Plant carotenoids are 40-carbon isoprenoids with polyene chains that may contain up to 15 conjugated double bonds. Because of their chemical properties carotenoids are essential components of all photosynthetic organisms. Xanthophylls, oxygenated forms of carotenes, are accessory pigments in the light-harvesting antennae of the chloroplasts, which are capable of transferring energy to the chlorophylls. They also quench triplet excited states in chlorophyll molecules by dissipating the excess excitation energy in a non-radiative manner, a process known as non-photochemical quenching (NPQ) (reviewed in 1. and 2.). This function is crucial to protect against chlorophyll bleaching in intense light. An additional important role of carotenoids in plants is to furnish flowers and fruits with distinct colors that are designed to attract animals. In chloroplasts, carotenoids play vital roles in photosynthesis and are indispensable, whereas in chromoplasts, they can be considered as secondary metabolites. Carotenoids in plants are also precursors for the synthesis of the hormone abscisic acid (ABA) 3. and 4..
Elucidation of the carotenoid biosynthesis pathway is a wonderful example of a successful interdisciplinary approach to studying plant biochemistry. The enzymes of this pathway exist in minute amounts and are very labile upon purifi-cation. These characteristics and the lack of genuine in vitro assays for any of the enzymes have hindered the usage of conventional biochemical investigation. Because the cloning of the genes for these enzymes could not rely on protein purification, molecular analysis required the use of various genetic methods. Cloning of the first genes took advantage of the fact that the pathway in plants is similar to that in cyanobacteria. Hence, the phytoene desaturase (Pds) gene was first isolated from mutants of Synechococcus sp. PCC7942 that were resistant to norflurazon, an inhibitor of PDS [5]. The gene was identified by its ability to confer herbicide resistance in the wild-type background. The cyanobacterial genes then served as molecular probes to isolate the plant orthologs. A similar methodology has been successfully used to clone the gene for lycopene β-cyclase [6].