Abstract
BackgroundBixin or annatto is a commercially important natural orange-red pigment derived from lycopene that is produced and stored in seeds of Bixa orellana L. An enzymatic pathway for bixin biosynthesis was inferred from homology of putative proteins encoded by differentially expressed seed cDNAs. Some activities were later validated in a heterologous system. Nevertheless, much of the pathway remains to be clarified. For example, it is essential to identify the methylerythritol phosphate (MEP) and carotenoid pathways genes.ResultsIn order to investigate the MEP, carotenoid, and bixin pathways genes, total RNA from young leaves and two different developmental stages of seeds from B. orellana were used for the construction of indexed mRNA libraries, sequenced on the Illumina HiSeq 2500 platform and assembled de novo using Velvet, CLC Genomics Workbench and CAP3 software. A total of 52,549 contigs were obtained with average length of 1,924 bp. Two phylogenetic analyses of inferred proteins, in one case encoded by thirteen general, single-copy cDNAs, in the other from carotenoid and MEP cDNAs, indicated that B. orellana is closely related to sister Malvales species cacao and cotton. Using homology, we identified 7 and 14 core gene products from the MEP and carotenoid pathways, respectively. Surprisingly, previously defined bixin pathway cDNAs were not present in our transcriptome. Here we propose a new set of gene products involved in bixin pathway.ConclusionThe identification and qRT-PCR quantification of cDNAs involved in annatto production suggest a hypothetical model for bixin biosynthesis that involve coordinated activation of some MEP, carotenoid and bixin pathway genes. These findings provide a better understanding of the mechanisms regulating these pathways and will facilitate the genetic improvement of B. orellana.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2065-4) contains supplementary material, which is available to authorized users.
Highlights
Bixin or annatto is a commercially important natural orange-red pigment derived from lycopene that is produced and stored in seeds of Bixa orellana L
All carotenoids are synthesized by consecutive condensations of isopentenyl diphosphate (IPP), which in turn is synthesized through the plastidial methylerythritol phosphate (MEP) pathway [6, 7]
De novo sequence assembly of B. orellana transcriptome To investigate the MEP, carotenoid, and bixin pathways genes, we sequenced the transcriptome of B. orellana using mRNA from young leaves and two different developmental stages of seeds (Fig. 1)
Summary
Bixin or annatto is a commercially important natural orange-red pigment derived from lycopene that is produced and stored in seeds of Bixa orellana L. Many plant species utilized for production of secondary metabolites that are important components of human diet, animal feed, medicines, biopesticides, and bioherbicides, have been subject of limited research and genetic improvement. This is the case of Bixa orellana L., achiote in Mexico, a species belonging to the Bixaceae. Carotenoids are yellow to red pigments synthesized by microorganisms and plants In plants, they accumulate in the plastids (chromoplasts) of flowers and fruits. All carotenoids are synthesized by consecutive condensations of isopentenyl diphosphate (IPP), which in turn is synthesized through the plastidial methylerythritol phosphate (MEP) pathway [6, 7]. The cyclization of lycopene denotes a central branch point in the carotenoid biosynthesis pathway, and the relative activity of epsilon-cyclase (ε-LYC) versus beta-cyclase (β-LYC) may determine the flow of carotenoids from lycopene to either α-carotene or β-carotene [8]
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