Exogenous Melatonin Modulates Rice Responses to Phloem-Feeding Insect Stress

The white-backed planthopper (WBPH; Sogatella furcifera) is a destructive phloem-feeding hemipteran that significantly limits rice productivity by depleting assimilates and transmitting viral pathogens. Although melatonin is recognised as a multifunctional plant-signalling molecule, its defensive role against WBPH remains inadequately characterised. In this study, we demonstrate that exogenous melatonin application markedly enhances rice resilience to WBPH through coordinated regulation of growth, defence signalling, antioxidant activity and ionic homoeostasis. Melatonin increased seedling survival by up to 384% and reduced early WBPH colonisation by 93%, while delaying symptom onset under sustained pest pressure. Oxidative damage was mitigated by decreasing H₂O₂ and O₂•- accumulation by 54% and 23%, respectively, elevating relative water content by 38%, and reducing electrolyte leakage by 44%. Melatonin also stimulated anthocyanin biosynthesis via transcriptional activation of PAL, CHS, F3H, DFR and ANS, and promoted endogenous melatonin synthesis by upregulating TDC, T5H, ASMT and SNAT. Phytohormonal profiling revealed increased levels of abscisic acid (82%) and salicylic acid (SA, 29%), alongside activation of jasmonic acid and SA pathways through induction of LOX, AOS, AOC, PR1, PR2 and NPR1. Antioxidant capacity was enhanced via elevated activities of APX, CAT, POD, SOD, ABTS and DPPH. Furthermore, melatonin restored Ca²⁺ homoeostasis and stimulated GABA shunt metabolism, resulting in a 97% increase in succinate accumulation. These findings highlight melatonin's multifaceted role in conferring protection against WBPH by integrating physiological, biochemical and molecular defences, underscoring its potential as a bio-regulatory agent to enhance rice resistance against phloem-feeding insects.