2). Schematic of the biosynthetic pathway that leads to monoterpenoid indole alkaloids in Madagascar periwinkle. However, several Arabidopsis mutants, identiﬁed through screens for FSM resistance, grow in the presence of FSM without apparent up-regulation of the MEP pathway or generation of MEP pathway products (Sauret-Güeto et al., 2006; Flores-Pérez et al., 2008, 2010; Van Ree et al., 2011). When 13 CO 2 is fed to photosynthesizing plant leaves, isoprene rapidly becomes labelled confirming the close relationship between isoprene synthesis and the Calvin-Benson cycle.48, 50, Bacteria, plants, and apicomplexan protozoa—such as malaria parasites—are able to produce isoprenoid precursors using an alternative pathway, the MEP pathway, which is a non-mevalonate pathway. The source of GAP and pyruvate for the MEP pathway in chloroplasts of plants is potentially more complex (Fig. It is well known that terpenoids are biosynthesized via two pathways in plants: the mevalonate (MVA) pathway in the cytosol and the methylerythritol phosphate (MEP) pathway in the plastids ().The 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is the rate-limiting enzyme of the MVA pathway, and mevinolin (MEV) is a highly specific inhibitor of HMGR . MEP pathway products (Sauret-Güeto et al., 2006). Two metabolic pathways exist for the biosynthesis of isopentenyl pyrophosphate and dimethylallyl pyrophosphate: The mevalonate pathway, predominantly used by plants and in a few insect species. the MVA pathway or the MEP pathway in null mutants or the complete inhibition of single pathway enzymes in wild-type plants treated with specific inhibitors results in a developmen-tal block and a seedling-lethal phenotype, indicating that the loss of one of the two pathways cannot be compensated by the remaining pathway. In the case of plants and certain protozoa, the biosynthesis of IPP/DMAPP takes place in plastid organelles. However, the role of IPI in each pathway and in plant development is unknown due to a lack of genetic studies using IPI-defective mutants. In addition, there are several reports demonstrating the metabolic cross-talk between MVA and MEP pathway in the biosynthesis of sesquiterpenes [4,84]. For instance, labeling experiments in snapdragon flowers have shown that sesquiterpenes were produced by cytosolic sTPSs using C 5 precursors made in plastids via the MEP pathway [68,85]. More recently, another pathway has been described, the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway, which is found in the plastids of plants (19). plants are derived from the MEP pathway (Eisenreich et al., 2001; Rodriguez-Concepcion and Boronat, 2002). In plants, IPP and DMAPP are synthesized via the cytosolic mevalonate (MVA) and plastidic methylerythritol phosphate (MEP) pathways, respectively. The three non-mevalonate (MEP) pathway genes DXS, DXR and MECS, and G10H are expressed in internal phloem parenchyma cells. Cordoba E, Salmi M, León P (2009) Unravelling the regulatory mechanisms that modulate the MEP pathway in higher plants. Unlike isoprenoid biosynthesis in other living organisms, prenyl-PP, as the precursor of all isoprenoids in plants, is synthesized by two independent pathways: the mevalonate (MVA) pathway in the cytoplasm and the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway in plastids. Introduction. The MEP pathway uses glyceraldehyde 3-phosphate and pyruvate as initial substrates to form deoxy-D-xylulose 5-phosphate (DXP); this is catalyzed by DXP synthase (DXS).