Carotenoids are a specific class of terpenoids, and consist of a C40 isoprenoid skeleton, often decorated with epoxy, hydroxy and/or keto groups (Giuliano et al. 2008). Carotenoids pigments are yellow-orange-red compounds that are found in fruits, flowers, leaves, and roots; whose attractive colours are formed by carotenoids. They represent one of the most diverse classes of lipophilic phytochemicals with over 1110 compounds identified so far (Carotenoid Database, Britton et al. 2004). They are synthesized in photosynthetic plants and algae, and non-photosynthetic organisms such as some fungi and bacteria (Stange and Flores 2012). They are also the dietary source of pigmentation of many fish, insects, crustaceans, and birds.
The health aspect of certain carotenoids is undisputed. The most well-known are lycopene and ß-carotene, high in for example tomato and carrot, play an important role in human health because of their powerful antioxidant properties and provitamin A activity (Bot et al. 2018). Vitamin A, or retinol, is important during embryonic development, vision, functioning of epithelial cells, glycoprotein synthesis, the immune system, red blood cell production, and growth (see also Chapter 2; Hammond 2015). In the retina, other carotenoids lutein and zeaxanthin have a role to filter and absorb UV and blue light. Vitamin A deficiency is a serious public health problem in low income countries in Asia, Africa, and Latin America (§2.3). Orange-fleshed sweet potato, yellow cassava, orange corn, and many other initiatives have been initiated to fight this deficiency . It is possible to breed for cultivars with more nutritious composition of carotenoids. All yellow genotypes of maize contain carotenoids, although the fraction of carotenoids with provitamin A activity (β-cryptoxanthin α- and β-carotene, which can be converted to vitamin A) is typically small (e.g. 10–20%) compared to zeaxanthin and lutein (each around 30–50% of total carotenoids) (Brenna and Berardo 2004; Howe and Tanumihardjo 2006). Alternatively, it is possible to directly introduce a phytonutrient by metabolic engineering. In this light, carotenoid accumulating golden rice was developed. Though recombinant DNA technology, Golden rice was engineered to accumulate β-carotene, the precursor for vitamin A (Ye et al. 2000; Al-Babili and Beyer 2005). Besides β-carotene, lycopene is also an important carotenoid in the human diet, and the nutritional role it may play has is a common topic for researchers (Clinton and Giovannucci 1998). In plants, carotenoids are synthesized in the plastids, where they protect against photooxidative stress from excessive light energy by quenching triplet chlorophylls, superoxide anion radicals and singlet oxygen (Niyogi 1999). Carotenoids accumulate in the thylakoid membranes of chloroplasts and participate in light harvesting in photosynthetic membranes (Cunningham and Gantt 1998). Their function is to transfer energy outside of the spectrum of chlorophyll (400-500 nm) to chlorophyll a during photosynthesis (Bramley 2002). Chloroplast can differentiate into chromoplasts, specialized plastids that synthesize and accumulate high amounts of carotenoids in lipid bodies or in crystalline structures inside these chromoplasts. They exist mainly in tissues that accumulate many carotenoids, flowers, fruits and roots (Bramley 2002). During tomato ripening, chloroplasts differentiate into chromoplasts and red tomatoes accumulate high amounts of lycopene (Sánchez-González et al. 2016). They also have a function in plant development, as they are precursors for the important plant hormones abscisic acid (ABA) and strigolactones (SL) (Auldridge et al. 2006; Giuliano et al. 2008). They are also important precursors of volatile compounds such as 6-Methyl-5-hepten-2-one and β-ionone, that are important for floral attraction of plant pollinators but also for an attractive flavour of food (Simkin et al. 2004; Farneti et al. 2015).
The biosynthesis of carotenoids in higher plants starts with IPP (isopentenyl phosphate), the C5 precursor from the MEP/DOXP pathway that is also the starting point for the biosynthesis of terpenoids (Hirschberg 2001; Giuliano et al. 2008). Four IPP molecules are converted to GGPP (geranylgeranyl diphosphate), and to 15-cis-phytoene, the first molecule in the carotenoid pathway, by phytoene synthase (PSY) (Figure 3.2). A series of four enzymatic reactions then lead to lycopene, the compound that gives tomatoes their characteristic red colour. In plants these reactions are performed by 4 different enzymes, however in bacteria, a single enzyme is capable to complete all these reactions. The pathway then splits towards either α-carotene or β-carotene, which can be hydroxylated into the xanthophylls lutein and zeaxanthin respectively. β-xanthophylls can be converted in violaxanthin and neoxanthin through the activity of NCED (9-cis-epoxycarotenoid dioxygenase). Apocarotenoids, such as ABA, strigolactones, and volatiles can be formed by the oxidative cleavage of carotenoids by carotenoid cleavage dioxygenases (CCDs) (Auldridge et al. 2006).
