Anthocyanin Metabolite and Transcriptome Sequencing Analysis Reveals White Flowers in Rhododendron dauricum var. albiflorum

Rhododendron dauricum is a cold-hardy shrub integrating ornamental, medicinal, and aromatic functions. Flower color is an important feature related to ornamental value for breeders and consumers. Nevertheless, the coloration mechanism of flower color in R. dauricum is still unclear. R. dauricum var. albiflorum is a white flower variety of R. dauricum. In this study, an integrative analysis of the metabolome and transcriptome was conducted between R. dauricum var. albiflorum and R. dauricum. A total of nine anthocyanins and two proanthocyanidins were differentially accumulated between R. dauricum var. albiflorum and R. dauricum. A decrease in malvidin-, delphinidin-, cyanidin-, peonidin-, and petunidin-based anthocyanins and an increase in procyanidin A2 and procyanidin B2 were responsible for the white flowers of R. dauricum var. albiflorum. Furthermore, a total of 4376 differentially expressed genes (DEGs) were identified using transcriptome sequencing. Integrated analysis of the metabolome and transcriptome showed that 21 DEGs encoding 9 enzymes (PAL, C4H, CHS, CHI, F3H, F3'5'H, DFR, ANR, and UFGT) were identified as structural genes involved in anthocyanin and proanthocyanidin biosynthesis, and 15 MYBs and 10 bHLHs were the transcriptional regulators of the anthocyanin biosynthesis pathways in R. dauricum var. albiflorum. Our results deepen the understanding of variations in azalea flower color, which is helpful for identifying important genes in the genetic engineering of azalea shrubs.