Seed Coating Synergies: Harnessing Plant Growth Regulators to Strengthen Soybean Nodulation and Stress Resilience

Soybean (Glycine max) is a globally important crop, but its productivity is often limited by suboptimal nodulation and nitrogen fixation, particularly under stress conditions. Bradyrhizobium diazoefficiens strain USDA110 is widely applied to enhance nodulation, yet its efficiency can be further improved by phytohormone modulation. This study examined the effects of seed coatings containing plant growth regulators (PGRs)—acetylsalicylic acid (ASA), aminoethoxyvinylglycine (AVG), indole-3-butyric acid (IBA), and 6-benzylaminopurine (BAP)—at varying concentrations from 5 – 500 nM, in combination with USDA110, on nodulation, nitrogenase activity, ethylene emission, physiological traits, and yield of soybean cultivar CM60. Laboratory assays identified 50 nM AVG, 5 nM IBA, and 5 nM ASA as optimal treatments, significantly enhancing nodule number and nitrogenase activity. Greenhouse trials under both well-watered and water-deficit conditions further demonstrated that USDA110 combined with AVG or IBA markedly improved photosynthesis, stomatal conductance, transpiration, plant height, and yield components compared with USDA110 or PGRs applied alone. Notably, USDA110 + AVG/IBA treatments sustained higher seed weight under drought, indicating strong synergistic effects in mitigating stress impacts. These findings highlighted that integrating USDA110 with specific PGRs represented a promising strategy to optimize nitrogen fixation and enhanced soybean productivity under both favorable and challenging conditions.