Figure 9.

Proposed organization of C₄₀ carotenoid formation and cleavage to apocarotenoids in a compartmentalized mycorrhizal root cell. C₄₀ carotenoids (exemplified by lactucaxanthin as the tentatively proposed precursor for AM-induced apocarotenoids) synthesized inside plastids are proposed to be cleaved by plastidial CCD7 to one C₂₇ aldehyde and one C₁₃ ketone intermediate, which both can be exported into the cytosol in an unmodified or modified state (white arrows). The C₂₇ intermediate in the cytosol is further cleaved by CCD1 to a C₁₄ and a second molecule of C₁₃, which are, together with the first C₁₃ molecule, modified to yield the oxidized C₁₃ and C₁₄ apocarotenoids (cyclohexenone and mycorradicin derivatives) found in mycorrhizal roots. For abbreviations, see Figure 1.