Melatonin: a multifaceted regulator of root development, stress responses, and hormonal crosstalk in horticultural crops

Melatonin is increasingly recognized as a multifunctional signaling molecule involved in plant growth regulation and stress adaptation. Recent studies have revealed that melatonin plays a pivotal role in shaping root system architecture (RSA) by modulating root growth dynamics, lateral root formation, and root-microbe interactions. This review highlights emerging evidence that melatonin regulates RSA through complex crosstalk with phytohormones, reactive oxygen species, and stress-responsive signaling pathways, rather than acting solely as a growth regulator. Importantly, accumulating evidence indicates that melatonin functions as an integrative regulator of RSA by coordinating multiple hormone signaling pathways, including auxin, jasmonic acid, ethylene, cytokinins, salicylic acid, and abscisic acid, in a concentration- and context-dependent manner. We further distinguish the regulatory effects of melatonin on root growth and root architectural remodeling and summarize the dose-dependent actions of melatonin under abiotic stress conditions. Beyond hormonal regulation, melatonin enhances root nutrient acquisition by modulating ion transporters, maintaining ion homeostasis, and optimizing root system architecture, thereby improving nitrogen, phosphorus, potassium, and micronutrient uptake under stress conditions. Emerging evidence also suggests that melatonin may indirectly influence root-microbe interactions by reshaping root physiology, redox status, and hormonal balance, contributing to improved stress resilience. By integrating molecular, physiological, and developmental perspectives, this review provides a conceptual framework for understanding melatonin-mediated root system plasticity and positions melatonin as an integrative regulator of root system architecture that links hormonal crosstalk, nutrient acquisition, and stress adaptation, offering insights into its potential applications in crop stress resilience improvement.