Response of glutamine synthetase and glutamate synthase isoforms to nitrogen sources in rice cell cultures

As a model system with no photorespiration and no long distance transport, rice cell cultures (Oryza saliva L. cv Sasanishiki) were used to investigate the effect of nitrogen sources on the levels of isoforms of glutamine synthetase (GS) and glutamate synthase (GOGAT). Isoforms of GS and GOGAT were analyzed by immunoblotting methods and their activities in early growth phase of the cells. Cytosolic type GS (41 kDa subunit) and NADH-GOGAT were the major isoforms in the rice cells grown in normal R-2 medium. However, contents of plastid type GS (44 kDa subunit) and Fd-GOGAT increased in response to NO_-³ supply. NADH-GOGAT activity also increased following the supply of NO_-³. In vitro translated products from poly(A)⁺RNA prepared from the cells showed that the precursor of plastid type GS (49 kDa) was detected at 48 h after the inoculation. Supply of NH₊⁴ resulted in an increase in NADH-GOGAT activity but had no effect on the levels of Fd-GOGAT, of polypeptides of the plastid type GS or of the corresponding mRNAs.