Cloning and functional characterization of a carotenoid cleavage dioxygenase 2 gene in safranal and crocin biosynthesis from Freesia hybrida

Safranal and crocin, commonly derived from the oxidative cleavage reaction of zeaxanthin in plants, are two kinds of apocarotenoids with versatile functions, which were only found in limited number of plant species. In this study, both metabolites were detected and varied concomitantly with the expression of carotenoid cleavage dioxygenase (CCD) genes in Freesia hybrida, Red River® and Ambiance cultivars. The newly isolated CCD, denoted here as FhCCD2, was phylogenetically clustered with other reported saffron CCD2s. Besides, ten introns were also observed in the genomic DNA sequence of FhCCD2 and the presence of N-terminal transporter peptide suggested its plastidial sub-localization. Biochemical analysis showed that the FhCCD2 cleaved zeaxanthin at the 7, 8 and 7', 8' double bonds to generate intermediates prerequisite for the biosynthesis of safranal and crocin. Further, gene transient expression analysis showed that the promoter of FhCCD2 was functional in Ambiance as well as Red River® cultivars, even with slight variation in their promoter sequence. At present, CCD2 proteins have only been found in Freesia and Crocus genus of Iridaceae family. Phylogenetic and intron position analysis infer that CCD2 perhaps emerged after the intron loss during evolutionary process of CCD1 or their shared ancestry.

Keywords: Apocarotenoid; CCD2; Crocin; Freesia hybrida; Safranal; Zeaxanthin.