Secondary metabolites are products of plant adaptation to environmental conditions, and elucidating their functions can provide novel strategies for enhancing stress resilience in agriculture and ecology. This study investigated drought-responsive secondary metabolites in the relict tree species Fraxinus mandshurica under two drought treatments: D1 (natural soil drying to ∼30% moisture) and D2 (maintained at ∼30% moisture). D1 caused leaf wilting and curling without major growth changes, whereas D2 led to severe leaf yellowing, marked reductions in plant height (-10.90%) and dry weight (-28.06% to −38.38%), and root remodeling. Leaf metabolomics identified 546 metabolites, with 23 differentialy accumulated metabolites between D1 and its control and 14 between D2 and its control (VIP > 1.3 and p < 0.05). Integrated transcriptomic and metabolomic analyses showed enrichment of plant hormone signal transduction and phenylpropanoid pathways under drought treatments. Abscisic acid (ABA) content and the expression of its biosynthesis genes increased, whereas jasmonic acid (JA) levels declined. In the phenylpropanoid pathway, genes for flavonoid and coumarin biosynthesis were upregulated, with increased total flavonoid and coumarin content and significant esculin (coumarin) accumulation in leaves (454.20–1459.90%) and roots (218.02–616.67%). Exogenous application of 50 μg·mL⁻¹ esculin reduced water loss and improved drought tolerance in both F. mandshurica and Arabidopsis thaliana . These results reveal dynamic drought-responsive metabolites in F. mandshurica seedlings and identify esculin as a potential cross-species drought tolerance biostimulant. This study revealed D1 (natural soil drying to ∼30% moisture) caused no significant growth impairment in Fraxinus mandshurica seedlings, while D2 (maintained soil moisture ∼30%) significantly reduced growth and triggered root remodeling. Leaf metabolomics detected 23 differential secondary metabolites between D1 and its control, 14 between D2 and its control. Integrated transcriptomic and metabolomic analyses revealed drought significantly disrupted plant hormone signal transduction and the phenylpropanoid pathway. Notably, drought sharply increased esculin content in Fraxinus mandshurica , and exogenous esculin application enhanced drought tolerance. • Secondary metabolites modulated by drought intensity were identified in Fraxinus mandshurica seedlings. • Drought intensity-dependent modulation primarily enriched in phytohormone signaling and phenylpropanoid biosynthesis pathway. • Sustained drought stress significantly induced esculin accumulation in leaves and roots of Fraxinus mandshurica seedlings. • Esculin significantly enhances drought tolerance in seedlings of both Fraxinus mandshurica and Arabidopsis thaliana.
Wang et al. (Sun,) studied this question.