Widely targeted metabolomic profiling combined with transcriptome analysis provides new insights into flavonoid biosynthesis of sweet orange 'Newhall' (C. sinensis) under magnesium stress

Sweet orange 'Newhall' peels (SOPs) of fruits are rich in flavonoids and have become popular in nutrition, foods, and medicine. However, the flavonoid components in SOPs and the molecular mechanism of flavonoid biosynthesis under magnesium stress are still unclear. Here, we performed an integrative analysis of the metabolome and transcriptome in SOPs from Mg-sufficiency (MS) and Mg-deficiency (MD) at three stages. A total of 1555 secondary metabolites were identified, including 740 flavonoid . A total of 740 individual flavonoids divided into eight classes were identified, and flavones and flavonols were considered the most abundant flavonoid components in SOPs. The total flavonoid content of SOPs in MD was higher than MS; it increased first and then decreased in MD under magnesium stress. Nine-quadrant analysis identified differentially abundant flavonoids in MS and MD peels, respectively, under magnesium stress. Weighted gene correlation network analysis (WGCNA) identified eight structural genes and four transcription factor genes involved in the regulation of flavonoid biosynthesis. Moreover, qPCR results verified the accuracy of the transcriptome data and the reliability of the candidate genes. Taken together, our results reveal the flavonoid components of SOPs and document changes in flavonoid metabolism under magnesium stress, providing valuable information for nutrition value and food utilization of SOPs.