Carotenoid analysis and functional characterization of lycopene cyclases in Zinnia elegans L.

Abstract

Carotenoids, a group of essential pigments for petal coloration in many species, could be generally categorized into linear and cyclic groups. The cyclization of the linear substrate lycopene at both end groups by β- and ε-cyclases (LCYBs and LCYEs, respectively) is a key branching point of the biosynthesis of cyclic carotenoids in flowers. Common zinnia (Zinnia elegans L.), an Asteraceae family plant that has been widely used in landscape greening, is best known for its brilliant flower colors. However, the mechanism of its petal coloration is not well studied, especially with respect to carotenoid pigmentation. In this study, total carotenoid content and carotenoid compositions of petals were analyzed in two common zinnia cultivars, 'Dreamland Red’ (DRE) and 'Dreamland Yellow’ (DY). Despite the lighter color of DY petals, the total carotenoid content in DY flowers was significantly higher than that in DRE. Metabolomic analysis revealed that the major carotenoids of both cultivars are cyclic carotenoids, with β-carotene being the most abundant. In addition, one LCYB, LCYE, and capsanthin/capsorubin synthase (CCS) gene were identified from Z. elegans by homology-based search in transcriptome. After cloning and sequencing, full-length open reading frame of each gene is exactly the same between two cultivars. Bacterial pigment complementation experiments revealed that ZeLCYB and ZeCCS can cyclize lycopene at both ends to produce β-carotene. Unlike most LCYEs characterized from higher plants, ZeLCYE produced predominantly ε-carotene in E. coli system. However, ZeLCYE is likely to have a lower affinity for lycopene or lower activity compared with ZeLCYB. Gene expression analysis showed that DY has higher expression levels of ZeLCYB and ZeCCS than DRE at specific developmental stages, consistent with its higher total carotenoid content. This work would lay a solid foundation for future studies on carotenoid accumulation in Z. elegans.