Light-responsive transcription factor ZeMYB32 negatively regulates ZeCCD4-2 to affect carotenoid accumulation in Zinnia elegans petals.

Carotenoids are essential pigments in ornamental plants, and their accumulation results from the interaction of internal developmental processes and external environmental factors. Light serves as a crucial environmental factor significantly influencing carotenoid synthesis and accumulation. Among various pigments, carotenoids function as an essential factor in petal coloring of Zinnia elegans ray floret. However, how light regulates carotenoid metabolism in Z. elegans petals remains unclear. This study identified a light-responsive transcription factor, ZeMYB32, that negatively regulates ZeCCD4-2 expression, impacting carotenoid metabolism in Z. elegans petals. Specifically, light affects the color parameters and carotenoid content of ray floret petals. After dark treatment, the brightness, yellowness, and color saturation increased significantly, while the greenness decreased markedly. Concurrently, the total carotenoid content rose substantially, and the total chlorophyll content fell. Transcriptomic analysis revealed that carotenoid cleavage dioxygenase 4 (ZeCCD4-2), a key structural gene in carotenoid metabolism, was significantly downregulated under dark treatment, with seven MYB transcription factor binding sites in its promoter region. Further analysis pinpointed ZeMYB32, a transcription factor from the S4 subfamily of Z. elegans, whose expression rose markedly after dark treatment. Yeast one-hybrid and dual-luciferase assays confirmed that ZeMYB32 binds to and suppresses the transcriptional activity of the ZeCCD4-2 promoter. Further functional studies based on the stable hairy root transformation system in Z. elegans revealed that the expression abundance of ZeCCD4-2 was significantly decreased in ZeMYB32 overexpressing roots. These findings enhance our understanding of how light regulates carotenoid metabolism in ornamental plant petals.