Melatonin Buffers Microplastic-induced Oxidative Stress in Rice by Restoring Redox Balance, Antioxidants Reprogramming and Phytohormone Homeostasis

Microplastic (MP) contamination poses an emerging threat to crop productivity. It is essential to find out cost-effective strategies to mitigate injurious effects of MPs on crops. We investigated the comparative phytotoxicity of four different microplastics (MPs, 4 mg L^− 1) i.e. polyethylene (PE), polyurethane (PU), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) as well as evaluated the protective role of melatonin (MEL, 6 mg L^− 1) in rice grown hydroponically. MPs and MEL in selected doses were added to individual glass containers having rice plants and various physiological and biochemical traits were recorded. All MPs significantly reduced biomass, photosynthetic pigments and gas exchange attributes. PE emerged as most hazardous MPs for rice plants. PE-MPs, decreased shoot and root biomasses by 61% and 58%, respectively, compared to the control. PE, PU, PET, and PVC also suppressed Pn significantly up to 52, 45, 36, and 21%, respectively. MP in root zone increased malondialdehyde (117%) and H₂O₂ levels (37%), altered antioxidant activities, disrupted osmolyte balance, and suppressed key defense-related metabolites. Hormonal homeostasis was markedly affected by all MPs, characterized by elevated abscisic acid (73% due to PE) and reduced indole-3-acetic acid and gibberellic acid levels, alongside increased (293% by PE) plasma membrane H⁺-ATPase activity. Exogenous MEL application substantially alleviated MP-induced toxicity. The MEL-treated plants were strongly associated with higher activities of antioxidant enzymes (SOD, CAT, POD, GR), improved redox balance (GSH/GSSG), enhanced osmo-protection and increased stress-related protective metabolites. PC1 clearly separated MP-only treatments from MP + MEL treatments, demonstrating that this axis represents a physiological stress-to-recovery gradient. PC2 primarily represented a hormonal and metabolic regulation dimension. Overall, these findings demonstrate that melatonin effectively buffers MP-induced stress in rice and emerge as a protective regulator to sustain crop performance under microplastic-contaminated environments.