Overexpression of BcbZIP134 negatively regulates the biosynthesis of saikosaponins

Jiao Xu,Su-Rui Wu,Chun Sui,Zhan-Yu Ge,Yan-Hong Xu,Jian-He Wei

doi:10.1007/s11240-019-01571-0

Abstract

Chaihu, the dried roots of several Bupleurum species, are widely used as herbal drugs. Triterpene saponins, as saikosaponins (SSs) in Chaihu, exhibit a wide range of pharmacological actions, including significant anti-inflammatory, immunomodulatory, hepatoprotective, antioxidant, anti-tumor and antiviral activities. To explore the regulatory biosynthesis of SS in Bupleurum chinense roots, we screened putative transcription factor (TF) genes from transcriptome data. Here, we report one of the bZIP TF genes, named BcbZIP134, which belongs to bZIP G subfamily. The expression of BcbZIP134 was elevated in MeJA-treated adventitious roots of B. chinense. No obvious differential expression of BcbZIP134 was detected in tissues (root, stem, leaf, flower and fruit) of B. chinense. Overexpressing BcbZIP134 in B. chinense hairy roots revealed that the content of the main SS monomers a, c and d were decreased compared to control hairy roots. Furthermore, the contents of a few other minor SS monomers were also affected when compared to the controls. The results indicate that BcbZIP134 may play a negative regulatory role in SS biosynthesis.

Highlights

Saikosaponins (SSs), which are triterpene saponins, are the main active components in Radix Bupleuri, the roots of some species from the genus Bupleurum
To identify candidate transcription factor (TF) genes that may be involved with SS biosynthesis (The putative biosynthetic pathway presented in our previous report (Sui et al 2011)), we first analyzed the category of the TFs based on their sequences obtained from the transcriptome data of B. chinense and B. scorzonerifolium
A total of 26 TF genes were assayed for their expression profiles, and the results were combined with the SSs content assays in MeJA-treated B. chinense adventitious roots

Summary

Introduction

Saikosaponins (SSs), which are triterpene saponins, are the main active components in Radix Bupleuri, the roots of some species from the genus Bupleurum. AaWRKY1 bound to the W-box of the promoter of the ADS gene and activated its expression (Ma et al 2009). In Medicago truncatula, two homologous jasmonate-inducible TFs of the basic helix-loop-helix family (bHLH), Triterpene saponin biosynthesis activating regulator (TSAR1) and TSAR2, were found directly binding to the N-box in the promoter of the gene encoding 3-hydroxy-3-methylglutaryl-coenzyme A reductase 1 (HMGR1) to enhance triterpene saponin biosynthesis (Mertens et al 2016). Another bHLH TF in glycyrrhiza uralensis, GubHLH3, was shown to regulate the expression of soyasaponin biosynthetic genes positively (Tamura et al 2018)

Methods

Results

Discussion

Conclusion