The fading of flower color is caused by changes in anthocyanin content during flower development in many plants, including lilac (Syringa oblata). However, the molecular regulatory mechanism of this phenomenon is still poorly understood. UDP-glucose: flavonoid 3-O-glucosyltransferase (UFGT) has a pivotal role in the formation of stable anthocyanins. Here, SoUFGT1 and three transcription factors, SoMYB44, SobHLH130, and SoNAC72, were identified and verified to participate in anthocyanin production in lilac. Overexpressing SoMYB44 promoted SoUFGT1 expression in lilac petals. The yeast one-hybrid (Y1H) and dual-luciferase (Dual-LUC) assays demonstrated that SoMYB44 activated SoUFGT1, thereby bolstering anthocyanin accumulation. The overexpression and silencing of SoNAC72 in petals revealed that it facilitated anthocyanin accumulation. The Y1H and Dual-LUC assays verified that SoNAC72 was capable of directly binding to the SoMYB44 promoter to activate the latter's expression. In addition, SobHLH130 was also displayed to mediate anthocyanin accumulation in petals. By using yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays, the interaction between SoMYB44 and SobHLH130 was confirmed. These results corroborated that SoNAC72 regulates SoMYB44 expression, and SoMYB44 interacts with SobHLH130 to trigger SoUFGT1 expression in lilac, which then contributes to their anthocyanin accumulation. In sum, along with lilac flower development, the lower expression of SoNAC72 and SobHLH130 reduces SoMYB44 transcripts and depresses transcriptional regulation of SoUFGT1, thus diminishing anthocyanin biosynthesis, leading to the fading of petal color. These study's findings provide valuable new insight for understanding the formation and regulatory mechanisms of flower color in lilac.

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