Spatial and Temporal Regulation of Flower Coloration in <i>Cymbidium lowianum</i>

Abstract

Flower color is a crucial trait that attracts pollinators and determines the ornamental value of floral crops. Cymbidium lowianum, one of the most important breeding parent of Cymbidium hybrids, has two flower morphs (normal and albino) that differ in flower lip color. However, the molecular mechanisms underlying flower color formation in C. lowianum are not well understood. In this study, comparative metabolomic analysis between normal and albino flower lip tissues indicated that cyanidin-3-O-glucoside content was significantly higher in red epichiles than in other lip tissues. This finding suggests that cyanidin-3-O-glucoside is responsible for color variation and differentiation in the lip in C. lowianum. We also found that red coloration in C. lowianum flower is correlated with high levels of F3'H expression; further, anthocyanins, carotenoids and chlorophyll coordinate to influence sepal and petal coloration during flower development. In transgenic Arabidopsis lines, overexpression of F3'H increased anthocyanin concentration, overexpression of BCH increased carotenoid concentration, whereas overexpression of HEMG and CHLI both increased chlorophyll concentration. Identification and assessment of several transcription factors revealed that MYB308-1 activates BCH, MYB111 and PIF4-2 activate HEMG and CHLI expression during flower development. Importantly, MYB14-1 shows interaction with PIF4-2, and appears to act as a connector between anthocyanin and chlorophyll biosynthesis by either activating F3'H expression or inhibiting CHLI expression. These results indicate that, in C. lowianum, variation in flower color and differentiation of lip color patterns are primarily regulated by the types and concentrations of flavonoids; further, carotenoids and chlorophyll also influence flower coloration during development.