Role of carotenoid cleavage dioxygenase 1 (CCD1) in apocarotenoid biogenesis revisited

摘要

Oxidative tailoring of C_(4o) carotenoids by double bond-specific cleavage enzymes (carotenoid cleavage dioxygenases, CCDs) gives rise to various apocarotenoids. AtCCDl generating C_(13), and C_(14) apocarotenoids and orthologous enzymes in other plants are the only CCDs acting in the cytosol, while the hitherto presumed C_(4o) substrate is localized in the plastid. A new model for CCD1 action arising from a RNAi-mediated CCD1 gene silencing study in mycorrhizal hairy roots of Medicago truncatula maysolve this contradiction. This approach unexpectedly resulted in the accumulation of C_(27) apocarotenoids but not C_(4o) carotenoids suggesting C_(27) as the main substrates for CCD1 in planta. It further implies a consecutive two-step cleavage process, in which another CCD performs the primary cleavage of C_(40) to C_(27) in the plastid followed by C_(27) export and further cleavage by CCD1 in the cytosol. We compare the specificities and subcellular locations of the various CCDs and propose the plastidial CCD7 to be the first player in mycorrhizal apocarotenoid biogenesis.