Spermidine supplementation protects photosynthesis under drought stress by increasing expression of antioxidants and growth in Phaseolus vulgaris L

• PEG induced drought stress significantly reduced the growth and physio-biochemical attributes of P. vulgaris L. • Foliar spraying of Spermidine attenuates the effects of drought stress by enhancing the growth and protects the photosynthetic capacity of P. vulgaris L. • Spermidine application enhances the activities of antioxidant as well as polyamine biosynthesis enzymes and lowered the levels of MDA, H 2 O 2 and Pro. • Antioxidant enzyme genes were upregulated and endogenous spermidine content elevated under Spermidine supplementation. Drought is one of the predominant climatic threats to farming that severely limits plant growth, survival and yields worldwide. Spermidine (Spd), a type of natural polyamine (PAs), has emerged as a potential master to counteract abiotic stresses in plants. However, the molecular mechanisms underlying the changes to physiological and biochemical attributes induced by Spd supplementation in drought stress tolerance of common bean ( Phaseolus vulgaris L.) are poorly understood. Therefore, this study was carried out to decipher the role of foliar spraying of Spd at 0, 0.25, 0.50, 0.75 and 1.00 mM in mitigation of PEG-6000 induced drought stress responses in P. vulagris . Application of 0.75 mM Spd maximally enhanced photosynthesis, PSII activity, growth, and reduced oxidative stress. Plants supplemented with Spd showed a substantial decrease in the ROS accumulation, induced the activities of peroxidase (POD), superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APOX). Spd supplementation also modulates the activity of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) enzymes, and increased the content of proline, ascorbic acid and reduced glutathione (GSH). Further, Spd treatment improved the expression of antioxidant genes that was related to protection of photosynthetic capacity under drought stress. The inclusion of Spd biosynthetic metabolic inhibitors, dicyclohexylamine (DCHA), resulted in additional oxidative stress and reduced PSII and growth attributes under drought stress. This study deciphers how Spd supplementation coordinates alleviation responses under drought stress by targeting interconnected physio-biochemical and molecular defense mechanisms, highlighting Spd’s potential as a novel stimulant molecule to enhance crop resilience in water-deficient conditions.