Fig. 1: Compartmentalization of resolved and unknown transport steps in the C. roseus MIA pathway.

Biosynthesis of iridoid glucosides derived from the MEP pathway occurs in the internal phloem-associated parenchyma (IPAP) and they are transported out via an unknown mechanism. Secoiridoids are moved into epidermal cells via CrNPF2.4-2.6¹⁰, where secologanin is imported into the vacuole by CrMATE1 (purple arrow). An unknown transporter concurrently imports tryptamine. In the vacuole, condensation of the two molecules by strictosidine synthase (STR) forms the central MIA intermediate, strictosidine, which is exported into the cytosol by CrNPF2.9¹⁴. Strictosidine-β-d-glucosidase (SGD) guides its substrate into the nucleus, where strictosidine aglycones accumulate. A series of enzymatic steps leading to catharanthine follows, where it is finally exported via CrTPT2¹⁵, accumulating in the laticifers/idioblasts, along with vindoline, mediated by unknown mechanisms. Uncharacterized transport steps are shown with gray arrows. Multiple reaction steps are shown with three conjoined arrows.