In many organisms, various enzymes mediate site-specific carotenoid cleavage to generate biologically active apocarotenoids. These carotenoid-derived products include provitamin A, hormones, and flavor and fragrance molecules. In plants, the CCD1 enzyme cleaves carotenoids at 9,10 (9,10) bonds to generate multiple apocarotenoid products. Here we systematically analyzed volatile apocarotenoids generated by maize CCD1 (ZmCCD1) from multiple carotenoid substrates. ZmCCD1 did not cleave geranylgeranyl diphosphate or phytoene but did cleave other linear and cyclic carotenoids, producing volatiles derived from 9,10 (9,10) bond cleavage. Additionally the Arabidopsis, maize, and tomato CCD1 enzymes all cleaved lycopene to generate 6-methyl-5-hepten-2-one. 6-Methyl-5-hepten-2-one, an important flavor volatile in tomato, was produced by cleavage of the 5,6 or 5,6 bond positions of lycopene but not geranylgeranyl diphosphate, -carotene, or phytoene. In vitro, ZmCCD1 cleaved linear and cyclic carotenoids with equal efficiency. Based on the pattern of apocarotenoid volatiles produced, we propose that CCD1 recognizes its cleavage site based on the saturation status between carbons 7 and 8 (7 and 8) and carbons 11 and 12 (11 and 12) as well as the methyl groups on carbons 5, 9, and 13 (5, 9, and 13).Apocarotenoids are terpenoid compounds derived from the oxidative cleavage of carotenoids (1). They are generated when double bonds in a carotenoid are cleaved by molecular oxygen, forming an aldehyde or ketone in each product at the site of cleavage. Carotenoids can be cleaved at any of their conjugated double bonds, resulting in a diverse set of apocarotenoids. This structural diversity is the consequence of the large number of carotenoid precursors (more than 600) and subsequent modifications such as oxidation, reduction, and conjugation. Although apocarotenoid formation can also occur via nonspecific oxidation, biologically active forms with regulatory functions are expected to be generated via site-specific cleavage.Apocarotenoids are widely distributed in nature and serve important biological functions. Examples of biologically active apocarotenoids include retinoids in animals (2), trisporic acid in fungi (3), and abscisic acid in higher plants (4). A variety of other biologically important compounds are believed to be derived by oxidative cleavage of carotenoids. Among these compounds are those associated with mycorrhizal colonization, including mycorradicin (5), blumenin (6), and strigolactone (7).Various enzymes mediate the site-specific carotenoid cleavage needed to generate biologically active apocarotenoids. The founding member of the carotenoid-cleaving enzymes is maize VIVIPAROUS14 (VP14) (8, 9). This 9-cis-epoxycarotenoid dioxygenase (NCED) generates abscisic acid via asymmetrical cleavage at the 11,12 double bond of neoxanthin and/or violaxanthin. NCED genes have been identified in a variety of plant species (9 -13). Using sequence similarity to VP14, the 15,15Ј-dioxygenases responsible for vitamin A biosynthesis w...