Comparative transcriptome analyses reveal genes related to pigmentation in the petals of a flower color variation cultivar of Rhododendron obtusum

Rhododendron is an important woody ornamental plant, and breeding varieties with different colors is a key research goal. Although there have been a few reports on the molecular mechanisms of flower colors and color patterning in Rhododendron, it is still largely unknown what factors regulate flower pigmentation in Rhododendron.In this study, the flower color variation cultivar 'Yanzhi Mi' and the wild-type (WT) cultivar 'Dayuanyangjin' were used as research objects, and the pigments and transcriptomes of their petals during five flower development stages were analyzed and compared. The results showed that derivatives of cyanidin, peonidin and pelargonidin might be responsible for the pink color of mutant petals and that the S2 stage was the key stage of flower color formation. In total, 412,910 transcripts and 2780 differentially expressed genes (DEGs) were identified in pairwise comparisons of WT and mutant petals. GO and KEGG enrichment analyses of the DEGs showed that 'DNA-binding transcription factor activity', 'Flavonoid biosynthesis' and 'Phenylpropanoid biosynthesis' were more active in mutant petals. Early anthocyanin pathway candidate DEGs (CHS3-CHS6, CHI, F3Hs and F3'H) were significantly correlated and were more highly expressed in mutant petals than in WT petals in the S2 stage. An R2R3-MYB unigene (TRINITY_DN55156_c1_g2) was upregulated approximately 10.5-fold in 'Yanzhi Mi' petals relative to 'Dayuanyangjin' petals in the S2 stage, and an R2R3-MYB unigene (TRINITY_DN59015_c3_g2) that was significantly downregulated in 'Yanzhi Mi' petals in the S2 stage was found to be closely related to Tca MYB112 in cacao.Taken together, the results of the present study could shed light on the molecular basis of anthocyanin biosynthesis in two Rhododendron obtusum cultivars and may provide a genetic resource for breeding varieties with different flower colors.