γ-Aminobutyric acid enhances nitrogen use efficiency in soybean through coordinated regulation of root architecture and nitrogen metabolism under low nitrogen stress

Abstract To investigate the physiological mechanisms by which exogenous γ-aminobutyric acid (GABA) alleviates low nitrogen (LN) stress in soybean (Glycine max L.), this study employed a sand culture system under LN conditions (2.9 mmol·L⁻¹, 1/5 of the normal nitrogen level of 14.5 mmol·L⁻¹). Nutrient solutions with normal nitrogen (CK) and LN (LN treatment) were applied from the V1 stage (designated as day 0), followed by root application of 5 mmol·L⁻¹ GABA for three consecutive days starting at the V2 stage (LN + GABA treatment). The effects of GABA on root and shoot morphology, nitrogen metabolism, and photosynthetic parameters were systematically analyzed. The results demonstrated that GABA enhances root system architecture and activity, thereby improving nitrogen acquisition capacity. This is accompanied by elevated activities of key nitrogen assimilation enzymes, including glutamine synthetase (GS) and glutamate synthase (GOGAT), which synergistically optimize nitrogen utilization efficiency. The coordinated regulation of carbon metabolism further stabilizes carbon-nitrogen balance, ensuring the integrity of chlorophyll synthesis and photosynthetic enzyme functionality. Consequently, GABA significantly improves photosynthetic performance and overall plant growth under LN stress. This study reveals a cascade regulatory mechanism involving root system architecture, nitrogen metabolism, carbon-nitrogen balance, and photosynthetic performance, providing a theoretical foundation for developing GABA-based biostimulants to enhance nitrogen use efficiency and support sustainable agriculture with reduced nitrogen fertilizer dependency.