Integrated metabolomics and transcriptomics indicate an antagonistic relationship between low temperature stress and exogenous abscisic acid in β-ionone synthesis in Osmanthus fragrans ‘Liuye Yingui’ flowers

Osmanthus fragrans is valued as an ornamental tree appreciated for its fragrance, in which β-ionone is one of the most critical volatile organic compounds (VOCs). The distribution of O. fragrans is mainly limited by low temperature (LT), which in other species can also induce the synthesis of endogenous ABA and the formation of floral aroma. However, the regulatory relationship between LT and ABA on the floral aroma quality in O. fragrans remains unclear. In this study, flowers of O. fragrans ‘Liuye Yingui’ were treated with LT, ABA, or LT+ABA. Metabolic analysis of VOCs and PCA indicated that all three treatments produced clusters well separated from the control group. However, while both of ABA and LT+ABA treatments decreased the content of β-ionone, the LT treatment alone caused an increase. Notably, a significant enrichment in the carotenoid biosynthesis pathway, which is closely associated with β-ionone synthesis was observed in the control vs. ABA and LT vs. LT+ABA groups, indicating that the ABA affects β-ionone contents mainly through its effects on the carotenoid biosynthesis pathway. By conducting an analysis of the weighted gene co-expression network, a module of genes most positively correlated with β-ionone was identified, in which one hub structural gene OfPDS3–2, as well as three transcriptional factors OfMYB187, OfATHB15, and OfTCP3 were indicated to have potentially important regulatory roles in β-ionone synthesis under LT and ABA stimuli. Together, these findings indicate that the promotion of β-ionone synthesis by LT can be negatively regulated by exogenous ABA and suggest useful genetic resources for the floral aroma improvement breeding in O. fragrans.