l-DOPA Impacts Nitrate and Ammonium Uptake and Their Assimilation into Amino Acids by Soybean (Glycine max L.) Plants

l-3,4-Dihydroxyphenylalanine (l-DOPA) is a non-protein amino acid with many biological roles, and it accumulates in species of the Fabaceae family. Our earlier work showed that l-DOPA reduces photosynthesis in soybean plants by inducing stomatal closure and limiting carbon metabolism. Because the photosynthetic assimilation of carbon is closely associated with nitrogen metabolism, we evaluated the effects of l-DOPA on nitrate and ammonium uptake and their assimilation into amino acids in soybean plants. l-DOPA inhibited nitrate and ammonium uptake by the roots and the conversion of nitrate to ammonium in the roots and leaves by nitrate and nitrite reductases. Ammonium incorporation into glutamate and glutamine by the action of the glutamine synthetase/glutamate synthase cycle was also reduced in both organs. In contrast, there was an evident accumulation of free amino acids in the roots and leaves, probably caused by the increased degradation of proteins. Taken together, our data revealed that l-DOPA affects the ability of soybean plants to uptake and assimilate inorganic nitrogen into amino acids.