Mutually Regulated AP2/ERF Gene Clusters Modulate Biosynthesis of Specialized Metabolites in Plants.

APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) gene clusters regulate the biosynthesis of diverse specialized metabolites, including steroidal glycoalkaloids in tomato (Solanum lycopersicum) and potato (S. tuberosum), nicotine in tobacco (Nicotiana tabacum), and pharmaceutically valuable terpenoid indole alkaloids (TIAs) in Madagascar periwinkle (Catharanthus roseus). However, the regulatory relationships between individual AP2/ERF genes within the cluster remain unexplored. We uncovered intra-cluster regulation of the C. roseus AP2/ERF regulatory circuit, which consists of ORCA3, ORCA4, and ORCA5. ORCA3 and ORCA5 activate ORCA4 by directly binding to a GC-rich motif in the ORCA4 promoter. ORCA5 regulates its own expression through a positive auto-regulatory loop, and indirectly activates ORCA3. In determining the functional conservation of AP2/ERF clusters in other plant species, we found that GC-rich motifs are present in the promoters of analogous AP2/ERF clusters in tobacco, tomato, and potato. Intra-cluster regulation is evident within the tobacco NICOTINE2 (NIC2) ERF cluster. Moreover, overexpression of ORCA5 in tobacco and of NIC2 ERF189 in C. roseus hairy roots activates nicotine and TIA pathway genes, respectively, suggesting that the AP2/ERFs are functionally equivalent and are likely to be interchangeable. Elucidation of the intra-cluster and mutual regulation of transcription factor gene clusters advances our understanding of the underlying molecular mechanism governing regulatory gene clusters in plants.