Melatonin Seed Coating Improves Soybean Growth in Association With Rhizosphere Physiological and Bacterial Community Shifts

Melatonin (MT), a potent bioactive molecule, regulates plant development and stress resistance. However, in complex field environments, how MT seed coating recruits soil microbes via root development modulation and exudate release to boost yield remains unclear. In this 2-year field study, five seed coating treatments with or without MT were used to investigate how MT-associated seed coating coordinates soybean growth with root architecture remodeling, exudate profile changes, rhizosphere microenvironment succession, and plant-microbe associations. Results indicated that MT seed coating treatment significantly increased IAA and GA content in roots by 66.3% and 38.2% compared to CK treatment, while inhibiting ABA content. This hormonal synergistic effect markedly enhanced root vitality by 18.4%, synchronously increased total root length and surface area by 54.5% and 62.6%, induced a significant increase in root tip number, and increased malic acid and citric acid contents in root exudates by 37.2% and 29.8% compared to CK. As a result, soybean yield increased by 10.8% under MT treatment. Rhizosphere analyses showed that MT-containing treatments were associated with higher nitrogen-related enzyme activities and improved inorganic nitrogen availability. Microbiome analyses indicated that stochastic processes remained dominant in bacterial community assembly, but MT reduced the relative contribution of stochasticity and promoted relatively stronger deterministic filtering. Correlation analyses showed that Sphingomonas- and Lysobacter-affiliated taxa were positively associated with growth-promoting hormones, tryptophan, root traits, and nitrogen-turnover indicators. Functional prediction suggested that MT-associated bacterial communities had enhanced potential for amino acid metabolism, membrane transport, and energy metabolism. These findings suggest that MT seed coating may promote soybean growth through coordinated changes in root hormonal balance, root architecture, exudate profiles, rhizosphere nitrogen availability, and microbial community structure.