SmCAD4-mediated lignin biosynthesis and improved root architecture are crucial for drought tolerance in Salvia miltiorrhiza.

key messageSmCAD4 orchestrates a synergistic defense against drought by simultaneously fortifying tissues with lignin and optimizing the root system for enhanced water uptake, a novel integrated mechanism. Drought stress is a primary environmental factor limiting the productivity and medicinal quality of crops such as Salvia miltiorrhiza. While cell wall lignification represents a fundamental adaptive strategy, the specific transcriptional networks coordinating these responses under water deficit remain largely uncharacterized. In this study, we identify the cinnamyl alcohol dehydrogenase gene SmCAD4 as a pivotal regulator of drought resilience in S. miltiorrhiza. Utilizing DNA affinity purification sequencing (DAP-seq) alongside molecular interaction assays including Y1H, Dual-LUC, and EMSA, we establish that SmCAD4 is a direct downstream target of the transcription factor SmDof32. SmDof32 specifically binds to a conserved AAAAG motif within the distal promoter region of SmCAD4 to activate its transcription, thereby defining a complete stress-responsive module. Overexpression of SmCAD4 significantly enhances plant survival under severe water deficit by triggering a synergistic dual mechanism. Biochemically, SmCAD4 promotes lignin deposition in vascular tissues to fortify cellular structures and improve water retention; morphologically, it drives the development of a robust root system architecture with increased length and branching for enhanced water acquisition. Furthermore, the heterologous expression of SmCAD4 in Arabidopsis thaliana consistently confers improved osmotic and drought tolerance, confirming its functional potential across different plant systems.Query This work deciphers a novel molecular pathway integrating biochemical reinforcement with morphological adaptation, highlighting the SmDof32-SmCAD4 module as a prime target for the genetic improvement of stress resilience in medicinal plants and broader agricultural crops.