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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Timing and Biosynthetic Potential for Carotenoid Accumulation in Genetically Diverse Germplasm of Maize^{1,[C],[W],[OA]}

Department of Biological Sciences, Lehman College, City University of New York, Bronx, New York 10468; and Graduate School and University Center-City University of New York, New York, New York 10016–4309

Enhancement of the carotenoid biosynthetic pathway in food crops benefits human health and adds commercial value of natural food colorants. However, predictable metabolic engineering or breeding is limited by the incomplete understanding of endogenous pathway regulation, including rate-controlling steps and timing of expression in carotenogenic tissues. The grass family (Poaceae) contains major crop staples, including maize (Zea mays), wheat (Triticum aestivum), rice (Oryza sativa), sorghum (Sorghum bicolor), and millet (Pennisetum glaucum). Maize carotenogenesis was investigated using a novel approach to discover genes encoding limiting biosynthetic steps in the nutritionally targeted seed endosperm. A combination of bioinformatics and cloning were first used to identify and map gene families encoding enzymes in maize and other grasses. These enzymes represented upstream pathways for isopentenyl diphosphate and geranylgeranyl diphosphate synthesis and the downstream carotenoid biosynthetic pathway, including conversion to abscisic acid. A maize germplasm collection was used for statistical testing of the correlation between carotenoid content and candidate gene transcript levels. Multiple pathway bottlenecks for isoprenoid biosynthesis and carotenoid biosynthesis were discovered in specific temporal windows of endosperm development. Transcript levels of paralogs encoding isoprenoid isopentenyl diphosphate and geranylgeranyl diphosphate-producing enzymes, DXS3, DXR, HDR, and GGPPS1, were found to positively correlate with endosperm carotenoid content. For carotenoid pathway enzymes, transcript levels for CrtISO inversely correlated with seed carotenoid content, as compared with positive correlation of PSY1 transcripts. Since zeaxanthin epoxidase (ZEP) depletes the carotenoid pool in subsequent conversion to abscisic acid, ZEP transcripts were examined. Carotenoid accumulation was found to be inversely associated with ZEP1 and ZEP2 transcript levels. Extension of the maize results using phylogenetic analysis identified orthologs in other grass species that may serve as potential metabolic engineering targets.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Eleanore T. Wurtzel (wurtzel{at}lehman.cuny.edu).

^[C] Some figures and tables in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.137042

^* Corresponding author; e-mail wurtzel{at}lehman.cuny.edu.

Received February 17, 2009; accepted March 27, 2009; published April 3, 2009.

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