Integrated metabolomics and transcriptomics analysis to characterize the formation mechanism of purple in Michelia spp. petals

Michelia are ornamental trees that belong to the Magnoliaceae family. The only species in the genus that exhibits purple flowers is Michelia crassipes, which has garnered significant interest. However, the mechanism underlying purple flower formation remains poorly understood, restricting its usage as an ornamental plant. We analysed transcriptomic and metabolomic data from four different colour of Michelia plants (M. crassipes, Michelia 'XiangFei', Michelia 'HuaHaoYueYuan', and M. figo) with flowers ranging from fuchsia to Indian red, rosy brown, and dark khaki to investigate the mechanism underlying the formation of purple petals in the Michelia flower. The results revealed a strong correlation between total anthocyanin content (TAC) and the formation of purple petals in Michelia. M. crassipes exhibited the highest TAC content (4.31 mg/g) and the colour closest to purple petals, M. 'XiangFei' (0.52 mg/g) and M. 'HuaHaoYueYuan' (0.39 mg/g) followed with lower TAC, and M. figo displayed the lowest TAC (0.13 mg/g). Metabolomic analysis detected 279 flavonoids, of which 24 key metabolites were confirmed to be the main contributors to the purple phenotype. Cyanidin 3-o-rutinoside, cyanidin 3-o-glucoside, delphinidin 3-o-rutinoside 7-o-glucoside, peonidin 3-o-rutinoside, and pelargonidin 3-o-rutinoside were responsible for the deep co -pigmentation of Michelia petals. In total, 22,997 differently expressed genes (DEGs) were identified using transcriptome analysis. Among these, 46 structural genes encoding enzymes (4CL, CHS, CHI, F3H, and DFR, among other) and seven transcription factors from gene families such as bHLH, bZIP, GeBP, MADS-MIKC, MYB, NAC and SNF2, were identified as candidate regulators responsible for anthocyanin accumulation and purple formation in Michelia. In addition, multiple members of MYB, bZIP and bHLH transcription factor families showed significant correlations with the 24 key metabolites. This study provides insight into the regulatory mechanism underlying the purple colour formation of Michelia and identifies several metabolites and potential genes that serve as a theoretical basis for the genetic improvement of Michelia.