Metabolic and transcriptomic analysis related to flavonoid biosynthesis during the color formation of Michelia crassipes tepal

Michelia crassipes is the only plant with purple flowers amongst Michelia species, and its tepals exhibit an obvious color change from green to purple. In this study, a combination of metabolic and transcriptomic analyses was conducted at three stages of tepals in Michelia crassipes: green tepal, purple spot-containing tepal, and totally purple tepal. Several classes of flavonoid compounds were detected and cyanidin 3-rutinoside and delphinidin 3-glucoside were the major anthocyanins underlying the purple color formation, along with co-pigmentation of flavone compounds represented by luteolin derivatives and flavonol compounds represented by kaempferol and quercetin derivatives. Transcriptome analysis revealed up-regulation of genes encoding enzymes involved in the conversion of phenylpropanoid for flavonoid biosynthesis in Stage 1 vs. Stage 2, whereas up-regulation of most flavonoid biosynthesis genes was observed in Stage 1 vs. Stage 3. MYB, bHLH, and WD40 isoforms, as well as other classes of transcriptional factors, also exhibited differential expression. In addition, differentially expressed genes putatively related to the transport of flavonoids were also identified. The results of the current study provide insight into the regulatory mechanism underlying the color transition from green to purple in Michelia crassipes tepals and describe a complicated network involving PAL, transporter genes, and transcription factors, specifically responsible for the emergence of purple color in Stage 1 vs. Stage 2.