Figure 8.

Proposed role for BUP transporters in opium poppy BIA biosynthesis. Opiate metabolic enzymes are largely restricted to either sieve elements or adjacent laticifers (Onoyovwe et al., 2013). The major alkaloids of the latex include the opiates thebaine, codeine, and morphine, in addition to minor amounts of intermediates (S)-N-methylcoclaurine, (S)-reticuline, and salutaridine. However, initial steps of BIA biosynthesis occur in the sieve elements, implying a transport mechanism to laticifers. Laticifer-localized, proton symport-mediated BUP activity (red) may act in concert with an unknown transport system (gray) to export alkaloid intermediates from the sieve element to the apoplast. Enzymes in green have been localized to sieve elements, whereas enzymes in purple are predominant in latex. Enzymes and alkaloids in gray are not detected in latex-free plant extract (Dastmalchi et al., 2019). 6OMT, norcoclaurine 6-O-methyltransferase; CNMT, coclaurine N-methyltransferase; NMCH, N-methylcoclaurine 3′-hydroxylase; 4′OMT, 3′-hydroxy-N-methylcoclaurine 4′-O-methyltransferase; CPR, cytochrome P450 reductase; NISO, neopinone isomerase; COR, codeinone reductase.