title: Gene duplication in the carotenoid biosynthetic pathway preceded evolution of the grasses.
abstract: Despite ongoing research on carotenoid biosynthesis in model organisms, there is a paucity of information on pathway regulation operating in the grasses (Poaceae), which include plants of world-wide agronomic importance. As a result, efforts to either breed for or metabolically engineer improvements in carotenoid content or composition in cereal crops have led to unexpected results. In comparison to maize (Zea mays), rice (Oryza sativa) accumulates no endosperm carotenoids, despite having a functional pathway in chloroplasts. To better understand why these two related grasses differ in endosperm carotenoid content, we began to characterize genes encoding phytoene synthase (PSY), since this nuclear-encoded enzyme appeared to catalyze a rate-controlling step in the plastid-localized biosynthetic pathway. The enzyme had been previously associated with the maize Y1 locus thought to be the only functional gene controlling PSY accumulation, though function of the Y1 gene product had never been demonstrated. We show that both maize and rice possess and express products from duplicate PSY genes, PSY1 (Y1) and PSY2; PSY1 transcript accumulation correlates with carotenoid-containing endosperm. Using a heterologous bacterial system, we demonstrate enzyme function of PSY1 and PSY2 that are largely conserved in sequence except for N- and C-terminal domains. By database mining and use of ortholog-specific universal PCR primers, we found that the PSY duplication is prevalent in at least eight subfamilies of the Poaceae, suggesting that this duplication event preceded evolution of the Poaceae. These findings will impact study of grass phylogeny and breeding of enhanced carotenoid content in an entire taxonomic group of plant crops critical for global food security.
authors.affiliation: Department of Biological Sciences, Lehman College, The City University of New York, Bronx, New York 10468, USA.
authors.forename: Cynthia E
authors.initials: CE
authors.lastname: Gallagher
authors.forename: Paul D
authors.initials: PD
authors.lastname: Matthews
authors.forename: Faqiang
authors.initials: F
authors.lastname: Li
authors.forename: Eleanore T
authors.initials: ET
authors.lastname: Wurtzel
mesh_terms: D001483:Base Sequence; D002338:Carotenoids / Q000096:biosynthesis*; D017931:DNA Primers; D018744:DNA, Plant / Q000235:genetics / Q000302:isolation & purification; D019143:Evolution, Molecular; D020440:Gene Duplication; D008969:Molecular Sequence Data; D012275:Oryza / Q000235:genetics; D010940:Plant Proteins / Q000235:genetics; D006109:Poaceae / Q000145:classification / Q000235:genetics*; D016133:Polymerase Chain Reaction; D003313:Zea mays / Q000235:genetics
chemical_list: D017931:DNA Primers; D018744:DNA, Plant; D010940:Plant Proteins; D002338:Carotenoids