bHLH Family Transcription Factors: Molecular Switches in Plant Specialized Metabolism
Highlights What are the main findings? Plant bHLH factors regulate specialized metabolism through direct, cascade, or hybrid architectures centered on signal-responsive modules such as JA-JAZ-MYC. Cross-species comparison identifies six determinants of bHLH output: signal gating, pathway topology, promoter and partner logic, spatial competence, feedback regulation, and evidence context. What are the implications of the main findings? Effective metabolic engineering should match the intervention to the pathway bottleneck and combine transcription-factor tuning with tissue-specific control, precursor supply, transport, and storage capacity. This framework supports more precise improvement of crop defense, food quality, and medicinal-metabolite production while minimizing growth penalties and toxicity. Abstract Plant specialized metabolites connect genetic programs and environmental responses with ecologically and economically valuable natural products. Their accumulation is rarely constitutive, varying instead with tissue identity, developmental stage, stress exposure, hormone signaling, and cellular storage capacity. This review examines basic helix-loop-helix (bHLH) transcription factors as regulatory switch points in plant specialized metabolism, with emphasis on the jasmonate-JAZ-MYC module. In resting tissues, JAZ repressors constrain MYC/bHLH activity; after wounding, herbivory, pathogen challenge, or elicitation, jasmonoyl-isoleucine triggers COI1-dependent JAZ turnover, releasing MYC factors to bind E-box/G-box motifs, recruit coregulators such as MED25, and activate biosynthetic genes or downstream transcription-factor cascades. Plant lineages have repeatedly adapted this regulatory logic to control terpenoids, alkaloids, phenylpropanoids, flavonoids, glucosinolates, phytoalexins, and related metabolites. Comparative examples include Arabidopsis sesquiterpenes and glucosinolates, Taxus taxanes, Artemisia artemisinin, Catharanthus terpenoid indole alkaloids, Salvia phenolic acids and tanshinones, Ginkgo terpene trilactones, rice diterpenoid phytoalexins, and cotton gossypol. Across these systems, bHLH output depends on dimer choice, promoter grammar, chromatin accessibility, hormone crosstalk, partner transcription factors, and cell-type competence. Six shared principles emerge: signal gating, topology matched to pathway architecture, partner-dependent promoter decoding, spatial competence, feedback rheostats, and evidence-dependent transferability. We further discuss evidence standards, multi-omics-guided factor discovery, miRNA-mediated post-transcriptional control, and engineering strategies for crop defense, food quality, medicinal-metabolite production, and synthetic biology. Unlike pathway- or MYC2-centered surveys, this review organizes the literature within a direct–cascade–hybrid framework that integrates promoter grammar, spatial competence, storage anatomy, and an explicit evidence hierarchy.