Impact of Putrescine on essential oil composition, free polyamines content and expression of related genes (DXR and TPS2) in Thymus daenensis under drought stress

Background Thymus daenensis a medicinal plant native to Iran, produces essential oils rich in thymol and carvacrol, known for their strong antioxidant and antimicrobial properties. Exposure to drought conditions affects the production pathways of secondary metabolites, potentially contributing to improved stress tolerance in plants. Polyamines (PAs), particularly putrescine (Put), play a crucial role in mitigating drought stress by regulating stomatal behavior, improving osmotic adjustment, and modulating oxidative stress responses. This study aimed to investigate the effects of exogenous Put on T. daenensis under drought stress, focusing on gene expression related to terpenoid biosynthesis, secondary metabolite accumulation, and free PA content. Results Six-week-old T. daenensis plants were subjected to drought stress induced by 15% (w/v) polyethylene glycol (PEG) 6000, with or without foliar application of Put. Gene expression analysis (RT-qPCR), HPLC-based PA quantification, and GC-MS profiling of essential oil composition were performed. Put treatment significantly increased endogenous Put and spermidine content in drought-stressed plants, whereas no significant changes were observed in non-stressed plants. Furthermore, Put application upregulated TPS2 but downregulated DXR expression in drought-stressed plants. Additionally, the relative contents of γ -terpinene and p -cymene—precursors of thymol and carvacrol—increased, while the contents of thymol and carvacrol decreased following Put treatment. Conclusions Exogenous application of putrescine modulates the expression of key genes in the terpenoid biosynthetic pathway and alters essential oil composition in T. daenensis under drought stress, potentially contributing to the plant’s adaptive responses.