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 (5, 50, and 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 more than 32% and 28%, as, respectively, compared to untreated seeds. Greenhouse trials in pots, both under well-watered and water stress conditions, showed that USDA110 + AVG/IBA significantly improved photosynthetic rate (+21 and +18% compared to USDA110 alone) and increased plant height. Notably, USDA110 + AVG/IBA treatments sustained higher seed weight under drought, increasing it by over 25%, 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.