GmNMHC5 Modulates Gibberellin Homeostasis to Balance Carbon–Nitrogen Metabolism and Enhance Protein Yield in Soybean

Soybean, a vital source of high-quality plant-based protein and oil, continues to be a focal crop for improving yield and quality. Gibberellins (GAs), as key regulators of plant growth and development, hold significant potential for enhancing these traits in soybean. In this study, previously developed GmNMHC5 mutant lines were utilized to assess GA levels, photosynthetic capacity, carbohydrate and nitrogen accumulation, as well as yield and quality characteristics during the seed-filling stage. Knockout of GmNMHC5 was found to enhance endogenous GA biosynthesis, resulting in increased plant height and improved carbon and nitrogen metabolism. These physiological changes contributed to significant increases in both seed weight and size, leading to higher accumulation of carbohydrate and protein reserves in mature seeds. Although total seed yield remained unchanged, protein yield per plant was significantly elevated in the GmNMHC5 knockout line. In contrast, overexpression of GmNMHC5 led to reduced GA levels, decreased plant height, and diminished aboveground dry matter accumulation, ultimately lowering protein yield. These findings indicate that targeted knockout of GmNMHC5 can enhance protein yield in soybean by modulating GA levels and optimizing carbon-nitrogen allocation. This study provides a theoretical basis and valuable genetic resources for the development of high-protein soybean cultivars.