Accumulation of high levels of free amino acids in soybean seeds through integration of mutations conferring seed protein deficiency

Soybean (Glycine max [L.] Merr.) seeds are rich in protein, most of which is contributed by the major storage proteins glycinin (11S globulin) and β-conglycinin (7S globulin). Null mutations for each of the subunits of these storage proteins were integrated by crossbreeding to yield a soybean line that lacks both glycinin and β-conglycinin components. In spite of the absence of these two major storage proteins, the mutant line grew and reproduced normally, and the nitrogen content of its dry seed was similar to that for wild-type cultivars. However, protein bodies appeared underdeveloped in the cotyledons of the integrated mutant line. Furthermore, whereas free amino acids contribute only 0.3–0.8% of the seed nitrogen content of wild-type varieties, they constituted 4.5–8.2% of the seed nitrogen content in the integrated mutant line, with arginine (Arg) being especially enriched in the mutant seeds. Seeds of the integrated mutant line thus appeared to compensate for the reduced nitrogen content in the form of glycinin and β-conglycinin by accumulating free amino acids as well as by increasing the expression of certain other seed proteins. These results indicate that soybean seeds are able to store nitrogen mostly in the form of either proteins or free amino acids.