Figure 11. Bixin biosynthesis model of gene regulation in bixin biosynthesis.

The black solid line represents the cell wall. The double green line represents the plastid. The grey circles represent plastoglobules (PGs). The structure in the form of “S” represent plastid stromules. Membrane-associated plastid complex enzymes: PSY, phytoene synthase; PDS, phytoene desaturase; ZDS, ζ-carotene desaturase; β-LCY, lycopene β-cyclase; ε-LCY, lycopene ε-cyclase; LYC, lycopene; β-Car, β-carotene; Zea, Zeaxanthin, B. ALDH., Bixin aldehyde; C₃₂ (C₃₂ intermediate). Bixin synthesis: BoCCD, Carotenoid cleavage dioxygenases; BoALDH, bixin aldehyde dioxygenase; BoMET norbixin methyltransferase (A) Bixin synthesis, is carried out in plastids: BoCCD4-3 cleaves lycopene symmetrically at position 5,6 (5′,6′) to produce bixin aldehyde and continues to bixin synthesis. Bixin could be transported by plastid stromules into BSCs. (B) Bixin synthesis is partially carried out in plastids and is finished in the cytosol. First BoCCD4-3 cleaves lycopene asymmetrically at position 5,6 or (5′,6′) and the resulting product (C₃₂) diffuses out of the plastid and is used by BoCCD1-1 to form bixin aldehyde.