Abstract
BackgroundCrocins are soluble apocarotenoids that mainly accumulate in the stigma tissue of Crocus sativus and provide the characteristic red color to saffron spice, in addition to being responsible for many of the medicinal properties of saffron. Crocin biosynthesis and accumulation in saffron is developmentally controlled, and the concentration of crocins increases as the stigma develops. Until now, little has been known about the molecular mechanisms governing crocin biosynthesis and accumulation. This study aimed to identify the first set of gene regulatory processes implicated in apocarotenoid biosynthesis and accumulation.ResultsA large-scale crocin-mediated RNA-seq analysis was performed on saffron and two other Crocus species at two early developmental stages coincident with the initiation of crocin biosynthesis and accumulation. Pairwise comparison of unigene abundance among the samples identified potential regulatory transcription factors (TFs) involved in crocin biosynthesis and accumulation. We found a total of 131 (up- and downregulated) TFs representing a broad range of TF families in the analyzed transcriptomes; by comparison with the transcriptomes from the same developmental stages from other Crocus species, a total of 11 TF were selected as candidate regulators controlling crocin biosynthesis and accumulation.ConclusionsOur study generated gene expression profiles of stigmas at two key developmental stages for apocarotenoid accumulation in three different Crocus species. Differential gene expression analyses allowed the identification of transcription factors that provide evidence of environmental and developmental control of the apocarotenoid biosynthetic pathway at the molecular level.
Highlights
Crocins are soluble apocarotenoids that mainly accumulate in the stigma tissue of Crocus sativus and provide the characteristic red color to saffron spice, in addition to being responsible for many of the medicinal properties of saffron
Both C. sativus and C. cartwrightianus belong to section Crocus, which are species that bloom in autumn and only accumulate crocins in the stigma tissue [24]
We dissected stigmas of these three Crocus species at two developmental stages focused on the transition from white to yellow stigmas because it is the beginning of crocetin biosynthesis and crocin accumulation (Fig. 1a)
Summary
Crocins are soluble apocarotenoids that mainly accumulate in the stigma tissue of Crocus sativus and provide the characteristic red color to saffron spice, in addition to being responsible for many of the medicinal properties of saffron. Crocin biosynthesis and accumulation in saffron is developmentally controlled, and the concentration of crocins increases as the stigma develops. The biosynthesis and accumulation of crocins in the stigma of saffron and in flowers of other Crocus species showed an increase parallel to the expression of precursor carotenogenic and apocarotenogenic genes [11,12,13,14,15], which represent a chromoplast-specific carotenoid pathway for crocin biosynthesis in Crocus [3]. Different strategies to control carotenoid biosynthesis and accumulation have been reported [16,17,18], and among them, transcriptional regulation of carotenogenic gene expression has been shown to be the major mechanism by which the biosynthesis and accumulation of specific carotenoids are regulated. Epigenetic regulation of genes involved in carotenoid synthesis and degradation, including histone- and/ or DNA-methylation, and RNA silencing at the posttranscriptional level affect carotenoids in plants
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