Melatonin as a multifaceted stress protector in rice: Mechanisms, synergies, and knowledge gaps

Rice productivity, a cornerstone of global food security, is increasingly threatened by a spectrum of abiotic and biotic stressors, including heavy metal toxicity, salinity, drought, temperature extremes, flooding/water logging, nutrient deficiencies, and pathogens or pest infestations. Melatonin (N-acetyl-5-methoxytryptamine), also referred to as phytomelatonin, was first identified in plants in 1995 and has since emerged as a potent antioxidant and versatile signaling molecule. It plays a critical role in integrating hormonal networks and modulating stress responses in plants, including rice. Both endogenous and exogenously applied melatonin enhance rice tolerance to multiple stress conditions by improving photosynthetic efficiency, reinforcing antioxidant defense systems, maintaining ionic and osmotic homeostasis, and regulating growth and development processes. In the context of biotic stress, melatonin activates innate immune mechanisms, including modulation of defense genes, synthesis of phytoalexins, and fortification of structural barriers, thereby enhancing resistance to pathogens and insect herbivores. Notably, combinatorial applications of melatonin with silicon and nano-zero-valent iron have demonstrated synergistic effects, significantly augmenting stress resilience under complex environmental conditions. Despite these advancements, key knowledge gaps persist regarding mechanistic understanding of melatonin signaling, particularly its interaction with the OsPMTR1 receptor, as well as its efficacy under multi-stress field scenarios. Moreover, melatonin's functional outcomes are influenced by rice genotype and environmental context, underscoring the need for optimized application strategies (such as foliar spray, seed priming, and root drenching), and precise dosage calibration to maximize protective benefits while avoiding phytotoxic effects. This review synthesizes current insights into melatonin biosynthesis, signaling pathways, and its multifaceted roles in rice stress physiology, while identifying critical knowledge gaps and underscoring its potential as an integrative and sustainable strategy for advancing climate-resilient rice production.