Carotenoids are essential photoprotective and antioxidant pigments synthesized by all photosynthetic organisms. Most carotenoid biosynthetic enzymes were thought to have evolved independently in bacteria and plants. For example, in bacteria, a single enzyme (CrtI) catalyzes the four desaturations leading from the colorless compound phytoene to the red compound lycopene, whereas plants require two desaturases (phytoene and -carotene desaturases) that are unrelated to the bacterial enzyme. We have demonstrated that carotenoid desaturation in plants requires a third distinct enzyme activity, the carotenoid isomerase (CRTISO), which, unlike phytoene and -carotene desaturases, apparently arose from a progenitor bacterial desaturase. The Arabidopsis CRTISO locus was identified by the partial inhibition of lutein synthesis in light-grown tissue and the accumulation of poly-cis -carotene precursors in dark-grown tissue of crtISO mutants. After positional cloning, enzymatic analysis of CRTISO expressed in Escherichia coli confirmed that the enzyme catalyzes the isomerization of poly-cis -carotenoids to all-trans -carotenoids. Etioplasts of darkgrown crtISO mutants accumulate acyclic poly-cis -carotenoids in place of cyclic all-trans -xanthophylls and also lack prolamellar bodies (PLBs), the lattice of tubular membranes that defines an etioplast. This demonstrates a requirement for carotenoid biosynthesis to form the PLB. The absence of PLBs in crtISO mutants demonstrates a function for this unique structure and carotenoids in facilitating chloroplast development during the first critical days of seedling germination and photomorphogenesis.

Carotenoid pigments are critical for plant survival, and carotenoid composition is tuned to the developmental stage, tissue, and to environmental stimuli. We report the cloning of the CAROTENOID CHLOROPLAST REGULATORY1 (CCR1) gene. The ccr1 mutant has increased shoot branching and altered carotenoid composition, namely, reduced lutein in leaves and accumulation of cis-carotenes in dark-grown seedlings. The CCR1 gene was previously isolated as EARLY FLOWERING IN SHORT DAYS and encodes a histone methyltransferase (SET DOMAIN GROUP 8) that methylates histone H3 on Lys 4 and/or 36 (H3K4 and H3K36). ccr1 plants show reduced trimethyl-H3K4 and increased dimethyl-H3K4 surrounding the CAROT-ENOID ISOMERASE (CRTISO) translation start site, which correlates with low levels of CRTISO mRNA. Microarrays of ccr1 revealed the downregulation of 85 genes, including CRTISO and genes associated with signaling and development, and upregulation of just 28 genes. The reduction in CRTISO transcript abundance explains the altered carotenoid profile. The changes in shoot branching are additive with more axillary branching mutants, but the altered carotenoid profile may partially affect shoot branching, potentially by perturbed biosynthesis of the carotenoid substrates of strigolactones. These results are consistent with SDG8 regulating shoot meristem activity and carotenoid biosynthesis by modifying the chromatin surrounding key genes, including CRTISO. Thus, the level of lutein, the most abundant carotenoid in higher plants that is critical for photosynthesis and photoprotection, appears to be regulated by a chromatin modifying enzyme in Arabidopsis thaliana.

Vitamin A deficiency, a global health burden, can be alleviated through provitamin A carotenoid biofortification of major crop staples such as maize (Zea mays) and other grasses in the Poaceae. If regulation of carotenoid biosynthesis was better understood, enhancement could be controlled by limiting b-carotene hydroxylation to compounds with lower or no nonprovitamin A activity. Natural maize genetic diversity enabled identification of hydroxylation genes associated with reduced endosperm provitamin A content. A novel approach was used to capture the genetic and biochemical diversity of a large germplasm collection, representing 80% of maize genetic diversity, without having to sample the entire collection. Metabolite data sorting was applied to select a 10-line genetically diverse subset representing biochemical extremes for maize kernel carotenoids. Transcript profiling led to discovery of the Hydroxylase3 locus that coincidently mapped to a carotene quantitative trait locus, thereby prompting investigation of allelic variation in a broader collection. Three natural alleles in 51 maize lines explained 78% of variation and approximately 11-fold difference in b-carotene relative to b-cryptoxanthin and 36% of the variation and 4-fold difference in absolute levels of b-carotene. A simple PCR assay to track and identify Hydroxylase3 alleles will be valuable for predicting nutritional content in genetically diverse cultivars found worldwide.