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.Carotenoids, a class of over 600 structures derived from isoprenoids, are synthesized by all photosynthetic organisms, some bacteria, and fungi. In plants, carotenoids are essential for plant growth and development; mutations blocking carotenoid accumulation have pleiotropic effects on chloroplast biogenesis and seed development (Robertson et al., 1978;Wurtzel, 1992). Carotenoids function as accessory pigments in photosynthesis, as photoprotectors preventing photooxidative damage, and as precursors to the plant hormone, abscisic acid (Hirschberg, 2001). The presence of carotenoids in plant endosperm tissue adds nutritional value; in humans and animals, dietary carotenoids are essential precursors to vitamin A and to retinoid compounds needed in development (Lee et al., 1981;Bendich and Olson, 1989). Nonprovitamin A carotenoids, such as lycopene, lutein, zeaxanthin, and others, also play beneficial roles in human health (Giovannucci et al., 1995;Kohlmeier et al., 1997;Sommerburg et al., 1998;Krinsky et al., 2003). The various roles of carotenoids affecting plant yield and nutritional potential has made them targets for breeding and metabolic engineering (Shewmaker et al., 1999;Matthews and Wurtzel, 2000;Ye et al., 2000;Davison, 2002;Blott et al., 2003;Gallagher et al., ...

Association between allelic variation at the Phytoene synthase 1 gene and yellow pigment content in the wheat grain

“…This hypothesis is supported by the independent observation of complete linkage between PSY-E1 and GYPC and is consistent with the knowledge that this gene aVects the accumulation of carotenoid pigments in the grains of other grass species. In maize, transcription proWles of PSY-1 (but not of PSY-2) correlate with carotenoid pigment accumulation in the grains (Gallagher et al 2004), and over-expression of PSY in Arabidopsis seeds results in increased levels of carotenoids (Lindgren et al 2003).…”

“…Pozniak et al (2007) found the Phytoene synthase 1 (PSY-B1) gene to be linked to a 7BL QTL for GYPC and suggested that it might be a good candidate gene for this QTL. The PSY gene is duplicated in the grasses, but only the PSY-1 paralogue shows a correlation between transcript accumulation and GYPC (Gallagher et al 2004). High levels of PSY-1 transcripts have been observed in the endosperm of yellow maize but not in white maize or in the white endosperm of rice (Gallagher et al 2004).…”

“…The PSY gene is duplicated in the grasses, but only the PSY-1 paralogue shows a correlation between transcript accumulation and GYPC (Gallagher et al 2004). High levels of PSY-1 transcripts have been observed in the endosperm of yellow maize but not in white maize or in the white endosperm of rice (Gallagher et al 2004).…”

PSY3, a New Member of the Phytoene Synthase Gene Family Conserved in the Poaceae and Regulator of Abiotic Stress-Induced Root Carotenogenesis

“…We previously showed that PSY was encoded by two paralogs, PSY1 and PSY2, in 12 species across eight subfamilies of the grasses (Poaceae; Gallagher et al, 2004) in comparison to a single PSY gene in Arabidopsis; we also demonstrated enzymatic functions for maize PSY1 and PSY2 and rice PSY2. While attempting to expand PSY studies in sorghum, which is an important cereal crop in Africa and other parts of the world, we discovered a novel PSY cDNA, which we termed PSY3.…”

“…The first enzyme in the plastid-localized carotenoid biosynthetic pathway, phytoene synthase (PSY), which is known to control flux to carotenoids in the seed (Gallagher et al, 2004), is nuclear encoded by a small gene family consisting of PSY1 and PSY2, which we had previously shown to exist throughout the Grasses (Gallagher et al, 2004). In the process of searching for orthologs of PSY1 and PSY2 in sorghum (Sorghum bicolor), a cultivated species in drought-and saltstressed environments, we stumbled upon a new and unrelated PSY gene, PSY3.…”