Fig. 2. Model for the alkaloid plant cell factories in Catharanthus roseus leaves. The biosynthesis of MIAs sequentially involves internal phloem associated parenchyma (IPAP) cells, pavimentous epidermal cells (upper and lower) and specialized cells (laticifers and idioblasts). The biosynthetic pathway leading to vindoline is shown in the margins of the leaf diagram. Known single enzymatic steps in each cell type are indicated by grey arrows and abbreviation of enzyme names. Broken grey arrows indicate unknown enzymatic steps. No specific cellular localization for NMT has been determined. A DNA helix is shown besides proteins encoded by genes characterized by exploiting C. roseus transcriptomic resources. In the leaf diagram, dashed arrows indicate conversion of pyruvate (A) and GAP (B) to loganic acid (C) in IPAP cells; conversion of loganic acid (C) to 16-methoxy-tabersonine (D) in epidermis (for simplification, only the conversion in the upper epidermis is shown) and conversion of desacetoxyvindoline (E) to vindoline (F) in laticifers and idioblasts. The blue arrows indicate intercellular metabolite translocation for loganic acid (C) and secretion to the leaf surface for catharantine. Broken blue arrows indicate that the translocated metabolite is uncharacterized/unknown. DXS, 1-deoxy-D-xylulose-5-phosphate (DXP) synthase; DXR, DXP reductoisomerase; CMS, 4- (cytidine 50-diphospho)-2C-methyl-D-erythritol (CM) synthase; CMK, CM kinase; MECS, 2C-methyl-D-erythritol-2,4-cyclodiphosphate (MEC) synthase; HDS, hydroxymethylbutenyl 4-diphosphate (HD) synthase; HDR, HD reductase; IDI, isopentenyl diphosphate isomerase; GPPS, geranyl diphosphate synthase; GES, geraniol synthase; G10H (CYP76B6), geraniol 10-hydroxylase; CPR, cytochrome P450-reductase; 10HGO, 10-hydroxygeraniol oxidoreductase; IS, iridoid synthase; 7DLGT, 7-deoxyloganetic acid glucosyltransferase; 7DLH, 7-deoxyloganic acid hydroxylase; LAMT, loganic acid O-methyltransferase; SLS (CYP72A1), secologanin synthase; TDC, tryptophan decarboxylase; STR, strictosidine synthase; SGD, strictosidine b-glucosidase; T16H2 (CYP71D351), tabersonine 16-hydroxylase 2; 16OMT, 16-hydroxytabersonine O-methyltransferase; TPT2, catharanthine transporter from pleiotropic drug resistance (PDR) family of ABC transporters; D4H, desacetoxyvindoline 4-hydroxylase; DAT, deacetylvindoline 4-Oacetyltransferase. DMAPP, dimethylallyl diphosphate; GAP, glyceraldehyde 3-phosphate; IPP, isopentenyl diphosphate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)