IbMBP3 counteracts IbHB007-mediated repression to regulate carotenoid metabolism in sweetpotato
Orange-fleshed sweetpotato (Ipomoea batatas (L.) Lam.), rich in carotenoids, serves as an ideal feedstock for natural carotenoid production. Although the carotenoid biosynthesis pathway has been extensively studied, its regulatory mechanism in sweetpotato remains relatively unclear. In this study, RNA-seq analysis was conducted on Jishu 35 (an orange-fleshed variety) and Jishu 36 (a white-fleshed variety). A BEL-like homeobox gene, designated as IbHB007, was found to be highly expressed in Jishu 36 but significantly suppressed in Jishu 35. In IbHB007-RNAi plants, the flesh color of storage roots changed from light yellow to orange, and the total carotenoid content was significantly higher compared to the wild type (WT). Results from qRT-PCR, Y1H and DLR assays indicated that IbHB007 directly targets the promoters of IbPDS and IbZDS and markedly suppresses their activity. Specifically, IbHB007 physically interacts with a MADS-box transcription factor IbMBP3. Through this interaction, IbMBP3 promotes the accumulation of carotenoids in storage roots by suppressing IbHB007-mediated transcriptional activation of IbPDS. Simultaneously, IbMBP3 also directly inhibits the expression of the carotenoid degradation gene IbCCD4. Together, the IbHB007-IbMBP3 heterodimer synergistically regulates carotenoid levels to maintain homeostasis. IbHB007 can also inhibit carotenoid biosynthesis in tomato by repressing the expression of SlPDS and SlZDS. These results suggest a potentially conserved function of IbHB007 in carotenoid metabolism across diverse species, indicating that it is a valuable candidate gene for breeding high-carotenoid plants.