Plant carotenoids: pigments for photoprotection, visual attraction, and human health.

Plant carotenoids are red, orange, and yellow lipid-soluble pigments found embedded in the membranes of chloroplasts and chromoplasts. Their color is masked by chlorophyll in photosynthetic tissues, but in late stages of plant development these pigments contribute to the bright colors of many flowers and fruits and the carrot root. Carotenoids protect photosynthetic organisms against potentially harmful photooxidative processes and are essential structural components of the photosynthetic antenna and reaction center complexes. In plants, some of these compounds are precursors of abscisic acid (ABA), a phytohormone that modulates developmental and stress processes (Koornneef, 1986). Carotenoids with provitamin A activity are essential components of the human diet, and there is considerable evidence that many carotenoids have anti-cancer activity. Carotenoids absorbed through the diet, and often metabolized into other compounds, are responsible for the color of familiar animals such as lobster, flamingo, and fish. Often unaware of the chemical nature of food colorants, humans consume large amounts of carotenoids added to foods primarily to please the eye (Klaüi and Bauernfeind, 1981). As with other natural pigments, carotenoids attracted the attention of 19th-century organic chemists. BCarotene was isolated in 1817, and the pigments of autumn leaves were identified as xanthophylls in 1837 (Isler, 1971). Classical geneticists identified carotenoid mutants in maize, tomato, and Arabidopsis during the early part of this century. However, difficulties with the biochemical properties of the enzymes of the pathway and the generally poor viability of mutants affected in carotenoid biosynthesis during early stages of development conspired to delay development of the field. Genetic studies of carotenoid biosynthesis in bacteria (reviewed in Armstrong, 1994) have played a fundamental role in the cloning and characterization of plant carotenoid genes. Since 1991, when a bacterial probe was first used to clone a plant gene for an enzyme of the pathway, progress has been rapid. However, much remains to be learned about the environmental and developmental regulation of the pathway, and many classic mutants have not yet been characterized at the molecular level. Recent reviews (Armstrong, 1994; Bartley et al., 1994; Sandmann, 1994) and two books on methods (Packer, 1992a,