The cDNA of the gene pds from tomato, encoding the carotenoid biosynthesis enzyme phytoene desaturase, was cloned, and its nucleotide sequence was determined. Cells of Escherichia coli that expressed the tomato pds gene could convert phytoene to zeta-carotene. This result suggests that one polypeptide, the product of the pds gene, can carry out phytoene desaturation in the carotenoid biosynthetic pathway. Transcripts of the pds gene accumulate in orange tomato fruit, indicating transcriptional control of pds expression during fruit ripening. The deduced amino acid sequence of phytoene desaturase indicates that this enzyme in tomato contains 583 amino acids that are highly conserved with respect to the homologous enzymes in cyanobacteria and algae. The deduced amino acid sequences of the phytoene desaturases from other microorganisms (purple bacteria and fungi) appear to be evolutionarily unrelated to those from green photosynthetic organisms.

Phytoene synthase (Psy) and phytoene desaturase (Pds) are the first dedicated enzymes of the plant carotenoid biosynthesis pathway. We report here the organ-specific and temporal expression of PDS and PSY in tomato plants. Light increases the carotenoid content of seedlings but has little effect on PDS and PSY expression. Expression of both genes is induced in seedlings of the phytoene-accumulating mutant ghost and in wild-type seedlings treated with the Pds inhibitor norflurazon. Roots, which contain the lowest levels of carotenoids in the plant, have also the lowest levels of PDS and PSY expression. In flowers, expression of both genes and carotenoid content are higher in petals and anthers than in sepals and carpels. During flower development, expression of both PDS and PSY increases more than 10-fold immediately before anthesis. During fruit development, PSY expression increases more than 20-fold, but PDS expression increases less than threefold. We concluded that PSY and PDS are differentially regulated by stress and developmental mechanisms that control carotenoid biosynthesis in leaves, flowers, and fruits. We also report that PDS maps to chromosome 3, and thus it does not correspond to the GHOST locus, which maps to chromosome 11.