Biochemical and molecular analysis of carotenoid biosynthesis in flavedo of orange (Citrus sinensis L.) during fruit development and maturation.

Changes in carotenoid content and composition and expression of carotenoid biosynthetic genes were analyzed in the flavedo of sweet orange (Citrus sinensis L. Osbeck, cv. Navelate) fruit during development and maturation. Lutein and all-E-violaxanthin were the major carotenoids in chloroplast-containing tissues. During fruit coloration, phytoene, beta-cryptoxanthin, zeaxanthin, and mainly (9Z)-violaxanthin progressively accumulated, and a large proportion of apocarotenoids was also found in the flavedo of full-colored fruits. We have cloned partial and full-length cDNAs corresponding to genes involved in early condensation and desaturase reactions [phytoene synthase (PSY), phytoene desaturase (PDS), and zeta-carotene desaturase (ZDS)], coupled redox reaction (plastid terminal oxidase), cyclizations [beta-lycopene cyclase (beta-LCY) and epsilon-lycopene cyclase (epsilon-LCY)], hydroxylation [beta-carotene hydroxylase (beta-CHX)], and epoxidation [zeaxanthin epoxidase (ZEP)] and analyzed their mRNA accumulation in the flavedo of fruits during development and ripening as compared with those of leaves. Collectively, the results indicated that PDS gene expression correlated with carotenoid content in developing fruit and that up-regulation of PSY and ZDS genes at the onset of fruit coloration would enhance the production of linear carotenes and the flux into the pathway. The shift from the beta,epsilon-branch to the beta,beta-branch of the pathway that originates the changes in carotenoid composition during fruit coloration may be explained by a down-regulation of epsilon-LCY and by the increase of the beta-CHX transcript.