Journal of Experimental Botany, Vol. 54, No. 391, pp. 2215-2230,
October 1, 2003
© 2003 Oxford University Press
Maize phytoene desaturase and 
-carotene desaturase catalyse a poly-Z desaturation pathway: implications for genetic engineering of carotenoid content among cereal crops
Received 5 March 2003; Accepted 9 June 2003
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Department of Biological Sciences, Lehman College and The Graduate School and University Center, The City University of New York, 250 Bedford Park Blvd. West, Bronx, NY 10468, USA
* Present address: Agronomy and Plant Genetics, University of Minnesota, 1991 Upper Buford Circle, St Paul, MN, USA. Present address: Human Genome Sciences Inc., Antibody Department, 9800 Medical Center Drive, Rockville, MD 20850, USA. To whom correspondence should be addressed. Fax: +1 718 960 7348/8236. E-mail: etwlc{at}cunyvm.cuny.edu
Carotene desaturation, an essential step in the biosynthesis of coloured carotenoids, has received much attention (1) as a target of bleaching herbicide action, (2) as a determinant of geometric isomer states of carotenoids and their metabolites, and (3) as a key modulator of accumulation and structural variability of carotenoids. Having previously isolated and functionally characterized the cDNA encoding the first enzyme in maize carotene desaturation, phytoene desaturase (PDS), the isolation and functional characterization of the second desaturase, a maize endosperm cDNA (2265 bp) encoding zetacarotene (
-carotene) desaturase (ZDS) is reported here. Functional analysis of the concerted actions of maize PDS and ZDS ex situ showed these enzymes to mediate a poly-Z desaturation pathway to the predominate geometric isomer 7,9,7',9'-tetra-Z-lycopene (poly-Z-lycopene or prolycopene), and not the all-trans substrate required of the downstream lycopene cyclase enzymes. This finding suggests a rate-controlling isomerase associated with the carotene desaturases as a corollary of a default poly-Z carotenoid biosynthetic pathway active in planta for maize. Comparative gene analysis between maize and rice revealed that genes encoding PDS and ZDS are single copy; the Zds cDNA characterized here was mapped to maize chromosome 7S and vp9 is suggested as a candidate locus for the structural gene while y9 is ruled out. Classical genetic resources were used to dissect the desaturation steps further and hydroxyphenylpyruvate dioxygenase was linked to the vp2 locus, narrowing candidate loci for an obligate isomerase in maize to only a few. Since the first functional analysis of the paired carotene desaturases for a cereal crop is reported here, the implications for the genetic modification of the pro-vitamin A content in cereal crops such as rice and maize, are discussed.
Key words: Biosynthesis, carotenoid, desaturase, genes, isoprenoid, maize, mutants, phytoene, rice, zetacarotene.
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