Integrated Physiological and Transcriptomic Analysis Reveals Transcription Factors Are Crucial for Melatonin-Mediated Drought Tolerance in E. ulmoides.

Eucommia ulmoides is a Chinese herbal medicine, and much attention has been paid to its tolerance mechanism under stress conditions. Among them, drought is a severe stress that affects the plant's growth and development. Here, we assessed the protective efficiency of different concentrations of melatonin in the leaves of Eucommia ulmoides under drought stress. Our study revealed how exogenous melatonin enhanced drought tolerance in E. ulmoides through integrated physiological and molecular mechanisms. Melatonin preserved photosynthetic capacity by upregulating genes like PsbS, stabilizing PSII, scavenging ROS, and activating antioxidant enzymes. Moreover, melatonin promoted the reconstruction of redox homeostasis by mediating MAPK and ABA signal transduction and endogenous hormone crosstalk signals to balance ROS homeostasis and stress gene expression. It also differentially regulated MYC transcription factors (e.g., EuMYC2/8), redirected jasmonic acid signaling from root growth to stress adaptation, and optimized carbon metabolism by promoting starch-to-sugar conversion and enhanced the phenylpropanoid flux, reinforcing cell walls and antioxidant defenses. Our results provide new insights into the morphological, physiological, and transcriptional responses in the leaves of E. ulmoides for drought stress and reveal the molecular mechanism of exogenous melatonin in improving the drought resistance ability in E. ulmoides.