SummaryIn higher plants, isopentenyl diphosphate (IPP) is synthesised both from the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) and from the cytosolic mevalonate (MVA) pathways. Primary metabolites, such as phytol group of chlorophylls, carotenoids and the plant hormones abscisic acid (ABA) and gibberellins (GAs) are derived directly from the MEP pathway. Many secondary metabolites, such as monoterpene indole alkaloids (MIAs) in Catharanthus roseus, are also synthesised from this source of IPP. Using Northern blot and in situ hybridisation experiments, we show that three MEP pathway genes (1-deoxy-D-xylulose 5-phosphate synthase (DXS), 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR ) and 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (MECS)) and the gene encoding geraniol 10-hydroxylase (G10H ), a cytochrome P450 monooxygenase involved in the ®rst committed step in the formation of iridoid monoterpenoids display identical cell-speci®c expression patterns. The co-localisation of these four transcripts to internal phloem parenchyma of young aerial organs of C. roseus adds a new level of complexity to the multicellular nature of MIA biosynthesis. We predict the translocation of pathway intermediates from the internal phloem parenchyma to the epidermis and, ultimately, to laticifers and idioblasts during MIA biosynthesis. Similarly, the translocation of intermediates from the phloem parenchyma is probably also required during the biosynthesis of hormones and photosynthetic primary metabolites derived from the MEP pathway.
The Apocynaceae Catharanthus roseus accumulates a number of monoterpene indole alkaloids (MIAs) that originate from the coupling of the indole and the iridoid pathways. The latter pathway is usually considered as limiting for MIA biosynthesis, but evidence is now strong that the precursors tryptamine (from the indole pathway) and secologanin (from the iridoid pathway) have to be provided within the cells in a concerted manner for sustained MIA synthesis. Secologanin is formed from isopentenyl diphosphate (IPP) in a number of steps, some of which are still unknown. However significant progress has been obtained recently with the characterisation of cDNAs encoding secologanin synthase and the two constituents of geraniol 10-hydroxylase (G10H). IPP itself is formed through both the plastidial methyl-erythritol phosphate (MEP) pathway and the cytosolic mevalonate (MVA) pathway. The MEP pathway comprises 7 steps of which 4 have been identified at the molecular level in C. roseus. This pathway plays a major role in the production of MIAs, but there is now evidence that the MVA pathway serves as a minor source of precursors for iridoid biosynthesis and/or contributes (through protein prenylation) to a fine regulation of the MEP gene expression. G10H is one of the key enzymes of the MIA pathway and the upregulation of the gene activity concomitantly with an increase in G10H activity and MIA production have been reported with various hormones and elicitors. Since regulatory genes encoding transcription factors acting on several genes of the MEP and terpenoid pathways are beginning to be characterised, metabolic engineering of the iridoid pathway could be a promising approach to control the metabolite flux towards secologanin and MIA production through biotechnological applications in the future.Keywords Madagascar periwinkle Á Cross talk Á Methyl-erythritol phosphate pathway Á Mevalonate pathway Á Secologanin-derived monoterpene indole alkaloids Á Terpenoid pathway Abbreviations AACT Acetoacetyl-CoA thiolase ADH (monoterpene primary) Alcohol dehydrogenase CDP-ME 4-Diphosphocytidyl-2-C-methyl-D-erythritol CMK 4-Diphosphocytidyl-2-C-methyl-D-erythritol kinase A. Oudin (&) Á M. Courtois Á M. Rideau Á M. Clastre EA 2106,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.