with crucial functions throughout a plant’s life, like chlorophylls,
tocopherols, gibberellic acid, and plastoquinones [1, 2].
The activity of PSY was found to be rate-limiting for carotenogenesis [3]. For instance, PSY is light-induced during photomorphogenesis providing enhanced formation of carotenoids for
photosynthesis, and is also induced upon abiotic stress to warrant
sufficient substrate amounts for the cleavage of xanthophylls into
abscisic acid [4–7]. Moreover, increased PSY transcript levels often
correlate with increased carotenoid contents in nonphotosynthetic
tissues rich in carotenoids, for instance daffodil flowers, tomato
fruits, or melons [8–10].
Consequently and corroborating its rate-limiting function,
PSY overexpression results in large increases in carotenoid levels
obtained through biotechnological approaches [11]. In carrot and
Arabidopsis roots as well as callus, carotenoid levels are elevated to
high levels through PSY overexpression which results in crystallization of β-carotene [12, 13].
PSY is subjected to a pronounced regulation at various levels
beside transcriptional activation, for example, posttranscriptionally
via its 5
0 UTR and posttranslationally, mediated by the OR proteins
[14–16]. In subsequent analysis, OR was found to mediate PSY
protein stability and turnover in association with the plastid clp
proteinase system [17]. PSY constitutes a small gene family with
up to three members in different taxa which sometimes developed
specificity for certain physiological requirements. For instance,
three PSY genes are found in tomato; tomato PSY2 is expressed
in leaves while PSY1 is fruit-specifically induced and a third homolog is thought to be involved in supporting root strigolactone
biosynthesis upon interaction with symbiotic arbuscular mycorrhizal fungi [9, 18, 19]. A functional diversification of the three PSY
genes in Poaceae is less obvious as two homologs are expressed in
leaves while a third version in induced upon abiotic stress [6, 20,
21]. While the endosperm-specific expression of PSY in rice
resulted only in the accumulation of the colorless phytoene, the
generation of Golden Rice required the coexpression of a bacterial
desaturase CrtI to proceed to lycopene which is then converted
into downstream carotenoids by endogeneous enzymes already
present in the endosperm in sufficient quantities [22]. However,
the origin of PSY determined the amount of carotenoids accumulating in Golden Rice seeds: for instance, overexpression of daffodil
PSY and tomato PSY1 yielded only 1 and 5 μg g
À1 total carotenoids, respectively, while overexpression of Poaceae PSY1 from
maize and rice yielded up to 30 μg g
À1 [23]. This suggests differences in intrinsic properties of PSY proteins from various plant taxa.
This observation is further supported by largely different carotenoid levels in fruits from different banana varieties while the PSY
paralogs differ only slightly in their amino acid sequences [24]. Furthermore, single nucleotide polymorphisms in PSY were shown to
40
Maurizio Camagna and Ralf Welsch
tocopherols, gibberellic acid, and plastoquinones [1, 2].
The activity of PSY was found to be rate-limiting for carotenogenesis [3]. For instance, PSY is light-induced during photomorphogenesis providing enhanced formation of carotenoids for
photosynthesis, and is also induced upon abiotic stress to warrant
sufficient substrate amounts for the cleavage of xanthophylls into
abscisic acid [4–7]. Moreover, increased PSY transcript levels often
correlate with increased carotenoid contents in nonphotosynthetic
tissues rich in carotenoids, for instance daffodil flowers, tomato
fruits, or melons [8–10].
Consequently and corroborating its rate-limiting function,
PSY overexpression results in large increases in carotenoid levels
obtained through biotechnological approaches [11]. In carrot and
Arabidopsis roots as well as callus, carotenoid levels are elevated to
high levels through PSY overexpression which results in crystallization of β-carotene [12, 13].
PSY is subjected to a pronounced regulation at various levels
beside transcriptional activation, for example, posttranscriptionally
via its 5
0 UTR and posttranslationally, mediated by the OR proteins
[14–16]. In subsequent analysis, OR was found to mediate PSY
protein stability and turnover in association with the plastid clp
proteinase system [17]. PSY constitutes a small gene family with
up to three members in different taxa which sometimes developed
specificity for certain physiological requirements. For instance,
three PSY genes are found in tomato; tomato PSY2 is expressed
in leaves while PSY1 is fruit-specifically induced and a third homolog is thought to be involved in supporting root strigolactone
biosynthesis upon interaction with symbiotic arbuscular mycorrhizal fungi [9, 18, 19]. A functional diversification of the three PSY
genes in Poaceae is less obvious as two homologs are expressed in
leaves while a third version in induced upon abiotic stress [6, 20,
21]. While the endosperm-specific expression of PSY in rice
resulted only in the accumulation of the colorless phytoene, the
generation of Golden Rice required the coexpression of a bacterial
desaturase CrtI to proceed to lycopene which is then converted
into downstream carotenoids by endogeneous enzymes already
present in the endosperm in sufficient quantities [22]. However,
the origin of PSY determined the amount of carotenoids accumulating in Golden Rice seeds: for instance, overexpression of daffodil
PSY and tomato PSY1 yielded only 1 and 5 μg g
À1 total carotenoids, respectively, while overexpression of Poaceae PSY1 from
maize and rice yielded up to 30 μg g
À1 [23]. This suggests differences in intrinsic properties of PSY proteins from various plant taxa.
This observation is further supported by largely different carotenoid levels in fruits from different banana varieties while the PSY
paralogs differ only slightly in their amino acid sequences [24]. Furthermore, single nucleotide polymorphisms in PSY were shown to
40
Maurizio Camagna and Ralf Welsch