Changes in the Content of Two Glutamate Synthase Proteins in Spikelets of Rice (Oryza sativa) Plants during Ripening

Nitrogen accumulation in the apical spikelets on the primary branches of the main stem of rice plants have been studied during the ripening process (0-35 d after flowering). The level of NADH-dependent glutamate synthase (GOGAT) protein and activity increased 4- and 6-fold, respectively, in the first 15 d after flowering. Maximum levels of NADH-GOGAT were found at that time when the spikelets had just begun to increase in dry weight and to accumulate storage proteins. Subsequently, both the level of NADH-GOGAT protein and its activity in spikelets declined rapidly. Although changes in ferredoxin (Fd)-dependent GOGAT paralleled changes in NADH-GOGAT, the relative abundance of NADH-GOGAT protein in the spikelets was about 3 times higher than that of Fd-GOGAT from 5 to 15 d after flowering. When the chaff (lemma and palea) was separated from the spikelets 10 d after the flowering, 16% of the NADH-GOGAT protein was found in the chaff and 84% in the young grain tissues (endosperm, testae, aleurone tissues, and embryo). On the other hand, Fd-GOGAT protein was distributed 52% in the chaff and 48% in the young grain tissues in spikelets of the same age. Activity of NADP-isocitrate dehydrogenase, which may generate the 2-oxoglutarate required for the GOGAT reactions, was much higher than that of total GOGAT activities on a spikelet basis during the ripening process. These results suggest that in rice plants NADH-GOGAT is responsible for the synthesis of glutamate from the glutamine that is transported from senescing tissues to the spikelets.