RESPONSE OF GLUTAMINE-SYNTHETASE ISOFORMS TO NITROGEN-SOURCES IN RICE(ORYZA-SATIVA L) ROOTS

Previous studies, using ion-exchange chromatography, reported that one isoform of glutamine synthetase (GS) existed in the cytosol of rice ( Oryza saliva L.) roots. We report that two isoforms of GS were observe d in rice roots using native polyacrylamide gel electrophoresis (PAGE) , GS activity staining and immunoblotting. One isoform (GSra) had a mo bility similar to that of cytosolic GS in rice leaves (GSI). This isof orm existed in seminal roots of germinating seeds and in the roots of seedlings grown in N-free and N-containing culture solutions. Another isoform (GSrb) moved more slowly than GSra and occurred only in the pr esence of external N. Both ammonium-N and nitrate-N induced GSrb, but the activity of GSrb was higher under ammonium-N than nitrate-N. After the removal of N source from the solution, GSrb activity and GSrb pro tein disappeared as judged by native-PAGE and immunoblotting but GSra remained relatively constant. It was estimated that GSrb contributed a bout 80% of total GS activity of the rice roots grown in the solution containing NH4+. SDS-PAGE and immunoblotting test showed that subunits of GSra, GSrb, and GS1 had the same molecular weight. Our results sug gest that GSra was a constitutive enzyme and GSrb was an enzyme induce d by external N. (C) 1997 Elsevier Science Ireland Ltd.