title: Changes in the Content of Two Glutamate Synthase Proteins in Spikelets of Rice (Oryza sativa) Plants during Ripening.
abstract: 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.
authors.affiliation: Laboratory of Plant Nutrition (T.H., T.M., K.O.) and Laboratory of Cell Biochemistry (T.Y.), Department of Applied Biological Chemistry, Faculty of Agriculture, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-Ku, Sendai 981, Japan.
authors.forename: T.
authors.initials: T
authors.lastname: Hayakawa
authors.forename: T.
authors.initials: T
authors.lastname: Yamaya
authors.forename: T.
authors.initials: T
authors.lastname: Mae
authors.forename: K.
authors.initials: K
authors.lastname: Ojima