Nitrogen assimilation in soybean nodules

In order to elucidate the pathways to assimilate the ammonia produced by Na-fixation in soybean nodules, UN-labeled compounds were administered to intact nodules or nodule slices pretreated with various inhibitors of nitrogen assimilation. After exposure to 15N3, 15N-incorporation into various nitrogenous compounds was investigated in attached nodules injected with methionine sulfoximine (MSX) or azaserine (AS). MSX treatment increased the 15N content of ammonia more than 6 times, however, depressed UN content of most of amides and amino acids. AS treatment enhanced 15N content of amido-N of glutamine as well as ammonia, but decreased amino-N of glutamine and most of amino acids. Experiments with nodule slices pretreated with MSX or AS solution and then fed with 15N-labeled ammonia or amido-15N of glutamine showed the same trends. Aminooxyacetate inhibited nitrogen flow from glutamic acid to other amino acids. These results strongly indicate that the ammonia produced by N2-fixation is assimilated by GS/GOGAT system to glutamic acid and then transaminated to various amino acids in situ. 15N-incorporation patterns in nodule slices fed with 15N-labeled ammonia, hydroxylamine, nitrite were similar, but nitrate seemed to be reduced in a definite compartment and assimilated similarly as in intact nodules fed with 15N2.