DIFFERENTIAL EXPRESSION OF GLUTAMINE-SYNTHETASE ISOFORMS IN TOMATO DETACHED LEAFLETS INFECTED WITH PSEUDOMONAS-SYRINGAE PV TOMATO

Expression of glutamine synthetase (GS) isoforms of tomato leaflets was investigated during the infection by Pseudomonas syringae pv. tomato. Glutamine synthetase activity decreased markedly when it was determined by synthetase assay, but when GS activity was evaluated by transferase assay a not so strong decline was observed. Two GS molecular forms were separated by ion-exchange chromatography in noninfected and in chlorotic detached leaves. In noninfected leaves the major peak eluted at about 0.22 M KCl; however, in chlorotic leaves the major peak eluted at about 0.15 M KCl. A GS polypeptide of 45 KDa was the major species detectable by Western blot analysis of noninfected leaves; however, a second band of 39 KDa was the major GS species detectable in chlorotic leaves. Western and Northern blot analysis from infected detached leaflets in different stages of pathogenesis showed a decrease of GS polypeptide of 45 KDa and its specific mRNAs and an increase of GS species of 39 KDa and its transcripts. Our results suggest that during the infection there is a change in the GS isoform pattern and cytosolic GS (GS-1) replace chloroplastic GS (GS-2).