An ABA-Responsive SmAREB1/SmbZIP1-SmBBX24 Transcriptional Cascade Regulates Phenolic Acid Biosynthesis in Salvia miltiorrhiza.

Danshen (Salvia miltiorrhiza) contains two major classes of bioactive constituents, including phenolic acids and tanshinones. These compounds are predominantly accumulated in the roots and hold substantial pharmacological and economic value. Abscisic acid (ABA), a crucial phytohormone regulating stress responses, is known to influence the biosynthesis of these metabolites, but the underlying molecular mechanisms remain largely elusive. Here, we identified and characterized an ABA-induced B-box (BBX) transcription factor, SmBBX24, which functions as a transcriptional activator. Our analyses indicated that SmBBX24 directly binds to the promoter of SmPAL1, a key gene in the phenolic acid biosynthetic pathway, and activates its expression, thereby promoting phenolic acid biosynthesis. Furthermore, we demonstrated that SmbZIP1 and SmAREB1, which are two ABA-responsive positive regulators of phenolic acid biosynthesis, synergistically activate SmBBX24 expression by binding to its promoter. Crucially, this synergistic activation is mediated by the physical formation of an SmbZIP1-SmAREB1 complex, which significantly enhances their transcriptional activity on the SmBBX24 promoter. Our findings reveal a novel ABA-responsive transcriptional cascade: SmAREB1/SmbZIP1 complex directly activates SmBBX24, which subsequently activates SmPAL1 to precisely modulate phenolic acid biosynthesis in S. miltiorrhiza roots. This study provides new mechanistic insights into ABA-mediated regulation of specialized metabolite biosynthesis and establishes a basis for future molecular breeding to boost active compound production in S. miltiorrhiza.