membranes. The absence of appropriate lipoprotein sequestering
substructures within amyloplasts perhaps restricts their capacity to
significantly synthesize and stably store carotenoids. Chloroplasts
that define plants are the photosynthetic plastids in green tissues.
Abundant carotenoids are metabolized during active photosynthesis [119]. The majority of carotenoids are as protein-bound in the
chloroplast thylakoid membranes, which serve as the sequestering
structures to promote carotenoid biosynthesis in plastid envelope
membranes, where the carotenogenic enzymes are primarily localized [37]. However, as indispensable components of the photosynthetic apparatus, carotenoids are synthesized tightly in
coordination with chlorophyll formation in chloroplasts. Such
tight regulation constrains excess carotenoid sequestration to maintain proper carotenoid and chlorophyll pigment ratio for photosynthesis. By contrast, chromoplasts as carotenoid storing plastids do
not have such a constraint. Sequestration in various carotenoidlipoprotein substructures enables chromoplasts to have high sink
strength or great capacity to synthesize and store carotenoids.
These sequestering substructures function not only in sequestering
the newly synthesized carotenoids inside chromoplast for stable
storage, but also in promoting continuous biosynthesis by driving
carotenoid products away from the biosynthesis sites [5, 114]. As a
result, alterations of both chromoplast numbers and compartment
sizes positively affect total carotenoid levels.
4.3 Chromoplast
Formation
Chromoplast biogenesis is critical important for high levels
of carotenoid accumulation [5]. However, the process is not understood. Chromoplasts can be derived from various types of
plastids, including chloroplasts, amyloplasts, and proplastids
[113, 114]. The direct conversion of chloroplasts into chromoplasts at initiation of carotenoid overproduction was observed
during tomato fruit ripening [120]. However, the factors or proteins that control chromoplast differentiation and/or transition
remain elusive.
The ORANGE (OR) protein, which was first identified from
orange curd cauliflower mutant, represents the only known protein
that acts as a molecular switch to initiate chromoplast differentiation [121, 122]. A single amino acid substitution in the OR protein
is responsible for chromoplast formation and carotenoid deposition
in orange melon fruit [45, 123]. The role of OR in chromoplast
biogenesis is also proved by its ectopic expression in potato, tomato
and Arabidopsis calli [124–126]. In the OR-induced chromoplasts,
carotenoids are sequestrated mainly in components of massive,
highly ordered sheets with membranous substructure as showed
by transmission electron microscopy [121, 125, 127]. In addition,
OR posttranslationally regulates PSY protein stability and activity to
promote carotenoid biosynthesis [43, 44, 46]. The dual roles of
OR in initiating chromoplast formation and regulating PSY activity
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
15
substructures within amyloplasts perhaps restricts their capacity to
significantly synthesize and stably store carotenoids. Chloroplasts
that define plants are the photosynthetic plastids in green tissues.
Abundant carotenoids are metabolized during active photosynthesis [119]. The majority of carotenoids are as protein-bound in the
chloroplast thylakoid membranes, which serve as the sequestering
structures to promote carotenoid biosynthesis in plastid envelope
membranes, where the carotenogenic enzymes are primarily localized [37]. However, as indispensable components of the photosynthetic apparatus, carotenoids are synthesized tightly in
coordination with chlorophyll formation in chloroplasts. Such
tight regulation constrains excess carotenoid sequestration to maintain proper carotenoid and chlorophyll pigment ratio for photosynthesis. By contrast, chromoplasts as carotenoid storing plastids do
not have such a constraint. Sequestration in various carotenoidlipoprotein substructures enables chromoplasts to have high sink
strength or great capacity to synthesize and store carotenoids.
These sequestering substructures function not only in sequestering
the newly synthesized carotenoids inside chromoplast for stable
storage, but also in promoting continuous biosynthesis by driving
carotenoid products away from the biosynthesis sites [5, 114]. As a
result, alterations of both chromoplast numbers and compartment
sizes positively affect total carotenoid levels.
4.3 Chromoplast
Formation
Chromoplast biogenesis is critical important for high levels
of carotenoid accumulation [5]. However, the process is not understood. Chromoplasts can be derived from various types of
plastids, including chloroplasts, amyloplasts, and proplastids
[113, 114]. The direct conversion of chloroplasts into chromoplasts at initiation of carotenoid overproduction was observed
during tomato fruit ripening [120]. However, the factors or proteins that control chromoplast differentiation and/or transition
remain elusive.
The ORANGE (OR) protein, which was first identified from
orange curd cauliflower mutant, represents the only known protein
that acts as a molecular switch to initiate chromoplast differentiation [121, 122]. A single amino acid substitution in the OR protein
is responsible for chromoplast formation and carotenoid deposition
in orange melon fruit [45, 123]. The role of OR in chromoplast
biogenesis is also proved by its ectopic expression in potato, tomato
and Arabidopsis calli [124–126]. In the OR-induced chromoplasts,
carotenoids are sequestrated mainly in components of massive,
highly ordered sheets with membranous substructure as showed
by transmission electron microscopy [121, 125, 127]. In addition,
OR posttranslationally regulates PSY protein stability and activity to
promote carotenoid biosynthesis [43, 44, 46]. The dual roles of
OR in initiating chromoplast formation and regulating PSY activity
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
15