Integrated untargeted metabolomic and transcriptomic analyses reveal OMT genes controlling polymethoxyflavonoid biosynthesis in the pericarp of Citrus reticulata ‘Chachi’

The dried pericarp of Citrus reticulata 'Chachi' (CRC) is commonly utilized in Traditional Chinese Medicine (TCM) and food consumption. Geqingpi (GQP), Sihuaqingpi (SHQP) and Guangchenpi (CRP) are TCM materials derived from different developmental stages of CRC pericarp, each possessing distinct medicinal properties. However, there is currently limited study on the active substances in these three stages. Metabolome data suggested that the discrepancies in flavonoid content, particularly polymethoxyflavonoids (PMFs), predominantly contribute to the variations across the three periods. Additionally, through multivariate statistical analysis, 7 potential biomarkers were identified and nobiletin (NOB) possessed the most significant contribution. O-methyltransferases (OMTs) play a crucial role to catalyze the formation of -OCH3 in PMFs. The analysis of transcriptome and genome data revealed the presence of 93 CrOMTs in CRC and 43 were expressed on pericarp. Further investigation identified that 28 of these genes were potentially responsible for catalyzing the biosynthesis of important PMFs such as NOB, sinensetin (SIN), and 3,3′,4′,5,6,7,8-heptamethoxyflavone (HEP). Our results may provide a new perspective for the study of GQP, SHQP and CRP, as well as the regulatory mechanism of PMFs in CRC.