Fig. 1

A schematic diagram of plant carotenoid metabolic pathway in plastids. Carotenoid biosynthesis utilizes the plastidial MEP pathway to supply the C5 precursor metabolites IPP and DMAPP. The first committed step in the carotenoid biosynthetic pathway involves the condensation of two C₂₀ GGPPs into C₄₀ carotenoid phytoene catalyzed by PSY, a major rate-limiting enzyme for carotenoid biosynthesis. Carotenoid degradation involves specific enzymatic oxidative cleavages by CCDs and NCEDs, nonspecific enzymes, and non-enzymatic oxidation to produce diverse apocarotenoids including phytohormones ABA and strigolactones. Phytohormones are highlighted with blue color. Various plastids provide different storage capacity to accumulate very little of carotenoids in etioplasts to massive amounts of carotenoids in chromoplasts. OR is the only known bona fide regulator of chromoplast biogenesis. MEP, methylerythritol 4-phosphate; GA3P, glyceraldehyde 3-phosphate; IPP, isopentenyl diphosphate; DMAPP, dimethylallyl diphosphate; GGPP, geranylgeranyl diphosphate; DXS, 1-deoxy-D-xylulose 5-phosphate synthase; GGPPS, GGPP synthase; PSY, phytoene synthase; PDS, phytoene desaturase; Z-ISO, ζ-carotene isomerase; ZDS, ζ-carotene desaturase; CrtISO, carotenoid isomerase; CrtI, bacterial phytoene desaturase; LCYE, lycopene ε-cyclase; LCYB, lycopene β-cyclase; BCH, β-carotene hydroxylase; CYP97A, cytochrome P450 carotene β-hydroxylase; CYP97C, cytochrome P450 carotene ε-hydroxylase; ZEP, zeaxanthin epoxidase; VDE, violaxanthin de-epoxidase; NXS, neoxanthin synthase; CCD, carotenoid cleavage dioxygenase; NCED, 9-cis-epoxycarotenoid dioxygenase; ZAS, zaxinone synthase; ABA, abscisic acid; OR, ORANGE protein