Transcriptome profiling of Bupleurum chinense DC. root provides new insights into the continuous inflorescence removal induced improvements to root growth and saikosaponin biosynthesis

The root of Bupleurum chinense DC. is the major source of Radix Bupleuri and widely used for the treatment of inflammatory disorders and infectious diseases in Asia. However, the excessive reproductive growth of B. chinense DC. strongly interferes with its root biomass accumulation and secondary metabolite biosynthesis. Although inflorescence removal or disbudding has been used to improve the yield and quality in rhizome crops, the molecular mechanism is unknown. Here, we compared the differences between the control groups and the continuous inflorescence removal (CIR) treatment groups of B. chinense DC. on physiological, chemical and molecular levels. The results of 2-year field experiments showed that the primary root length and biomass accumulation, as well as the saikosaponin a and d (SS-a and SS-d) contents were all increased under CIR treatment. Illumina sequencing of cDNA libraries prepared from the roots of control groups and CIR groups during four flowering stages identified 172, 243, 1974 and 3024 differentially expressed genes (DEGs) respectively. Most DEGs were involved in brassinolide (BR) signal transduction, jasmonic acid (JA) signal tranduction, 2-methyl-d-erythritol-4-phosphate pathway/Mevalonate (MVA/MEP) pathways and SS pathways. There were 295 differentially expressed transcription factors (TFs) including members of the MYB, bHLH, bZIP, and NAC families that also induced by CIR treatment. Irrigation experiments further showed that 0.05 mg/L exogenous BR improved the root fresh weight and the root/shoot ratio of B. chinense DC. mainly through cyclin D3, BIR1, BSK1 and ARFs; the SS-a and SS-d contents in the roots of B. chinense DC. were increased under 100 mM exogenous Me-JA treatment, most genes involved in MVA, MEP and SS pathways were activated. In all, this study not only provides new insights into the fundamental mechanism underlying the poorly studied inflorescence removal process in B. chinense DC. and other rhizome crops, but also represents an important resource for future research on gene functions during CIR treatment or the reproductive stage.