Abstract
Rhododendron simsii Planchon is an important ornamental species in the northern hemisphere. Flower color is an important objective of Rhododendron breeding programs. However, information on anthocyanin synthesis in R. simsii is limited. In this research, the regulatory mechanism of anthocyanin biosynthesis in R. simsii was performed through the integrated analysis of metabolome and RNA-seq. A total of 805 and 513 metabolites were screened by positive and negative ionization modes, respectively, In total, 79 flavonoids contained seven anthocyanidins, 42 flavanones, 10 flavans, 13 flavones, and seven flavonols. Methylated and glycosylated derivatives took up the most. Differentially accumulated metabolites were mainly involved in "flavone and flavonol biosynthesis", "cyanoamino acid metabolism", "pyrimidine metabolism", and "phenylalanine metabolism" pathways. For flavonoid biosynthesis, different expression of shikimate O-hydroxycinnamoyltransferase, caffeoyl-CoA O-methyltransferase, flavonoid 3'-monooxygenase, flavonol synthase, dihydroflavonol 4-reductase/flavanone 4-reductase, F3'5'H, chalcone synthase, leucoanthocyanidin reductase, and 5-O-(4-coumaroyl)-D-quinate 3'-monooxygenase genes ultimately led to different accumulations of quercetin, myricetin, cyanidin, and eriodictyol. In flavone and flavonol biosynthesis pathway, differential expression of F3'5'H, flavonoid 3'-monooxygenase and flavonol-3-O-glucoside/galactoside glucosyltransferase genes led to the differential accumulation of quercetin, isovitexin, and laricitrin. This research will provide a biochemical basis for further modification of flower color and genetic breeding in R. simsii and related Rhododendron species.