Combined drought and high temperature stress enhances saikosaponin biosynthesis in Bupleurum Chinense through multi-level regulatory mechanisms.

backgroundAs an important medicinal plant, the synthesis of active ingredient saikosaponin in Bupleurum chinense DC. is significantly influenced by environmental factor stress. However, there is limited understanding of how secondary metabolites in medicinal plants change under the multiple combined stresses.

resultsThis study investigated the changes in saikosaponins content in Bupleurum chinense under drought combined with temperature stress, as well as the potential molecular and ecological mechanisms involved. The experimental plants were divided into four groups: control (CK), drought with low temperature (DL), drought with medium temperature (DM), and drought with high temperature (DH). DL, DM, and DH inhibited plant height and root length, with DH exhibiting the strongest inhibitory effect. The degree of stress followed the order DH > DM > DL > CK. B. chinense adapted to the combined stresses by modulating osmotic regulatory substances and protective enzyme activities. Under combined stress, five enzymes in the terpenoid synthesis pathway showed positive changes. The content of saikosaponins, specifically saikosaponin a and saikosaponin d, increased significantly under DH treatment in the short term. On day 4 of treatment, their levels reached 4.52, 2.07, and 1.54 mg/g, representing increases of 40.82%, 32.76%, and 36.97%, respectively, compared to CK. Abscisic acid levels under DL, DM, and DH were 12.90-, 6.38-, and 16.27-fold higher, respectively, than those under CK. High-quality transcriptome sequencing revealed active changes in gene expression profiles. Weighted gene co-expression network analysis identified the turquoise module genes (12,753 target genes), which were strongly correlated with physiological indices, plant hormones, functional enzymes, and saikosaponins. Saikosaponins synthesis was found to be regulated by multiple transcription factors and functional genes, with distinct regulatory networks governing abscisic acid and saikosaponins.

conclusionsOur findings demonstrate that the synthesis and accumulation of saikosaponins exhibit a "short-term promotional effect" under combined stress, regulated by multiple factors. This study elucidates the molecular and ecological mechanisms underlying the rapid accumulation of saikosaponins and provides technical insights for ecological strategies to support B. chinense high-quality cultivation.