Melatonin‐Mediated Modulation of Polyamines Enhances Drought Tolerance in Foxtail Millet ( Setaria italica L.)

ABSTRACT Melatonin have been proposed as a potential alternative in enhancing drought tolerance. However, the mechanisms underlying remain poorly understood. This study tested the hypothesis that melatonin improves drought tolerance by regulating polyamine metabolism. Hydroponically grown foxtail millet ( Setaria italica L.) seedlings were subjected to drought stress induced by polyethylene glycol (PEG‐6000, 10%), with or without melatonin application. The results showed that exogenous melatonin improved drought tolerance by improving water uptake, increasing root hydraulic conductivity (L pr ) and whole‐plant hydraulic conductance (K plant ) by 37% and 33%, respectively. Melatonin also significantly increased the activities of key enzymes involved in polyamine biosynthesis, specifically arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and S‐adenosyl‐ l ‐methionine decarboxylase (SAMDC), while concurrently decreasing the activities of polyamine degrading enzymes, namely polyamine oxidase (PAO) and diamine oxidase (DAO). Additionally, melatonin elevated the levels of putrescine (Put), spermidine (Spd), and spermine (Spm) in both roots and leaves. Transcriptome analysis further indicated that melatonin upregulated genes related to polyamine biosynthesis. To further verify the role of polyamines in melatonin‐induced drought tolerance, seedlings were treated with exogenous Spd or a polyamine biosynthesis inhibitor (dicyclohexylammonium sulfate, DCHA) under combined drought and melatonin conditions. Exogenous Spd produced effects similar to those of melatonin under drought stress, while DCHA abolished the melatonin‐induced drought tolerance and reduced its beneficial effects on root water absorption. These findings demonstrate that polyamines are involved in melatonin‐induced drought tolerance in foxtail millet, supporting the positive role of melatonin in modulating drought tolerance.