4 Carotenoid Sequstration and Storage in Plants
4.1 Site
and Structure
for Carotenoid Storage
Plastids are the organelles for carotenoid biosynthesis and storage
in plant cells [5]. Chloroplasts and chromoplasts are two types of
plastids that accumulate high levels of carotenoids, although almost
all plastids including etioplasts and amyloplasts possess the ability
for carotenoid biosynthesis [37, 112]. Chloroplasts primarily synthesize and accumulate lutein, β-carotene, violaxanthin, and neoxanthin, along with minor amounts of zeaxanthin and α-carotene in
green tissues. The majority of these carotenoids are located in the
thylakoid membranes, where they function in light harvesting and
photoprotection processes. Amyloplasts produce and store mainly
xanthophylls such as lutein, zeaxanthin and violaxanthin in many
agronomically important starchy grain seeds, as well as in potato
tubers and cassava roots. Among all types of plastids, chromoplasts
are specialized for massive storage of a vast range of carotenoid
pigments in different plant organs including colored flowers, fruits,
and vegetables [5, 113–115].
As carotenoid-accumulating plastids, chromoplasts have various morphologies. Depending on internal substructures for
carotenoid-sequestering, chromoplasts are classified into five
major types as globular, tubular, crystalline, fibrillar and membranous [5, 114, 115]. More than one type of chromoplasts can
coexist in a species [116]. Plastoglobules (lipid droplets) are the
major site for carotenoid storage in globular chromoplasts with
apolar and ester carotenoids in the core of plastoglobules and
polar carotenoids associated with surrounding membrane
[117]. In tubular chromoplasts, carotenoids are usually sequestered
in lipid monolayer tubes branching extensively cross the stroma.
Crystalline chromoplasts are typically found in tissues overaccumulating lycopene and β-carotene. Those carotenoids in crystalline chromoplasts are deposited as red or orange crystals surrounded by lipid bilayers. In fibrillar chromoplasts, carotenoids are
sequestered in those spindle shaped fibrils. Membranous chromoplasts contain multilayers of condensed whirly membrane structures and the stacked membranes provide sites for carotenoid
storage. The various carotenoid sequestering substructures in chromoplasts likely affect the kinds and levels of carotenoids accumulated in different plant tissues [5, 118].
4.2 Carotenoid
Sequestration
in Plastids
Plastid substructures are critically important for carotenoid accumulation because they affect both carotenoid biosynthesis and
storage capacity [5, 118]. Various types of plastids are formed
with distinctive functions in plants, and they have dramatically
different sink strength or capacity to synthesize and sequester carotenoids. Amyloplasts as starch-storing plastids in general synthesize and store relatively low levels of carotenoids in plastid
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Tianhu Sun et al.
4.1 Site
and Structure
for Carotenoid Storage
Plastids are the organelles for carotenoid biosynthesis and storage
in plant cells [5]. Chloroplasts and chromoplasts are two types of
plastids that accumulate high levels of carotenoids, although almost
all plastids including etioplasts and amyloplasts possess the ability
for carotenoid biosynthesis [37, 112]. Chloroplasts primarily synthesize and accumulate lutein, β-carotene, violaxanthin, and neoxanthin, along with minor amounts of zeaxanthin and α-carotene in
green tissues. The majority of these carotenoids are located in the
thylakoid membranes, where they function in light harvesting and
photoprotection processes. Amyloplasts produce and store mainly
xanthophylls such as lutein, zeaxanthin and violaxanthin in many
agronomically important starchy grain seeds, as well as in potato
tubers and cassava roots. Among all types of plastids, chromoplasts
are specialized for massive storage of a vast range of carotenoid
pigments in different plant organs including colored flowers, fruits,
and vegetables [5, 113–115].
As carotenoid-accumulating plastids, chromoplasts have various morphologies. Depending on internal substructures for
carotenoid-sequestering, chromoplasts are classified into five
major types as globular, tubular, crystalline, fibrillar and membranous [5, 114, 115]. More than one type of chromoplasts can
coexist in a species [116]. Plastoglobules (lipid droplets) are the
major site for carotenoid storage in globular chromoplasts with
apolar and ester carotenoids in the core of plastoglobules and
polar carotenoids associated with surrounding membrane
[117]. In tubular chromoplasts, carotenoids are usually sequestered
in lipid monolayer tubes branching extensively cross the stroma.
Crystalline chromoplasts are typically found in tissues overaccumulating lycopene and β-carotene. Those carotenoids in crystalline chromoplasts are deposited as red or orange crystals surrounded by lipid bilayers. In fibrillar chromoplasts, carotenoids are
sequestered in those spindle shaped fibrils. Membranous chromoplasts contain multilayers of condensed whirly membrane structures and the stacked membranes provide sites for carotenoid
storage. The various carotenoid sequestering substructures in chromoplasts likely affect the kinds and levels of carotenoids accumulated in different plant tissues [5, 118].
4.2 Carotenoid
Sequestration
in Plastids
Plastid substructures are critically important for carotenoid accumulation because they affect both carotenoid biosynthesis and
storage capacity [5, 118]. Various types of plastids are formed
with distinctive functions in plants, and they have dramatically
different sink strength or capacity to synthesize and sequester carotenoids. Amyloplasts as starch-storing plastids in general synthesize and store relatively low levels of carotenoids in plastid
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Tianhu Sun et al.