Comparative transcriptome analysis reveals abscisic acid-induced bHLH transcription factors involved in saikosaponin biosynthesis in <i>Bupleurum chinense</i> DC

<i>Bupleurum chinense</i> DC. a medicinal plant valued for saikosaponins (SSs) with antipyretic and hepatoprotective properties, faces constrained SS biosynthesis mediated by abscisic acid (ABA) during growth. Basic helix-loop-helix (bHLH) transcription factors (TFs) are hypothesized to participate in ABA signaling cascades, but their mechanistic role in SS regulation remains undefined. In this study, 20 differentially expressed <i>BcbHLH</i> genes were identified by transcriptomic profiling of ABA-induced hairy roots, with four MYC-family candidates (<i>BcbHLH1-BcbHLH4</i>) demonstrating ABA-responsive regulatory potential. ABA exposure (100 or 200 μmol/L, 24-72 h) induced dose-dependent SS reduction, while correlation analyses revealed coordinated expression between <i>BcbHLH1-BcHMGR</i> (<i>r</i> = 0.62) and <i>BcbHLH4-BcBAS</i> (<i>r</i> = 0.78), pinpointing these TFs as critical nodes in SS pathway modulation. Tissue-specific profiling showed predominant <i>BcbHLH</i> expression in stems and young leaves, with nuclear localization confirming their transcriptional regulatory organelles. BcbHLH3/4 exhibited transcriptional activation activity in the MYC_N domain, while molecular docking predicted 11th Arginine in the HLH domain as essential for G-box DNA binding. Collectively, our findings suggest that BcbHLH1-BcbHLH4 may serve as potential switches for fine-tuning ABA responsiveness in SS biosynthesis. Strategic manipulation of <i>BcbHLH</i> activity through genetic engineering approaches such as CRISPR-based editing or overexpression could alleviate ABA-mediated biosynthetic repression. Furthermore, precision engineering of the critical functional domain in BcbHLH could enhance promoter-binding activity to target genes and improve SS biosynthesis efficiency. These findings provide a reference framework for harnessing transcriptional regulators to optimize SS production in <i>Bupleurum chinense</i> DC.