Natural Genetic Variation in Lycopene Epsilon Cyclase Tapped for Maize Biofortification

Abstract

Dietary vitamin A deficiency causes eye disease in 40 million children each year and places 140 to 250 million at risk for health disorders. Many children in sub-Saharan Africa subsist on maize-based diets. Maize displays considerable natural variation for carotenoid composition, including vitamin A precursors alpha-carotene, beta-carotene, and beta-cryptoxanthin. Through association analysis, linkage mapping, expression analysis, and mutagenesis, we show that variation at the lycopene epsilon cyclase (lcyE) locus alters flux down alpha-carotene versus beta-carotene branches of the carotenoid pathway. Four natural lcyE polymorphisms explained 58% of the variation in these two branches and a threefold difference in provitamin A compounds. Selection of favorable lcyE alleles with inexpensive molecular markers will now enable developing-country breeders to more effectively produce maize grain with higher provitamin A levels.

Cloning and comparative analysis of carotenoid β-hydroxylase genes provides new insights into carotenoid metabolism in tetraploid (Triticum turgidum ssp. durum) and hexaploid (Triticum aestivum) wheat grains

“…A key step for lutein biosynthesis, which also impacts the division between β,ε-and β,β-branch carotenoid formation, is the introduction of an ε-ring, catalyzed by LCY-E . It was shown that naturally occurring polymorphisms of ZmLCY-E account for 58% of variations in the β,ε-/β,β-carotene branch ratios of the carotenoid pathway in maize grains (Harjes et al, 2008). Interestingly, while LCY-E expression was generally decreased in developing tetraploid wheat grains (similar to the reducing trend of lutein accumulation), it showed a steady increase during hexaploid wheat grain development after an initial drop at grain 2, a pattern that somewhat resembles HYD1 expression (Fig.…”

Nutritionally Enhanced Staple Food Crops

“…Carotenoids in plants play a crucial role in photosynthesis, membrane stability, growth, and development (Menkir et al 2008). Unlike other agronomically important traits, only limited germplasm sets have been assessed for b-carotene, predominantly in chickpea, maize, pearl millet, sorghum, and wheat (Table 3.6), with some lines accumulating b-carotene as high as 7.6 mg g À1 in pearl millet (Hash et al 1997) and 15.6 mg g À1 in maize (Harjes et al 2008;Menkir et al 2008). Genotypic differences for b-carotene have also been reported in rice: some germplasm had shown b-carotene in unpolished seeds while others having no b-carotene in unpolished seeds.…”

“…Pathway branching and hydroxylation are therefore key determinants in controlling vitamin A levels. Polymorphism at the LCYE locus in maize explained 58% of the variation in a-and b-carotene and a threefold difference in provitamin A compounds (Harjes et al 2008), while a rare genetic variant, b-carotene hydroxylase 1 (crtRB1), increases b-carotene substantially in maize grains . Further, metabolite sorting of a germplasm collection identified 10 genetically diverse subsets representing biochemical extremes for maize kernel carotenoids and transcript profiling of this subset led to the discovery of the Hydroxylase 3 locus that coincidently mapped to a carotene QTL (Chander et al 2008).…”

“…Further, metabolite sorting of a germplasm collection identified 10 genetically diverse subsets representing biochemical extremes for maize kernel carotenoids and transcript profiling of this subset led to the discovery of the Hydroxylase 3 locus that coincidently mapped to a carotene QTL (Chander et al 2008). The natural alleles at Hydroxylase 3 locus contribute 78% of variation and approximately 11-fold differences in b-carotene relative to b-cryptoxanthin and 36% of the variation and fourfold difference in absolute levels of b-carotene (Harjes et al 2008). The reduction in HYD3 transcripts leads to reduced conversion of b-carotene to downstream xanthophylls, causing b-carotene to accumulate (Vallabhaneni et al 2009).…”

“…The reduction in HYD3 transcripts leads to reduced conversion of b-carotene to downstream xanthophylls, causing b-carotene to accumulate (Vallabhaneni et al 2009). Genetics tests such as the HYD3 assay (Vallabhaneni et al 2009) together with the previously described LCYE assay (Harjes et al 2008) may be used to select germplasm containing optimal HYD3 and LCYE alleles in breeding programs, which will lead to higher b-carotene levels in maize endosperm when both genes are highly expressed than either with optimal alleles of either gene alone. Furthermore, the experimental evidence from the association and linkage mapping reveals that crtRB1 underlies a principal QTL associated with b-carotene concentration and conversion in maize kernels and crtRB1 alleles associated with reduced transcript expressions correlate well with higher b-carotene .…”