Transcriptome Analysis Reveals Candidate Genes Involved in Anthocyanin Biosynthesis in Flowers of the Pagoda Tree ( Sophora japonica L.)

The flowers of pagoda tree (Sophora japonica L.) are white, which limited its ornamental value and caused few researches on its flower color and anthocyanin accumulation. A pink flower mutant (PM) was discovered, which provided an opportunity to investigate anthocyanin biosynthesis in pagoda tree flowers. In this study, the anthocyanin and flavone/flavonol contents in petals of PM and wild type (WT; white flower) were measured. The content of total anthocyanins was significantly higher in the PM than in the WT at four floral stages (S1–S4). There were no significant differences of total flavonol content between these two varieties at each floral stage, indicating that the flavone/flavonol content did not influence flower color, while anthocyanin was a key factor to determine pink flower coloration. Then, transcriptome analysis was performed by using the petals of PM and WT at S1–S4. Differentially expressed genes encoding chalcone synthase, flavanone 3-hydroxylase, flavonol synthase, flavanone 3′-hydroxylase, dihydroflavonol-4-reductase, anthocyanidin synthase, and UDP-glucose: flavonoid-3-O-glucosyltransferase were identified. Importantly, we found that SjDFR2 might be the key gene in anthocyanin accumulation in petals. Transcription factors, SjPAP1, SjMYB111, SjMYB1b, SjMYBL2, SjTT8, and WD40 protein SjTTG1, might play critical roles in regulation of anthocyanin accumulation in petals. Moreover, regulatory factors involved in the phytohormone signal pathway (SjIAA19, SjIAA27, SjAXR3, SjARF20, and SjERS1), and light signal pathway (SjHY5), were also screened out. These results will provide some new insights into the control of flower color in pagoda tree, as well as some candidate genes to breed novel flower colors using molecular methods.