Nodule Protein Synthesis and Nitrogenase Activity of Soybeans Exposed to Fixed Nitrogen

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Abstract
Nitrate or ammonium was added to soybean (G. max L. Merrill cv. Corsoy) plants grown in plastic 10 days after nodules first appeared. By the 3rd day of treatment with 10 mM nitrate, nitrogenase specific activity (per unit nodule wt) had decreased to 15-25% of that of untreated plants. Longer incubations and higher concentrations of nitrate had no greater effect. Exogenous nitrate or ammonium resulted in slower nodule growth and decreased total protein synthesis in the bacterial and the plant portion of the nodule (as measured by incorporation of 35S). Two-dimensional gel electrophoresis revealed that the nitrogenase components were not repressed or degraded relative to other bacteroid [Rhizobium] proteins. In the presence of an optimal C source, the nitrogenase specific activity of nodules detached from nitrate-treated plants was equivalent to that of nodules from untreated plants. These results are consistent with models that proposed decreased availability or utilization of photosynthate in root nodules when legumes are exposed to fixed N.