Integrative metabolome and transcriptome analyses reveal the molecular mechanisms involved in cucurbitacin accumulation and bitterness in Luffa fruits

The fruits of Luffa in the Cucurbitaceae family are characterized by a bitter taste, which is mainly due to cucurbitacin. However, the types of cucurbitacins related to bitterness and the molecular mechanism of cucurbitacin accumulation during Luffa fruit growth and development are poorly understood. Here, we identified a total of 633 metabolites in Luffa , including alkaloids, flavonoids, and terpenoids, with 364 key metabolites showing significant differences between bitter and non-bitter varieties. Cucurbitacins B and D were significantly more abundant in bitter Luffa and showed an upward trend between non-bitter and extra-bitter varieties. Transcriptome analysis revealed 25,577 differentially expressed genes (DEGs) in these Luffa varieties, including the upregulation of biosynthetic pathways of terpenoids in bitter fruits. Integrative metabolite profiling and transcriptome analyses showed that LaCBS , encoding cucurbitadienol synthase, is a hub gene for the first committed step in cucurbitacin biosynthesis, located within gene clusters comprising LaP450 s and LaACT . In addition, two LaCBS genes are located within two gene clusters, alongside LaP450s and LaACT, in the interspecific hybrid of Luffa . A total of 746 DEGs were identified as transcription factors, including 103 LabHLH family members; two LabHLHs positively correlated to LaCBS and LaACT. Heat and abscisic acid stress activated the biosynthesis pathway of cucurbitacins. These findings help to elucidate the molecular mechanism of cucurbitacin biosynthesis in the bitter fruits of Luffa .