THE SPECIFICITY OF METHIONINE SULFOXIMINE AND AZASERINE INHIBITION INPLANT-TISSUES

According to the current paradigm, an inhibition of nitrate and ammoni um assimilation by methionine sulfoximine (MSX) and azaserine indicate s the importance of glutamine synthetase (GS) and glutamate synthase ( GOGAT) in the assimilation of ammonia and the production of glutamate. Inhibitors of GS and GOGAT also result in altered levels of asparagin e and glutamine. These amides have been shown to inhibit the induction of nitrate uptake and its reduction to ammonium ions. In the current experiments, the expected reduction in glutamine levels were found whe n we used MSX; with azaserine, however, levels of both amides were hig her. In roots, MSX additions resulted in enhanced levels of nitrate re ductase (NR) as expected. In shoot tissues, on the other hand, MSX add itions resulted in an inhibition of the NADH dependent NR and a seven- fold enhancement of the NAD(P)H dependent NR. With azaserine there was a severe inhibition of NR as would be predicted with an increase in t he levels of either of the amides. Thus although MSX and azaserine do have direct effects respectively on GS and GOGAT, there are other fund amental effects on the assimilation of nitrate caused by 1) altered le vels of nitrogen metabolites, and 2) effect of the inhibitors themselv es at other sites in the nitrate assimilation pathway. (C) 1998 Elsevi er Science Ltd. All rights reserved.