Revealing the key aromatic compounds in Malus 'Lollipop' flowers by transcriptome and metabolome

The ornamental crabapple Malus (M.) 'Lollipop' is renowned for its compact growth and fragrant flowers. This study aims to elucidate the biosynthesis molecular mechanism of volatile organic compounds (VOCs) across four developmental stages of the M. 'Lollipop' flowers using metabolomics and transcriptomics analyses. Gas chromatography-mass spectrometry (GC-MS) identified 29 VOCs (aliphatic derivatives, benzenes, and alkanes) in M. 'Lollipop' flowers. Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) analysis highlights 14 key differential aromatic compounds (VIP ≥ 1), featuring (Z)-3-hexen-1-yl acetate in stage 1, methyl benzoate in stage 2, benzyl alcohol and linalool in stage 3, and camphene and (Z)-3-hexen-1-ol in stage 4. (Z)-3-hexen-1-yl acetate was identified as a co-primary constituent in the four flowering stages, designated as a key and floral contributing metabolite (variable importance in projection (VIP) ≥ 1& odor activity value (OAV) ≥ 1). RNA sequencing revealed key genes including CAT, DXS, MVD, HMGCR, FDPS, and TPSc in camphene and linalool synthesis, aroA, ADT, PDT, PAL, BEBT1, SDR, 4CL, CNL, and BALDH for benzyl alcohol, benzaldehyde, and methyl benzoate production. And PLA2G, SPLA2, TGL4, LOX2S and ADH1 in (Z)-3-hexen-1-yl acetate and (Z)-3-hexen-1-ol synthesis. 24 transcription factors (TFs) were predicted to be closely linked to genes involved in VOC synthesis. The findings above deepen our comprehension of the floral scent in crabapple, laying a foundation for further investigations into their functions and potential industrial applications.