Jasmonate-responsive group IX AP2/ERF transcription factors control the biosynthesis of benzylisoquinoline alkaloids

Benzylisoquinoline alkaloids (BIAs) are a class of compounds produced by various plants, but the genetic factors controlling their synthesis remain incompletely understood. Jasmonate (JA)-responsive group IX APETALA2/Ethylene Responsive Factor (AP2/ERF) transcription factors (TFs) have been identified as regulators of specialized plant metabolism, including BIAs biosynthesis. However, the specific roles of these TFs in BIAs production have not been well-characterized.

In a study involving Coptis japonica, five novel group IX AP2/ERF TFs, named Benzylisoquinoline alkaloid Jasmonate-responsive AP2/ERF (BJE1-5), were isolated. Phylogenetic analysis showed that BJE proteins form separate subclades from group IXa, which comprises well-known AP2/ERF TFs involved in alkaloid biosynthesis. Transient expression analyses in C. japonica protoplasts revealed that certain BJEs, notably CjBJE3 and CjBJE5, positively regulated BIA biosynthetic genes via a mutual regulatory network among BJE members.

Moreover, the expression of CjBJE3 was regulated by CjbHLH1, a unique-type basic helix-loop-helix (bHLH) transcription factor exclusive to BIA-producing plants. When heterologously expressed in cultured Eschscholzia californica cells, CjBJE3 and CjBJE5 significantly boosted overall BIA production, particularly by upregulating end-product benzophenanthridine BIAs, which implicates several unidentified biosynthetic genes within the genome.

These results suggest that BIA-producing species possess a specialized regulatory network involving CjbHLH1 and BJE TFs, offering valuable insights for the discovery of novel biosynthetic enzymes.