Fig. 2

Functions of OR family proteins in carotenoid biosynthesis and carotenoid accumulation in plastids. Both ORANGE family proteins (representing by OR here) and ORANGE^ORANGE (OR^OR) isoforms are chaperones for phytoene synthase (PSY) in plastids and directly regulate carotenoid biosynthesis via modulating the stability of PSY, the key rate limiting and first committed enzyme of carotenoid pathway. OR^OR gains an additional function to promote biogenesis of chromoplasts, the specific plastid with strong sink strength for carotenoid sequestration and storage, leading to higher capacity of carotenoid accumulation. Thus, OR^OR has dual functions in regulating both carotenoid biosynthesis and chromoplast biogenesis and represents an effective genetic tool for carotenoid biofortification. A simplified carotenoid biosynthesis pathway is outlined. DMAPP, dimethylallyl diphosphate; IPP, isopentenyl diphosphate; GGPP, geranylgeranyl diphosphate. The carotenoid pathway is associated the production of plant hormones of gibberellin (GA) from GGPP, strigolactone (SL) from β-carotene, and abscisic acid (ABA) from violaxanthin and neoxanthin. Some major carotenoids accumulated in chromoplasts are listed