# Dynamics of Monoterpene Formation in Spike Lavender Plants
## 1 Introduction  
Terpenes are the largest and most diverse family of natural products. Within these compounds both primary metabolites (hormones, carotenoids, chlorophylls and sterols), necessary for plant growth and survival, and secondary metabolites, that are not directly involved in growth and/or development, can be found [1; 2; 3; 4; 5]. The latter compounds have also considerable commercial importance due to their uses in food, perfume, cosmetic and pharmaceutical industries [6; 7; 8; 9]. All terpen originate from the universal C5 precursor IPP and its isomer DMAPP. In plants, both compounds are formed through two pathways [10; 11]: the cytosolic MVA pathway, which comprises several enzymatic reactions from acetyl coenzymeA condensation through mevalonate iphosphate decarboxylation, and the plastitial MEP pathway starting from the reaction between pyruvate and glyceraldehyde-3-phosphate (Figure 1). The MVA pathway is primarily regulated at the level of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR) [1; 12], and it is known that HMGR activity regulates the metabolic flux through the MVA pathway and the eventual production of the isoprenoid end-products [13; 14]. The scenario seems to be more complex in the case of the MEP pathway. In fact, the MEP pathway can be regulated by several enzymes, including 1-deoxy-d-xylulose 5-phosphate (DXP) synthase (DXS), DXP reductoisomerase (DXR) [15] and hydroxymethyl-butenyl 4-diphosphate (HMBPP) reductase (HDR) [13; 15; 16]. Generally, it is assumed that the MVA pathway is responsible for providing precursors for the synthesis of sesquiterpenenes and triterpenenes, while the MEP pathway donates precursors for monoterepenes, diterpenenes and tetraterpenes [17]. Although both pathways are thought to operate independently under normal conditions, interactions between them have been repeatedly reported [18]. Thus, experiments with labelled products and/or treatments with specific inhibitors (mevinoline and fosmidomycin that respectively block the MVA and MEP pathways) illustrate that the compartmental division between the two pathways is not complete since common metabolites from both pathways can be exchanged, in both directions, through the plastitial membrane [11; 19; 20]. Therefore, the relative contribution of the MVA and MEP pathways for the biosynthesis of plant terpen and the factors controlling this distribution especially under physiological conditions of growing plants remain uncertain for each plant species. It is still assumed, however, that monoterepenes are primarily synthesized in the plastids via the MEP pathway-derived IPP and DMAPP [21]. Spike lavender (_Lavandula latifolia_ Medicus) is an aromatic shrub of high economic importance due to its essential oil, composed mainly of monoterepenes [22]. The main constituents in spike lavender oil from flowers are the monoterepenes linalool, cineole and camphor, while the main constituents  
Figure 1: General scheme of the terpene synthesis pathways in plants and their inhibitors. ABA: abscisic acid. CLO: comazone. DMAPP: isomer dimethylallyl diphosphate. DXP: 1-deoxy-d-xylulose 5-phosphate. DXR: DXP reductoisomerase. DXS: DXP synthase. FPP: famesyl diphosphate. FSM: fosmidomycin. GGPP: geranyldigonbarle. G3P: d-glyceraldehyde-3 phosphate. HMG-CoA: 3-hydroxy-3-methylglutaryl-coenzyme A. HMGR: HMG-CoA reductase. IPP: isopentenyl diphosphate. MEP: methyl-d-erythitol-4-phosphate. MEV: mevinoline. MVA: mevalonate [10; 11]  
in the oil from leaves are cineole and camphor [23]. All monoterpenes are derived from geranyl diphosphate molecule (GPP). In the case of camphor, GPP is transformed via cyclisation of linaloyl pyrophosphate into bornyl pyrophosphate, followed by a hydrolysis reaction that produces borneol, which finally is oxidized to camphor. In the case of cineole, the GPP intermediate is first converted into \(\alpha\)-terpineol by the \(\alpha\)-terpineol synthase and this monoterpene is converted into cineole by the cineole synthase [24] (Figure 2).