Transcriptome and metabolome reveal key genes in the phenylpropane pathway to regulate the floral fragrance of Rhododendron fortunei

Abstract Background: Floral aroma is an important plant trait and it is one of the main traits of Rhododendron varieties which affect their ornamental value. The volatile compounds of floral aroma are secondary metabolites. In order to reveal the key genes of the phenylpropane pathway, which regulates the fragrance of Rhododendron fortunei, we performed a comprehensive transcriptome and metabolome analysis of the petals of the petals of two alpine rhododendrons from two different varieties: the scented Rhododendron fortunei and the non-scented Rhododendron ‘Nova Zembla’. Results: Transcriptomic and qRT-PCR results showed that nine candidate genes were highly expressed in R. fortunei, but down-regulated in R. ‘Nova Zembla’. The correlation analysis of candidate genes and metabolites revealed nine key genes involved in the regulation of floral aroma in the phenylpropane pathway. Among them, EGS was significantly positively correlated with various volatile benzene/phenylpropane compounds, and significantly negatively correlated with various non-volatile compounds; CCoAOMT, PAL, C4H,and BALDH were significantly negatively correlated with various volatile benzene/phenylpropane compounds, and significantly positively correlated with various non-volatile compounds. CCR, CAD, 4CL, and EGSwere significantly negatively correlated with various benzene/phenylpropane compounds. The validation of RfSAMT proved that RfSAMT gene regulated the synthesis of aromatic substances in R. fortunei. Conclusion: The findings of this study indicated that key candidate genes and metabolites involved in the biosynthetic pathway of phenylpropane may regulate the fragrance of R. fortunei. This lays a foundation for further research on the molecular mechanism of fragrance synthesis in the genus Rhododendron.