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Background  Birch ( Betula platyphylla  Suk.) contains triterpenoids with anti-HIV and anti-tumor pharmacological activities. However, the natural abundance of these triterpenoids is low, and their chemical synthesis is costly. Transcription factors have the ability to regulate the metabolite pathways of triterpenoids via multi-gene control, thereby improving metabolite yield. Thus, transcription factors have the potential to facilitate the production of birch triterpenoids. Plant bHLH (basic helix-loop-helix) transcription factors play important roles in stress response and secondary metabolism.    Results  In this study, we cloned two genes,  BpMYC4  and  BpbHLH9 , that encode bHLH transcription factors in  Betula platyphylla  Suk. The open reading frame (ORF) of  BpMYC4  was 1452 bp and encoded 483 amino acids, while the ORF of  BpbHLH9  was 1140 bp and encoded 379 amino acids. The proteins of  BpMYC4  and  BpbHLH9  were localized in the cell membrane and nucleus. The tissue-specific expression patterns revealed that  BpMYC4  expression in leaves was similar to that in the stem and higher than in the roots. The expression of  BpbHLH9  was higher in the leaves than in the root and stem. The expressions of  BpMYC4  and  BpbHLH9  increased after treatment with abscisic acid, methyl jasmonate, and gibberellin and decreased after treatment with ethephon. The promoters of  BpMYC4  and  BpbHLH9  were isolated using a genome walking approach, and 900-bp and 1064-bp promoter sequences were obtained for  BpMYC4  and  BpbHLH9 , respectively. The ORF of  BpbHLH9  was ligated into yeast expression plasmid pYES3 and introduced into INVScl and INVScl1-pYES2-SS yeast strains. The squalene and total triterpenoid contents in the different INVScl1 transformants decreased in the following order INVScl1-pYES-SS-bHLH9 > INVScl1-pYES3-bHLH9 > INVScl1-pYES2- BpSS > INVScl-pYES2. In BpbHLH9 transgenic birch, the relative expression of the genes that encodes for enzymes critical for triterpenoid synthesis showed a different level of up-regulation compair with wild birch(control), and the contents of betulinic acid, oleanolic acid and betulin in bHLH9–8 transgenic birch were increased by 11.35%, 88.34% and 23.02% compared to in wild birch, respectively.    Conclusions  Our results showed that the modulation of  BpbHLH9  by different hormones affected triterpenoid synthesis and triterpenoid contents. This is the first report of the cloning of  BpbHLH9 , and the findings are important for understanding the regulatory role of  BpbHLH9  in the synthesis of birch triterpenoids.    Electronic supplementary material  The online version of this article (10.1186/s12870-017-1150-z) contains supplementary material, which is available to authorized users.

Cloning and expression of BpMYC4 and BpbHLH9 genes and the role of BpbHLH9 in triterpenoid synthesis in birch