Melatonin Enhances Peanut Productivity by Enriching Root-Associated Nitrogen-Fixing Bacteria

Melatonin, a pleiotropic phytohormone, is widely recognized as a promising bio-stimulant, yet its integrative effects on root development, yield gain, and microbiome assembly in legumes remain underexplored. In this study, we investigated the effects of melatonin seed treatment across three peanut genotypes, focusing on plant productivity and the composition and structure of bacterial communities in root, rhizosphere, and bulk soil compartments. Melatonin treatment substantially improved root biomass, nodulation, nitrogen balance index, and yield-related traits, with the highest response observed in the genotype Xinbaihua 16. Amplicon sequencing revealed that melatonin induced distinct genotype and compartment specific shifts in bacterial community composition, with the root bacteria showing the increased remodeling, including a 45.9% increase in unique amplicon sequence variants (ASVs). Melatonin selectively enriched key Proteobacteria taxa such as Rhizobium, Sphingomonas, and Enterobacter hormaechei, known for their plant-growth promoting and biocontrol capabilities. Notably, melatonin-enriched taxa also included widely recognized nitrogen-fixing symbionts such as Pararhizobium and Ensifer, underscoring a direct link between melatonin-induced microbiome shifts and enhanced nitrogen acquisition capacity. Co-occurrence network analysis indicated that melatonin-treated roots harbored more complex bacterial networks, and Modules 3 and 4, dominated by melatonin-induced Proteobacteria, were strongly correlated with most plant traits. Collectively these findings highlight melatonin dual role as a bio-stimulant and microbiome modulator, promoting a functionally enriched and responsive bacteria that support enhanced plant performance. This study provides novel insights into the melatonin-mediated coordination of plant performance and bacterial assembly, offering a foundation for microbiome-informed crop improvement strategies.