4-phosphate (MEP) pathway, was identified for the biosynthesis of
prenyl diphosphate precursors of plastidic isoprenoids including
carotenoids around the same period of time [8, 9]. A great deal is
now known about the biochemistry of carotenoid biosynthesis
[1, 2, 10]. Carotenoid degradation occurs via nonenzymatic oxidation and enzymatic oxidation. Carotenoid cleavage dioxygenases
(CCDs) catalyze oxidative cleavage of carotenoids to form various
apocarotenoids [11, 12]. Their roles in the biosynthesis of phytohormones abscisic acid (ABA) and strigolactones are well elucidated
[13, 14]. Since carotenoids are synthesized and stored in plastids in
plants, the plastid sink strength or sequestration capacity is critically
important for carotenoid accumulation [5, 15]. The biosynthesis
activity, degradation rate, and stable storage in plastids define the
final carotenoid content in plant tissues.
2 Carotenoid Biosynthesis
2.1 Plastidial
Precursors
for Carotenoid
Biosynthesis in Plants
Plant carotenoid biosynthesis occurs exclusively in plastids and the
plastid-localized MEP pathway provides precursors for carotenoid
production. The MEP pathway utilizes pyruvate and glyceraldehyde 3-phosphate (GAP) to produce 5-carbon building blocks
isopentenyl diphosphate (IPP) and dimethylallyl diphosphate
(DMAPP) (Fig. 1). Deoxy-D-xylulose 5-phosphate synthase
(DXS) and reductoisomerase (DXR) are two major ratedetermining enzymes in the MEP pathway [9, 16]. By addition of
three IPP molecules to DMAPP, geranylgeranyl diphosphate
(GGPP) is produced by GGPP synthase (GGPS). GGPP is the
direct precursor for carotenoid biosynthesis, as well as for many
other important plastidial isoprenoids, such as gibberellins, chlorophylls, tocopherols, and plastoquinones.
Various evidence support the impact of precursor supply from
the MEP pathway on carotenoid biosynthesis. By overexpression
of DXS and DXR, carotenoid levels increase in plants [17–19]. Null
mutants in the MEP pathway cause albino phenotype due to lack of
precursors for the biosynthesis of carotenoids and other plastidial
isoprenoids [20]. While a recent work indicates that GGPS activity
also influences carotenoid accumulation [21], overexpression of
GGPS usually exerts minimal impact on total carotenoid levels,
likely due to the fact that GGPP is the precursor for many plastidial
isoprenoids. Through direct interactions with different enzymes,
GGPS has been shown to channel GGPP to different plastidial
isoprenoid biosynthetic pathways [22, 23]. A recent study shows
that enzyme fusion of GGPS and phytoene synthase (PSY) can
effectively direct metabolic flux into carotenogenesis [24].
4
Tianhu Sun et al.
prenyl diphosphate precursors of plastidic isoprenoids including
carotenoids around the same period of time [8, 9]. A great deal is
now known about the biochemistry of carotenoid biosynthesis
[1, 2, 10]. Carotenoid degradation occurs via nonenzymatic oxidation and enzymatic oxidation. Carotenoid cleavage dioxygenases
(CCDs) catalyze oxidative cleavage of carotenoids to form various
apocarotenoids [11, 12]. Their roles in the biosynthesis of phytohormones abscisic acid (ABA) and strigolactones are well elucidated
[13, 14]. Since carotenoids are synthesized and stored in plastids in
plants, the plastid sink strength or sequestration capacity is critically
important for carotenoid accumulation [5, 15]. The biosynthesis
activity, degradation rate, and stable storage in plastids define the
final carotenoid content in plant tissues.
2 Carotenoid Biosynthesis
2.1 Plastidial
Precursors
for Carotenoid
Biosynthesis in Plants
Plant carotenoid biosynthesis occurs exclusively in plastids and the
plastid-localized MEP pathway provides precursors for carotenoid
production. The MEP pathway utilizes pyruvate and glyceraldehyde 3-phosphate (GAP) to produce 5-carbon building blocks
isopentenyl diphosphate (IPP) and dimethylallyl diphosphate
(DMAPP) (Fig. 1). Deoxy-D-xylulose 5-phosphate synthase
(DXS) and reductoisomerase (DXR) are two major ratedetermining enzymes in the MEP pathway [9, 16]. By addition of
three IPP molecules to DMAPP, geranylgeranyl diphosphate
(GGPP) is produced by GGPP synthase (GGPS). GGPP is the
direct precursor for carotenoid biosynthesis, as well as for many
other important plastidial isoprenoids, such as gibberellins, chlorophylls, tocopherols, and plastoquinones.
Various evidence support the impact of precursor supply from
the MEP pathway on carotenoid biosynthesis. By overexpression
of DXS and DXR, carotenoid levels increase in plants [17–19]. Null
mutants in the MEP pathway cause albino phenotype due to lack of
precursors for the biosynthesis of carotenoids and other plastidial
isoprenoids [20]. While a recent work indicates that GGPS activity
also influences carotenoid accumulation [21], overexpression of
GGPS usually exerts minimal impact on total carotenoid levels,
likely due to the fact that GGPP is the precursor for many plastidial
isoprenoids. Through direct interactions with different enzymes,
GGPS has been shown to channel GGPP to different plastidial
isoprenoid biosynthetic pathways [22, 23]. A recent study shows
that enzyme fusion of GGPS and phytoene synthase (PSY) can
effectively direct metabolic flux into carotenogenesis [24].
4
Tianhu Sun et al.