crocoll et al., 2010). The further steps to thymol and carvacrol are most likely performed by cytochrome P450 monooxygenases. We currently identify and characterize the enzymes catalyzing these latter steps.