recently discovered growth regulators anchorene and zaxinone
[34, 35], pigments, such as crocin in saffron [36], citraurin in citrus
fruits [37], and the fungal neurosporaxanthin [38, 39], and volatiles, such as safranal [40, 41], geranial [42], and β-ionone
[43, 44]. All of these bioactive metabolites (named apocarotenoids)
are generated by oxidative cleavage of carotenoids, which is generally catalyzed by carotenoid cleavage dioxygenases (CCDs) or by
nonenzymatic oxidation caused by reactive oxygen species (ROSs)
[25, 32, 45, 46]. In Arabidopsis, there are five different types of
CCDs. The nine-cis-epoxycarotenoid cleavage dioxygenases
(NCED represented by 5 enzymes; NCED2, 3, 5, 6, and 9) are
involved in the biosynthesis of abscisic acid, while the four other
CCD types, designated as CCD1, CCD4, CCD7, and CCD8 exert
different biological functions and have, accordingly, different substrates and regiospecificities [23, 25, 44, 47]. For example, CCD1
enzymes catalyze the conversion of carotenoids into a plenitude of
volatiles, such as geranial, pseudoionone, and β-ionone, and are
likely involved in scavenging of destructed carotenoids/apocarotenoids; the Arabidopsis CCD4 converts all-trans-β-carotene into
β-ionone and all-trans-β-apo-10
0 -carotenal and determines carotenoid content; The strigolactone biosynthesis enzyme CCD7
(MAX3) cleaves 9-cis-β-carotene to form β-ionone and
9-cis-β-apo-10
0 -carotenal, the substrate of CCD8 (MAX4) that
forms the central strigolactone biosynthesis intermediate
carlactone.
Carotenoid cleavage yields dialdehyde products (DIALs),
besides monocarbonyl apocarotenoids that have been in the focus
of carotenoid research. Carotenoid-derived DIALs are defined as a
class of carotenoid oxidative products incorporating two aldehyde
functional groups, which are produced from repeated oxidative
cleavage of double bonds within carotenoids or apocarotenoids
(Fig. 1). Recent in vitro studies showed that several plant and
cyanobacterial CCDs can cleave carotenoids or apocarotenoids to
produce DIALs. Adrian et al expected that all-trans-β-apo-100 -carotenal can be cleaved by CCD8 to form all-trans-4-methyl2,4,6-octatrienedial [48], which has been confirmed in Wang
et al.’s work. Scherzinger et al. demonstrated that cyanobacterial
retinal-forming enzymes can catalyze the conversion of
β-apo-8
0 -carotenoids to all-trans-2,6-dimethyl-2,4,6-octatrienedial [49]. Ilg et al. reported tomato CCDs 1A and 1B convert
many apocarotenoid substrates including apo-8
0 -lycopenal,
apo-10
0 -lycopenal, apo-12
0 -lycopenal, and apo-15
0 -lycopenal in
in vitro assays, leading to a series DIALs with different carbon
chains (e.g., all-trans-2,6-dimethyl-2,4,6,8-decatetraenedial, all-trans-3,7-dimethyl-2,4,6,8-decatetraenedial, all-trans-2,6,11-trimethyl-2,4,6,8,10-dodecapentaenedial,
all-trans-2,6,11trimethyl-2,4,6,8,10,12-quattuordecahexaenedial,
all-trans2,7,11-trimethyl-2,4,6,8,10,12-quattuordecahexaenedial,
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Jianing Mi et al.
[34, 35], pigments, such as crocin in saffron [36], citraurin in citrus
fruits [37], and the fungal neurosporaxanthin [38, 39], and volatiles, such as safranal [40, 41], geranial [42], and β-ionone
[43, 44]. All of these bioactive metabolites (named apocarotenoids)
are generated by oxidative cleavage of carotenoids, which is generally catalyzed by carotenoid cleavage dioxygenases (CCDs) or by
nonenzymatic oxidation caused by reactive oxygen species (ROSs)
[25, 32, 45, 46]. In Arabidopsis, there are five different types of
CCDs. The nine-cis-epoxycarotenoid cleavage dioxygenases
(NCED represented by 5 enzymes; NCED2, 3, 5, 6, and 9) are
involved in the biosynthesis of abscisic acid, while the four other
CCD types, designated as CCD1, CCD4, CCD7, and CCD8 exert
different biological functions and have, accordingly, different substrates and regiospecificities [23, 25, 44, 47]. For example, CCD1
enzymes catalyze the conversion of carotenoids into a plenitude of
volatiles, such as geranial, pseudoionone, and β-ionone, and are
likely involved in scavenging of destructed carotenoids/apocarotenoids; the Arabidopsis CCD4 converts all-trans-β-carotene into
β-ionone and all-trans-β-apo-10
0 -carotenal and determines carotenoid content; The strigolactone biosynthesis enzyme CCD7
(MAX3) cleaves 9-cis-β-carotene to form β-ionone and
9-cis-β-apo-10
0 -carotenal, the substrate of CCD8 (MAX4) that
forms the central strigolactone biosynthesis intermediate
carlactone.
Carotenoid cleavage yields dialdehyde products (DIALs),
besides monocarbonyl apocarotenoids that have been in the focus
of carotenoid research. Carotenoid-derived DIALs are defined as a
class of carotenoid oxidative products incorporating two aldehyde
functional groups, which are produced from repeated oxidative
cleavage of double bonds within carotenoids or apocarotenoids
(Fig. 1). Recent in vitro studies showed that several plant and
cyanobacterial CCDs can cleave carotenoids or apocarotenoids to
produce DIALs. Adrian et al expected that all-trans-β-apo-100 -carotenal can be cleaved by CCD8 to form all-trans-4-methyl2,4,6-octatrienedial [48], which has been confirmed in Wang
et al.’s work. Scherzinger et al. demonstrated that cyanobacterial
retinal-forming enzymes can catalyze the conversion of
β-apo-8
0 -carotenoids to all-trans-2,6-dimethyl-2,4,6-octatrienedial [49]. Ilg et al. reported tomato CCDs 1A and 1B convert
many apocarotenoid substrates including apo-8
0 -lycopenal,
apo-10
0 -lycopenal, apo-12
0 -lycopenal, and apo-15
0 -lycopenal in
in vitro assays, leading to a series DIALs with different carbon
chains (e.g., all-trans-2,6-dimethyl-2,4,6,8-decatetraenedial, all-trans-3,7-dimethyl-2,4,6,8-decatetraenedial, all-trans-2,6,11-trimethyl-2,4,6,8,10-dodecapentaenedial,
all-trans-2,6,11trimethyl-2,4,6,8,10,12-quattuordecahexaenedial,
all-trans2,7,11-trimethyl-2,4,6,8,10,12-quattuordecahexaenedial,
178
Jianing Mi et al.