Integrated Transcriptomic Profiling Reveals Candidate Genes for Huperzine A Biosynthesis in Huperzia serrata

Huperzine A (HupA) is a natural Lycopodium alkaloid known for its potent neuroprotective properties through the inhibition of acetylcholinesterase. Nevertheless, the limited understanding of its biosynthesis restricts its broader application. This study integrates full-length and second-generation transcriptomes with the quantification of HupA and its precursor, huperzine B, across various tissues of Huperzia serrata, the primary source plant. By employing phylogenetic clustering, expression profiling, and correlation analysis between gene expression and metabolite abundance, we identified 71 candidate genes from seven enzyme families potentially involved in the synthesis of the HupA backbone, including lysine/ornithine decarboxylases, copper amine oxidases (CAOs), chalcone synthases, and cytochrome P450 monooxygenases. Additionally, 28 genes from two families were identified for modification reactions, specifically 2-oxoglutarate/Fe(II)-dependent dioxygenases and caffeoyl shikimate esterases. Comparative analysis between young and mature leaves revealed 3801 genes with higher expression in young leaves, with 84 showing a high correlation with HupA content across seven families. Protein–protein interaction network analysis indicated possible interactions with transcription factors from the MYB, NF-YC, GRAS, ERF, BHLH, and SAP families. Functional validation of two candidate CAOs in planta confirmed their catalytic roles in amine/alkaloid metabolism. This study provides a theoretical foundation and a set of candidate genes for elucidating the biosynthetic pathway of HupA and related alkaloids in H. serrata.