Urease-null soybean (eu3-a) under salt and copper stress: nitrogen metabolism, antioxidant defense, and arginine pathway genes.

Abstract

Main conclusion: The eu3-a mutant exhibited greater tolerance to salt stress but increased sensitivity to copper stress, with distinct impacts on nitrogen metabolism, photosynthesis, and antioxidant responses. The eu3-a soybean mutant is urease-null, lacking all urease activity responsible for catalyzing the hydrolysis of urea into ammonia and carbon dioxide. In this study, the urease-null eu3-a soybean mutant was used to assess the saline and copper stresses on nitrogen metabolism. Seeds of eu3-a/eu3-a and the corresponding dominant homozygous Eu3 precursor line were collectively referred to as near-isogenic lines (NILs). Experiments were conducted under hydroponic conditions using plants at the reproductive stage (R1-R2) and subjected to either salinity stress (NaCl: 0, 50, and 100 mM) or copper stress (CuCl₂: 0, 10, and 50 µM) over a 5-day treatment period. The following parameters were assessed in leaf tissue: photosynthetic performance, antioxidant enzyme activity, levels of nitrogenous compounds, and the expression of genes encoding key enzymes in the arginine-derived metabolic network. Overall, salinity imposed more severe physiological disruptions than copper in both lines, as evidenced by an approximately 88% reduction in photosynthetic performance under 100 mM of salt. Both stresses impaired nitrogen metabolism, increasing ammonia levels and reducing nitrate concentration. Interestingly, eu3-a plants demonstrated enhanced tolerance to salt stress relative to Eu3 plants, but this trend was not observed under copper stress. Future work should address nitrogen-related enzymatic activities associated with urease metabolism and elucidate the non-enzymatic antioxidant mechanisms contributing to stress tolerance in eu3-a soybean plants under salt and copper stress.