The table also highlights the knowledge gaps in pathway discovery, and genes used for secondary modifications of cannabinoids. Table 1 presents an overview of currently known pathway genes. Corresponding signal peptides might serve as indicators for selection of the encoding genes. Hence, DBR and DHAC should localize to the cytosol, the RmaPT to the plastid, and the PAS to the oil body. Assuming that the biosynthesis of the bibenzyl cannabinoids is distributed across the liverwort cells in a similar manner as the biosynthetic pathways in Cannabis and Rhododendron, transcripts encoding the respective enzymes should be expressed accordingly. Other putative enzymes involved in the biosynthesis of perrottetinenic acid are a double-bond reductase (DBR) that generates dihydrocinnamoyl-CoA from the phenylpropanoid precursor, an aromatic prenyltransferase ( RmaPT), and an oxidocyclase (perrottetinenic acid synthase, PAS). This PKS is most likely responsible for chain elongation of the aromatic precursor ( Figure 4). dauricum prenyltransferase 1 Δ 9-THCA, Δ 9-tetrahydrocannabinolic acid Δ 9-THCAS, Δ 9-tetrahydrocannabinolic acid synthase. sativa aromatic prenyltransferase 4 DCA, daurichromenic acid DCAS, daurichromenic acid synthase GA, grifolic acid HPL, hydroperoxide lyase LOX, lipoxygenase MEP, methylerythritol 4-phosphate OAC, olivetolic acid cyclase OLS, olivetol synthase ORS, orselinic acid synthase RdPT1, R. Abbreviations: AAE1, acyl-activating enzyme 1 ADH, alcohol dehydrogenase CBCA, cannabichromenic acid CBCAS, cannabichromenic acid synthase CBDA, cannabidiolic acid CBDAS, cannabidiolic acid synthase CBGA, cannabigerolic acid CBGAS, cannabigerolic acid synthase CsaPT4, C. Polyketide formation takes place in the cytosol, prenylation in the plastid, and oxidocyclization and storage in the apoplast. Question marks indicate unknown transport mechanisms. Enzymes are located in the cytosol (yellow), plastids (green), or apoplastic space (white).
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