Plant carotenoid cleavage oxygenases and their apocarotenoid products

The oxidative cleavage of carotenoids leads to the production of apocarotenoids and is catalyzed by a family of carotenoid cleavage dioxygenases (CCDs). CCDs often exhibit substrate promiscuity, which probably contributes to the diversity of apocarotenoids found in nature. Biologically and commercially important apocarotenoids include the phytohormone abscisic acid, the visual and signaling molecules retinal and retinoic acid, and the aromatic volatile b-ionone. Unexpected properties associated with the CCD catalytic products emphasize their role in many aspects of plant growth and development. For instance, CCD7 and CCD8 produce a novel, graft-transmissible hormone that controls axillary shoot growth in plants. Here, CCDs are discussed according to their roles in the biosynthesis of these products. Recent studies regarding their mechanism of action are also addressed.