![]() ![]() The first carotenoid in the committed pathway is phytoene, which is produced by the enzyme phytoene synthase through a condensation of two molecules of geranylgeranyl diphosphate. In plants, carotenoids are synthesized within the plastids from the central isoprenoid pathway (reviewed by Hirschberg, 2001) ( Figure 1). Indeed, although the bulk of carotenoids in higher plants occur in the all- trans configuration, different cis-isomers exist as well, but in small proportions. These double bonds enable the formation of cis- trans geometric isomers in various positions along the molecule. The polyene chain in carotenoids contains up to 15 conjugated double bonds, a feature that is responsible for their characteristic absorption spectra and specific photochemical properties ( Goodwin, 1980 Britton et al., 1998). Carotenoids are mainly 40-carbon isoprenoids, which consist of eight isoprene units. They also furnish distinctive yellow, orange, and red colors to fruit and flowers to attract animals. They assist in harvesting light energy and protect the photosynthetic apparatus against harmful reactive oxygen species that are produced by overexcitation of chlorophyll. The function of carotene isomerase in plants presumably is to enable carotenoid biosynthesis to occur in the dark and in nonphotosynthetic tissues.Ĭarotenoid pigments are essential components in all photosynthetic organisms. CRTISO from tomato is expressed in all green tissues but is upregulated during fruit ripening and in flowers. In tangerine mic, loss of function is attributable to a deletion mutation in CRTISO, and in tangerine 3183, expression of this gene is impaired. Two alleles of tangerine have been investigated. CRTISO is a redox-type enzyme structurally related to the bacterial-type phytoene desaturase CRTI. Our data indicate that the tangerine gene, designated CRTISO, encodes an authentic carotenoid isomerase that is required during carotenoid desaturation. Fruit of tangerine are orange and accumulate prolycopene (7Z,9Z,7′Z,9′Z-tetra- cis-lycopene) instead of the all- trans-lycopene, which normally is synthesized in the wild type. To elucidate the molecular mechanism of carotenoid isomerization, we have taken a genetic map-based approach to clone the tangerine locus from tomato. However, the cis-trans isomerization of carotenoids, which is known to occur in vivo, has remained a mystery since its discovery five decades ago. Carotenoid biosynthesis in plants has been described at the molecular level for most of the biochemical steps in the pathway. ![]()
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