The biosynthesis of carotenoid pigments
Carotenoids provide many of the red, orange and yellow pigments in
fruit, as seen in citrus (Citrus spp.), tomato (Solanum
lycopersicon ), capsicum/pepper (Capsicum spp.) and papaya
(Carica papaya ). Carotenoids are essential for photoprotection
and integral to the photosynthetic machinery and in green tissues are
co-ordinately regulated with chlorophylls (Frank & Cogdell 1996). The
pathway is located in plastids and derived from the
mevalonate-independent pathway (MEP) pathway, which generates the
C20 compound geranylgeranyl pyrophosphate (GGPP)
(Cunningham & Gantt 1998). The condensation of two GGPP molecules
provides the first carotenoids, the colourless phytoene, and is
catalysed by phytoene synthase (PSY) (Figure 4). This is often seen as
the rate-limiting step for carotenoid synthesis, regulating downstream
pathway flux. A two-step desaturation of phytoene, by phytoene
desaturase (PDS), catalyses phytoene into phytofluene and then
zeta-carotene, which is converted to di-cis zeta-carotene by
zeta-carotene isomerase (Z-ISO). Desaturation by zeta-carotene
desaturase (ZDS) and isomerization by carotene isomerase (CRTISO)
produces lycopene and from this the pathway branches, with lycopene beta
cyclases (LCYB) producing alpha-carotene and the nutritional vitamin A
precursor, β-carotene (Moise, Al-Babili & Wurtzel 2014). The conversion
of alpha-carotene to lutein and beta-carotene to zeaxanthin is through
the action of carotene hydroxylases. Beyond these steps, other
carotenoids such as xanthophylls, including astaxanthin, are generated
and finally, through the actions of nine-cis -epoxycarotenoid
dioxygenases (NCEDs) and carotenoid cleavage dioxygenases (CCDs), the
phytohormone abscisic acid (ABA) and a range of apocarotenoids are
produced (Giuliano 2014). Carotenoid accumulation is further influenced
by cell physiology, specifically the number of available carotenoid
storage plastids (Cazzonelli & Pogson 2010).