CDNA cloning of the phytoene synthase (PSY) and expression analysis of PSY and carotenoid cleavage dioxygenase genes in Osmanthus fragrans

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In the carotenoid biosynthesis pathway, phytoene synthase (PSY) catalyzes the dimerization of two molecules of geranylgeranyl pyrophosphate (GGPP) to phytoene and has been shown to be a rate-limiting enzyme for the synthesis of carotenoids. In this study, a PCR approach was used to isolate a phytoene synthase cDNA from flower of Osmanthus fragrans. It contained a 1314 bp CDS. The predicted protein (49.4 kDa) displayed a sequence of 438 amino acid residues. Phylogenetic analysis demonstrated that the O. fragrans PSY gene (OfPSY) clustered with Gardenia jasminoides, Coffea canephora, Lycium barbarum and Mespilus germanica. Moreover, the OfPSY sequence relates closely with other PSY sequences of higher plants. We detected flower’s carotenoid content and analysed OfPSY and carotenoid cleavage dioxygenase genes (OfCCD1 and OfCCD4) in two cultivars of O. fragrans with varied coloration, including O. fragrans “Yingui” (light yellow) and “Dangui” (orange) at different floral stages. We observed that “Dangui” can accumulate carotenoids when OfPSY transcript was already constant, and this showed that the OfPSY does not play a role in the accumulation of total carotenoid in this cultivar. However, the transcripts of OfCCD4 in “Yingui” flower petals were clearly higher than that in “Dangui”. This was consistent with the ionone content in “Yingui” flower petals, which was significantly higher than that in “Dangui”. This response could contribute to explain the different accumulation of carotenoids in petals of the 2 cultivars.