in where it is then converted by alcohol dehydrogenase to abscisic
aldehyde and oxidized by abscisic aldehyde oxidase into ABA [69].
The enzymes CCD7 and CCD8 are involved in the production
of strigolactones, the phytohormone inhibiting plant shoot branching, promoting germination of parasitic plants, and establishing
mycorrhizal symbiosis. CCD7 and CCD8 cleave β-carotene
sequentially to form carlactone (Fig. 2), the precursor for strigolactone biosynthesis [70]. β-carotene is initially isomerized by the
carotenoid isomerase D27 to form 9-cis-β-carotene. CCD7 cleaves
9-cis-β-carotene to produce β-apo-10
0 -carotenal and β-ionone. The
β-apo-10
0 -carotenal is then further catalyzed by CCD8 to form
carlactone for subsequent synthesis of strigolactones [71].
Both CCD1 and CCD4 recognize a large number of carotenoids and cleave them at various double bond locations to produce
a wide array of apocarotenoid products such as bixin, saffron,
α-ionone, β-ionone, β-cyclocitral, and β-citraurin (Fig. 2). CCD1
catabolizes carotenoid such as ζ-carotene, pro-lycopene, lycopene,
α-carotene, β-carotene, β-cryptoxanthin, and zeaxanthin, whereas
CCD4 degrades α-carotene, lutein, β-carotene, β-cryptoxanthin,
and zeaxanthin [2]. Interestingly, CCD1 has cytosolic localization
and is suggested to act as scavenger of apocarotenoids. Indeed,
apocarotenoids are substrates of several CCDs [72]. The newly
discovered CCD2 cleaves zeaxanthin to form crocetin in the Crocus
species [73, 74].
In contrast to the excentric cleavage of carotenoids by CCDs in
plants, central cleavage of dietary carotenes at the 15, 15
0 double
bond by CCOs plays a critical role for vitamin A production in
mammals including humans. Human genome encodes three
CCOs. Two members β-carotene-15,15
0 -oxygenase (BCO1) and
β-carotene-9
0 ,10
0 -oxygenase (BCO2) are involved in vitamin A
formation [75]. BCO1 cleaves β-carotene at central 15,15
0 position
to produce two molecules of vitamin A. BCO2 catabolizes carotenes such as β-carotene and β-cryptoxanthin at 9
0 ,10
0 and 9,10
double bond sites to produce C27 apocarotenoid, which can be
further cleaved by BCO1 for vitamin A production. BCO2 also
cleaves a range of carotenoids to form various apocarotenoid products and plays a critical role in controlling carotenoid homeostasis
in animal tissues [76]. Since no CCDs with central cleavage activities are known from plants, plants do not specifically produce
vitamin A.
3.3 Carotenoid
Degradation
and Carotenoid
Content in Plants
While there is no evidence that the activities of NCEDs and
CCD7/8 affect carotenoid homeostasis in plants, CCD1 and
CCD4 have been shown to influence carotenoid content in various
plant species [77–79]. The loss-of-function mutation of CCD1 and
CCD4 in Arabidopsis leads to more carotenoid accumulation in
seeds [63, 79]. A significant negative correlation between CCD1
copy number and carotenoid content was also reported in a panel of
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
11
aldehyde and oxidized by abscisic aldehyde oxidase into ABA [69].
The enzymes CCD7 and CCD8 are involved in the production
of strigolactones, the phytohormone inhibiting plant shoot branching, promoting germination of parasitic plants, and establishing
mycorrhizal symbiosis. CCD7 and CCD8 cleave β-carotene
sequentially to form carlactone (Fig. 2), the precursor for strigolactone biosynthesis [70]. β-carotene is initially isomerized by the
carotenoid isomerase D27 to form 9-cis-β-carotene. CCD7 cleaves
9-cis-β-carotene to produce β-apo-10
0 -carotenal and β-ionone. The
β-apo-10
0 -carotenal is then further catalyzed by CCD8 to form
carlactone for subsequent synthesis of strigolactones [71].
Both CCD1 and CCD4 recognize a large number of carotenoids and cleave them at various double bond locations to produce
a wide array of apocarotenoid products such as bixin, saffron,
α-ionone, β-ionone, β-cyclocitral, and β-citraurin (Fig. 2). CCD1
catabolizes carotenoid such as ζ-carotene, pro-lycopene, lycopene,
α-carotene, β-carotene, β-cryptoxanthin, and zeaxanthin, whereas
CCD4 degrades α-carotene, lutein, β-carotene, β-cryptoxanthin,
and zeaxanthin [2]. Interestingly, CCD1 has cytosolic localization
and is suggested to act as scavenger of apocarotenoids. Indeed,
apocarotenoids are substrates of several CCDs [72]. The newly
discovered CCD2 cleaves zeaxanthin to form crocetin in the Crocus
species [73, 74].
In contrast to the excentric cleavage of carotenoids by CCDs in
plants, central cleavage of dietary carotenes at the 15, 15
0 double
bond by CCOs plays a critical role for vitamin A production in
mammals including humans. Human genome encodes three
CCOs. Two members β-carotene-15,15
0 -oxygenase (BCO1) and
β-carotene-9
0 ,10
0 -oxygenase (BCO2) are involved in vitamin A
formation [75]. BCO1 cleaves β-carotene at central 15,15
0 position
to produce two molecules of vitamin A. BCO2 catabolizes carotenes such as β-carotene and β-cryptoxanthin at 9
0 ,10
0 and 9,10
double bond sites to produce C27 apocarotenoid, which can be
further cleaved by BCO1 for vitamin A production. BCO2 also
cleaves a range of carotenoids to form various apocarotenoid products and plays a critical role in controlling carotenoid homeostasis
in animal tissues [76]. Since no CCDs with central cleavage activities are known from plants, plants do not specifically produce
vitamin A.
3.3 Carotenoid
Degradation
and Carotenoid
Content in Plants
While there is no evidence that the activities of NCEDs and
CCD7/8 affect carotenoid homeostasis in plants, CCD1 and
CCD4 have been shown to influence carotenoid content in various
plant species [77–79]. The loss-of-function mutation of CCD1 and
CCD4 in Arabidopsis leads to more carotenoid accumulation in
seeds [63, 79]. A significant negative correlation between CCD1
copy number and carotenoid content was also reported in a panel of
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
11