maize inbred lines [80]. However, CCD1 activity is also known to
have minimal impact on the carotenoid content in other plant
species [81, 82]. Negative correlations between CCD4 activity
and total carotenoid levels are clearly demonstrated in peach and
in chrysanthemum [77, 78, 83].
Nonspecific oxidation of carotenoids also mediates carotenoid
content and is a noticeable problem particularly during postharvesting storage. Both enzymatic and nonenzymatic degradation of
carotenoids can prevail in living cells, and the nonenzymatic turnover is predominant in mature dry grains [84]. Indeed, investigation of Arabidopsis mutants defect in CCD genes reveals that the
oxidized carotenoid metabolites β-cyclocitral and β-ionone are not
reduced during photoxidative stress [85], indicating the contribution of nonspecific oxidation of carotenoids. In durum wheat, the
lipoxygenase activity was found to be negatively correlated to total
carotenoid concentration [86]. During postharvesting storage of
carotenoid enriched grains, the rapid turnover of β-carotene with
the occurrence of β-apo-14
0 -carotenal and β-apo-13-carotenone,
which are not the cleavage products of known CCDs, indicates the
prevalence and contribution of nonenzymatic destruction in
controlling carotenoid levels [84].
3.4 Carotenoid
Degradation and Plant
Aroma, Pigmentation,
and Signaling
Carotenoid degradation to produce apocarotenoid volatiles particularly the C13 derivatives contributes to scent and aroma of plants
(Fig. 2). In rice, CCD1 cleaves lycopene to produce geranial and
also other carotenoids to form ketone products [87]. The temporal
expression of CCD1 is essential for the emission of apocarotenoid
volatiles in grape berries and petunia flowers [88, 89]. Carotenoid
turnover also provides a large number of volatiles, such as β-ionone,
β-cyclocytral, geranial, neral and farnesylacetone in melon fruit
[90], and cis-pseudoionone, geranial, and farnesylacetone in
tomato fruits [81]. In saffron flower, CCD4 activity contributes
to the emission of volatile β-ionone during stigma development
[91].
Carotenoid degradation also affects fruit and flower pigmentation (Fig. 2). In the flavedo of some citrus fruit, CCD4 catalyzes
the cleavage of β-cryptoxanthin and zeaxanthin to produce
β-citraurin, which gives the fruit their characteristic color [92]. In
peach, CCD4 was found to control flesh color. The expression of
CCD4 promotes carotenoid turnover and results in white-fleshed
peach, while lack of CCD4 activity reduces carotenoid degradation
to produce yellow-flesh fruit [83]. In chrysanthemum, CCD4a was
showed to express strongly in the petals of white flower but weakly
in yellow petals [77]. Knockdown CCD4a activity by RNAi in
‘Jimba’, the most popular white-flowered chrysanthemum cultivar
in Japan, changes the petal colors to yellow with significant carotenoid accumulation [93]. CCD2 cleaves zeaxanthin to give crocetin pigment in the Crocus species [73, 74]. Bixin, a diapocarotenoid
12
Tianhu Sun et al.
have minimal impact on the carotenoid content in other plant
species [81, 82]. Negative correlations between CCD4 activity
and total carotenoid levels are clearly demonstrated in peach and
in chrysanthemum [77, 78, 83].
Nonspecific oxidation of carotenoids also mediates carotenoid
content and is a noticeable problem particularly during postharvesting storage. Both enzymatic and nonenzymatic degradation of
carotenoids can prevail in living cells, and the nonenzymatic turnover is predominant in mature dry grains [84]. Indeed, investigation of Arabidopsis mutants defect in CCD genes reveals that the
oxidized carotenoid metabolites β-cyclocitral and β-ionone are not
reduced during photoxidative stress [85], indicating the contribution of nonspecific oxidation of carotenoids. In durum wheat, the
lipoxygenase activity was found to be negatively correlated to total
carotenoid concentration [86]. During postharvesting storage of
carotenoid enriched grains, the rapid turnover of β-carotene with
the occurrence of β-apo-14
0 -carotenal and β-apo-13-carotenone,
which are not the cleavage products of known CCDs, indicates the
prevalence and contribution of nonenzymatic destruction in
controlling carotenoid levels [84].
3.4 Carotenoid
Degradation and Plant
Aroma, Pigmentation,
and Signaling
Carotenoid degradation to produce apocarotenoid volatiles particularly the C13 derivatives contributes to scent and aroma of plants
(Fig. 2). In rice, CCD1 cleaves lycopene to produce geranial and
also other carotenoids to form ketone products [87]. The temporal
expression of CCD1 is essential for the emission of apocarotenoid
volatiles in grape berries and petunia flowers [88, 89]. Carotenoid
turnover also provides a large number of volatiles, such as β-ionone,
β-cyclocytral, geranial, neral and farnesylacetone in melon fruit
[90], and cis-pseudoionone, geranial, and farnesylacetone in
tomato fruits [81]. In saffron flower, CCD4 activity contributes
to the emission of volatile β-ionone during stigma development
[91].
Carotenoid degradation also affects fruit and flower pigmentation (Fig. 2). In the flavedo of some citrus fruit, CCD4 catalyzes
the cleavage of β-cryptoxanthin and zeaxanthin to produce
β-citraurin, which gives the fruit their characteristic color [92]. In
peach, CCD4 was found to control flesh color. The expression of
CCD4 promotes carotenoid turnover and results in white-fleshed
peach, while lack of CCD4 activity reduces carotenoid degradation
to produce yellow-flesh fruit [83]. In chrysanthemum, CCD4a was
showed to express strongly in the petals of white flower but weakly
in yellow petals [77]. Knockdown CCD4a activity by RNAi in
‘Jimba’, the most popular white-flowered chrysanthemum cultivar
in Japan, changes the petal colors to yellow with significant carotenoid accumulation [93]. CCD2 cleaves zeaxanthin to give crocetin pigment in the Crocus species [73, 74]. Bixin, a diapocarotenoid
12
Tianhu Sun et al.