Integrated metabolic and transcriptional analysis reveals the role of carotenoid cleavage dioxygenase 4 (IbCCD4) in carotenoid accumulation in sweetpotato tuberous roots

Background: Plant carotenoids are essential for human health, having wide uses in dietary supplements, food colorants, animal feed additives, and cosmetics. With the increasing demand for natural carotenoids, plant carotenoids have gained great interest in both academic and industry research worldwide. Orange-fleshed sweetpotato (Ipomoea batatas) enriched with carotenoids is an ideal feedstock for producing natural carotenoids. However, limited information is available regarding the molecular mechanism responsible for carotenoid metabolism in sweetpotato tuberous roots. Results: In this study, metabolic profiling of carotenoids and gene expression analysis were conducted at six tuberous root developmental stages of three sweetpotato varieties with different flesh colors. The correlations between the expression of carotenoid metabolic genes and carotenoid levels suggested that the carotenoid cleavage dioxygenase 4 (IbCCD4) and 9-cis-epoxycarotenoid cleavage dioxygenases 3 (IbNCED3) play important roles in the regulation of carotenoid contents in sweetpotato. Transgenic experiments confirmed that the total carotenoid content decreased in the tuberous roots of IbCCD4-overexpressing sweetpotato. In addition, IbCCD4 may be regulated by two stress-related transcription factors, IbWRKY20 and IbCBF2, implying that the carotenoid accumulation in sweeetpotato is possibly fine-tuned in responses to stress signals. Conclusions: A set of key genes were revealed to be responsible for carotenoid accumulation in sweetpotato, with IbCCD4 acts as a crucial player. Our findings provided new insights into carotenoid metabolism in sweetpotato tuberous roots and insinuated IbCCD4 to be a target gene in the development of new sweetpotato varieties with high carotenoid production.