Evidence of ammonium assimilation via the glutamine synthetase-glutamate synthase system in rice seedling roots

Abstract

When rice seedling roots were fed 15N-ammonium for 1 hr, the amide nitrogen of glutamine showed the highest 15N abundance. Moreover, glutamine amino, glutamic acid, aspartic acid and alanine showed higher 15N abundance than ammonium did. In roots whose GS activity was inhibited with MS, both the amount of ammonium and its 15N abundance were increased. In contrast, both the amount of all examined amino acids containing glutamic acid and their 15N abundance decreased in roots whose GS activity was inhibited. From these results, it could be concluded that the first step of ammonium assimilation in rice seedling roots was mainly glutamine synthesis by GS and the second was glutamic acid formation by the GOGAT system. The results of an experiment using 15N glutamine also supported this conclusion.