Integrating stress responses and secondary metabolism: the regulatory functions of histone deacetylases in medicinal plants.

Medicinal plants are an important source of pharmacologically active secondary metabolites. However, their cultivation is often constrained by environmental stresses such as extreme temperatures, drought, salinity and ultraviolet radiation. These stresses reshape the biosynthesis of secondary metabolites including alkaloids, phenolic compounds and terpenoids through coordinated signaling, transcriptional reprogramming and chromatin remodeling. Epigenetic regulation provides a reversible mechanism by which plants translate environmental cues into stable but adjustable gene-expression states. Among epigenetic regulators, histone deacetylases have emerged as important modulators of stress responses and specialized metabolism. This review summarizes current knowledge of histone deacetylase families in plants, with emphasis on their roles in stress signaling and secondary metabolite biosynthesis in medicinal plants. We discuss evidence from phenylpropanoid, terpenoid and alkaloid-related pathways, and distinguish transcriptional mechanisms from post-translational regulation of metabolic enzymes. We further propose an integrative framework in which histone deacetylases function as regulatory nodes that connect hormone signaling, stress-responsive transcription factors, chromatin accessibility and metabolic flux. This framework provides insight into plant adaptation and suggests potential strategies for improving the yield and stability of bioactive compounds in medicinal plants.