Isomerization of 9,15,9'-tri-cis-ζ-carotene mediated by 15-cis-ζ-carotene isomerase Z-ISO is a critical step in the biosynthesis of carotenoids, which define fruit color. The tomato clade (Solanum section Lycopersicon) comprises the cultivated tomato (Solanum lycopersicum) and 12 related wild species differing in fruit color and, thus, represents a good model for studying carotenogenesis in fleshy fruit. In this study, we identified homologous Z-ISO genes, including 5'-UTRs and promoter regions, in 12 S. lycopersicum cultivars and 5 wild tomato species (red-fruited Solanum pimpinellifolium, yellow-fruited Solanum cheesmaniae, and green-fruited Solanum chilense, Solanum habrochaites, and Solanum pennellii). Z-ISO homologs had a highly conserved structure, suggesting that Z-ISO performs a similar function in tomato species despite the difference in their fruit color. Z-ISO transcription levels positively correlated with the carotenoid content in ripe fruit of the tomatoes. An analysis of the Z-ISO promoter and 5'-UTR sequences revealed over 130 cis-regulatory elements involved in response to light, stresses, and hormones, and in the binding of transcription factors. Green- and red/yellow-fruited Solanum species differed in the number and position of cis-elements, indicating changes in the transcriptional regulation of Z-ISO expression during tomato evolution, which likely contribute to the difference in fruit color.
Keywords: 15-cis-ζ-carotene isomerase Z-ISO; SNPs; Solanum section Lycopersicon; carotenogenesis; fruit color; promoter analysis.