Carotenoid Metabolism: the Biosynthesis, Regulation, and Beyond

Carotenoids are indispensable to plants and play a critical role in human nutrition and health. Significant progress has been made in our understanding of carotenoid metabolism in plants. The biosynthetic pathway has been extensively studied. Nearly all the genes encoding the biosynthetic enzymes have been isolated and characterized from various organisms. In recent years, there is an increasing body of work on the signaling pathways and plastid development, which might provide global control of carotenoid biosynthesis and accumulation. Here we will highlight recent progress on the biosynthesis, regulation, and metabolic engineering of carotenoids in plants, as well as the future research towards elucidating the regulatory mechanisms and metabolic network that control carotenoid metabolism. 3 Carotenoids represent a diverse group of pigments widely distributed in nature. They contribute to the red, orange, and yellow colors found in many flowers, fruits and vegetables. Carotenoids are synthesized de novo not only in all photosynthetic organisms, such as plants (including algae) and cyanobacteria but also in some non-photosynthetic bacteria and fungi. In plants, carotenoids fulfill a variety of critical functions. They serve as accessory pigments to harvest light for photosynthesis and constitute the basic structural units of photosynthesis apparatus. They act as photoprotectors for plants to adapt high light stress and furnish flowers and fruits with distinct colors to attract insects and animals for pollination and seed dispersal. In addition, oxidative cleavage of carotenoids produces apocarotenoids. Some of the apocarotenoids are signals in plant development, serve as antifungal agents, and contribute to the flavor and aroma of flowers and fruits (Auldridge et al. 2006). The phytohormone abscisic acid, the most well known apocarotenoid derivative, regulates a wide range of biological processes in plants. Carotenoids are important not only to those organisms where they are synthesized, but also to animals and humans. Carotenoids have long been recognized as essential nutrients and important health beneficial compounds (Fraser and Bramley 2004). As animals and humans are unable to synthesize carotenoids de novo, they have to depend on diet for these essential products. " Pro-vitamin A " carotenoids, such as β-carotene and α-carotene, provide the primary dietary sources of vitamin A. The deficiency of vitamin A is one of the most noticeable nutritional problems in many parts of the world and affects 140–250 million children under 5 years of age (World Health Organization). Food biofortification with enhanced provitamin A carotenoids offers a sustainable way to combat vitamin A …