Co‐expression of three MEP pathway genes and geraniol 10‐hydroxylase in internal phloem parenchyma of Catharanthus roseus implicates multicellular translocation of intermediates during the biosynthesis of monoterpene indole alkaloids and isoprenoid‐derived primary metabolites

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Abstract
In 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 first committed step in the formation of iridoid monoterpenoids display identical cell‐specific 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.