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).